A combination of favorable materials properties such as a high breakdown field, high polarization-doped two-dimensional electron gas density, high electron mobility, and high peak electron saturation velocity make the (Al,Ga)N material system of great interest for power switching devices. In order to take advantage of the intrinsic benefits offered by this material system, key challenges associated with surface passivation, epitaxial growth, leakage, gate dielectrics, and electric field management must be addressed in power switching transistors. In this talk, recent materials and device innovations addressing these challenges and enabling high-efficiency high-frequency power switch operation will be overviewed. The potential impact of high-speed high-voltage (Al,Ga)N transistors on power switching and RF applications will be discussed.

GaN-based materials are high attractive for power electron devices because of their combination of high breakdown field and high conductivity - particularly in two dimensional electron gases that form at the AlGaN/GaN interface. We present recent results on improved conductivity in AlGaN/AlN/GaN and InAlN/AlN/GaN heterostructures based on precise layer and composition control in both plasma-assisted molecular beam epitaxy (PAMBE) and ammonia molecular beam epitaxy (NH₃ MBE). In a series of systematic experiments, we elucidate the role of threading dislocations in gate leakage, channel conductivity, and three-terminal breakdown voltage. We also will present new data on high flux PAMBE growth and record bulk electron mobility in NH₃ MBE GaN - we place these later two results in the context of vertical GaN-based structures for high voltage operation.

AlGaN/GaN High Electron Mobility Transistors (HEMTs) are well suited for high power applications in the frequency range from 2 to 100 GHz due to a high breakdown voltage and large charge densities in the channel even without doping the AlGaN barrier. It is assumed that the two-dimensional electron gas (2DEG) in the channel originate from surface donors [1]. Moreover, due to the small distance between channel and surface, it can be expected that the 2DEG may be changed by altering the surface of the HEMT structure.
Plasma based techniques like deposition and dry etching are widely used in the fabrication scheme of modern semiconductor devices. In particular, plasma treatment on the surface of the active transistor area should be avoided because it may affect the electronic properties of the transistors strongly. In the case of AlGaN/GaN HEMTs it has been shown that fluorine plasma treatment depletes the 2DEG channel from electrons and thus changes the transistor characteristics from normally on to normally off. The observed depletion has been attributed to an enhanced surface potential. We have shown recently that incorporation into the AlGaN barrier also enhances impurity scattering of the channel electrons [2].
In this work, we have systematically studied the influence of fluorine and nitrogen based plasma processes on the electronic properties of AlGaN/GaN HEMT structures having different barrier thicknesses, and aluminum content. Also, the influence of a GaN-cap layer with different thicknesses has been investigated. The electrical properties of the 2DEG were characterized by Hall measurements, while photo electron spectroscopy and x-ray reflectivity were used to analyze material properties. The results which will be presented in more detail at the conference can be summarized as follows:
(i) Plasma treatment affects the 2DEG properties primarily by changes of the surface potential.
(ii) Hard plasma treatments reduce the mobility in the 2DEG strongly, which may be explained by enhanced scattering due to the incorporation of negatively charged fluorine ions.
(iii) Annealing in nitrogen at 425°C leads to complete recovery of the electronic properties except for samples which were subjected to hard plasma treatment.
The results of these investigations have been applied for the fabrication of HEMT devices with improved characteristics.
[1] J.P. Ibbetson et al. Appl. Phys. Lett. 77 (2000) 250.
[2] St. Linkohr, et al., Phys. Status Solidi C 9 (2012) 938.

III-nitride (III-N) materials system and related bipolar devices, e.g. PIN diodes and transistors offer greatly increased power-switching performance and a dramatic theoretical advantage in the standard Figures of Merit. Although extensive efforts were spent on the development of III-N unipolar devices such as heterojunction field effect transistors, recent developments of III-N heterojunction bipolar transistors (HBTs), in particular GaN/InGaN HBTs, have shown that III-N bipolar transistors could able be suitable for next-generation high-power switching and amplification. We have previously reported npn GaN/In_0.03Ga_0.97N HBTs with high current gain (h_fe > 100) and high current density (J_C > 20 kA/cm²) for devices grown on sapphire substrates [[1],[2],[3]]. The microwave performance of InGaN/GaN HBTs with f_T = 8GHz were also reported [[4]]. However, these devices were prone to degradation under a high current stressing condition and the achievable J_C and the d.c. power density (P_d.c.) were limited for GaN/InGaN HBTs grown on sapphire. In this paper, we report on a drastic InGaN HBT performance improvement using a palladium-based metallization for the base layer contact. When compared to a typical nickel-based metallization, the Pd-based contact demonstrated a much improved contact stability and is less susceptible to the high-current-stressing related device degradation. The fabricated GaN/InGaN HBTs using the newly developed metal scheme also shows a record J_C > 94 kA/cm² and P_d.c.> 1.3 MW/cm² for a device with an emitter area (A_E) of 11.7 mm². To the best of our knowledge, the results represent the highest current and power handling performance achievable for III-N HBTs grown on sapphire substrates to date. We also report on State-of-the-Art GaN PIN diodes with a reverse blocking voltage of 800V and a specific on-resistance of (R_ONA) of 0.28 mOmega;-cm² at a current density (J) of 2.5 kA/cm². The Baliga Figure of Merit of these devices is 2.5 GW/cm². The on-state resistance represents the lowest value a semiconductor power rectifier could achieve in the 800-V range.
References:
[[1]] S.-C. Shen, et al., “GaN/InGaN Heterojunction Bipolar Transistors with fT > 5 GHz,” IEEE Electron Device Letters, vol. 32, no. 8, pp. 1065-1067, August 2011.
[[2]] Y.-C. Lee, et al., "GaN/InGaN heterojunction bipolar transistors with ultra-high d.c. power density (>3MW/cm²)," Phys. Status Solidi A, vol.209, no. 3, pp.497, 2012
[[3]] Yun Zhang, et al., “High- performance GaN/ InGaN double heterojunction bipolar transistors with power density >240 kW/cm², ” Phys. Status Solidi C, vol. 8, no. 7-8, pp. 2451-2453, 2011
[[4]] R. D. Dupuis, et.,al., “III-N High-Power Bipolar Transistors,” in ECS Transactions, vol. 58, no. 4, pp. 261-267, 2013

There is a great interest in developing rectifying and switching power devices based on wide bandgap materials such as SiC and GaN, while silicon power devices are approaching material physical limits. To date, a majority of GaN power device development effort has been directed toward lateral devices, such as high-electron mobility transistors, fabricated in thin layers of GaN that are grown on foreign substrates. By fabricating power semiconductor devices on bulk GaN substrates with low defect density, it is possible to realize the fundamental material limit potential of GaN including true avalanche breakdown capability.
Here we describe materials-related issues associated with bulk-GaN-based power electronic devices, including challenges of MOCVD growth, estimates of fundamental properties such as electron mobility and breakdown field; and their temperature-dependence and their impact on performance. Vertical GaN pn junction diodes with breakdown voltages up to 3.7kV have been fabricated on bulk GaN substrates. Large-are diodes with 400A pulsed-current capability and 700V reverse breakdown are also demonstrated. However, due to design and process complexities it has been difficult to demonstrate high performance vertical GaN transistors. These devices lag behind the lateral GaN transistors and their SiC-based counterparts. Here we report vertical GaN transistors with breakdown voltages of 1.5kV fabricated on bulk GaN substrates. These transistors operate in a normally-off condition, with a specific on-resistance of 2.2mOmega;-cm².

GaN is a promising material for the development of power electronic devices due to its superior material properties. A problem for device design is that the break-down voltages for lateral and vertical GaN is still limited by field crowding, both at the sample edges and at defects. While field termination techniques such as incorporating field plates or damage implants have been implemented, device breakdown voltages are still well below theoretical GaN limits. In this paper, we propose a novel wrap around field plate termination technique for a vertical device fabricated on a free standing GaN substrate.In the wrap around structure, the field plate is designed so that it controls the entire device area, unlike conventional field plates which simply shift field crowding away from the sample edge. 2D finite element simulations using ATLAS show that this produces a uniform field distribution, similar that of an infinite plate capacitor, with no field crowding at the edges. The wrap around structure outperforms conventional field plate structure, and shows breakdown voltages beyond 1400 V for a 5um thick epilayer vertical Schottky barrier diode.

Continuous scaling of Si transistors has enabled us to sustain Moore&’s law for over 30 years. However, the physical gate length has already reached 30nm in the current 65nm technology node and is expected to reach 10nm within the next few years. This is widely believed to be the physical limit, beyond which we have to look at alternatives to scaling in order to continue enjoying the benefits of Moore&’s law. III-nitrides offer unique advantages over other materials for high frequency and high power applications. Although, in terms of mean electron velocity, these predictions are much less than those possible with GaAs, GaN has a larger peak electron velocity, larger saturation velocity, higher breakdown voltage and thermal stability, making it ideal for use as a channel material in microwave and high frequency integrated circuits. Successful realisation of these devices requires Schottky contacts with large Schottky barriers and good thermal stability. The wide and direct bandgap (3.4eV) of GaN results in a lower leakage current and consequently, an ability to operate at higher temperatures. These properties make GaN an important material for high frequency and high power applications. Schottky diodes have advantages like high operating frequency, fast switching speed and low forward voltage drop. As a result, Schottky diodes are widely used in a variety of RF and microwave applications like varactors, detectors, mixers, multipliers and low-voltage reference circuits. In the study of semiconductor surfaces, the metal-insulator-semiconductor Schottky diode is an important device. The performance of semiconductor devices is closely related to their surface conditions and an understanding of surface physics with the help of MIS diodes is of great importance to device operation.
GaN (n-type) films were grown using plasma assisted molecular beam epitaxy (PA-MBE). Pt/HfO₂/n-GaN metal-insulator-semiconductor Schottky diodes were fabricated using standard lithography techniques. 10nm thick HfO₂ was deposited using RF sputtering as the insulator layer. Conventional Pt/n-GaN metal-semiconductor Schottky diodes were also fabricated. The performance of the Pt/n-GaN (MS) structure was compared with the Pt/HfO2/n-GaN (MIS) structure . IV and CV measurements were carried out to confirm the Schottky nature of contacts.
It was found that introduction of the HfO₂ layer resulted in a decrease in leakage current, slight increase of barrier height and a reduction in the ideality factor. Mechanism of current transport in MS and MIS structures were studied. Diode parameters like barrier height and ideality factor were extracted and were found to be temperature dependant. This indicates that there are inhomogenities in the barrier height at the interface. The experimental observations were modelled by assuming a Gaussian distribution of barrier heights at the interface. Results were validated using industry standard device simulator Silvaco Atlas.

GaN electronic devices despite already achieving outstanding device performance have still limitations in reliability resulting in a restricted safe-operating area (SOA) which could be dramatically increased once trapping and thermal challenges have been addressed. This may even involve the beneficial use of dislocations which have been shown to be able to reduce dynamic Ron effects. Degradation mechanisms have also not been fully understood yet. Our latest results in this field will be presented, using a combination of electrical, optical experimental device assessment and device simulation.

The development of GaN on Si substrates is a critical technology for potential low cost power electronics. These devices can accommodate faster switching speeds, hotter temperatures, and high voltages needed for power electronics applications. However, the lattice mismatch and difference in crystal structure between 111 Si and c-axis hexagonal GaN requires the use of buffer layers in order to grow device quality epitaxial layers. For lateral high electron mobility transistors, these interfacial layers act as a potential source of increased thermal boundary resistance which impedes heat flow out of the GaN on Si devices. In addition, these interfacial layers impact the growth and residual stress in the GaN epitaxial layer which can play a role in device reliability. In this work, we explore the impact of the use of AlN and super lattice (SL) interfacial layers on the thermal boundary resistance and residual stress AlGaN/GaN heterostructures grown on 111 Si. Transient domain thermoreflectance measurements were used to measure the thermal boundary resistance on device stack with increasing complexity beginning with samples of AlN or SL on Si followed by GaN/Al/Si or Gan/SL/Si. In addition the thickness of the GaN layer was increased clearly showing the impact of the GaN layer thickness on the intrinsic thermal conductivity of the GaN layer. The residual strain in the GaN layer was measured using Raman spectroscopy and photoluminescence. Data show that large thermal boundary resistances can be introduced through the use of SL interfacial layers, depending on the number of periods used. In addition, a strong thickness dependence in thermal conductivity exists for the GaN layers. These results suggest that while SL layers may help with the residual stress in the GaN layer, care must be taken in their design in order to minimize the impact on interfacial thermal boundary resistance for GaN on Si devices.

AlGaN/GaN high electron mobility transistors (HEMTs) are promising for high frequency high power device applications in space-based communication systems, due to their unique combination of strong transport properties and radiation hardness. Nevertheless, these devices do degrade after being exposed to large doses of high energy particles. One of the commonly observed degradation features is the shift of threshold voltage (V_th). In particular, proton irradiation has been reported to cause a positive V_th shift in both HEMTs and AlGaN/GaN structures.^[1] Although this effect is generally attributed to the radiation-induced deep levels, the detailed degradation mechanism is not well established yet. Our previous work has revealed that proton irradiation introduces compensating centers in n-type GaN,^[2] therefore, it is of particular interest to investigate the possible correlation between the radiation-induced traps in the GaN buffer and V_th shifts in proton-irradiated AlGaN/GaN structures. Here, we explored the V_th shift as a function of proton fluence for two AlGaN/GaN structures with different GaN buffers. The V_th evolutions for these two samples were significantly different, clearly revealing the impact from the GaN layer. By comparing experimental and simulated results, the V_th shift behavior can be explained by radiation-induced buffer traps.
The two AlGaN/GaN heterostructures studied here consist of identical 26 nm-thick Al_0.2Ga_0.8N/800 nm-thick GaN structures, but with different doping densities in the GaN layers; one was unintentionally doped (UID) while the other one was doped with 4×10¹⁶ cm^-3 Si. The samples were processed into Schottky diodes and exposed to 1.8 MeV proton radiation of different fluences varying from 5×10¹² cm^-2 to 1×10¹⁴ cm^-2. Although both samples exhibit positive V_th shifts after radiation, their evolutions are substantially different. For the sample with UID-GaN, V_th increases rapidly at low fluence. In contrast, for the sample with Si-doped GaN, there is a clear threshold behavior for V_th shift: the V_th is almost constant at low fluences, and starts to increase drastically only after a fluence of 3×10¹³ cm^-2. The simulated band structure shows that large density of irradiation-induced acceptors in GaN can significantly lower the Fermi level in buffer, thus positively shifts V_th in AlGaN/GaN. As this effect on the Fermi level position depends on the ratio between the densities of traps and background doping, a higher trap density, therefore, a higher fluence, is needed for such a behavior to be revealed in the sample with doped GaN. Using the trap energies and densities obtained from our study in n-type GaN, the simulated V_th evolutions match the experimental data, revealing that the proton radiation-induced traps in GaN causes the V_th shift in these AlGaN/GaN structures.
[1] X. Hu, et. al., IEEE Trans. Nucl. Sci.50, 1791 (2003)
[2] Z. Zhang, et al., Appl. Phys. Lett. 103, 42102 (2013)

GaN-based High Electron Mobility Transistors (HEMTs) are contenders for replacing existing Si and GaAs devices in high-power radio-frequency applications. AlGaN/GaN HEMTs take advantage of sheet of highly mobile electrons, confined in a two-dimensional electron gas (2DEG), that forms at the heterointerface and are easily modulated by an applied bias. Unfortunately, high-power operating conditions often result in unpredictable and catastrophic device degradation. In previous work, we correlated the formation of clusters of defects under the drain edge of the gate as a function of device lifetime and performance. Evolution of strain in the AlGaN layer was utilized to estimate the sheet polarization charge density over time. However, the formation mechanisms of these defects as well as contribution of existing threading dislocations have not been fully investigated. As such, quantitative analysis on the evolution of defects is needed to further understand device failure mechanisms.
In this work we employed in-situ Transmission Electron Microscopy (TEM) to investigate degradation of a pristine transistor under electrical bias. Specifically, we compared device degradation during both off- and on-state stresses. Off-state stressing causes the device to enter a pinch off state, wherein the 2DEG is depleted of mobile carriers and the device is only subject to electric field effects. On-state stressing causes the device to experience both electric field and thermal effects caused by accelerated electrons. These experiments resulted in the formation and increase in density of defects as well as shortening of the threading dislocations under bias.
Coupling in-situ experiments with geometric phase analysis and NanoMEGAS orientation mapping, we quantified the evolution of strain and dislocations within the devices. After biasing, High-angle annular dark-field imaging and electron energy loss spectroscopy were performed on the bias-induced defects in order to further analyze their nature and electronic state, and also to investigate possible diffusion of gate metals in the AlGaN layer. This research serves to improve the current understanding of physical degradation of GaN HEMTs during operation and can lead to improved prediction models and better device reliability.

In this study, we have investigated GaN channel layer to suppress drain lag, which is an important parameter for switching performance. During the drain-lag test, the device with our proposed GaN channel layer showed a reduction in trap effect.
GaN HEMTs is a promising device for high-voltage switching and high power amplifier (PA) in RF power applications due to its high breakdown voltage and high electron mobility. A lot of research has been carried out to reveal the mechanism of the trap effect [1-2]. However, in terms of GaN channel layer, the relationship between channel layer quality and drain lag has not yet been fully investigated. In this report, we applied a trap-reduced GaN channel to improve drain-lag performance.
We fabricated GaN HEMTs on a SiC substrate with a normal GaN channel layer (epi-A) and a quality controlled layer (epi-B). After we fabricated an Al-based metal as an ohmic electrode, we annealed it to form an ohmic contact. Both samples showed that specific on-resistance were about 5×10^-6 Omega;cm². SiN passivation film was deposited on the surface and the opening was formed for the Schottky contact area by dry etching. The Schottky electrode was Ni/Au. These devices were used to investigate drain-lag performance.
First, we evaluated the crystal quality by using X-ray diffraction (XRD). The value of the GaN channel tilt angle of epi-A and epi-B were 209 arcsec and 243 arcsec, respectively. The twist angle was also assessed by using XRD. However, from the results, there was no difference of the twist angle between epi-A and epi-B. Additionally, electron mobility and sheet resistance were almost the same value at RT.
Second, we measured devices whose gate periphery and width were 1.8 mm and 0.5 µm, respectively. To investigate the trap effect, drain-lag performance was evaluated and trapping time constant (tau;), which is the maximum value of δ(I_d)/δ(time) calculated from drain lag performance, was also measured. In the measurement, the time period was 10 msec, V_ds = 50 V and V_g were set for I_d to be 20 mA/mm. In the pinch-of state, V_gs = -3 V, V_ds = 100 V and time width was 100 mu;sec. We also surveyed the device with epi-A for comparison.
As a result, tau; were 6.4×10^-5 sec and 1.6×10^-5 sec for the devices with epi-A and epi-B, respectively. This result suggested that the GaN channel tilt angle influenced the drain-lag performance.
[1] A. R. Arehert, A. Sasikumar, G. D. Via, B. Winningham, B. Poling, E. Heller, and S. A. Ringel, IEDM10-465 20.1.1-20.1.4
[2] I. Rosetto, M. Meneghini, G. Meneghesso, E. Zanoni, Microelectronics Reliability 53 (2013) 1456-1460.

Silicon has the lowest cost among all suitable substrates for III-nitride crystal growth and the thermal conductivity of Si is large enough to make it competitive with other substrates for both high voltage and RF applications. Silicon substrates are available in diameters of beyond 300 mm although the substrate size available for nitride growth is limited by suitable deposition equipment and the requirement of (111) substrate orientation. Nitride-based structures grown on Si can be processed in well-established low-cost device fabrication lines and significant effort is underway worldwide to adopt Si-based processes to enable complete integration of GaN-on-Si wafers into these high volume foundries.
Despite the tremendous promise and progress in GaN-on-Si technology, challenges associated with epitaxial growth of large diameter wafers remain. In this work, we will summarize current status of GaN HEMT growth on (111) on-axis silicon substrates varying from 4 to 8 inch in diameter. All substrates used for this study were per SEMI standard thickness. The total thickness of the HEMT epilayers was in the range of 2 - 5 µm.
Detailed in-situ wafer curvature and temperature monitoring was performed to analyze evolution of the MOCVD growth process. The maximum wafer curvature during growth of identical HEMT structures decreased with increasing wafer diameter. For 8 inch substrates, the in-situ wafer curvature peaked at about 100 km^-1 of convex bow. The temperature of the wafer surface was measured in-situ using optical thermometry. Temperature deviated more significantly across 8 inch wafers as compared to 4 and 6 inch. Moreover, the temperature profile across 8 inch wafers changed from nearly flat at the beginning of the growth to convex (hotter at center and colder at edge) in the middle of the process, further to concave (colder at center and hotter at edge) at the end. The change of the temperature profile from convex to concave occurred when the wafer shape was slightly convex, illustrating the complexities associated with maintaining uniform layer thicknesses, doping concentrations, and other film properties in the presence of significant wafer bow.
Typically, as-grown wafers were slightly concave with the warp of less than 30 µm for thinner (~2mu;m) structures and less than 50 µm for thicker (~5mu;m) ones. Good crystal quality and electron transport properties were routinely obtained. Contactless Eddy-current mapping of 8 inch HEMT wafers produced sheet resistance in the range of 400 - 450 Ohm/sq with standard deviation of about 2%. Spreading resistance analysis of the nitride-silicon interface revealed modest to low parasitic conductivity with reasonable uniformity across the 8 inch diameter. Additionally, DC device characteristics from 8 inch HEMT wafers will be presented and discussed.

Gallium Nitride (GaN) is arguably the best suited semiconductor for the next generation of power transistors and diodes. Not only the combination of high critical electric field and excellent transport properties enables much better performance than Si-based power electronics, but also the momentum gained by this semiconductor family thanks to GaN-based solid state lighting and the early prototypes of GaN power devices will be difficult to match by any other material in the future.
This talk will summarize some of the work done by my group at MIT to improve the performance of GaN power transistors and diodes, as well as to benchmark the performance of GaN devices in power electronic circuits. Material growth, device technology and design, reliability, thermal dissipation and packaging, and final system-level application are all important issues that need to be carefully analyzed in order to get the maximum performance out of this amazing semiconductor.
We will also describe the recent progress to seamlessly integrate GaN power transistors and state-of-the-art Si control electronics on the same chip. This heterogeneous integration, which is being developed for 8" wafers, also allows for the integration of magnetic components in order to demonstrate fully integrated power circuits. In these circuits, the high switching frequency of GaN devices is key to minimize the real estate use by the power electronics and lower the cost of the entire integrated system. The proposed integration has important applications in future power management, advanced RF power electronics, and high temperature electronics.
Acknowledgements.- This work has been partially funded by the DARPA DAHI program, the DOE GIGA program, the ARPA-E ADEPT and SWITCHES programs, and the ONR PECASE program.

AlGaN based high-electron-mobility-transistors (HEMTs) grown on silicon are the focus of considerable research efforts, due to the availability of low-cost, large-diameter wafers and the potential for integration with Si-based technologies. However, epitaxy of GaNonSi is challenging compared to growth on sapphire or SiC because of eutectic Ga-Si reactions, and the larger mismatch in lattice constant and thermal expansion coefficient. The large lattice-mismatch between Si and GaN can result in high misfit and threading dislocation density in epilayers, along with significant intrinsic strain causing wafer bow/deformation (either concave or convex) during growth. A pronounced wafer bow leads to large gradients in growth temperature across the wafer, resulting in non-uniformity in epilayer thickness, alloy composition, and device performance. Compressive intrinsic stress is intentionally built in the GaN layer during epitaxy to compensate for the large tensile thermal stress occurring during cool-down. Typically, AlN is used as a nucleation layer to avoid melt-back etching of Si and initiate compressive stress build up in GaN. Reducing the overall compensated stress prevents epilayer cracks, large concave wafer bow, and possible wafer breakage. Intermediate layers such as graded or stepped Al_xGa_1-xN layers [J. Appl. Phys. 113, pp023510 (2013)] and strained AlN/(Al)GaN superlattices (SLs) [Appl. Phys. Lett. 79, pp3239 (2001)] have been proven effective in building compressive intrinsic stress and filter dislocations within the growth plane.
In this presentation, we are reporting on stress control using AlN/GaN SLs for epitaxy of GaN on 200 mm Si substrates ranging in thickness from 0.725 to 1.0 mm. All growths are carried out in a single wafer state-of-art Veeco Turbodisc MOCVD system. Compressive and tensile stress can be precisely adjusted via thickness of the AlN and GaN layers in the SLs. At constant period thickness, the effect of growth conditions, such as growth rate, V/III ratio, and growth temperature, on film stress have been investigated and will be discussed. Crack-free 2 mu;m GaN layers were grown over structures containing 50-100 pairs of AlN/GaN SLs having periods of 3-5nm/15-20nm. The wafer bow at room temperature can be consistently controlled below ±10 mu;m on 200 mm Si. Smooth surfaces with excellent crystal quality are observed; roughness of 0.22 nm in a 5x5 mu;m² AFM scan, and 352 and 375 arcsecs FWHM for (002)omega; and (102)omega; XRD rocking curves are, respectively. The correlation between the residual stress and epilayer cracks will be discussed. Effective dislocation filtering and sharp interfaces between AlN and GaN were confirmed by transmission electron microscope (TEM), and omega-2 theta scans along GaN (006) direction by high resolution X-Ray Diffraction (HRXRD).

AlGaN/GaN high electron mobility transistors (HEMTs) are ideal for high-frequency, high power and high temperature devices. One main and urgent issue that degrades this device is a large leakage current. To address this issue, metal oxide semiconductor HEMTs with high-k dielctrics such as Al₂O₃ have been proposed. Prior studies have reported that the Ga-oxide at the Al₂O₃/GaN interface on a capped GaN/AlGaN/GaN substrate contributes a high density of interface trap states and a high voltage shift for MOSHEMTs.^1,2 However the impact of Ga-oxide on uncapped Al₂O₃/AlGaN/GaN MOSHEMTs is still not clear. In particular, the reactivity of the GaN capped surface and exposed AlGaN surface is very different (e.g a higher oxide or oxynitride concentration on the thin GaN capped surface).³ We have recently reported the chemical impact of N₂ plasma and forming gas (FG) plasma on uncapped AlGaN/GaN.⁴ However, whether the plasma treatments result in device degradation is unclear. In this study,⁵ we use capacitor C-V measurements and in-situ XPS characterization to reveal the impact of thermal oxide, N₂ and FG plasma pretreatments on the threshold voltage behavior. The in situ XPS characterization provides accurate interface chemistry information to define the impact of interface on the electric behavior. The results show that the increased formation of oxide/oxynitride contributes to the D_it at the high-k/AlGaN interace, and treatments with a N₂ or FG plasma reduces D_it by passivating oxide related defects, with the FG plasma more effective. The threshold voltage instability in the AlGaN/GaN devices without a GaN cap is likely due to plasma pretreatments, and not the low concentration of interfacial oxide on AlGaN. This work is supported by the AFOSR Asian Office of Aerospace Research and Development (AOARD) under Grand No. FA2386- 11- 1- 4077.
References
¹ S. Yang, Z. Tang, K. Wong, Y. Lin, C. Liu, Y. Lu, S. Huang, and K.J. Chen, IEEE Electron Device Lett. 34, 1497 (2013).
² S. Ozaki, T. Ohki, M. Kanamura, T. Imada, N. Nakamura, N. Okamoto, T. Miyajima, and T. Kikkawa, CS MANTECH Conf. 1 (2012).
³ D.W. Johnson, R.T.P. Lee, R.J.W. Hill, M.H. Wong, G. Bersuker, E.L. Piner, P.D. Kirsch, and H.R. Harris, IEEE Trans. Electron Devices 60, 3197 (2013).
⁴ X. Qin, H. Dong, B. Brennan, A. Azcatl, J. Kim, and R.M. Wallace, Appl. Phys. Lett. 221604, (2013).
⁵ X. Qin, A. Lucero, A. Azcatl, J. Kim, and R. M. Wallace, submitted, (2014)

The development of InGaN-based LEDs/LDs with operated in the whole visible range are anxiously awaited for the application to the next generation displays. Three primary colors integrated LEDs with high directional radiation profiles will contribute to the development of projection type LED displays, retina light-scanning personal displays as well as high functional solid state lightings. InGaN-based nanocolumn LEDs, which consist of periodically arranged nanocolumns, exhibit excellent innate qualities of RGB emission control by the structural parameter of nanocolumn^{1, 2)} and directional radiation beam property³⁾. The periodic structure of nanocolumn arrays leads to the nanocolumn photonic crystal effect, which has led to the optically pumped lasing of InGaN/GaN nanocolumns^{5, 6)}. The diffraction at the photonic band edge generates a high directional radiation beam with a small radiation angle⁷⁾.
In this study, we fabricated InGaN-based triangular lattice nanocolumn arrays with the lattice constants L of 200-350 nm by Ti-mask selective-area-growth of rf-plasma molecular beam epitaxy^{3, 4)}. InGaN/GaN multiple quantum wells were integrated into the GaN-based pn junction nanocolumn arrays. The emission wavelengths of the nanocolumn arrays were controlled with the structural parameters of diameter D, height H, and L; based on the technology, monolithic integration of four emission colors InGaN-based nanocolumn LEDs was demonstrated. The nanocolumn photonic crystal LEDs were fabricated and the directionality of radiation beam profiles were systematically investigated. The nanocolumn LEDs with the small radiation angle of ±20^o operated at 571 nm with a yellow emission. Design rule of the nanocolumn system for photonic crystal emitters was experimentally studied. Furthermore, the radiation beam profile of nanocolumn laser operation based on photonic crystal scheme was investigated under the optical excitation. From the angular dependency of the light intensity, the photonic band diagram was evaluated and it was evinced that the laser operation occurred at the photonic band edge.
Acknowledgement: This research was supported by a Grant-in-Aid for Specially Promoted Research (24000013) from the Ministry of Education, Culture, Sports, Science and Technology of Japan.
References
1) H. Sekiguchi, K. Kishino et al., Appl. Phys. Lett. 96 , 231104 (2010).
2) K. Kishino et al., Appl. Phys. Express 6, 012101 (2013).
3) H. Sekiguchi, K. Kishino et al., Appl. Phys. Express 1 , 124002 (2008).
4) K. Kishino et al., J. Cryst.Growth 311, 2063 (2009).
5) T. Kouno, K. Kishino et al., Opt. Express 17, 20440 (2009).
6) S. Ishizawa, K. Kishino et al., Appl. Phys. Express 4, 055001 (2011).
7) E. Rangel et al., Appl. Phys. Lett. 98, 081104 (2011).

GaN-based light-emitting diodes (LEDs) have recently emerged as excellent devices for the fabrication of the next generation light sources: thanks to the efforts of the scientific and industrial communities, white LEDs with efficacy in excess of 150 lm/W have already been demonstrated, thus clearing the way for a massive adoption of LEDs in various fields (lighting, automotive, biomedical, optogenetics, hellip;).
However, several factors still limit the performance and the reliability of InGaN-based LEDs, as briefly described in the following:
(i) GaN-based LEDs are usually grown on foreign substrates (silicon carbide, sapphire and silicon), due to the lack of GaN substrates of reasonable size and cost; this fact has a negative impact on the crystalline quality of the devices, resulting in high densities of dislocations, whose presence limits the efficiency and the reliability of the LEDs
(ii) At low current densities, the efficiency of InGaN LEDs is limited by Schockley-Read-Hall (SRH) recombination; on the other hand, at high current levels efficiency is governed by the so-called efficiency droop, whose physical origin is still under discussion
(iii) In normal operation, LEDs are supposed to work at high current density (>70-100 A/cm²) and temperature (Tj up to 150 °C) levels. This may favor the occurrence of several degradation mechanisms (generation of defects, degradation of the contacts, creation of leakage paths, hellip;) that limit the lifetime of LEDs well below the targets of this technology (50 000 h)
We present an extensive analysis of the physical mechanisms that limit the performance and the reliability of InGaN-based light-emitting diodes. Based on recent experimental results, we analyze the following relevant issues
a) origin of non-radiative (SRH) losses in InGaN LEDs: based on combined optical measurements, differential lifetime investigation, and deep level transient spectroscopy (DLTS), we demonstrate that the radiative efficiency of LEDs is strongly dependent on the crystalline quality of the devices. We discuss the role of threading dislocations in favoring the efficiency losses, and we present a detailed description of the properties of the deep levels responsible for SRH recombination in GaN-based LEDs
b) physical mechanisms responsible for the degradation of LEDs: we discuss the most critical degradation mechanisms, namely the generation of defects induced by high current densities, the degradation of the ohmic contacts, the generation of parasitic leakage paths; the discussion is based on recent results obtained by various analytical techniques, including electroluminescence (time-resolved and spatially resolved), DLTS, AFM, hellip; The role of the main driving forces (current, temperature, optical power) and the related acceleration factors will be described and discussed
The experimental data will be critically compared to previous literature reports throughout the presentation, to provide an exhaustive overview of the topic.

In_xGa_1-xN quantum wells (QWs) are widely employed as efficient blue emitters in solid state lighting, but improving the efficiency and achieving higher indium mole fractions to produce bright green emission remain important goals. While fundamental understanding of the factors controlling efficiency can contribute to advances in the technology, it is challenging to fully characterize the relationships between light emission and the spatial distribution of indium given the high density of dislocations threading through very thin QWs. Routine characterization techniques such X-ray diffraction (XRD) and PL mapping on the millimeter scale play important roles in the empirical optimization of growth conditions, but they fail to capture important nanoscale structure-property relationships. We have therefore conducted site-specific analysis of InGaN quantum wells through correlated micro-PL and atom probe tomography to elucidate the relationships between emission wavelength, efficiency, and defect distribution. Baseline studies on continuous In_xGa_1-xN QWs show good agreement between XRD and APT analysis of QW composition and width, though XRD fits are improved by incorporating composition profiles extracted from APT reconstructions, which have previously been shown to be reliable. [1] Hydrogen dosing after the QW growth is found to improve PL efficiency. Hydrogen preferentially etches indium rich regions, leading to the formation of discontinuous quantum wells with gaps. Standard XRD analysis underestimates the indium composition when continuous layers are assumed and transmission electron microscopy measurements average over depth. In contrast, APT reconstructions capture the three-dimensional distribution of indium with sub-nanometer resolution. In the etched sample, micro-PL mapping shows that red-shifted emitting regions are more efficient; we hypothesize that indium rich regions near trench-like defects are preferentially etched, leading to increased carrier localization in the remaining portions of the QWs. The highest indium content in the discontinuous QWs is 2% lower than continuous QWs, which is consistent with the observed overall blue-shift. More generally, these analyses indicate the important role that three decomposition mapping can play in the further development of In_xGa_1-xN QWs for energy efficient solid-state lighting.
The work at Sandia was supported by the Sandia&’s Solid-State Lighting Science Energy Frontier Research Center, funded by the US Department of Energy, Office of Basic Energy Sciences. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Company, for the U.S. Department of Energy&’s National Nuclear Security Administration under contract DE-AC04-94AL85000.
Reference
1. Riley, J., et al. Appl. Phys. Lett. 2014, 104, 152102.

Introduction
In GaInN-based light emitting diodes (LEDs), a decreases of external quantum efficiency with an increase of current injection, the so-called efficiency droop, has been reported[1]. Electron overflow has been suggested as one of the possible reasons[2]. In the past, a methods of a direct measurement of electron overflow has been reported[3], in which the second active regions were added on electron blocking(EB) layers in LEDs to monitor the emission caused by the overflowing electrons at the second active regions. We modified the LED structures to obtain high-quality second active regions and investigated electron overflow of blue-LEDs with two different EB layers, a p-AlGaN and a p-GaN.
Experiment
The LED structure consisted of an n-GaN layer, five periods of Si doped GaInN / GaN QWs as the first active region (blue), a p-type EB layer (p-AlGaN or p-GaN), a 60-nm -thick undoped GaN two periods of undoped GaInN/GaN QWs as the second active region (violet), a p-GaN layer, and a p+-GaN contact layer. The 60 nm-thick undoped GaN was used to reduce unintentional Mg incorporation into the second active region. Emission spectra and intensities were observed under various current injections.
Results and discussion
We measured photoluminescence (PL) spectra of the LED structures to investigate the second active region qualities by comparing the PL intensities. Clear PL emission spectra from the second active regions were observed from the two LEDs. Thus, the qualities of the second active regions were enough high to evaluate the electron overflow. Next, we measured emission spectra under various current injections. The LED with the p-AlGaN EB layer showed the emission from second active region and it increased with an increase of current injection. On the other hand, no emission from the second active region was observed from the LED with p-GaN EB layer. These results shows that electrons were overflowing in case of p-AlGaN EB layer but no in case of the p-GaN EB layer. Our simulation results on the injection efficiency of the LEDs suggest that large positive polarization charges at the GaN barrier/p-AlGaN EB layer interface are responsible for the electron overflow even though AlGaN has wider bandgap than GaN. Higher Mg doping at the GaN/AlGaN interface is suggested to neutralize the positive polarization charges and suppress the electron overflow.
Furthermore, the LED with p-GaN showing no electron overflow still showed clear efficiency droop measured under a pulsed operation.
Therefore, the origin of efficiency droop is not the electron overflow, but the other mechanism.
References: [1] J. Piprek Phys. Status Solidi A, 1-9 (2010), [2] Min-Ho Kim et al. Appl. Phys. Lett. 91 183507 (2007), [3] Kenneth J. Vampola et al. Appl. Phys. Lett. 94 061116 (2009)
Acknowledge: This study was supported by MEXT-Grants-in-Aid for Scientific Research
Specially Promoted Research (#25000011) and Grant-in-Aid for Scientific Research (B) (#26286045).

Trivalent europium (Eu³⁺) ions have been widely used as red-emitting phosphors in cathode ray-tube and plasma display panels. In these applications, the ions are doped into an insulator and red emission is mainly obtained by optical excitation. Eu-doped GaN (GaN:Eu) has been identified as a promising red emitter because it has excellent luminescence properties in the red spectral region resulting from the specific optical properties of rare-earth (RE) materials, such as a sharp, intense, and temperature-independent emission peak due to the intra-4f shell transitions.
We have succeeded in growing GaN:Eu layers with high crystalline quality by organometallic vapor phase epitaxy, and have demonstrated for the #64257;rst time a low-voltage current-injected red emission from p-type GaN (p-GaN)/GaN:Eu/n-type GaN (n-GaN) LEDs [1]. A main emission line with a half width of less than 1 nm was observed at 621 nm, which can be assigned to the ⁵D₀-⁷F₂ transition of Eu³⁺ ions. By optimizing growth conditions and device structures in our laboratory, the light output power has been increasing steadily to sub-mW at an injected current of 20 mA, which is the highest value ever reported. In this contribution, preferential energy transfer from the GaN host to Eu ions, depending on local structures, is demonstrated in GaN:Eu.
Combined excitation-excitation spectroscopy (CEES) using a wavelength-tunable laser revealed that there are at least eight kinds of luminescent Eu sites (OMVPE1-8) with different local structures. From the comparison of photoluminescence (PL) intensities, OMVPE4 was clearly dominant in GaN:Eu. The quantitative evaluation of the Eu luminescent sites, which was based on the excitation cross section and radiative decay time of each luminescent site, showed that more than 80% of Eu ions are incorporated as OMVPE4, while the faction of OMVPE7 is a few % [2,3]. However, the PL spectrum under indirect excitation using a He-Cd laser was markedly different from a concentration-weighed spectrum under resonant excitation, which indicates that a minority site of OMVPE7 exhibits highly efficient energy transfer from the GaN host to the luminescent site. In Eu,Mg-codoped GaN (GaN:Eu,Mg), a few of novel luminescent sites due to Eu and Mg were formed together with conventional Eu luminescent sites [4,5]. The Eu-Mg sites were still minority sites but they were also excited efficiently through the GaN host [6]. All the results imply that the energy transfer is strictly controlled by local structures around Eu ions.
[1] A. Nishikawa et al., Appl. Phys. Express 2, 071004 (2009). [2] N. Woodward et al., Appl. Phys. Lett. 98, 011102 (2011). [3] R. Wakamatsu et al., J. Appl. Phys. 114, 043501 (2013). [4] D. Lee et al., Appl. Phys. Lett. 100, 171904 (2012). [5] D. Lee et al., Appl. Phys. Lett. 102, 141904 (2013). [6] B. Michell et al. Phys. Rev. B 88, 121202(R) (2013).

Eu-doped GaN (GaN:Eu) is a promising material for the active layer in nitride-based red light-emitting diodes (LEDs). Recently, we have demonstrated the first GaN:Eu-based red LED operating at low voltage. However, for practical applications it is necessary to improve the light output. One method for enhancing the luminescence output is to increase the concentration of Eu ions. However, for normal growth temperatures and high Eu concentrations, the formation of precipitation on the sample surface was observed. In this contribution, we will discuss the properties of the Eu precipitation and demonstrate how lowering the growth temperature can reduce it.
For GaN:Eu samples grown at lower temperatures, mirror-like surface with no precipitations was obtained and the room-temperature photoluminescence intensity was approximately doubled as compared to samples grown at the standard temperature. In general, lowering the growth temperature of GaN is considered to increase the defect density. We propose a model that explains how this can lead to the improvement of output intensity. It appears that Eu ions can recover the quality of the grown crystal, while defects around Eu ions can mediate energy transfer from the host. In addition, low-temperature photoluminescence of these samples showed some features which can be explained by the reconfiguration of Eu centers.

Light-emitting diodes (LEDs) are becoming increasingly important in everyday life because of their superior efficiency, much longer lifetime and low pollution compared to incandescent lighting. Even though solid-state lighting based on III-V materials has become commercially available, improvements of the phosphors needed to produce white light are highly beneficial. One way to reach this is improving of the starting materials such as AlN.
By decreasing the particle size and size distribution and by increasing purity of the AlN nanoparticles it is possible to reduce the energy required for phosphor production and improve their color purity and efficiency [1, 2]. Chemical Vapor Synthesis (CVS) is one of the methods able to produce such nanoparticles by reaction of precursors, triethylaluminium (TEAl) and ammonia, in the gas phase. However, the nanoparticles are inevitably exposed to air/oxygen at some point which may cause structural changes in the nanoparticles. These changes are investigated by XAFS spectroscopy, as well as X-ray diffraction. The structural results are supported by investigating the samples using HRSEM in combination with EDS.
[1] X.-J. Wang, R.-J. Xie, B. Dierre, T. Takeda, T. Suehiro, N. Hirosaki, T. Sekiguchi, H. Lib and Z. Sun, Dalton Trans., 43 (2014) 6120
[2] H.-L. Li, R.-J. Xie, N. Hirosaki, T. Suehiro, and Y. Yajima, J. Electrochem. Soc., 155 (2008) J175

Nitride-based vertical-cavity surface emitting lasers (VCSELs) have been achieved[1]. In order to obtain low threshold current, a periodic gain structure (PGS) has been used in infrared VCSELs. We have used nitride-based LED structures in which two active regions emitting different wavelengths were intentionally used to investigate the carrier distribution in each active region. In this study, we investigate Mg concentration dependence in the p-GaN intermediate layer on an emission intensity ratio from two the different active regions. Based on the experimental results, we calculate and estimate an optimum Mg concentration in the intermediate layer to obtain a well-balanced intensity ratio of the two identical active regions, the PGS.
LED test structures were included two different groups of GaInN 3QWs. A p-side active region emitting 440-460 nm and a n-side active region emitting 390-420 nm separated with a 45-nm-thick GaN/p-GaN/GaN intermediate layer were included. The p-GaN layer was Mg-doped to control the carrier distributions in the two active regions. Three samples with different Mg concentrations in the intermediate layers were prepared. SIMS results revealed that the Mg concentrations for the three samples were 0, 5x10¹⁸, and 8x10¹⁸ cm^-3. In addition, the SIMS results also indicated additional Mg incorporations into the p-side active regions. The emission intensity ratios between the two active regions of the three samples were measured at the range from 1.0 to 2.5 kA/cm². We found that the ratios were very stable at the measured ranges. The emission intensity ratio of the n-side active region to the two active regions was linearly increased from 0 to 0.65 with an increase of Mg concentration from 0 to 8x10¹⁸ cm^-3. The calculated emission intensity ratio including the influence of Mg incorporation into the p-side active region showed a good agreement with the experimental emission intensity ratio.
It is impossible to experimentally measure the emission intensity ratio of the two identical active regions, PGS. Thus, we calculated optimum Mg concentration in the intermediate layer for the PGS as follows. We calculated the intensity ratios of not only the PGS but also the two different active regions which locations were exchanged in the structure we measured above. We found that a well-balanced intensity ratio, 0.5, of the PGS were theoretically obtained at 4x10¹⁸ cm^-3 of Mg concentration. Also we found that this Mg concentration corresponded with an average of Mg concentrations of the two samples, one contained 450nm p-side/410nm n-side active region (Mg: 6.5 x10¹⁸ cm^-3) and the other contained 410nm p-side/450nm n-side active region (Mg: 2.1 x10¹⁸ cm^-3). The results encourage us to optimize carrier distribution even in the PGS identical active region case.
[1] D. kasahara, et al., Appl. Phys. Exp. 4(2011)072103.
This study was supported by MEXT-Supported Program for the Strategic Research Foundation at Private Universities, 2012-2016.

A wide-bandgap semiconductor, 6H-SiC is a promising rare-earth-free and thermally stable fluorescent material for the white light generation, although it is an indirect bandgap semiconductor. Two donor-acceptor pairs of N-B and N-Al in 6H-SiC can create broad emission spectra, which cover whole range of the visible spectrum. However, there are some non-radiative recombination centers such are Z_1/2 states, dislocations and surface states, and they should be eliminated to obtain a high emission efficiency.
By employing an advanced closed-space sublimation growth method called “Fast Sublimation Growth Process” (FSGP), a relatively high growth rate of 50-100 mu;m/h is available at low growth temperature of around 1750 °C. A high quality 6H-SiC epilayer doped with N and B was grown on 3.5°-misoriented commercial 6H-SiC substrate. The epilayer has a narrow FWHM of X-ray rocking curve (0006) of less than 20 arcsec, a long carrier lifetime of more than 10 mu;sec, and strong DAP emission with a peak wavelength of 580-600 nm. However, it still contains a kind of localized non-radiative recombination centers, which is proven by the results of mu;-PCD decay curve and internal quantum efficiency.
In this talk, crystal growth, optical properties of N and B doped fluorescent SiC will be described. In addition, carrier relaxation mechanism including localized non-radiative recombination in donor-and-acceptor-doped 6H-SiC are also discussed.

Monolithic InGaN/Si tandem solar cells combine the bandgap tunability of InGaN and low-cost, wide availability of Si, to offer a cost-effective approach towards achieving broad solar spectral absorption and high efficiency photovoltaics. However, the quality of high indium content InGaN grown on Si, and even on GaN, thus far, requires substantial improvement for efficient device performance. This arises from the large mismatches in lattice constants and thermal expansion coefficients, high vapor pressure of N₂ over InN, and phase separation. In an attempt to overcome these challenges, we depart from the commonly employed practice of growing InGaN thin films, and explore the heteroepitaxy of dense highly ordered arrays of mesoscopic InGaN pyramids.
The hexagonal InGaN pyramids, arranged in a dense hexagonal array, were heteroepitaxially grown by metal-organic chemical vapor deposition (MOCVD) on AlN/Si(111) substrate through a porous SiN_y template defined by nanoimprint lithography (NIL). Bound by six {1-101} facets, the InGaN pyramids have a center-to-center spacing of 300 nm, with the base edges of adjacent pyramids parallel and touching each other. Compared to control InGaN thin films grown on bare AlN/Si substrates, the InGaN pyramid array exhibits higher In incorporation of ~25.7% versus ~19.3% due to strain relaxation effects. Further, a greater than five-fold increase in integrated phtoluminecence (PL) intensitiy is obtained, indicating substantial improvement in crystalline quality. The intense photoluminescence peaks at 2.25 eV which is in the green spectral region. The occurence of intense photoluminescence at the relatively high indium content is a very promising result, considering the highly lattice mismatched growth on AlN/Si(111) substrates. Further, average reflectance over the 300 - 1000 nm wavelength range is significantly reduced from 22.3 % to 8.0% due to light trapping effects. Considering these characteristics, we posit that our selective area heteroepitaxial approach can form a key step towards achieving improved crystalline quality InGaN on AlN-buffered Si substrates for the development of high efficiency InGaN/Si hybrid tandem solar cells.

GaN, AlN and their related materials have a potential for the application to high electron mobility transistors (HEMTs). In their development, the control of surface states is a crucial issue [1]. Miao et al. reported that the densities and energy positions of surface states depended on the conditions of surface oxides [2]. Thus, it is an important issue to observe the surface conditions. On the other hand, many threading dislocations (TDs) are included in the GaN and its related crystals at present. It was reported that the surface states of open cores gathering with TDs was similar to the m-plane surface [3]. In this paper, we report the angle-resolved XPS spectra of +c, -c and m-GaN with different surface-oxidation conditions. The Schottky barrier properties of +c, -c-GaN crystals are also discussed. The n-type free-stand GaN layer grown by halide vapor phase epitaxy (HVPE) [4] was used for the measurements in this study. The differential energy of AR-XPS spectra observed at the extracted angles of 0° and 40°. Here, the angle means the angular value between the axis of the analyzer and the incident X-ray beam. The differential energy is proportional to the amount of the electric field in the near surface. It was clarified that the amount of a band bending in m-GaN was a middle of that in c⁺- and c^--GaN. Now, the carrier concentration of +c, -c and m-GaN crystals is same. The results indicate that the position of the surface Fermi level of m-GaN is a middle of that in c⁺- and c^--GaN. The surface Fermi level of m-GaN will leads to the partial reduction of Schottky barrier height in the +c-GaN diodes.
[1] M. Higashiwaki et al., J. Appl. Phys. 108, 063719 (2010).
[2] M. S. Miao et al., J. Appl. Phys. 107, 123713 (2010).
[3] J. Elsner et al., Phys. Rev. Lett. 79, 3672 (1997).
[4] A. Usui et al., Jpn. J. Appl. Phys. 36, L899 (1997).

The operating gate junction temperature in an AlGaN/GaN High Electron Mobility Transistors (HEMTs) is an important parameter used to predict the electrical performance of a device. At high temperatures, thermal degradation effects reduce the device&’s reliability and lifetime. To ensure GaN HEMTs operate under a certain temperature, Micro-Raman spectroscopy is used to estimate the gate junction temperature by averaging through the depth of the channel. Due to the presence of T-gate structures and gate connected field plates, Raman spectroscopy is often not able to measure the direct channel of heat generation. In contrast, Gate Metal Resistance Thermometry, a novel technique, can be used to directly measure the gate junction temperature. This technique takes advantage of the low gate leakage found in GaN-on-SiC HEMTs using a four point measurement to accurately measure the gate resistance across a single channel. The gate junction temperatures for devices with different gate widths are compared while the effect of bias conditions on these devices is also investigated. A comparison of gate junction temperatures obtained from micro-Raman spectroscopy and Gate Resistance Thermometry confirms the accuracy of this novel technique. A 3-D thermal finite element model is also presented to further verify this method.

The growth of gallium nitride and its ternary alloy with indium nitride (In_xGa_1-xN) directly on silicon allows for the development of energy conversion devices such as tandem solar cells and photoelectrodes for water-splitting devices. This system allows for flexibility in heterostructure design in several ways. The bandgap energy of the top layer can be modulated through In/Ga alloying, the electrostatics of the InGaN/Si heterojunction can be changed through doping of both materials, and the synthesis of nanostructures allows for a template to investigate the use of low-dimensional structures for such applications.
In this work, chemical vapor deposition was used to synthesize GaN polycrystalline films on silicon by reacting anhydrous ammonia gas with gallium metal vapor. By tuning the amount of source materials and the specific deposition parameters, the morphology is varied from pyramid-shaped crystallites to vertically aligned sheets. Further investigations of materials synthesis show that GaN grows out of liquid gallium deposited on silicon when exposed to ammonia gas. Additionally, gold nanoparticles are used as growth catalysts for synthesizing GaN nanostructures on silicon. With directed design towards developing a heterojunction photocathode for the hydrogen evolution reaction, undoped GaN was deposited on p+ silicon to form a photoactive layer. Electrochemical measurements in aqueous and non-aqueous solutions, combined with impedance spectroscopy, analyze GaN/Si electrostatics and photoeffects.

Gallium nitride and its alloys with InN and AlN are key components which are currently used to produce blue LEDs and laser diodes. GaN is also very promising material for high-power electronics. The hydride vapor phase epitaxy (HVPE) technique, which is currently used for the growth of thick GaN films or freestanding templates, has several advantages including a very low density of dislocations, lower cost of production as compared to MBE and MOCVD techniques, and a low concentration of point defects. Photoluminescence (PL) is a powerful tool for studying points defects in wide-bandgap semiconductors, yet most of the defects in this material remain unidentified and not well understood. We have prepared and investigated a large number of undoped films grown by HVPE on sapphire under different growth conditions. Steady-state and time-resolved PL was employed to extract valuable information about point defects in this material. The concentrations of the defects were determined from the dependence of PL intensity on excitation intensity. Defects with concentrations in the range of 10¹⁴-10¹⁶ cm^-3 can be detected and analyzed by PL methods. This range falls below the detection limit of other techniques such as secondary-ion mass spectrometry. One of the main PL bands in our HVPE GaN is the red luminescence (RL) band with a maximum at 1.8 eV and a zero-phonon line at 2.36 eV. The RL band is caused by transitions from a shallow donor (at low temperature) or from the conduction band (above 50 K) to an unknown deep acceptor having an energy level 1.130 eV above the valence band. In high-quality GaN, the dominant defect-related PL band is the green luminescence (GL) band with a maximum at about 2.4 eV. This PL band can be easily recognized in time-resolved PL measurements due to its exponential decay even at low temperatures, with a characteristic lifetime of 1-2 mu;s. We preliminarily attribute the GL band to transitions of electrons from the conduction band to the 0/+ level of the isolated C_N defect. The yellow luminescence (YL) band, related to transitions via the -/0 level of the same defect, has a maximum at 2.1 eV and can be observed only for some high-purity samples.

Coherent control of a lattice vibration in bulk solid (mode-selective phonon excitation: MSPE) is one of the attractive methods in the solid state physics because it becomes a powerful tool for the study of ultrafast lattice dynamics (e.g. electron-phonon interaction and phonon-phonon interaction). Not only for that, MSPE can control electronic, magnetic, and structural phases of materials. A mid-infrared (MIR) pulse laser could be a useful photon source for MSPE. However, the MSPE by MIR pulse laser has not been demonstrated directly. In this study, we have successfully demonstrated MSPE in a silicon carbide (SiC) by the MIR pulse laser.
Anti-Stokes Raman scattering spectroscopy was used for direct observation of the phonon states. A MIR free-electron laser (FEL) was used as the wavelength-tunable light source for this measurement. The wavelength of the FEL was tuned to 10.4 mm which is the same as the target phonon mode in 6H-SiC. A second harmonic of Nd-YAG laser (532 nm) was used as the probe light for the anti-Stokes Raman scattering spectroscopy. Irradiation on the cooled sample at 14 K produced a scattering band whose peak located where the Raman shift corresponds to the photon energy of 119 meV (10.4 mm). This phenomenon was induced by MSPE through the irradiation of MIR-FEL, whose photon energy matched to the photon-absorption of a particular phonon mode. As the conclusion, MSPE by a MIR pulse laser with a bulk SiC was demonstrated directly by anti-Stokes Raman scattering spectroscopy.

We report on the reduction of threading dislocation density (TDD) of GaN epilayer grown on the {111} exposed surface of a V-groove formed on Si(100) substrates by metalorganic chemical vapor deposition (MOCVD). The V-grooves were either aligned parallel or misoriented at an angle α (α = 2, 4 or 6) towards the [011] crystallographic direction. The intentional introduction of misorientations of the masked trenches resulted steps and terraces on the exposed {111} surface. It is found that the full width at half maximum (FWHM) of both out-of-plane (0002) and in-plane (10-11) rocking curves of GaN grown on the 6^o misoriented template was narrower by a factor of 2 compared to the other samples indicating a reduction in both screw and edge dislocation densities (TD). The lithographically defined grooves are defined as either discontinuous or continuous based on whether there are spaces separating the grooves or not. Raman scattering results revealed that, GaN grown on the discontinuous V-grooved samples was unintentionally doped with free carriers from the SiN_x mask, resulting in the disappearance of the A₁(LO) phonons mode. Transmission electron microscopy (TEM) results confirmed the TDD reduction of GaN grown on the 6^o misoriented template via dislocation bending towards the edge facets of the 3D AlN buffer islands that nucleated on the vicinal Si{111} interface. Vicinal Si(100) substrate with step height (19 ± 3 nm) and terrace length (450 ± 20 nm) of certain geometric parameters are found to be effective in reducing the total TDD in GaN epilayer.

AlGaN/GaN high electron mobility transistors (HEMTs) are excellent candidate for next generation power switching device due to their unique material properties such as wide band gap, high saturation voltage, and high breakdown electric field [1]. For enhanced performance of these devices, low-resistance ohmic contacts are required with smooth surface morphology. The Ti/Al/XY/Au ohmic contact, where XY is a diffusion barrier layer for Au (e.g., Ni, Mo, Pt), is most widely used for ohmic contact in AlGaN/GaN heterostructures [2]. However, this Au-based metallization schemes has very bumpy surface morphology and poor edge acuity. Because of contamination issue, Au-based contacts are also not available in Silicon fabs [3].
In this work, we used TiN layer to prevent oxidation during high temperature annealing. TiN-based and Au-based ohmic contacts to AlGaN/GaN HEMTs were fabricated for analyzing electrical characterization, surface morphology and edge acuity. The layer structures for the device consisted of i-GaN(cap 20#8491;)/n-Al0.25Ga0.75N(200#8491;)/i-GaN(3000#8491;) grown by metal-organic chemical vapor deposition on a Si substrate. We used circular transmission line method (CTLM) to measure specific contact resistance and inner radius is 100#13211; and gap spacing ranging from 4 to 48#13211;. Prior to multilayer deposition, the samples were dipped in HCl:deinozed water (1:1) solution for 3 min to remove native oxide. Ohmic metallization schemes were deposited by electron beam evaporation and then samples were delineated using lift-off process by stripper. The CTLM samples were annealed in flowing nitrogen ambient at various temperatures and times using rapid thermal annealing chamber. The I-V characteristic was measured by four-probe method [Keithley 4200/SCS]. In order to analyze the surface morphology and edge acuity, scanning electron microscopy (SEM) [JEOL-JSM7100F] and atomic force microscope (AFM) ware carried out. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) were performed to investigate the cross-sectional microstructures and various compounds.

Si-doped n-type GaN is generally used in nitride-based light-emitting diodes (LEDs), where Ti/Al/Ti/Au multi-layer electrode is deposited as a cathode ohmic contact. The light created in the active layer reflects on the back side of a substrate in a flip-chip-type LED, and a part of the light returns to the cathode electrode. Therefore, the light absorption loss in the cathode electrode may not be negligible. However, formation of high reflective ohmic contact to n-type GaN is difficult. In this study, a high Si-doped n-type Al_0.03Ga_0.97N with a carrier concentration of 2.5 × 10¹⁹cm^-3 was introduced instead of the Si dope n-type GaN, whose maximum carrier concentration of 8.0 × 10¹⁸cm^-3. Since the AlGaN layer maintains a flat surface even though with the high Si-doping concentration, it has an advantage to form cathode ohmic contact by the high reflective material. A stack of a high-reflective Ag film and ITO oxidation protection film were depositted on the high Si-doped Al_0.03Ga_0.97N layer, and the I-V chracteristic was investigated with a variation of the annealing condition. Furthermore, we also show an improved performance of a nitride-based near UV-LED with the Ag/ITO ohmic contact.
The Ag/ITO stacked film was deposited on n-type GaN (as a reference) and n-type Al_0.03Ga_0.97N by sputtering technique, and rapid thermal annealing at 475#8451; was performed in an ambient of nitrogen and oxygen mixture (volume ratio of 7 to 3). This annealing condition is an optimized one for a good ohmic contact property to p-type GaN. The I-V curve for Ag/ITO on the n-AlGaN layer exhibited a good ohmic property, due to high Si-doping concentration. Then, this contact was applied to the flip-chip-type LED (l = 400 nm). The LED with the conventional Ti/Al/Ti/Au cathode contact was also fabricated as a reference. In L-I characteristics, the device with the Ag/ITO cathode was shown 20% higher light output than the Ti/Al/Ti/Au cathode, because of the high reflectance.
In conclusion, we presented a high reflective Ag/ITO cathode ohmic contact by using highly doped n-type Al_0.03Ga_0.97N (carrier concentration: 2.5 × 10¹⁹cm^-3). As a result of applying it to the flip-chip-type LED (l = 400 nm), significant improvement of light output power was obtained.

Because high current flows in and out through the metal contacts, low resistance ohmic contact with high thermal stability and good surface morphology is inevitable for high power and high frequency operation of the HEMT. As a metallization scheme for ohmic contact on AlGaN/GaN structure, Ti/Al/Ni/Au has been widely used due to the low resistive ohmic property. However, there exists rough surface morphology for Ti/Al/Ni/Au to obtain low resistance ohmic contact because the metal contact has to be annealed at high temperature above 800 C. In this study, we proposed and examined an effective process using Al₂O₃ particle to improve the surface morphology of a Ti/Al/Ni/Au ohmic contact on an AlGaN/GaN heterostructure.
To investigate the effect of Al₂O₃ particle on the ohmic contact, the AlGaN/GaN structure was selectively etched to define the isolated active region. The ohmic contact region was patterned on the active region of the substrates and Al₂O₃ particles were scattered on the region by using a spin coating of solution with the particles. The typical diameter of Al₂O₃ particles used in our experiment was 300 nm. Most of the particle on the substrate parted from each other with a distance of less than 1 µm. After that, Ti/Al/Ni/Au metal stack was deposited by an e-beam evaporation method on the patterned substrates and lifted off. All of the substrates were thermally treated at various temperatures using a rapid thermal annealing system under a N₂ ambient.
As a result, there was a marked improvement on the surface morphology of a Ti/Al/Ni/Au ohmic contact on AlGaN-GaN heterostructure by Al2O3 particle that was scattered on the surface of the structure. The root-mean square surface roughness of ohmic metal prepared with Al2O3 particle of typical diameter of 300 nm revealed half of the roughness value for the metal without Al2O3 particle after alloying at 850 C for 60s. Also, the enlargement of alloyed lump was limited to a finite dimension even at the higher temperature and the longer annealing time. The reason for the improvement of the surface roughness appears to be that the scattered Al₂O₃ particles limit the large area excess reaction of the ohmic metal on the substrate surface. The proposed process will be useful in device fabrication applications due to its simplicity.

SiC and Group III Nitride (such as GaN, AlGaN et.al.) semiconductors are expected to be utilized in next generation high power devices due to higher performance and ability to operate at higher temperatures. Presently, SiC-based FET's (field effect transistor) operate at around 20 GHz and GaN-based FET's at around 150 GHz.¹⁾ The performance is determined by the individual layer quality in the heterostructure (such as GaN/AlGaN et. al.) and resultant 2DEG. Thus, nitride/SiC epitaxy growth technique is expected to improve SiC-based FET&’s frequency performance. Two methods of growth of GaN on SiC are as follows: (1.) GaN grown on SiC via a buffer layer.^2),3) It is hard to fabricate the GaN/SiC vertical high power device structure for the AlN buffer&’s high resistance and the GaN buffer&’s bad crystal quality. (2.) GaN ELO (epitaxial lateral overgrowth) technique.⁴⁾ The high defect density between the window regions will lead to breakdown when applying high power. It is necessary to develop a new growth method to realize high frequency and high power device.
We developed a new GaN on SiC growth method by using of a single 2-dimension-growth step. Using this method, an undoped GaN layer was successfully grown on SiC by MOVPE (metalorganic vapour phase epitaxy). Before the GaN growth we applied a TMA treat process without NH₃ to prevent the reaction between the source gas (TMG and NH₃) and the SiC substrate. 1.5 mu;m of undoped crack-free GaN was grown on 6H-SiC (Si-face). The V/III ratio was 2161. The X-ray omega;-scan&’s peak showed (0002) FWHM (full width at half maximum) of 241 arcsec and (10-12) FWHM of 297 arcsec. Smooth morphology was confirmed by optical microscope. The good surface morphology was also confirmed by AFM (atomic force microscope) measurement. The GaN surface&’s RMS (root mean square) is found to be 0.156 nm. We also measured the sample&’s vertical resistance. Ti/Al/Ti/Au contact was deposited on GaN surface following a 650#8451; annealing in nitrogen gas environment. The Ni/Au contact was deposited on backside of SiC substrate. From the I-V characteristic, the vertical resistance of GaN/SiC structure was found to be around 82.1Omega;. It is expected to further reduce the vertical resistance by growing the n-GaN/SiC structure. We also expect the GaN and/or AlN hetero SiC-based vertical FET could both achieving high frequency and high power performance.
1) Patrick Hindle, Microwave Journal (2010) June 9.
2) F. A. Ponce, B. S. Krusor, J. S. Major Jr, W. E. Plano, and D. F. Welch, J. Appl. Phys 67 (1995) 410.
3) M. Lahreche, M. Leroux, M. Laugt, M. Vaille, B. Beaumont, and P. Gibart, J. Appl. Phys 87 (2000) 577.
4) J. K. Jeong, J. Choi, H. J. Kim, H. Seo, H. J. Kim, E. Yoon, C. S. Hwang, H. J. Kim, Journal of Crystal Growth 276 (2005) 407.

Surface orientation of a wafer, often called surface miscut or misorientation, represents the angle between surface and its nearest principal crystallographic plane, and the azimuthal direction at which the angle occurs. The surface orientation of sapphire wafer for LED substrate is created intentionally since it has a strong influence on the quality and the physical properties of GaN epitaxial layer deposited on the wafer. The horizontal component of the surface orientation of sapphire wafers for LED devices is usually controlled to be 0.20±0.05°, and the vertical component to be 0.0±0.1°, and the GaN deposited on the wafer is reported to have an improved crystalline quality.[1] Since the surface orientation is very important for device applications, an accurate measurement or precise control of the orientation of sapphire single crystal wafer is essential for good quality of the devices. A variety of measurement methods for the surface orientation using X-ray diffractometric techniques have been reported. Of those methods, the determination of the surface orientation according to the SEMI method[2] is very rapid and simple, so this method is usually used for quality assurance in in-line measurements of mass production. However, the method assumes that the surface normal of a wafer is parallel to the rotation axis of the goniometer employed. If there is any tilt or misalignment in the surface normal with respect to the rotation axis, then the tilt induces errors in the surface orientation. Therefore, the method needs a calibration with a standard for surface orientation. We developed a theoretical model to determine the surface orientation of a wafer precisely through the measurements of high-resolution X-ray rocking curves, and the model can be applied even to the case that the surface normal is not parallel to the rotation axis. Using the model, we determined the surface orientation of 150 mm sapphire single crystal wafer accurately. The horizontal component of the surface orientation was 0.198°, the vertical component 0.033°, and the expanded uncertainty of the components was ±0.005°. The determined orientation was then compared with the results from the SEMI method. The two results were consistent with each other when the misalignment of the sample surface normal with respect to the rotation axis is incorporated. The method we developed does not require an orientation standard or precise alignment of a sample with a goniometer. The method also enables us to obtain the misalignment of the sample surface normal with respect to the rotation axis. In addition, a round-robin test for surface orientations of sapphire wafers has been carried out, and the result will be also presented. [1] A. Nakamura et al., Phys. Stat. Sol. (c) 5(6), 2007 (2008). [2] SEMI MF26-0305, 2011

ZnO is emerging as one of the materials of choice for UV applications. It has a relatively benign chemical nature, a deep excitonic energy level, and a direct bandgap of ~ 3.4 eV. The latter two properties make ZnO a highly efficient light-emitter at and above room temperature. Alloying ZnO with certain atomic constituents can add new optical and electronic functionalities to ZnO. This research is on ZnO-based alloys with by-design optical properties in the blue to the UV part of the spectrum. Two alloy systems are being achieved and researched: Mg_xZn_1-xO, that enables tunability to the deep UV range: ~ 3.3 - 6 eV (376 -207 nm), and ZnS_xO_1-x that enables tunability to the blue/visible range: ~ 3.3 - 2.6 eV (376 - 477 nm). The alloys are thin films grown via a magnetron sputtering technique and have granular morphology. For the Mg_xZn_1-xO alloy system, we will present annealing studies for overcoming the inherent phase segregation tendency, reduction of intrinsic defects, and enhancement of the UV luminescence. It was found that annealing under an Argon environment significantly improved the material and optical properties of films due to the removal of intrinsic defects. The ZnS_xO_1-x system, like the Mg_xZn_1-xO alloys, should in principle exhibit phase segregation. However, our research indicates that this alloy system has energetically favorable intermediate phases such as zinc sulfate (ZnSO₄) which significantly impacts material and optical properties. The removal of this undesirable compound will be discussed in terms of the sputtering growth conditions. This work is supported by the National Science Foundation under Grant No. DMR-1202532

High quality single-crystal sapphire wafer is used as substrate for GaN LEDs. Manufacturing processes of sapphire wafer typically consists of ingot slicing, lapping, DMP (Diamond Mechanical Polishing) and CMP (Chemical-Mechanical Polishing). Polishing processes are utilized to remove mechanical surface damage from the lapped wafer and produce a mirror-like wafer surface. The quality of surface of the sapphire wafer affects the performance of LEDs as well as the production yield. Therefore, the wafer surface must be free of micro-cracks, scratches, and residual surface damage.
In wafer manufacturing industry, surface defects of sapphire wafer are usually inspected by optical microscopy, laser scattering, AFM, TEM, and XRD. For the increase in demand for high quality and size of sapphire wafer, the conventional surface inspection techniques are becoming less efficient due to the very small measuring area of these methods. Unlike these conventional techniques, X-ray topography (XRT) technique can investigate not only the crystallographic defects such as dislocations and surface damage but also the distributions of defects in a whole range wafer. In principle, residual surface damage is imaged by the strain field of damage due to the extreme strain sensitivity of XRT.
In the present work, a series of experiments has been conducted for the assessment of residual surface damage of 4-inch sapphire wafer by means of X-ray transmission topography. To quantitatively assess the residual surface damage, two features were used, which are the magnitude of the diffuse scattering in a reciprocal space map (RSM) measured by high-resolution XRD, and the degree of image contrast of an X-ray topograph. The magnitude of diffuse scattering was determined by the FWTTM (full width at one ten thousandth maximum) of (00#8729;6) rocking curves extracted from RSMs, and the FWTTM value represents the degree of relative residual surface damage in sapphire wafer. In addition, the degree of image contrast of an X-ray topograph was determined by a grayscale level of shades of gray ranging from 0 to 255. We found that the results between the FWTTM value and the grayscale level are consistent with each other, and verified the validity of XRT for the measurement of residual surface damage of sapphire wafer.
In conclusion, XRT allows us to distinguish the residual surface damage area from the damage free area, and provides the metrics to quantify residual surface damage by a grayscale level of X-ray topograph for large diameter wafer.

Realization of efficient laser diodes with UV emission from ~260-360 nm would enable many applications including fluorescence-based biological agent detection, water purification and higher-density optical storage. Currently, Al_xGa_1-xN alloys are the semiconductor of choice for solid-state, UV emitters. However, the lack of a lattice-matched substrate for mid-range compositions (~0.2 < x ~0.8) makes it difficult to grow thick epilayers without out cracking and with a threading dislocation density (TDD) less than 10⁹ cm^-2. Developing methods to lower the TDD in AlGaN epilayers is critical to achieving efficient, reliable UV emitters. Previously, Al_xGa_1-xN templates (x = 0.18-0.3) with TDD in the low 10⁸ cm^-2 were reported for AlGaN overgrown on faceted GaN (11-22) pyramids [2] and on etched trenches in GaN [1] or AlGaN [3].
Here, we report a distinct method for fabricating crack-free AlGaN templates with TDDs in the low 10⁸ cm^-2. In our approach, we also employ AlGaN overgrowth of Al_0.3Ga_0.7N templates but patterned with submicron-wide-mesas on a 2 mu;m pitch. Overgrowth of narrow mesas yields spatially-uniform TDDs of 2-3 x 10⁸ cm^-2, eliminating the need to align laser diodes to lower-defect striped regions. Unlike previously-reported faceted GaN templates which absorb l < 365 nm, these AlGaN templates have an AlN buffer layer and thus are suitable for bottom-emitting UV LEDs. In addition, we used 1.3 mm-thick sapphire substrates (3x thicker than standard 2-inch wafers) for epi-growth to eliminate layer cracking and to reduce wafer bow to less than 15 mu;m, enabling fabrication of narrow device features over the wafer. Using this approach we have demonstrated similarly low TDDs in Al_0.7Ga_0.3N epilayers which are relevant to the UV-C region.
We applied this AlGaN template approach to achieve room-temperature, pulsed-current operation of AlGaN-based UV laser diodes with emission at 352 nm. The resulting ridge-waveguide lasers have threshold current densities of ~22 kA/cm² and were operated to peak output powers of > 2.5 mW per facet. Lasers were fabricated with 2-4 mu;m-wide ridges, 0.7 to 1.3 mm-long cavities and etched facets. The heterostructure design differed from previous reports [1, 2] by utilizing doped waveguide layers and an electron blocking layer between the MQWs and p-waveguide layer. Optical pumping of heterostructures employing GaN/AlGaN MQWs, Al_0.2Ga_0.8N waveguides, a Al_0.3Ga_0.7N bottom cladding and etched facets on low-TDD AlGaN templates yielded room-temperature lasing at 346 nm with a threshold of 50 kW/cm². This low optically-pumped lasing threshold suggests that optimization of the p-type cladding and waveguide doping profiles to minimize optical loss while maintaining effective current injection will lead to lower threshold current densities.
[1.] Iida, et. al. Jpn. J. Appl. Phys. 43 L499 (2004)
[2.] Yoshida, et. al. Jpn. J. Appl. Phys. 46 5782 (2007)
[3.] Tsuzuki, et. al. Phys. Stat. Sol. (a) 206 1199 (2009)

High reflecting Distributed Bragg Reflectors (DBRs) are the building blocks for future nitride based UV vertical emitting laser structures and polariton devices. However, to assure a broad stop band width and high reflectivity along with a high structural quality, a trade-off between ensuring a large refractive index contrast and low lattice mismatch within the AlGaN based DBRs is necessary. For our targeted emission wavelength of 270 nm, a good compromise for AlN/AlGaN DBR having an Al composition of 0.65 is found. At the given wavelength a reasonable high refractive index contrast of 6 % between the two semiconductors is found. However, in order to achieve a reflectivity above 99 % being crucial for polariton devices thick structures consisting of more than 25 layer periods have to be considered. We report on the MOVPE growth of Al_0.65Ga_0.35N/AlN DBRs on sapphire substrate. Different template structures including an AlN and Al_0.85Ga_0.15N structure were used. The latter provided an Al composition at the average value of AlN and Al_0.65Ga_0.35N layers used within the DBR. The dislocation and strain state of both will be analyzed in order to increase the number of layer pairs and therefore the maximum reflectivity of our UV DBRs. For a direct comparison of both structures, first a 20.5 layer pair Al_0.65Ga_0.35N/AlN DBR was grown on top of the different templates. The structural quality was investigated using XRD theta;/2theta; scans. Pronounced periodic superlattice fringes indicate a homogeneous thickness distribution across the DBR stack. The high periodicity and smooth interfaces between the single lambda;/4 DBR layers was verified by cross section SEM investigations. The optical properties of the DBR structures grown on different templates were characterized by reflectivity measurements. Both exhibit similar reflectivity spectra showing a pronounced spectral region of high reflectivity of 8 nm centered at 272 nm. The maximum reflectivity within the stop band reaches 93 % for both. However, with increasing the number of layer pairs, the DBR on the AlN template began to relax at about 23 pairs. Otherwise, the Al_0.85Ga_0.15N template allows for a relief of the accumulated total strain energy across the whole structure. Consequently, the total DBR thickness before cracking was increased by 10 % as equivalent to 3 layer pairs. The improved DBR structure of 25.5 layer pairs grown on top of the Al_0.85Ga_0.15N template exhibit an increased maximum reflectivity of 97 % and a stop band width of 6.8 nm centered at 270.1 nm.

For advanced optoelectronic and electronic devices, like light emitting diodes (LEDs), or laser diodes (LDs) Al containing cladding layers or barriers are necessary. The commonly used hexagonal Al_xGa_1#8209;xN/GaN heterostructures show an inherently strong spontaneous polarization oriented along the hexagonal c-axis as well as strain induced piezoelectric polarization. Such polarization induced electric fields in strained quantum wells can cause the spatial separation of electrons and holes resulting in a severe reduction of optical recombination efficiency. Using the metastable cubic modifications of AlN and GaN such piezoelectric effects can be avoided if the samples are grown in (001) direction.
In this contribution we report on the optical properties of cubic AlN/GaN asymmetric multi quantum wells (MQW) structures on 3C-SiC/Si (001) substrates grown by radio-frequency plasma-assisted molecular beam epitaxy (MBE). Scanning transmission electron microscopy (STEM) and spatially resolved cathodo-luminescence (CL) at room temperature and at low temperature are used to characterize the optical properties of the cubic AlN/GaN MQW structures. An increasing CL emission intensity with increasing film thickness due to the improved crystal quality was observed. This correlation can be directly connected to the reduction of the linewidth of x-ray rocking curves with increasing film thickness of the c#8209;GaN films. Defects like stacking faults (SFs) on the {111} planes, which also can be considered as hexagonal inclusions in the cubic crystal matrix, lead to a decrease of the CL emission intensity. With low temperature CL line scans also monolayer fluctuations of the QWs have been detected and the observed transition energies agree well with solutions calculated using a one-dimensional (1D) Schrödinger-Poisson simulator.

Introduction
Nitride-based tunnel junctions provide more flexible current paths in various optoelectronic devices. The tunnel junctions have been used in tandem solar cells and in vertical cavity surface emitting lasers. Furthermore the tunnel junctions should be useful as contact layers in UV-LEDs since a use of resistive p-AlGaN layers can be minimized. In such an application not only low resistive but also low absorptive tunnel junctions are necessary. In this study, we obtained low resistive nitride-based tunnel junctions consisting of 2nm heavily Mg-doped GaInN and 3nm heavily Si-doped GaN, resulting in relatively low absorption due to only 5nm thickness of the junction.
Experiments and Results
The samples were grown on c-plane sapphire by MOVPE. Test structures consist of standard blue LEDs, tunnel junctions and 0.4 µm n-GaN layers. The tunnel junctions were annealed under oxygen ambient at 725 ^oC for 30min for Mg activation after forming sidewalls of the p-layers [1]. We fabricated two different sizes of LEDs, a 310-µm-square and 35-µm-diameter, with the tunnel junctions and a standard LED with a p-contact for comparison. We measured I-V characteristics and estimated voltage drops of tunnel junctions to compare with standard LED.
In the first test structure, the tunnel junction consisted of 2~3 nm heavily Mg-doped GaInN/15nm hevily Si-doped GaN. InN mole fractions were changed from 0.2 to 0.4. In the measurements of the 310-µm-square LEDs, we obteined the smallest voltage drop, 0.06 V at 20 mA, from 2 nm Ga_0.6In_0.4N tunnel junction. In the measurements of the 35-µm-diameter LEDs, the smallest voltage drop, 1.5 V at 5 kA/cm², was also obtained from the same tunnel junction. This result indicates that a specific resistance of the tunnel junction is about 3.1 x 10^-4 Omega;cm².
In the second test structure, the tunnel junction consisted of 2 nm heavily Mg-doped Ga_0.6In_0.4N/3nm hevily Si-doped GaN. The 310 µm-square LEDs with the wafer showed higher operating voltage than the LED with 2 nm Ga_0.6In_0.4N/15 nm GaN tunnel junction. The thinner GaN layer caused high voltage drop at the tunnel junction. After increasing Si doping to double at 3nm Si-doped GaN, we obteined a comparable voltage drop, 0.15V at 20mA, to our lowest resistive tunnel junction. Now the total thickness of the absorptive tunnel junction is 5nm, meaning that the absorption loss is estomated to be 5%. Our low resistive and thin nitride-based tunnel junction is beneficial to high-efficiency UV-LED.
Reference:
[1] Y. Kuwano, et al., Jpn. J. Appl. Phys. Lett. 52, 08JK12 (2013).
Acknowledgements: This work was supported by MEXT-Grants-in-Aid for Scientific Research Specially Promoted Research (#25000011) and Grant-in-Aid for Scientific Research (B) (#26286045), and the Ministry of Education, Culture, Sports, Science and Technology Supported Program for the Strategic Research Foundation at Private Universities, 2012-2016.

We investigate the relationship between grain boundaries and the optical properties of AlN-rich AlGaN/AlGaN quantum wells (QWs). The samples under investigation were grown by MOCVD on c-sapphire. Each sample consists of a 5 period QW structure, with the AlN concentration of the quantum barrier (QB) set at a nominal value of 73%, and that of the QWs varying between 35% and 65%, giving 257-284 nm band edge emission. We excite the samples below and above the bandgap of the quantum barrier using 244 nm and 229 nm second harmonic lines of an Ar⁺ laser. The grains are observed to have strong excitonic confinement characteristics, which causes a deviation from the usual ‘S-shape&’ photoluminescence (PL) temperature dependence. This tendency progressively intensifies with increasing grain boundary area. PL spectra show that the QWs have a dominant effect on the energy of the near-bandedge peak at temperatures below 150 K, whereas the confinement properties of the grains come to bear beyond 150 K. Room temperature cathodoluminescence hyperspectral imaging using an environmental SEM is used to compare emission from inside grains and near the grain boundaries. While our results show that the grain sizes have a strong modulating effect on the extent of confinement, we observe that the grain boundaries have no effect on the peak intensities of the AlGaN/AlGaN samples, which suggests that there is a distinction between the low-energy shoulder usually observed in photoluminescence spectra and lateral compositional inhomogeneity, which causes grain growth. We investigate the carrier dynamics in the QW using excitation below and above the QB bandgap and power dependent time-resolved PL spectroscopy.

Ternary AlGaN alloys are a promising active medium of deep ultraviolet (DUV) light-emitting devices since their bandgap energy covers from UV-A to UV-C wavelengths down to 210 nm. In most of DUV light-emitting diodes (LEDs) reported to date, c-plane AlGaN multiple quantum wells (MQWs) are used as a light-emitting medium. However, built-in internal electric fields induced by the immobile charges of opposite signs at the well-barrier interfaces due to spontaneous and piezoelectric polarization discontinuities separate electron and hole (e-h) wavefunctions to the opposite interfaces along the c-axis. Consequently, overlap integral of the e-h wavefunctions decreases and emission probability awfully decreases. One of the ways to recover this wavefunction overlap is to intentionally modulate the band profile.
In this presentation, we will show substantial recovery of the wavefunction overlap by modulating the alloy compositions of the wells and barriers of AlGaN MQWs along the c-axis. Such out-of-plane compositional modulation can offer polarization-induced three-dimensional (3D) immobile charges in the modulated layer to intentionally design the energy band profile. The use of this 3D immobile charges has been reported for obtaining a p-type AlGaN layer by linearly grading the alloy composition [1].
To quantify the effects of (i) linear gradation and (ii) quadratic modulation in the compositions of the wells and barriers, energy diagrams of conduction and valence bands and the square of the overlap integral, I², of e-h wavefunctions were calculated for the MQWs through self-consistent Shrödinger-Poisson equations using SiLENSe (STR Inc.). Calculated MQW structure consisted of Si-doped (10¹⁸ cm^-3) Al_xGa_1-xN barriers and Al_yGa_1-yN wells (x > y) coherently grown on a strain-free Al0.80Ga0.20N layer. The values of x and y were varied between 0.60 and 0.80. Also, the well width was varied from 1 to 4 nm and the barrier width was varied from 3 to 15 nm.
When x and y were linearly changing, I² of e-h wavefunctions did not change remarkably compared to that of constant compositions being about 60%. On the other hand, I² were substantially improved up to 75% when the quadratic compositional modulation was adopted. In this case, x and y were carefully chosen to achieve nearly flat conduction band profile in the wells. Similar strategy for obtaining nearly flat valence band profile in the wells gave the I² improvement up to 86%. These results confirm that out-of-plane compositional modulation technique proposed in this study is an effective method for improving emission efficiency of AlGaN MQWs.
This work was supported by the Network Joint Research Center for Materials and Devices.
[1] J. Simon, et al., Science 327, 60 (2010).

High AlN mole fraction AlGaN MQWs have attracted attention as a promising active layer of UV-C LEDs. To improve their IQE, gross concentration of nonradiative recombination centers (NRCs) must be decreased, especially in the region where QW excitons recombine. One of the practical ways to improve IQE of the near-band-edge (NBE) emission is doping Si in the wells, barriers, or both layers. A variety of models to explain this effect have been proposed, such as Coulomb screening of the internal fields that weakens quantum-confined Stark effects (QCSEs), interface improvements by the Si masking effects, and reduction in NRC concentrations. For Al_0.6Ga_0.4N MQWs doped by Si, Murotani et al.¹⁾ have speculated that improvement in interface quality and the decrease in NRC concentration in the wells were the reason for the improved IQE of Si-doped wells. However, exciton recombination dynamics have not been investigated. Here we quantify cathodoluminescence (CL) lifetimes and IQE of the NBE emission in Si-doped Al_0.6Ga_0.4N MQWs¹⁾ at 300K using the spatio-time-resolved cathodoluminescence (STRCL) equipment.^2,3) The changes in those values and spatial inhomogeneity of the peak energy and intensity for the MQWs of different Si-doped regions are compared to clarify the reason for the IQE improvement.³⁾
The samples were 50-period Al_0.6Ga_0.4N/Al_0.75Ga_0.25N QWs⁴⁾ of (i) unintentionally-doped structure, (ii) Si-doped wells, (iii) Si-doped barriers, and (iv) entirely Si-doped structure. They were grown by low-pressure MOVPE on an AlN / (0001) Al₂O₃ epitaxial templates. Remarkable difference was found in the NBE CL intensity of the samples at room-temperature, similar to the results described in Ref. 3: samples (ii) and (iv) exhibited the strongest NBE emission, followed by (iii). The fast decay lifetimes (tau;₁) for the samples (ii) and (iv) were much longer than that of (i) and (iii), reflecting different NRC concentrations. Because the band calculation indicated that overlap integral of electron and hole wavefunctions does not change remarkably by Si-doping, remarkable changes in tau;₁ and IQE must be caused by the decrease in NRC concentrations in the wells and interfaces. Another remarkable change by Si-doping in the wells was seen in better spatial homogeneity of panchromatic NBE CL intensity and increased area size exhibiting the same CL peak energy. The latter may reflect an increased terrace width, similar to the case for low temperature growth of GaN. Therefore, improved thickness uniformity by Si-doping, possibly due to improved surface mobility, combined with the H₃SiNH₂ reactant doping, would decrease the NRC concentration.
This work was supported in part by The Asahi Glass Foundation and AOARD monitored by G. Jessen.
[1] Murotani et al., APL 101, 042110 (2012). [2] Furusawa et al., APL 103, 052108 (2013). [3] Chichibu et al., APL 103, 142103 (2013). [4] Fukuyo et al., JJAP 52, 01AF03 (2013).

GaN-based light-emitting diodes (LEDs) have been widely used in TVs, Cell Phones, and traffic signals due to their excellent properties. Although bendable and flexible light emitters are needed in many applications, high growth temperature of GaN makes it difficult to be directly grown on a flexible substrate, such as polyethylene terephthalate (PET) and paper. To fabricate an flexible optoelectronic devices, graphene can be a replacement material of indium tin oxide due to its excellent mechanical durability and high electrical conductivity. In our experiments, we successfully fabricate the flexible GaN-based LEDs on PET and paper substrate by the combination of laser lift-off technique and simple transfer method of graphene.
Firstly, the sapphire substrate where the conventional GaN-based LEDs are grown was removed by laser lift-off (LLO) process in order to make GaN LEDs freestanding. PET and paper were used as a flexible substrate. Ultra-violet (UV) curing epoxy was spin-coated on the substrates as an adhesive between substrate and GaN-based LEDs. Then, freestanding GaN LEDs were transferred on the substrates, followed by UV curing. In addition, graphene layer was used as a back-side electrode of the vertical GaN LEDs on paper. Graphene layer grown on Cu foil by chemical vapor deposition method was deposited on a paper substrate by simple wet-based transfer method. And then, LLO-GaN LED was fixed on graphene layers using a carbon tape. The electrical and optical properties at different bending conditions were characterized. The electrical properties showed a little reduction of the forward current when they were bended. The peak position of electro-luminescence spectra barely changed. Details of the fabrication processes and results will be presented.

Solid-state lighting technology has been booming in recent years due to its wide use in lighting and displays. White light-emitting diodes (WLEDs), which are the most desirable of all LEDs for general illumination, are expected to be the dominant source of lighting in the near future. The most economically viable WLEDs are phosphor-converted (PC). In a PC-WLED, a one-chip LED is coated with a single phosphor or a mixture of phosphors to generate white light, such as a blue LED coated with a yellow phosphor. Current commercially available yellow phosphors are heavily dependent on rare-earth elements. Due to a potential supply crisis, the search for rare-earth-free yellow phosphors is becoming a pressing matter. Here we build a new rare-earth-free yellow hybrid phosphor by anchoring a preselected organic chromophore into a framework structure with metal nodes. The immobilization of the organic chromophore shifts the compound&’s emission further into the yellow region, with quantum efficiency comparable to the commercially available phosphor YAG:Ce³⁺. This new compound is subject to a broad range of excitation energy, from UV to blue light. Coating a blue LED with this new phosphor readily generates white light with high luminous efficacy. Our new rare-earth-free yellow phosphor thus demonstrates great potential for use in PC-WLEDs.

In this study, we present the results on the room-temperature (RT) oriented crystallization of β-Ga₂O₃ thin films on sapphire (0001) substrates, which were obtained by pulsed laser annealing of amorphous Ga₂O₃ thin films. β-Ga₂O₃ is a wide band gap and deep-UV transparent semiconductor with a band gap of 4.9 eV, that those crystalline thin films are promising for wide variety of UV optoelectronic applications and future high-power devices[1]. Low-temperature crystallization of these thin films would be effective to suppress excessive grain growth and obtain ultra-smooth surfaces and sharp interfaces in consideration of developing multilayered devices. However, there are currently few reports about fabrication technique of β-Ga₂O₃ crystalline thin films at relatively low-temperature (< 700 K)[2]. On the other hand, excimer laser annealing have been widely used for solid-phase crystallization of amorphous thin films at low-temperature, for example in the manufacture of poly-Si TFT&’s[3]. So far, we reported RT epitaxial growth of nitride or oxide thin films such as AlN, ZnO, NiO, α-Al₂O₃, etc. by pulsed laser deposition (PLD)[4]. Here, oriented crystallization of β-Ga₂O₃ thin films was obtained at RT (approximately 200C) by KrF excimer laser annealing of amorphous Ga₂O₃ thin films also grown at RT. In experimental, amorphous Ga₂O₃ thin films were grown on sapphire (0001) substrates at RT by PLD method using a focused KrF excimer laser (lambda;: 248 nm, pulse duration: 20 ns, fluence: 1.5 J/cm²) and a sintered target of β-Ga₂O₃. The grown amorphous films were then laser-annealed by irradiating the non-focused KrF excimer laser (fluence: 0.2 J/cm²) from the film surface in air at RT. Crystallization of the Ga₂O₃ thin films and their orientation were examined by x-ray diffraction (XRD) and reflection high-energy electron diffraction (RHEED). The sample after laser annealing was found to become the oriented β-Ga₂O₃ crystalline thin film. The alteration of surface morphology and roughness was also observed for the annealed film. Furthermore, the influence of the laser intensity, annealing time and the film thickness on crystallinity, surface morphology and optical property of Ga₂O₃ thin films were investigated to discuss the mechanisms of the present excimer laser crystallization at RT.
[1] M. Higashiwaki, K. Sasaki, A. Kuramata, T. Masui and S. Yamakoshi, Appl. Phys. Lett.100, 013504 (2012).
[2] F.B. Zhang, K. Saito, T. Tanaka, M. Nishio and Q.X. Guo, J. Cryst. Growth387, 96 (2014).
[3] T. Sameshima, S. Usui and M. Sekiya, IEEE Electron Device Lett.EDL-7, 276 (1986).
[4] M. Yoshimoto, R. Yamauchi, D. Shiojiri, G. Tan, S. Kaneko and A. Matsuda, J. Ceram. Soc. Japan121, 1 (2013)

Micron-sized single-crystals with round shapes are extremely difficult to fabricate since faceted surfaces normally appear, reflecting the atomic arrangements. Only micro-particles with smooth surfaces of polymers, glass, or ceramics are available, while single-crystalline semiconductor microspheres, which could provide significant advantages in any applications, e.g. optical micro-resonators, have not been fabricated. Recently, we reported on the fabrication of single-crystalline microspheres of semiconductors with anisotropic crystal structures that tend to grow in shapes with low symmetry, such as ZnO and CdSe, after laser ablation in superfluid helium [Sci. Rep. 4, 5186, (2014)]. These microspheres possess surfaces with atomic-level smoothness and stability in ambient condition. Furthermore, the microspheres show very high sphericity, which means a flexible melt of the material, whose shape is determined by surface tension, is ideally cooled to grow in crystalline form to maintain the highly spherical shape.
Here we expand the target materials and investigate the optical properties of the fabricated microspheres. ZnSe as well as ZnO and CdSe single-crystalline microspheres were successfully fabricated and showed efficient lasing in ultraviolet and/or visible region. Their nanoparticles are chemically synthesized, however their microspheres have been very difficult to fabricate because of the anisotropic crystal structures. In micro-photonics, the morphology and crystallinity of the microspheres are very important to obtain high Q values in whispering-gallery modes (WGMs).
We succeeded in the fabrication of the single-crystalline ZnO, CdSe, and ZnSe microspheres with high sphericity by laser ablation in superfluid helium. Fabricated these particles were observed with transmission (TEM) or scanning electron microscopes. Micro-photoluminescence (PL) measurements for single microspheres were also carried out at room temperature. The TEM images indicate that fabricated particles are highly spherical without faceted structures. All spots in electron diffraction pattern taken from the single microspheres can be indexed in those of bulk crystals. These results indicate that the fabricated microspheres are single-crystals. Moreover, PL spectra of the single microspheres showed several WGMs in the low energy sides of the band edges as excitation intensity increased; the power dependence of some sharp modes show superlinear behavior above some threshold values, indicating lasing at room temperature. The application of this fabrication technique to GaN is under way.

The AlInP material system has the highest band-gap (both direct and indirect) amongst the non-nitride III-V semiconductors and is potentially useful in devices such as amber and yellow LEDs, photo-detectors and multi-junction solar-cells, and hetero-junction bipolar transistors. Indirect band-gap Al_0.5In_0.5P lattice-matched to GaAs and direct band-gap (greater than 2.1 eV) AlInP grown on InGaAs compositionally graded buffers are characterized using diffraction contrast TEM, HAADF-STEM and luminescence spectroscopy. Under certain growth-conditions, the microstructure of AlInP concurrently exhibits CuPt-B ordering and phase-separation. The evolution of phase-separation from random composition fluctuations to a periodic profile in the lateral direction is observed at a range of temperatures (600 °C - 700 °C). The periodicity is limited by the aluminum adatom surface-diffusion length and is anisotropic in nature. Increasing the growth temperature reduces the degree of phase-separation and increases its length-scale. No phase-separation or ordering is detected at 750 °C. Exploiting the slow evolution of phase-separation, we demonstrate that it is possible to create ordered-quantum-wells with minimal phase-separation. Alternatively, doping can be used to eliminate ordering and retain phase-separation. Finally, lattice-mismatched direct band-gap AlInP with low threading dislocation densities and with room-temperature photoluminescence comparable to AlGaInP lattice-matched to GaAs is demonstrated. A maximum room-temperature direct band-gap of 2.33eV is measured in Al_0.43In_0.57P at a 0.6% lattice-mismatch from GaAs, beyond which the material transitions to having an indirect band-gap. The impact of phase-separation and CuPt-B ordering on luminescence from lattice-mismatched AlInP is studied. Together, these results further our understanding of phase-separation in MOCVD-grown mixed III-V materials and provides an opportunity to exploit non-random alloys in future applications using lattice-matched or lattice-mismatched substrates.

The US Department of Energy is in the process of creating a new Institute to address the manufacturing, reliability and cost challanges for wide bandgap devices based on SiC and GaN technologies. The main objectives of the Institute are to promote the transfer of fabrication technologies to large commercial foundries with a view to lower the cost and improve reliability using 100 mm and 150 mm substrates, and promote rapid adoption of SiC and GaN based power devices in Power Electronic Systems from power levels of 100 W to 1 MW. These systems include, for example, Traction Drives, Solar Converters, Motor Drives, Power Supplies etc. An overview of manufacturing challanges and systems applications will be provided.

With the introduction of SiC power MOSFETs into the commercial market place, it is critically important to determine the reliability of these devices. A number of potential issues need to be addressed, including the stability of the device threshold voltage, and the reliability of both the body diode and the gate oxide. This work focuses on the stability of the threshold voltage of SiC power MOSFETs and the basic mechanisms that affect this stability.
Limiting the shift in the threshold voltage can be critically important because an ideal switch has minimal leakage current in the OFF state and very low resistance in the ON state. But an excess negative shift of the threshold voltage under high-temperature reverse-bias (HTRB) conditions can lead to a critical increase in OFF-state leakage current and potential device failure. In a similar fashion, a large positive shift of the threshold voltage may occur under high-temperature gate-bias (HTGB) conditions, wherein a positive bias-temperature stress is applied. This can lead to a significant increase in the ON-state resistance and thus a decrease in device efficiency.
Although a number of different types of interfacial charge are present either in the insulating gate oxide or at its interface with the SiC conduction channel, the primary defects are near-interfacial oxide traps. Even commercial-grade devices exhibit a gate-bias-induced threshold-voltage instability effect, wherein a positive-bias stress shifts the threshold positively, and a negative-bias stress shifts the threshold negatively. This effect is repeatable, and caused by the direct tunneling of electrons either into or out of these near-interfacial oxide traps, depending on the applied gate bias. These effects are exaggerated for longer bias-stress times or greater oxide electric fields. Not surprisingly, the magnitude of this instability is affected by the device processing, in particular whether a post-oxidation anneal was performed. The measurement conditions following the gate-bias stress have a strong effect on the degree of instability observed.
Much larger threshold-voltage instabilities are observed when gate-bias stressing occurs at elevated temperature, compared with room-temperature stressing. This is very likely due to the activation of additional oxide traps, which can then participate in the oxide-trap charging process. Details of these mechanisms will be discussed.
Finally, improved reliability test standards need to be developed specifically for SiC power MOSFETs. Existing standards (based on Si technology) have been demonstrated to be inadequate in that the same device may be deemed to have both passed and failed, simply as a consequence of the delay in the measurement that is allowed under present standards.
In conclusion, although significant improvements in the threshold-voltage stability have been demonstrated in state-of-the-art commercial devices, the issue has not yet been fully resolved.

Basal plane dislocations (BPDs) in 4H-SiC epilayers lead to degradation of bipolar devices performance through the formation of Shockley stacking faults. Defect selective etching of substrates prior to growth is performed to induce the conversion of substrate BPDs to electrically benign threading edge dislocations (TEDs) at the substrate/ epilayer interface.¹ It was recently reported that the locations of TEDs were shifted compared to the BPDs.² The shift was along the BPD line directions toward the up-step direction. In this work, transmission electron microscopy (TEM) is used to analyze the conversion process and explain the dislocation shifts.
A 4H-SiC epilayer is grown on a 4° off-cut 4H-SiC substrate. The substrate was etched by KOH-NaOH-MgO prior to growth and the epilayer was etched by KOH after growth. Locations of dislocation etch pits were recorded before and after growth. Cross-sectional TEM specimens were prepared at several converted BPDs.
TEM analysis showed that the conversion point is below the substrate/epilayer interface for converted screw BPDs. However, the conversion point is exactly at the interface for converted mixed (i.e. non-screw) BPDs. A linear relationship is also observed between the conversion depths below the epilayer surface and the shift distances along the step-flow direction.
Based on the experimental observations, the dislocation shift is attributed to a combined effect of two different mechanisms. The 1^st mechanism is the in situ H₂ etching prior to growth, which leads to removal of a certain thickness of the substrate causing a change in the location where BPDs intersect the substrate surface. The H₂ etching leads to a shift in the TED locations along the BPD lines towards the up-step direction. The 2^nd mechanism is the glide of TEDs after dislocation conversion. TED glide leads to a shift in the TED locations along the dislocation Burgers vector direction towards the up-step direction. The TED glide is in agreement with the observation of conversion points below the substrate/ epilayer interface, as well as the linear relationship between conversion depths and shift distances. The glide of TEDs after conversion is confirmed by analysis of BPD conversion in another epilayer grown with no H₂ etching prior to growth.
Glide of TEDs after conversion was previously observed during annealing of SiC epilayers³ however, it has not been reported during epitaxial growth. TED glide is energetically favorable for all converted screw BPDs, as it decreases the total dislocation length. TED glide moves the conversion point deeper in the epilayer even below the substrate epilayer interface. Hence, it could present a viable means for removing BPDs from the epilayer during growth.
References:
¹ Zhang et al., Appl. Phys. Lett. 89, 081910 (2006).
² Song et al., J. Cryst. Growth 371, 94 (2013).
³ Zhang et al., J. Appl. Phys. 111, 123512 (2012).

In MEMS and semiconductor processes, plasma-etching is widely employed to make high aspect ratio contact holes on a substrate surface. However, there are critical problems in the nano-scale plasma-etching processes such as the generation of deformed holes and a decrease in aspect ratio. In particular, in SiC etching, it is difficult to get a high etching rate and selectivity because SiC is harder than other typical MEMS and semiconductor materials such as Si and SiO₂. To eliminate those problems, effects of etchant species on etching are investigated by experiments. However, experimental methods are difficult to reveal an atomistic etching mechanism, and surface reactions in SiC etching are still unclear. In this study, to investigate the different etching mechanism depending on etchant species, we developed an original etching simulator based on tight-binding quantum chemical molecular dynamics (TB-QCMD) method,^1-2 and applied it to the elucidation of surface reactions in SiC etching.
In the etching simulations, 4H-SiC (000-1) surface is continuously irradiated with SF₃ and SF₅ radicals with the kinetic energy of 200 eV, which are the most generated species in SF₆ plasma. During the simulations, SF₃ and SF₅ radicals cause chemical reactions on the SiC surface, and dissociation of Si-C bonds is observed. In both the etching by SF₃ and SF₅ radicals, irradiated S and F atoms make bonds with Si and C atoms, and Si-F, C-F, Si-S, and C-S bonds are generated. Moreover, we also observe the generation of SiF_x and CF_x (x = 1 ~ 4) molecules, which accords with experimental results. In particular, SiF_x molecules are generated more than CF_x molecules in both the simulations. Although we also observe the generation of SiS and SiS₂ molecules, these molecules are generated less than SiF_x molecules. Then, it is understood that F atoms of the etchant radicals contribute to etching of both Si and C atoms, and Si atom vapors more easily than C atom. Eventually, many Si-C bonds are dissociated on the SiC surface, and etching holes are generated. In comparison of the numbers of dissociated Si-C bonds, SF₅ radicals dissociate more Si-C bonds, and promote etching more rapidly than SF₃ radicals. This is because SF₅ radicals provide many dissociated F atoms at bombardments, and those F atoms make many Si-F and C-F bonds with the dissociation of Si-C bonds. Then, SF₅ radical have an advantage in the etching rate. On the other hand, in the etching by SF₃ radicals, Si-S and C-S bonds are generated more than that by SF₅ radicals. These bonds are likely to remain on the SiC surface although a part of Si-S bonds vapors as SiS and SiS₂ molecules. Then, SF₃ radical has less etching rate than SF₅ radical. We successfully revealed the different SiC etching mechanisms between SF₅ and SF₃ radicals with our original TB-QCMD etching simulator.
[1] H. Ito et al., Jpn. J. Appl. Phys., 52 (2013) 026502.
[2] T. Kuwahara, H. Ito et al., J. Phys. Chem. C, 117 (2013) 15602.

Silicon carbide based metal -oxide -silicon field effect transistors (MOSFETs) have enormous promise in high power and high temperature applications. However, the great promise of this technology is limited by relatively low effective channel mobilities and, under some circumstances, bias temperature instability problems. We have investigated silicon carbide-silicon dioxide interface and near interface defects with a variety of electrically detected magnetic resonance (EDMR) techniques and conventional electrical measurements including charge pumping and gated diode recombination current measurements. In our study, the EDMR measurements largely utilized spin dependent recombination; measurements were carried out at multiple frequencies which included 220MHz,350MHz, 9.5GHz, and 16GHz and were made over a temperature range of 20 to 175 Centigrade.The devices investigated included both p and n channel transistors provided by multiple manufactures. Measured devices were prepared under a range of technologically relevant processing conditions. EDMR measurements have essentially all of the (unrivaled) analytical power of electron paramagnetic resonance (EPR) with the additional advantages that they can provide the sensitivity required to observe defect centers in fully processed transistors as well as provide information about spatial distribution and (to some extent) energy levels. We have observed multiple defect centers in these silicon carbide transistors with EDMR. These defects include silicon vacancies on the silicon carbide side of the SiC/dielectric interface as well as oxide E' centers. E' centers are paramagnetic sites involving an unpaired elecron asociated with a silicon atom backbonded to oxygen atoms. These E' defects are often holes trapped at oxygen vacancies. We also observe at least one defect likely associated with nitrogen. Of likely relevance to bias temperatutre instability issues, we observe multiple defects associated with hydrogen atoms, one an E' hydrogen complex. Several defects exhibit EDMR amplitudes which quite significantly increase during bias temperature stressing and then recover after the bias temperature stress is removed, a result which suggests a role for them in bias temperature instability phenomena.

4H-SiC MOSFETs are expected to be the enabling technology for advanced future power electronics applications. Higher electron channel mobility is extremely desirable for next generation devices, in conjunction with stable normally-off operation at higher temperatures (~200°C-225°C). The higher performance and reliability associated with such developments is expected to have a significant positive impact on the device cost. Passivation of interface traps by nitridation at high temperatures is currently the industry standard method for achieving acceptable channel mobility. But even with nitridation, the channel mobility is still only about 5% of the bulk mobility of 4H-SiC. It is therefore highly desirable to develop gate oxidation and channel engineering processes for further enhancement of the channel mobility. In parallel, it is imperative to form a detailed understanding of the transport limiting mechanism in this unique MOS system. In this talk, recent results obtained using ultra-thin surface ‘counter-doping&’ layers using n-type dopants phosphorus, arsenic and antimony as interfacial impurities will be reported. These processes, in conjunction with NO annealing result in at least a factor of two higher channel mobility (at typical oxide fields of 3-4 MV/cm) without substantial reduction of the threshold voltage. The influence of surface counter-doping and transverse electric field on electron transport will be discussed. The primary channel resistance limiting mechanisms, namely, trapping, Coulomb scattering, Surface phonon and surface roughness scattering will be detailed. Results will be presented for MOSFETs fabricated on the conventional (0001) Si-face (relevant for DMOSFETs) and the (11-20) ‘a-face&’ of 4H-SiC (relevant for trench MOSFETs). A ‘universal mobility&’ model for 4H-SiC MOSFETs will be introduced and compared with the well-known universal mobility MOSFET behavior in Si MOSFETs. The ‘universal mobility&’ results will be used to explain typical ‘mobility-threshold inverse relationship&’ observed in NO annealed 4H-SiC MOSFETs. Key differences between the nitrogen and Phosphorus passivated devices will be also be highlighted in this context. In addition, stability and reliability concerns with these novel interfaces will also be discussed.

The interface between 4H-SiC and SiO₂ in metal oxide semiconductor field effect transistor (MOSFET) devices contains a high density of electrically active defects, which adversely affect the performance of SiC-based microelectronic devices by lowering the electron mobility. The electrical properties of these devices can be improved by a number of treatments, the most prevalent of which is a nitric oxide (NO) post-anneal. In addition to this, devices fabricated on different crystallographic faces of SiC, or with varying miscut at the interface show markedly different electronic performance. Our work on NO annealed devices has shown an inverse relationship between anneal time and the width of the transition layer at this interface, which is correlated with improved channel mobility, increased N interfacial density, and decreased charged interface trap density. We have performed a similar investigation on devices fabricated on various orientations (of the SiC substrate) and compared their interfacial qualities.
We present a characterization of the transition layer at the 4H-SiC/SiO₂ interface in SiC MOSFET devices using high resolution transmission electron microscopy (HRTEM), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), and spatially resolved electron energy-loss spectroscopy (EELS). In addition, we have characterized the effect of an NO post-anneal on interfacial states using X-ray photoemission spectroscopy (XPS), utilizing an improved chemical depth-profiling technique that will be described. Using the TEM techniques, we have investigated SiC/SiO₂ interfaces fabricated on the Si-face, with and without miscut, as well as on the a-face. Transition layer information was obtained by chemical analysis through observation of the shift of the Si L_2,3 EELS edge across the interface. Structural characterization of the interface was achieved by quantification of the interfacial roughness via HRTEM image observation and strain at the interface computed by geometric phase analysis (GPA). The results of these characterizations are discussed in terms of interface quality and its effect on device properties.
Our results demonstrate the importance of controlling the quality of the oxide-semiconductor interface in SiC power electronics and our methods provide a framework for analyzing devices processed under a range of various conditions.
*Supported by ARL under Grants No. W911NF-11-2-0044 and W911NF-07-2-0046, and NSF GRFP Grant No. DGE 1322106

GaN growth on three-dimensionally structured silicon surfaces is of significant interest as a pathway to mitigate the effects of lattice and thermal expansion mismatches and to modify the light extraction characteristics of light emitting devices. The prior work has mainly focused on the growth of GaN(0001) on the top surface of etched structures formed on Si(111) substrates. However, there is also interest in understanding GaN nucleation and growth on the sidewalls of 3D structures and investigating the effects of substrate orientation on this process. Previous work in our group investigated GaN growth on Si pillar arrays fabricated by deep reactive ion etching (DRIE) of Si substrates with various crystallographic orientations. It was found that GaN grew preferentially on the sidewalls of Si pillars etched in Si(001) substrates such that GaN(0002)//Si{110}. Consequently, it may be possible to use this approach to fabricate non-polar GaN films on (001)Si substrates via preferential growth off of etched sidewalls.
The studies presented in this work utilized trench structures which were etched by DRIE into Si(001) substrates which resulted in vertical sidewalls of Si trenches that were of the {110} family of planes. A pre-growth H₂ anneal at the growth temperature was used to reduce the roughness of the sidewalls of the Si trenches which arose from the DRIE process. After the pre-growth H₂ anneal, a 90 nm AlN layer was grown followed by GaN growth. Initial short (~17 min) growths showed preferential GaN nucleation on the Si{110} sidewalls of trench structures with little nucleation on the top Si(001) surface. Longer growth times resulted in the formation of GaN “fin” structures which vertically overgrew the top Si(001) trench surface. Cross-sectional TEM studies showed that GaN fin structures grown with a V/III of 8000 consisted of three regions with distinct crystallographic orientations including a non-polar GaN region originating from the Si{110} sidewall and two semi-polar regions which seemed to originate from trench corners. It was hypothesized that the semi-polar regions at the trench corners were nucleating on small facets at the Si trench corners which formed as a result of H₂ etching during the pre-growth anneal. Evidence was given to support this hypothesis by reducing the length of the pre-growth H₂ anneal which resulted in reduced H₂ etching of trench corners and therefore reduced formation of semi-polar GaN orientations. Additional studies showed further reduction in the presence of semi-polar GaN by decreasing the V/III and increasing the growth temperature; both of which resulted in improved lateral atomic diffusion to the Si{110} sidewalls and a single orientation, non-polar GaN fin. The use of trench structures was shown as a viable method for the formation of non-polar GaN on Si(001) substrates. Efforts to laterally overgrow Si trenches in order to obtain a coalesced non-polar GaN film will also be discussed.

GaN epilayers were grown on m-plane (10-10) sapphire substrates using plasma assisted molecular beam epitaxy. Impact of nitridation on structural and optical properties of GaN film was investigated. The film grown on a nitridated surface resulted in a nonpolar (10-10) orientation while without nitridation caused a semipolar (11-22) orientation. The high resolution X-ray diffraction studies confirmed the orientation of the GaN films. X-ray rocking curve showed better crystallinity of semipolar as compared to nonpolar GaN. Atomic force microscopy showed smoother films in case of nonpolar GaN which might be in account of the nitridation treatment. Room temperature photoluminescence study showed nonpolar GaN to have higher value of compressive strain as compared to semipolar GaN film, which was further confirmed by room temperature Raman spectroscopy. Despite the fact that it is difficult to obtain high-quality nonpolar material due to the planar anisotropic nature of the growth mode, we hereby report the development of non-polar GaN of usable quality, on an m-plane sapphire, involving controlled steps of nitridation.

Non-polar a-plane InGaN films were grown on a r-plane sapphire substrate by plasma assisted molecular beam epitaxy (PAMBE). The growth temperature and Indium flux were varied to optimize the desired composition of In_0.17Ga_0.83N on the (11-20) a-plane GaN epilayer grown on a (1-102) r-plane sapphire substrate. The structural, morphological and optical properties of the optimized composition has been studied. It was found that highly a-axis oriented InGaN epilayers with no phase separation can be grown at 550 °C with In/Ga flux ratio of 0.61. The composition of indium incorporation in single phase InGaN films was found to be 17% as estimated by high resolution X-ray diffraction. The room temperature band gap energy of single phase InGaN layers was determined by photoluminescence measurement and found to be around 2.36 eV. The rectifying behavior of the I-V curve indicates the existence of Schottky barrier at the InGaN and GaN interface. The Schottky barrier height (phi;_b) and the ideality factor (eta;) for the InGaN/GaN heterostructures were calculated.

In GaN-based optoelectronic devices, modification of surface morphology is a very important issue for improving the device performances. Various surface texturing techniques of GaN have been developed by employing wet-chemical and dry etching. Wet-chemical etch has several advantages compared with dry etch such as high selectivity, damage-free etch, and high etching rate. Most of wet etching studies have been focused on polar GaN thin film with KOH solutions. However, only few papers reported the effects of wet-chemical etch on semi- and non-polar GaN films using acid solutions. We investigated the selective etching behaviors on various crystallographic orientations of GaN films with KOH and H₃PO₄ solutions.
Polar (000-1), semi-polar (11-22), and non-polar (11-20) GaN thin films grown on sapphire were prepared. 2M KOH and 85 wt% H₃PO₄ solution were used to etch the GaN films. Firstly, every sample was immersed in 2M KOH solution, followed by etching using H₃PO₄ solution. After each etching process, samples were rinsed in deionized water and dried under a flow of nitrogen. The temperature of the etchants was fixed at 100^oC using hot-plate with thermocouple during the etching process (20 minute). The surface morphologies of each sample were characterized by scanning electron microscope (SEM), which clearly showed the newly exposed crystal plane of the GaN films after the etching process using KOH and H₃PO₄ solutions. For example, hexagonal pyramids on KOH etched polar (000-1) GaN film were changed to dodecagonal pyramids after H₃PO₄ etching. We also measured atomic ratio between the gallium and nitrogen on each plane of GaN using EDX system. There is a strong relationship between population of dangling bonds on GaN surface and resistance of wet-chemical etch. Detailed etching behaviors and mechanism will be presented.

Direct optical emitters for solid-state lighting (SSL) necessarily require wide band gap (> 2 eV) semiconductors, and the demand for power electronics (PE) switching at ever higher voltages pushes the desired semiconductor band gap larger still (> 3.4 eV). The InGaAlN alloy system is able to span this band gap range. Commercial SSL products based on InGaN/GaN quantum well light emitting diodes (LEDs) are well established, GaN-based PE products are emerging, and AlGaN-based PE are being considered to utilize their much larger band gap compared to GaN. Continued improvements in the performance and reliability of all of these devices are required to broaden their application spaces and accelerate consumer adoption.
Understanding and controlling deleterious crystal defects are important aspects for such improvement. This is because thermodynamics drives increasingly facile defect formation as the semiconductor band gap increases. Defects can act as traps and non-radiative recombination centers that reduce switching speed and optical efficiency. Understanding the physical origin of such defects, their dependence on growth conditions and alloy composition, and their influence on optical and electrical properties enable rational strategies to mitigate or eliminate their deleterious effects.
This talk will discuss the application of deep level optical spectroscopy (DLOS) for quantitative deep level defect characterization in InGaN/GaN LEDs critical to SSL and high Al mole fraction AlGaN thin films and heterostructures that will provide the basis for post-GaN PE devices. The influence of Indium mole fraction on deep level defect incorporation in InGaN quantum wells within blue- and green-emitting LEDs will be discussed. The role of these quantum well defects in the on-going “green gap” problem that hampers the efficiency of green LEDs and laser diodes will be considered. DLOS study of n-Al_0.8Ga_0.2N:Si films as a function of growth conditions will presented, demonstrating that new growth strategies should be adopted to control point defect incorporation and optimize the electrical properties of high Al mole fraction AlGaN for Al mole fractions significantly greater than 0.6.

Native AlN substrates allow for the creation of low-strain pseudomorphic interfaces with high Al content AlGaN and have dislocation densities lower than 1000 cm^-2. AlGaN devices grown on these substrates have been essential components to deep-UV LEDs and lasers. Additionally, they offer the possibility of high-power electronic devices because of their high carrier mobility, high breakdown voltage, low dynamic loss at high frequency, and wide tunable bandgap. Obtaining optimal functionality from these wide band gap materials requires an understanding of the properties of native as well as intentional and un-intentional impurity defects introduced during growth.
Here, we present the results of density functional theory (DFT) calculations with hybrid exchange correlation functionals to determine properties of native and impurity defects in both AlN and high Al content AlGaN. Defect formation energies, predicted optical signatures, and equilibrium concentrations will be presented. These results are compared to photoluminescence and absorption measurements of AlN samples grown by physical vapor transport (PVT), metal-organic chemical vapor deposition (MOCVD), and hydride vapor-phase epitaxy (HVPE). DFT results are compared to photoluminescence measurements of AlGaN grown by MOCVD.
We find that there are multiple sources of a deep-UV absorption between 4.5 and 5.0 eV in AlN. We show that the presence of the isolated carbon substitutional C_N^- absorbs at 4.7 eV and emits at 3.9 eV. A nitrogen-vacancy donor is found to predominantly compensate the carbon acceptor. This results in a donor-acceptor pair recombination, which is predicted at 2.7 eV. DAP emission at 2.8 eV is confirmed by photoluminescence-excitation (PLE) measurements. The role of complexes in the properties of AlN will also be covered. The energies, optical properties, and local vibrational modes of a variety of three carbon defects will be analyzed. We also explore the formation of a Si-C nearest neighbor complex. This complex has been identified as important in eliminating unwanted optical absorption without removal of un-intentional carbon. Our experimental results confirm that in samples with high Si and high C, the C-Si absorption and emission signature are both present, while the isolated carbon signature and carbon DAP signature are not present. Finally, we will discuss the well-known broad near-ultra-violet emission peak between 3.0 and 4.0 eV that is observed when the oxygen and silicon content is high and the carbon content is low. We will present results that show that this emission is caused by complexes between a cation vacancy and one or more substitutional oxygen impurities (V_Al+nO_N).

Gallium nitride is a III-nitride semiconductor with direct, wide (3.4 eV) bandgap. The III-nitride semiconductor family has a wide range of applications in optoelectronics, high-power and high frequency devices. However, the material still faces challenges related to, e.g., light emission efficiency at high currents in the case of LEDs and extension of lifetime of laser diodes. The exact origins of these defects are still under debate. Positron annihilation spectroscopy is a powerful yet nondestructive tool in studying neutral and negative vacancy type defects in crystalline matter, such as, semiconductors [1]. This is possible as positrons can get trapped at defects, and the annihilation radiation carries information of local atomic and electronic structure. In practice this, changes in positron lifetime and positron-electron momentum distribution are monitored to identify and quantify vacancy defects.
We present a combined experimental and computational study of vacancy-hydrogen complexes in GaN. We show that Ga vacancy-oxygen-multihydrogen complexes can be identified through their annihilation characteristics. These defects have been suggested to exist at abundant concentrations in MOCVD GaN thin films [2] and ammonothermal GaN bulk crystals [3]. Importantly, Ga vacancies complexed with 1-2 hydrogen atoms are readily detected with positrons and introduce deep electronic states in the gap, while the V_Ga-3H are invisible to positrons and neutral. It seems that hydrogen can be stripped from the latter by a high enough current density [2], suggesting a possible cause for degradation of optical properties under lasing conditions.
[1] F. Tuomisto and I. Makkonen, Rev. Mod. Phys. 85, 1583 (2013).
[2] H. Nykänen et al., Appl. Phys. Lett. 100, 122105 (2012).
[3] F. Tuomisto et al., J. Crystal Growth, in press.

The effects of doping on the minority charge carrier concentration in GaN remains a controversial issue, with many theoretical studies producing contradicting results. The standard method to model defects in crystals is the plane-wave supercell approach, which has disadvantages related to unwanted interaction between periodic images of defects. We present results of calculations based on a hybrid quantum mechanical/molecular mechanical embedded cluster approach to modelling defect formation associated with divalent metal dopants in GaN, with a particular focus on the technologically crucial case of Mg doping. As our approach does not employ periodic boundary conditions, provides access to the vacuum level, and allows the use of high level quantum chemistry approximations, accurate and unambiguous defect levels can be determined. From our calculations we find that a substantial amount of experimentally determined optical data can be attributed to the N vacancy in GaN, which is a charge-compensating defect for Group 2 dopant incorporation. Our calculated defect levels associated with the dopants are in excellent agreement with experiment where available. Furthermore, we definitively show that standard hybrid density functionals are inadequate for treating these defects and that double exchange is necessary for an accurate description.

In recent years, III-nitride materials - traditionally used in light-emitting diodes, high power and high temperature electronic devices, and short-wavelength photodetectors - have been attracting interest in less traditional fields such as photo-electrochemistry and photocatalysis [1,2,3,4]. The transport and recombination of photo-generated charges at the surface are crucial for all optoelectronic applications based on GaN. In general, processes involving photo-generated charges have very long time-scales in wide gap semiconductors (“persistent photoconductivity”) [5], the origin of which is not well understood. Here, we show that by combining conductance and contact potential difference (CPD) measurements in a consistent and systematic way very valuable information can be obtained about the kinetics of charges photo-generated at and close to the GaN surface. We compare the processes involved in the accumulation and decay of charge carriers generated via excitation with below (visible) and above (UV) band gap light with varying illumination intensity, where electrons and holes are generated far and close to the surface, respectively. In this way, we can draw a more complete picture about the underlying processes that dominate recombination in the different regions of the material. (0001) GaN layers grown by metal-organic chemical vapor deposition unintentionally n-type doped with 1.4×10¹⁷ electrons/cm³ were used for this investigation. In particular, we probe the role played by localized deep defect states in the kinetics of photo-generated charges. These states are responsible for the deep trapping of photo-generated electrons in the space-charge region close to the surface, which explains the slow response of photocurrent under UV illumination. These states are also involved in the transfer of electrons back to the surface after illumination, which results in the slow recovery of the CPD response after illumination. Moreover, due to the presence of these deep defect levels both electrons and holes can be generated (far from the surface) with below band gap illumination. However, capture of photo-holes at the surface is in this case less efficient than in the case of holes generated with UV illumination (close to the surface), where electron-hole separation can take place more efficiently due to surface band bending.
[1] J. Howgate et al., Adv. Mater. 22, 2632 (2010)
[2] S. Schäfer et al., J. Phys. Chem. C 116, 22281 (2012)
[3] S. Schäfer et al., J. Am. Chem. Soc. 134, 12528 (2012)
[4] F.Schuster et al., Nano Lett. 12, 2199 (2012)
[5] C. H. Qiu and J. I. Pankove, Appl. Phys. Lett. 70, 1983 (1997)

High AlN mole fraction AlGaN quantum wells have attracted attention for applications in DUV LEDs. To increase the external quantum efficiency (EQE) of the near-band-edge (NBE) emission, the use of low threading dislocation (TD) density bulk AlN substrates^1,2) is an essential approach. However, as EQE of DUV LEDs is still limited to 10%,^3,4) internal quantum efficiency (IQE), injection efficiency, and light extraction efficiency must be pursued. Among these, IQE is an intrinsic material talent and is a fraction of radiative rate over the sum of radiative and nonradiative rates; i.e. IQE=(1+tau;_R/tau;_NR)^-1, where tau;_R and tau;_NR are the radiative and nonradiative lifetimes, respectively. To understand what dominates and how we increase tau;_NR and decrease tau;_R, quantitative analysis of the lifetimes is mandatory, especially in view of the structural and point defects. However, due to limited availability of a desirable DUV femtosecond excitation source, only a few papers^5-7) have dealt with the emission dynamics of AlN and there has been no reported result on optical investigation of AlN using spectroscopic tools with sufficiently high spatial and temporal resolution.
In this presentation, we show the results of spatio-time-resolved cathodoluminescence (STRCL) spectroscopy⁸⁾ on various quality AlN epilayers grown on the bulk AlN substrates prepared by physical vapor transport.¹⁾ Reflecting the low TD density (lower than 10⁴ cm^-2), room-temperature CL intensity images mapped at the free A-exciton energy exhibited homogeneous contrasts. Then we show well-resolved low-temperature excitonic CL peaks to verify the exciton binding energy (51.3 meV). Low-temperature CL peaks at 6.0415 and 6.0287 eV, which were polarized parallel and perpendicular, respectively, to the c-axis, exhibited identical risetimes and short lifetimes, and the latter coincided with the temporal delay of neutral donor (Al) -bound exciton emissions. These results support the assumption that the two peaks originate from the recombination of free A-excitons of irreducible representations Γ₁ and Γ₅, respectively.
This work was supported in parts by NEDO by METI and The Asahi Glass Foundation, Japan, and AOARD budget monitored by G. Jessen.
[1] Rice et al., JAP 108, 043510 (2010). [2] Grandusky et al., Solid-State Electron. 78, 127 (2012). [3] See for example Pernot et al., APEX 3, 061004 (2010); Shatalov et al., ibid5, 082101 (2012) and references cited therein. [4] Kinoshita et al., APEX 5, 122101 (2012) [5] Nam et al., APL 82, 1694 (2003). [6] Onuma et al., APL 96, 061906 (2010). [7] Chichibu et al., APL 97, 201904 (2010). [8] Ishikawa et al., APL 101, 212106 (2012).

GaN is considered a model system for group III-N semiconductors. One of the controversial issues in the field of III-N semiconductors is the effect dislocations have on the electronic properties of the GaN epilayers. A-type screw dislocations are the most important threading dislocation in III-N heterostructures grown on nonpolar and/or semipolar substrates. Heterostructures with these orientations allow to reduce the quantum confined Stark effect and thereby to improve the efficiency of light emitting devices. Due to their importance we have therefore studied structure and electronic character of screw dislocations in GaN. For this defect the dislocation core is fully coordinated and electronic states are exclusively induced by its strain field. According to the conventional picture based on deformation potentials the shear strain field of a screw dislocation deforms the p-type VBM states but it does not affect the s-type CBM. Thus, excitons are formed by binding electrons to bound holes by Coulomb interactions.
In the present study we combine Density Functional Theory (DFT) calculations with cathodoluminescence (CL) and photoluminescence (PL) characterization experiments and we investigate the unusually strong excitonic emission at individual a-type screw dislocations in GaN. In order to investigate the electronic structure of both the core as well as the long range strain field of a screw dislocation we developed and applied a quasicontinuum approach that combines first principles calculations with elasticity theory. In a first step the effect of the dislocation core on the electronic structure has been investigated by calculating the electronic structure of a 400 atom supercell containing a pair of mutually compensating screw dislocations. In a second step and in order to address the effect of the long range strain field on the band edges we computed the strain field of an infinite array of screw dislocations in the quadrupol configuration. Then, we considered a series of bulk supercells which are placed at various positions in the strain field and deformed accordingly. The electronic structure of these supecells is then derived by DFT calculations and is used to sampled all relevant strain states in our geometry. Our results, clearly indicate that the shear stress applied by a screw dislocation bends both VBM (upwards) and CBM (downwards) states. A close analysis of our DFT calculations showed that the unexpected downward shift of the CBM is the result of rehybridization of the s-type lowest unoccupied state with the next highest p-like state. Temperature dependent PL and CL spectral map measurements on freshly induced a-type screw dislocation in HVPE grown GaN are in full agreement with this picture: Screw dislocations can be assumed as natural quantum wires that bound excitons and allow for radiative transitions.

Optical polarization properties of m-plane Al_1-xIn_xN epilayers grown by metalorganic vapor phase epitaxy were investigated both theoretically and experimentally. Approximately 500~600-nm-thick Al_1-xIn_xN epilayers subsequently grown on a 1-mm-thick GaN homoepitaxial underlayer on an m-plane freestanding GaN substrate prepared by hydride vapor phase epitaxy were examined.
At first, high-resolution x-ray reciprocal space mapping (X-RSM) measurement was carried out to quantify anisotropic strains of the Al_1-xIn_xN epilayers. Because of m-plane growth, a- and c- axes were anisotropically strained when a coherent growth was maintained. The X-RSM analysis confirmed that the Al_1-xIn_xN films of 0.15 < x le; 0.32 were fully strained, and the ones of x le; 0.15 were partially relaxed. We note that a- and c- axes of Al_1-xIn_xN alloys match those of GaN for x = 0.177 and x = 0.282, respectively.
Theoretical investigation was then carried out to correlate the anisotropic strains and optical polarization characteristics. For simplicity, the m-plane Al_1-xIn_xN layers were assumed to be fully strained. Valence band structures including the energies and polarization oscillator strengths were computed based on the k#8729;p perturbation technique. The results implied that valence bands were significantly mixed and showed anticrossing behavior at x = 0.17 and x = 0.20. Actually, there are two important x values for crystal symmetry in this regime. One is x = 0.177, where a-axis of Al_1-xIn_xN matches to that of GaN. Another particular x is 0.195, where the strains in a- and m-axes become the same. In this case, both the axes have slight compressive strains of -0.25% and crystal symmetry of Al_1-xIn_xN is C_6v, while it is C_2v for x ne; 0.195.
Consequently, the polarization oscillator strength of the optical transition between the conduction band and the topmost valence band was drastically changed at around x = 0.17. For x < 0.17, the oscillator strength had finite value only for the direction along m-axis and other components of the oscillator strength were nearly zero, where we represent this polarization as E//m in the usual manner. In this case, light extraction along surface normal might be difficult. For x > 0.17, on the other hand, most of the oscillator strength was focused along c-axis (E//c) and surface emission could be allowed. We will compare the experimental results with theoretically predicted optical characteristics described above in the conference.

High Al-content AlGaN alloys are attractive for fabrication of deep-UV optoelectronic devices such as light emitting diodes (LEDs) and laser diodes (LDs). Nitride-based LEDs and electrically pumped LDs have been demonstrated with wavelengths as short as 210 nm and 354.7 nm, respectively. Optically pumped laser cavities have been demonstrated with wavelengths as low as 242 nm but these had a high threshold pump power of 1.2 MW/cm². Lower threshold pump powers can be achieved by using low dislocation density (<10⁴ cm^-2) bulk AlN substrates. A low threshold pump power and high quantum efficiency are prerequisites for the realization of electrically pumped lasers.
We have carried out an extensive study on Al_xGa_1-xN/Al_yGa_1-yN multiple-quantum well (MQW) and Al_xGa_1-xN/AlN double heterojunction (DH) structures grown on AlN substrates that were targeted to emit between 235 nm and 280 nm. The structures were grown on 500 nm of pseudomorphic undoped Al_xGa_1-xN on AlN substrates using low-pressure metal-organic chemical vapor deposition (MOCVD). Well width, barrier width, and V/III ratio were varied to determine their dependence on lasing characteristics. Laser cavities were formed by cleaving wafers into 0.2 - 1.5 mm wide stripes. Narrow cavities were realized by first thinning the samples to 0.15 mm. They were characterized at room temperature (RT) and at 3 K (LT) using a 193 nm ArF excimer laser where the lowest RT pump power threshold was 50 kW/cm². All lasers had a peak FWHM ranging from 0.3 nm to 0.8 nm above threshold. There was a relatively small difference in RT and LT threshold indicating that the structures have high internal quantum efficiency. Longitudinal modes with line widths as narrow as 0.014 nm were observed where the mode separation was directly dependent on the cavity width. The stimulated emission was transverse electrically (TE) and transverse magnetically (TM) polarized for wavelengths above and below 245 nm respectively. It was found that a higher V/III ratio during growth resulted in brighter emitters with lower lasing thresholds. This is attributed to a reduction in point defect incorporation during growth. In conclusion, deep UV-C photo-pumped lasers have been realized on bulk AlN substrates with low threshold pump power. These promising results will further aid the fabrication of deep UV-C LDs in the near future.

Strain-induced piezoelectric fields within GaInN quantum wells grown on polar GaN are blamed as a potential reason for the fairly low efficiency of green LEDs. Therefore, non- and semipolar growth directions gained special attention over the recent years leading to reduced or even completely absent internal electric fields in such structures. Besides using semipolar GaN substrates cut from thick c-plane wafers, which are still very small in size and very expensive, foreign wafers like sapphire cut in non-c directions can be taken to achieve this goal. However, such less polar structures typically contain large dislocation and stacking fault densities. Therefore, we have concentrated on an approach, where metalorganic vapour phase epitaxial (MOVPE) growth is initiated by nucleating on inclined c-plane-like side-facets prepared by etching grooves into adequately oriented sapphire wafers. By this approach, we are still driving the growth mainly in the conventional c-direction. After coalescence of such initially striped nitride structures, they form large area planar semipolar surfaces on which GaInN quantum well structures can be grown. Following approach, several semipolar planes including (10-11), (11-22), and (20-21) can be produced on n-plane (11-23), r-plane (10#8209;12), and s-plane (22-43) sapphire wafers, respectively. By carefully optimizing the growth conditions and applying various defect reduction methods, we were able to decrease the dislocation and stacking fault densities substantially. Excellent structural properties have been obtained, being evident from very narrow X-ray diffraction peaks in the range of 200 arcsec and photoluminescence spectra dominated by the excitonic peaks. Only very weak stacking fault related signals were visible, probably due to the lateral overgrowth of defect-rich areas in our stripes from the neighbour stripes. Overgrowth by hydride vapour phase epitaxy typically leads to further improved layer qualities. GaInN quantum well structures grown on such semipolar (10-11) and (11-22) layers show strong luminescence at about 500 nm indicating that also on such planes large amounts of In can be incorporated. It should be noticed that even more In is needed for such semipolar quantum wells as compared to polar c-plane structures, because of the significantly reduced quantum confined Stark effect. First LED test structures show fair electroluminescence, although the doping profile of these structures is not yet optimized. In this contribution, we will mainly address the defect reduction mechanisms which we observed. Moreover, we have analysed the anisotropic strain and the resulting bow of our layers as a consequence of the growth conditions. Thermally induced strain may be helpful for the self-separation of thick HVPE-grown layers, whereas a too large bow would be a problem for later surface polishing.

Deep-UV (DUV) emitters have numerous applications. But most of the current DUV emitters like excimer lasers have large size. Recently, the III-N DUV emitters have drawn great attention due to suitable direct bandgap which can lead to compact size. Recently, low-threshold optically-pumped DUV lasers containing AlGaN multiple-quantum wells (MQWs) were demonstrated by using c-plane bulk AlN substrates. The AlN substrates have low-dislocation density and can reduce lattice mismatch, thus leading to active regions with low-dislocation density. But because of the high cost for the AlN substrates, it is more desirable to grow DUV lasers on inexpensive sapphire substrates.
In this abstract, we present stimulated emission from optically-pumped AlGaN MQW DUV lasers grown on (0001) sapphire substrates. The lowest threshold were obtained from the lasing at 249 nm (“the 249-nm laser”) and 256 nm (“the 256-nm laser”).
The AlGaN MQW laser structures were grown on c-plane sapphire substrates in a 3×2” MOCVD reactor. The laser structure firstly comprised an AlN template layer deposited on the sapphire substrate. The total dislocation density of the template layer was estimated to be 2.5×109/cm2 by TEM experiments, which represents one of the lowest dislocation densities reported for planar AlN/sapphire templates. The RMS surface roughness was 0.12 nm determined by 5×5 #61549;m2 AFM scans. Thus the template layer provided a relatively good crystal quality and smooth surface for growth of the AlGaN structure.
An AlGaN waveguide layer, AlGaN MQWs and a thin AlGaN cap layer were grown on the template layer sequentially. All the epitaxial layers were pseudomorphically grown confirmed by XRD (105) RSM. The 5×5 #61549;m2 AFM scans show RMS roughness of the finished structure was 0.55 nm, which was close to that of a comparable laser structure grown on an AlN substrate and thereby suggest relatively low dislocation density. The wafers were cleaved into laser cavities by mechanical scribing from back side of the substrate. The cavities were optically pumped at room temperature by a 193-nm excimer laser.
Power-dependent PL spectra of the 249-nm laser and 256-nm laser were obtained. The difference of the emission wavelengths between the two lasers was due to normal shift of sample condition across a two-inch diameter wafer grown by the MOCVD. The 249-nm laser and the 256-nm laser demonstrated thresholds of 90 kW/cm2 and 61 kW/cm2, respectively. Spectral linewidth of both the 249-nm laser and the 256-nm laser reduced with increasing pumping power density and reached 1.6 nm, indicating stimulated emission. The thresholds are more than an-order-of-magnitude lower than the previously-reported optically-pumped DUV laser grown on 4H-SiC substrates. In addition, the thresholds are comparable with the reported state-of-the-art optically-pumped AlGaN DUV lasers grown on bulk AlN substrates lasing at 266 nm, suggesting excellent candidacy of sapphire substrates for III-N DUV laser diodes.

Bulk AlN crystals with high structural perfection are considered as most promising substrate material for optoelectronic deep UV devices based on AlGaN layers with high Al content. From today&’s point of view, only bulk growth by Physical Vapour Transport (PVT) can provide crystals with highest crystalline quality (dislocation densities < 10⁴ cm^-2, no small-angle grain boundaries), which result in high quantum efficiencies of the UV-C device if this structural perfection can be kept in homoepitaxy.
In this work, we report on the PVT growth of high quality single crystalline AlN with continuous crystal enlargement and on the effect of the impurity/dopant concentrations of O, C, and Si on the optical and electrical properties of the bulk AlN crystals.
The developed growth technique comprises (i) the spontaneous nucleation of freestanding AlN crystals for the initial seed preparation and (ii) subsequent growth runs on AlN seeds cut from such crystals. The structural quality of the grown material is evaluated by rocking curves, X-ray Lang topography, and defect-selective wet chemical etching. Currently, up to Oslash;15 mm crystals with rocking FWHM values between 13 and 21 arcsec across the whole surface area are grown by subsequent growth experiments.
The impurity concentrations are investigated by secondary ion mass spectrometry (SIMS). In conjunction with UV-VIS spectrometry and Fourier-transform infrared (FTIR) absorption measurements a tri-carbon defect complex is correlated with a strong absorption band at 265 nm which impairs the application relevant deep UV transparency. The appearance and quenching of this absorption band will be discussed in dependence of the Fermi level which is correlated to the ratio of [C] to [O] + [Si]. Furthermore, the free carrier concentration in the samples is investigated in dependence of different growth facets by temperature dependent optical transmission imaging.
We will show that the impurity concentrations and thus the electical and optical properties can be controlled to some extent by adjusting the growth temperature or the thermal field-induced crystal faceting. Also, we have successfully grown Si-doped AlN bulk single crystals by adding silicon containing ceramics to the source material. These samples show a weakly n-type electrical behavior as evidenced by Hall effect and resistivity/admittance measurements.

Introduction
Acidic ammonothermal method is one of the most promising techniques which enables the mass production of large diameter bulk GaN crystal.High power light emitting diodes and laser diodes are now fabricated on GaN substrates made by hydride vapor phase epitaxy, but to realize electrical devices such as a vertical GaN-based power switching device, large-diameter GaN substrates are essential.Acidic ammonothermal method is expected for high purity, strain free, low dislocation, large scale and low cost bulk GaN manufacturing, similar to hydrothermal method for quartz crystal mass production.
We have been studying the characteristics of supercritical NH₃ using ammonium halides as mineralizers, and succeeded in growing the GaN crystals faster than 100mm/day along Ga-polar direction ^[1]. We have tried in-autoclave gas phase mineralizer synthesis ^[2], metal Ga nutrient ^[3] and also found that acidic ammonothermal crystals can be used for substrates for epitaxy ^[4].
To realize large scale ammonothermal GaN production, the pressure is one of the most important issues.The capacity of the superalloy autoclave depends on the operating pressure, because the size of the Ni-Cr material is limited.
Experimental procedure
The solubility was measured by weight loss method.The ammonothermal growth of GaN crystals was carried out using 4 kinds of ammonium halide mineralizers. To prevent the Ni-based superalloy autoclave from corroding, the inner wall was covered with Pt. A circular Pt baffle plate with holes was placed at the center of the autoclave to separate the growth region and nutrient region. After charging the precursor and the seed crystals, ammonium halide powder was added. Then, filter-purified NH₃ gas was fed into the autoclave. The charging amount was controlled to obtain the designated pressure for the temperatures during the growth period. The autoclave was heated using a two-zone vertical furnace.
Results and discussion
Because of the small inner diameter of the high pressure tubing, it is difficult to keep high purity environment only by vacuum deaeration.We have found that the deoxidizer is effective to grow high purity crystals.
The crystal quality and the growth rate strongly depend on mineralizer species.We have also studied the dependence on temperature and pressure, and found it possible to control growth direction by optimizing the growth condition.Finally, we have found that the growth rate faster than 1000mm/day can be achieved. Based on these studies and optimization, we have succeeded in growing high purity crystal with x-ray rocking curve FWHM less than 25arcsec at the pressure condition around 100MPa.
[1] D. Tomida et al., J.Cryst.Growth 353, 59 (2012).
[2] D. Tomida et al., J.Cryst.Growth 348, 80 (2012).
[3] Q. Bao et al., Cryst.Eng.Comm. 14, 3351 (2012).
[4] S. F. Chichibu et al., Appl. Phys. Express 4, 045501 (2011).

This presentation will describe results recently obtained with pulsed electron beam deposition (PED) of GaN on c-plane sapphire, silicon (111), and 2 nm germanium coated silicon (111) substrates. The PED technique is potentially useful for growth of III-nitrides at lower substrate temperatures, a capability that can allow use of new buffer layer materials, introduction of chemically dissimilar lattice-matched materials, and help solve wafer bowing and cracking problems during growth. The introduction of this technique could lead to improvements in device quality and fabrication of vertical LED structures. In this study, GaN was deposited on sapphire at a substrate temperature of 750°C, and on silicon (111) and Ge/Si (111) at 600°C in a UHP N₂ (15 mTorr) environment, without any surface pre-treatment such as pre-nitridation. A high power electron gun pulse (Neocera, Inc) was used to ablate the GaN target (1” dia. x 0.250” thick, 99.99% pure) stationed at a 5 cm vertical distance from the substrate. The electron pulses were generated at 15KV, 0.3 J/pulse at 1 Hz for initial few nm of growth, and then increased to a 3 Hz pulse rate. Scanning electron microcopy (SEM), X-ray diffraction (XRD), Rutherford backscattering (RBS), and optical absorption characterization were performed. SEM imaging confirmed a rough surface morphology with the presence of 30 nm to 300 nm scaled GaN crystallites for the GaN/Sapphire sample, while smaller but more coalesced crystallites of 30-50 nm size were observed for the GaN/Si (111) and GaN/Ge/Si (111) samples. The average film thickness was 350 nm, yielding a growth rate of 0.16 angstrom/pulse. XRD theta;-2theta; scans from 2theta; = 0° to 2theta; = 70° for the GaN on sapphire sample showed only two other peaks, besides the peaks from the sapphire, near 2theta; = 34.6°. The peaks near 2theta; = 34.6° consist of a stronger peak at 34.668° and a much weaker peak at 36.903°. These peaks correspond to the (0002) and (10-11) orientations for GaN, respectively. XRD scans for the GaN/Si (111) and GaN/Ge/Si (111) samples showed only a polar GaN (0002) peak at 34.7°. The XRD results clearly show that the deposited GaN material is not polycrystalline. Optical absorption spectroscopy over a 1.2 eV to 6.2 eV spectral range, for the GaN/Sapphire sample, showed an abrupt absorption edge at 3.4 eV, a clear indication of interband transitions in binary GaN. These results confirm that our PED-grown GaN is highly c-axis oriented and suitable for the initial growth of GaN on various substrate materials.

Investigation of hydrostatic strain and change in dislocation densities of edge and screw dislocations have been studied before and after passivation of amorphous silicon nitride (a-Si₃N₄) layer on Al_0.3Ga_0.7N/GaN heterostructure by non-destructive high resolution x-ray diffraction (HRXRD) analysis. Group III-nitrides materials are quite attractive for high power, high frequency, high temperature applications at microwave frequency due to their high band gap and high break down voltage. Epitaxially grown III-nitrides films give rise to high strain and as well as defect densities due to various parameters such as lattice mismatch, dopant, unintentional impurities, thermal expansion coefficient etc. The high dislocation density arises due to unusually large lattice mismatch in hetero epitaxial films and also between the epitaxial layer and substrate. Al_0.3Ga_0.7N/GaN group III-nitride heterostructure has been grown by plasma assisted molecular beam epitaxy (PAMBE) technique on c-plane sapphire substrate and an amorphous silicon nitride(a- Si₃N₄) film of 40 nm thickness as a passivation layer has been deposited on the AlGaN layer by plasma enhanced chemical vapor deposition (PECVD) technique. The highly strain sensitive nature of HRXRD gives a remarkable strain-stress and structural analysis of heteroepitaxial films. The well-known Bragg&’s law and Vegard&’s law have been used to evaluate hydrostatic strain of Al_0.3Ga_0.7N/GaN heterostructure which is expressed in terms in-plane strain (ε_in) and out of plane strain (ε_out). The out-of-plan and in-plane strains before and after passivation are evaluated from HRXRD (omega;-2theta;) diffraction profile across (002) plane and asymmetric (omega;-2theta;) diffraction profile across (105) plane respectively. After passivation strain is increased at heteroepitaxially grown Al_0.3Ga_0.7N/GaN structure, which imply the reduction of surface states at the interface of dielectrics and AlGaN layer. The edge and screw dislocation densities was found out from full width at half maxima (FWHM) of triple axis Omega_Rel (002) and skew symmetric (102) diffraction profile respectively. After passivation on Al_0.3Ga_0.7N/GaN heterostructure dislocation density is found to increase due to the formation of dislocation boundary at the interface of passivation layer and next epitaxial layer. This boundary generates dislocation microstructure such as dislocation pile ups. After passivation due to reduction of surface states issues and enhancement of hydrostatic strain, the Al_0.3Ga_0.7N/GaN heterostructure would be more effective to increase strain induced polarization charge effects as well as 2DEG carrier density for high density and high power HEMT applications.

A GaInN/GaN heterostructure system is widely used to high-brightness blue and green LEDs and high power violet laser diodes. The application of GaInN films in high-efficiency solar cells is also expected, because the bandgap of GaInN alloys ranges from 0.65 to 3.43 eV. Although there are many reports of the analysis of the relaxation process in GaInN/GaN heterostructure system characterized by TEM, XRD reciprocal space mapping (XRD-map), and in situ XRD, an understanding of the critical layer thickness at which misfit dislocations (MDs) are introduced in the GaInN/GaN heterostructure is insufficient. For instance, a large fluctuation exists in the reported value of the critical thickness in GaInN/GaN heterostructure system. In addition, the measurement accuracy of the critical film thickness is unknown.
In this study, we investigated the measurement accuracy of the critical film thickness in GaInN/GaN heterostructure system characterized by in situ XRD attached with metalorganic vapor phase epitaxy (MOVPE) reactor. The GaInN layers were grown on GaN template (threading dislocation density: ~3 × 10⁸ cm^-2) on sapphire substrate covered with low-temperature buffer layer and freestanding GaN substrates (threading dislocation density: <3 × 10⁶ cm^-2). We evaluated the GaInN films with symmetric (0002) Bragg diffraction using an in situ XRD system. The X-ray was focused on the sample surface using a Johansson curved crystal mirror. By using this method, the incidence angle of the X-rays can be changed without moving the X-ray source. Moreover, the diffracted X-ray was detected by a 1D X-ray CCD. By using this method, the scattered X-ray can be detected without moving the substrate and detector. By using this configuration, this system realized the equivalent of a (0002) 2theta;/omega; scan without requiring the use of an analyzer crystal in 1 s during the rotation of the wafers. Although the resolution limit decreases slightly, this in situ XRD system is able to perform the equivalent of a (0002) 2theta;/omega; scan at a resolution of 1 arcsec. In this setup, the full width at half maximum (FWHM) is both controlled by the dispersion in the lattice constant c and the mosaicity of the crystal. We observed the GaInN surface structure and defects close to the GaInN/GaN heterointerface by SEM, CL, AFM and TEM, respectively. Strain relaxation was also evaluated by a typical ex situ XRD-map.
By comparing in situ XRD measurement results and ex situ characterization such as TEM, CL, AFM, SEM, and XRD-map, we found that it is possible to accurately determine the critical thickness of MDs in the GaInN are introduced by analyzing the FWHM of the in situ XRD spectrum from GaInN. In particular, we found that measurement accuracy of the critical thickness by this in situ XRD system is higher than that by XRD-maps. Moreover, critical thickness of MD in GaInN was significant depends on the dislocation density in the GaN underlying layer.

The growth of InGaN multiple quantum wells by metalorganic chemical vapor deposition (MOCVD) has been well established over the years for the fabrication of active layers in LEDs and laser diodes. Recently, there has been interest in the growth of thicker InGaN layers to serve as absorber materials in solar cells and strain-reducing base layers for deep-green LEDs. However, the growth of thicker layers is challenging. The miscibility gap between InN and GaN leads to InGaN phase separation and/or indium fluctuations. In addition, reduced indium incorporation and a high density of V-pits are typically observed in InGaN films grown in the metal-polar direction. Alternatively, the growth of films in the N-polar direction offers advantages that are attractive for the growth of thick InGaN layers. N-polar growth enables higher indium incorporation and V-pits are less favorable to form. Despite these advantages, there have been limited studies examining the growth of thick N-polar InGaN by MOCVD.
In this study, in-situ stress measurements were used to monitor the growth stress of thick (>120nm) InGaN layers. Both N-polar and metal-polar InGaN growth was carried out under identical conditions (780°C, 300 Torr, V/III=2450). Metal-polar InGaN films were grown on GaN templates with an AlN buffer on Si-face (0001) 6H-SiC substrates. In the case of N-polar InGaN, films were grown on N-polar GaN templates that were grown via a two-step GaN process. The two-step GaN consisted of an initial low-temperature GaN layer on an AlN buffer followed by a main high-temperature GaN layer. The N-polar GaN templates were grown on 4 degree vicinal C-face (0001) 4H-SiC and 6H-SiC substrates. The N-polar InGaN films were free of hexagonal hillocks and V-pits and exhibited a reduced 20x20um RMS roughness of 1.95nm in comparison to RMS roughness of 5.71nm for metal-polar InGaN. X-ray analysis revealed a higher indium fraction of 20.5% for N-polar InGaN in comparison to 7.5% for metal-polar InGaN. X-ray rocking curve (XRC) FWHM of the (0002) reflection for both InGaN film polarities were 486 arcsec, indicating comparable levels of mosaic tilt. However, XRC FWHM of the (10-15) reflection revealed lower extent of twist for N-polar InGaN films. From in-situ curvature measurements, metal-polar InGaN films grew under a constant compressive stress on the GaN layer. N-polar InGaN films however grew initially under low stress which gradually evolved to compressive stress. The differences in stress evolution are likely to originate from the type and density of defects as well as the stress state of the GaN template which are a focus of current studies. The combination of in-situ stress measurements and ex-situ characterization are used to elucidate differences in stress evolution as a function of InGaN film polarity.

Recently, a novel quasi ternary InGaN alloy system utilizing digital alloying technology has attracted much attention because of its potential application for high performance photonic devices with high In composition InGaN. The fundamental structure in such alloys is the (InN)_m/(GaN)_n short period superlattices (SPSs) based on the SMART technology depositing/embedding ultimately thin “one monolayer (1-ML)” InN coherently on/in GaN at remarkably higher temperatures than the conventional critical one. SMART stands for Superstructure Magic Alloys fabricated at Raised Temperature and we have proposed coherent band engineering in InGaN quasi ternary alloys with the SMART technology.
The SMART is a quite challenging technology, however, in particular it is true when the growth temperature is getting higher and the GaN barrier thickness is approaching such ultimately thin as a few monolayers; i.e. In coverage tends to be fractional at higher growth temperatures and the strain in SPSs quickly increases with decreasing the GaN barrier thickness. Then it is quite important to study how those different SPS structures affect their band engineering, in particular optical properties. In this paper we have investigated the band-engineering behavior of the (InN)₁/(GaN)_n SPS system focusing on optical properties such as excitation-power dependence of photoluminescence (PL) intensity/peak position and detailed reflectance/absorption spectroscopy.
(InN)₁/(GaN)_n (n =4-9) SPSs with 50-100 periods were grown on GaN templates (GaN/Al₂O₃) by plasma-assisted molecular beam epitaxy (MBE). All samples were capped with a 10-nm-thick GaN layer. First, we have confirmed that the desired periodic structure has been successfully fabricated through XRD satellite peaks in 2theta;-omega; spectra.
The effective In composition was estimated by the XRD 0th peak, and basically it strongly depends on the growth temperature. It is normally lower than the theoretical value. For example, it is 10.2% for the SPS sample with n=4, for which the theoretical one is 20%. This indicates that the coverage of InN is not unity but rather a fractional monolayer due to the high growth temperature.
In the case of SPSs with nge;7, it was found that the band-edge emission located at the energy in the vicinity of the PL peak of 1ML-InN quantum well (QW) embedded in thick GaN barriers. On the other hand, the band-edge emission of SPSs with n=4 was found at lower energy side than that for the 1ML-InN QWs. The band-edge position well agreed with that from reflectance and absorption spectra. These are the first experimental observations of quasi InGaN ternary alloy behavior with the (InN)₁/(GaN)_n SPSs.
In summary, we have proposed a novel ternary InGaN alloy system with SMART (InN)₁/(GaN)_n SPSs and for the first time experimentally confirmed their quasi ternary behavior.

Al_0.83In_0.17N is lattice matched to c-plane GaN and exhibits a low refractive index. Thus it is possible to obtain crack-free 42-pair AlInN/GaN DBRs with a reflectivity higher than 99%¹. On the other hand, a low growth rate (~0.24mu;m/h) and a high In/Al supply ratio (~2) in AlInN growth are needed to obtain a smooth surface. The low growth rate causes an enormous amount of growth time, over 20 hours, for fabricating the AlInN/GaN DBRs. A high growth rate, however, resulted in a rough surface². We then investigated high growth rate of AlInN and obtained 0.7µm/h with smooth surface by optimizing the growth conditions. The AlInN/GaN DBR grown under 0.5µm/h was successfully obtained, showing a reflectivity of 99%.
All the samples were grown by MOVPE. Less than 100 nm thick AlInN single layers were grown on GaN templates. The growth temperature for AlInN was 785~800 oC. The In/Al ratio was fixed to be 0.6, which is smaller than a conventional value. The growth pressures for AlInN layers were 75~ 300 Torr. As the growth pressure was decreased from 300 to 75 torr, the growth rate was increased from 0.2 to 0.33mu;m/h, and surface morphology was improved from 1.2 to 0.3 nm as a RMS value measured by AFM. The high growth rate could be caused by a suppression of the parasitic reaction. The growth efficiency of the AlInN layer showing 0.33mu;m/h was close to that of GaN. Regarding the improvement of surface morphology simultaneously observed, details are unknown, but may be related to the suppression of the parasitic reaction.
Next, AlInN with higher growth rates, 0.5~0.7mu;m/h, were grown by increasing a supply of MO to 1.5~2 times. Even though the growth rate was much higher than that typically used, we obtained smooth AlInN layers showing 0.4 nm as a RMS value. Higher In incorporation efficiency was simultaneously observed with an increase of the growth rate. Thus we were able to use high growth temperature, 830 oC, while sufficient InN incorporation was obtained, resulting in sufficient migration and smooth surface.
Finally, 40-pair AlInN/GaN DBRs were grown with 0.5mu;m/h. Thicknesses of AlInN and GaN were 45nm and 42nm, respectively, designed as 410 nm wavelength. It took 12 hours to finish the DBR growth, which was almost half of a conventional case. The DBR showed 0.3 nm as a RMS value and 99% as a reflectivity. The AlInN growth conditions we found will lead to a realization of practical nitride-based blue VCSELs.
ACKNOWLEDGMENTS
This study was supported by MEXT-Supported Program for the Strategic Research Foundation at Private Universities, 2012-2016.
REFERENCES
1. G.Cosendey, J-F.Carlin,et, Appl.Phys.Lett 98 181111(2011)
2. T.C.Sadler et al. J.CrystalGrowth314 (2011)

Developing substrates for GaN thin film with small lattice mismatch, appropriate structural and thermal properties, and competitive price is highly promising for advanced opto-electronic device applications. In this context, (-201)-oriented β-Ga₂O₃ can be considered as one of the potential wide band gap substrate since it combines the transparency of Al₂O₃ and the conductivity of SiC, which are commonly used substrates. We have performed the structural and optical characterization of a MOCVD grown GaN epilayer on (-201) oriented β-Ga₂O₃ using different techniques such as X-ray diffraction (XRD), Atomic Force Microscopy (AFM), Raman Spectroscopy, and Photoluminescence (PL). XRD shows a high quality single-phase wurtzite GaN thin film on (-201) β-Ga₂O₃. In-plane epitaxial orientation relationships between the β-Ga₂O₃ and the GaN thin film are defined by (010) β-Ga₂O₃ || (11-20) GaN and (-201) β-Ga₂O₃ || (0001) GaN leading to a lattice mismatch of ~4.7%. The GaN lattice parameters, determined by the Bond method, show a slight compressive strain. The rocking curve width analysis reveals impressive results for the first growth experiment with little optimization of growth parameters. AFM analysis shows a smooth surface with mean square roughness of 0.68nm with a threading dislocation density (TDD) of ~10⁸ cm^-2. The photoluminescence analysis at 6K and room temperature (RT) exhibits a high intensity (nearly 1.5 times higher at RT than that of a commercial GaN film on Al₂O₃ having a TDD of ~4×10⁷cm^-2), sharp bandedge emission peak and a very weak yellow luminescence. We have observed that GaN/β-Ga₂O₃ at RT exhibits Internal Quantum Efficiency (IQE) of ~37%, which is remarkably high. The Raman spectra analysis compliments our XRD results. The temperature dependent absorption experiment and time resolved photoluminescence give detailed insight in the recombination mechanism of the GaN material. The strain effect on the optical properties (across the whole 2 inch GaN wafer) also studied by Raman, XRD and time-resolved PL.