The interplay of plastic and elastic relaxation mechanisms in strained epitaxial films is a complex balance of the energetics and kinetics of the evolving system. We will review the use of in-situ electron microscopy methods to quantify the kinetics of misfit dislocation generation and to map defect microstructures as functions of growth and processing conditions in such thin film systems, specifically in Ge(x)Si(1-x)/Si. This work has led to enormous volumes of data (e.g. hundreds of hours of video, thousands of photographic negatives). In the spirit of the US “Materials Genome Initiative” we seek to develop new methodologies to collate, organize and analyze this data to develop new understanding and avenues for process control through consideration of the whole body of data. We present a new methodology for organizing and illuminating complex interdependent dislocation kinetic mechanisms, that is related to the phylogenetic methods used in bio-informatics and evolutionary biology. Within our analogous “materials cladogram”, different branches indicate the evolution of different structures from a common original structure, based upon different kinetic pathways. This approach can capture both the points of divergence at which different structures emerge during a growth or processing sequence, and the relevant kinetic parameters that define the structure of divergent branches. The development of such pathways can be mapped using simulations that capture and integrate the essential quantitative kinetic descriptions derived from the experiments (e.g. activation parameters for dislocation glide, dislocation nucleation rates, dislocation interaction processeshellip;). Comparing the generated maps with large numbers of specific experimental observations then allows refinement of the simulation structure and increased accuracy in the determination of the relevant kinetic parameters, eventually enabling generation of new cladogram branches by simulation alone. Ultimately, this can provide the structure for a processing map that captures the set of different kinetic pathways and resulting structures for a given system, and helps define the key experimental parameters required for extension to new systems.
We acknowledge the contributions of David Sandler (RPI). Original experimental work in collaboration with J. Bean, J. Floro (U. Virginia); F. Ross (IBM); and E. Stach (BNL).

Metamorphic buffer layers (MBLs) are of great interest for the development of new semiconductor devices with alloy compositions that are not typically feasible due to the high defect density resulting from the mismatched epitaxy. MBLs grown by hydride vapor phase epitaxy (HVPE) are especially promising because they can achieve a high degree of strain relaxation while depositing thick layers that enable the use of chemical mechanical planarization. While MBLs have been in use for quite some time, the mechanisms which govern dislocation generation and propagation, strain relaxation, and tilting are still unclear. HVPE-grown MBLs provide a unique tool for understanding these processes, as a wide range of layer thicknesses, beyond what is typically employed in MOVPE and MBE, can be employed allowing relaxation to be observed at many different stages of growth. A combination of TEM, high resolution reciprocal space mapping (RSM), and electron microprobe was employed to gain a clearer picture of the compositional and strain states of the various layers in a series of HVPE-grown InxGa1-xAs MBLs. It was found that there are dislocations lying perpendicular to the growth direction in the constant-composition capping layer of the MBL that lie above the final compositional interface. These dislocations were correlated with RSM data that indicate that the capping layer in these step-graded MBLs is partially relaxed. Since it was observed that the majority of the capping layer closest to the surface is defect free, it appears that these dislocations have climbed from sources present at or near the last compositional interface, relaxing the lower portion of the cap. The upper portion of the cap remains nearly fully strained with respect to the previous composition step. This is in contrast to the commonly assumed mechanism, in which dislocation loops are thought to nucleate at the surface and then propagate down towards the nearest compositional interface. Tilting behavior in these layers was measured by x-ray diffraction omega-phi mapping. It was found that tilt magnitude typically increased with xInAs and did not depend on grading style (linear vs. step-grading). The direction of the tilt was initially random on nominally (100) oriented substrates and changed as grading continued, appearing to ‘twist&’ around the growth direction. MBLs grown on 4° miscut substrates tilted in the opposite direction of the miscut, and the tilt magnitude for a given composition was greater.

Group III-Sb materials show the best hole transport among III-V materials and are considered as promising candidates for future p-type MOSFETs in all III-V CMOS technology. Silicon as a universal microelectronics platform is especially attractive substrate, and III-Sb films with low-defect density and smooth surface are critical for achieving commercial viability of devices made of these materials. However, large lattice mismatch and non-polar nature of Si substrate present a challenge for growth of high quality III-V materials. Molecular beam epitaxy of GaSb and strained InGaSb quantum wells were employed using metamorphic buffers, GaSb/AlSb superlattice or AlGaSb layers. In both cases, the growth was initiated on Sb-soaked Si substrate with thin AlSb nucleation layer. A 10 nm thick Al₂O₃ gate oxide was deposited either by in-situ reactive evaporation of Al in 10-6 Torr oxygen ambient, or by ex-situ atomic layer deposition. E-beam evaporated nickel was used as a gate metal to fabricate MOS capacitors, which were evaluated by measuring C-V and I-V characteristics. Both n- and p-type GaSb MOS capacitors were studied with various thicknesses III-Sb structures and consequently, with different defect densities. Be-doped p-type GaSb MOS capacitors demonstrated similar characteristics to MOSCaps grown on GaAs substrates. In contrast, we observed a p-type MOSCap behavior in GaSb:Te on Si devices despite Te doping of up to 5x10 ¹⁷ cm^-3 in the structures with thin, <1.5mu;m III-Sb structures. Thicker structures with dislocation densities in the top layers <10⁸ cm^-2 have shown normal n-type C-V behavior similar to the structures grown on GaSb and GaAs substrates, but with significantly faster minority carriers generation/recombination rates. SIMS analysis did not reveal noticeable Si diffusion from the substrate, nor other impurities were detected in GaSb. We believe that native defects, generated in GaSb grown on non-polar high-mismatch Si substrate contributed to the observed polarity inversion. Specific morphological features on the surface, were further analyzed using TEM, FIB/SEM and AFM. Comparable structures grown on GaAs demonstrate atomically smooth surface with single atomic steps seen in both AFM and SEM. Two major types of surface topography defects were found: shorter crystallographically aligned straight streaks (i), and longer winding loops (ii). Cross-sectional TEM reveals the density ~6x10⁸ cm^-2 of threading dislocations within top layer of EPI structure with total thickness of 1.25 mu;m, and ~2x10⁴ cm^-1 of planar defects. The correlation of the TEM images with the surface morphology features reveals that the microtwins are responsible for the features (i) and create surface steps with the height equal to the number of faulted planes. The features (ii) are due to the antiphase domain boundaries. That is confirmed by direct imaging of the high/low-z Sb-Ga dumbbell STEM contrast.

Nitride semiconductors offer many unique and beneficial properties for a new generation of electronic devices. AlGaN/GaN high electron mobility transistors (HEMTs) are used in applications where high-power and high-frequency devices are needed. Unfortunately, high-power operating conditions result in unpredictable and catastrophic device degradation. Various techniques have been used to detect and investigate the degradation mechanisms of these devices, including cathodoluminescence spectroscopy, atomic force microscopy, and TEM. However, the formation mechanism of these cracks was not investigated as a function of operating time. As such, quantitative analysis on evolutions of defects and piezoelectric polarization is needed to further understand device failure mechanisms.
The degradation of AlGaN/GaN HEMTs devices was quantified as a function of defect generation and overall strain evolution in the AlGaN layer using high-resolution transmission electron microscopy (HRTEM) techniques. Observations of device cross-sections on ex-situ biased devices revealed that the formation of defects occurred mainly on the drain side of the gate. Geometric phase analysis (GPA) of HRTEM images indicated that the tensile strain decreased from +1.67% in the unbiased device to +1.17% after sufficiently long bias duration. Based on our observations, we propose three different regimes under which a HEMT device undergoes physical degradation during its lifetime. In-situ TEM biasing experiments were also carried out on lift-out devices in order to characterize formation of defects during the application of bias. Real-time observation of generation of defects and formation of physical damage provides a fundamental understanding of the unknown reliability of HEMTs under application of bias, which will contribute toward their functionality during application.

Recently, semipolar (11-22) GaN film has been widely studied to achieve higher brightness and longer wavelength LEDs. In device fabrication process, all etching processes of semipolar GaN-based LEDs have been performed by conventional dry etching process which requires long time for vacuum, high cost, many toxic gas and professional person. For these reasons, even though the needs of wet etching process for GaN-based LEDs were increased, the wet etching technique has been only used to study of c-plane GaN due to its difficulties of wet etching process. Furthermore, a few research groups reported on the wet etching phenomena of nonpolar a-plane GaN films. However, there is no report on the wet etching study of semipolar (11-22) Mg-doped GaN film. In this study, we comparatively investigated the wet etching properties of semipolar (11-22) undoped and Mg-doped GaN films.
We grew 2.0 mu;m-thick semipolar (11-22) undoped and Mg-doped GaN on m-plane sapphire substrate using MOCVD. After then, both samples were chemically etched by using three major wet etching parameters, such as temperature (55~115 oC), time (1~3 min for undoped GaN, 1~90 min for Mg-doped GaN) and the mole (2~6 mol) of KOH solution. SEM and AFM were used to study the surface morphologies. The etching damage was characterized by room temperature PL. Based on SEM analyses, we found that undoped and Mg-doped GaN films exhibited same wet etching behaviors which were the increase of etching rate with increasing solution temperature, etching time and solution mole number. However, the etching rate of Mg-doped GaN was two times lower than that of undoped GaN film. We suspected that the surface band bending effect would induce the disappearance of electron near a surface, resulting in the reduction of wet-etching reaction. In this point, we will discuss etching mechanism in detail. In addition, both etched samples represented two kinds of etched-surface planes such as (0001) c- and {1-100} m-plane. However, the etched-area ratio of c-plane to m-plane of undoped GaN was almost same, whereas Mg-doped GaN exhibited low etched-area ratio of c- to m-plane after wet etching process. In addition, Mg-doped GaN represented clearer boundary between c-plane and m-plane than undoped GaN. From these results, we suggested that Mg atom of semipolar (11-22) GaN would decrease etching rate toward c-direction rather than m-direction. Furthermore, PL intensity of undoped GaN film was increased and then decreased with increasing solution temperature and number of moles, whereas PL intensity of Mg-doped GaN film was slightly decreased and then rapidly increased with increasing solution temperature and number of mole. Based on these results, we believed that Mg atom would decrease wet etching rate of semipolar (11-22) GaN film due to surface band bending effect, resulting in the improvement of optical properties compared to dry etching process, but it will be further studied to clarify the detailed etching mechanism

The band gap of InGaN alloys can be tuned to cover the whole visible spectrum of light by varying the concentrations of In and Ga. Hence, this materials family is interesting for optoelectronic devices such as light emitting diodes. The quality of these devices is affected by the existence of defects.

In this work, we model electronic and atomic structures of InGaN alloys, with and without vacancies, and InN/GaN superlattices, and positrons states in these structures. In InGaN, we study the effect of varying In and Ga concentrations on the measured Doppler spectrum, and the effect of the alloying on the annihilation signal produced by vacancies. In the case of the InN/GaN superlattices, we extend our previous study [1] to the limit of digital superlattices, i.e., the limit of small number of In layers. Our results are used to estimate the usefulness of positron annihilation spectroscopy [2] in studying these structures and vacancy defects therein.
[1] I. Makkonen, A. Snicker, M. J. Puska, J.-M. Mäki, and F. Tuomisto, Phys. Rev. B 82, 041307(R) (2010).
[2] F. Tuomisto and I. Makkonen, Defect identification in semiconductors with positron annihilation: experiment and theory, Reviews of Modern Physics, to be published.

Carbon is a common impurity in the nitride semiconductors, often incorporated during growth but also used intentionally to create semi-insulating GaN. The properties of carbon impurities are not fully understood: although experiments show that C-doping leads to resistive GaN, previous theoretical work suggested that C_N should act as a shallow acceptor. In this work we employ hybrid density functional calculations to investigate the electrical and optical properties of the carbon impurity in InN, AlN, and GaN. This method overcomes the band-gap problem of traditional density functional theory, allowing for the quantitative prediction of defect transition levels and formation energies. Our results indicate that C_N acts as a deep acceptor in all nitrides, and can give rise to deep, broad luminescence signals that have been observed in C-doped material. We also investigate the stability of C interstitials and C substituting on the cation site to determine how C will influence the electrical conductivity of the nitride semiconductors. Our results explain experimental observations on carbon&’s role in conductivity of InN, and its effects on AlGaN/GaN transistors.

Carrier-trapping phenomena in GaN-based HEMTs, current collapses, present a major limitation on actual device performances at high frequencies. In this study, focusing on inevitable C impurity for MOCVD growth of AlGaN/GaN hetero-structures, we have screened deep-level defects in the bulk region of the GaN buffer layer and then have investigated a detailed relation between the C-related deep-level defects and turn-on recovery characteristics in the AlGaN/GaN-based Schottky barrier diodes (SBDs).
Three kinds of Al_0.24GaN/GaN hetero-structures (25nm/3mu;m) were grown on c-Al₂O₃ substrates by MOCVD. The growth temperature of the GaN buffer layer was varied at 1120, 1150, and 1170°C. All the samples were confirmed to exhibit typical 2DEG properties. The C concentrations were uniformly ~6x10¹⁶, ~2x10¹⁶, and ~1x10¹⁶cm^-3 or less, respectively. After growth, planar dot-and-plane SBDs were fabricated on these samples, using Ni as a Schottky metal, and then were characterized by means of I-V, C-V, and steady-state photo-capacitance spectroscopy (SSPC) measurements. For the simple estimation of carrier trapping in the bulk region, turn-on current recovery characteristics at V_G of +2.0V after the off-state at V_G of -30V for 60min were measured under various optical illuminations using a Xe lamp coupled with three kinds of long-pass filters of 540, 390, and 370nm.
From SSPC measurements, all the samples show five photoemission states with their onsets at ~1.70, ~2.07, ~2.26, ~2.75, and ~3.23eV below the conduction band. Among them, the ~2.07, ~2.75, and ~3.23eV levels are significantly enhanced with decreasing the growth temperature of the GaN buffer layer. These specific levels are probably produced by the C impurity incorporation into the GaN buffer layer, because the C incorporation tends to be enhanced at the low growth temperature for MOCVD growth, resulting in the deep-level formation of the residual C impurities and the Ga vacancies. The ~2.07, ~2.75, and ~3.23eV levels observed are presumably attributable to V_Ga and/or V_Ga-O_N, V_Ga-C_N, and C_N, respectively. From carrier-trapping measurements, the recovery time in the dark is ~372s, which slow value is due to the 2DEG carrier-trapping at the deep-level traps in the bulk region under the turn-on state. In sharp contrast, the recovery time significantly shortens down to ~67s by the white light illumination without any filters. Additionally, the recovery time is mostly unchanged by the optical illumination with the 540nm filter, compared to that in the dark. On the other hand, the recovery time becomes significantly shorter under the illuminations with the 390 and 370nm filters. These experimental results indicate that the ~2.75 and ~3.23eV levels are strongly responsible for the turn-on recovery characteristics, that is, the residual C impurity on the N sites turns out to be closely related to carrier-trapping phenomena in the bulk region of AlGaN/GaN hetero-structures.

Compound semiconductor alloys are widely used in electronic and optoelectronic devices, and point defects created by radiation or during growth can degrade device performance. Defect diffusion controls the rate of defect annealing, and thus quantitative models of defect diffusion are an essential component when predicting device performance in the presence of defects. In an alloy, defect energies and characteristics are sensitive to the occupations of nearby sites (e.g., whether nearby group-III sites in InGaAs contain In or Ga) and thus vary with location in the alloy. A defect can become trapped in energetically favorable regions of the alloy with profound consequence for the rate of defect diffusion. We have recently developed a model of defect diffusion in alloys that combines Kohn-Sham Density Functional Theory calculations, the Cluster Expansion approach, and kinetic Monte-Carlo simulations. We will describe this model and its application to diffusion of the As interstitial in InGaAs.
Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under Contract DE-AC04-94AL85000.

The interest in achieving high-quality large-scale synthesis of GaN crystals is driving the development of bulk growth methods such as the ammonothermal technique. To optimize the growth of undoped and n-doped GaN crystals using this method, an assesment of the crystal quality, dopant incorporation and sample homogeneity is needed. Raman scattering is a nondestructive tool that can be used for structural and optical semiconductor characterization at the submicron level. Moreover, in polar compounds such as GaN, the study of the longitudinal-optic phonon-plasmon coupled modes (LOPCMs) can be used to determine the free electron density in a local, contactless way. This work presents the results of a confocal micro-Raman study of ammonothemally-grown Si-doped GaN. The distribution of the free electron density in the sample is studied and discussed.
We performed polarized Raman measurements on the N-polarity and Ga-polarity faces of the sample. The spectra obtained in all scattering configurations are in accordance with the selection rules and the Raman peaks are narrow, indicating a high crystalline quality. In the N-polarity face both L⁺ and L^- branches of the LOPCMs are simultaneously observed whereas in the Ga-polarity face no signal from coupled modes is detected. From the N-polarity face, we have performed depth profiling of the charge density at 5 mu;m-steps by using confocal micro-Raman spectroscopy. We have implemented a dielectric model based in the Lindhard-Mermin electronic susceptibility to extract the free electron density from the LOPCMs in the Raman spectra. Our results reveal a electron density of 3,3E19 cm^-3 at the surface. The charge density increases by about 40% over the first 50 mu;m as the probing depth increases. Deeper in the sample the free electron density saturates at 4,6E19 cm^-3, a value close to that given by Hall measurements. This free electron density distribution has been assessed through lateral measurements using a motorized stage to record Raman spectra at 30 mu;m steps from the surface to the seed.
Electron density variations may have a strong impact on device performance, particularly in high-power devices. Therefore the free electron density gradient found in our work stresses the need for assessment of charge homogeneity in ammonothermally-grown GaN. Whereas Hall measurements provide an average value of the carrier concentration, our results demonstrate that confocal micro-Raman spectroscopy is a useful technique to obtain reliable values of the local free electron density distribution in doped semiconductor materials.

GaAs_1-xBi_x alloys are of interest because of their potential to improve the properties of conventional III-V semiconductor devices and to extend them to longer infrared wavelengths. The bandgap energy of GaAs_1-xBi_x alloys decreases strongly as the Bi fraction is increased. And the lattice constant increase of GaAs_1-xBi_x is much smaller than that of In_xGa_1-xAs at the same bandgap energy, considerably reducing constraints related to lattice mismatch strain for GaAs_1-xBi_x/GaAs heterostructures. However, Bi is incorporated into GaAs films grown by molecular beam expitaxy (MBE) only at growth temperatures below about 400 ^oC, for which significant defect concentrations in GaAs films have been reported. To investigate the properties of GaAsBi films and the effects of Bi incorporation on electrically active defects, we have performed deep level transient spectroscopy (DLTS) measurements on GaAsBi films grown at 330 ^oC and on GaAs films grown both at standard temperatures (560-580 ^oC) and temperatures as low as 300 ^oC.
C-doped (p-type) GaAs layers grown by MBE at substrate temperatures of 560-350 ^oC showed similar concentrations, ~10¹⁵ cm^-3, of a defect energy level at E_V+0.56 eV. In contrast, trap concentrations in Si-doped (n-type) GaAs layers grown at standard temperature, 560-580 ^oC, are le;4x10¹³ cm^-3. Defect concentrations increase strongly to 10¹⁶ cm^-3 in an n-type GaAs layer grown at 390 ^oC and to 10¹⁸ cm^-3 when the growth temperature was further reduced to 330 ^oC, where the energy level of the dominant defect is E_C-0.40 eV [1]. When only 0.3% Bi is incorporated into n-type GaAs at 330 ^oC, the formation of the E_C-0.40 eV trap is suppressed completely, reducing the total trap concentration by more than a factor of 20. Other electron traps, including the dominant ones having energy levels at E_C-0.66 eV and E_C-0.80 eV, are present in similar concentrations in both GaAs and dilute GaAs_1-xBi_x layers grown at 330 ^oC and, therefore, apparently result from the low growth temperatures. Both are point defect complexes involving an arsenic atom on a gallium lattice site (As_Ga) as expected for MBE growth at these low temperatures.
1. P.M. Mooney, et al., J. Appl. Phys. 113, 133708 (2013).

InAs/GaAs quantum dots (QDs) grown by Stranski-Krastanov mode have wide application in photo-detectors, lasers, etc. Methods such as thermal annealing, laser induced thermal annealing and ion implantation have been used to tune the optical emission of QDs via interface intermixing. Among this, ion implantation can provide more homogeneous intermixing along the growth direction by adjustment of implantation energy. Optical emission efficiency enhancement of InAs QDs was reported by passivation of dislocation with proton implantation [1]. In this work, we have studied the effect of implantation of Li^- ions of energy 50keV with a dose ranging from 3x10¹¹ to 8x10¹¹ ions cm^-2 on the optoelectronic properties of 2.7 ML InAs QDs capped with a combination of 3nm In_0.15Ga_0.85As and 50nm GaAs (grown by Solid State MBE) using photoluminescence (PL) experiment. A significant red shift from 1131 nm to 1180 nm of the PL peak along with the lowering of activation energy (from 220 meV to 130 meV), full-width at half-maxima (FWHM) (from 68 meV to 52 meV) and integrated emission intensity compared to that of the as-grown sample was noted with the increase of dose from 3 x10¹¹ to 6x10¹¹ ions cm^-2. At a higher dose of 8x10¹¹ ions cm^-2, a shift of PL peak to 1151 nm, with significant increase in activation energy (236 meV) and three-fold increase in integrated emission intensity compared to that of the as grown sample was noted with little increase in FWHM value (55 meV). The red shift of the PL peak with the increase of dose from 3 x10¹¹ to 6x10¹¹ ions cm^-2 is due to increase in In atom segregation from InGaAs layer followed by its incorporation on InAs QDs thereby increasing its height, thus reducing the confinement(in the growth direction), activation energy and integrated emission intensity[2]. The reduction of FWHM shows more QDs reaching uniformity with increase in InAs coverage. At higher implantation dose of 8x10¹¹ ions cm^-2, the effect of excess In atom segregation from InGaAs layer was more significant in increasing the confinement [3] compared to the reduction in confinement due to increase in dot height, thus leading to a blue shift of PL peak and increase in activation energy. The high In atom concentration gradient between InAs QDs and InGaAs layer generated a quasi electric field, accelerating carriers from InGaAs layer to the QDs, which increases carrier capture efficiency and hence integrated emission efficiency [3]. From temperature dependent PL study, evidence of annihilation of defect state was observed along with the tunability of different optical properties with different Li^- implantation dose. DST, India is acknowledged.
1. Hydrogen in Semiconductor-II, Semiconductors and Semimetals Vol. 61
(Academic, New York, 1999)
2. IEEE TRANS.NANOTECH, VOL. 5,683-686 (2006)
3. Appl. Phys.Lett. 82,2802(2003)

Near-IR quantum dot (QD) lasers are attractive sources as pump lasers and transmitters for satellite communications systems due to three-dimensional localization of carriers that makes them more suitable for radiation hardening than lattice-matched or strained quantum well (QW) lasers. A few research groups have reported encouraging results of QD lasers showing enhanced resistance to radiation damage, but complete understanding of physical mechanisms leading to this enhancement is still lacking. Also, it is well known that point defects induced by proton irradiation behave as nonradiative recombination centers (NRCs), but the role that NRCs play in reliability and degradation mechanisms in QD lasers is not well understood.
We studied carrier dynamics in InAs-GaAs QD laser structures with RT photoluminescence (PL) peaks at ~ 1 µm and also in broad-area InAs-GaAs QD laser diodes at ~1150 nm. All of our samples were grown by MBE and InAs QDs were self-assembled by the Stranski-Krastanov process. For InAs QD laser structures, InAs QDs were clad by 100 nm thick GaAs waveguide layers and then AlGaAs cladding layers. AR-HR coated broad-area InAs QD lasers had 200 µm wide waveguides, ~3.6 mm long cavities, and window regions introduced by forming openings in the backside n-contacts. We employed time-resolved PL (TR-PL) techniques to measure carrier lifetimes in a series of InAs-GaAs QD laser structures grown under different conditions. These structures were irradiated with protons (energies of 10 - 50 MeV and the fixed fluence of 10¹²/cm²) and pre- and post-proton irradiation lifetimes were compared. All PL decay curves from these structures showed initial faster components followed by slower components. Slower component lifetimes were 23 - 42 ns before irradiation, but the lifetimes were significantly reduced to 8 - 13 ns after irradiation. We also measured lifetimes from irradiated QD samples that were subsequently annealed in RTA at various temperatures between 100 and 650°C. Unlike strained-layer InGaAs-AlGaAs single QW lasers at 975 nm that showed a single decay component with typical lifetimes of 7 - 8 ns, all QD lasers that we studied showed faster components followed by slower components with significantly long lifetimes of > 400 ns. We will report on our TR-PL results from the InAs QD samples.

Colloidal quantum dot (QD)-based electroluminescent light emitting diodes (QLEDs) have been considered as one of future display technologies, due to narrow emission bandwidth, color tunability by controlling QD size, and economic solution processibility. Such prominent optical properties have guided ceaseless devotions on the development of high performance QLEDs and positive outlook, but unfortunately their commercialization is restricted by the use of hazardous substances in general QDs (i.e., Cd and Pb). Although tight encapsulation technology and recycling policy might mitigate such environmental concerns, fundamental solutions are urgently needed to exclude toxic heavy metal elements and thus to promote the persistent development of QLEDs
Herein, we demonstrate bright, efficient and environmentally-benign green-emitting InP QLEDs exhibiting EQE as high as 3.46 % and brightness up to 3,900 cd m-2, 10-fold increase in device efficiency and 5-fold increase in brightness compared with previous reports. This breakthrough is achieved by the tailored device architecture as well as the structural formulation of InP@ZnSeS core@shell heterostrctured QDs, enabling direct charge carrier injection into QDs and efficient exciton recombination. The direct charge carrier (particularly electron) injection within QDs is assisted by a solution processed, thin conjugated polyelectrolyte layer (poly[(9,9-bis(3prime;-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-ioctylfluorene) (PFN)), forming an interfacial dipole layer between ZnO electron transport layer and QDs. The vacuum level shift induced by PFN reduces the electron injection barrier into QDs and promotes the charge balance within QDs. On the basis of this finely-organized device structure, we employ highly efficient InP QDs with a thick composition-gradient ZnSeS shell (PL QY > 70 % with ~ 6 monolayers of ZnSeS alloy layers). The thick ZnSeS composition-gradient shell provides a sufficient potential barrier for the effective confinement of electrically-generated excitons within InP core away from the surface states. As a result of the finely-tuned device structure along with the protection of excitons from surface state-mediated quenching, the radiative recombination of excitons was considerably enhanced, realizing InP QLEDs with high efficiency and brightness.

ZnO has been attractive for optoelectronic semiconductor material due to its wide band-gap (3.37 eV) and a large exciton binding energy of 60 meV. However, since ZnO film has been mostly grown to the c-axis, it would represent spontaneous and piezoelectric polarizations, resulting in quantum confinement Stark effect (QCSE). Therefore, a few research groups have reported nonpolar (11-20) and (10-10) GaN film to achieve high efficiency optoelectronic devices. Moreover, ZnO nanostructures have been currently focused on improving optoelectronic applications because of their high surface volume ratio, surface tailoring ability, improved solubility, and multi-functionality to open up many new possibilities in various applications. In this work, we investigated the growth and characterization of non-polar (10-10) ZnO nanorods (NRs) grown on m-plane sapphire with double seed layer of Al-doping ZnO (AZO)/ZnO by using hydrothermal synthesis.
We prepared 80 nm-thick ZnO film and 6 nm-thick AZO film as a seed layer on m-plane sapphire substrates using atomic layer deposition (ALD) system. The results of high resolution X-ray diffraction exhibited that high quality nonpolar (10-10) ZnO film would be successfully grown on m-plane sapphire substrate. By using nonpolar (10-10) AZO/ZnO double seed layer, ZnO NRs were synthesized by using hydrothermal growth methods. The precursors of ZnO synthesis were zinc nitrate and hexamethylenetetramine (HMT). Nonpolar (10-10) AZO/ZnO/sapphire templates were placed in a heated solution of zinc nitrate and HMT for 1 hr at 90 °C. From SEM analyses, we found the askance non-polar (10-10) ZnO NRs on nonpolar (10-10) AZO/ZnO/sapphire templates. PL band edge emission intensity of nonpolar (10-10) ZnO NRs was 14.5 times higher than that of ZnO seed layer grown by ALD, which would be explained by high extraction efficiency and no QCSE. In addition, deep level emission was shifted from 500 nm to 570 nm and significantly increased by growing nonpolar (10-10) ZnO NRs. It may be due to generation of point defect during hydrothermal synthesis mechanism. In this presentation, we will further report the optical improvement of nonpolar (10-10) ZnO NRs and its application for light-emitting diodes.

The Y2 ordering induced changes in the optical properties, including crystal field splitting, spin-orbit splitting, band gap and valence band splittings, for AlxGa1-xAs, GaxIn1-xAs, GaxIn1-xP, GaAsxSb1-x and InPxSb1-x are studied using first-principles calculations. These properties are provided as a function of the degree of long range order. For the partially ordered materials, we explain the trends of the changes in the crystal field splitting and band gap narrowing. The change of spin-orbit splitting is found to be positive and small. For the fully ordered materials, we compare Y2 with other orderings and find that Y2 has a large and negative crystal field splitting and negative spin-orbit bowing parameter. The calculated data can be useful in analyzing experimental results and deriving the ordering parameters of partially ordered samples.

Current deep level transient spectroscopy (DLTS) with a bipolar rectangular weighting function in the unit of coulomb [1] has been applied to characterize traps in high-resistivity MOCVD GaN doped with carbon. Electrons or holes generated by the illumination of the above-band-gap light are captured by traps and then emitted during the light-off period, resulting in the current transient [2]. The sample was carbon-doped high-resistivity GaN grown by MOCVD on SiC substrate. The doping concentration of carbon was 1x10¹⁷ cm^-3. Ohmic contacts were formed by electron beam evaporation of Ti/Al. The 355-nm LED was used as an optical source of the above-band-gap light. DLTS measurements were performed under isothermal condition in the temperature range from 290 to 330 K. At 300 K, one trap is observed with the emission time constant of 0.3 s. The thermal emission activation energy for the trap in high-resistivity GaN is estimated to be 0.88 eV from the Arrhenius plot of the emission time constants in the temperature range from 290 to 330 K. This value of 0.88 eV is close to the thermal emission activation energy of 0.86 eV for the carbon-related hole trap observed in n-GaN codoped with silicon and carbon by minority carrier transient spectroscopy for Schottky diodes [3]. It is thought that the 0.88 eV trap in high resistivity GaN is associated with carbon-related defects, although a slight difference in emission time constants is found between 0.88 eV trap and 0.86 eV trap.
[1] Y. Tokuda, T. Shibata, H. Naitou, T. Katou and M. Katayama, Materials Research Society Fall meeting, U3.11, 2011.
[2]Ch. Hurtes, M. Boulou, A. Mitonneau, and D. Bois, Appl. Phys, Lett. 32, 821 (1978).
[3] U. Honda, Y. Yamada, Y. Tokuda, and K. Shiojima, Jpn. J. Appl. Phys. 51, 04DF04 (2012).

We carry out integrated optical and electrical modeling studies to predict the limiting efficiencies of III-V nanowire on silicon solar cells.
In the first part of our study, we calculate the optical absorption of vertically oriented nanowire arrays. We find that optimized arrays have a larger integrated broadband absorption across the solar spectrum than a thin film of the same height.
We then consider the incorporation of III-V nanowire arrays in tandem cells, where one junction lies in the III-V wires, and the other junction in a silicon substrate. We consider four different III-V materials, with band gaps above and below the optimal value. We calculate the limiting efficiency in an idealized model. We show that for a sub-optimal band gap in the top cell, efficiency is increased by decreasing the absorption in the nanowires to achieve current matching. For a band gap above the optimal value, the efficiency is highest when the nanowires absorb all light above the band gap. We optimize the structural parameters of the nanowire arrays to achieve the highest efficiency within a detailed balance model. We find that higher than 30% detailed balance efficiency can be achieved using 1 mu;m-tall nanowire arrays.
Sample device simulations are then conducted to compare different junction geometries and the effect of surface conditions. We find that radial junctions are more robust to the presence of surface recombination. For the axial junction, we design a passivation scheme for the nanowire array focusing on GaAs, in particular. AlGaAs is used as a shell layer. Simulation results show that this passivation design greatly improves the short circuit current and open circuit voltage relative to an unpassivated device.

Semiconductor nanomaterials have been well documented for their potential and impacts in various areas including light emitting diodes (LEDs), solar cells, and bio-imaging. For the various applications, there is an compelling demand on high-quality nanocrystals with synthetic reproducibility and high particle yield. Accordingly, fundamental understanding of the chemistry affecting nucleation and growth as well as the resulting size and size distribution should be appreciated for rational design. I will discuss our latest experimental results towards such chemistry. Hopefully, such a presentation will bring insight into rational design of high-quality nanocrystals to fulfill their unprecedented potential expected in the near future. Colloidal semiconductor nanocrystals (NCs) have been synthesized from single-source precursors which consist of the metallic and nonmetallic elements of the semiconductor constituents in a single molecule, and from separated metallic-element and nonmetallic-element precursors. The former is called single-source precursor approaches (SSPAs). The latter, dual-source precursor approaches (DSPAs) to binary ME semiconductor nanocrystals (NCs), commonly uses metal carboxylates ( (RCOO)2M such as M = Zn, Cd, Cu, In, and Pb) and phosphine chalcogenides (such as R3P=X where X = S, Se, and Te) as the precursors. To explore the common mechanism for the formation of the various semiconductor NCs, CdSe was investigated in detail as a model system. A probable mechanism is put forward for the formation of the NCs from both the SSPA and DSPA in 1-octadecene (ODE) at ambient temperatures. This proposed mechanism offers new avenues to optimize the design of low-temperature approaches to various semiconductor nanomaterials.

Semiconductor fine particles have received a great interest in wide fields such as photonics, catalyst chemistry, and so on. Among those, in micro-photonics microspheres have attracted much attention as optical microcavities with high quality factor (Q value). The morphology and crystalline quality of the microspheres are very important to obtain the high Q value. However, it has been very difficult to fabricate semiconductor microspheres with both high sphericity and single-crystalline nature. Here we succeeded in the fabrication of the single-crystalline microspheres of some semiconductors by laser ablation in superfluid helium.
ZnO has been widely studied as an efficient ultraviolet and visible light source. Thus the ZnO microcavities such as single-crystalline micro- or nano-wires have been fabricated and investigated intensively, but a ZnO microsphere is difficult to fabricate because of its wurtzite structure. Here we successfully fabricated ZnO microspheres by laser ablation in superfluid helium. We performed transmission electron microscopy of the fabricated microspheres to verify their morphology and crystalline quality. The results indicate that the microspheres are highly spherical without faceted structure which is a characteristic of ZnO crystal and also show uniform diffraction contrasts, which show that they have very few dislocations or defects. Moreover, it was clarified that the fabricated ZnO microspheres were single crystals from the electron diffraction patterns. All the diffraction spots are indexed by those of a ZnO bulk crystal with wurtzite structure. In addition, uniform lattice fringes were clearly observed around the edge of the ZnO microspheres. This result shows the fabricated ZnO microspheres are single crystals with good crystalline quality even near their surfaces. We also investigated optical properties of the fabricated ZnO microspheres with the morphology as discussed above by micro-photoluminescence measurement. Then, we observed efficient lasing with a threshold of 100 W/cm2, which is much smaller than that of other ZnO microcavities with different morphology, e.g. microwires. This result shows the fabricated ZnO microspheres have extremely high sphericity and good crystalline quality, which provides efficient lasing.
We applied this fabrication technique to other materials. We also successfully fabricated single-crystalline microspheres of CeO2 with cubic structure, which has been widely studied as an efficient catalyst. Finally, we fabricated single-crystalline microspheres of CdSe with wurtzite structure, which has attracted much attention as a visible light source. Thus, the novel method, laser ablation in superfluid helium, should be applicable to many materials, not only oxides but non-oxides, regardless of the crystal structure, e.g. wurtzite or cubic.

Repetitive deposition of ultrathin InAs submonolayers (SML) separated by GaAs matrix material results in an SML stack that provides 0D recombination centers embedded in a 2D background layer which is due to agglomeration and segregation of indium during the growth process. Controlling the vertical distance between single InAs SML depositions allows tuning of the SML stack recombination energy inside the near infrared spectral range. For example, this would allow tuning of the small inhomogeneously broadened SML density of states to be resonant with a laser resonator mode.
We present a system consisting of an SML stack grown on top of a layer of InAs Stranski-Krastanov grown quantum dots (SK QDs) that are separated by a spacer of thickness d. Using spacers in the range of a few nanometers enable the coupling of both nanostructured systems. Energy resonance of SK QDs excited states and SML stack states allows an energy transfer which introduces an additional decay channel for carriers which are localized inside the SML-stack. Thus, there are mainly two concurrent decay processes for the SML carriers at low temperature: first the radiative recombination, and second the transfer into the quantum dots with time constant tau;_SML-QD. We have investigated and found evidence for the impact of this coupling using time-integrated photoluminescence and time-resolved measurements. The influence of this coupling becomes clearly visible in the excitation density dependent behavior of the SML PL, it is observed directly in PLE measurements and dominates the SML PL recombination dynamics.
The observed PL dynamics cannot be simply described by the superposition of exponential decays, as saturation effects due to state blocking in the SK QDs are present. Therefore, a rate equation system (RES) taking into account Pauli-blocking of QD states is essential for the description of the dynamics. We present a model which is used to describe our experimental data and to derive the time constant tau;_SML-QD of the charge carrier transfer process from the SML into the SK QDs, which exhibit a strong dependence on the spacer thickness d.

The fabrication of optoelectronic devices on unconventional substrates such as silicon, amorphous glass, plastics, and metals would permit the creation of large-sized flexible displays and complex optoelectronic circuits. In particular, monolithic integration of compound semiconductor photonic devices with silicon (Si)-based electronic circuits would create a new field combining electronics with photonics, facilitating intra- and inter-chip communications. Accordingly, tremendous efforts have been made related to direct growth of films on such substrates. However, those films have been of poor quality because of mismatches in lattice constants and thermal expansion coefficients between the film and the substrates. This has made it difficult to fabricate sophisticated devices such as a laser that requires significantly better material quality. The challenging issue facing heteroepitaxial growth can be resolved using an appropriate intermediate layer. Recently, intermediate layers, such as titanium and graphene, were used to improve the quality of gallium nitride (GaN) thin films grown on amorphous substrates, which enabled the fabrication of light-emitting diodes. For sophisticated devices such as a laser, however, better structural and optical qualities of the materials prepared on graphene films are still required.
Here, we present a novel approach to grow high-quality GaN microdisks on graphene dots via epitaxial lateral overgrowth (ELOG) for laser applications. Highly crystalline GaN microdisks having hexagonal facets were grown on amorphous silicon oxide layers formed on Si using micro-patterned graphene films as a nucleation layer. Cathodoluminescence spectroscopy and transmission electron microscope analyses showed that ELOG of GaN enhanced the material quality. The microdisk having hexagonal facets showed whispering-gallery-mode lasing with a Q-factor of 1200 at room temperature. The approach presented here for growing high-quality GaN microdisks, even on an amorphous silicon oxide layer, using patterned, transferable graphene films offers a promising and general route to fabricating high-quality light sources and photovoltaic and electronic devices on various substrates.

Sensor networks play a key role in various fields, including health/environment monitoring, defense technology, and artificial skins. Traditional wireless sensors require a battery as a power source, which might lead to problems such as adding weight to the whole system, limited life time, high cost for replacement, or potential hazard to the environment. To solve these problems, the battery-free self-powered sensor, which could scavenge energy from the environment as the power source, is highly desirable. In this regard, our group discovered that the nanogenerators could serve as active sensors to actively detect the mechanical vibrations without using a battery. By active sensor we mean that the sensor automatically gives an electric output signal without applying an external power source, which can be used to directly quantify the mechanical triggering applied onto the nanogenerator.
Here in this paper, we introduce the up-to-date progress on the explorations of active sensors based on both piezoelectric and triboelectric nanogenerators. We fabricated the piezoelectric nanogenerators based on ZnO nanowires grown on flexible PDMS or PVDF substrates by the wet chemical approach. Its applications were demonstrated for transportation monitoring and ambient wind velocity detection. Moreover, the ZnO nanowires could even be grown on an elastic spring component to function as a self-powered balance for active weight measurement. On the other hand, the triboelectric nanogenerator was fabricated based on micro-patterned polymers. The power generation of the pyramid-featured device far surpassed that exhibited by the unstructured films, and gave an output voltage of up to 18 V at a current density of ~0.13 uA/cm2. Furthermore, the as-prepared nanogenerator can be applied as a self-powered pressure sensor for sensing a water droplet (8 mg) and a falling feather (20 mg). The triboelectric nanogenerator could also be employed to actively detect the ambient magnetic field.
Reference:
Nano Energy, 2013, 2(1), 75-81;
Energy and Environmental Science, 2012, 5(9), 8528-8533;
Energy and Environmental Science, 2013, 6(4), 1164-1169;
Nano Letters, 2012, 12(6), 3109-3114;
ACS Nano, 2012, 6(11), 10378-10383.

Newly generated, heterostructured semiconducting nanocrystals (NCs) of mixed dimensionality like CdSe/CdS dot-rod structures (DRs) have an enormous potential as biological markers. These DRs feature high photoluminescence quantum yield (QY) achieving values up to 80% and giant extinction coefficients resulting in an improved brightness. The two-photon absorption cross-section of these elongated NCs is increased compared to spherical quantum dots.
A high quality CdSe core with wurtzite crystalline structure is essential for a successful DRs synthesis. Consequently the need of a reliable and reproducible CdSe core production is required, which can&’t be fulfilled with traditional batch methods. Therefore, we have developed a continuous flow synthesis approach to produce these CdSe cores. This method guarantees automation, reproducibility and control over the nanoparticles properties. The reactor set-up is split into two different sections to mimic the classical hot-injection methodology: On the one hand there is a microfluidic mixing chamber for nucleation, which can be heated up to 350 °C. On the other hand there is a growth oven, in which the NC&’s size can be tailored by adjusting the flow speed and temperature.
Furthermore the biofunctionalization of these DRs will be shown, too. The phase transfer approach is based on an amphiphilic PI-PEO diblock polymer generating a micelle formation around the NCs. This results in QY values up to 70% in water and features extraordinary fluorescence stability.

Recently developed high aspect ratio (1D) semiconductor nanostructures, such as nanowires, have been proposed to be a potential next generation technology, envisioned as providing a nanoscale framework of both interconnects and functional elements for the ‘bottom up&’ approach. With reliable synthesis routes now established, a number of nanowire based electronic and optoelectronic devices have been demonstrated including field-effect transistors, lasers, photodetectors, single electron memory and solar cell devices. In this talk, I will review some challenging issues related to the growth and characterization of III-V semiconductor nanowires grown by metal-organic chemical vapor deposition using Au as catalyst to understand the growth mechanism and their effects on optical properties. In addition, I also show some results from our prototype nanowire devices including solar cell and laser devices.
In summary, we achieved precise control over crystal structure either in ZB or WZ phases for GaAs and InAs nanowires, by carefully tuning the growth parameters. Vertically aligned pure ZB nanowires were achieved using a low growth temperature combined with a high V/III ratio. On the other hand, a high growth temperature combined with a low V/III ratio produced pure WZ nanowires. This tunability of ZB and WZ structures not only will enhance the performance of nanowire devices but also opens new possibilities for engineering nanowire devices.
Due to the large surface-to-volume ratio inherent to nanowires, non-radiative carrier recombination has dominated in GaAs core-only nanowires. Coating the GaAs nanowire core with an AlxGa1-xAs shell can greatly reduce the surface states of GaAs nanowires and nearly intrinsic exciton lifetimes have been obtained at low temperature. Our recent study has also demonstrated a minority carrier lifetime of 1.5 ± 0.43 ns at room temperature.
Modeling of Nanowire lasers has also been carried out by calculating the threshold gain for nanowire guided modes as a function of nanowire diameter and length. Based on these calculations, we have optimized the structure design for nanowire laser devices. The prototype GaAs/AlGaAs nanowire laser devices were fabricated and optically pumped laser operation was demonstrated at room temperature.
The prototype solar cell device exhibited a spectrally broad photo-response and the J-V characteristics clearly show its photovoltaic properties. A power conversion efficiency of 3.56% was obtained, which is a good value for GaAs nanowire-based solar cell devices. We also developed a technique based upon two-photon induced photocurrent that provides a submicrometer resolution, three-dimensional mapping of photocurrent in these devices.
Acknowledgments: This research is supported by the Australian Research Council and Australian National Fabrication Facility established under Australian Government NCRIS Program.

Compound semiconducting nanowires are promising building blocks for several nanoelectronic devices yet the inability to reliably control their growth morphology is a major challenge. Here, we report the Au-catalyzed vapor-liquid-solid (VLS) growth of GaN nanowires with controlled growth direction, surface polarity and surface roughness. We develop a theoretical model that relates the growth form to the kinetic frustration induced by variations in the V(N)/III(Ga) ratio across the growing nanowire front. The model predictions are validated by the trends in the as- grown morphologies induced by systematic variations in the catalyst particle size and processing conditions. The principles of shape selection highlighted by our study pave the way for morphological control of technologically relevant compound semiconductor nanowires.

GaN, as a direct wide band gap semiconductor, has become an ideal choice for fabricating ultraviolet (UV) photo detectors (PDs) due to its chemical stability of being able to work in harsh environments where UV detection needed. After decades of investigations, Schottky-contacted GaN based UV PDs, whose performances are controlled by the Schottky Barrier Height (SBH) at local contact, have been proved excellent with high sensitivity, impressive responding and reset time as well as good detection limit. Piezo-phototronic effect provides an effective way to tune the SBH at metal-semiconductor (MS) interface by utilizing the piezopotential produced inside the piezoelectric nanostructures when under externally applied strains, which can work as a strain “gate” to control the electron transport process of the devices.(1) In this work, piezo-phototronic effect was employed to tune the SBH and hence enhance the performances of Schottky-contacted metal-semiconductor-metal (MSM) structured(2) GaN nanobelt (NB) based PDs. In general, the response level of PDs was obviously enhanced by piezo-phototronic effect when applying strains on devices. The optimized external strain, indicating an optimal SBH at MS contact, was found to be -0.53%, under which the responsivity of the PD was increased by 18%. Moreover, the sensitivity of GaN NB based PDs was enhanced by from 22% to 31% under a -0.53% compressive strain, when illuminated by 325 nm laser of light intensity ranging from 12 W/cm2 to 2 W/cm2. Carefully studying the mechanism using band structure diagrams reveals that the observed optoelectronic behavior were resulted from the change of SBH caused by external strains as well as light intensity. This work provides an applicable way by using piezo-phototronic effect to enhance the performances of PDs made of not only GaN, but also other wurzite family materials.
References:
1. Wang, Z. L. Progress in Piezotronics and Piezo-Phototronics. Adv Mater 2012, 24, (34), 4632-4646.
2. Dong, L.; Niu, S. M.; Pan, C. F.; Yu, R. M.; Zhang, Y.; Wang, Z. L. Piezo-Phototronic Effect of CdSe Nanowires. Adv Mater 2012, 24, (40), 5470-5475.