Symposium EQ01—Ultra-Wide Bandgap Materials and Devices

There is growing recognition that upcoming challenges/opportunities such as the future electricity grid, beyond 5G communication, space-based installation and military communication and reconnaissance will be substantially advanced by ultra-wide bandgap semiconductors (UWBS). The UWBS, with a bandgap greater than 4 eV, and displaying high thermal conductivity and high carrier mobility most often discussed include diamond, AlN and AlGaN, Ga2O3 and (AlGa)2O3, and BN and BAlN. This talk will briefly summarize how the different ideal characteristics could favor specific materials groups for separate sets of applications. Basic materials properties and electrical characteristics are considered for p-i-n diodes including doping and transport properties. Here, the similarity of diamond to vacuum transport is described in terms of space charge limited current. Diamond is a bipolar material with p- and n-type doping with both high mobility band transport and hopping transport. Strategies to achieve desired electron and hole transport including conduction modulation are crucial for device configurations. The properties of dielectric-diamond interfaces are integral for communication applications. Oxides are usually considered for the dielectric layers, but recent research has considered fluorides, nitrides and multilayer dielectric structures. Here, band alignment, polarization doping, charge transfer doping, and interface states are of interest to achieve predicted performance. The impact of defects and impurities are now being identified in high performance diodes, and the electrical characteristics may help guide the materials development. Heterostructures and interface thermal properties will be crucial to integration of diamond with other UWBS materials. There is growing anticipation and even expectation that UWBG electronics will develop to play important roles in the future electricity grid and communications. For power electronics research co-design teams are being formed which span from the materials to grid design and all steps in between with a goal of effectively guiding the research at all levels - materials, devices, systems, architecture.
Acknowledgement: Research supported by the US Department of Energy through the ULTRA Energy Frontier Research Center, NASA and the National Science Foundation.

Since the inverter circuit is composed of upper and lower arms of n-channel field effect transistors (n-FETs), dead time and delay in the bootstrap circuit are inevitable. In a complementary inverter with upper n-FET and lower p-channel FET (p-FET), however, the gate and source potentials are the same for both FETs [1]. There is only one gate drive circuit, and no need for synchronization between the upper and lower gate drivers. The dead time is eliminated and MHz operation can be achieved to reduce heavy inductive part such as filters and transformers. An ideal compact power system can be realized. However, high-voltage C-FET has not been developed yet. The reason is that there was no high-voltage p-FET with performance close to that of n-FET. In GaN or SiC, the performance of p-FET is about 1/100 of that of n-FET. On the other hand, diamond lateral p-FETs that use 2 Dimensional Hole Gas (2DHG) for the channel have high current drive characteristics with high breakdown voltages [2,3]. They are far superior to SiC and GaN p-FETs, and can be compared with those lateral n-FETs in normally-on type. In this study vertical diamond FETs and normally off diamond FETs with high performance have been investigated.
Vertical FETs: C-H surface has been studied for the channel of FETs since 1994 [4]. Not only the 2DHG formed beneath the C-H surface, but also the low surface state density is advantageous for MOSFET application. The regrown homo epitaxial layer in a trench structure of vertical MOSFET has no distinct interfacial defects at the side walls observed by TEM. All most the same hole conductance as that in horizontally grown layer is obtained in trench walls even though interfaces are inclined to be far from low index planes such as (001), (111), or (110) etc.. Holes from a source on the top are controlled by a gate near the edge of a trench, pass through drift layer on the side walls and reach the p⁺ substrate at the bottom of trench [5,6]. The vertical MOSFETs exhibit high breakdown voltage of 580V [6]. The current density exceeds 10,000 Acm^-2 [4] and the real current exceeds 3 A at large channel width >10 mm at present. The minimum specific on resistance decreased to be 2 mΩcm² [4].
Normally-off high performance diamond p-FET with C-Si MOS interface:For high voltage switching, normally-off with large threshold voltage (Vth) more than 3V is desired in the power device application. Alternative interface for C-H is C-Si which appears as oxidized Si termination. From its negative χ [7], the valence band maximum position is near that of C-H indicating p-type tendency. C-Si connects diamond to SiO₂, which is a traditional insulator, but the most reliable gate oxide in MOSFETs. The most advantageous function of C-Si interface is normally-off operation with large Vth >5V without deteriorating MOSFET performance such as current density and transconductance [8]. C-O-Si can also connect diamond to SiO₂, but C-O interface bonding is not suitable for current modulation. C-Si can replace some of the functions of C-H such as channel for normally-off FET.
Combined with vertical device structure and C-Si channel for normally-off, diamond FETs can contribute to the complementary high power circuit topology which will open a new era of power electronics.
[1] K. Okuda, T. Isobe, H. Tadano, N. Iwamuro Proc. Eur. Conf. Power Electron. Appl., Sep. 2016, pp. 1–10.
[2] H. Kawarada et al. Sci. Rep. 7, 42368, (2017)
[3] N. C. Saha, M. Kasu et al ., IEEE Elec Dev Lett 42, 903 (2021)
[4] H. Kawarada et al. Appl. Phys. Lett. 65, 1563 (1994). H. Kawarada Surf. Sci. Rep. 26, 205 (1996).
[5] N. Oi, H. Kawarada et al. Scientific Reports, 8, 10660 (2018), M. Iwataki, H. Kawarada et al., Elec. Dev. Lett. 41, 111 (2020).
[6] J. Tsunoda, H. Kawarada et al.,(submitted).
[7] A. Schenk, A. Tadich, M. Sear, K. M. O’Donnell, L. Ley, A. Stacey, C. Pakes, Appl. Phys. Lett. 106, 191603 (2015).
[8] W. Fei, H. Kawarada et al. Appl. Phys. Lett. 116 212103 (2020).

The use of synthetic p-type diamond for high power electronic applications has been of significant interest, and the success in the performance of such devices highly depends on the control of the growth process of diamond films. Despite recent advances in the synthesis of chemical vapor deposition (CVD) single crystal diamond (SCD), the device performance still remains far below the theoretical capacities.¹ Therefore, the optimization of the deposition conditions of B-doped layers has a crucial role in the improvement of electronic devices such as Schottky barrier diodes.
In a previous study² it was shown that by increasing the methane concentration from 0.5% to 3% in the CVD plasma, the boron incorporation increased from 10¹⁷ cm^-3 to well above the Mott transition 10²¹ cm^-3, leading to heavily B-doped SCD films (p⁺). This fact was corroborated by first-principle density functional theory calculations. It was revealed that the presence of CH₂ sites has no significant impact on the binding energy of a boron atom. However, the CH₂ sites enhance the chance of a so-called H-defect site formation, i.e. a desorption of an H atom on the diamond surface thereby providing additional boron binding sites.
In this work, the previously studied p⁺ layers² were used to fabricate pseudo-vertical Schottky barrier diodes to investigate the effect of methane concentration during the growth of B-doped layers on the eventual device performances. Within this framework, the optimized growth conditions were used to deposit lightly B-doped diamond layers (p^-) with an acceptor concentration of (1.0 0.5) 10¹⁶ cm^-3, as measured by the Hall effect, on the said p⁺ layers. These p⁺ layers were grown by microwave plasma-enhanced CVD on 3 3 mm² Ib (100)-oriented high pressure, high temperature substrates. Ti (40 nm) / Au (60 nm) and Zr (30nm) / Au (50 nm) contacts were deposited on the p⁺ and p^- layers as the Ohmic and Schottky contacts, respectively.
The electrical characteristics of the rectifiers exhibited good reproducibility regardless of the methane concentration used during the deposition of the p⁺ layers and the size of diodes, indicating the uniformity of the Zr / diamond interface over large areas. In addition, a very low reverse current of 10^-12 A was measured, pointing to the high crystalline quality of the grown layers. The forward operation current was boosted proportionally from the rectifiers with the p⁺ layer grown at the lowest [CH₄]/[H₂] of 0. 5%, to the diodes with the p⁺ layer grown at the highest [CH₄]/[H₂] of 3 %. This is associated with the increase of boron incorporation in the diamond layers by increasing the methane concentration during the CVD growth. The highest current density (10³ A/cm²) was found for the diodes with the p⁺ layers grown at 2% and 3% methane concentrations. The capacitance-voltage measurements of the diodes showed a punch-through behavior with a very low doping level (10¹⁴ cm^-3).
References:
H. Umezawa et al., Diam. Relat. Mater., 24, 201-205, 2012.
R. Rouzbahani et al., Carbon, 172, 463-473, 2021.

Irradiation of matter with a femtosecond laser pulse can commonly induce structural transition to a disordered state, such as amorphization, or creation of defects. Graphitization of diamond is a counterexample, as it is an order-to-order (solid-to-solid) phase transition. Graphitization was observed by applying soft x-ray in experiments by Tavella et al. [High Energy Density Physics 24, 22 (2017)]. While the graphitization mechanism was considered to be interatomic bond breaking that forms sp² instead of sp³ bonds, how photoexcitation causes such bond change remains elusive. To elucidate the atomic mechanism of photo-induced graphitization of diamond, we performed nonadiabatic quantum molecular dynamics (NAQMD) simulations. The simulations were performed using a system consisting of 64 carbon atoms under microcanonical ensemble. Collapse of electronic band gap was observed by exciting 3% of valence electrons including s electrons by 40 eV, inducing the graphitization. This excitation condition corresponds to the experiments by Tavella et al. We will discuss detailed analysis of photo-induced changes in electronic structures and how they lead to graphitization.

Amongst the traditional wide band-gap materials diamond has suffered in terms of the relative immaturity of both substrate and processing technology. However, recent developments in both these aspects have now made the development of diamond devices for power devices a realistic prospect; within the framework of the European GreenDiamond collaboration (2015-2020), all aspects required for the realisation of the first HVDC diamond based converter were considered. These included diamond device design, substrate qualification/preparation, diamond epitaxy, device processing technology, packaging and power converter design. At the heart of field effect transistor technology lies the MOS channel; breakthrough technology in the formation of γ-Alumina gate oxide with controlled band offset allowing an effective barrier to hole leakage from the channel has been demonstrated [1]. Device design specifically suited to diamond’s wide band-gap properties is also essential, an example being the ‘deep-depletion’ diamond MOSFET concept also recently demonstrated [2]. In terms of diamond MESFET technology, the use of ‘Reverse-blocking’ devices where the drain on the diamond device structure is Schottky, as opposed to the conventional ohmic contact, has proved highly effective [3]. Diamond FET structures that rely upon low damage etching procedures have also been developed [4]. This paper will use what has been learnt from the GreenDiamond programme to highlight the current status of diamond FET technology and discuss the challenges that must still be met.
[1] Canas et al., Appl. Surf. Sci., 535 (2021) 146301
[2] Masante et al., J. Phys. D: Appl. Phys. 54 (2021) 233002
[3] Canas et al., IEEE Trans Elec. Dev., (2021) DOI 10.1109/TED.2021.3117237
[4] Hicks et al., Sci. Rep., 9 (2019)15619
This work was supported by the European Commission “GREENDIAMOND” through the H2020 Large Project under Grant SEP-210184415.

Micro/nano-electro-mechanical systems (MEMS/NEMS) incorporating miniature scale mechanical elements into electronic circuits are of interest in applications ranging from atomic resolution mass spectrometers¹ to heated stages for in-situ material characterisation². For devices utilising resonating structures, the frequency of operation is proportional to the acoustic velocity of the material from which they are fabricated, making diamond with its unrivalled value of 18000 m/s ideally suited for use in high frequency NEMS with minimal dissipation from scaling induced losses. While offering the benefits of diamond but in thin-film form, the use of polycrystalline diamond (PCD) for smaller scale NEMS however is often prohibited by its considerable columnar growth induced roughness, complicating fabrication and resulting in significant surface associated dissipation³. In addition, the concentration of strain at the attachment points of typically used geometries is expected to radiate energy away from the device and lead to substantial loss that scales with frequency⁴. Through combining the use of chemical mechanical polished (CMP) stock with advanced geometries it is therefore expected that dissipation can be minimised in thin-film diamond based NEMS⁵. The addition of high mechanical strength, high thermal conductivity, and tailorable conductivity meanwhile also makes diamond a suitable candidate for the fabrication of ultra-high temperature and single material micro-hotplates, with electromigration or structural failure limiting temperatures to 1500 °C with routinely used materials². The aim of this work is to therefore investigate the utilisation and optimisation of PCD in the aforementioned devices, enabling the next generation of diamond NEMS/MEMS.
To this end, metallised doubly clamped and ‘free-free’ geometry resonators incorporating flexural supports were fabricated from intrinsic PCD films with and without CMP. The devices were then placed in a Quantum Design Physical Property Measurement System at cryogenic temperatures, and actuated magnetomotively through sweeping the frequency of an applied AC signal while in the presence of a static magnetic field^6,7. Upon comparing the resulting resonances, reductions in dissipation were observed upon both switching from rough to polished films and the addition of 2^nd mode flexural supports. In addition, suspended type PCD micro-hotplates with dimensions of the order of 100 μm were fabricated on SOI substrates and tested at pressures below 10^-4 mbar. The spectral emission and composition of the resulting devices were studied with spectrometry and in-situ Raman spectroscopy, with temperatures of up to 1900 °C reached through self-heating at power consumptions of less than 200 mW.
¹ K. Jensen et al., Nat Nano 3, 533–537 (2008).
² R. G. Spruit et al., J Microelectromech Syst. 26, 1165–1182 (2017).
³ X. M. H. Huang et al., New J. Phys. 7, 247 (2005).
⁴ M. Imboden et al., Appl. Phys. Lett. 90, 173502 (2007).
⁵ E. L. H. Thomas et al., Carbon 68, 473–479 (2014).
⁶ K. L. Ekinci et al., Appl. Phys. Lett. 81, 2253–2255 (2002).
⁷ T. Bautze et al., Carbon 72, 100–105 (2014).

Aluminum Nitride (AlN) is an over 80-year-old insulator that has to date, shown little promise to be converted to a semiconductor via doping because of limitations originating from well-known impurity compensation and DX center defects. Until our recent report on the use of Be incorporated at low temperatures (to avoid compensating donors normally present at high temperature due to growth system outgassing) no substantial p-type doping was reported in AlN. For n-type doping, both silicon and oxygen impurities are known to form DX centers at high concentration leading to a loss of shallow donors in favor of deep states. These processes are also known to be exasperated by Al vacancies that are prevalent at high temperatures. This has led AlN to be used primarily as an insulator - until now. Low temperature, Al rich growth methods have shown the ability to control the compensation finally making it possible to have both n and p-type AlN and its first practical pn diodes. Using commercially scalable crystal synthesis methods that produce less compensating impurities and new dopant elements, we demonstrate: a) for the first time well behaved bulk semiconducting functionality in AlN making it the largest bandgap semiconductor ever demonstrated; b) substantial bulk p-type conduction for the first-time demonstration with hole concentrations p=3.1×10¹⁸ cm^-3; c) massive improvement in bulk n-type conduction with electron concentrations n=6×10¹⁸ cm^-3, which is nearly 6000 times the prior state-of-the-art; d) and the first PN AlN diode with a nearly ideal turn-on voltage of ~6 V for a 6.1 eV bandgap semiconductor and ~six decades of current rectification. The understanding of this advance is in its infancy and present devices show higher than desired series resistance and leakage current. The high series resistance results from high contact resistances that may have an ultimate fundamental limit for these extreme bandgap devices but are presently only limited by etching/fabrication concerns that have known engineering solutions. Likewise, the leakage currents observed are speculated to result from re-growth interfacial defects known in the literature that also have existing engineering solutions making the promise for higher performance, near-future devices outstanding. Early evidence for and speculation on why low temperature synthesis methods are so efficient in controlling doping in AlN will be presented with specific emphasis given to surface chemistry control, kinetics, lattice strain and impurity compensation. TEM, XRD, SIMS and electrical characterization will be presented. Assuming solutions to the above contact and interfacial defects are found, a wide variety of AlN-based applications should be enabled by this advance that will revolutionize deep ultraviolet light-based viral and bacterial sterilization, polymer curing, lithography, laser machining, high-temperature, high-voltage, and high-power electronics.

High quality N-polar AlN epilayers were grown and characterized on N-face bulk AlN substrates by plasma assisted molecular beam epitaxy. This is the first step towards achieving N-polar AlN based high-power RF electronics. While the successful molecular beam homoepitaxy of metal polar AlN on single-crystal AlN substrates has been recently demonstrated [1,2] there has been no work reported thus far on the homoepitaxy of N-polar AlN on bulk substrates owing to the challenges of surface cleaning of the highly chemically reactive N-face of Aluminum Nitride. Due to the ultrawide-bandgap, direct bandgap and excellent photonic properties, high thermal conductivity and high piezoelectricity of AlN, it provides a promising platform for next generation nitride electronics and photonics. [3]
High-structural-quality N-polar AlN (000-1) bulk substrates with a dislocation density of 10⁴ cm^–2 produced by Asahi Kasei were used as the substrates in this study. With a combination of in situ thermal deoxidation and Al-assisted thermal desorption at ≈ 1000C in a high vacuum environment monitored with reflection high-energy electron diffraction (RHEED), native surface oxides and impurities were removed from the N-face of the AlN surface prior to the homoepitaxy. Subsequent epitaxial growth of 1 μm thick AlN layer on the in situ cleaned substrates exhibited smooth surface morphologies measured by atomic force microscopy (AFM) to be of rms roughness <0.8 nm over a 20 μm x 20 μm area with clean and wide atomic terraces on the surface.
In situ RHEED reconstructions and KOH etching studies were used to study the polarity of the as-grown samples. N-polarity was confirmed by observing strong c(6x12) RHEED reconstructions on a thin 6-7 nm GaN epilayer grown over the 1 m AlN epitaxial layer. Homoepitaxial grown 1 μm thick AlN samples were etched in 2% KOH solution for 10s at 80C, the surface was vigorously etched. Cross sectional scanning electron microscopy scan of the etched surface revealed triangular shaped pyramids over a large coverage area confirming the N-polarity. Some small regions were unetched which were attributed to inversion domains arising from metal polar nucleation due to defects on the native substrate itself. In addition, the incorporation of chemical impurities in the homoepitaxially grown AlN was studied by SIMS and the impurity concentrations which revealed that Si, O, H and C concentrations are very low, all near the detection limit throughout the epitaxial layer away from the nucleating interface.
In conclusion, these results indicate that with the help of in situ Al assisted cleaning procedure we have achieved the epitaxial growth of excellent structural quality N-polar AlN on single crystal AlN substrates with very few inversion domain incorporations. Thus, this serves as a good starting point for epitaxial growth of device layers for N-polar heterostructures devices and an important first step in achieving N-polar RF electronics on AlN platform.
References:
[1] Y. Cho, C. S. Chang, K. Lee, M. Gong, K. Nomoto, M. Toita, L. Schowalter, D. Muller, D. Jena, and H. G. Xing, Applied Physics Letters 116, 172106 (2020).
[2] K. Lee, Y. Cho, L. J. Schowalter, M. Toita, H. G. Xing, and D. Jena, Applied Physics Letters 116, 262102 (2020).
[3] A. L. Hickman, R. Chaudhuri, S. J. Bader, K. Nomoto, L. Li, J. C. Hwang, H. G. Xing, and D. Jena, Semiconductor Science and Technology 36, 044001 (2021).

Far-ultraviolet (far-UV) light near 230 nm in wavelength can be used for disinfection and sterilization without harming the human body because it is absorbed by the stratum corneum, which is an outer layer of the skin without nucleic acids [1]. Since AlN has both strong optical nonlinearity and high transparency in the UV region, it is useful for wavelength conversion device application including far-UV light-emitting devices. We have proposed a transverse quasi-phase matched (T-QPM) waveguide wavelength conversion device in which the nonlinear optical coefficient is spatially modulated in the vertical direction by stacking the polarity-inverted nitride semiconductor layers. In our previous study, the polarity-inverted AlN T-QPM device was fabricated and blue second harmonic generation (SHG) was demonstrated [2]. Recently, a method for inverting the polarity of AlN by epitaxial crystal growth has been developed [3]. In the polarity-inverted AlN T-QPM device for far-UV SHG, the thickness of the AlN is about 300 nm, but it is difficult to fabricate such a thin polarity-inverted structure by epitaxial crystal growth. On the other hand, the enhancement of the efficiency can be also expected by employing an upper amorphous core instead of the upper polarity inverted AlN core in comparison with a single AlN core waveguide. In this paper, we propose the non-polar/polar T-QPM waveguide consisting of an amorphous upper core and an AlN lower core and designed for 230 nm far-UV.
The design was carried out using a channel waveguide consisting of HfO₂ as the non-polar material, and c-oriented AlN as the polar material. Sapphire and SiO₂ were used as the substrate and the cladding, respectively. To use the largest nonlinear optical tensor component d₃₃ of AlN, the TM molarizations were used for both fundamental and SH waves. The thickness of the T-QPM waveguides is determined so that it satisfies the modal dispersion phase matching condition between the 460 nm fundamental wave of TM₀₀ mode (TM₀₀^F) and the 230 nm SH wave of nth-order TM mode (TM_0n^SH, n ≥ 1). The nonlinear coupling coefficient κ between TM₀₀^F and TM_0n^SH is calculated by κ=2ωε₀/4∬[E^F(x,y)]²d₃₃E^SH(x,y)dxdy,(1) where ω is an angular frequency of the fundamental wave, and E^F and E^SH are the transverse electric field distributions of TM₀₀^F and TM_0n^SH, respectively. In a single AlN waveguide, κ becomes almost zero because E^SH has positive and negative antinodes. High κ is expected by spatially modulating d₃₃ with inverting the polarity of AlN at one node of the E^SH. In the non-polar/polar T-QPM waveguide with a channel width of 1.2 µm, the maximum κ of 1.6 W^−1/2cm⁻¹ was obtained for TM₀₂^SH . In this design, the thicknesses of the upper HfO₂ core and the lower AlN core were 120 nm and 75 nm, respectively. For comparison, the maximum κ of the polarity inverted AlN T-QPM waveguide was calculated as 4.4 W^−1/2cm⁻¹ for TM₀₃^SH.
With pumping by an InGaN laser with a wall plug efficiency (WPE) of 40%, the conversion efficiency η of 75% is required to obtain the total WPE of 30%, which is comparable to that of light-emitting diodes. It was found that a device length of 2.6 cm was required to obtain the η of 75% with the HfO₂/AlN T-QPM waveguide excited at 100 mW. It can be expected to realize a compact and high efficiency far-UV light source by combining a InGaN laser and the HfO₂/AlN T-QPM waveguide.
This work was partly supported by JSPS KAKENHI grant numbers JP16H06415, JP17H01063, JP17H05335, and JP19H02631.
[1] T. Fukui et al., PLoS ONE 15, e0235948 (2020).
[2] A. Yamauchi et al., LEDIA 2019, 8-3 (2019).
[3] K, Shojiki et al., CGCT-8, C01-01-05, (2021)

N-polar aluminum nitride (AlN) is an important building block for next-generation high-power RF electronics due to its highest electrical resistivity and thermal conductivity in the nitride semiconductor family.[1] For example, the current state-of-the-art performance achieved in N-polar GaN/AlGaN high electron mobility transistors (HEMTs) can potentially be further boosted by replacing AlGaN buffer with AlN.[2]
Although N-polar AlN has been synthesized on different foreign substrates such as Si, SiC and sapphire,[3-5] the development of homoepitaxial growth technique is highly desired to maximize the performance of N-polar AlN-based devices. However, to the best of the authors’ knowledge, successful MBE homoepitaxy of N-polar AlN has not been reported yet.
In this work, we report successful homoepitaxial growth of N-polar AlN by MBE on large-area cost-effective sputtered face-to-face annealed N-polar AlN/c-plane sapphire templates.[5] Direct growth leads to film with inverted Al-polarity. But with in-situ Al-assisted cleaning [6,7] before growth, the epilayer is found to maintain the N-polarity. The MBE-grown N-polar AlN epilayer is electrically insulating and has pit-free smooth surface with a root-mean-square (rms) roughness of 0.3 nm in a 10x10 μm² area with parallel atomic steps. Narrow X-ray rocking curves (XRCs) with full width at half of maximums (FWHMs) of 14/380 arcsec across AlN (0002)/(10-12) reflections indicate high structural quality of the N-polar AlN epilayer. Moreover, clear near band-edge photoluminescence (PL) emission peaks are observed on samples with MBE-grown N-polar AlN epilayers at room temperature. This peak is absent on the bare AlN substrates, implying the suppression of non-radiative recombination centers in the MBE-grown epitaxial N-polar AlN layers. These results suggest the significant potential of MBE homoepitaxy for preparation of electronics-grade N-polar AlN and are important milestones towards N-polar RF electronics based on AlN platform.
The authors at Cornell University acknowledge financial support from the Cornell Center for Materials Research (CCMR)—a NSF MRSEC program (No. DMR-1719875); ULTRA, an Energy Frontier Research Center funded by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES), under Award No. DE-SC0021230; and AFOSR Grant No. FA9550-20-1-0148. This work uses the CESI Shared Facilities partly sponsored by NSF No. MRI DMR-1631282 and Kavli Institute at Cornell (KIC).
References:
[1] A. L. Hickman, R. Chaudhuri, S. J. Bader, K. Nomoto, L. Li, J. C. Hwang, H. G. Xing, and D. Jena, Semiconductor Science and Technology 36, 044001 (2021).
[2] J. Lemettinen, H. Okumura, T. Palacios, and S. Suihkonen, Applied Physics Express 11, 101002 (2018).
[3] S. Dasgupta, F. Wu, J. Speck, and U. Mishra, Applied Physics Letters 94, 151906 (2009).
[4] J. Lemettinen, H. Okumura, I. Kim, C. Kauppinen, T. Palacios, and S. Suihkonen, Journal of Crystal Growth 487, 12 (2018).
[5] K. Shojiki, K. Uesugi, S. Kuboya, and H. Miyake, Journal of Crystal Growth 574, 126309 (2021).
[6] Y. Cho, C. S. Chang, K. Lee, M. Gong, K. Nomoto, M. Toita, L. Schowalter, D. Muller, D. Jena, and H. G. Xing, Applied Physics Letters 116, 172106 (2020).
[7] K. Lee, Y. Cho, L. J. Schowalter, M. Toita, H. G. Xing, and D. Jena, Applied Physics Letters 116, 262102 (2020).

Wide bandgap (WBG) materials are promising for next generation electronics due to their potential for operation at high frequencies and high power. These unique capabilities make WBG materials ideal for radio frequency (RF) devices, power electronics, and optoelectronics [1], as well as heat-spreading layers [2], which are of great interest for heterogeneous integration (HI) of systems. However, HI of WBG semiconductors demands two critical thermal and material considerations: back-end-of-the-line (BEOL) compatible deposition temperatures (<600 °C) and high thermal conductivity for heat dissipation at high operating temperatures. WBG materials that fulfill both criteria are extremely difficult to obtain and require fundamental thermal and electrical evaluation [3].

Thermal conductivities of non-metallic materials are largely governed by phonon transport, which is strongly dependent on material crystallinity and, by extension, synthesis temperature. Common passivation layers such as silicon oxide and silicon nitride can be deposited at low temperatures, but only as amorphous, defective films with low thermal conductivities. Conversely, the high thermal conductivities of bulk crystalline WBG materials such as diamond, aluminum nitride (AlN) [4], and cubic boron nitride (cBN) [5] are facilitated by strong covalent bonds and a high degree of crystallinity, both of which are typically achieved only by deposition or synthesis at high temperatures (e.g. > 900 °C).

Here, we present an electrical and thermal study of wide bandgap (~5-6 eV) nitrides (AlN, BN) deposited via sputtering and atomic layer deposition (ALD) at BEOL-compatible temperatures. For electrical measurements, we first fabricate metal-insulator-metal (MIM) structures by sputtering platinum bottom electrodes on silicon substrates. For the thermal measurements we prepare bare silicon substrates. We then deposit thin film polycrystalline nitride films of varying thicknesses (from 10 to 800 nm): BN via electron enhanced atomic layer deposition (EE-ALD) [6], and AlN via sputtering and atomic layer annealing (ALA) [2]. We use shadow masks to deposit top electrodes (gold for BN and platinum for AlN with titanium adhesion layers for both) and top aluminum transducers for the thermal measurements. We use the MIM structures toquantify electrical resistivity, dielectric constant, and breakdown field, and measure the cross-plane thermal conductivity of nitrides using time domain thermo-reflectance (TDTR).

BN deposited at room temperature via EE-ALD demonstrates resistivity on the order of 10⁹ Ω-m, breakdown field of ~0.05 V/nm, and relative dielectric constant ~5. Polycrystalline AlN sputtered at < 100 °C has resistivity on the order of 10¹⁰ Ω-m, and notably high thermal conductivity: 18 to 48 W/m/K for film thicknesses from 100 to 800 nm at room temperature. These high thermal conductivities are exceptional compared to other AlN thin films of similar thicknesses [4], and demonstrate that deposition of low-defect, high-quality AlN is possible at BEOL-compatible temperatures. After measurement, we also use scanning electron microscopy and transmission electron microscopy to gain insight into the role of the columnar grain structure on directional thermal transport within these films.

The novel low deposition temperatures and electrically insulating yet thermally conductive nature of BN and AlN offer promising applications for RF and power electronics, heat spreaders and heat management solutions within HI systems. This fundamental thermal and electrical study is a first step towards realizing the practical implementation of BEOL-compatible WBG nitrides.

[1] D. Garrido-Diez, I. Baraia, IEEE ECMSM (2017)
[2] S. Ueda, et al., IEEE VLSI-TSA (2020)
[3] J. Millan, et al., IEEE Trans. Pow. Electron., 29, 5, 2155-2163 (2013)
[4] R. Xu, et al., J. Appl. Phys., 126, 185105 (2019)
[5] K. Chen, et al., Science, 367, 6477 (2020)
[6] J. K. Sprenger, et al., J. Phys. Chem. C, 122, 17 (2018)

High quality ultra-wide bandgap materials and devices require high quality substrates. Homoepitaxial substrates for ultra-wide band gap semiconductors like AlGaN are prohibitively expensive and alternative heteroepitaxial schemes to date have been complex and resulted in poorly performing devices. We have identified TaC as a high temperature stable material as capable of serving as a low cost and high-quality lattice-matched heteroepitaxial template for AlGaN. The (111) plane of rock-salt TaC has a lattice parameter perfectly lattice matched to the hexagonal (001) plane of AlGaN. Cost-effective substrates for high quality AlGaN growth are a critical component to the development of vertical devices needed for next generation high power electronics.
In this work we grow (111)-oriented epitaxial TaC on Al₂O₃ substrates by RF sputtering. We explore structure and crystallinity as a function of sputter gun power, plasma incidence angle, chamber pressure, and film thickness. We find temperatures above 800 C and pressures of 5 mTorr result in pure-phase (111) oriented TaC. Rocking curves confirm the presence of a (113) peak; these plane angles geometrically differentiate the TaC layer from a competing Ta₂C phase. X-ray diffraction confirms registry to the oxygen sublattice of the underlying sapphire substrate. Transmission electron micrographs also show epitaxial registry of the grown layer to the substrate.
We also investigate crystallinity improvements resulting from annealing TaC films at. We find the optimal annealing geometry is a face-to-face arrangement, which appears to restrict oxidation and evaporation of the thin film layers. Precedent with AlN thin films suggests that face to face annealing promotes recrystallization while maintaining a local partial pressure near the surface of the film. [1] XRD of TaC films before and after annealing above 1500 C shows sharper peaks with reduced full width half max for the (111) plane while still maintaining phase purity. We also assess surface roughness and dislocation density before and after annealing. Our results indicate TaC virtual substrates may be a commercially viable path forward for growth of AlGaN devices.
[1] Miyake, H., Lin, C.-H., Tokoro, K. & Hiramatsu, K. Preparation of high-quality AlN on sapphire by high-temperature face-to-face annealing. Journal of Crystal Growth 456, 155–159 (2016).

AlN, as an ultra-wide bandgap semiconductor (UWBG), is an alternative for high power devices and deep-UV optoelectronics. However, significant challenges in efficient n-type doping and point defect control in AlN epitaxy has precluded the development of AlN based devices. Si is typically employed as n-type dopant in AlN with activation energy of about 300 meV originally attributed to the DX^-1 formation. Recently, Bagheri et al. provided a direct proof of single electron occupancy for donors in Al rich AlGaN and AlN instead of DX^-1 formation.¹ This makes controllable n-type doping via epitaxy in AlN feasible but only if proper point defect management is attained. Si doped AlN exhibits a “knee behavior” resulting in a conductivity and carrier concentration maxima at a specific Si concentration. Hence a high doping limit exists for Si in AlN that lowers the maximum achievable carrier concentration that are necessary for AlN based optoelectronics. Similarly, a low doping limit (a minimum achievable carrier concentration with a corresponding maximum mobility) exists in AlN, similar to that in AlGaN and GaN, which makes unfeasible the implementation of AlN power electronics that require low doped drift regions. These two challenges translate into a major “point defect problem” that exists in AlN which should be solved for implementation of this technology.
In this work, we demonstrate a systematic chemical potential control (CPC) based point defect control scheme where we relate the growth environment variables to the defect formation energy by determining and controlling the impurity chemical potentials and systematically engineer the growth environment accordingly for minimal point defect incorporation or generation.
Accordingly, threading dislocations, C_N, and V_III-nSi_III complexes were identified as the primary defects responsible for the doping limits in AlN grown by metalorganic chemical vapor deposition (MOCVD). We demonstrate control over the knee point (improving the maximum carrier concentration by inhibiting the formation of complexes) and low doping limit (achieving minimum carrier concentrations along with the maximum mobility by reducing the carbon incorporation) via controlling the chemical potentials of III/N, growth temperature and reduction of threading dislocation density. Doping below 10¹⁸ cm^-3 Si concentration was severely restricted by dislocations on sapphire substrate (TDD~10⁹ cm^-2) independent of growth condition. On AlN single crystal, efficient doping up to 5×10¹⁷ cm^-3 Si was achieved by reduction of dislocation density and carbon incorporation under a N-rich growth environment. An increase in the N chemical potential and a 200°C increase in the growth temperature led to a mobility increase from 40 to 200 cm²/Vs at a carrier concentration of 10¹⁵ cm^-3 in Si doped AlN.
This work opens pathways for controllable n-type doping in AlN providing one of the main building blocks for the realization of UWBG-based high power devices and deep-UV optoelectronics.
1. Bagheri, P. et al. On the Ge shallow-to-deep level transition in Al-rich AlGaN. J. Appl. Phys. 130, 055702 (2021).

Alloying GaN with Al increases the band gap but decreases the electron mobility due to alloy scattering. Using predictive first-principles calculations, we find that Al compositions greater than 75% are required to obtain even a two-fold increase of the Baliga Figure of Merit (BFOM) compared to GaN. At such high compositions, shallow donors undergo the DX transition making impurity doping difficult without severely limiting the mobility. Electrons in atomically ordered compounds do not undergo alloy scattering, therefore a large increase in the figure of merit may be possible for lower Al compositions where impurity doping is efficient. Atomically thin, chemically ordered superlattices of AlN and GaN have been known to grow experimentally under appropriate growth conditions. Using many-body perturbation theory and density functional perturbation theory, we uncover the electronic properties of atomically thin AlN/GaN superlattices with an Al to Ga ratio of 1:1, corresponding to 50% Al composition. We predict band gaps up to 4.91 eV and in-plane mobility that is up to 3x higher than that of random Al_0.5Ga_0.5N. Using a modified BFOM that accounts for the dopant activation energy, we predict that atomically thin AlN/GaN superlattices have the highest modified BFOM among several prominent ultra-wide band-gap materials, including GaN, AlN, random AlGaN, β-Ga2O3, cBN, and diamond.

Wide-bandgap (WBG) GaN and ultra-wide-bandgap (UWBG) AlGaN alloys are appealing semiconductor materials for the next-generation of high-voltage power devices due to their superior material properties. Practical power devices such as merged-PiN-Schottky (MPS) diodes and junction field effect transistors (JFETs) require selective areas of p-type semiconductor surrounded by n-type material. This selective area p-type doping is typically achieved using implantation and thermal annealing to form the p-well in Si and SiC based power devices. However, ion implantation presents challenges in GaN and requires specialized equipment for the high-pressure and high temperature annealing for dopant activation while implantation into AlGaN alloys has received limited attention.

We have investigated selective-area-regrowth (SArG) to epitaxially grow p-type GaN and AlGaN in place of dopant implantation. Successful pn diode formation by SArG requires a process to remove residual crystalline damage resulting from the inductively coupled plasma (ICP) etch typically used to form the p-well and a method to remove the elevated level of Si found at the regrowth interface of a surface that had been previously exposed to air. For the case of SArG of p-GaN on air-exposed, blanket ICP etched n-type GaN, we will present the novel use of a fluorine-based precursor for in-situ etching of GaN in the MOCVD chamber. Unlike chlorine (TBCl and CCl4) and bromine (CBr4) -based precursors we have studied, we obtained a smooth surface following in-situ fluorine etching of air-exposed GaN. Schottky barrier diodes (SBDs) formed by shadow mask evaporation on in-situ fluorine etched n-GaN that had been previously ICP etched showed reverse leakage currents to -40 V that are equal to those of SBDs formed on as-grown n-GaN layers. The in-situ fluorine/ICP etched diodes had reverse leakage currents more than 3 orders of magnitude lower than those formed on n-GaN layers that had only experienced ICP etching. This suggests that the in-situ fluorine etch was effective at removing the residual damage of the ICP etch process. Furthermore, we discuss the use of in-situ fluorine etching to reduce the concentration of Si at a regrown GaN interface.

Unlike the case for GaN, we find that pn junction formation by SArG of p-Al_0.3Ga_0.7N on air-exposed and ICP etched n-Al_0.3Ga_0.7N drift layers to be tolerant to residual etch damage and elevated concentration of Si at the regrowth interface. We will present pn junction diodes formed by p-Al_0.3Ga_0.7N regrowth on blanket ICP-etched n-Al_0.3Ga_0.7N drift layers with performance which matches that of continuously grown diodes that have achieved a breakdown voltage of 1.5 kV. Furthermore, we will describe a pn junction diode where the anode consisted of p-Al_0.3Ga_0.7N regrown in ICP etched p-wells in a n-Al_0.3Ga_0.7N drift layer with current-voltage characteristics equal to diodes grown without growth interruption that have achieved a breakdown voltage of 1.8 kV. These demonstrations support the promise of AlGaN alloys for realizing practical, kilovolt-class power diodes and transistors for next generation power systems.

This work was supported in part by the Laboratory Directed Research and Development program at Sandia National Laboratories and in part by the Advanced Research Projects Agency – Energy (ARPA-E), U.S. Department of Energy under the PNDIODES program directed by Dr. Isik Kizilyalli. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA-0003525. The views expressed in the presentation do not necessarily represent the views of the U.S. Department of Energy or the United States Government.

Ultrawide-bandgap (UWBG) materials such as AlGaN are suitable candidates for emerging high-power and high frequency electronics owing to its superior electric field handling capability [1]. Also, on the rise are N-polar GaN/AlGaN HEMTs for their very promising RF performance, often considered more advantageous compared to its Ga-polar counterpart due to the lower parasitic resistance and better scalability [2]. N-polar AlGaN channel can bring remarkable performance advantage to RF and power electronics. While experimental realization of Ga-polar AlGaN/AlGaN HEMT has recently been reported [3], N-polar AlGaN/AlGaN HEMT structures are yet to be realized experimentally. In this work, we report the first experimental realization of N-polar AlGaN/AlGaN HEMT using metal organic chemical vapor deposition (MOCVD) technique.
In this study, we started the growth with a 0.6 µm-thick Al_0.15Ga_0.85N buffer layer on 4° miscut sapphire c-plane substrate. Next, we grew the barrier Al_xGa_1-xN layer and the channel Al_yGa_1-yN layer, each of them being 25 nm thick. We varied the mole fractions of Al (x>y) for the barrier and the channel layers to achieve AlGaN/AlGaN HEMT structures with different stoichiometries. We used a fixed 100 mbar chamber pressure while varying the flow rate for the precursors (TMGa, TMAl and NH₃) and the temperature to achieve the desired stoichiometries. As an example, the parameters we used for the growth of Al_0.15Ga_0.85N buffer layer are: NH₃: 1466 sccm, TMGa: 45.6 sccm, TMAl: 33 sccm, temperature: 1130°C and pressure: 100 mbar.
We demonstrated four different HEMT structures with Al mole fraction y=20%, 30%, 60% and 75% in the Al_yGa_1-yN channel layer while the corresponding Al mole fractions (x) in the Al_xGa_1-xN barrier layer are 60%, 60%, 100% and 100%, respectively. The Al mole fractions were determined using TCAD simulations such that there is always a two-dimensional electron gas formation in the channel. Our X-ray diffraction (XRD) measurements (2θ-ω scan) confirmed the stoichiometries for the HEMT structures. From the XRD peak positions of the AlGaN and the AlN, we used Vegard’s law to determine the Al mole fractions. Atomic force microscopy imaging showed a good surface morphology with a root mean square surface roughness of <1 nm for all the samples under 5 µm x 5 µm scan size. Additionally, all our samples showed hexagonal hillock-like structures after KOH etching, confirming the N-polarity of the grown samples.
Next, we explored the effect of Al mole fraction in the mobility of the HEMT structures using Hall measurement method. The HEMT structure with Al_0.20Ga_0.80N channel layer showed an average mobility of 120 cm²/V/s. The mobility values for conduction parallel to the multiatomic steps can be higher (∼20%) than the measured average Hall mobility here. The mobility shows a decreasing trend with the increasing Al mole fraction in the channel layer showing a mobility of 25 cm²/V/s for Al_0.75Ga_0.25N channel. Such reduction in mobility can be attributed to the alloy scattering within the lattice structure.
The calculated alloy scattering limited mobility from simulated TCAD structures closely followed our experimental results. The mobility was calculated using the simulated 2DEG electron density and alloy compositions for the corresponding HEMT structures [4]. The highest mobility that can be achieved at 85% Al composition is ∼255 cm²/V/s.
In summary, we experimentally demonstrated the MOCVD growth of N-polar AlGaN/AlGaN HEMT with varying stoichiometries. These results could enable numerous high power and high frequency electronic applications using AlGaN-channel HEMTs.
We acknowledge the funding from ONR (Award No N00014-19-1-2611- P00004) and Dr. Paul Maki for supporting this work.
[1] J. Tsao et al., Adv. Electron. Mater. 4, (2018)
[2] O. Koksaldi et al., IEEE EDL 39, (2018)
[3] A. Baca et al., ECS J. Solid State Sci. Technol. 6 (2017)
[4] M. Coltrin et al., ECS J. Solid State Sci. Technol 6 (2017)

Ultrawide bandgap (UWBG) semiconductors are of significant interest for their wide applications in optoelectronic devices such as ultraviolet light emitting diodes (UV LEDs), UV lasers, Schottky barrier diodes, solar-blind UV photodetectors, and high-electron-mobility transistors (HEMTs). Poor Schottky contacts, for example, in III-nitrides based photodetectors or HEMTs resulted in severe leakage current, high power consumption, and device failure. Furthermore, considering that AlGaN with a high aluminum content is relatively rarely studied compared to GaN, research on developing a high quality Schottky contacts with superior reliability on AlGaN is required to improve the performance of AlGaN-based optoelectronic devices.
In this study, we investigated the electrical properties of Ag-Pd-Cu (APC) alloy Schottky contacts on n-AlGaN under various annealing temperature. V/Al/Ti/Au (30/200/10/50 nm) were deposited on n-AlGaN as ohmic contacts, and annealed at 900 °C for 1 min in N₂ atmosphere. APC films as Schottky contacts were deposited on the inner Schottky region, followed by annealing at 300, 500, 700 °C for 1min in air. For comparison, conventional Ni/Au (20/20 nm) Schottky contacts were also applied after annealing at 500 °C for 1 min in air.
The Schottky barrier heights (SBHs) were measured using the current-voltage (I-V) method, the barrier inhomogeneity model, and the capacitance-voltage (C-V) method. The SBHs of APC contacts obtained by I-V method were in the range of 0.822-0.972 eV, depending on the annealing temperature. The SBHs of APC contacts measured by using the barrier inhomogeneity model and C-V method were in the range of 1.62-1.80 eV and 1.81-2.19 eV, respectively. On the other hand, the SBHs of Ni/Au contacts measured by I-V method, the barrier inhomogeneity model, and the C-V method were 0.788-0.820 eV, 1.43-1.69 eV, and 1.48-1.66 eV, respectively. In addition, we observed that APC samples showed more uniform leakage current properties compared to Ni/Au samples. The relative standard deviation (RSD) of leakage current at reverse bias region of annealed APC samples exhibited lower values (〈0.3) than that of Ni/Au samples (〉0.5), which means APC can provide a better reliability in chip fabrication process.
The X-ray photoemission spectroscopy (XPS) were used to observe the shift of the Ga 3d core levels of the annealed APC samples toward lower binding energies after annealing, as compared to that of as-dep samples. The outdiffusion of Ga atoms from n-AlGaN was observed by the scanning transmission electron microscope (STEM) elemental mapping. The scanning electron microscope (SEM) images showed the enhanced thermal stability of APC films. The better electrical properties and reliabilities of APC Schottky contacts with various annealing temperature will be discussed based on XPS, STEM and SEM results.

GaN and its alloys have been widely utilized in optoelectronics, photonics, and electronics. Due to its large band gap (3.4 eV), strong critical electric field (3.4 MV cm^-1) and high electron mobility (~1200 cm² V^-1 s^-1), GaN exhibits great potential for high power electronics. Vertical GaN PN diode with 5kV breakdown voltage has been demonstrated [1]. To achieve vertical GaN power devices with V_BR>10 kV, one requires the epitaxy of high quality GaN films with thick drift layer and low controllable doping with high mobility. Typical MOCVD GaN films with high mobilities were grown with growth rate (GR) of 2-3 µm hr^-1. Thus, it is important to develop and understand MOCVD GaN growth with fast GR.

Our previous work [2] has demonstrated that laser-assisted metalorganic chemical vapor deposition (LA-MOCVD) can effectively facilitate NH₃ decomposition and, thus, to achieve high-quality GaN films with low carbon (C) and fast GR. Low [C] at 5.5E15 cm^-3 was achieved in the LA-MOCVD GaN film grown with the GR of 4 µm hr^-1. LA-MOCVD GaN exhibited lower [C] incorporation at low input V/III ratio and high GRs as compared to the conventional MOCVD GaN growth. High room-temperature mobilities over 1000 cm² V^-1 s^-1 were achieved by LA-MOCVD with a GR of 4.5 µm hr^-1. The LA-MOCVD GaN growth technique provides a promising approach to obtain high-quality, low-C GaN with fast GR for power device applications.
In this work, the LA-MOCVD GaN growth window was further extended to a higher GR (>7 µm hr^-1) regime. The effect of growth conditions on both MOCVD and LA-MOCVD GaN growths at high GR regime was studied. It was found that the process gas (H₂) flow plays an important role on the growth rate, surface morphology and impurity incorporation. The GRs of both MOCVD and LA-MOCVD GaN increase with the reduction of alkyl flows. The growth rate of LA-MOCVD GaN was similar as that of regular MOCVD GaN at high alkyl flow rates. With the same total process flow rate, redistributing more H₂ flow to hydride injection increases the GR of regular MOCVD GaN but results in a significant reduction of LA-MOCVD GaN GR. The [C] incorporation is dependent on the process gas flow/distribution and positively related to growth rates. The corresponding LA-MOCVD growths show about 40% reduction of [C] under all process gas flow/distribution conditions. It shows that higher hydride flow could effectively suppress the [C] incorporation with both regular MOCVD and LA-MOCVD GaN. Under this condition, relatively low [C] was achieved on both regular MOCVD (5.8E16 cm^-3, GR=11 µm hr^-1) and LA-MOCVD GaN (2.1E16 cm^-3, GR=7.5 µm hr^-1), which represent one of the best reported results with the similar growth rates [3].
Using the optimized growth condition, recently we demonstrated the GaN vertical PN diode with V_BR of ~5 kV with low leakage current. The drift layer thickness was ~30 µm grown with a growth rate of 5 µm hr^-1. The doping was controlled at ~ 3E15 cm^-3. Ion-implanted guard ring structure was applied for the field management. The R_on was 0.88 mΩcm². The Baliga’s figure of merit (BFOM) was over 27 GW/cm², which is the highest among the reported values [1].
In summary, we demonstrated LA-MOCVD GaN growth with fast growth rates. The effect of growth conditions on both MOCVD and LA-MOCVD GaN growths at high GR regime was systematically studied. LA-MOCVD provides an enabling route to achieve high-quality GaN epitaxy with low-C and fast GR simultaneously. The demonstrated GaN vertical PN diode has a breakdown voltage close to 5kV, with a record BFOM over 27 GW/cm².
Acknowledgment: ARPA-E (DE-AR0001036), and U.S. Department of Energy's Office of Energy Efficiency and Renewable Energy (EERE) under the Advanced Manufacturing Office, FY18/FY19 Lab Call.
References:
[1] H. Ohta, et al, Jpn. J. Appl. Phys. 58, SCCD03 (2019).
[2] Y. Zhang, et al., Phys. Status Solidi RRL, 15, 2100202 (2021).
[3] T. Ciarkowski, et al., Materials 12, 2455 (2019).

Multifunctional perovskite oxides exhibit a wide array of exotic properties often not observed in conventional semiconductors, including multiferroicity, high sheet charge densities and dielectric constants (κ), superconductivity, and tunable electronic phases. Advances in epitaxial growth techniques in recent years have enabled the monolithic integration of these functional oxides with semiconductor materials including Si, Ge, GaAs, and more recently, GaN. Development of new electronics on wide and ultra-wide bandgap semiconductors such as GaN and AlN is especially attractive in the field of RF and power electronics, due to the increased saturation electron velocity, thermal and chemical stability, and larger breakdown electric field of these materials when compared to conventional semiconductors such as Si. One limitation to increasing output power density is the high peak electric field that occurs as operating voltage is increased, leading to premature breakdown. “Extreme-κ” (>100) dielectric layers have been proposed as an approach to reduce peak electric fields and enhance breakdown voltage in a device. Perovskite oxides such as SrTiO₃ or CaTiO₃ can be considered “extreme-κ” materials in this sense, as their bulk κ values are >100. However, κ values in thin film amorphous perovskite oxides are significantly lower (typically ~20).
In this work, we demonstrate high permittivity thin films of Sr_1-xCa_xTiO₃ (SCTO), a solid solution of SrTiO₃ and CaTiO₃, epitaxially grown on AlGaN/GaN/SiC high-electron-mobility transistor heterostructures via RF-plasma-assisted oxide molecular beam epitaxy. The structural phase mismatch between the perovskite SCTO and wurtzite GaN is overcome by the deposition of a 1 nm rutile TiO₂ buffer layer on the GaN, which results in a (111), single-phase out-of-plane orientation of the SCTO layers. We investigate the structural and electrical properties of SCTO/AlGaN/GaN heterostructures with compositions ranging from x = 0 to x = 0.53 and oxide film thicknesses ranging from 7 nm to 126 nm. Current-voltage measurements show up to a 5 order of magnitude reduction in vertical leakage of the SCTO samples when compared to a Schottky-contacted control sample. Capacitance-voltage measurements show minimal hysteresis, an extracted κ value as high as 290, and a fixed positive interface charge density of 2.38 ×10¹³ cm^-2 at the SCTO/AlGaN interface. RF characterization of interdigitated capacitors fabricated using the SCTO films grown on unintentionally doped GaN/SiC shows that the films maintain their high permittivity values at 2 GHz and only exhibit a slight reduction in κ with increased lateral electric fields. These results demonstrate the epitaxial integration of an “extreme κ” functional oxide with GaN that can potentially improve electric field management in RF high-electron-mobility transistors. Moreover, the epitaxial connection between a perovskite and wurtzite crystal structure enables the subsequent expansion of perovskite oxides onto ultra-wide bandgap semiconductors such as high-Al-content AlGaN and AlN to realize next-generation multifunctional oxide/nitride hybrid electronics.
This work is supported by the Office of Naval Research.

This work reports vertical GaN PN diodes with over 5 kV breakdown voltage. The devices were fabricated on a 2” Ammonothermal NEAT GaN substrate. The electron concentration of the substrate is > 8 ×10¹⁸ cm⁻³ with an average dislocation density of ~ 1×10⁵ cm⁻². The drift layer was grown by metalorganic chemical vapor deposition (MOCVD) at a growth rate of 5 μm/hr. The epi-layers on top of the substrate consist of 2 μm thick n+ layer (2×10¹⁸ cm^-3) followed by ~ 28 μm n^- drift layer (1×10¹⁵ cm^-3), and a 500 nm p-GaN layer (1×10¹⁷ cm^-3) topped with a thin p⁺-20 nm GaN layer (5×10¹⁷ cm^-3). The device has a size of 500 μm in diameter. The device structure was designed based on analytical calculations and numerical simulations. After optimization in numerical simulations, a design with 6 Guard Rings (GR) was chosen for electrical field management. In addition to the GRs, a 1.7 μm spin-on-glass and a field plate design were included. Simulation results show that the electric field is peaked at 3.5 MV/cm at the edge of the GR under a reverse bias of 6.4 kV. The GRs were implemented by nitrogen ion implantation at a dose of 3.2×10¹³ ions/cm⁻². A Ti/Al based alloyed contact was used as the back cathode for n-GaN and a Ni/Pt/Au based alloyed contact was used as top anode for p-contact. The n-contact resistance was measured to be 6.1×10^-6 Ωcm² and the p-contact resistance was 9.2×10^-5 Ωcm², respectively. The device exhibited an avalanche breakdown at 5093 V, showing a remarkable agreement with numerical simulation with a breakdown efficiency of > 79%. The reverse leakage current density is below 1×10^-7 A/cm² until 200 V and ~ 1×10^-5 A/cm² at 3 kV. For forward characteristics, the device has a turn-on voltage of 3.85 V at 100 A/cm² and has a current density of over 500 A/cm² below 5.0 V. The specific on resistance for this device is 2.45 mΩ.cm². Considering the measured breakdown voltage and specific on-resistance, the device has a Baliga figure of merit of 10.6 GW/cm². Considering the p-contact resistivity from the TLM structure next to the device (1.4 mΩcm²) and the specific on-resistance of the device, the electron mobility in n-drift layer is approximately 1000 cm²/Vs. To the best of our knowledge, this is the first GaN pn power diode that demonstrates over 5 kV breakdown with a drift layer of less than 30 μm.

Acknowledgment: ARPA-E (DE-AR0001036), and U.S. Department of Energy's Office of Energy Efficiency and Renewable Energy (EERE) under the Advanced Manufacturing Office, FY18/FY19 Lab Call.

The ferromagnetic form of GaN may have an enormous technological relevance due to the already dominating role of the nitride family in light industry, high-frequency, and high-power electronics. In particular, the existence of sizable piezoelectromagnetic coupling has been evidenced recently in homogeneous (single-phase) (Ga,Mn)N [1], an effect that opens the door for realization of external electric field driven, repeatable magnetization reversal. It is, therefore, very important to understand the physics which governs the behavior and magnetic properties of transition metals in nitrides.
In this paper we report on a comprehensive material characterization with the main emphasis put on the magnetic properties of the Mn impurity in bulk GaN. Our one inch in diameter single crystals were crystallized by halide vapor phase epitaxy [2] with Mn concentration ranging from 1×10¹⁸ to 5×10¹⁹ cm^-3. The material is probed by secondary ion mass spectrometry, x-ray diffraction, electron paramagnetic resonance (EPR) and superconducting quantum interference device magnetometry. Vital clues on the charge state of Mn ions comes from the analysis of the magnetic anisotropy, which in combination with EPR points to the Mn³⁺ (d⁴) configuration. Such a center is characterized by a very strong single ion magnetic anisotropy. The trigonal-symmetry surrounding and Jahn-Teller effect lead to splitting of the five lowest spin quantum levels (characterized by spin quantum numbers m_S = -2,-1,0,1,2). Around H = 0 the ground state is composed mostly of m_S = 0 and with the increase of magnetic field m_S = -1 and m_S = -2 become sequentially the lowest states, leading to a well-developed staircase-like magnetization curve at sub-Kelvin temperatures. The unique shape and positions of the magnetization steps measured at the magnetically hard axis configuration (parallel to the wurtzite c-axis of GaN) are accurately described using a single ion crystal fie2ld approach relevant to Mn³⁺ state. The full understanding is achieved by taking into account the tetrahedral cubic field with trigonal distortion, spin-orbit interaction, static tetragonal Jahn-Teller distortion, and magnetic field [3]. The obtained perfect agreement between the experimental m(H) and theory means that for such strong dilutions there is no need to invoke terms corresponding to Mn-Mn interactions (pair or triplets) and that the concentration of magnetically isotropic Mn²⁺ centers is negligibly small in the samples. The latter finding is of great importance for commercialization of industry-relevant, high quality, insulating substrates since it implies a highly insulating property of these crystals.
This work has been supported by the National Science Centre, Poland through OPUS (DEC-2018/31/B/ST3/03438) grant and TEAM TECH program of the Foundation for Polish Science co-financed by the European Union under the European Regional Development Fund (No.POIR.04.04.00-00-5CEB/17-00).
[1] D. Sztenkiel et al., Nature Comm. 7, 13232 (2016).
[2] M. Bockowski et al., Journal of Crystal Growth 499, 1-7 (2018).
[3] D. Sztenkiel et al., New J. Phys. 22, 123016 (2020).

Commercially available GaN-based electronics and optoelectronics devices are fabricated mainly on Ga-polar GaN substrates. However, recent results from the University of California Santa Barbara have shown superior performance of N-polar GaN high electron mobility transistors (HEMTs) in W-band compared with the Ga-polar GaN HEMTs. Meanwhile, research and progress on Nitrogen-polar films have been limited due to the fact that it is very hard to grow films of high quality, unlike the metal-polar case.
Although AlGaN has been commonly used as the barrier in GaN HEMTs, InAlN could be potentially a great candidate to enable high charge in HEMTs with scaled channels. In particular, In_0.18Al_0.82N which is lattice-matched to GaN, has a wide bandgap, large spontaneous polarization, and high refractive index contrast with GaN. Therefore, it is attractive as the barrier material in (HEMTs) and UV light emitting diodes (LEDs).
Regardless, the growth of InAlN is challenging due to a large difference in the thermal stability of InN and AlN. Therefore, InAlN has been traditionally grown at temperatures much lower than optimized growth temperature for GaN using both metal-organic chemical vapor deposition (MOCVD) and MBE [1-3]. Lower growth temperature leads to higher incorporation of impurities such as oxygen and carbon. Additionally, the lower growth temperature of InAlN would require several growth interruptions to change the substrate temperature for the growth of other (Al,Ga)N layers in the epi-structure. In this work, we are going to present the first demonstration of high-temperature growth of InAlN on N-polar GaN substrate by plasma assisted molecular epitaxy (PAMBE).

A GenXplor (MBE) system was used for the epitaxial growth of samples in this work. First, a 200 nm-thick GaN layer was grown at 750 °C to ensure a smooth surface morphology. The growth was then interrupted to change the substrate temperature for the growth of InAlN film. A number of InAlN samples were grown at different temperatures ranging from 600 °C to 800 °C. By tuning the Al and In fluxes, InAlN films with InN mole fraction ranging from 8% to 37% was achieved. The surface morphology of samples was characterized using atomic force microscopy (AFM). The alloy composition was extracted using high-resolution X-Ray diffraction (HRXRD). In0.18Al0.82N film with smooth surface morphology and high structural quality was achieved at 750 °C (the optimum temperature for growth of N-polar GaN).

1- Appl. Phys. Lett. 104, 072107 (2014).
2- AIP Advances 6, 035211 (2016);
3- Mater Renew Sustain Energy 2, 10 (2013).

Point defects and their charge transition levels in Ga₂O₃ are critical to its success in high power electronics. When investigating defects or transition levels, however, often little information is known about the defect itself; rather, identification is made by monitoring carriers excited to or from the defect. Time-dependent photo-induced electron paramagnetic resonance (photo-EPR) offers the opposite approach: little is known about the carriers, but the identity of the defect is clear. Basically, the technique is an optical absorption measurement performed on a specific, known defect. The process consists of 1) identifying an EPR spectrum with a specific defect structure; 2) monitoring the amplitude of the EPR during illumination with selected wavelengths, and 3) Fitting the time-dependent data with a series of rate equations representing the type of transition. An optical cross section spectrum, σ(E), obtained from the data is compared to one which includes the defect level (E_D) as well as defect-lattice interaction or relaxation energy as parameters.
Photo-EPR is performed with a conventional 10 GHz EPR spectrometer and a series of LEDs from 0.7 to 4.4 eV. Samples studied include bulk Fe- or Mg-doped Ga₂O₃ grown by the Czochralski or floating zone methods. Samples were examined with X-ray diffraction to determine the orientation of the crystal faces and secondary ion mass spectrometry to determine the concentration of the typical impurities, Si, Fe, and Ir. The concentration of Fe and Mg in the intentionally doped samples were 5-7x10¹⁸ cm^-3 and 6-30x10¹⁸ cm^-3, respectively. All samples were insulating.
The defect levels for Fe^2+/3+ and Mg^-/0 were directly probed using the photo-EPR method. Data obtained from both Mg-doped and Fe-doped samples yielded a defect level of 0.6 eV below the conduction band edge for the 2+ to 3+ transition of Fe. As an optical technique, we are also able to extract the lattice relaxation energy, which for this transition is 0.7 eV. The transition level is similar to that determined by several other groups and the relaxation energy confirms the value calculated from density functional theory [1,2]. Theoretical calculations for the defect level for neutral to negative transition of Mg was reported to be ~ 1 – 1.5 eV above the valence band maximum (VBM) with a lattice relaxation of 1.1 eV [3-5]. Recent experimental measurements, however, place the level only 0.6 eV above VBM [6]. Our results, which agree with both the defect level and relaxation energy predicted by theory, are 1.35 eV above VBM (E_d) and 1.1 eV (relaxation energy). The Fe and Mg-related process represent direct defect-to-band transitions. In addition to these, the talk will highlight the role of other point defects such as Ir and the much-discussed V_Ga, both of which are observable with EPR.
The work was supported by the National Science Foundation under DMR-1904325
1. M. E. Ingebrigtsen et al, Appl. Phys. Lett. 112, 042104 (2018).
2. C. A. Lenyk et al , J. Appl. Phys. 126, 245701 (2019).
3. Q. D. Ho et al, J. Appl. Phys. 124, 145702 (2018).
5. A. Kyrtsos et al, Appl. Phys. Lett. 112, 032108 (2018).
5. J. L. Lyons, Semicond. Sci. Technol. 33, 05LT02 (2018).
6. C. A. Lenyk et al, Appl. Phys. Lett. 116, 142101 (2020).

Gallium oxide (Ga₂O₃) is an interesting ultra wide band gap semiconductor, currently arresting attention of the semiconductor research community. Since native point defects in semiconductors are electrically active, learning about their energetics is of paramount importance. Notably, there is a spectacular progress reached on this matter from the theoretical perspective in semiconductors in general and in Ga₂O₃ in particular. For example, for gallium vacancy (V_Ga) – known as probably most fundamental defect in Ga₂O₃ – the energetics were calculated in a wide range of the Fermi level (E_F) and the chemical potential [1-2]. On the other hand, the correlations of these theoretical data with experimental values are immature. Two of our recent publications [3-4] were dealing with these challenges, and the present contribution aims to extend these interpretations. Conceptually, we used radiation effects and impurity diffusion measurements to interconnect the data with the V_Ga energetics in the context of theory.
Specifically, we explored, so-called “dose rate effect” in Ga₂O₃, measured its activation energy of 0.8 eV, and associated this energy with the migration barrier for V_Ga, limiting the defect annealing process during the irradiation [3]. Further, we studied silicon (Si) diffusion in Ga₂O₃ and measured its activation energy. Importantly, the interpretation of the data unveiled that the diffusion occurs via neutral and single negatively charged point defects, in our interpretation V_Ga in the corresponding charge states, with the activation of 3.2 and 5.4 eV, respectively [4]. Again, the added value of these data is in comparison with theory [1-2]. Firstly, it is not the triply negative charge state of V_Ga that mediates Si diffusion, even though its lowest formation energy for the n-type material is envisaged by theory. That was interpreted in terms of the E_F lowering at the high temperatures [5] of the Si diffusion process. Secondly, neglecting the binding energy and accounting for 0.8 eV as the V_Ga migration energy [3], the upper limit of the V_Ga formation energy in neutral and single negatively charged states can be readily estimated as 2.4 eV and 4.6 eV, respectively, consistently with the theory [1-2].
In summary, we made interpretations of the gallium vacancy energetics in gallium oxide as extracted from the radiation phenomena and diffusion esperiments.
REFERENCES
[1]. Kyrtsos, M. Matsubara, and E. Bellotti, Migration mechanisms and diffusion barriers of vacancies in Ga2O3, Phys Rev. B 95, 245202 (2017).
[2] H. Peelaers, J. L. Lyons, J. B. Varley, and C. G. Van de Walle, Deep acceptors and their diffusion in Ga₂O₃, APL Mater. 7, 22519 (2019).
[3] A.Azarov, V.Venkatachalapathy, E.Monakhov, A.Kuznetsov, Dominating migration barrier for intrinsic defects in Ga₂O₃: dose-rate effect measurements Appl.Phys.Lett. 118, 232101 (2021)
[4] Activation energy of silicon diffusion in Ga₂O₃: roles of the mediating defects charge states and phase modification, A.Azarov, V.Venkatachalapathy, L.Vines, E.Monakhov, I-H. Lee, A Kuznetsov Appl.Phys.Lett, accepted: 15 October 2021 (doi: 10.1063/5.0070045)
[5] E.Chikoidze, A.Fellous, A.Perez-Tomas, G.Sauthier, T.Tchelidze, C.Ton-That, T. T. Huynh, M. Phillips, S. Russell, M. Jennings, B. Berini, F. Jomard, and Y. Dumont, P-type b-gallium oxide: A new perspective for power and optoelectronic devices, Mater. Today Phys. 3, 118 (2017).

Atomic scale scanning transmission electron microscopy (STEM) was used to study the formation of point and extended defects, as well as phase transformations in Si-implanted β-Ga₂O₃. Quantitative analysis of the atomic column intensities in STEM images acquired with an absolute scale, when combined with precise electron scattering simulations, can directly visualize the detailed structure of atomic and nanoscale defects in materials. For example, our previous studies have revealed the formation of different types of point and extended defects, including the interstitial-divacancy complexes in β-Ga₂O₃ and planar defects and phase transition in (Al_xGa_1−x)₂O₃ that directly correlate with Al incorporation into the lattice. In the present study, we performed a correlative study on the structural change and defect formation in Si implanted β-Ga₂O₃ (edge-defined, film-fed (EFG)-grown (001) β-Ga₂O₃ substrate) as a function of Si dose, using a combination of STEM and secondary ion mass spectrometry (SIMS). Peak Si concentrations of 10¹⁸-10²¹ cm^-3 were investigated. Different types of point defects and their complexes were observed in lower Si concentrations (< ~ 10¹⁹ cm^-3), which include cation interstitials and substitutional atoms into the oxygen positions. The types and concentrations of those defects change as a function of the depth of the implantation. The implication of the observed defects to electronic properties will be discussed. High concentration of point defects at a local region also led to the formation of a unique type of extended defect, which apparently involves a large strain field that extends up to a few tens of nanometers. At higher Si concentrations (> 10²⁰ cm^-3), the structure tends to transform into different Ga₂O₃ phases, including γ-Ga₂O₃ which, according to our previous investigation, has a close relationship to the extended defects in β-Ga₂O₃. In situ annealing of the samples was performed to understand the structural evolution and diffusion dynamics of the implanted materials. The precise atomic scale information on defect formation and their evolution provides an important guidance to understand and control the ion implantation of Ga₂O₃ materials and devices which is crucial to advance them to next generation ultrawide-bandgap applications.

β-AlGaO has recently been emerged as a promising semiconductor material for next generation high power and radio frequency electronic applications due to its tunable energy bandgap, high breakdown field strength and the ability to achieve large electron mobility by modulation doping at β-AlGaO/GaO interfaces. Metalorganic chemical vapor deposition (MOCVD) of β-AlGaO films have been demonstrated with a great control of uniformity, composition, and high purity [1-4]. While high quality and high-Al composition β-AlGaO films are advantageous for the development of high-power devices, the quality of the dielectric-semiconductor interfaces is also crucial for reliable high-performance transistors. Due to its good compatibility with β-Ga₂O₃, Al₂O₃ has been widely used as a gate dielectric for β-Ga₂O₃ and β-AlGaO based devices. The deposition of Al₂O₃ is mostly performed by using ex-situ method, such as atomic layer deposition (ALD) technique, which can lead to interface contamination due to the exposure to the ambient. Recently, in-situ MOCVD growth of dielectric Al₂O₃/(010) β-Ga₂O₃ has been demonstrated with improved interface quality as compared to other conventional dielectric deposition techniques [5]. However, the investigation of the interface quality and the band offsets at in-situ MOCVD deposited Al₂O₃/β-AlGaO interfaces are still lacking. In this work, for the first time, we have investigated the interface quality and experimentally determined the band offsets of in-situ MOCVD dielectric Al₂O₃/β-AlGaO interfaces with various Al compositions along different orientations. Moreover, the band offsets determined for in-situ Al₂O₃ are also compared with ALD (ex-situ) Al₂O₃/β-AlGaO interfaces.

Phase pure β-AlGaO epitaxial films [(010), Al = 0, 0.17, 0.35], [(100), Al = 0, 0.17, 0.52] and [(-201), Al = 0, 0.17, 0.48] were grown on (010), (100) and (-201) β-Ga₂O₃ substrates by MOCVD. Al₂O₃ layer was deposited on top of β-AlGaO films by using both in-situ (MOCVD) and ex-situ (ALD) deposition techniques. The valence (VBO) and conduction (CBO) band offsets at Al₂O₃/β-AlGaO interfaces were determined by XPS. The XRD and high-resolution HR-STEM imaging confirm the growth of amorphous Al₂O₃ films with abrupt Al₂O₃/β-AlGaO interfaces. The bandgaps estimated by examining the onset of the inelastic energy loss spectra are measured to be 6.91 eV and 6.88 eV for in-situ and ex-situ deposited Al₂O₃, respectively, which are consistent with previous reports on ALD Al₂O₃. The CBOs varying from 2.0 eV (Al=0) to 1.53 eV (Al=0.35) were determined from in-situ Al₂O₃/(010) β-AlGaO interfaces. Similarly, the CBO of 2.18 eV (Al=0) and 1.07 eV (Al=0.52) were measured for (100) orientation. The CBOs at Al₂O₃/(-201) β-AlGaO interfaces vary from 2.14 eV (Al=0) to 1.06 eV (Al=0.48). For ex-situ ALD deposited Al₂O₃, the CBOs were revealed to be varied between 2.09-1.73 eV [(010), Al=0-0.35], 2.32-1.23 eV [(100), Al=0-0.52] and 3.01-1.96 eV [(-201), Al=0-0.48], indicating a distinct dependence of the band offsets on different orientations and dielectric deposition techniques.

In summary, we investigated the interface quality and band offsets of in-situ MOCVD and ex-situ ALD deposited Al₂O₃/β-AlGaO interfaces for different orientations with the variation of Al compositions. Findings from this study will guide future designs of β-Ga₂O₃ and β-AlGaO based structures and devices.
Acknowledgment: The authors acknowledge the funding support from the Air Force Office of Scientific Research No. FA9550-18-1-0479 (AFOSR, Dr. Ali Sayir), and NSF (1810041, 2019753).
References:
1. Bhuiyan et al., APL Mater. 8, 031104 (2020).
2. Bhuiyan et al., Cryst. Growth Des. 20, 6722 (2020).
3. Bhuiyan et al., Appl. Phys. Lett. 117, 142107 (2020).
4. Bhuiyan et al., J. Vac. Sci. Technol. A 39, 063207 (2021).
5. Roy et al., arXiv:2103.15280 (2021).

Two-dimensional electron gases (2DEGs) with high room-temperature (RT) mobility in wide-bandgap stannates perovskite is fascinating for oxide electronics, particularly to high-electron-mobility-transistors (HEMT). The experimental realization of 2DEGs formation via modulation doping in oxide heterostructure, however, has far been limited to date. Here, we report the structure and transport properties of 2DEGs in a modulation-doped BaSnO₃/SrSnO₃ heterostructure grown by molecular-beam epitaxy. Utilizing the band diagram based on the 1D-Poisson’s equation and the energy band offset between BaSnO₃ and SrSnO₃, we experimentally achieved the transfer of mobile electron carrier from the 1.6 nm thick La-doped SrSnO₃ to the undoped BaSnO₃. The non-destructive capacitance-voltage measurement by mercury probe at RT indicates the 2D carrier confinement at the SrSnO₃/BaSnO₃ interface. The RT 2DEG mobility is measured as high as 130 cm²V^–1s^–1 with a 2D carrier density of 1.1×10¹³ cm^-2, which is the highest RT 2DEGs mobility among the modulation-doped perovskite oxide interface. Our work suggests the tremendous potential of stannate 2DEGs for the next generation oxide-nanoelectronics.

β-Ga₂O₃ has recently become the focal point of ultra-wide bandgap semiconductor research for power electronics due to its high breakdown field (~ 8 MV/cm) and the availability of high-quality native substrates. It is also receptive to multiple dopants used to control its n-type conductivity, realizing wide doping ranges from 2x10¹⁴ to >3x10²⁰ cm^-3 using Ge and Si impurities for films grown in an MOCVD method. On the other hand, β-(Al_xGa_1-x)₂O₃ (AlGaO) ternary alloys have been widely studied to modulate the bandgap of Ga₂O₃, which is crucial to design and realize power electronic devices with great flexibility. However, achieving a high doping concentration in the β-(Al_xGa_1-x)₂O₃ layers remains challenging. In this presentation, we report on the MOCVD growth of heavily doped (>10²⁰ 1/cm³) strained β-(Al_xGa_1-x)₂O₃/β-Ga₂O₃ layers with an Al composition of up to 25%. Triethylgallium (TEGa), triethylaluminium (TEAl), and silane were used as a source for Ga, Al, and Si and oxygen for oxidation. The layer thickness and Al content in the β-(Al_xGa_1-x)₂O₃ films were estimated from the Omega-2Thetta XRD scans, whereas Hall measurements were utilized to determine electron mobility and free carrier concentration. β-(Al_xGa_1-x)₂O₃/β-Ga₂O₃ layers with free carrier concentrations ranging between 5x10¹⁶ and >1x10²⁰ cm^-3 were demonstrated for AlGaO films with different Al content. The effects of AlGaO layer thickness and Al content in the incorporation of silicon were studied. Free carrier concentration in the AlGaO layer was found to decrease with Al content and the increase of layer thickness due to relaxation. This presentation will discuss the growth of high-quality β-(Al_xGa_1-x)₂O₃/β-Ga₂O₃ superlattice structure with sharp interfaces and sub-nanometer surface roughness. We will also present the realization of a modulation-doped two-dimensional electron gas (2DEG) in β-(Al_0.2Ga_0.8)₂O₃/Ga₂O₃ heterostructure by silicon delta doping as well as demonstration of delta-doped Ga₂O₃/(Al_xGa_1-x)₂O₃/Ga₂O₃ double heterostructures--both building blocks of lateral power field-effect transistors.

Due to its expected high spontaneous electrical polarization and the possibility of polarization doping at heterointerfaces, the metastable orthorhombic κ-phase of Ga₂O₃ and its indium and aluminum alloy systems are an interesting material class. Here we report on the advantages and pitfalls of X-ray photoelectron spectroscopy for the characterization of κ-([Al,In]_xGa_1-x)₂O₃ thin films and quantum well systems and discuss a potential application in quantum-well infrared photodetectors.
The determined band alignments of the alloy systems to MgO reveal the formation of a type I heterojunction for all compositions, with conduction band offsets of at least 1.4 eV, providing excellent electron confinement [1]. We further found that the conduction band offsets in the alloy systems are mainly determined by the evolution of the bandgaps. Therefore, tunable conduction band offsets of up to 1.1 eV could allow for sub-level transition energies in corresponding quantum wells from the IR to the visible regime.
[1] T. Schultz et al., Band Offsets at κ-([Al,In]_xGa_1–x)₂O₃/MgO Interfaces, ACS Appl. Mater. Interfaces 12, 8879-8885 (2020)

Ga₂O₃ received tremendously increasing interest within the last decade. This is due to its large bandgap of about 5 eV and the correspondingly large expected electric breakdown field of about 8 MV/cm rendering it promising for power device applications [1]. Ga₂O₃ can be stabilized in at least four different polymorphs, the thermodynamically stable monoclinic β-phase as well as the metastable rhombohedral α-, orthorhombic κ-, and the defect spinel γ-modification. Among those, the α-phase exhibits the largest bandgap of about 5.3 eV as well as the largest expected electric breakdown field. Additionally, it features the same crystal structure as α-Al₂O₃ and can therefore be fabricated with high crystalline quality on all common epitaxial planes of cost-effective sapphire substrates. Especially the prismatic m-plane shows promising properties of α-Ga₂O₃ thin films such as increased stability as well as larger electron mobilities [2,3]. Sophisticated power devices such as HEMTs require high-quality heterostructures comprising α-Ga₂O₃ as well as α-(Al_xGa_1-x)₂O_3, whose bandgap increases with x. For the controlled growth of such ideally pseudomorphically strained heterostructures, knowledge of the elastic tensor and the relaxation processes of the material is required, which is lacking up to now in literature especially for the growth on the prismatic a- and m-planes of sapphire.
Here, we present a detailed X-ray diffraction study on α-(Al_xGa_1-x)₂O₃ thin films deposited by combinatorial pulsed laser deposition in the full composition range 0≤x≤1 on a- as well as m-plane sapphire substrates. We investigate both pseudomorphically strained and relaxed thin film layers with different thickness. We achieved pseudomorphic growth on m-plane sapphire for Al-contents as low as x~0.45 for layers of about 10 nm thickness. By reciprocal space map measurements, we will demonstrate a fundamental difference for the growth on a- and m-epitaxial planes. Pseudomorphic α-(Al_xGa_1-x)₂O₃ layers on m-plane sapphire show a distinct shear strain e’₅ along the in-plane c-axis direction, which is in contrast to samples on a-plane sapphire. Similarly, relaxed m-plane α-(Al_xGa_1-x)₂O₃ exhibits a global lattice tilt in c-axis direction that is missing on the a-plane. We modeled the lattice constants as well as the shear strain of the pseudomorphic layers as function of x for both epitaxial planes based on our elastic theory for rhombohedral heterostructures [4]. We will show that the reason for this behavior is the non-vanishing C₁₄ component of the stress-strain tensor both for α-Ga₂O₃ and α-Al₂O₃ that contributes strongly only for the m-epitaxial plane. Additionally, we are able to confirm the up to this date only theoretically calculated value of C₁₄ for α-Ga₂O₃ of about 17.3 GPa [5]. By geometric considerations of the possible slip planes and Burger’s vectors for a- and m-epitaxial planes as well as the modeling of the angle of the global lattice tilt as function of x, we will explain the occurrence of the lattice tilt only for the m-plane as well as identify possible microscopic relaxation mechanisms. The results are summarized in [6].
[1] Zhang et al., APL Mater. 8, 020906 (2020)
[2] Jinno et al., Sci. Adv. 7, eabd5891 (2021)
[3] Akaiwa et al., Phys. Status Solidi A 217, 1900632 (2020)
[4] Grundmann, J. Appl. Phys. 124, 185302 (2018)
[5] Furthmüller, Phys. Rev. B 93, 115204 (2016)
[6] Kneiß et al., J. Mater. Res., online (2021), DOI: 10.1557/s43578-021-00375-3

Wide band-gap semiconductors like SiC and GaN are effective alternatives to Si for mid-voltage applications. However, for high voltage applications (above 1.2 kV), large conduction loss limits the maximally allowed power and none of the current semiconductors present viable solutions. Motivated by this and the lack of p-type ultra-wide band-gap semiconductors, we identify a metastable AlScO₃ perovskite that has both high p-type Baliga figure of merit and thermal conductivity, using the database from our previous search [1]. AlScO₃ was studied previously for its relevance in geology but its electronic structure and dopability remain uncharacterized. Using first-principles methods, we predict that AlScO₃ has an indirect band gap of ~8.0 eV, making it particularly promising for high-voltage and high-temperature applications. From defect calculations, we find that cation and oxygen vacancies are the dominant native defects and that AlScO₃ is only p-type dopable. In addition, Mg, as a p-type dopant, is predicted to have achievable concentrations of ~10¹⁷ cm³ for samples synthesized at O-rich conditions and hole ionization energy of ~0.49 eV. Such ionization energy is comparable to the p-type rutile GeO₂ and among the lowest for typical ionization energies of small hole polarons (SHP) bound to defects across a range of wide bandgap oxides (0.5-1.0 eV). Ionization energies of free SHP can be generally estimated by the Landau-Pekar model [2] and relatively small hole effective mass (averaged over different directions, various bands close to the valence band maximum, and various hole pockets) for AlScO₃ (~1.74) accounts for its small ionization energy of free SHP (~0.06 eV), despite its large band gap. In addition, our preliminary results suggest a correlation between ionization energies of SHP bound to acceptors and the materials ionic dielectric constants. In the meeting, we will discuss how discovery of p-doping in a high-pressure metastable material like AlScO₃, along with rocksalt ZnO [3], could open a new search direction for more p-type UWBG materials with low ionization energies of bound SHP.

[1] P. Gorai et al. Energy Environ. Sci., 2019,12, 3338
[2] W. Sio et al. Phys Rev. B 99, 235139(2019)
[3] A. Goyal et al. Phys. Rev. Materials 2, 084603 (2018)

The ultra-wide bandgap (4.8 eV) of gallium oxide (Ga₂O₃) makes it suitable not only for high power electronics but also for high power photonics. Photonic cavities delimited by distributed Bragg reflectors (DBRs) were achieved in both undoped and chromium doped Ga₂O₃ micro- and nanowires by patterning periodic holes with a focused ion beam (FIB) microscope. Due to the strong enhancement of the end facet reflectivity, tunable resonant peaks with improved finesse and quality factors with respect to the unpatterned wires were measured by micro-photoluminescence (μ-PL) and thoroughly analyzed [1, 2]. Furthermore, the temperature-dependent shift of the resonant peaks due to refractive index change was precisely modelled, which has very recently resulted in the application of these optical microcavities as wide dynamic range thermometers [3].
Despite the large enhancement in the performance of these resonators by FIB patterning, there is still room for improvement. Firstly, we have found that, in our finest FIB-fabricated cavities, around 20% of the emitted light is scattered at the periodic DBR holes. Furthermore, part of the PL emission is lost due to not fulfilling the total internal reflection condition. Hence, alternative methods for the DBR fabrication on the Ga₂O₃ nanowires should be explored.
Atomic Layer Deposition (ALD) allows for the growth of thickness-controlled conformal thin films. The possibility of encapsulating Ga₂O₃:Cr nanowire cavities by ALD-grown TiO₂/Al₂O₃ superlattices as DBR structures is investigated. The optical performance of such cavities is modelled and compared to experimental analysis using reflectivity and μ-PL measurements.
[1] M. Alonso-Orts, E. Nogales, J. M. San Juan, M. L. Nó, J. Piqueras, and B. Méndez, Physical Review Applied 9, 064004 (2018).
[2] M. Alonso-Orts, G. Chilla, R. Hötzel E. Nogales, J. M. San Juan, M. L. Nó, M. Eickhoff, and B. Méndez, Optics Letters 46, 278-281 (2021).
[3] M. Alonso-Orts, D. Carrasco, J. M. San Juan, M. L. Nó, A. de Andrés, E. Nogales, B. Méndez, Small (in press, 2021).

β-Ga₂O₃ has an ultrawide bandgap energy (4.5 eV) and is attracting attention as a next-generation semiconductor material for power devices with high breakdown voltage and low loss. For improving the performance of β-Ga₂O₃ devices, it is necessary to characterize the crystal defects that cause the degradation of electrical properties. However, major defect characterization methods for β-Ga₂O₃ crystals, such as the etch-pit method and transmission electron microscopy (TEM), require destructive sample processing, and the nondestructive characterization method is limited to X-ray topography. Therefore, there is a need for non-destructive observation technology. In this study, three-dimensional (3D) imaging of multiphoton-excitation photoluminescence (MPPL) was performed for the nondestructive defect evaluation of β-Ga₂O₃ crystals. In β-Ga₂O₃, the carrier dynamics near the defects are unknown, so it is unclear which emission can be utilized to visualize them. Therefore, we investigated the emission wavelengths emitted from β-Ga₂O₃ by the MPPL spectrum, and then performed 3D MPPL imaging observation.
MPPL measurements were carried out at room temperature using a homoepitaxially-grown β-Ga₂O₃ sample with a thickness of 6.8 µm by the halide vapor phase epitaxy on a (001) Sn-doped substrate with a threading dislocation density of about 10⁴ cm⁻². A femtosecond laser with a wavelength of 1030 nm (1.2 eV) was used as the pump source. The laser light was focused inside the sample with an objective lens. MPPL occurs at the focal point via multiple photon absorption and subsequent carrier relaxation. 3D MPPL intensity images were taken with scanning the focal point and constructed by detecting the emission intensity near the peak wavelength of the MPPL spectrum with photomultiplier tubes through corresponding wavelength filters.
First, the MPPL measurement was performed under the reflection geometry, which can detect visible wavelength regions. The MPPL spectra from the substrate measured at a focal depth of 10 µm had emission peaks at around 3.0 eV (UVL) and 2.3 eV (YL). The UVL and YL intensities were proportional to approximately the fourth and third powers of the excitation intensity, respectively. Considering the bandgap energy of β-Ga₂O₃, the carrier excitation processes of the UVL and YL are above-gap excitation and below-gap excitation, respectively. The UVL emission subsequently occurred through impurity levels or the formation of self-trapped excitons (STE). Deeper levels of impurities mediate the YL. The UVL image has bright contrast along the [010] direction in both the homoepitaxial layer and the substrate, while YL is found only in the substrate and is non-uniformly distributed. These luminescence distributions may be attributed to the non-uniform incorporation of different impurities originating from the growth habit. In addition, nanopipes were observed as stick-like shadows extending along the [010] direction in the substrate. It would be due to the scattering of PL caused by the refractive index change in the nanopipes.
Next, we constructed a transmission configuration MPPL measurement system to detect MPPL with ultraviolet wavelength region. Using a quartz stage and parabolic mirror enabled the detection of higher energies than those on the reflective side. The MPPL spectrum from the substrate at a depth of 10 µm showed a narrow emission peaked at 343 nm (3.6 eV), which is the third harmonic generation (THG) of the excitation laser at the wavelength of 1030 nm, and two broad UVL emissions around 3.3–3.6 eV and 3.7 eV. Both UVL emissions are due to four-photon absorption and subsequent via donor-acceptor pair (DAP) transition or STE. Because the intensity of THG depends on the crystal structure and orientation, its 3D imaging can characterize the local crystal structure.
In conclusion, MPPL imaging can be used for β-Ga₂O₃ to visualize the 3D growth behavior, crystalline defects, and local crystal structure.

CdS emitter/buffer layer has been predominantly used to achieve high conversion efficiency in commercial Cu (In, Ga) (S, Se)₂ and CdTe thin film photovoltaic cells. Despite the demonstrated efficiencies of devices with CdS, the electron-hole pairs generated in the CdS emitter layer do not contribute towards the photocurrent due to significant reduction of the quantum efficiencies for wavelengths < 550 nm. With advent of MgZnO (MZO), CdS buffer layer is not essential. Wide band gap of MZO ensures appropriate band offset at the TCO/absorber interface. A conduction band offset of ~ +0.2 eV which is representative of a small spike across the TCO/CdTe interface is preferred to deter interface recombination. MZO has challenges associated with defective interface, chemical stability, and doping; however, its emergence suggests TCO alloys as attractive alternatives to CdS.
An exciting alternative to MZO are Ga₂O₃ based TCO layers, which have demonstrated tremendous improvements in interfacial defect passivation in crystalline silicon and dye sensitized solar cells, leading to record high V_oc. Ga₂O₃ shows a transparency to wavelengths ~ 250 nm. Additionally, Ga₂O₃ also demonstrate relative ease of fabrication in both bulk crystals as well as thin films structures along with wider control over electrical properties from semi-insulating to degenerate doping. All these features make Ga₂O₃ as a promising candidate for TCO layers in CdTe photovoltaics.
In line with the objective, we employed theoretical approach to evaluate Ga₂O₃ and Ga₂O₃ based alloys as a potential TCO layer in CdTe thin film photovoltaic cells. We utilized density functional theory (DFT) methods to calculate the band offsets across TCO/CdTe interfaces. A very large band gap for Ga₂O₃ (~4.6-5.0 eV) would lead to a significantly high conduction band offset across the TCO/CdTe interface. Alloying with In₂O₃ lowers the band gap of pure Ga₂O₃. We further investigated the impact of In₂O₃ alloying on the conduction band offset across TCO/CdTe interfaces. We observe, In₂O₃ alloying lowers the conduction band offset from +0.67 eV (for pure Ga₂O₃) to +0.18 eV (for InGaO₃ [50:50 alloy]). This conduction band offset for InGaO₃/CdTe matches closely to the preferred optimum value of +0.2 eV. Based on these results, we further performed numerical cell modeling with objective of evaluating the impact of band offsets on the cell performance. The numerical cell modeling reports the maximum theoretical conversion efficiency of 17.59% for a device employing an n-InGaO₃ buffer layer with InGaO₃ TCO construction. Our results suggest that alloys of In₂O₃ and Ga₂O₃ may be attractive alternatives to MZO for tailoring optimal conduction band offsets of the buffer and TCO layers in high efficiency CdTe thin film solar cells.
This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

Beta-Gallium Oxide (β-Ga₂O₃) is a promising material for power electronics due to its ultra-wide band gap of ~ 4.8 eV, a reasonable electron mobility, and the availability of bulk substrates. Nonetheless, β-Ga₂O₃ transistors reported so far still suffer from low current density compared to GaN devices. Thus, forming high aspect ratio channel structures to achieve lower on-resistance is still crucial for β-Ga₂O₃ transistors. Although through RIE, β-Ga₂O₃ vertical transistors with 9.27 aspect ratio has been fabricated,[1] the high energy ion induced damage from RIE degrades the device performance, leading to a limited effective channel mobility. In addition, the ion-induced damages result in a hysteresis in all β-Ga₂O₃ transistor utilizing RIE process (ref?). On the other hand, β-Ga₂O₃ fins with high aspect ratio and low interface trap density have been demonstrated by the metal-assisted chemical etching (MacEtch) process (ref). Moreover, an almost hysteresis-free CV loop was achieved on the MacEtch-formed β-Ga₂O₃ structures, making it suitable for β-Ga₂O₃ transistor fabrication. In this work, we demonstrate hysteresis-free β-Ga₂O₃ FinFETs with 1.5 mΩ-cm² specific on-resistance (R_on,sp) and over 10:1 aspect ratio through the MacEtch process.
A 2 μm-thick lightly doped β-Ga₂O₃ layer was grown on a (010) Fe-doped semi-insulating substrate by MOCVD, with Si doping concentration of ~ 4×10¹⁷ cm^-3. Then, channel and source/drain region were defined through lithography followed by 20 nm Pt MacEtch catalyst deposition with an ebeam evaporator. After a standard lift-off process, the samples were immersed in a mixture of hydrofluoric acid (HF, 49%) and potassium persulfate (K₂S₂O₈) under deep UV illumination to etch and form fin-shaped channels and source/drain mesa. Then, Al₂O₃ was deposited through an ALD process followed by 1 min RTA with N₂ ambient. A Ti/Au gold gate electrodes were sequentially deposited on the high-k layer to form the gate stack. Finally, 1 min RTA with N₂ ambient was applied for source/drain ohmic contact formation. Through the MacEtch process, β-Ga₂O₃ FinFETs with high aspect ratio channels and smooth sidewalls were produced. The top width and fin height are ~ 170nm and ~2.1 μm, respectively, leading to an overall 10:1 aspect ratio. In addition, slight lateral etching on the β-Ga₂O₃ fins also occurred during the MacEtch process, resulting in the trapezoid-like channel shapes.
DC transfer characteristics of the β-Ga₂O₃ FinFETs were measured and the drive current obtained is ~1.9×10^-5A (4.34A/m when normalized to 2×Fin height + top width) under V_ds=5V, V_ov=5V and L_g=1μm. The on/off ratio is around 10⁶ and gate leakage current is at only ~ 10 pA level, suggesting a gate stack with good electrostatic control and low leakage. A dependence of threshold voltages on the fin dimension is also observed, causing the FinFETs to shift from depletion mode (normally on) to enhancement mode (normally off) as the fin width decreases. This tunability of the threshold voltage provides the freedom to choose transistor operation mode depends on the application. The subthreshold swings (SS) are extracted to be 101 mV/dec., indicating the interface trap density is low on the MacEtch-formed sidewalls. Moreover, all the transfer characteristics demonstrate almost zero hysteresis. The largest hysteresis is only 55 mV clockwise, which is small compared to the typical 120 – 800 mV hysteresis of reported β-Ga₂O₃ FETs.[1-3] This hysteresis-free characteristic could be attributed to the absence of RIE-induced ion damages and traps by the virtual of the MacEtch. The R_on,sp extracted from the slope of low V_ds region is 1.5mΩ-cm² with V_ov = 6 V, which is low compared to other published β-Ga₂O₃ FETs (ref). Detailed output characteristics and breakdown measurement of β-Ga₂O₃ FinFET will be presented.
Acknowledgement: NSF ECCS 18-09946.
[1] W.Li, et al., 2019 IEDM, 2019. [2] K.Chabak et al., APL, 2016 [3] Z.Hu et al., EDL, 2018

We investigate atomic layer deposited aluminium doped zinc magnesium oxide films with varying Al content as wide bandgap transparent conducting films. While Aluminium doping of zinc oxide increases the electrical conductivity, it decreases the transmission due to absorption close to the band edge [1]. On the other hand, with addition of Mg, the optical properties and bandgap of ZnO can be tuned from 3.3ev upto ~4ev making them well-suited for optoelectronic devices and photovoltaic applications [2].
ALD has been used to investigate both aluminium doped zinc oxide and magnesium doped zinc oxide [2][3], however, previous studies on aluminium doped zinc magnesium oxide widely involved sputtering and the ALD studies on Al: Zn_1-xMg_xO were limited to few ratios [4]. Also, the ALD grown Al: Zn_1-xMg_xO adopted the optimal Al doping concentration of around 3% and did not investigate the interplay between Al and Mg doping.
Optimizing a quaternary process by mixing three binary ALD process involves immense challenges [5]. We achieve high degree of control on composition by optimizing the growth conditions with supercycle parameters such as pulse ratios and bilayer period. With the advantage of highly conformal thin film growth, we study ALD grown Al doped Zn_1-xMg_xO with x varying from 0.2 to 0.3 and different aluminium doping concentrations from underdoped to over doped cases. We elucidate the effect of doping on the band alignment, electrical and optical properties.
[1]. Zhang, Wu, et al. "Tailoring of optical and electrical properties of transparent and conductive Al-doped ZnO films by adjustment of Al concentration." Materials Science in Semiconductor Processing 74 (2018): 147-153.
[2]. Törndahl, T., et al. ‘Atomic Layer Deposition of Zn1−xMgxO Buffer Layers for Cu (In,Ga)Se2 Solar Cells’. Progress in Photovoltaics: Research and Applications, vol. 15, no. 3, 2007, pp. 225–35. Wiley Online Library, doi:10.1002/pip.733.
[3]. Peng, Qing, Anil U. Mane, and Jeffrey W. Elam. "Nanometer-Thick Mg x Zn (1–x) O Ternary Films for Photovoltaics." ACS Applied Nano Materials 3.8 (2020): 7732-7742.
[4]. Fleischer, Karsten, et al. Aluminium Doped Zn 1 − x Mg x O – a Transparent Conducting Oxide with Tunable Optical and Electrical Properties. 2012..
[5]. Mackus, Adriaan JM, et al. "Synthesis of doped, ternary, and quaternary materials by atomic layer deposition: a review." Chemistry of Materials 31.4 (2018): 1142-1183.

GaN high electron mobility transistors (HEMT) are becoming the mainstream for high frequency and power switching applications. Devices and circuits based on these emerging materials are more suited to operate at higher voltages and temperatures than Si-based devices owing to their superior physical properties. Recently, AlGaN/GaN based high electron mobility transistors (HEMTs) on low cost silicon substrate have been extensively demonstrated as attractive candidates for next generation power devices in the 100-650V range with low on-resistances. On the other hand, Ultra-Wide Band Gap (UWBG) materials such as AlN that has a bandgap of 6.2 eV are attracting attention for pushing the limits and address new requirements of high voltage power devices. In this presentation, we will discuss various device designs and preliminary results already showing the advantages and benefits of these new material systems for high voltage applications.

Ultra-wide-bandgap III-Nitride semiconductors such as GaN and AlGaN alloys have the potential to enable highly compact power circuits that operate with higher efficiency, greater power density, and function under extreme environments. In this contribution we use parameter-free density function theory (DFT) to improve our understanding of p-AlGaN//p-GaN interfaces found in diodes. We improve on DFT computed optical properties using a self-consistent DFT-ACBN0 (Hubbard-U) approach and find for example that the DFT bandgap for AlGaN, ~2.8 eV, increases with DFT-ACBN0 to ~4.5 eV, in excellent agreement with experimental results. Our results show that ordered and disordered bulk-AlGaN are energetically near-degenerate. Surprisingly, we find that stoichiometric surfaces and interfaces derived from AlGaN and GaN are metallic, in contrast to the corresponding insulating bulk systems. The only exception is p-doped disordered AlGaN, with a small indirect band-gap of ~0.2 eV. These results suggest that disordered p-doped AlGaN is a promising materials template for obtaining a gapped material and improving diode functionality. The introduction of carbon spacers in AlGaN//GaN interfaces for restoring a finite bandgap is likely unsuccessful since the carbon modified interfaces are predicted to remain metallic. Instead, we find evidence that C-C edges can induce amorphization in the carbon spacer. In summary, our preliminary results suggest that AlGaN//GaN interfaces with and without carbon spacers tend to decrease diode performance through bandgap closure, a tendency that may be avoided by increasing the abundance of disordered AlGaN in diode devices. SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525.

Ultrawide bandgap (UWBG) semiconductor β-Ga₂O₃ has attracted tremendous research interest in power electronics, RF electronics, and photonics due to its large bandgap of 4.9 eV and higher breakdown field of ~ 8 MV/cm. However, due to the lack of p-type doping, most of the demonstrated power devices are unipolar such as Schottky barrier diodes (SBDs). However, SBDs suffer from high leakage current, low breakdown voltages, and thermal instability due to low Schottky barrier. To overcome these issues, this work aims at studying β-Ga₂O₃ based junction barrier Schottky (JBS) rectifiers, which combines the advantages of SBDs and p-n diodes.
Other p-type materials such as GaN and NiO have been investigated to form heterojunctions with β-Ga₂O₃. Among them, GaN is a very promising candidate since both materials have wide bandgaps, and they can be epitaxially grown on each other using metalorganic chemical vapor deposition (MOCVD) or molecular beam epitaxy (MBE). Our preliminary work on mechanically exfoliated β-Ga₂O₃/GaN p-n junction showed decent electrical properties. In this work, β-Ga₂O₃/p-GaN JBS rectifiers are systematically investigated for efficient power electronics applications using SILVACO TCAD simulation. The JBS devices will be compared with conventional β-Ga₂O₃ SBD and β-Ga₂O₃ SBD with p-GaN guard rings (GR). All the devices have 500 nm heavily doped β-Ga₂O₃ contact layer and 5 µm unintentionally doped β-Ga₂O₃ drift layer. Platinum (Pt) with a work function of 5.65 eV is used as the top contact (anode/Schottky contact), and titanium/gold (Ti/Au) metal stacks are used as the bottom contact (cathode/ohmic contact). The forward characteristics such as on-resistance and turn-on voltage and the reverse breakdown voltages of the three devices will be compared to demonstrate the advantages of JBS rectifiers. The JBS structure will be optimized with different p-GaN widths, depths, shapes, and spacing between p-GaN regions. This work can provide critical guidance for the development of advanced β-Ga₂O₃ based high-voltage and high-power rectifiers.

Due to its ultra-wide bandgap, high critical electric field, and large Baliga's figure of merit (BFOM), beta-phase gallium oxide (β-Ga₂O₃) has attracted significant research attention for high-power, high-voltage, and high-frequency applications. Compared with lateral devices such as high electron mobility transistors (HEMTs), vertical devices have tremendous advantages in power electronics, including higher voltage and current handling capability, immunity to surface-related issues, simpler cooling requirement, smaller chip area, and easiness in scalability. Recently, β-Ga₂O₃ based current aperture vertical electron transistors (CAVETs) have been demonstrated. However, the device performance is still far from the β-Ga₂O₃ material limit, and several major challenges need to be addressed and, such as low breakdown voltage, high off-state leakage, small gate voltage swing, and difficulty in realizing enhancement mode (E-mode).
In this work, we perform the first comprehensive design and modeling of β-Ga₂O₃ CAVETs and explore their performance limit using TCAD SILVACO simulation. The simulated CAVETs structure consisted of an n⁺-Ga₂O₃ substrate, an 8 um Ga₂O₃ drift layer, a 0.8 um Ga₂O₃ aperture layer containing an aperture with the length of 20 um, two current blocking layers (CBLs) located at both sides of the aperture formed by nitrogen implantation, a 0.15 um Ga₂O₃ channel layer, two n⁺-Ga₂O₃ contact regions for the source and drain, and a 50 nm Al₂O₃ layer as gate dielectric. The length of gate, gate to source, and gate to drain is 40 um, 10 um, and 9 um, respectively. For the mobility simulation of Ga₂O₃, field-dependent saturation velocity mobility model and thermal dependent mobility model were introduced. For the breakdown simulation of the devices, lattice heating model and impact ionization model were incorporated. Furthermore, the incomplete ionization model was used for both donors and accepters in Ga₂O₃, which is critical for the accurate simulation of the CBL region. The device model was calibrated with experimental results such as transfer curves and breakdown characteristics. It was found that the threshold voltage (V_TH) of the devices was sensitive to and increased with the doping concentration in the channel layer. When the doping concentration in the channel layer was lower than 5×10¹⁷ cm^-3, the device changed from the depletion mode to the enhancement mode. Under zero bias, electric fields were concentrated between the gate and the CBL with a low breakdown voltage (BV) of 260 V, which is caused by insufficient gate control over the channel. Several gate structures will be investigated for the optimal BV-R_ON performance, including p-Gate, deep-recessed gate and p-type recessed gate. These results can provide critical references for the future development of high-performance vertical β-Ga₂O₃ power electronics.

Post-synthesis control of oxygen vacancy (V_O) defects in metal oxides would improve material properties such as electron conductivity and mobility, ferromagnetism, piezoelectricity, superconductivity, electrical breakdown resistance, charge-carrier recombination rates, light absorption, and chemical reactions. V_O forms readily during synthesis or heating at highly variable concentrations that depend sensitively upon processing history. Methods abound to foster V_O formation, but many applications would benefit from V_O lessening. High-temperature annealing in oxidizing gases to diminish V_O concentrations is commonly used but alters the concentrations of other defects.
Creation of oxygen interstitials (O_i) to annihilate V_O offers an attractive alternative because O_i often exhibits greater thermodynamic stability under O-rich conditions as well as lower diffusional hopping barriers. O_i formation via anti-Frenkel dissociation in the bulk suffers from high energetic barriers, so use of O_i for defect engineering has been rarely used. However, oxide surfaces create O_i efficiently from adsorbed O atoms after monolayer-level surface poisons have been removed.^1,2 Many oxides dissociate H₂O at least partially, offering reason to believe that suitably prepared surfaces inject O_i while submerged under water. Yet the adsorbed atoms must first be created before injection.
The present work uses isotopic O self-diffusion measurements demonstrate facile interstitial injection from surfaces of ZnO, TiO₂, and Ga₂O₃ contacted with liquid water at only a few tens of degrees Celsius (< 80 °C). Adsorbed poisons were removed before exposing single-crystal metal oxide specimens to liquid H₂¹⁸O. At the highest temperatures, ¹⁸O from the water reaches depths up to 20 nm for ZnO and 70 nm for TiO₂. For Ga₂O₃, the characteristic penetration depth is nearly 2 mm. First-principles calculations for all three oxides reveal O injection mechanisms that mimic bulk site hopping and exhibit similarly low activation barriers.
This injection phenomenon provides opportunities for near-surface vacancy regulation in thin films, nanoparticles and porous oxides. More broadly, post-synthesis removal of poisons and exposure to liquid water opens possibilities for defect control that are not available to existing techniques, potentially generalizing to many other oxides.

References
1. Jeong, H., Ertekin, E. & Seebauer, E. G. Kinetic Control of Oxygen Interstitial Interaction with TiO2(110) via the Surface Fermi Energy. Langmuir 36, 12632–12648 (2020).
2. Jeong, H., Li, M., Kuang, J., Ertekin, E. & Seebauer, E. G. Mechanism of Creation and Destruction of Oxygen Interstitial Atoms by Nonpolar Zinc Oxide(10-10) Surfaces. Phys. Chem. Chem. Phys. 23, 16423–16435 (2021).

Two dimensional materials possess many unique properties. From the electronic structural point of view, one can find conductive, semiconducting and insulating single nanosheets. Here we report a wide band gap insulator (band gap energy up to 7.1-8.2 eV) nanosheets obtained by delamination of synthetic fluorohectorite clay testifying these nanosheets to be one of the largest band gap insulators in the world. The band gap has been determined by electron energy loss spectroscopy in a high resolution transmission electron microscope. Fluorohectorite clay 1 nm thick single nanosheets can be synthetized with high-aspect ratio and lateral dimensions up to dozens of microns. This property renders the synthetic fluorohectorite nanosheets promising candidates for practical applications in electronic devices, serving as insulating nanosheets for deposition of graphene and various (semi)conductive 2D materials by self-assembly.

We reported the first operation of an inversion channel MOSFET using diamond semiconductor [1]. Here, we precisely controlled the MOS interface using an atomic layer deposition of Al₂O₃ film on OH-terminated diamond (111) surface [2] and pn-junctions using selective growth of heavily B doped layers on n-type diamond body for the formation of source/drain. The field-effect mobility μ_FE of the p-channel diamond MOSFET was 8 cm²/Vs at the highest, which is much lower than the hole mobility of 6,300 cm²/Vs at RT [3]. Recently, we have investigated the interface state density D_it dependence of μ_FE in inversion channel diamond MOSFETs [4,5] and developed the process technologies for improving the inversion channel diamond MOSFETs [6-9]. We will report the progress in the inversion channel diamond MOSFETs.
Acknowledgements
This work was partially supported by MEXT-Program for Creation of Innovative Core Technology for Power Electronics Grant Number JPJ009777, Adaptable and Seamless Technology transfer Program through Target-driven R&D (A-STEP) from Japan Science and Technology Agency (JST) Grant Number JPMJTR20RA, Kanazawa University SAKIGAKE Project 2020, a NEDO Feasibility Study Program (Uncharted Territory Challenge 2050) grant number 19101600-0, JST FOREST Program (Grant Number JPMJFR20353078, Japan), and JSPS KAKENHI grant numbers JP18KK0383, JP20K14773, and JP21H01363.
References
[1] T. Matsumoto, N. Tokuda et al., Sci. Rep. 6 (2016) 31585.
[2] R. Yoshida, N. Tokuda et al., Appl. Surf. Sci. 458 (2018) 222.
[3] I. Akimoto, N. Naka, N. Tokuda, Diamond. Diamond Relat. Mater. 63 (2016) 38.
[4] T. Matsumoto, N. Tokuda et al., Appl. Phys. Lett. 114 (2019) 242101.
[5] X. Zhang, N. Tokuda et al., J. Mater. Res. DOI:10.1557/s43578-021-00317-z.
[6] M. Nagai, N. Tokuda et al., Sci. Rep. 8 (2018) 6687.
[7] M. Nagai, N. Tokuda et al., Diam. Relat. Mater. 103 (2020) 107713.
[8] T. Tabakoya, N. Tokuda et al., Diam. Relat. Mater. 114 (2021) 108294.
[9] K. Sakauchi, N. Tokuda et al., Diam. Relat. Mater. 116 (2021) 108390.