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This paper reviews materials challenges and recent progress for selective area regrowth and doping for vertical gallium nitride (GaN) power devices. The purpose is to realize randomly placed, reliable, contactable, and generally useable laterally patterned p-n junctions, which are the building blocks for various advanced power rectifiers and transistors. The general regrowth process and regrowth dynamics in trenches were discussed, where the effects of trench geometries, growth methods, and bulk substrates were elucidated. Comprehensive materials characterization techniques were utilized to analyze the regrown structures, including scanning electron microscopy, transmission electron microscopy, atom probe tomography, scanning probe microscopy, and secondary-ion mass spectrometry. Cathodoluminescence and secondary electrons in scanning electron microscopy and atom probe tomography were used to achieve lateral and vertical dopant profiling at a sub-micron scale. The regrowth interface after dry etching accumulated a high density of impurities and charges, contributing to the formation of a p+-n+ tunneling junction. This hypothesis was further confirmed by the electrostatic potential profile at the regrowth interface using electron holography. Novel etching technologies were investigated to improve the regrowth interface. It was found that low-power dry etching significantly reduced the interfacial charges and the reverse leakage currents of regrown p-n junctions. Photoelectrochemical wet etching was found to be effective in reducing deep-level defects near the regrowth interface. Atomic layer etching uses self-limiting chemical processes, thus removing the damaged layers without inducing further etching damage. Tertiarybutylchloride-based in situ etching may serve as an alternative etching method to dry etching with reduced etching damage. In terms of devices, regrown p-n junctions with low leakage currents and vertical junction field-effect transistors were demonstrated. Further improvements in selective area regrowth and associated devices can be expected using regrowth optimization and regrowth interface engineering via surface treatments and low-damage etching. These results represent an important step towards realizing selective area regrowth and doping for high performance GaN power electronics devices and systems.

Ultrawide bandgap Al0.7Ga0.3N MESFETs with refractory Tungsten Schottky and Ohmic contacts are studied in 300–675 K environments. Variable-temperature dc electrical transport reveals large ON-state drain current densities for an AlGaN device: 209 mA/mm at 300 K and 156 mA/mm at 675 K in the ON-state (25% reduction). Drain and gate currents are only weakly temperature-dependent, suggesting potential for engineering temperature invariant operation. The ON-/ OFF-ratio is limited by OFF-state leakage through the gate, which is attributed to damage from sputter deposition. Future work using refractory metals with larger work functions that are deposited by electron beam deposition is proposed.

Advanced GaN power devices are promising for many applications in high power electronics but performance limitations due to material quality in etched-and-regrown junctions prevent their widespread use. Carrier diffusion length is a critical parameter that not only determines device performance but is also a diagnostic of material quality. Here we present the use of electron-beam induced current to measure carrier diffusion lengths in continuously grown and etched-and-regrown GaN pin diodes as models for interfaces in more complex devices. Variations in the quality of the etched-and-regrown junctions are observed and shown to be due to the degradation of the n-type material. We observe an etched-and-regrown junction with properties comparable to a continuously grown junction.

Aluminum gallium nitride (AlGaN) high electron mobility transistors (HEMTs) are candidates for next-generation power conversion and radio frequency (RF) applications. AlxGa1-xN channel HEMT devices (x = 0.3, x = 0.7) were investigated using multiple in-situ thermal characterization methods and electro-thermal simulation. The thermal conductivity, contact resistivity, and channel mobility were characterized as a function of temperature to understand and compare the heat generation profile and electro-thermal transport within these devices. In contrast to GaN-based HEMTs, the electrical output characteristics of Al0.70Ga0.30 N channel HEMTs exhibit remarkably lower sensitivity to the ambient temperature rise. Also, during 10kHz pulsed operation, the difference in peak temperature between the AlGaN channel HEMTs and GaN HEMTs reduced significantly.

The ability to form pristine interfaces after etching and regrowth of GaN is a prerequisite for epitaxial selective area doping, which in turn is needed for the formation of lateral PN junctions and advanced device architectures. In this work, we report the electrical properties of etched-and-regrown GaN PN diodes using an in situ Cl-based precursor, tertiary butylchloride (TBCl). We demonstrated a regrowth diode with I-V characteristics approaching that from a continuously grown reference diode. The sources of unintentional contamination from the silicon (Si) impurity and the mediating effect of Si during the TBCl etching are also investigated in this study. This work points to the potential of in situ TBCl etching toward the realization of GaN lateral PN junctions.

GaN p-n diodes were formed by selective area regrowth on freestanding GaN substrates using a dry etch, followed by post-etch surface treatment to reduce etch-induced defects, and subsequent regrowth into wells. Etched-and-regrown diodes with a 150 μm diameter achieved 840 V operation at 0.5 A/cm 2 reverse current leakage and a specific on-resistance of 1.2 mΩ·cm 2 . Etched-and-regrown diodes were compared with planar, regrown diodes without etching on the same wafer. Both types of diodes exhibited similar forward and reverse electrical characteristics, which indicate that etch-induced defectivity of the junction was sufficiently mitigated so as not to be the primary cause for leakage. An area dependence for forward and reverse leakage current density was observed, suggesting that the mesa sidewall provided a leakage path.

Vertical c-plane GaN p–n diodes, where the p-GaN layer is formed by epitaxial regrowth using metal–organic chemical-vapor deposition, are reported. Current–voltage (I–V) performance similar to continuously grown p–n diodes is demonstrated, including low reverse leakage current and reverse breakdown voltage in excess of −600 V, despite the lack of field management structures to increase the reverse breakdown voltage. Secondary-ion mass spectrometry analysis of regrown interfaces reveals that the primary source of Si at the regrown interface (“Si spike”) is ex situ contamination during wafer handling prior to loading for regrowth. Continuously grown p–n diodes with intentional Si doping at the p–n junction do not show degraded I–V performance for Si sheet concentrations

We report high-voltage regrown nonpolar ${m}$ -plane p-n diodes on freestanding GaN substrates. A high blocking voltage of 540 V at ~1 mA/cm $^{\textsf {2}}$ (corresponding to an electric field of E ~ 3.35 MV/cm), turn- ON voltages between 2.9 and 3.1 V, specific on-resistance of 1.7 $\text{m}\Omega \cdot \textsf {cm}^{\textsf {2}}$ at 300 A/cm $^{\textsf {2}}$ , and a minimum ideality factor of 1.7 were obtained for the regrown diodes. Our results suggest that Si, O, and C interfacial impurity levels up to $\textsf {2}\times \textsf {10}^{\textsf {17}}$ cm $^{-\textsf {3}}$ , $\textsf {8}\times \textsf {10}^{\textsf {17}}$ cm $^{-\textsf {3}}$ , and $\textsf {1}\times \textsf {10}^{\textsf {19}}$ cm $^{-\textsf {3}}$ , respectively, at the metallurgical junction of ${m}$ -plane, p-n diodes do not result in very early breakdown in the reverse bias although the off-state leakage current in the forward bias is affected. The impact of the growth interruption/regrowth on diode performance is also investigated.

Ultra-wide bandgap (~ 4.8 eV) beta phase gallium oxide (β-Ga2O3) grown by metal organic chemical vapor deposition (MOCVD) has demonstrated promising electronic transport properties with room temperature electron mobilities reaching 194 cm2/V-s and background doping as low as 9×1014 cm-3 [Zeng et al, Appl. Phys. Lett. 114, 250601 (2019)]. Commensurate with these values is a total trap concentration that is ~10x lower, with a different distribution of states throughout the bandgap than what has been observed for β-Ga2O3 grown by other methods [Zhang et al., Appl. Phys. Lett. 108, 052105 (2016), Farzana et al, Appl. Phys. Lett. 123, 161410 (2018)]. Given the promise of MOCVD-grown β-Ga2O3, a deeper understanding of the nature of defects in this material is of interest. This work provides a comprehensive picture of the current state of knowledge regarding deep levels in MOCVD-grown β-Ga2O3, including trapping properties, energy and concentration distributions in the bandgap, potential physical sources, and comparisons with other growth methods. By applying a suite of complementary defect spectroscopy methods-deep level optical spectroscopy, deep level transient spectroscopy, and admittance spectroscopy, quantitative characterization of defect states within the ~ 4.8 eV bandgap is possible. We find that, through systematically varying growth conditions, differing trends in concentrations for individual states are observed, implying that growth optimization is possible. Combined with observations made after high energy particle irradiation, we can differentiate between states of intrinsic and extrinsic origin.

This paper describes the development of vertical GaN PN diodes for high-voltage applications. A centerpiece of this work is the creation of a foundry effort that incorporates epitaxial growth, wafer metrology, device design, processing, and characterization, and reliability evaluation and failure analysis. A parallel effort aims to develop very high voltage (up to 20 kV) GaN PN diodes for use as devices to protect the electric grid against electromagnetic pulses.

Researchers have been extensively studying wide-bandgap (WBG) semiconductor materials such as gallium nitride (GaN) with an aim to accomplish an improvement in size, weight, and power (SWaP) of power electronics beyond current devices based on silicon (Si). However, the increased operating power densities and reduced areal footprints of WBG device technologies result in significant levels of self-heating that can ultimately restrict device operation through performance degradation, reliability issues, and failure. Typically, self-heating in WBG devices is studied using a single measurement technique while operating the device under steady-state direct current (DC) measurement conditions. However, for switching applications, this steady-state thermal characterization may lose significance since high power dissipation occurs during fast transient switching events. Therefore, it can be useful to probe the WBG devices under transient measurement conditions in order to better understand the thermal dynamics of these systems in practical applications. In this work, the transient thermal dynamics of an AlGaN/GaN high electron mobility transistor (HEMT) were studied using thermoreflectance thermal imaging and Raman thermometry. Also, the proper use of iterative pulsed measurement schemes such as thermoreflectance thermal imaging to determine the steady-state operating temperature of devices is discussed. These studies are followed with subsequent transient thermal characterization to accurately probe the self-heating from steady-state down to sub-microsecond pulse conditions using both thermoreflectance thermal imaging and Raman thermometry with temporal resolutions down to 15 ns.

We report on ultra-wide bandgap (UWBG) Al0.7Ga0.3N channel metal–oxide–semiconductor field-effect transistors (MOSFETs) grown by metal-organic chemical vapor deposition. Employing reverse Al composition graded ohmic contact layers and 20 nm Al2O3 gate-dielectric, 250 nm thick Al0.7Ga0.3N:Si channel MOSFETs resulted in the maximum current density of 0.5 A/mm, which is the highest value reported for AlGaN channels with Al composition >0.25. Transistors with a gate-drain spacing ( $L_{\mathrm{ GD}})$ of $1.7~\mu \text{m}$ demonstrated a breakdown voltage ( $V_{\mathrm{ DG}})$ of ~620 V, translating in an average lateral breakdown field of ~3.6 MV/cm. This work establishes UWBG AlGaN as a promising candidate for advanced RF applications.

Understanding the impact of high-energy electron radiation on device characteristics remains critical for the expanding use of semiconductor electronics in space-borne applications and other radiation harsh environments. Here, we report on in situ measurements of high-energy electron radiation effects on the hole diffusion length in low threading dislocation density homoepitaxial bulk n-GaN Schottky diodes using electron beam induced current (EBIC) in high-voltage scanning electron microscopy mode. Despite the large interaction volume in this system, quantitative EBIC imaging is possible due to the sustained collimation of the incident electron beam. This approach enables direct measurement of electron radiation effects without having to thin the specimen. Using a combination of experimental EBIC measurements and Monte Carlo simulations of electron trajectories, we determine a hole diffusion length of 264 ± 11 nm for n-GaN. Irradiation with 200 kV electron beam with an accumulated dose of 24 × 1016 electrons cm−2 led to an approximate 35% decrease in the minority carrier diffusion length.

Devices based on GaN have shown great promise for high power electronics, including their potential use as radiation tolerant components. An important step to realizing high power diodes is the design and implementation of an edge termination to mitigate field crowding, which can lead to premature breakdown. However, little is known about the effects of radiation on edge termination functionality. We experimentally examine the effects of proton irradiation on multiple field ring edge terminations in high power vertical GaN PIN diodes using in operando imaging with electron beam induced current (EBIC). We find that exposure to proton irradiation influences field spreading in the edge termination as well as carrier transport near the anode. By using depth-dependent EBIC measurements of hole diffusion length in homoepitaxial n-GaN we demonstrate that the carrier transport effect is due to a reduction in hole diffusion length following proton irradiation.

Simulations of reverse breakdown behavior of GaN power diodes with junction termination extensions (JTEs) are presented. The p-type JTE is located at the edge of the main p-n-junction, and under reverse bias, the charge in the JTE causes spreading and reduction of the peak electric fields to avoid premature avalanche breakdown. To determine the available charge in the JTE, it is shown that the electric field under reverse bias causes severe band bending within the JTE and full ionization of the Mg acceptor. Therefore, all the Mg dopants contribute charge and determine the performance of the JTE. The dependence of the breakdown voltage on the JTE’s acceptor concentration and thickness is shown. When the JTE is properly designed, the simulations show improved reverse breakdown behavior and breakdown efficiencies approaching 98% of the ideal limit for planar geometry. Finally, the challenges of creating JTEs within GaN power diodes are discussed.

Combined with recess etching, Al-rich III-N high electron mobility transistors (HEMTs) can be treated with a reactive ion etch plasma to implant F- ions into the HEMT's near surface region for a positive threshold voltage $(V_{TH})$ shift to achieve enhancement-mode (e-mode) operation. These HEMTs, along with depletion-mode (d-mode) controls that lack fluorine treatment, were evaluated for F- ion stability using step-stress and fixed-bias stress experiments. Step-stress experiments identified parametric shifts as a function of the drain-voltage $(V_{DS})$ stress prior to catastrophic failure that occurred at ${\it V_{DS}}$ ranging between 70–75 V. Fixed bias stressing at $V_{DS}=50\mathrm{V}$ was conducted at $190\ ^{\circ}\mathrm{C}$ Both e- and d- mode HEMTs exhibited a negative $V_{TH}$ shift of $0.6-1.0 \mathrm{V}$ during early time stressing at 190°C, with minor on-resistance effects, but both HEMT types were thereafter stable up to 4 hours. The early time changes are common to both e-mode and d-mode HEMTs and the F-induced ${\it V_{TH}}$ delta between e- and d-mode HEMTs remains intact within the bias-temperature stressing conditions of this work.

This chapter discusses the motivation for the use of Ultra-Wide-Bandgap Aluminum Gallium Nitride semiconductors for power switching and radio-frequency applications. A review of the relevant figures of merit for both vertical and lateral power switching devices, as well as lateral radio-frequency devices, is presented, demonstrating the potential superior performance of these devices relative to Gallium Nitride. Additionally, representative results from the literature for each device type are reviewed, highlighting recent progress as well as areas for further research.

Tunnel junction devices grown monolithically by metal organic chemical vapor deposition were optimized for minimization of the tunnel junction voltage drop. Two device structures were studied: an all-GaN homojunction tunnel junction and a graded InGaN heterojunction-based tunnel junction. This work reports a record-low voltage drop in the graded-InGaN heterojunction based tunnel junction device structure achieving a de-embedded tunnel junction voltage drop of 0.17 V at 100 A/cm2. The experimental data were compared with a theoretical model developed through technology computer-aided design (TCAD) simulations that offer a physics-based approach to understanding the key components of the design space, which lead to a more efficient tunnel junction.

We carefully investigate three important effects including postgrowth activation annealing, delta (δ) dose and magnesium (Mg) buildup delay as well as experimentally demonstrate their influence on the electrical properties of GaN homojunction p–n diodes with a tunnel junction (TJ). The diodes were monolithically grown by metalorganic chemical vapor deposition (MOCVD) in a single growth step. By optimizing the annealing parameters for Mg activation, δ-dose for both donors and acceptors at TJ interfaces, and p +-GaN layer thickness, a significant improvement in tunneling properties is achieved. For the TJs embedded within the continuously-grown, all-MOCVD GaN diode structures, ultra-low voltage penalties of 158 mV and 490 mV are obtained at current densities of 20 A cm−2 and 100 A cm−2, respectively. The diodes with the engineered TJs show a record-low differential resistivity of 1.6 × 10−4 Ω cm2 at 5 kA cm−2.

Ammonothermal growth of bulk gallium nitride (GaN) crystals is considered the most suitable method to meet the demand for high quality bulk substrates for power electronics. A non-destructive evaluation of defect content in state-of-the-art ammonothermal substrates has been carried out by synchrotron X-ray topography. Using a monochromatic beam in grazing incidence geometry, high resolution X-ray topographs reveal the various dislocation types present. Ray-tracing simulations that were modified to take both surface relaxation and absorption effects into account allowed improved correlation with observed dislocation contrast so that the Burgers vectors of the dislocations could be determined. The images show the very high quality of the ammonothermal GaN substrate wafers which contain low densities of threading dislocations (TDs) but are free of basal plane dislocations (BPDs). Threading mixed dislocations (TMDs) were found to be dominant among the TDs, and the overall TD density (TDD) of a 1-inch wafer was found to be as low as 5.16 × 103 cm−2.

High critical electric field (EC) and a large saturation velocity in ultra-wide bandgap transistors (UWBG) such as Al-rich AlGaN and ß-Ga2O3 is stimulating research into these devices [1,2]. This research is aimed at producing devices that may one day excel in power switching [3] and RF applications [4]. Figure-of-merit (FOM) analysis supports the possibility that Al-rich AlGaN transistors will outperform other WBG and UWBG transistors [5,6]. In spite of this promise, the UWBG devices do not yet perform well enough reach the promise evidenced by the FOM analysis. The favorable resolution of several important issues may be the key to broad acceptance of Al-rich AlGaN transistors. Until that time, Al-rich AlGaN transistors will begin to find utility in less mainstream uses. Recent developments are encouraging for the acceptance of Al-rich AlGaN in harsh environments. This talk will describe multiple research efforts to develop Al-rich AlGaN into the material of choice for next-generation power electronics applications and compare these research results to other WBG transistors where appropriate. These transistors generally benefit from the materials and fabrication similarity to AlGaN/GaN. They are grown epitaxially in the same metal organic chemical vapor deposition reactors and on mostly the same substrates. They will also benefit from similar fabrication infrastructure that is in place for AlGaN/GaN. Likewise, the AlGaN alloys support a wide diversity of device types. Depending on the targeted application, the AlGaN transistors may be depletion-mode (normally-on) or enhancement-mode (normally-off) transistors. Diversity in device types also includes various types of device designs that have been investigated, including metal-semiconductor field effect transistors, high electron mobility transistors (HEMTs), metal-insulator-semiconductor transistors, metal-oxide transistors, and polarization doped transistors. We will also describe the progress in maturing this transistor technology along a number of fronts. They are improving towards high current density at a rapid rate through efforts to improve the mobility, efforts to improve the Ohmic contacts, and by means of aggressively scaling the device dimensions. The particular strengths of the Al-rich AlGaN transistor include high electric field at breakdown, which is already more than 2-times that of GaN, the high ION/IOFF ratio in the transistor, now exceeding 109 in multiple devices, and in their high temperature operation with multiple reports in the 300-500°C range. A recent comparison of an Al0.85Ga0.15N/Al0.7Ga0.3N HEMT against AlGaN/GaN showed a superior ION/IOFF ratio at 500°C and also shows a favorable extrapolation to the 500-800°C temperature range for Al0.7Ga0.3N but not for GaN [7]. Such results are beginning to suggest that AlGaN may one day become the preferred transistor for operation at extreme temperatures. Details regarding ongoing work to optimize device performance for Al-rich AlGaN transistors will be presented in the talk. [1] A.G. Baca et al., J. Vac. Science and Technology, 40, 1600501 (2020). [2] M. Higashiwaki and G. H. Jessen, Appl. Phys. Lett. 112, 060401 (2018). [3] R. J. Kaplar et al., J. Solid-State Sci. Technol., 6, Q3061 (2017). [4] R. J. Kaplar et al., Proc. GOMAC, Albuquerque, NM, USA (2019), pp. 587-591. [5] Coltrin et al., J. Solid-State Sci. Technol., 6, S3114 (2017). [6] S. Bajaj et al., Appl. Phys. Lett. 105, 263503 (2014). [7] P. H. Carey et al., J. Electron Devices Society, 7, 444 (2019). This work was supported by the Laboratory Directed Research and Development program at Sandia National Laboratories. 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.

Steady-state photocapacitance (SSPC) was conducted on nonpolar m-plane GaN n-type Schottky diodes to evaluate the defects induced by inductively coupled plasma (ICP) dry etching in etched-and-regrown unipolar structures. An ∼10× increase in the near-midgap Ec – 1.9 eV level compared to an as-grown material was observed. Defect levels associated with regrowth without an etch were also investigated. The defects in the regrown structure (without an etch) are highly spatially localized to the regrowth interface. Subsequently, by depth profiling an etched-and-regrown sample, we show that the intensities of the defect-related SSPC features associated with dry etching depend strongly on the depth away from the regrowth interface, which is also reported previously [Nedy et al., Semicond. Sci. Technol. 30, 085019 (2015); Fang et al., Jpn. J. Appl. Phys. 42, 4207–4212 (2003); and Cao et al., IEEE Trans. Electron Devices 47, 1320–1324 (2000)]. A photoelectrochemical etching (PEC) method and a wet AZ400K treatment are also introduced to reduce the etch-induced deep levels. A significant reduction in the density of deep levels is observed in the sample that was treated with PEC etching after dry etching and prior to regrowth. An ∼2× reduction in the density of Ec – 1.9 eV level compared to a reference etched-and-regrown structure was observed upon the application of PEC etching treatment prior to the regrowth. The PEC etching method is promising for reducing defects in selective-area doping for vertical power switching structures with complex geometries [Meyers et al., J. Electron. Mater. 49, 3481–3489 (2020)].

Vertical GaN power diodes with a bilayer edge termination (ET) are demonstrated. The GaN p-n junction is formed on a low threading dislocation defect density ( $10^{4}-10^{5}$ cm $^{-2}$ ) GaN substrate, and has a 15- $\mu \text{m}$ -thick n-type drift layer with a free carrier concentration of $5\times 10^{15}$ cm $^{-3}$ . The ET structure is formed by $N$ implantation into the p+-GaN epilayer just outside the p-type contact to create compensating defects. The implant defect profile may be approximated by a bilayer structure consisting of a fully compensated layer near the surface, followed by a 90% compensated (p) layer near the n-type drift region. These devices exhibit avalanche breakdown as high as 2.6 kV at room temperature. Simulations show that the ET created by implantation is an effective way to laterally distribute the electric field over a large area. This increases the voltage at which impact ionization occurs and leads to the observed higher breakdown voltages.

Electrical performance and defect characterization of vertical GaN P-i-N diodes before and after irradiation with 2.5 MeV protons and neutrons is investigated. Devices exhibit increase in specific on-resistance following irradiation with protons and neutrons, indicating displacement damage introduces defects into the p-GaN and n- drift regions of the device that impact on-state device performance. The breakdown voltage of these devices, initially above 1700 V, is observed to decrease only slightly for particle fluence

A sidewall activation process was optimized for buried magnesium-doped p-GaN layers yielding a significant reduction in tunnel junction-enabled light emitting diode (LED) forward voltage. This buried activation enabled the realization of cascaded blue LEDs with fully transparent GaN homojunction tunnel junctions. The initial optimization of buried p-GaN activation was performed on PN junctions grown by metal organic chemical vapor deposition (MOCVD) buried under hybrid tunnel junctions grown by MOCVD and molecular beam epitaxy. Next the activation process was implemented in cascaded blue LEDs emitting at 450 nm, which were enabled by fully transparent GaN homojunction tunnel junctions. The tunnel junction-enabled multi-active region blue LEDs were grown monolithically by MOCVD. This work demonstrates a state-of-the-art tunnel junction-enabled cascaded LED utilizing homojunction tunnel junctions which do not contain any heterojunction interface.

Gallium nitride substrates grown by the hydride vapor phase epitaxy (HVPE) method using a patterned growth process have been characterized by synchrotron monochromatic beam X-ray topography in the grazing incidence geometry. Images reveal a starkly heterogeneous distribution of dislocations with areas as large as 0.3 mm2 containing threading dislocation densities below 103 cm−2 in between a grid of strain centers with higher threading dislocation densities (>104 cm−2). Basal plane dislocation densities in these areas are as low as 104 cm−2. By comparing the recorded images of dislocations with ray tracing simulations of expected dislocations in GaN, the Burgers vectors of the dislocations have been determined. The distribution of threading screw/mixed dislocations (TSDs/TMDs), threading edge dislocations (TEDs) and basal plane dislocations (BPDs) is discussed with implications for fabrication of power devices.

AlGaN polarization-doped field-effect transistors were characterized by DC and pulsed measurements from room temperature to 500 °C in ambient. DC current-voltage characteristics demonstrated only a 70% reduction in on-state current from 25 to 500 °C and full gate modulation, regardless of the operating temperature. Near ideal gate lag measurement was realized across the temperature range that is indicative of a high-quality substrate and sufficient surface passivation. The ability for operation at high temperature is enabled by the high Schottky barrier height from the Ni/Au gate contact, with values of 2.05 and 2.76 eV at 25 and 500 °C, respectively. The high barrier height due to the insulatorlike aluminum nitride layer leads to an ION/IOFF ratio of 1.5 × 109 and 6 × 103 at room temperature and 500 °C, respectively. Transmission electron microscopy was used to confirm the stability of the heterostructure even after an extended high-temperature operation with only minor interdiffusion of the Ni/Au Schottky contact. The use of refractory metals in all contacts will be key to ensure a stable extended high-temperature operation.

Research results for AlGaN-channel transistors are reviewed as they have progressed from low Al-content and long-channel devices to Al-rich and short-channel RF devices. Figure of merit (FOM) analysis shows encouraging comparisons relative to today’s state-of-the-art GaN devices for high Al-content and elevated temperatures. Critical electric field (EC), which fuels the AlGaN transistor FOM for high Al-composition, is not measured directly, but average gate-drain electric field at breakdown is substantially better in multiple reported AlGaN-channel devices compared to GaN. Challenges for AlGaN include the constraints arising from relatively low room temperature mobility dominated by ternary alloy scattering and the difficulty of making low-resistivity Ohmic contacts to high Al-content materials. Nevertheless, considerable progress has been made recently in the formation of low-resistivity Ohmic contacts to Al-rich AlGaN by using reverse compositional grading in the semiconductor, whereby a contact to a lower-Al alloy (or even to GaN) is made. Specific contact resistivity (ρc) approaching ρc ∼ 2 × 10−6 Ω cm2 to AlGaN devices with 70% Al-content in the channel has been reported. Along with scaling of the channel length and tailoring of the threshold voltage, this has enabled a dramatic increase in the current density, which has now reached 0.6 A/mm. Excellent ION/IOFF current ratios have been reported for Schottky-gated structures, in some cases exceeding 109. Encouraging RF performance in Al-rich transistors has been reported as well, with fT and fmax demonstrated in the tens of gigahertz range for devices with less than 150 nm gates. Al-rich transistors have also shown lesser current degradation over temperature than GaN in extreme high-temperature environments up to 500 °C, while maintaining ION/IOFF ratios of ∼106 at 500 °C. Finally, enhancement-mode devices along with initial reliability and radiation results have been reported for Al-rich AlGaN transistors. The Al-rich transistors promise to be a very broad and exciting field with much more progress expected in the coming years as this technology matures.

GaN is an attractive material for high-power electronics due to its wide bandgap and large breakdown field. Verticalgeometry devices are of interest due to their high blocking voltage and small form factor. One challenge for realizing complex vertical devices is the regrowth of low-leakage-current p-n junctions within selectively defined regions of the wafer. Presently, regrown p-n junctions exhibit higher leakage current than continuously grown p-n junctions, possibly due to impurity incorporation at the regrowth interfaces, which consist of c-plane and non-basal planes. Here, we study the interfacial impurity incorporation induced by various growth interruptions and regrowth conditions on m-plane p-n junctions on free-standing GaN substrates. The following interruption types were investigated: (1) sample in the main MOCVD chamber for 10 min, (2) sample in the MOCVD load lock for 10 min, (3) sample outside the MOCVD for 10 min, and (4) sample outside the MOCVD for one week. Regrowth after the interruptions was performed on two different samples under n-GaN and p-GaN growth conditions, respectively. Secondary ion mass spectrometry (SIMS) analysis indicated interfacial silicon spikes with concentrations ranging from 5e16 cm-3 to 2e18 cm-3 for the n-GaN growth conditions and 2e16 cm-3 to 5e18 cm-3 for the p-GaN growth conditions. Oxygen spikes with concentrations ~1e17 cm-3 were observed at the regrowth interfaces. Carbon impurity levels did not spike at the regrowth interfaces under either set of growth conditions. We have correlated the effects of these interfacial impurities with the reverse leakage current and breakdown voltage of regrown m-plane p-n junctions.

The impact of dry-etch-induced defects on the electrical performance of regrown, c-plane, GaN p-n diodes where the p-GaN layer is formed by epitaxial regrowth using metal-organic, chemical-vapor deposition was investigated. Diode leakage increased significantly for etched-and-regrown diodes compared to continuously grown diodes, suggesting a defect-mediated leakage mechanism. Deep level optical spectroscopy (DLOS) techniques were used to identify energy levels and densities of defect states to understand etch-induced damage in regrown devices. DLOS results showed the creation of an emergent, mid-gap defect state at 1.90 eV below the conduction band edge for etched-and-regrown diodes. Reduction in both the reverse leakage and the concentration of the 1.90 eV mid-gap state was achieved using a wet chemical treatment on the etched surface before regrowth, suggesting that the 1.90 eV deep level contributes to increased leakage and premature breakdown but can be mitigated with proper post-etch treatments to achieve >600 V reverse breakdown operation.

Improvements in radio frequency and power electronics can potentially be realized with ultrawide bandgap materials such as aluminum gallium nitride (AlxGa1−xN). Multidimensional thermal characterization of an Al0.30Ga0.70N channel high electron mobility transistor (HEMT) was done using Raman spectroscopy and thermoreflectance thermal imaging to experimentally determine the lateral and vertical steady-state operating temperature profiles. An electrothermal model of the Al0.30Ga0.70N channel HEMT was developed to validate the experimental results and investigate potential device-level thermal management. While the low thermal conductivity of this III-N ternary alloy system results in more device self-heating at room temperature, the temperature insensitive thermal and electrical output characteristics of AlxGa1−xN may open the door for extreme temperature applications.

High aluminum content AIGaN channel high electron mobility transistors (HEMTs) are an attractive candidate for the next generation of RF and high-power applications amongst ultra-wide bandgap materials $(\mathrm{E}_{\mathrm{G}} > 3.4\mathrm{eV})$ . This is due to numerous beneficial intrinsic material properties, such as high saturation velocity and large critical electric field. The built-in polarization electric field populates the 2-dimensional electron gas (2DEG) which cannot be depleted entirely at zero bias by the Schottky contact for a reasonable barrier thickness. As was observed in GaN channel HEMTs, these AIGaN channel devices intrinsically operate as depletion mode (d-mode). While d-mode devices are preferable for RF applications, high power applications typically require enhancement mode (e-mode) operation in order to guarantee system reliability [1]. Through the use of a $\mathrm{p}-\mathrm{Al}_{0.3}\mathrm{Ga}_{0.7}\mathrm{N}$ gate on an AIGaN channel HEMT, this work shows the capability to achieve enhancement-mode operation with a +0.3V threshold voltage, high saturated drain current $(> 60\mathrm{mA}/\mathrm{mm})$ , no gate hysteresis $(\Delta \mathrm{V} , and exceptionally low gate leakage current of $ even under high forward bias of $\mathrm{V}_{\mathrm{GS}}=8\mathrm{V}$ .

We measure the electron density dependence of carrier mobility in ultra-wide bandgap Al0.85Ga0.15N/Al0.7Ga0.3N heterostructures, using only Au/Pt Schottky contact deposition and without the need for Ohmic contacts. With this technique, we measure mobility over a two-dimensional electron gas density range from 1010 to 1013 cm-2 at an AlGaN/AlGaN heterojunction. At room temperature, subthreshold mobility was 4 cm2/Vs and peak mobility 155 cm2/Vs. Peak mobility decreased with temperature as T-0.86 suggesting alloy scattering as the dominant scattering mechanism.

Demonstration of Al0.3Ga0.7N PN diodes grown with breakdown voltages in excess of 1600 V is reported. The total epilayer thickness is 9.1 μm and was grown by metal-organic vapour-phase epitaxy on 1.3-mm-thick sapphire in order to achieve crack-free structures. A junction termination edge structure was employed to control the lateral electric fields. A current density of 3.5 kA/cm2 was achieved under DC forward bias and a reverse leakage current

Emerging ultrawide bandgap semiconductor materials are logical candidates for applications that exploit the large critical electric field (E C ) associated with these devices. For semiconductor devices, E C scales approximately as E g 2 5 [1]. With a 25% larger bandgap and an approximately 73% larger E C than GaN, Al 0.3 Ga 0.7 N-channel high election mobility transistors (HEMTs) might be viable candidates for harsh environment electronics or short wavelength photo-transistors. In either case, transistor quality factors such as minimal off-state leakage currents and subthreshold slope factor are important metrics.

In this work, we investigate the use of nanoscale polarization engineering to achieve efficient hole injection from metals to ultra-wide band gap AlGaN, and we show that UV-reflective aluminum (Al) layers can be used for hole injection into p-AlGaN. The dependence of tunneling on the work function of the metal was investigated, and it was found that highly reflective Al metal layers can enable efficient hole injection into p-AlGaN, despite the relatively low work function of Al. Efficient tunneling hole injection was confirmed by light emission at 326 nm with on-wafer peak external quantum efficiency and wall-plug efficiency of 2.65% and 1.55%, respectively. A high power density of 83.7 W/cm2 was measured at 1200 kA/cm2. The metal/semiconductor tunnel junction structure demonstrated here could provide significant advantages for efficient and manufacturable device topologies for high power UV emitters.

The Energy Frontier Research Center (EFRC) for Solid-State Lighting Science (SSLS) is one of 46 EFRCs initiated in 2009 to conduct basic and use-inspired research relevant to energy technologies. The overarching theme of the SSLS EFRC is the exploration of energy conversion in tailored photonic structures. In this article we review highlights from the research of the SSLS EFRC. Major research themes include: studies of the materials properties and emission characteristics of III-nitride semiconductor nanowires; development of new phosphors and II–VI quantum dots for use as wavelength downconverters; fundamental understanding of competing radiative and nonradiative processes in current-generation, planar light-emitting diode architectures; understanding of the electrical, optical, and structural properties of defects in InGaN materials and heterostructures; exploring ways to enhance spontaneous emission through modification of the environment in which the emission takes place; and investigating routes such...