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The valence and conduction band offsets at the interfaces between NiO/AlN, SiO2/AlN, Al2O3/AlN, and ITO/AlN heterointerfaces were determined via x-ray photoelectron spectroscopy using the standard Kraut technique. These represent systems that potentially would be used for p-n junctions, gate dielectrics, and improved Ohmic contacts to AlN, respectively. The band alignments at NiO/AlN interfaces are nested, type-I heterojunctions with a conduction band offset of −0.38 eV and a valence band offset of −1.89 eV. The SiO2/AlN interfaces are also nested gap, type-I alignment with a conduction band offset of 1.50 eV and a valence band offset of 0.63 eV. The Al2O3/AlN interfaces are type-II (staggered) heterojunctions with a conduction band offset of −0.47 eV and a valence band offset of 0.6 eV. Finally, the ITO/AlN interfaces are type-II (staggered) heterojunctions with conduction band offsets of −2.73 eV and valence band offsets of 0.06 eV. The use of a thin layer of ITO between a metal and the AlN is a potential approach for reducing contact resistance on power electronic devices, while SiO2 is an attractive candidate for surface passivation or gate dielectric formation on AlN. Given the band alignment of the Al2O3, it would only be useful as a passivation layer. Similarly, the use of NiO as a p-type layer to AlN does not have a favorable band alignment for efficient injection of holes into the AlN. [ABSTRACT FROM AUTHOR]

β-Ga2O3 is an ultrawide bandgap semiconductor that is attracting much attention for applications in next-generation high-power, deep UV, and extreme-environment devices. Hydrogen impurities have been found to have a strong effect on the electrical properties of β-Ga2O3. This Tutorial is a survey of what has been learned about O–H centers in β-Ga2O3 from their vibrational properties. More than a dozen, O–H centers have been discovered by infrared absorption spectroscopy. Theory predicts defect structures with H trapped at split configurations of a Ga(1) vacancy that are consistent with the isotope and polarization dependence of the O–H vibrational spectra that have been measured by experiment. Furthermore, O–H centers in β-Ga2O3 have been found to evolve upon thermal annealing, giving defect reactions that modify conductivity. While much progress has been made toward understanding the microscopic properties and reactions of O–H centers in β-Ga2O3, many questions are discussed that remain unanswered. A goal of this Tutorial is to inspire future research that might solve these puzzles. [ABSTRACT FROM AUTHOR]

Strain plays an important role in the performance and reliability of AlGaN/GaN high electron mobility transistors (HEMTs). However, the impact of strain on the performance of proton irradiated GaN HEMTs is yet unknown. In this study, we investigated the effects of strain relaxation on the properties of proton irradiated AlGaN/GaN HEMTs. Controlled strain relief is achieved locally using the substrate micro-trench technique. The strain relieved devices experienced a relatively smaller increase of strain after 5 MeV proton irradiation at a fluence of 5 × 1014 cm−2 compared to the non-strain relieved devices, i.e., the pristine devices. After proton irradiation, both pristine and strain relieved devices demonstrate a reduction of drain saturation current (Ids,sat), maximum transconductance (Gm), carrier density (ns), and mobility (μn). Depending on the bias conditions the pristine devices exhibit up to 32% reduction of Ids,sat, 38% reduction of Gm, 15% reduction of ns, and 48% reduction of μn values. In contrast, the strain relieved devices show only up to 13% reduction of Ids,sat, 11% reduction of Gm, 9% reduction of ns, and 30% reduction of μn values. In addition, the locally strain relieved devices show smaller positive shift of threshold voltage compared to the pristine devices after proton irradiation. The less detrimental impact of proton irradiation on the transport properties of strain relieved devices could be attributed to reduced point defect density producing lower trap center densities, and evolution of lower operation related stresses due to lower initial residual strain. [ABSTRACT FROM AUTHOR]

Junction barrier Schottky design Ga2O3 rectifiers allow for a combination of low turn-on voltage and high breakdown voltage. Ni/Au/Ga2O3 Schottky rectifiers and NiO/Ga2O3 heterojunction rectifiers were fabricated on the same wafer and the percentage of the relative areas and diameters of each were varied from pure Schottky devices to pure heterojunction devices. The on-voltage increased from 0.6 V for Schottky rectifiers to 2.4 V for heterojunction rectifiers, with a monotonic decrease in forward current at fixed bias of 5 V from 375 nA cm−2 to 175 nA cm−2. Conversely, the breakdown voltage increased monotonically as the proportion of heterojunction area increased, from 1.2 kV for Schottky rectifiers to 6.2 kV for pure heterojunction devices. Breakdown mostly was initiated at the edge of the anode contact but could also occur at the transition region from the Schottky contact to NiO edge termination. The Baliga figure of merit increased with both the relative percentage of area and diameter of the heterojunction contact from 0.2 GW cm−2 to 3 GW cm−2, while the energy loss during switching also increased from 2 to 3.9 W cm−2. These trends illustrate the trade-offs of Schottky versus pn junctions for the operation of Ga2O3 rectifiers. [ABSTRACT FROM AUTHOR]

p-NiO/n-Ga2O3 heterojunction (HJ) diodes exhibit much larger changes in their properties upon 1.1 MeV proton irradiation than Schottky diodes (SDs) prepared on the same material. In p-NiO/Ga2O3 HJ diodes, the narrow region adjacent to the HJ boundary is found to contain a high density of relatively deep centers with levels near EC-0.17 eV and a depleted region in the immediate vicinity of the HJ boundary. The series resistance of the HJ diodes is slightly higher than for the Schottky diodes and shows a temperature dependence with activation energy ~ 0.12 eV, like the temperature dependence of the NiO film resistivity. Irradiation with 1.1 MeV protons leads to a decrease of the hole concentration in the NiO, with a high carrier removal rate of ~ 1.3 × 105 cm−1 and a strong compensation of the interfacial region where the concentration of the EC-0.17 eV centers decreases with a high rate of ~ 7 × 103 cm−1. The combined action of these two effects gives rise to the much stronger increase of the series resistance of the HJ diodes compared to Schottky diodes. The observed differences between the radiation response of the HJs and SDs cannot be credibly attributed to the changes of the density of any of the deep electron and hole traps detected in our experiments. [ABSTRACT FROM AUTHOR]

Minority carrier diffusion length in undoped p-type gallium oxide was measured at various temperatures as a function of electron beam charge injection by electron beam-induced current technique in situ using a scanning electron microscope. The results demonstrate that charge injection into p-type β-gallium oxide leads to a significant linear increase in minority carrier diffusion length followed by its saturation. The effect was ascribed to trapping of non-equilibrium electrons (generated by a primary electron beam) on metastable native defect levels in the material, which in turn blocks recombination through these levels. While previous studies of the same material were focused on probing a non-equilibrium carrier recombination by purely optical means (cathodoluminescence), in this work, the impact of charge injection on minority carrier diffusion was investigated. The activation energy of ∼0.072 eV, obtained for the phenomenon of interest, is consistent with the involvement of Ga vacancy-related defects. [ABSTRACT FROM AUTHOR]

β-Ga2O3 has attracted much recent attention as a promising ultrawide bandgap semiconductor. Hydrogen can affect the conductivity of β-Ga2O3 through the introduction of shallow donors and the passivation of deep acceptors. The introduction of H or D into β-Ga2O3 by annealing in an H2 or D2 ambient at elevated temperature produces different classes of O–H or O–D centers. This work is a study of the interaction of D with V Ga 1 and V Ga 2 deep acceptors as well as other impurities and native defects in Ga2O3 by infrared spectroscopy and the complementary theory. (We focus primarily on the deuterium isotope of hydrogen because the vibrational modes of O–D centers can be detected with a higher signal-to-noise ratio than those of O–H.) O–D centers in β-Ga2O3 evolve upon annealing in an inert ambient and are transformed from one type of O–D center into another. These reactions affect the compensation of unintentional shallow donors by deep acceptors that are passivated by D. Defects involving additional impurities in β-Ga2O3 compete with V Ga deep acceptors for D and modify the deuterium-related reactions that occur. The defect reactions that occur when D is introduced by annealing in a D2 ambient appear to be simpler than those observed for other introduction methods and provide a foundation for understanding the D-related reactions that can occur in more complicated situations. [ABSTRACT FROM AUTHOR]

Three different metal stacks, namely Ti/Au, Ni/Au and Sc/Au, were examined as Ohmic metal contacts to Si-doped, n-type (4.1 × 1019 cm−3), 300-nm thick (Al0.18Ga0.82)2O3 layers grown by metal-organic chemical vapor deposition. This is a typical composition used for (AlxGa1-x)2O3 /Ga2O3 heterostructure field effect transistors. The effects of postdepositional annealing (300–475 °C) were examined through circular transfer length method (CTLM) measurements to determine both the transfer resistance and specific contact resistivity. The lowest resistances were achieved with Ti/Au, with specific contact resistivity 1.2 × 10–4 Ω·cm2 and transfer resistance 3.82 Ω·mm for as-deposited contacts. Annealing was found to degrade both of these resistances in all cases from the as-deposited values, even though the AGO sheet resistance decreased slightly, from 1191 Ω/□ to 905 Ω/□ after annealing at 475 °C. The temperature dependence of specific contact resistivity is also investigated. [ABSTRACT FROM AUTHOR]

Vertical geometry NiO/Ga2O3 heterojunction (HJ) rectifiers and Ni/Au Schottky rectifiers fabricated on the same wafer and each with the same diameter (100 μm) were operated at 77–473 K to compare their capabilities in space-like environments. The HJ rectifiers suffer a 4-order reduction in forward current at 77 K due to the freeze-out of acceptors in the NiO, leading to MIS-type operation. By sharp contrast, the Schottky rectifiers have higher forward current and lower on-resistance at 77 K compared to 298 K due to improved carrier mobility. The on/off ratio of Schottky rectifiers at 77 K becomes similar to HJ rectifiers at 298–473 K. The reverse recovery time of Schottky rectifiers is reduced from 20 ns at 273 K to 16 ns at 77 K, while HJ rectifiers cannot be switched at this temperature. While the latter are superior for high-temperature applications, the former are better suited to cryogenic operation. [ABSTRACT FROM AUTHOR]

Switching of vertical 6.1 kV/4A NiO/Ga2O3 rectifiers from voltages up to 1.45 kV showed reverse recovery times of 75 ns, current slew rate of 39.0 A/μs, and energy loss of ∼105 μW. These are the highest switching voltages reported for Ga2O3 rectifiers. To place the results in context, commercial 3.3 kV/5 A SiC merged PiN Schottky diodes showed reverse recovery times of 20 ns, current slew rate of 47.5 A/μs, and energy loss of ∼67 μW. The validity of comparing unpackaged experimental Ga2O3 diodes with commercial Si or SiC diodes without considering their differences in chip size and consequently in capacitive charge and ON-resistance is restricted. However, the results show the rapid progress in these devices when compared to commercial SiC rectifiers. [ABSTRACT FROM AUTHOR]

The dry etching of high crystal quality c-plane AlN grown by metal organic chemical vapor deposition was examined as a function of source and chuck power in inductively coupled plasmas of Cl2/Ar or Cl2/Ar/CHF3. Maximum etch rates of ∼1500 Å min−1 were obtained at high powers, with selectivity over SiO2 up to 3. The as-etched surfaces in Cl2/Ar/CHF3 have F-related residues, which can be removed in NH4OH solutions. The Al-polar basal plane was found to etch slowly in either KOH or H3PO4 liquid formulations with extensive formation of hexagonal etch pits related to dislocations. The activation energies for KOH- or H3PO4-based wet etching rates within these pits were 124 and 183 kJ/mol, respectively, which are indicative of reaction-limited etching. [ABSTRACT FROM AUTHOR]

The use of the low work function (4.5 eV) tungsten (W) as a rectifying contact was studied to obtain low on-voltages in W/Ga2O3 Schottky rectifiers and NiO/Ga2O3 heterojunction rectifiers that were simultaneously fabricated on a single wafer. The devices were produced with varying proportions of relative areas and diameters, encompassing a spectrum from pure Schottky Barrier Diode (SBD) to pure Heterojunction Diode (HJD) configurations. The turn-on voltages with W contacts ranged from 0.22 V for pure Schottky rectifiers to 1.50 V for pure HJDs, compared to 0.66 and 1.77 V, respectively, for Ni/Au contacts. The reverse recovery times ranged from 31.2 to 33.5 ns for pure Schottky and heterojunction rectifiers. Switching energy losses for the SBD with W contacts were ∼20% of those for HJDs. The reverse breakdown voltages ranged from 600 V for SBDs to 2400 V for HJDs. These are the lowest reported turn-on voltage values for 600/1200 V-class Ga2O3 rectifiers that extend the range of applications of these devices down to the voltages of interest for electric vehicle charging applications. [ABSTRACT FROM AUTHOR]

It has recently been demonstrated that electron beam injection into p-type β-gallium oxide leads to a significant linear increase in minority carrier diffusion length with injection duration, followed by its saturation. The effect was ascribed to trapping of non-equilibrium electrons (generated by a primary electron beam) at meta-stable native defect levels in the material, which in turn blocks recombination through these levels. In this work, in contrast to previous studies, the effect of electron injection in p-type Ga2O3 was investigated using cathodoluminescence technique in situ in scanning electron microscope, thus providing insight into minority carrier lifetime behavior under electron beam irradiation. The activation energy of ∼0.3 eV, obtained for the phenomenon of interest, is consistent with the involvement of Ga vacancy-related defects. [ABSTRACT FROM AUTHOR]

Hydrogen in β-Ga2O3 passivates shallow impurities and deep-level defects and can have a strong effect on conductivity. More than a dozen O–D vibrational lines have been reported for β-Ga2O3 treated with the heavy isotope of hydrogen, deuterium. To explain the large number of O–D centers that have been observed, the involvement of additional nearby defects and impurities has been proposed. A few O–H centers have been associated with specific impurities that were introduced intentionally during crystal growth. However, definitive assignments of O–H and O–D vibrational lines associated with important adventitious impurities, such as Si and Fe, have been difficult. A set of well-characterized Si-doped β-Ga2O3 epitaxial layers with different layer thicknesses has been deuterated and investigated by vibrational spectroscopy to provide new evidence for the assignment of a line at 2577 cm−1 to an OD–Si complex. The vibrational properties of several of the reported OD-impurity complexes are consistent with the existence of a family of defects with a V Ga 1 ic − D center at their core that is perturbed by a nearby impurity. [ABSTRACT FROM AUTHOR]

High-power electronics, such as GaN high electron mobility transistors (HEMTs), are expected to perform reliably in high-temperature conditions. This study aims to gain an understanding of the microscopic origin of both material and device vulnerabilities to high temperatures by real-time monitoring of the onset of structural degradation under varying temperature conditions. This is achieved by operating GaN HEMT devices in situ inside a transmission electron microscope (TEM). Electron-transparent specimens are prepared from a bulk device and heated up to 800 °C. High-resolution TEM (HRTEM), scanning TEM (STEM), energy-dispersive x-ray spectroscopy (EDS), and geometric phase analysis (GPA) are performed to evaluate crystal quality, material diffusion, and strain propagation in the sample before and after heating. Gate contact area reduction is visible from 470 °C accompanied by Ni/Au intermixing near the gate/AlGaN interface. Elevated temperatures induce significant out-of-plane lattice expansion at the SiNx/GaN/AlGaN interface, as revealed by geometry-phase GPA strain maps, while in-plane strains remain relatively consistent. Exposure to temperatures exceeding 500 °C leads to almost two orders of magnitude increase in leakage current in bulk devices in this study, which complements the results from our TEM experiment. The findings of this study offer real-time visual insights into identifying the initial location of degradation and highlight the impact of temperature on the bulk device's structure, electrical properties, and material degradation. [ABSTRACT FROM AUTHOR]

Films of α-Ga2O3 grown by Halide Vapor Phase Epitaxy (HVPE) were irradiated with protons at energies of 330, 400, and 460 keV with fluences 6 × 1015 cm−2 and with 7 MeV C4+ ions with a fluence of 1.3 × 1013 cm−2 and characterized by a suite of measurements, including Photoinduced Transient Current Spectroscopy (PICTS), Thermally Stimulated Current (TSC), Microcathodoluminescence (MCL), Capacitance–frequency (C–f), photocapacitance and Admittance Spectroscopy (AS), as well as by Positron Annihilation Spectroscopy (PAS). Proton irradiation creates a conducting layer near the peak of the ion distribution and vacancy defects distribution and introduces deep traps at Ec-0.25, 0.8, and 1.4 eV associated with Ga interstitials, gallium–oxygen divacancies VGa–VO, and oxygen vacancies VO. Similar defects were observed in C implanted samples. The PAS results can also be interpreted by assuming that the observed changes are due to the introduction of VGa and VGa–VO. [ABSTRACT FROM AUTHOR]

We report on growth and electrical properties of α-Ga2O3 films prepared by halide vapor phase epitaxy (HVPE) at 500 °C on α-Cr2O3 buffers predeposited on sapphire by magnetron sputtering. The α-Cr2O3 buffers showed a wide microcathodoluminescence (MCL) peak near 350 nm corresponding to the α-Cr2O3 bandgap and a sharp MCL line near 700 nm due to the Cr+ intracenter transition. Ohmic contacts to Cr2O3 were made with both Ti/Au or Ni, producing linear current–voltage (I–V) characteristics over a wide temperature range with an activation energy of conductivity of ∼75 meV. The sign of thermoelectric power indicated p-type conductivity of the buffers. Sn-doped, 2-μm-thick α-Ga2O3 films prepared on this buffer by HVPE showed donor ionization energies of 0.2–0.25 eV, while undoped films were resistive with the Fermi level pinned at EC of 0.3 eV. The I–V and capacitance–voltage (C–V) characteristics of Ni Schottky diodes on Sn-doped samples using a Cr2O3 buffer indicated the presence of two face-to-face junctions, one between n-Ga2O3 and p-Cr2O3, the other due to the Ni Schottky diode with n-Ga2O3. The spectral dependence of the photocurrent measured on the structure showed the presence of three major deep traps with optical ionization thresholds near 1.3, 2, and 2.8 eV. Photoinduced current transient spectroscopy spectra of the structures were dominated by deep traps with an ionization energy of 0.95 eV. These experiments suggest another pathway to obtain p–n heterojunctions in the α-Ga2O3 system. [ABSTRACT FROM AUTHOR]

Beta gallium oxide (β‐Ga2O3) has emerged as a highly promising semiconductor material with an ultrawide bandgap ranging from 4.5 to 4.9 eV for future applications in power electronics, optoelectronics, as well as gas and ultraviolet (UV) radiation sensors. Here, surface adsorption and air damping behavior of doubly clamped β‐Ga2O3 nanomechanical resonators are probed and systemically studied by measuring the resonance characteristics under different gas and pressure conditions. High responsivities of resonance to pressure are obtained by heating the devices up to 300 °C to induce an accelerated adsorption–desorption process. The initial surface conditions of the β‐Ga2O3 thin film play important roles in affecting the resonant behavior. UV ozone treatment proves effective in altering the initial surface conditions of β‐Ga2O3 nanosheets by eliminating physisorbed contaminants and filling oxygen vacancy defects residing on the surface, resulting in a consequential and discernible modification of the resonance behavior of β‐Ga2O3 nanomechanical resonators. The surface adsorption and desorption processes in β‐Ga2O3 demonstrate clear reversibility by exposing the UV treated β‐Ga2O3 to air. This study attains first‐hand information on how the surface conditions of β‐Ga2O3 affect its mechanical properties, and helps guide future development of transducers via β‐Ga2O3 nanoelectromechanical systems (NEMS) for pressure sensing applications, especially in harsh environments. [ABSTRACT FROM AUTHOR]

The addition of CHF3 to Cl2/Ar inductively coupled plasmas operating at low dc self-biases (<100 V, corresponding to incident ion energies <125 eV) leads to etch selectivity for Ga2O3 over (Al0.18Ga0.82)2O3 of >30, with a maximum value of 55. By sharp contrast, without CHF3, the etching is nonselective over a large range of source and rf chuck powers. We focused on low ion energy conditions that would be required for device fabrication. This result has a direct application to selective removal of Ga2O3 contact layers to expose underlying (Al0.18Ga0.82)2O3 donor layers in high-electron-mobility transistor structures. It is expected that formation of nonvolatile AlF3 species helps produce this selectivity. X-ray photoelectron spectroscopy does detect F residues on the etched surface for the Cl2/Ar/ CHF3 plasma chemistry. [ABSTRACT FROM AUTHOR]

Substitutional impurities in β-Ga2O3 are used to make the material n-type or semi-insulating. Several O–H and O–D vibrational lines for complexes that involve impurities that are shallow donors and deep acceptors have been reported recently. The present article compares and contrasts the vibrational properties of complexes that involve shallow donors (OD-Si and OD-Ge) with complexes that involve deep acceptors (OD-Fe and OD-Mg). Theoretical analysis suggests that these results arise from defect complexes based on a shifted configuration of the Ga(1) vacancy with a trapped H atom and a nearby impurity. [ABSTRACT FROM AUTHOR]