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Heterojunctions (HJs) of p-NiO/n-Ga2O3were prepared by deposition of thin films of p-NiO by ion beam sputtering on bulk nominally undoped (100) oriented n-Ga2O3samples cleaved from Czochralski-grown (010) oriented crystal. Electrical properties and deep traps spectra were studied and compared with those obtained for similar samples with Ni Schottky diodes. Characteristic features of the NiO/Ga2O3HJs are the built-in voltage of over 2 V compared to 1 V in Ni Schottky diodes, a strong frequency dispersion of capacitance due to the presence of high density over 1018cm−3of Ec-0.16 eV traps in the thin region adjacent to the NiO/Ga2O3interface. For heterojunction diodes the strong increase of reverse current occurs at a much higher voltage than for Schottky diodes (120 V versus 60 V).

Deep trap spectra and carrier diffusion lengths were measured for unintentionally doped β-Ga2O3bulk crystals with (100) orientation. The 20-mm diameter, 15-mm length boule was pulled by the Czochralski method from gallium oxide in (010) direction. It is found that the net density of shallow donors in (100) plates cleaved from the crystal was 2.6 × 1017 cm−3, with ionization energies of 0.05 eV measured from admittance spectra. Three deep electron traps with respective ionization energies of 0.6 eV (concentration 1.1 × 1014 cm−3), 0.8 eV (concentration 3.9 × 1016 cm−3) and 1.1 eV (concentration 8.9 × 1015 cm−3) were detected by Deep Level Transient Spectroscopy. The dominant 0.8 eV trap is associated with the E2 centers due to Fe acceptors, the two other traps are the well documented E1 and E3 centers. The major deep acceptors in the lower half of the bandgap have optical ionization threshold of 2.3 eV and concentration of 4 × 1015 cm−3and are believed to be due to the split Ga vacancies acceptors. The diffusion length of non-equilibrium charge carriers was 90 nm. The electrical properties of these (100) oriented crystals grown by Czochralski are quite similar to those synthesized by the undoped Edge-defined Film-Fed Growth technique.

The properties of orthorhombic κ-Ga2O3films grown by Epitaxial Lateral Overgrowth (ELOG) were studied by Scanning Transmission Electron Microscopy (STEM), X-ray diffraction, capacitance-voltage profiling, Microcathodoluminescence (MCL) spectroscopy and imaging. ELOG mask was formed by deposition of SiO2stripes on TiO2buffer prepared on basal plane sapphire, with the stripes going along the [112̅0] direction of sapphire. κ-Ga2O3ELOG growth was performed using Halide Vapor Phase Epitaxy (HVPE), with ELOG wing of the structure formed by lateral overgrowth over the 20 μm-wide SiO2stripes, while growth in between the stripes proceeded initially by vertical growth in the 5-μm-wide windows. TEM analysis showed that the material in the windows comprised 120orotational nanodomains typical of κ-Ga2O3, while, in the wing regions, the material was single-domain monocrystalline. The films were conducting, with the net donor density close to 1013cm−3. The data suggested the material in the windows have much higher resistance than in the wings. MCL spectra and imaging revealed much higher density of nonradiative recombination centers in the windows than in the wings.

There is currently great interest in ultra-wide bandgap semiconductors for their applicability in power switching electronics with improved efficiency compared to current technologies and also to solar-blind UV detection. One of the most promising materials is Ga2O3, available in large area bulk crystals and as exfoliated nano-layers (nanobelts, nanomembranes, and nanosheets). One aspect of this material that has not been widely recognized is the sensitivity of its surface to environment. The goal of this brief focus article is to provide some insight into the mechanisms and defects that underlie this effect and explain inconsistencies in the literature.

The characteristics of NiO/β-(Al0.21Ga0.79)2O3/Ga2O3 heterojunction lateral geometry rectifiers with the epitaxial layers grown by metal organic chemical vapor deposition were measured over a temperature range from 25 °C–225 °C. The forward current increased with temperature, while the on-state resistance decreased from 360 Ω.cm2at 25 °C to 30 Ω.cm2at 225 °C. The forward turn-on voltage was reduced from 4 V at 25 °C to 1.9 V at 225 °C. The reverse breakdown voltage at room temperature was ∼4.2 kV, with a temperature coefficient of −16.5 V K−1. This negative temperature coefficient precludes avalanche being the breakdown mechanism and indicates that defects still dominate the reverse conduction characteristics. The corresponding power figures-of-merit were 0.27–0.49 MW.cm−2. The maximum on/off ratios improved with temperature from 2105 at 25 °C to 3 × 107 at 225 °C when switching from 5 V forward to 0 V. The high temperature performance of the NiO/β-(Al0.21Ga0.79)2O3/Ga2O3 lateral rectifiers is promising if the current rate of optimization continues.

Neutrons generated through charge-exchange 9Be (p; ni) 9Be reactions, with energies ranging from 0–33 MeV and an average energy of ∼9.8 MeV were used to irradiate conventional Schottky Ga2O3rectifiers and NiO/Ga2O3p-n heterojunction rectifiers to fluences of 1.1–2.2 × 1014cm−2. The breakdown voltage was improved after irradiation for the Schottky rectifiers but was highly degraded for their NiO/Ga2O3counterparts. This may be a result of extended defect zones within the NiO. After irradiation, the switching characteristics were degraded and irradiated samples of both types could not survive switching above 0.7 A or 400 V, whereas reference samples were robust to 1 A and 1 kV. The carrier removal rate in both types of devices was ∼45 cm−1. The forward currents and on-state resistances were only slightly degraded by neutron irradiation.

Deep centers and their influence on photocurrent spectra and transients were studied for interdigitated photoresistors on α-Ga2O3undoped semi-insulating films grown by Halide Vapor Phase Epitaxy (HVPE) on sapphire. Characterization involving current-voltage measurements in the dark and with monochromatic illumination with photons with energies from 1.35 eV to 4.9 eV, Thermally Stimulated Current (TSC), Photoinduced Current Transients Spectroscopy (PICTS) showed the Fermi level in the dark was pinned at Ec−0.8 eV, with other prominent centers being deep acceptors with optical thresholds near 2.3 eV and 4.9 eV and deep traps with levels at Ec−0.5 eV, Ec−0.6 eV. Measurements of photocurrent transients produced by illumination with photon energies 2.3 eV and 4.9 eV and Electron Beam Induced Current (EBIC) imaging point to the high sensitivity and external quantum efficiency values being due to hole trapping enhancing the lifetime of electrons and inherently linked with the long photocurrent transients. The photocurrent transients are stretched exponents, indicating the strong contribution of the presence of centers with barriers for electron capture and/or of potential fluctuations.

The κ-Ga2O3polytype is attracting attention because of its high spontaneous electric polarization, which exceeds that of III-Nitrides. However, little is known of its transport and photoconductive properties. The electron beam induced current gain effect in Schottky barriers prepared on thick films of κ-Ga2O3has been studied. It is shown that the gain originates in the depletion region of the Schottky barrier. It is demonstrated that the induced current gain takes place only in some local regions, several which increases with applied bias. Such unusual behavior can be explained by an inhomogeneous distribution of hole traps or by a formation of conductive channels under applied bias.

In this study, the response to a heavy-ion strike and the resulting single effect burnout on beta-Ga2O3Schottky diodes with biased field rings is investigated via TCAD. The model used to simulate the device under high-reverse bias is validated using experimental current-voltage (I-V) curves. A field ring configuration for the device demonstrates an improved charge removal after simulated heavy-ion strikes. If the time scale for charge removal is faster than single event burnout, this can be an effective mechanism for reducing the effect of single ion strikes. This study explores various configurations of the termination structure and shows the impact of different design parameters in terms of a transient response after the ion strike.

Ga2O3layers with thickness from 10 to 86 μm were grown by halide vapor phase epitaxy (HVPE) on GaN(0001)/sapphire templates in a hot wall reactor at 570 °C, with the growth rate of about 3–4 μm h−1. The grown layers consisted of pure (001)-oriented κ-Ga2O3polymorph with no admixture of β-Ga2O3or α-Ga2O3phases. The narrowest (004) X-ray rocking curves were observed for 13–20 μm thick κ-Ga2O3layers. A further increase in thickness results in deterioration of the crystal quality which is indicated by the broadening of rocking curves. Electrical measurements of the thick layers revealed that they were n-type, with the concentration of shallow donors gradually decreasing from ∼1016 cm−3to ∼1015 cm−3. Deep level transient spectroscopy (DLTS) measurements revealed the presence of deep traps with levels near Ec−0.3 eV, Ec−0.6 eV, Ec−0.7 eV, Ec−0.8 eV, Ec−1 eV, with the Ec−0.8 eV being predominant.

Two-inch diameter α-Ga2O3films with thickness ∼4 μm were grown on basal plane sapphire by Halide Vapor Phase Epitaxy (HVPE) and doped with Sn in the top ∼1 μm from the surface. These films were characterized with High-Resolution X-ray Diffraction (HRXRD), Scanning Electron Microscope (SEM) imaging in the Secondary Electron (SE) and Micro-cathodoluminescence (MCL) modes, contactless sheet resistivity mapping, capacitance-voltage, current-voltage, admittance spectra, and Deep Level Transient Spectroscopy (DLTS) measurements. The edge and screw dislocations densities estimated from HRXRD data were respectively 7.4 × 109 cm−2and 1.5 × 107cm−2, while the films had a smooth surface with a low density (∼103 cm−2) of circular openings with diameters between 10 and 100 μm. The sheet resistivity of the films varied over the entire 2-inch diameter from 200 to 500 Ω square−1. The net donor concentration was ∼1018 cm−3near the surface and increased to ∼4 × 1018cm−3deeper inside the sample. The deep traps observed in admittance and DLTS spectra had levels at Ec−0.25 eV and Ec−0.35 eV, with concentration ∼1015 cm−3and Ec−1 eV with concentration ∼1016 cm−3.

BCl3is an attractive plasma etchant for oxides because it is a Lewis acid used to scavenge native oxides on many semiconductors due to the strong B–O bonding. We investigated BCl3-based dry etching of the NiO/Ga2O3heterojunction system. BCl3/Ar Inductively Coupled Plasmas produced maximum etch rates for NiO up to 300 Å.min−1and 800 Å.min−1for β-Ga2O3under moderate plasma power conditions suitable for low damage pattern transfer. The selectivity for NiO: Ga2O3was <1 under all conditions. The ion energy threshold for initiation of etching of NiO was between 35–60 eV, depending on the condition and the etch mechanism was ion-driven, as determined by the linear dependence of etch rate on the square root of ion energy incident on the surface. By sharp contrast, the etching of Ga2O3had a stronger chemical component, without a well-defined ion energy threshold. The as-etched NiO and Ga2O3surfaces show chlorine residues, which can be removed on both materials by the standard 1NH4OH: 10H2O or 1HCl: 10H2O rinses used for native oxide removal. According to the location of the Cl 2p3/2peak, the Cl is ionically bonded.

The switching performance of unpackaged vertical geometry NiO/β-Ga2O3rectifiers with a reverse breakdown voltage of 1.76 kV (0.1 cm diameter, 7.85 × 10−3cm2area) and an absolute forward current of 1.9 A fabricated on 20 μm thick epitaxial β-Ga2O3drift layers and a double layer of NiO to optimize breakdown and contact resistance was measured with an inductive load test circuit. The Baliga figure-of-merit of the devices was 261 MW.cm−2, with differential on-state resistance of 11.86 mΩ.cm2. The recovery characteristics for these rectifiers switching from forward current of 1 A to reverse off-state voltage of −550 V showed a measurement-parasitic-limited recovery time (trr) of 101 ns, with a peak current value of 1.4 A for switching from 640 V. The reverse recovery time was limited by extrinsic parasitic and thus does not represent the intrinsic device characteristics. There was no significant dependence of trron switching voltage or forward current.

Patterning of NiO/Ga2O3heterojunctions requires development of selective wet and dry etch processes. Solutions of 1:4 HNO3:H2O exhibited measurable etch rates for NiO above 40 °C and activation energy for wet etching of 172.9 kJ.mol−1(41.3 kCal.mol−1, 1.8 eV atom−1), which is firmly in the reaction-limited regime. The selectivity over β-Ga2O3was infinite for temperatures up to 55 °C. The strong negative enthalpy for producing the etch product Ga(OH)4suggests HNO3-based wet etching of NiO occurs via formation and dissolution of hydroxides. For dry etching, Cl2/Ar Inductively Coupled Plasmas produced etch rates for NiO up to 800 Å.min−1, with maximum selectivities of <1 over β-Ga2O3. The ion energy threshold for initiation of etching of NiO was ∼55 eV and the etch mechanism was ion-driven, as determined the linear dependence of etch rate on the square root of ion energy incident on the surface.

We present a review of the published experimental and simulation radiation damage results in Ga2O3. All of the polytypes of Ga2O3are expected to show similar radiation resistance as GaN and SiC, considering their average bond strengths. However, this is not enough to explain the orders of magnitude difference of the relative resistance to radiation damage of these materials compared to GaAs and dynamic annealing of defects is much more effective in Ga2O3. It is important to examine the effect of all types of radiation, given that Ga2O3devices will potentially be deployed both in space and terrestrial applications. Octahedral gallium monovacancies are the main defects produced under most radiation conditions because of the larger cross-section for interaction compared to oxygen vacancies. Proton irradiation introduces two main paramagnetic defects in Ga2O3, which are stable at room temperature. Charge carrier removal can be explained by Fermi-level pinning far from the conduction band minimum due to gallium interstitials (Gai), vacancies (VGa), and antisites (GaO). One of the most important parameters to establish is the carrier removal rate for each type of radiation, since this directly impacts the current in devices such as transistors or rectifiers. When compared to the displacement damage predicted by the Stopping and Range of Ions in Matter(SRIM) code, the carrier removal rates are generally much lower and take into account the electrical nature of the defects created. With few experimental or simulation studies on single event effects (SEE) in Ga2O3, it is apparent that while other wide bandgap semiconductors like SiC and GaN are robust against displacement damage and total ionizing dose, they display significant vulnerability to single event effects at high Linear Energy Transfer (LET) and at much lower biases than expected. We have analyzed the transient response of β-Ga2O3rectifiers to heavy-ion strikes via TCAD simulations. Using field metal rings improves the breakdown voltage and biasing those rings can help control the breakdown voltage. Such biased rings help in the removal of the charge deposited by the ion strike.

AlGaN/GaN high electron mobility transistors (HEMTs) with polar and nonpolar ZnO nanowires modified gate exhibit significant changes in channel conductance upon expose to different concentration of carbon monoxide (CO) at room temperature. The ZnO nanowires, grown by chemical vapor deposition (CVD), with perfect crystal quality will attach CO molecule and release electrons, which will lead to a change of surface charge in the gate region of the HEMTs, inducing a higher positive charge on the AlGaN surface, and increasing the piezoinduced charge density in the HEMTs channel. These electrons create an image positive charge on the gate region for the required neutrality, thus increasing the drain current of the HEMTs. The HEMTs source-drain current was highly dependent on the CO concentration. The limit of detection achieved was 400 ppm and 3200ppm in the open cavity with continuous gas flow using a 50x50m2gate sensing area for polar and nonpolar ZnO nanowire gated HEMTs sensor.

Properties of GaN radiation detectors are analyzed. It is shown that present day epitaxial material is suitable for detection of -particles with the charge collection efficiency close to 100%. Such detectors can operate at temperatures of at least 60°C and withstand irradiation with reactor neutrons fluences higher than 1015 cm-2. They keep the collection efficiency at 30% even after irradiation with 2×1016 cm-2neutron fluence. Registration of thermal neutrons with GaN detectors can also be achieved by using 10B converter and the efficiency of registration is determined by the 10B conversion efficiency from neutrons to low energy a-particles.

X Ray Photoelectron Spectroscopy was used to measure valence band offsets for Al2O3deposited by Atomic Layer Deposition on α-(AlxGa1-x)2O3alloys over a wide range of Al contents, x, from 0.26–0.74, corresponding to a bandgap range from 5.8–7 eV. These alloys were grown by Pulsed Laser Deposition. The band alignments were type I (nested) at x <0.5, with valence band offsets 0.13 eV for x = 0.26 and x = 0.46. At higher Al contents, the band alignment was a staggered alignment, with valence band offsets of − 0.07 eV for x = 0.58 and −0.17 for x = 0.74, ie. negative valence band offsets in both cases. The conduction band offsets are also small at these high Al contents, being only 0.07 eV at x = 0.74. The wide bandgap of the α-(AlxGa1-x)2O3alloys makes it difficult to find dielectrics with nested band alignments over the entire composition range.

The optical and transport properties of Asdoped ZnO films are examined. The films are grown by pulsedlaser deposition using As2O3as the source of As and O3O2as the oxidant. The films are epitaxial, single phase, and oriented in the ZnO [002] direction. Low temperature photoluminescence spectra at 20 K show for the formation of acceptor states associated with the As, although Hall measurements indicate that the films doped with 0.2 at As are ntype. From the photoluminescence spectra, the binding energy of the Asrelated acceptor was estimated to be 155 meV. © 2008 WILEYVCH Verlag GmbH & Co. KGaA, Weinheim

The properties of ZnO thin films grown with ozone are examined. Annealing studies were performed on ZnO films grown by pulsedlaser deposition using either O2and an O2O3gas mixture as the oxidant. The carrier density of ZnO films grown with pure O2generally decreases upon annealing in 1 atm O2. In contrast, the ntype carrier density for ZnO films grown with O2O3mixture gas increased with O2annealing. The results indicate that acceptor states, created via growth in ozone, are annihilated with postannealing. This suggests that the ozonerelated acceptor states are metastable. © 2008 WILEYVCH Verlag GmbH & Co. KGaA, Weinheim

High density nanoporous patterns were imprinted on bulk ZnO single-crystal substrates. The nanostamps were prepared via replication from nanoporous anodized alumina. Using the nanoimprinting technique, nanopatterns were successfully transferred onto the ZnO surface. Here we demonstrate that the light emitting efficiency of ZnO is highly enhanced through the simple nano-texturing. This simple method is applicable to any other light emitting diodes where the antireflective surface coating is desirable. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

PDA(Polydiacetylene)-supramolecules were successfully immobilized on the surface of Sc2O3/GaN/Sapphire structures for use as selective NH3 gas sensors. The Sc2O3 was epitaxially grown on GaN/Sapphire templates by molecular beam epitaxy (MBE) to replace the non-uniform native Ga2O3. The GaN-based PDA gas sensors showed excellent selectivity for ammonia detection after the end-functional group was modified to respond to this specific gas species. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In this paper we review our recent results from in-depth investigations of physical mechanisms which govern efficiency of several processes important for future spintronic devises, such as spin alignment within diluted magnetic semiconductors (DMS), spin injection from DMS to non-magnetic spin detectors (SDs) and also spin depolarization within SD. Spin-injection structures based on II–VIs (e.g. ZnMnSe/Zn(Cd)Se) and III–Vs (e.g. GaMnN/Ga(In)N) were studied as model cases. Exciton spin relaxation within ZnMnSe DMS, important for spin alignment, was found to critically depend on Zeeman splitting of the exciton states and is largely facilitated by involvement of longitudinal optical (LO) phonons. Optical spin injection in ZnMnSe/Zn(Cd)Se was shown to be governed by (i) commonly believed tunneling of individual carriers or excitons and (ii) energy transfer via localized excitons and spatially separated localized electron–hole pairs (LEHP) located within DMS. Unexpectedly, the latter mechanism is in fact found to dominate spin injections. We shall also show that spin depolarization in the studied structures is essentially determined by ef- ficient spin relaxation within non-magnetic spin detectors, which is an important factor limiting efficiency of spin detection. Detailed physical mechanisms leading to efficient spin depolarization will be discussed. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report on the measurement of stress in bulk, free-standing GaN templates under external bending using micro-Raman scattering. A 488 nm Ar-ion laser beam was scanned in a cross-sectional geometry across the GaN template held under external stress conditions. Our experiment showed that the top half of the bulk GaN was under ∼73 MPa of tensile stress while the bottom half of the GaN template was under 40 MPa of compressive stress when bent by a press screw. This data is very helpful to understand both the optical and mechanical properties of bulk GaN and for calibrating and optimizing GaN-based pressure sensors. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Bulk ZnO Schottky rectifiers with Pt rectifying contacts were exposed to 40 MeV protons at fluences from 5 × 109 to 5 × 1010 cm–2. These doses correspond to that received in more than 10 or 100 years, respectively, in low earth satellite orbit. The reverse breakdown voltage of the ZnO diodes increased from ∼3.2 V (taken at a current density of 0.1 A/cm2) in unirradiated devices to ∼3.9 V after the highest proton dose. The effective barrier height decreased with proton dose from 0.37 eV to 0.35 eV while the diode ideality factor increased from 1.8 to 1.9 for the 5 × 1010 dose. The very high tolerance of the ZnO diodes to high doses of energetic protons suggests that this material is well-suited to applications requiring high radiation tolerance. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The photo-response of phosphorus-doped ZnO thin films is reported. In particular, room temperature photoluminescence and ultraviolet photoconductivity are measured for epitaxial films grown by pulsed laser deposition. For the as-deposited films, the near band-edge emission in P-doped films is reduced in intensity and shifts to higher energy relative to the undoped materials. Annealing in 100 Torr of oxygen decreases the near band-edge photoluminescence intensity for both the undoped and P-doped material. However, an enhancement in the visible orange-red emission due to defect states in the gap is observed with oxygen annealing, which correlates with a decrease in conductivity. The visible luminescence is consistent with radiative recombination involving deep states. Ultraviolet photoconductivity is observed for the annealed films, and is most notable for the phosphorus-doped materials. These results are consistent with the phosphorus introducing a deep level in the gap that is heavily compensated by a defect-related shallow donor in the as-deposited films. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

This review focuses on understanding and optimization of several key aspects of GaN device processing. A novel rapid thermal processing up to 1500°C, in conjunction with AlN encapsulation, has been developed. The activation processes of implanted Si or Group VI donors, and common acceptors in GaN by using this ultrahigh-temperature annealing, along with its effects on surface degradation, dopant redistribution, and damage removal have been examined. 1400 degrees has proven to be the optimum temperature to achieve high activation efficiency and to repair the ion-induced lattice defects. Ion implantation was also employed to create high-resistivity GaN. Damage-related isolation with sheet resistances of 1012Ω/square in n-GaN and 1010Ω/square in p-GaN has been achieved by implant of O and transition metal elements. The effects of surface cleanliness on characteristics of GaN Schottky contacts have been investigated, and the reduction in barrier height was correlated with removing the native oxide that forms an insulating layer on the conventionally cleaned surface. W alloys have been deposited on Si-implanted samples and Mg-doped epilayers to achieve ohmic contacts with low resistance and better thermal stability than the existing non refractory contact schemes. Dry etching damage in GaN has been studied systematically using Schottky diode measurements. Wet chemical etching and thermal annealing processes have been developed to restore the ion-degraded material properties.Based on these technical improvements, attempts have been made to demonstrate GaN-based bipolar transistors. The devices operated in common base mode at current densities up to 3.6 kAcm−2and temperatures up to 300°C. The key issues that currently limit the device performance, such as high base resistance, poor impurity control, and defects resulting from the heteroepitaxial growth, have been addressed. A physically based simulation suggested that GaN bipolar devices may still suffer from small minority-carrier lifetime in the absence of aforementioned processing problems.

To achieve low leakage in GaN-based power devices and improve reliability in optoelectronic devices such as laser diodes, it is necessary to reduce dislocation density in GaN epitaxial layers and control their introduction during processing. We have previously shown that dislocations can be introduced at room temperature in GaN. The effect of electron-beam irradiation at fixed points on the shift of such freshly introduced dislocations in GaN has been studied. It is observed that dislocations can be displaced at distances up to 10–15 μm from the beam position. The analysis carried out allows to conclude that the main reason limiting the dislocation travelling distance in GaN is the existence of large density of pinning defects.

The thermal stability of n/n+β-Ga2O3epitaxial layer/substrate structures with sputtered ITO on both sides to act as rectifying contacts on the lightly doped layer and Ohmic on the heavily doped substrate is reported. The resistivity of the ITO deposited separately on Si decreased from 1.83 × 10−3Ω.cm as-deposited to 3.6 × 10−4Ω.cm after 300 °C anneal, with only minor reductions at higher temperatures (2.8 × 10−4Ω.cm after 600 °C anneals). The Schottky barrier height also decreased with annealing, from 0.98 eV in the as-deposited samples to 0.85 eV after 500 °C annealing. The reverse breakdown voltage exhibited a negative temperature coefficient of −0.46 V.C−1up to an annealing temperature of 400 °C and degraded faster at higher temperatures. Transmission Electron Microscopy showed significant reaction at the ITO and Ga2O3interface above 300 °C, with a very degraded contact stack after annealing at 500 °C.

Valence band offsets for SiO2deposited by Atomic Layer Deposition on α-(AlxGa1-x)2O3alloys with x = 0.26–0.74 were measured by X-ray Photoelectron Spectroscopy. The samples were grown with a continuous composition spread to enable investigations of the band alignment as a function of the alloy composition. From measurement of the core levels in the alloys, the bandgaps were determined to range from 5.8 eV (x = 0.26) to 7 eV (x = 0.74). These are consistent with previous measurements by transmission spectroscopy. The valence band offsets of SiO2with these alloys of different composition were, respectively, were −1.2 eV for x = 0.26, −0.2 eV for x = 0.42, 0.2 eV for x = 0.58 and 0.4 eV for x = 0.74. All of these band offsets are too low for most device applications. Given the bandgap of the SiO2was 8.7 eV, this led to conduction band offsets of 4.1 eV (x = 0.26) to 1.3 eV (x = 0.74). The band alignments were of the desired nested configuration for x > 0.5, but at lower Al contents the conduction band offsets were negative, with a staggered band alignment. This shows the challenge of finding appropriate dielectrics for this ultra-wide bandgap semiconductor system.

We investigated inductively coupled plasma (ICP) etching of both In-containing (InP, InAs, and InSb) and Ga-containing compound semiconductors (GaAs, GaP, and GaSb) in two new chemistries: BI3and BBr3with addition of Ar. Etch rates as high as 2 µm·min−1were obtained for InP in both types of discharge while for GaAs maximum rates were 1 and 2.5 µm·min−1, respectively, in BI3and BBr3. The rates were strongly dependent on plasma composition, ICP source power and radio frquency chuck power. BI3etching produced much smoother surfaces on both GaAs and InP, while maintaining the near-surface stoichiometry. Etch selectivities ≥ 10 were obtained for GaAs and InP over SiO2and SiNxmasks. The BI3chemistry appears attractive as an universal etchant for In-based and Ga-based compound semiconductors.

A review of the properties of hydrogen in crystalline semiconductors is presented. The equilibrium lattice positions of the various states of hydrogen are detailed, together with the reactions of atomic hydrogen with shallow and deep level impurities that passivate their electrical activity. Evidence for several charge states of mobile hydrogen provides a consistent picture for both the temperature dependence of its diffusivity and the chemical reactions with shallow level dopants. The electrical and optical characteristics of hydrogen-related defects in both elemental and compound semiconductors are discussed, along with the surface damage caused by hydrogen bombardment. The bonding configurations of hydrogen on semiconductor surfaces and the prevalence of its incorporation during many benign processing steps are reviewed. We conclude by identifying the most important areas for future effort.

Electron cyclotron resonance (ECR) plasma etching with additional rf-biasing produces etch rates = 2,500 A/min for InGaP and AlInP in CH4/H2/Ar. These rates are an order of magnitude or much higher than for reactive ion etching conditions (RIE) carried out in the same reactor. N2 addition to CH4/H2/Ar can enhance the InGaP etch rates at low flow rates, while at higher concentrations it provides an etch-stop reaction. The InGaP and AlInP etched under ECR conditions have somewhat rougher morphologies and different stoichiometries up to ˜200 Å from the sur face relative to the RIE samples.

Dry etching of common masking materials used in GaAs device technology, was examined down to temperatures of −30°C. The etch rates of SiNx, SiO2, and W in SF6/Ar are reduced below 0°C, but the anisotropy of the etching is improved at low temperature. Microwave enhancement of the SF6/Ar discharges produces increases in etch rates of several times at 25°C, but much lower increases at −30°C substrate temperature. The underlying GaAs surface shows increased S and F coverage after low-temperature etching, but these species are readily removerd either by anex-situwet chemical cleaning step or an in-situ H2plasma exposure. Photoresist etching is less sensitive to temperature, and anisotropic profiles are produced between −30 and +60°C in pure O2discharges.

Microwave discharges of HBr/H2/Ar and H/H2/Ar with additional do biasing of the sample were used to etch InP, GaAs, and AlGaAs at temperatures between 50–250°C. The etch rates increase by factors of 3–50 and 5–9, respectively, for HBr-and HI-based discharges over this temperature range, but display non-Arrhenius behavior. The etched surfaces became very rough above ~ 100°C for InP with either discharge chemistry due to preferential loss of P, while GaAs and AlGaAs are more tolerant of the elevated temperature etching. The near-surface electrical properties of InP are severely degraded by etch temperatures above ~ 100°C, while extensive hydrogen in-diffusion occurs in GaAs and AlGaAs under these conditions, leading to dopant passivation which can be reversed by annealing at ~400°C.