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Point defects were controllably introduced into 270 nm AlGaN Light-Emitting Diodes (LEDs) by 5 MeV electron and 1.1 MeV proton irradiations to examine the effect on electrical and luminescent characteristics. The 5 MeV electron irradiations had only a marginal effect on the charge distribution, current flow, and electroluminescence of LEDs for electron fluences up to 4.5 × 1016 e/cm2 that cause very strong degradation in green, blue, and near-UV LEDs. This lack of changes is attributed to the much higher charge densities in quantum-wells (QWs) and quantum barriers (QBs) of the 270 nm LEDs and to higher bond strength in high Al mole fraction AlGaN layers. By contrast, irradiation with 1.1 MeV protons with a fluence of 1016 p/cm2 leads to more than two orders of magnitude decrease in charge density in the QWs and QBs, a strong increase in the series resistance, and the emergence of deep electron traps near Ec-0.5 eV. The difference is explained by a much higher density of primary defects produced by protons. The observed effects are compared to changes in performance caused by aging after high driving current. [ABSTRACT FROM AUTHOR]

270 nm deep-ultraviolet AlGaN light-emitting diodes were irradiated with either neutrons or 15 MeV protons. Neutrons produced via charge-exchange reactions of 9Be with protons exhibited energy ranges from 0 to 33 MeV, with an average energy of approximately 9.8 MeV. The fluences ranged from 1.1 × 1014 to 2.2 × 1014 neutrons cm−2 and 1013 or 1014 protons cm−2. Two primary degradation modes were observed: increased trap-assisted tunneling, indicated by an initial reduction in turn-on voltage, and a decrease in carrier concentration, shown by reduced forward current due to deep state formation. For instance, 15 MeV proton irradiation resulted in more than an order of magnitude reduction in reverse current at a fluence of 1014 ions cm−2. The decrease in subthreshold leakage current at higher fluences of neutrons and protons is attributed to defect-induced carrier trapping, thereby reducing layer conductivity across the p-n junction. Emission intensity decreased with fluence for both protons and neutrons, without a measurable increase in midgap emission. The nonlinear degradation in current and light output with fluence suggests strong dynamic recombination of defects during irradiation of high aluminum alloys. [ABSTRACT FROM AUTHOR]

Co-60 gamma irradiation of SiC merged-PiN Schottky (MPS) diodes up to fluences of 1 Mrad (Si) produces increases in both forward and reverse current, with less damage when the devices are biased during irradiation. Subsequent injection of minority carriers by forward biasing at 300 K can partially produce some damage recovery, but at high forward biases also can lead to further degradation of the devices, even in the absence of radiation damage. Recombination-enhanced annealing by carrier injection overall is not an effective technique for recovering gamma-induced damage in SiC MPS diodes, especially when compared to other near athermal methods like electron wind force annealing. [ABSTRACT FROM AUTHOR]

The effect of doping in the drift layer and the thickness and extent of extension beyond the cathode contact of a NiO bilayer in vertical NiO/β-Ga2O3 rectifiers is reported. Decreasing the drift layer doping from 8 × 1015 to 6.7 × 1015 cm−3 produced an increase in reverse breakdown voltage (VB) from 7.7 to 8.9 kV, the highest reported to date for small diameter devices (100 μm). Increasing the bottom NiO layer from 10 to 20 nm did not affect the forward current–voltage characteristics but did reduce reverse leakage current for wider guard rings and reduced the reverse recovery switching time. The NiO extension beyond the cathode metal to form guard rings had only a slight effect (∼5%) in reverse breakdown voltage. The use of NiO to form a pn heterojunction made a huge improvement in VB compared to conventional Schottky rectifiers, where the breakdown voltage was ∼1 kV. The on-state resistance (RON) was increased from 7.1 m Ω cm2 in Schottky rectifiers fabricated on the same wafer to 7.9 m Ω cm2 in heterojunctions. The maximum power figure of merit (VB)2/RON was 10.2 GW cm−2 for the 100 μm NiO/Ga2O3 devices. We also fabricated large area (1 mm2) devices on the same wafer, achieving VB of 4 kV and 4.1 A forward current. The figure-of-merit was 9 GW cm−2 for these devices. These parameters are the highest reported for large area Ga2O3 rectifiers. Both the small area and large area devices have performance exceeding the unipolar power device performance of both SiC and GaN. [ABSTRACT FROM AUTHOR]

Defects created in lightly Sn-doped (2 × 1016 cm−3) (010)-oriented bulk β-Ga2O3 implanted with 1.2 MeV, 3 × 1015 cm−2 197Au+ ions before and after treatment in hydrogen plasmas at 330 °C were studied by X-ray measurements, Rutherford backscattering spectra, capacitance–voltage, current–voltage, admittance spectra and deep level transient spectroscopy. Au implantation creates defects that produce total depletion of carriers in the top 1.5 µm and introduces electron traps with energy levels at Ec-0.7 eV, Ec-1.05 eV, Ec-0.45 eV, and deep acceptors with optical ionization thresholds near 1.3 eV, 2.3 eV and 3.1 eV, similar to the centers dominating the spectra of deep traps in β-Ga2O3. Hydrogen plasma treatment greatly enhances the photocurrent and photo-capacitance and decreases the width of the insulating layer produced by Au implantation. The results can be explained by hydrogen passivation of the triply charged Ga vacancies and doubly charged split Ga vacancies acceptors in the implanted region, returning part of this region to n-type conductivity. [ABSTRACT FROM AUTHOR]

Changes induced by irradiation with 1.1 MeV protons in the transport properties and deep trap spectra of thick (>80 μm) undoped κ-Ga2O3 layers grown on sapphire are reported. Prior to irradiation, the films had a donor concentration of ∼1015 cm−3, with the two dominant donors having ionization energies of 0.25 and 0.15 eV, respectively. The main electron traps were located at Ec−0.7 eV. Deep acceptor spectra measured by capacitance-voltage profiling under illumination showed optical ionization thresholds near 2, 2.8, and 3.4 eV. The diffusion length of nonequilibrium charge carriers for ɛ-Ga2O3 was 70 ± 5 nm prior to irradiation. After irradiation with 1.1 MeV protons to a fluence of 1014 cm−2, there was total depletion of mobile charge carriers in the top 4.5 μm of the film, close to the estimated proton range. The carrier removal rate was 10–20 cm−1, a factor of 5–10 lower than in β-Ga2O3, while the concentration of deep acceptors in the lower half of the bandgap and the diffusion length showed no significant change. [ABSTRACT FROM AUTHOR]

NiO/β-(AlxGa1−x)2O3/Ga2O3 heterojunction lateral geometry rectifiers with diameter 50–100 μm exhibited maximum reverse breakdown voltages >7 kV, showing the advantage of increasing the bandgap using the β-(AlxGa1−x)2O3 alloy. This Si-doped alloy layer was grown by metal organic chemical vapor deposition with an Al composition of ∼21%. On-state resistances were in the range of 50–2180 Ω cm2, leading to power figures-of-merit up to 0.72 MW cm−2. The forward turn-on voltage was in the range of 2.3–2.5 V, with maximum on/off ratios >700 when switching from 5 V forward to reverse biases up to −100 V. Transmission line measurements showed the specific contact resistance was 0.12 Ω cm2. The breakdown voltage is among the highest reported for any lateral geometry Ga2O3-based rectifier. [ABSTRACT FROM AUTHOR]

Vertical geometry NiO/β n-Ga2O/n+ Ga2O3 heterojunction rectifiers with contact sizes from 50 to 200 μm diameter showed breakdown voltages (VB) up to 7.5 kV for drift region carrier concentration of 8 × 1015 cm−3. This exceeds the unipolar 1D limit for SiC and was achieved without substrate thinning or annealing of the epi layer structure. The power figure-of-merit, VB2/RON, was 6.2 GW cm−2, where RON is the on-state resistance (9.3–14.7 mΩ cm2). The average electric field strength was 7.56 MV/cm, approaching the maximum for β-Ga2O3. The on–off ratio switching from 5 to 0 V was 2 × 1013, while it was 3 × 1010–2 × 1011 switching to 100 V. The turn-on voltage was in the range 1.9–2.1 V for the different contact diameters, while the reverse current density was in the range 2 × 10−8–2 × 10−9 A cm−2 at −100 V. The reverse recovery time was 21 ns, while the forward current density was >100 A/cm2 at 5 V. [ABSTRACT FROM AUTHOR]

The electric field dependence of emission rate of the deep traps with level near Ec−0.6 eV, so-called E1 traps, was studied by means of deep level transient spectroscopy measurements over a wide range of applied voltages. The traps were initially introduced by 900 °C ampoule annealing in molecular hydrogen. The results indicate the activation energy of the centers and the ratio of high-field to low-field electron emission rates at a fixed temperature scale as the square root of electric field, suggesting that the centers behave as deep donors. The possible microscopic nature of the centers in view of recent theoretical calculations is discussed. The most likely candidates for the E1 centers are SiGa1–H or SnGa2–H complexes. [ABSTRACT FROM AUTHOR]

There is increasing interest in α-polytype Ga2O3 for power device applications, but there are few published reports on dielectrics for this material. Finding a dielectric with large band offsets for both valence and conduction bands is especially challenging given its large bandgap of 5.1 eV. One option is HfSiO4 deposited by atomic layer deposition (ALD), which provides conformal, low damage deposition and has a bandgap of 7 eV. The valence band offset of the HfSiO4/Ga2O3 heterointerface was measured using x-ray photoelectron spectroscopy. The single-crystal α-Ga2O3 was grown by halide vapor phase epitaxy on sapphire substrates. The valence band offset was 0.82 ± 0.20 eV (staggered gap, type-II alignment) for ALD HfSiO4 on α-Ga0.2O3. The corresponding conduction band offset was −2.72 ± 0.45 eV, providing no barrier to electrons moving into Ga2O3. [ABSTRACT FROM AUTHOR]

The characteristics of sputtered NiO for use in pn heterojunctions with Ga2O3 were investigated as a function of sputtering parameters and postdeposition annealing temperature. The oxygen/ nickel and Ni2O3/NiO ratios, as well as the bandgap and resistivity, increased as a function of O2/Ar gas flow ratio. For example, the bandgap increased from 3.7 to 3.9 eV and the resistivity increased from 0.1 to 2.9 Ω cm for the O2/Ar ratio increasing from 1/30 to 1/3. By sharp contrast, the bandgap and Ni2O3/NiO ratio decreased monotonically with postdeposition annealing temperatures up to 600 °C, but the density of films increased due to a higher fraction of NiO being present. Hydrogen is readily incorporated into NiO during exposure to plasmas, as delineated by secondary ion mass spectrometry measurements on deuterated films. The band alignments of NiO films were type II-staggered gaps with both α- and β-Ga2O3. The breakdown voltage of NiO/β-Ga2O3 heterojunction rectifiers was also a strong function of the O2/Ar flow ratio during deposition, with values of 1350 V for 1/3 and 830 V for 1/30. [ABSTRACT FROM AUTHOR]

There is increasing interest in the alpha polytype of Ga2O3 because of its even larger bandgap than the more studied beta polytype, but in common with the latter, there is no viable p-type doping technology. One option is to use p-type oxides to realize heterojunctions and NiO is one of the candidate oxides. The band alignment of sputtered NiO on α-Ga2O3 remains type II, staggered gap for annealing temperatures up to 600 °C, showing that this is a viable approach for hole injection in power electronic devices based on the alpha polytype of Ga2O3. The magnitude of both the conduction and valence band offsets increases with temperature up to 500 °C, but then is stable to 600 °C. For the as-deposited NiO/α-Ga2O3 heterojunction, ΔEV = −2.8 and ΔEC = 1.6 eV, while after 600 °C annealing the corresponding values are ΔEV = −4.4 and ΔEC = 3.02 eV. These values are 1−2 eV larger than for the NiO/β-Ga2O3 heterojunction. [ABSTRACT FROM AUTHOR]

The temperature-dependent behavior of on/off ratio and reverse recovery time in vertical heterojunction p-NiO/β n-Ga2O/n+ Ga2O3 rectifiers was investigated over the temperature range of 25–300 °C. The device characteristics in forward bias showed evidence of multiple current transport mechanisms and were found to be dependent on the applied bias voltages and temperatures. The on–off ratio decreased from 3 × 106 at 25 °C to 2.5 × 104 at 300 °C for switching to 100 V reverse bias. For 200 μm diameter rectifiers, the reverse recovery time of ∼21 ns was independent of temperature, with the Irr monotonically increasing from 15.1 mA at 25 °C to 25.6 mA at 250 °C and dropping at 300 °C. The dI/dt increased from 4.2 to 4.6 A/μs over this temperature range. The turn-on voltage decreased from 2.9 V at 25 °C to 1.7 V at 300 °C. The temperature coefficient of breakdown voltage was negative and does not support the presence of avalanche breakdown in NiO/β-Ga2O3 rectifiers. The energy loss during switching from 100 V was in the range 23–31 μJ over the temperature range investigated. [ABSTRACT FROM AUTHOR]

The (SmxGa1−x)2O3 alloy system is a potential new dielectric for compound semiconductors such as GaAs. Using molecular beam epitaxy under metal-modulated growth conditions, we grew the binary oxide, Sm2O3, at two substrate temperatures (100 and 500 °C) and optimized the structural, morphological, and electrical properties of the films. Decreasing the Sm cell temperature suppressed the formation of the monoclinic phase and promoted the growth of the cubic phase. Next, the ternary oxide, (SmxGa1−x)2O3, was deposited to investigate the effects of Ga incorporation. Optimization experiments were used to determine the effects of substrate temperature and samarium cell temperature (i.e., growth rate) on film stoichiometry, phase distribution, and microstructure in these films. Films grown at 500 °C showed significant surface roughness and the presence of multiple crystalline phases. Since all of the Sm-based oxides (i.e., samarium oxide with and without gallium) were found to have unbonded Sm metal, annealing experiments were carried out in oxygen and forming gas to determine the effects of annealing on film stoichiometry. The motivation behind annealing in forming gas was to see whether this commonly used technique for reducing interface densities could improve the film quality. GaAs metal-oxide-semiconductor diodes with (SmxGa1−x)2O3 showed breakdown fields at 1 mA/cm2 of 4.35 MV/cm, which decreased with increasing Sm unbonded metal content in the films. [ABSTRACT FROM AUTHOR]

Focused Ga+ ion milling of lightly Si-doped, n-type Ga2O3 was performed with 2–30 kV ions at normal incidence and beam currents that were a function of beam voltage, 65 nA for 30 kV, 26 nA for 10 kV, 13 nA for 5 kV, and 7.1 nA for 2 kV, to keep the milling depth constant at 100 nm. Approximate milling rates were 15, 6, 2.75, and 1.5 μm3/s for 30, 10, 5, and 2 kV, respectively. The electrical effects of the ion damage were characterized by Schottky barrier height and diode ideality factor on vertical rectifier structures comprising 10 μm epitaxial n-Ga2O3 on n+ Ga2O3 substrates, while the structural damage was imaged by transmission electron microscopy. The reverse bias leakage was largely unaffected even by milling at 30 kV beam energy, while the forward current-voltage characteristics showed significant deterioration at 5 kV, with an increase in the ideality factor from 1.25 to 2.25. The I–V characteristics no longer showed rectification for the 30 kV condition. Subsequent annealing up to 400 °C produced substantial recovery of the I–V characteristics for all beam energies and was sufficient to restore the initial ideality factor completely for beam energies up to 5 kV. Even the 30 kV-exposed rectifiers showed a recovery of the ideality factor to 1.8. The surface morphology of the ion-milled Ga2O3 was smooth even at 30 kV ion energy, with no evidence for preferential sputtering of the oxygen. The surface region was not amorphized by extended ion milling (35 min) at 5 kV with the samples held at 25 °C, as determined by electron diffraction patterns, and significant recovery of the lattice order was observed after annealing at 400 °C. [ABSTRACT FROM AUTHOR]