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  1. 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 thesemore »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.« less
    Free, publicly-accessible full text available July 1, 2024
  2. 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.
    Free, publicly-accessible full text available May 1, 2024
  3. The energy and beam current dependence of Ga+focused ion beam milling damage on the sidewall of vertical rectifiers fabricated on n-type Ga2O3was investigated with 5–30 kV ions and beam currents from 1.3–20 nA. The sidewall damage was introduced by etching a mesa along one edge of existing Ga2O3rectifiers. We employed on-state resistance, forward and reverse leakage current, Schottky barrier height, and diode ideality factor from the vertical rectifiers as potential measures of the extent of the ion-induced sidewall damage. Rectifiers of different diameters were exposed to the ion beams and the “zero-area” parameters extracted by extrapolating to zero area and normalizing for milling depth. Forward currents degraded with exposure to any of our beam conductions, while reverse current was unaffected. On-state resistance was found to be most sensitive of the device parameters to Ga+beam energy and current. Beam current was the most important parameter in creating sidewall damage. Use of subsequent lower beam energies and currents after an initial 30 kV mill sequence was able to reduce residual damage effects but not to the point of initial lower beam current exposures.

  4. NiO/β-(Al x Ga 1− x ) 2 O 3 /Ga 2 O 3 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-(Al x Ga 1− x ) 2 O 3 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 cm 2 , 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 cm 2 . The breakdown voltage is among the highest reported for any lateral geometry Ga 2 O 3 -based rectifier.
    Free, publicly-accessible full text available May 1, 2024
  5. There is increasing interest in α-polytype Ga 2 O 3 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 HfSiO 4 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 HfSiO 4 /Ga 2 O 3 heterointerface was measured using x-ray photoelectron spectroscopy. The single-crystal α-Ga 2 O 3 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 HfSiO 4 on α-Ga 0.2 O 3 . The corresponding conduction band offset was −2.72 ± 0.45 eV, providing no barrier to electrons moving into Ga 2 O 3 .
    Free, publicly-accessible full text available March 1, 2024
  6. The temperature-dependent behavior of on/off ratio and reverse recovery time in vertical heterojunction p-NiO/β n-Ga 2 O/n + Ga 2 O 3 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 × 10 6 at 25 °C to 2.5 × 10 4 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 I rr 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-Ga 2 O 3 rectifiers. The energy loss during switching from 100 V was in the range 23–31  μJ over the temperature range investigated.
    Free, publicly-accessible full text available December 1, 2023
  7. NiO/Ga 2 O 3 heterojunction rectifiers were exposed to 1 Mrad fluences of Co-60 γ-rays either with or without reverse biases. While there is a small component of Compton electrons (600 keV), generated via the interaction of 1.17 and 1.33 MeV gamma photons with the semiconductor, which in turn can lead to displacement damage, most of the energy is lost to ionization. The effect of the exposure to radiation is a 1000× reduction in forward current and a 100× increase in reverse current in the rectifiers, which is independent of whether the devices were biased during this step. The on–off ratio is also reduced by almost five orders of magnitude. There is a slight reduction in carrier concentration in the Ga 2 O 3 drift region, with an effective carrier removal rate of <4 cm −1 . The changes in electrical characteristics are reversible by application of short forward current pulses during repeated measurement of the current–voltage characteristics at room temperature. There are no permanent total ionizing dose effects present in the rectifiers to 1 Mad fluences, which along with their resistance to displacement damage effects indicate that these devices may be well-suited to harsh terrestrial and space radiation applications if appropriate bias sequencesmore »are implemented to reverse the radiation-induced changes.« less
    Free, publicly-accessible full text available January 7, 2024
  8. The characteristics of sputtered NiO for use in pn heterojunctions with Ga 2 O 3 were investigated as a function of sputtering parameters and postdeposition annealing temperature. The oxygen/ nickel and Ni 2 O 3 /NiO ratios, as well as the bandgap and resistivity, increased as a function of O 2 /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 O 2 /Ar ratio increasing from 1/30 to 1/3. By sharp contrast, the bandgap and Ni 2 O 3 /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-Ga 2 O 3 . The breakdown voltage of NiO/β-Ga 2 O 3 heterojunction rectifiers was also a strong function of the O 2 /Ar flow ratio during deposition, with values of 1350 V for 1/3 and 830 V for 1/30.
    Free, publicly-accessible full text available January 1, 2024
  9. The band alignments of two candidate dielectrics for ScAlN, namely, SiO 2 and Al 2 O 2 , were obtained by x-ray photoelectron spectroscopy. We compared the effect of deposition method on the valence band offsets of both sputtered and atomic layer deposition films of SiO 2 and Al 2 O 3 on Sc 0.27 Al 0.73 N (bandgap 5.1 eV) films. The band alignments are type I (straddled gap) for SiO 2 and type II (staggered gap) for Al 2 O 3 . The deposition methods make a large difference in relative valence band offsets, in the range 0.4–0.5 eV for both SiO 2 and Al 2 O 3 . The absolute valence band offsets were 2.1 or 2.6 eV for SiO 2 and 1.5 or 1.9 eV for Al 2 O 3 on ScAlN. Conduction band offsets derived from these valence band offsets, and the measured bandgaps were then in the range 1.0–1.1 eV for SiO 2 and 0.30–0.70 eV for Al 2 O 3 . These latter differences can be partially ascribed to changes in bandgap for the case of SiO 2 deposited by the two different methods, but not for Al 2 O 3 , where the bandgap as independent of depositionmore »method. Since both dielectrics can be selectively removed from ScAlN, they are promising as gate dielectrics for transistor structures.« less
    Free, publicly-accessible full text available December 21, 2023
  10. BCl 3 is 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 BCl 3 -based dry etching of the NiO/Ga 2 O 3 heterojunction system. BCl 3 /Ar Inductively Coupled Plasmas produced maximum etch rates for NiO up to 300 Å.min −1 and 800 Å.min −1 for β -Ga 2 O 3 under moderate plasma power conditions suitable for low damage pattern transfer. The selectivity for NiO: Ga 2 O 3 was <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 Ga 2 O 3 had a stronger chemical component, without a well-defined ion energy threshold. The as-etched NiO and Ga 2 O 3 surfaces show chlorine residues, which can be removed on both materials by the standard 1NH 4 OH: 10H 2 O or 1HCl: 10H 2 O rinses used for native oxide removal. Accordingmore »to the location of the Cl 2p 3/2 peak, the Cl is ionically bonded.« less
    Free, publicly-accessible full text available November 1, 2023