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The performance of ultra-wide bandgap semiconductors like ${\beta }$ -Ga2O3 is critically dependent on achieving high average electric fields within the active region of the device. In this article, we show that dielectrics like BaTiO3 with extremely high dielectric constant can provide an efficient field management strategy by improving the uniformity of electric field profile within the gate–drain region of lateral field-effect transistors. Using this strategy, we achieved high average breakdown field of 1.5 and 4 MV/cm at gate–drain spacing ( ${L}_{\text {gd}}$ ) of 6 and $0.5~{\mu }\text{m}$ , respectively in ${\beta }$ -Ga2O3, at a high channel sheet charge density of $1.6\,\,{\times }\,\,10^{{13}}$ cm−2. The high channel charge density along with the high breakdown field enabled a record power figure of merit ( ${V}_{\text {br}}^{{2}}/{R}_{\text {ON}}$ ) of 376 MW/cm2 at a gate–drain spacing of ${3}~{\mu }\text{m}$ .

Superlattices composed of either monoclinic μ-Fe2O3 or β-(AlxGa1−x)2O3 with β-Ga2O3 spacers are grown on (010) β-Ga2O3 substrates using plasma-assisted molecular beam epitaxy. High-resolution x-ray diffraction data are quantitatively fit using commercial dynamical x-ray diffraction software (LEPTOS) to obtain layer thicknesses, strain, and compositions. The strain state of β-(AlxGa1−x)2O3 and μ-Fe2O3 superlattices as characterized using reciprocal space maps in the symmetric (020) and asymmetric (420) diffraction conditions indicates coherent growths that are strained to the (010) β-Ga2O3 lattice. β-(AlxGa1−x)2O3 and μ-Fe2O3 superlattices grown at hotter substrate temperatures result in crystal structures with better coherency and reduced defects compared to colder growths. The growth rate of μ-Fe2O3 is ∼2.6 nm/min at Tsub = 700 °C and drops to ∼1.6 nm/min at Tsub = 800 °C due to increased Fe interdiffusion at hotter substrate temperatures. Scanning transmission electron microscopy data of a μ-Fe2O3 superlattice grown at Tsub = 700 °C confirm that there is significant diffusion of Fe atoms into β-Ga2O3 layers.

As an ultra-wide bandgap semiconductor, $\beta $ -Ga2O3 has attracted great attention for high-power, high-voltage, and optoelectronic applications. However, until now, high-frequency performance of gallium oxide devices has been limited to relatively low current gain cutoff frequencies below 5 GHz. Here, we show that highly localized delta-doping designs can enable high-sheet-charge density to enable devices with short gate lengths that allow high-frequency operation. Field-effect transistors with a gate length of 120 nm on such delta-doped $\beta $ -Ga2O3 are reported here with extrinsic unity current gain frequency of 27 GHz. The device has a peak drain current of 260 mA/mm, transconductance (gm) of 44 mS/mm, and three-terminal off-state breakdown voltage of 150 V. These results demonstrate that the potential of $\beta $ -Ga2O3 for future RF and millimeter-wave device applications.

We report on the high-field transport characteristics and saturation velocity in a modulation-doped $\beta $ -(AlxGa1–x)2O3/Ga2O3 heterostructure. The formation of a 2-D electron gas (2DEG) in the modulation-doped structure was confirmed from the Hall measurements, and the 2DEG channel mobility increased from 143 cm $^{2}/\text{V}\cdot \text{s}$ at room temperature to 1520 cm $^{2}/\text{V}\cdot \text{s}$ at 50 K. The high electron mobility at 50 K made it feasible to achieve velocity saturation inside the channel. The saturation velocity was estimated based on both pulsed current–voltage measurements and small-signal radio frequency (RF) measurements. The measured velocity–field profile suggested a saturation velocity above $1.1\times 10^{7}$ cm/s at 50 K. The small-signal RF characteristics were measured for the fabricated modulation-doped field-effect transistors with a Pt-based Schottky contact. The current gain cutoff frequency ( $\text{f}_{\text {t}}$ ) and maximum oscillation frequency ( $\text{f}_{\text {max}}$ ) showed significant increases from 4.0/11.8 GHz at room temperature to 17.4/40.8 GHz at 50 K for the device with gate length of $\text{L}_{{\textsf {G}}} = 0.61\,\,\mu \text{m}$ . The analysis of the low temperature $\text{f}_{\text {t}}$ based on device simulations indicated a peak velocity of $1.2\times 10^{7}$ cm/s. The three-terminal off-state breakdown measurement further suggested an average breakdown field of 3.22 MV/cm. The high saturation velocity and high breakdown field in $\beta $ -Ga2O3 make it a promising candidate for high-power and high-frequency device applications.

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.

Threshold voltage instability was observed on $\beta $ -Ga2O3 transistors using double-pulsed current–voltage and constant drain current deep level transient spectroscopy (DLTS) measurements. A total instability of 0.78 V was attributed to two distinct trap levels, at ${E}_{C}$ -0.70 and ${E}_{C}$ -0.77 eV, which need to be mitigated for future applications. The traps are likely located near the gate–drain edge and below the delta-doped layer, which is determined through the DLTS technique and an understanding of the fill and empty biasing conditions. The trap modulation was consistent with a gate leakage-based trap filling mechanism, which was demonstrated. It is likely that Fe is playing a role in the observed dispersion due to the close proximity of the Fe substrate.

The influence of growth temperature on the distribution (concentrations and energy levels) of individual defect states in metal organic chemical vapor deposition-grown, Si-doped β-Ga2O3 is investigated. A combination of deep level thermal transient/optical spectroscopies and admittance spectroscopy (AS) was used to quantitatively monitor the evolution of trap states throughout the ∼4.8 eV bandgap. States are observed at EC-0.12 eV by AS; at EC-0.4 eV by deep level transient spectroscopy; and at EC-1.2 eV, EC-2.0 eV, and EC-4.4 eV by deep level optical spectroscopy, and showed different dependencies on growth temperatures ranging from 800 °C to 920 °C. The EC-0.4 eV and EC-4.4 eV states both displayed a strong reduction in its concentration with increasing growth temperature, whereas no consistent trends were seen for the states at EC-1.2 eV and 2.0 eV over the temperature range studied. In contrast, the concentration of the EC-0.12 eV trap monotonically increased over the same range of increasing growth temperature, which tracked a slight, monotonic increase in overall Si concentration measured by secondary ion mass spectroscopy with growth temperature. The opposing trends in concentrations for some of these states shifted the dominant deep level in the bandgap from the EC-4.4 eV state at the lowest growth temperature explored here to the EC-0.12 eV state at the highest growth temperature. The shifting dominance of various bandgap states can have important ramifications on β-Ga2O3 device behavior, and the different trends for these deep levels cannot only guide further growth optimization but also advance the identification of their physical sources.

This report discusses the design and demonstration of β-(Al0.17Ga0.83)2O3/Ga2O3 modulation doped heterostructures to achieve high sheet charge density. The use of a thin spacer layer between the Si delta-doping and the heterojunction interface was investigated in a β-(AlGa)2O3/Ga2O3 modulation doped structure. It is shown that this strategy enables a higher two-dimensional electron gas (2DEG) sheet charge density up to 4.7 × 1012 cm−2 with an effective mobility of 150 cm2/V s. The presence of a degenerate 2DEG channel was confirmed by the measurement of a low temperature effective mobility of 375 cm2/V s and the lack of carrier freeze out from low temperature capacitance voltage measurements. The electron density of 4.7 × 1012 cm−2 is the highest reported 2DEG density obtained without parallel conducting channels in a β-(AlxGa(1−x))2O3/Ga2O3 heterostructure system.

The results of a detailed investigation of electrically active defects in metal-organic chemical vapor deposition (MOCVD)-grown β-Ga2O3 (010) epitaxial layers are described. A combination of deep level optical spectroscopy (DLOS), deep level transient (thermal) spectroscopy (DLTS), and admittance spectroscopy (AS) is used to quantitatively map the energy levels, cross sections, and concentrations of traps across the entire ∼4.8 eV bandgap. States are observed at EC-0.12 eV by AS; at EC-0.4 eV by DLTS; and at EC-1.2 eV, EC-2.0 eV, and EC-4.4 eV by DLOS. While each of these states have been reported for β-Ga2O3 grown by molecular-beam epitaxy (MBE) and edge-defined film fed grown (EFG), with the exception of the EC-0.4 eV trap, there is both a significantly different distribution in the concentration of these states and an overall ∼10× reduction in the total trap concentration. This reduction is consistent with the high mobility and low background compensating acceptor concentrations that have been reported for MOCVD-grown (010) β-Ga2O3. Here, it is observed that the EC-0.12 eV state dominates the overall trap concentration, in marked contrast with prior studies of EFG and MBE material where the state at EC-4.4 eV has dominated the trap spectrum. This sheds light on possible physical sources for this ubiquitous DLOS feature in β-Ga2O3. The substantial reduction in trap concentration for MOCVD material implies great promise for future high performance MOCVD-grown β-Ga2O3 devices.

We report on the origin of high Si flux observed during the use of Si as a doping source in plasma assisted MBE growth of β-Ga2O3. We show on the basis of secondary ion mass spectroscopy analysis that Si flux is not limited by the vapor pressure of Si but by the formation of volatile SiO. The low sublimation energy of SiO leads to a weak dependence of the SiO flux of Si cell temperature and a strong dependence on the background oxygen pressure. Extended exposure to activated oxygen results in reduction of SiO flux due to the formation of SiO2 on the Si surface. The work reported provides key understanding for incorporating Si into future oxide-based semiconductor heterostructure and device MBE growth.

Two buffer traps at EC-0.7 eV and EC-0.8 eV have been individually identified as causing threshold voltage and on-resistance instabilities in β-Ga2O3 Si ∂-doped transistors grown by plasma-assisted molecular beam epitaxy (PAMBE) on semi-insulating Fe doped β-Ga2O3 substrates. The instabilities are characterized using double-pulsed current-voltage and isothermal constant drain current deep level transient spectroscopy. The defect spectra are compared between transistors grown using two different unintentionally doped buffer layer thicknesses of 100 nm and 600 nm. The EC-0.8 eV trap was not seen using the thicker buffer and is shown to correlate with the presence of residual Fe in thePAMBE buffer layer. The EC-0.7 eV trap was unchanged in concentration and is revealed as the dominating source of the threshold voltage instability. This trap is consistent with the characteristics of a previously reported intrinsic point defect [Ingebrigtsen et al., APL Mater. 7, 022510 (2019)]. The EC-0.7 eV trap is responsible for ∼70% of the total threshold voltage shift in the 100 nm thick buffer transistor and 100% in the 600 nm thick buffer transistor, which indicates growth optimization is needed to improve β-Ga2O3 transistor stability.Two buffer traps at EC-0.7 eV and EC-0.8 eV have been individually identified as causing threshold voltage and on-resistance instabilities in β-Ga2O3 Si ∂-doped transistors grown by plasma-assisted molecular beam epitaxy (PAMBE) on semi-insulating Fe doped β-Ga2O3 substrates. The instabilities are characterized using double-pulsed current-voltage and isothermal constant drain current deep level transient spectroscopy. The defect spectra are compared between transistors grown using two different unintentionally doped buffer layer thicknesses of 100 nm and 600 nm. The EC-0.8 eV trap was not seen using the thicker buffer and is shown to correlate with the presence of residual Fe in thePAMBE buffer layer. The EC-0.7 eV trap was unchanged in concentration and is revealed as the dominating source of the threshold voltage instability. This trap is consistent with the characteristics of a previously reported intrinsic point defect [Ingebrigtsen et al., APL Mater. 7, 022510 (2019)]. The EC-0.7 eV trap is respons...

BaSnO_{3}, a high mobility perovskite oxide, is an attractive material for oxide-based electronic devices. However, in addition to low-field mobility, high-field transport properties such as the saturation velocity of carriers play a major role in determining device performance. We report on the experimental measurement of electron saturation velocity in La-doped BaSnO_{3} thin films for a range of doping densities. Predicted saturation velocities based on a simple LO-phonon emission model using an effective LO phonon energy of 120 meV show good agreement with measurements of velocity saturation in La-doped BaSnO_{3} films.. Density-dependent saturation velocity in the range of 1.6x10^{7} cm/s reducing to 2x10^{6} cm/s is predicted for {\delta}-doped BaSnO3 channels with carrier densities ranging from 10^{13} cm^{-2} to 2x10^{14} cm^{-2} respectively. These results are expected to aid the informed design of BaSnO3 as the active material for high-charge density electronic transistors., Comment: 23 pages, 10 figures, 2 tables

Traps in β-Ga2O3 keep devices, such as power transistors, photodetectors, and diodes, from working ideally. This manifests itself as short-term (ns to ms time scales) to longer term (day to week long) instabilities in terminal characteristics including dynamic on-resistance and threshold voltage and as increased leakage currents that degrade breakdown voltage and noise equivalent power. Using both conventional deep level transient and optical spectroscopies (DLTS/DLOS) and direct, quantitative defect spectroscopy on transistors namely constant drain current DLTS (CID-DLTS), the trap energies and concentrations in test structures and metal-semiconductor field effect transistors (MESFETs) are quantified and compared with the instabilities in the MESFET threshold voltage and dynamic on-resistance are correlated showing which traps matter in these MESFETs. A critical step in defect spectroscopy is to identify the sources of defects to enable strategies to eliminate the defects or bring them down to tolerable levels. Our group and the community have made great strides in beginning to identify the defects throughout the bandgap. Using high energy proton and neutron irradiation to create native defects and controlled studies with Fe, the two primary defects that cause threshold voltage instability in MESFETs will be discussed along with mitigation strategies.

We report on the effect of iron (Fe)-doped semi-insulating buffers on the electron transport and DC-RF dispersion in Si delta (δ)-doped β-Ga2O3 metal-semiconductor field effect transistors. The effect of the distance between the 2-dimensional electron gas and the Fe-doped region was investigated, and Fe doping in the buffer was found to have a significant effect on the transport properties. It was found that buffers thicker than 600 nm can enable better transport and dispersion properties for field effect transistors, while maintaining relatively low parasitic buffer leakage. This work can provide guidance for the use of Fe-doped insulating buffers for future Ga2O3 based electronics.