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In this work, bulk Czochralski-grown single crystals of 10 mol. % Al2O3 alloyed B-Ga2O3 - monoclinic 10% AGO or B-(Al0.1Ga0.9)2O3 - are obtained, which show +0.20 eV increase in the bandgap compared with unintentionally doped B-Ga2O3. Further, growths of 33% AGO - B-(Al0.33Ga0.67)2O3 - and 50% AGO - B-(Al0.5Ga0.5)2O3 or B-AlGaO3 - produce polycrystalline single-phase monoclinic material (B-AGO). All three compositions are investigated by x-ray diffraction, Raman spectroscopy, optical absorption, and 27Al nuclear magnetic resonance (NMR). By investigating single phase B-AGO over a large range of Al2O3 concentrations (10 - 50 mol. %), broad trends in the lattice parameter, vibrational modes, optical bandgap, and crystallographic site preference are determined. All lattice parameters show a linear trend with Al incorporation. According to NMR, aluminum incorporates on both crystallographic sites of B-Ga2O3, with a slight preference for the octahedral (GaII) site, which becomes more disordered with increasing Al. Single crystals of 10% AGO were also characterized by x-ray rocking curve, transmission electron microscopy, purity (glow discharge mass spectroscopy and x-ray fluorescence), optical transmission (200 nm - 20 um wavelengths), and resistivity. These measurements suggest that electrical compensation by impurity acceptor doping is not the likely explanation for high resistivity, but rather the shift of a hydrogen level from a shallow donor to a deep acceptor due to Al alloying. .. Cont. This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Journal of Applied Physics 131 155702.

We report on the realization of record low resistance Ohmic contacts to MOVPE-grown heavily Si-doped $β$-Ga$_2$O$_3$ and $β$-(Al$_x$Ga$_1-x$)$_2$ O$_3$ epitaxial films. Transfer length measurement (TLM) patterns were fabricated on the heavily Si-doped homoepitaxial $β$-Ga$_2$O$_3$ films with electron concentration (n) ranging from 1.77 to 3.23e20 cm^-3. Record low specific contact resistance and total contact resistance (Rc) of 1.62e-7 Ohm.cm^2 and 0.023 Ohm.mm were realized for $β$-Ga$_2$O$_3$: Si films with n > 3e20 cm^-3. TLM structures were also fabricated on heavily Si doped coherently strained $β$-(Al$_x$Ga$_1-x$)$_2$ O$_3$ (x=12%, 17% and 22%) films. The film with 12% Al composition (n=1.23e20 cm^-3) showed \r{ho}c of 5.85e-6 Ohm.cm^2, but it increased to 2.19e-4 Ohm.cm^2 for a layer with a 22% Al composition. Annealing the samples post metal deposition has generally led to a decrease in contact resistance, but for high Al content $β$-(Al$_x$Ga$_1-x$)$_2$ O$_3$, the contact resistance did not change significantly after the annealing process. The low contact resistance values measured in this work are very promising for the fabrication of high frequency power devices.

We demonstrate a new substrate cleaning and buffer growth scheme in $β$-Ga$_2$O$_3$ epitaxial thin films using metalorganic vapor phase epitaxy (MOVPE). For the channel structure, a low-temperature (LT, 600 $^\circ$C) undoped Ga$_2$O$_3$ buffer is grown followed by transition layers to a high-temperature (HT, 810 $^\circ$C) Si-doped Ga$_2$O$_3$ channel layers without growth interruption. The (010) Ga$_2$O$_3$ Fe-doped substrate cleaning uses solvent cleaning followed by an additional HF (49% in water) treatment for 30 mins before the epilayer growth. This step is shown to compensate the parasitic Si channel at the epilayer-substrate interface that originates from the substrate polishing process or contamination from the ambient. SIMS analysis shows the Si peak atomic density at the substrate interface is several times lower than the Fe atomic density in the substrate - indicating full compensation. The elimination of the parasitic electron channel at the epi-substrate interface was also verified by electrical (capacitance-voltage profiling) measurements. In the LT-grown buffer layers, it is seen that the Fe forward decay tail from the substrate is very sharp with a decay rate of $\sim$ 9 nm$/$dec. These channels show record high electron mobility in the range of 196 - 85 cm$^2$/Vs in unintentionally doped and Si-doped films in the doping range of 2$\times$10$^{16}$ to 1$\times$10$^{20}$ cm$^{-3}$. Si delta-doped channels were also grown utilizing this substrate cleaning and the hybrid LT-buffers. Record high electron Hall mobility of 110 cm$^2$/Vs was measured for sheet charge density of 9.2$\times$10$^{12}$ cm$^{-2}$. This substrate cleaning combined with the LT-buffer scheme shows the potential of designing Si-doped $β$-Ga$_2$O$_3$ channels with exceptional transport properties for high performance gallium oxide-based electron devices., 20 pages, 10 figures

Ulta-wide bandgap semiconductors based on $\beta$-Ga$_2$O$_3$ offer the potential to achieve higher power switching performance, efficiency, and lower manufacturing cost than today's wide bandgap power semiconductors. However, the most critical challenge to the commercialization of Ga$_2$O$_3$ electronics is overheating, which impacts the device's performance and reliability. We fabricated a Ga$_2$O$_3$/4H-SiC composite wafer using a fusion-bonding method. A low temperature ($\le$ 600 $^{\circ}$C) epitaxy and device processing approach based on low-temperature (LT) metalorganic vapor phase epitaxy is developed to grow a Ga$_2$O$_3$ epitaxial channel layer on the composite wafer and subsequently fabricate into Ga$_2$O$_3$ power MOSFETs. This LT approach is essential to preserve the structural integrity of the composite wafer. These LT-grown epitaxial Ga$_2$O$_3$ MOSFETs deliver high thermal performance (56% reduction in channel temperature), high voltage blocking capabilities up to 2.45 kV, and power figures of merit of $\sim$ 300 MW/cm$^2$, which is a record high for any heterogeneously integrated Ga$_2$O$_3$ devices reported to date. This work is the first realization of multi-kilovolt homoepitaxial Ga$_2$O$_3$ power MOSFETs fabricated on a composite substrate with high heat transfer performance which delivers state-of-the-art power density values while running much cooler than those on native substrates. Thermal characterization and modeling results reveal that a Ga$_2$O$_3$/diamond composite wafer with a reduced Ga$_2$O$_3$ thickness ($\sim$ 1 $\mu$m) and thinner bonding interlayer ($, Comment: 17 pages, 9 figures

In this letter, fin-shape tri-gate $β$-Ga$_{2}$O$_{3}$ lateral MESFETs are demonstrated with a high power figure of merit of 0.95 GW/cm$^{2}$ - a record high for any $β$-Ga$_{2}$O$_{3}$ transistor to date. A low-temperature undoped buffer-channel stack design is developed which demonstrates record high Hall and drift electron mobilities in doped $β$-Ga$_{2}$O$_{3}$ channels allowing for low ON resistances R$_{ON}$ in $β$-Ga$_{2}$O$_{3}$ MESFETs. Fin-widths (W$_{fin}$) were 1.2-1.5 $μ$m and there were 25 fins (N$_{fin}$) per device with a trench depth of $\sim$1$μ$m. A $β$-Ga$_2$O$_3$ MESFET with a source-drain length of 6.4 $μ$m exhibits a high ON current (187 mA/mm), low R$_{ON}$ (20.5 $Ω$.mm) and a high average breakdown field (4.2 MV/cm). All devices show very low reverse leakage until catastrophic breakdown for breakdown voltages scaled from 1.1kV to $\sim$3kV. This work demonstrates the potential of channel engineering in improving $β$-Ga$_{2}$O$_{3}$ device performance toward lower conduction losses for low-to-medium voltage applications., 4 pages, 5 pages

We report on the growth and characterization of Ge-doped \b{eta}-Ga2O3 thin films using a solid germanium source. \b{eta}-Ga2O3 thin films were grown using a low-pressure chemical vapor deposition (LPCVD) reactor with either an oxygen or gallium delivery tube. Films were grown on 6 degree offcut sapphire and (010) \b{eta}-Ga2O3 substrates with growth rates between 0.5 - 22 {\mu}m/hr. By controlling the germanium vapor pressure, a wide range of Hall carrier concentrations between 10^17 - 10^19 cm-3 were achieved. Low-temperature Hall data revealed a difference in donor incorporation depending on the reactor configuration. At low growth rates, germanium occupied a single donor energy level between 8 - 10 meV. At higher growth rates, germanium doping predominantly results in a deeper donor energy level at 85 meV. This work shows the effect of reactor design and growth regime on the kinetics of impurity incorporation. Studying donor incorporation in \b{eta}-Ga2O3 is important for the design of high-power electronic devices., Comment: 16 pages,4 figures

This paper presents vertical (001) oriented $β$-Ga$_2$O$_3$ field plated (FP) Schottky barrier diode (SBD) with a novel extreme permittivity dielectric field oxide. A thin drift layer of 1.7 $μm$ was used to enable a punch-through (PT) field profile and very low differential specific on-resistance (R$_{on-sp}$) of 0.32 m$Ω$-cm$^{2}$. The extreme permittivity field plate oxide facilitated the lateral spread of the electric field profile beyond the field plate edge and enabled a breakdown voltage ($V_{br}$) of 687 V. The edge termination efficiency increases from 13.5 $\%$ for non-field plated structure to 63 $\%$ for high permittivity field plate structure. The surface breakdown electric field was extracted to be 5.45 MV/cm at the center of the anode region using TCAD simulations. The high permittivity field plated SBD demonstrated a record high Baliga figure of merit (BFOM) of 1.47 GW/cm$^{2}$ showing the potential of Ga$_2$O$_3$ power devices for multi-kilovolt class applications.

In this work, we use photoluminescence spectroscopy (PL) to monitor changes in the UV, UV', blue, and green emission bands from n-type (010) Ga2O3 films grown by metalorganic vapor phase epitaxy (MOVPE) induced by annealing at different temperatures under O2 ambient. Annealing at successively higher temperatures decreases the overall PL yield and UV intensity at nearly the same rates, indicating the increase in formation of at least one non-radiative defect type. Simultaneously, the PL yield ratios of blue/UV and green/UV increase, suggesting that defects associated with these emissions increase in concentration with O2 annealing. Utilizing the different absorption coefficients of 240 and 266 nm polarization-dependent excitation, we find an overall activation energy for the generation of non-radiative defects of 0.69 eV in the bulk but 1.55 eV near the surface. We also deduce activation energies for the green emission-related defects of 0.60 eV near the surface and 0.89-0.92 eV through the films, whereas the blue-related defects have activation energy in the range 0.43-0.62 eV for all depths. Lastly, we observe hillock surface morphologies and Cr diffusion from the substrate into the film for temperatures above 1050 oC. These observations are consistent with the formation and diffusion of VGa and its complexes as a dominant process during O2 annealing, but further work will be necessary to determine which defects and complexes provide radiative and non-radiative recombination channels and the detailed kinetic processes occurring at surfaces and in bulk amongst defect populations., 14 pages, 3 figures

The quasi-static anisotropic permittivity parameters of electrically insulating gallium oxide (beta-Ga2O3) were determined by terahertz spectroscopy. Polarization-resolved frequency domain spectroscopy in the spectral range from 200 GHz to 1 THz was carried out on bulk crystals along different orientations. Principal directions for permittivity were determined along crystallographic axes c, and b, and reciprocal lattice direction a*. No significant frequency dispersion in the real part of dielectric permittivity was observed in the measured spectral range. Our results are in excellent agreement with recent radio-frequency capacitance measurements as well as with extrapolations from recent infrared measurements of phonon mode and high frequency contributions, and close the knowledge gap for these parameters in the terahertz spectral range. Our results are important for applications of beta-Ga2O3 in high-frequency electronic devices, Comment: Main Manuscript is 6 pages long with 3 figures and 1 table. The submission also contains a Supplementary Info section

We report on low-temperature MOVPE growth of silicon delta-doped \b{eta}-Ga2O3 films with low FWHM. The as-grown films are characterized using Secondary-ion mass spectroscopy, Capacitance-Voltage and Hall techniques. SIMS measurements show that surface segregation is the chief cause of large FWHM in MOVPE-grown films. The surface segregation coefficient (R) is observed to reduce with reduction in the growth temperature. Films grown at 600 {\deg}C show an electron concentration of 9.7 x 1012 cm-2 and a FWHM of 3.2 nm. High resolution scanning/transmission electron microscopy of the epitaxial film did not reveal any significant observable degradation in crystal quality of the delta sheet and surrounding regions. Hall measurements of delta-doped film on Fe-doped substrate showed a sheet charge density of 6.1 x 1012 cm-2 and carrier mobility of 83 cm2/V. s. Realization of sharp delta doping profiles in MOVPE-grown \b{eta}-Ga2O3 is promising for high performance device applications., Comment: 10 pages, 5 figures

We report on the modeling of polarization-induced two-dimensional electron gas (2DEG) formation at $��$-AlGaO3 / $��$-Ga2O3 heterointerface and the effect of spontaneous polarization (Psp) reversal on 2DEG density in $��$-Ga2O3 /$��$-AlGaO3 / $��$-Ga2O3 double heterostructures. Density-functional theory (DFT) is utilized to calculate the material properties of $��$-Ga2O3 and $��$-AlGaO3 alloys. Using Schrodinger-Poisson solver along with DFT calculated parameters, the 2DEG density is calculated as a function of barrier type and thickness. By optimizing the layer thicknesses of $��$-Ga2O3/$��$-AlGaO3/$��$-Ga2O3 heterostructures, charge contrast ratios exceeding 1600 are obtained. This computational study indicates the high potential for $��$-Ga2O3-based heterostructure devices for non-volatile memories and neuromorphic applications.

In this work, we report on the growth of high-mobility $��$-Ga$_2$O$_3$ homoepitaxial thin films grown at a temperature much lower than the conventional growth temperature window for metalorganic vapor phase epitaxy. Low-temperature $��$-Ga$_2$O$_3$ thin films grown at 600$^{\circ}$C on Fe-doped (010) bulk substrates exhibits remarkable crystalline quality which is evident from the measured room temperature Hall mobility of 186 cm$^2$/Vs for the unintentionally doped films. N-type doping is achieved by using Si as a dopant and controllable doping in the range of 2$\times$10$^{16}$ - 2$\times$10$^{19}$ cm$^{-3}$ is studied. Si incorporation and activation is studied by comparing silicon concentration from secondary ion mass spectroscopy (SIMS) and electron concentration from temperature-dependent Hall measurements. The films exhibit high purity (low C and H concentrations) with a very low concentration of compensating acceptors (2$\times$10$^{15}$ cm$^{-3}$) even at this growth temperature. Additionally, abrupt doping profile with forward decay of $\sim$ 5nm/dec (10 times improvement compared to what is observed for thin films grown at 810$^{\circ}$C) is demonstrated by growing at a lower temperature., 5 pages, 6 figures

This paper reports on the modulation of Schottky barrier heights (SBH) on three different orientations of $��$-Ga$_2$O$_3$ by insertion of an ultra-thin SiO$_2$ dielectric interlayer at the metal-semiconductor junction, which can potentially lower the Fermi-level pinning (FLP) effect due to metal-induced gap states (MIGS). Pt and Ni metal-semiconductor (MS) and metal-interlayer-semiconductor (MIS) Schottky barrier diodes were fabricated on bulk n-type doped $��$-Ga$_2$O$_3$ single crystal substrates along the (010), (-201) and (100) orientations and were characterized by room temperature current-voltage (I-V) and capacitance-voltage (C-V) measurements. Pt MIS diodes exhibited 0.53 eV and 0.37 eV increment in SBH along the (010) and (-201) orientations respectively as compared to their respective MS counterparts. The highest SBH of 1.81 eV was achieved on the (010)-oriented MIS SBD using Pt metal. The MIS SBDs on (100)-oriented substrates exhibited a dramatic increment ($>$1.5$\times$) in SBH as well as reduction in reverse leakage current. The use of thin dielectric interlayers can be an efficient experimental method to modulate SBH of metal/Ga$_2$O$_3$ junctions., 8 pages, 10 figures, 2 tables

We report on n-type degenerate doping in MOVPE grown \b{eta}-(Al0.26Ga0.74)2O3 epitaxial thin films and modulation doping in \b{eta}-(Al0.26Ga0.74)2O3/\b{eta}-Ga2O3 heterostructure. Alloy composition is confirmed using HRXRD measurements. Carrier concentration in the thin films is proportional to the silane molar flow. Room temperature hall measurements showed a high carrier concentration of 6x1018-7.3x1019 cm-3 with a corresponding electron mobility of 53-27 cm2/V.s in uniformly-doped \b{eta}-(Al0.26Ga0.74)2O3 layers. Modulation doping is used to realize a total electron sheet charge of 2.3x1012 cm-2 in a \b{eta}-(Al0.26Ga0.74)2O3/\b{eta}-Ga2O3 heterostructure using a uniformly-doped \b{eta}-(Al0.26Ga0.74)2O3 barrier layer and a thin spacer layer.

We report on the direct measurement of two-dimensional sheet charge density dependence of electron transport in AlGaN/GaN high electron mobility transistors. Pulsed IV measurements established increasing electron velocities with decreasing sheet charge densities, resulting in saturation velocity of 1.9 x 10^7 cm/s at a low sheet charge density of 7.8 x 10^11 cm-2. A new optical phonon emission-based electron velocity model for GaN is also presented. It accommodates stimulated LO phonon emission which clamps the electron velocity with strong electron-phonon interaction and long LO phonon lifetime in GaN. A comparison with the measured density-dependent saturation velocity shows that it captures the dependence rather well. Finally, the experimental result is applied in TCAD-based device simulator to predict DC and small signal characteristics of a reported GaN HEMT. Good agreement between the simulated and reported experimental results validated the measurement presented in this report and established accurate modeling of GaN HEMTs., Comment: 11 pages, 7 figures

Layered metal dichalcogenide materials are a family of semiconductors with a wide range of energy band gaps and properties, and potential to open up new areas of physics and technology applications. However, obtaining high crystal quality thin films over a large area remains a challenge. Here we show that chemical vapor deposition (CVD) can be used to achieve large area electronic grade single crystal Molybdenum Disulfide (MoS2) thin films with the highest mobility reported in CVD grown films so far. Growth temperature and choice of substrate were found to critically impact the quality of film grown, and high temperature growth on (0001) orientated sapphire yielded highly oriented single crystal MoS2 films for the first time. Films grown under optimal conditions were found to be of high structural quality from high-resolution X-ray diffraction, transmission electron microscopy, and Raman measurements, approaching the quality of reference geological MoS2. Photoluminescence and electrical measurements confirmed the growth of optically active MoS2 with a low background carrier concentration, and high mobility. The CVD method reported here for the growth of high quality MoS2 thin films paves the way towards growth of a variety of layered 2D chalcogenide semiconductors and their heterostructures.