We report mercury-probe measurement of the carrier mobility in the two-dimensional electron gas (2DEG) of the epi-structure for GaN-based high electron mobility transistors (HEMTs). This nondestructive method requires no contact metal deposition. The mobility is extracted from the dielectric relaxation frequency of the 2DEG moderated by the barrier layer of the HEMT epi-structure, which is manifested as the inflection frequency of the gate capacitance versus frequency curve. We define the sheet capacitance as the in-plane capacitance of a square thin film to elucidate 2DEG’s large dielectric relaxation frequency and HEMT’s superior high-frequency performance.

The ionoluminescence response of the ultraviolet emission bands in wurtzite GaN was investigated in situ under 1.75 MeV O2+ irradiation from 68 to 200 K. A systematic decrease in luminescence intensity with increasing ion fluence was observed, evidencing the formation of non-radiative recombination centers. The quenching behavior exhibits a pronounced temperature dependence, reflecting the complex competition between defect formation, migration, and recovery processes. Arrhenius analysis of thermal quenching at the onset of irradiation revealed an activation energy of 105 meV for the dominant non-radiative process. The beam-induced cross sections associated with non-radiative recombination were obtained by fitting an analytical model to the ion fluence dependent yield curves. The cross sections were found to increase as temperature is reduced, indicating enhanced efficiency of the non-radiative channel when defect mobility and/or dynamic annealing are suppressed. This study provides insights into the role that radiation-induced structural defects may play on electronic transport properties in GaN-based devices used in low temperature radiation environments, such as those found in satellites and spacecraft.

We demonstrate the epitaxial growth of single-phase (100) BaTiO3 films on (100) β-Ga2O3 substrates at substrate temperatures ranging from 600 to 700 °C using molecular-beam epitaxy. Characterization of a 47 nm thick BaTiO3 film by atomic force microscopy reveals a step-and-terrace morphology with unit-cell-high BaTiO3 steps and an rms surface roughness of 0.26 nm. Scanning transmission electron microscopy (STEM) images show that in some regions the β-Ga2O3 substrate terminates with a (100)A plane as it transitions to BaTiO3 and in other regions with a (100)B plane. The (100) BaTiO3 films are fully relaxed and consist of a mixture of two types of a-axis domains: a1 and a2. The orientation relationship determined by X-ray diffraction and confirmed by STEM is (100) BaTiO3 || (100) β-Ga2O3 and [011] BaTiO3||[010] β-Ga2O3. Despite the average linear lattice mismatch of 3.8%, BaTiO3 films with rocking curve full width at half maximum widths as narrow as 28 arc sec are achieved. From capacitance–voltage measurements on a metal–oxide–semiconductor capacitor structure with a-axis BaTiO3 as the oxide layer and Si-doped β-Ga2O3 as the semiconducting layer, we extract a dielectric constant of K11 = 670 for the BaTiO3 epitaxially integrated with (100) β-Ga2O3. We anticipate that this high-K epitaxial dielectric will be useful for electric-field management in β-Ga2O3-based device structures.

Among ultrawide bandgap semiconductors, beta-Ga2O3 is particularly promising for high power and frequency applications. For devices, n-type concentrations above 1019 cm-3 are required. Ge is a promising alternative n-type dopant with an ionic radius similar to Ga. Homoepitaxial (010) beta-Ga2O3 films were implanted with Ge to form 50 and 100 nm box concentrations of 3 & times; 1019 and 5 & times; 1019 cm-3, with damage ranging from 1.2 to 2.0 displacements per atom. For lower damage implants, optimized anneals in ultrahigh purity N-2 at 950-1000 degrees C for 5-10 min resulted in an R-S of 600-700 Omega/square, mobilities of 60-70 cm(2)/V s, and a Ge activation of up to 40%. For higher damage implants, activation dropped to 23% with similar mobilities. Ge diffusion, measured by secondary ion mass spectrometry, showed the formation of a Ge "clustering peak" with a concentration exceeding the initial implant following anneals in N-2 or O-2 at 950-1050 degrees C. Beyond this peak, minimal Ge diffusion occurred for N-2 anneals at 950 degrees C, but at 1050 degrees C, non-Fickian diffusion extended to >200 nm. Electrical activation data suggest that clustered Ge is electrically inactive. To understand Ge clustering, several samples were characterized by synchrotron x-ray diffraction. Second-phase precipitates were observed in as-implanted samples, which then fully dissolved after furnace annealing in N-2 at 1050 degrees C. Diffraction peaks suggest that these implant-induced precipitates may be related to a high pressure Pa3 phase of GeO2 and may evolve during anneals to explain the Ge clustering. Ultimately, we believe that Ge clustering limits the activation of implanted Ge at high concentrations.

Ultra-wide bandgap (UWBG) semiconductor materials are predicted to greatly improve the performance of next-generation power electronics. beta-Ga2O3 is a promising UWBG material due to its large critical electric field strength, readily accessible n-type doping, and the availability of native substrates. However, no power converters using only beta-Ga2O3 devices have been demonstrated yet because the field has prioritized low periphery devices with large unnormalized on-resistances. In this paper, we present the first demonstration of a dc-dc converter using beta-Ga2O3 lateral field effect transistors with large periphery. We first fabricate and package the transistors then place them into the high and low side switch of a buck converter circuit. The devices are tested at 45 V input voltage, 2.7 W output power, and the converter achieves an efficiency of over 90% under various operating conditions.

A digital alloy is a superlattice-like nanostructure formed by stacking ultra-thin (<= 4 monolayers) AlN and GaN layers periodically. Digital alloys allow for the tunability of the bandgap and electrical transport behavior. However, for them to be explored for electronic device applications, it is crucial that we determine their thermal properties, as this greatly impacts the thermal resistance and heat spreading within a device. Here we investigate the thermal properties of various AlN/AlGaN and AlxGa1-xN/AlyGa1-yN digital alloys (where x and y are the associated alloy composition) are investigated using the combined techniques of time-domain thermoreflectance and steady-state thermoreflectance. A highly anisotropic thermal conductivity of 9.6 W/m-K (cross-plane) and 39.8 W/m-K (in-plane) was measured for an AlN/AlGaN digital alloy (0.86/5.93 nm period thickness), while all measured AlxGa1-xN/AlyGa1-yN digital alloys measured a thermal conductivity of 2.9-3.3 W/m-K (cross-plane) and 8.6 W/m-K (in-plane). To investigate the influence of these thermal properties have on in-planedevice thermal transport, a number of die-level thermal management approaches are investigated on an AlGaN metal-semiconductor field-effect transistor using numerical simulations. The effects of the various cooling approaches on the device channel temperature were comprehensively investigated, along with guidance for material selection to enable the most effective thermal solutions. Specifically, we investigate the influence of substrate material, top-side heat spreader thickness/thermal conductivity, digital alloy thickness, and flip-chip design. Overall, this numerical study shows that it is possible to achieve high power digital alloy device operation with appropriate die-level thermal management solutions.

We present a fundamental study of the band alignment at the interface of HfZrO4 (HZO) with Ge-doped Ga2O3. Ge is an alternative n-type dopant for the wide band gap Ga2O3 due to its shallow donor level and favorable MBE growth conditions. In the perspective of using the ferroelectric polarization of hafnia based oxides, we have used a stack of HZO on highly Ge doped Ga2O3, the latter providing high carrier density. Electrical contacts were ensured by a TiN top electrode deposited on the HZO and an Au pad on the Ge:Ga2O3. The band alignment was measured by carrying out hard X-ray photoelectron spectroscopy (HAXPES) with in situ bias application across the HZO and following the evolution of both HZO and Ga2O3 energy level. Complementary high-resolution transmission electron microscopy (HRTEM) provided structural confirmation of the polar orthorhombic phase however electrical characterization showed that charge injection and trapping at the interface prevents stabilizing the ferroelectric polarization in HZO. The band alignment in the presence of a leaky HZO layer is therefore dominated by the bias induced band skewing.

The formation of AlN/AlGaN short period superlattices (SPSLs) was investigated though the introduction of a constant Ga overpressure during the metal modulated epitaxy (MME) growth of AlN. A combination of x-ray diffraction (XRD) and scanning transmission electron microscopy (STEM) analyses found that control over the Al composition in the AlGaN layer was achieved through modulating the Ga beam equivalent pressure (BEP), with a minimum partial pressure of 3 × 10−7 Torr needed for Ga to incorporate at a growth temperature of 825 °C. A minimum Al composition in the AlGaN layer of 72% was achieved for a Ga BEP of 1 × 10−6 Torr using this method. An apparent limit of the AlGaN layer thickness of 3–4 ML indicated that the incorporation of Ga was confined to the consumption region of the MME growth process. Determination of this behavior made clear the requirements of having both XRD and STEM in order to be able to fully characterize the SPSL layer structure. Finally, AFM imaging highlighted that the presence of Ga on the surface behaved as a surfactant, with a minimum RMS roughness of 0.46 nm achieved at the maximum Ga BEP of 1 × 10−6 Torr.

Interface roughness (IFR) scattering significantly impacts the mobility of two-dimensional electron gases (2DEGs) in heterostructures. While existing models for IFR scattering have advanced our understanding, they have notable limitations. The model developed by Jin et al. in 2007, while incorporating a realistic barrier height and roughness-induced changes in potential and subband wave-functions, employs a first-order roughness expansion. The formulation introduced by Lizzit et al. in 2014, although avoiding the first-order approximation for better higher-order effect modeling, omits IFR-induced change in electron density distribution. To address these limitations, we introduce a novel model that comprehensively accounts for all IFR-induced effects while avoiding any expansion approximations, by incorporating IFR-modified subband energies and wave-functions obtained from the numerical solution of the Schr & ouml;dinger equation during the calculation of IFR scattering matrix elements. In addition, we have included models for other relevant scattering mechanisms, including charged dislocation lines, ionized impurities, acoustic phonons, and polar optical phonons. A comprehensive numerical analysis of carrier mobility has been performed for an AlN/GaN high electron mobility transistor, yielding results consistent with experimental data. Furthermore, to investigate the impact of device architecture on 2DEG mobility, we study the effects of layer thickness and modulation doping profiles in AlN/GaN digital alloys. Our findings reveal strategies for engineering high mobility at elevated 2DEG concentrations, potentially advancing the development of high-performance semiconductor devices. (c) 2024 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution-NonCommercialNoDerivs 4.0 International (CC BY-NC-ND) license (https://creativecommons.org/licenses/by-nc-nd/4.0/). https://doi.org/10.1063/5.0229570

We present a comprehensive investigation of electrically active defects and transport properties in commercial (201) edge edge-defined film-fed growth beta-Ga2O3 using admittance spectroscopy measurements from 14 K up to 450 K. Isothermal capacitance-frequency measurements were conducted from 410 to 450 K to resolve a defect similar to 0.8 eV below the conduction band edge and compared with deep-level transient spectroscopy and isothermal capacitance transient spectroscopy measurements. We report significant non-Arrhenius behavior of a defect similar to 100 meV below the conduction band and apply the Arrhenius transformation matching method to extract its temperature-dependent activation energy and capture cross-section in the temperature range of 75 to 195 K. At low temperatures (< 50 K), we use bias-dependent admittance spectroscopy to extract the electron mobility in beta-Ga2O3 from the modified dielectric relaxation frequency. Finally, we discuss the potential of admittance spectroscopy for defect characterization in wide bandgap semiconductors in terms of the defect detection range, instrument requirements, and the frequency-temperature experiment space.

We report the use of suboxide molecular-beam epitaxy (S-MBE) to grow α-(AlxGa1−x)2O3 films on (110) sapphire substrates over the 0 < x < 0.95 range of aluminum content. In S-MBE, 99.98% of the gallium-containing molecular beam arrives at the substrate in a preoxidized form as gallium suboxide (Ga2O). This bypasses the rate-limiting step of conventional MBE for the growth of gallium oxide (Ga2O3) from a gallium molecular beam and allows us to grow fully epitaxial α-(AlxGa1−x)2O3 films at growth rates exceeding 1 µm/h and relatively low substrate temperature (Tsub = 605 ± 15 °C). The ability to grow α-(AlxGa1−x)2O3 over the nominally full composition range is confirmed by Vegard’s law applied to the x-ray diffraction data and by optical bandgap measurements with ultraviolet–visible spectroscopy. We show that S-MBE allows straightforward composition control and bandgap selection for α-(AlxGa1−x)2O3 films as the aluminum incorporation x in the film is linear with the relative flux ratio of aluminum to Ga2O. The films are characterized by atomic-force microscopy, x-ray diffraction, and scanning transmission electron microscopy (STEM). These α-(AlxGa1−x)2O3 films grown by S-MBE at record growth rates exhibit a rocking curve full width at half maximum of ≊ 12 arc secs, rms roughness <1 nm, and are fully commensurate for x ≥ 0.5 for 20–50 nm thick films. STEM imaging of the x = 0.78 sample reveals high structural quality and uniform composition. Despite the high structural quality of the films, our attempts at doping with silicon result in highly insulating films.

Optimizing thermal anneals of Si-implanted β-Ga2O3 is critical for low resistance contacts and selective area doping. We report the impact of annealing ambient, temperature, and time on the activation of room temperature ion-implanted Si in β-Ga2O3 at concentrations from 5 × 1018 to 1 × 1020 cm−3, demonstrating full activation (>80% activation, mobilities >70 cm2/V s) with contact resistances below 0.29 Ω mm. Homoepitaxial β-Ga2O3 films, grown by plasma-assisted molecular beam epitaxy on Fe-doped (010) substrates, were implanted at multiple energies to yield 100 nm box profiles of 5 × 1018, 5 × 1019, and 1 × 1020 cm−3. Anneals were performed in an ultra-high vacuum-compatible quartz furnace at 1 bar with well-controlled gas compositions. To maintain β-Ga2O3 stability, pO2 must be greater than 10−9 bar. Anneals up to pO2 = 1 bar achieve full activation at 5 × 1018 cm−3, while 5 × 1019 cm−3 must be annealed with pO2 ≤ 10−4 bar, and 1 × 1020 cm−3 requires pO2 < 10−6 bar. Water vapor prevents activation and must be maintained below 10−8 bar. Activation is achieved for anneal temperatures as low as 850 °C with mobility increasing with anneal temperatures up to 1050 °C, though Si diffusion has been reported above 950 °C. At 950 °C, activation is maximized between 5 and 20 min with longer times resulting in decreased carrier activation (over-annealing). This over-annealing is significant for concentrations above 5 × 1019 cm−3 and occurs rapidly at 1 × 1020 cm−3. Rutherford backscattering spectrometry (channeling) suggests that damage recovery is seeded from remnant aligned β-Ga2O3 that remains after implantation; this conclusion is also supported by scanning transmission electron microscopy showing retention of the β-phase with inclusions that resemble the γ-phase.

We present a study of Ge segregation at the surface of highly germanium-doped gallium oxide (2.5 x 10(20) cm(-3) nominal doping level) grown by molecular beam epitaxy. We probed the dopant concentration as a function of depth by hard x-ray photoelectron spectroscopy and standard laboratory photoemission spectroscopy. We notably found that there is germanium segregation within the top 2 nm where its concentration is 3 times the nominal doping level. This increased dopant concentration leads to a threefold enhancement of surface conductivity. The results suggest a reliable method for delta doping for power electronics applications.

Traditionally, elemental Ga and Si have been used to supply Ga and Si, respectively, in molecular beam epitaxy (MBE) to grow Si-doped β-Ga2O3. In this work, we investigated the feasibility of enhancing the β-Ga2O3 growth rate by using a Ga-suboxide precursor in a plasma-assisted MBE. Additionally, Si doping of β-Ga2O3 using diluted disilane and Ga-suboxide as the Si and Ga precursors, respectively, was studied. The growth rate and film quality under different suboxide fluxes were inspected. We found that Si concentration has an inverse relationship with Ga2O flux due to atom competition. A room-temperature mobility of 115 cm2/V s was measured for an electron concentration of 1.2 × 1017 cm−3 on the sample grown using a Ga2O beam equivalent pressure of 1.1 × 10−7 Torr and a disilane flow rate of 0.006 sccm. Temperature-dependent Hall characterization was performed on this sample, revealing compensating acceptor and neutral impurity densities of 2.70 × 1015 and 8.23 × 1017 cm−3, respectively.

We demonstrate the junction-moderated dielectric relaxation method to measure the in-plane electron mobility in beta-Ga2O3 epitaxial layers. Unlike the Hall technique and channel mobility measurement in field-effect transistors, this method does not require the deposition of permanent metal contacts. Rather, it measures the bias voltage and frequency dependence of the equivalent capacitance of the Mercury/beta-Ga2O3/Mercury structure consisting of a Schottky contact, a quasi-neutral thin film semiconductor, and an Ohmic contact connected in series. The intrinsic dielectric relaxation of the bulk beta-Ga2O3 semiconductor typically occurs at similar to 1012 s-1, but when moderated by the Mercury/beta-Ga2O3 Schottky junction, it manifests itself as an inflection in the capacitance-frequency characteristics at a much lower frequency of similar to 106 s-1 within the range of most capacitance measuring instruments. Using carrier density and layer thickness determined from capacitance-voltage measurement, we extract the electron mobility of beta-Ga2O3 from the junction-moderated dielectric relaxation frequency.

Development of high-temperature compatible electronic devices is desired for several applications. Electronic devices made with wide bandgap and ultra-wide bandgap materials are suitable for these high temperature applications. In this article, we presented high temperature device data obtained from p-Ga203 field-effect transistors (FETs) and AIGaN/GaN high-electron mobility transistors (HEMTs) at temperatures up to 500 oC, We discussed device details, systematically analyzed the electrical data by considering insights obtained from materials characterization, and explained the variation in device parameters (such as transconductance, threshold voltage, contact resistance, gate leakage) with temperature and time.

Development of high-temperature compatible electronic devices is desired for several applications. Electronic devices made with wide bandgap and ultra-wide bandgap materials are suitable for these high temperature applications. In this article, we presented high temperature device data obtained from p-Ga203 field-effect transistors (FETs) and AIGaN/GaN high-electron mobility transistors (HEMTs) at temperatures up to 500 oC, We discussed device details, systematically analyzed the electrical data by considering insights obtained from materials characterization, and explained the variation in device parameters (such as transconductance, threshold voltage, contact resistance, gate leakage) with temperature and time.

We investigate the intermixing effects of doping profile and the carrier emission from deep levels in the capacitance-voltage measurement of the wide bandgap semiconductor material of 8-Ga2O3. Specifically, we find that the spatial non-uniformity of doping measured under practical conditions is substantially contributed by artifacts due to carrier emission from deep levels. We develop a procedure to measure the hysteresis in cyclic capacitance-voltage experiments in dark and at room temperature for probing of the deep levels contributing to the apparent doping profile. Analysis of this hysteresis in the dynamic electrostatic framework of a Schottky junction containing deep levels enables more accurate determination of the doping density and its spatial distribution, and simultaneously the extraction of energy, density, and capture cross-section of the deep levels.