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β-Ga2O3 is of great interest for power electronic devices with efficiency beyond current generation Si, 4H-SiC, and GaN devices due to its large breakdown electric field of ∼8 MV/cm. However, taking advantage of this large field strength in power diodes requires device engineering to reduce leakage current that arises at high electric fields. In this work, we elucidate the current transport mechanisms of metal/TiO2/β-Ga2O3 diodes, showing that thermionic emission is an excellent descriptor of current in forward and reverse bias. It is shown that tunneling current is greatly suppressed, and consequently, that the diodes with the TiO2 interlayer can block orders of magnitude more current than Schottky barrier diodes with the same barrier heights. Finally, a 1200 V diode structure is designed based on the derived transport models, and calculated on- and off-state current characteristics closely align with those of state-of-the-art 4H-SiC commercial devices, indicating that this diode structure is ready to enable the realization of β-Ga2O3 power diodes. [ABSTRACT FROM AUTHOR]

A simple and inexpensive thermal oxidation process was performed to synthesize gallium oxide (Ga2O3) nanowires using Ag thin film as a catalyst at 800 °C and 1000 °C to understand the effect of the silver catalyst on the nanowire growth. The effect of doping and orientation of the substrates on the growth of Ga2O3 nanowires on single-crystal gallium arsenide (GaAs) wafers in atmosphere were investigated. A comprehensive study of the oxide film and nanowire growth was performed using various characterization techniques including XRD, SEM, EDS, focused ion beam (FIB), XPS and STEM. Based on the characterization results, we believe that Ag thin film produces Ag nanoparticles at high temperatures and enhances the reaction between oxygen and gallium, contributing to denser and longer Ga2O3 nanowires compared to those grown without silver catalyst. This process can be optimized for large-scale production of high-quality, dense, and long nanowires.

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. [ABSTRACT FROM AUTHOR]

A new methodology for the systematic study of thermoelectric generator (TEG) design and economic analysis is presented, with the objective of assessing the performance and financial feasibility of small-scale TEG installations, for 4 leading candidate thermoelectric materials. Temperature of a steam trap pipe surface were measured at the University of California Davis Pilot Brewery, and device performance was modeled using the finite-element modeling software ANSYS. The model integrated temperature-dependent material properties from leading candidate thermoelectric materials and experimental time-variant temperature data. Calculated power outputs were utilized in a net present value (NPV) framework to assess the financial feasibility and economic implications of small scale TEG installations, as well as to address the aspects of TEG research, design and implementation which have potential for rapid and substantive improvement. This model, along with case study results, provides a powerful platform for analyzing the performance of real-world systems and can be used to predict where further technological development on TEG materials and devices would be most effective. It is found that a BiSbTe based TEG generated the highest power output at the measured temperatures and consequently resulted in the highest NPV at the end of 25 years. Sensitivity analysis of the NPV revealed a strong dependence on the heat-exchanger cost, highlighting the importance of efficient heat transfer design. The zT necessary for a 7-year payback period as a function of the capital cost and hot-side temperature was also calculated for a SiGe based TEG.

A variety of nanostructured materials are produced through template-based synthesis methods, including zero-dimensional, one-dimensional, and two-dimensional structures. These span different forms such as nanoparticles, nanowires, nanotubes, nanoflakes, and nanosheets. Many physical characteristics of these materials such as the shape and size can be finely controlled through template selection and as a result, their properties as well. Reviewed here are several examples of these nanomaterials, with emphasis specifically on the templates and synthesis routes used to produce the final nanostructures. In the first section, the templates have been discussed while in the second section, their corresponding synthesis methods have been briefly reviewed, and lastly in the third section, applications of the materials themselves are highlighted. Some examples of the templates frequently encountered are organic structure directing agents, surfactants, polymers, carbon frameworks, colloidal sol-gels, inorganic frameworks, and nanoporous membranes. Synthesis methods that adopt these templates include emulsion-based routes and template-filling approaches, such as self-assembly, electrodeposition, electroless deposition, vapor deposition, and other methods including layer-by-layer and lithography. Template-based synthesized nanomaterials are frequently encountered in select fields such as solar energy, thermoelectric materials, catalysis, biomedical applications, and magnetowetting of surfaces.

This work demonstrates TiO2/β-Ga2O3 metal–dielectric–semiconductor (MDS) diodes with an average breakdown field beyond the material limits of SiC and GaN. These MDS diodes have lower conduction losses and higher breakdown voltage (Vbk) than the cofabricated Schottky barrier diodes (SBDs), simultaneously improving both on- and off-state parameters that are typically in competition with each other. With further optimized field management using p-NiO guard rings (GRs), the Ni/TiO2/β-Ga2O3 MDS diodes present a path to realistically utilize the high critical field of Ga2O3 without large forward conduction losses from a high-barrier junction. MDS diodes showed a lower Von (0.8 V) than the SBDs (1.1 V) from linear extrapolation of the current density-voltage (J-V) curve. The MDS diode had higher Vbk of 1190 V (3.0 MV/cm) compared to 685 V (2.3 MV/cm) for the SBD, and the MDS diode with the p-NiO guard ring saw further improvement with Vbk of 1776 V (3.7 MV/cm) compared to 826 V (2.5 MV/cm) for GR SBD. The BFOM (Vbk2/Ron,sp) of 518 MW/cm2 for the GR HJD is competitive with other literature results. A new figure of merit that includes the impact of turn on voltage is also proposed and demonstrated in this paper, which highlights how diodes perform in a practical high-power operation. This full paper is derived from the proceeding abstract of Willams et al. [IEEE Device Research Conference, Santa Barbara, CA, 25-28 June 2023 (IEEE, New York, 2023)]. [ABSTRACT FROM AUTHOR]

Low resistance non-alloyed ohmic contacts are realized by a metal-first process on homoepitaxial, heavily n+ doped (010) β-Ga2O3. The resulting contacts have a contact resistance (Rc) as low as 0.23 Ω-mm on an as-grown sample and exhibit nearly linear ohmic behavior even without a post-metallization anneal. The metal-first process was applied to form non-alloyed contacts on n+ (010) β-Ga2O3 grown by metal-organic chemical vapor deposition (MOCVD) as well as suboxide molecular beam epitaxy. Identical contacts fabricated on similar MOCVD samples by conventional liftoff processing exhibit highly rectifying Schottky behavior. Re-processing using the metal-first process after removal of the poor contacts by conventional methods does not improve the contacts; however, addition of a Ga-flux polishing step followed by re-processing using a metal-first process again results in low resistance, nearly linear ohmic contacts. The liftoff process, therefore, does not reliably render nearly linear ohmic behavior in non-alloyed contacts. Furthermore, no interface contamination was detected by x-ray photoelectron spectroscopy. This suggests that during the initial liftoff processing, a detrimental layer may form at the interface, likely modification of the Ga2O3 surface, that is not removable during the contact removal process but that can be removed by Ga-flux polishing. [ABSTRACT FROM AUTHOR]

We report the use of suboxide molecular-beam epitaxy (S-MBE) to grow β-Ga2O3 at a growth rate of ∼1 µm/h with control of the silicon doping concentration from 5 × 1016 to 1019 cm−3. In S-MBE, pre-oxidized gallium in the form of a molecular beam that is 99.98% Ga2O, i.e., gallium suboxide, is supplied. Directly supplying Ga2O to the growth surface bypasses the rate-limiting first step of the two-step reaction mechanism involved in the growth of β-Ga2O3 by conventional MBE. As a result, a growth rate of ∼1 µm/h is readily achieved at a relatively low growth temperature (Tsub ≈ 525 °C), resulting in films with high structural perfection and smooth surfaces (rms roughness of <2 nm on ∼1 µm thick films). Silicon-containing oxide sources (SiO and SiO2) producing an SiO suboxide molecular beam are used to dope the β-Ga2O3 layers. Temperature-dependent Hall effect measurements on a 1 µm thick film with a mobile carrier concentration of 2.7 × 1017 cm−3 reveal a room-temperature mobility of 124 cm2 V−1 s−1 that increases to 627 cm2 V−1 s−1 at 76 K; the silicon dopants are found to exhibit an activation energy of 27 meV. We also demonstrate working metal–semiconductor field-effect transistors made from these silicon-doped β-Ga2O3 films grown by S-MBE at growth rates of ∼1 µm/h. [ABSTRACT FROM AUTHOR]

We demonstrated 500 °C operation of field-effect transistors made using ultra-wide bandgap semiconductor β-Ga2O3. Metal–semiconductor field-effect transistors were fabricated using epitaxial conductive films grown on an insulating β-Ga2O3 substrate, TiW refractory metal gates, and Si-implanted source/drain contacts. Devices were characterized in DC mode at different temperatures up to 500 °C in vacuum. These variable-temperature measurements showed a reduction in gate modulation of the drain current due to an increase in gate leakage across the gate/semiconductor Schottky barrier. Devices exhibited a reduction in transconductance with increasing temperature; despite this, drain current increased with temperature due to a reduction in threshold voltage caused by the de-trapping of electrons from deep-level traps. Devices also showed negligible change in semiconductor epitaxy and source/drain contacts, hence demonstrated recovery to their room-temperature electrical properties after the devices were tested intermittently at different high temperatures in vacuum. The mechanism of gate leakage was also explored, which implicated the presence of different conduction mechanisms at different temperatures and gate electric fields. [ABSTRACT FROM AUTHOR]

We demonstrate a passivated MESFET fabricated on (010) Si-doped $\beta $ -Ga2O3 with breakdown over 2.4 kV without field plates, high Power Figure of Merit (PFOM), and high estimated Huang’s Material Figure of Merit (HMFOM), owing to low gate charge and high breakdown. MESFETs with 13 $\mu \text{m}$ source-drain spacing and 75 nm channel exhibited a current density of 61 mA/mm, peak transconductance of 27 mS/mm, and on-resistance of ${133}~\Omega \cdot \text {mm}$. The device showed a PFOM competitive with state-of-the-art $\beta $ -Ga2O3 devices and a record high estimated HMFOM for a $\beta $ -Ga2O3 device, competitive with commercial wide-band gap devices. This demonstrates high-performance $\beta $ -Ga2O3 devices as viable multi-kV high-voltage power switches. [ABSTRACT FROM AUTHOR]

The fabrication and transfer via spontaneous delamination of large-area, high-quality β-Ga2O3 nanoscale thin films with centimeter-scale dimensions is demonstrated via thermal oxidation of reusable GaAs substrates. The films are characterized by using X-ray diffraction, energy dispersive spectroscopy, Raman spectroscopy, and scanning electron microscopy. The film demonstrates good mechanical flexibility facilitating reliable transfer. The β-Ga2O3 film's optical band gap and Schottky barrier height with gold are 4.8 and 1.03 eV, respectively. Electrical and optical properties of the transferable β-Ga2O3 thin films exhibit a potential for application in solar-blind photodetectors. The scale of the β-Ga2O3 films transferred herein exceeds what the research community has reported to-date by more than four orders of magnitude. The macroscopic dimensions of the transferred films may offer a remedy for the low thermal conductance of β-Ga2O3 via transfer onto substrates with high thermal conductivity. [ABSTRACT FROM AUTHOR]

The self-assembly of heterogeneousmesoscale systems is mediatedby long-range interactions, including van der Waals forces. Diversemesoscale architectures, built of optically and morphologically anisotropicelements such as DNA, collagen, single-walled carbon nanotubes, andinorganic materials, require a tool to calculate the forces, torques,interaction energies, and Hamaker coefficients that govern assemblyin such systems. The mesoscale Lifshitz theory of van der Waals interactionscan accurately describe solvent and temperature effects, retardation,and optically and morphologically anisotropic materials for cylindricaland planar interaction geometries. The Gecko Hamakeropen-science software implementation of this theory enables newand sophisticated insights into the properties of important organic/inorganicsystems: interactions show an extended range of magnitudes and retardationrates, DNA interactions show an imprint of base pair composition,certain SWCNT interactions display retardation-dependent nonmonotonicity,and interactions are mapped across a range of material systems inorder to facilitate rational mesoscale design. [ABSTRACT FROM AUTHOR]

The interband optical properties of single-crystal berlinite AlPO4 have been investigated in the vacuum ultraviolet ( VUV) range using VUV spectroscopy and spectroscopic ellipsometry. The complex optical properties were directly determined from 0.8 to 45 eV. Band gap energies, index of refraction and complex dielectric functions, oscillator index sum rule, and energy loss functions were calculated through Kramers-Kronig transformation. Direct and indirect band gap energies of AlPO4 over the absorption coefficient range of 33-11 000 cm−1 are 8.06 and 7.89 eV, respectively. The index of refraction at 2 eV, nvis, is 1.51. The interband transition features at 10.4, 11.4, 14.2, 16.2, 17.3, 21, 22.5, 24.5, and 31 eV were indexed and correlated with the electronic structure of AlPO4. Strong similarities were observed between AlPO4 and its structural isomorph SiO2 in the exciton and interband transitions, resulting from the similarity of their constituent tetrahedra and the strong electron localization therein. The London dispersion spectrum for AlPO4 was calculated, and the Hamaker coefficients for AlPO4 with various interlayers were calculated using the Lifshitz method. These results elucidate the role of phosphate complex anions on the electronic structure and van der Waals forces in important organic and inorganic systems. [ABSTRACT FROM AUTHOR]

A simple and inexpensive thermal oxidation process was performed to synthesize gallium oxide (Ga2O3) nanowires using Ag thin film as a catalyst at 800 °C and 1000 °C to understand the effect of the silver catalyst on the nanowire growth. The effect of doping and orientation of the substrates on the growth of Ga2O3 nanowires on single-crystal gallium arsenide (GaAs) wafers in atmosphere were investigated. A comprehensive study of the oxide film and nanowire growth was performed using various characterization techniques including XRD, SEM, EDS, focused ion beam (FIB), XPS and STEM. Based on the characterization results, we believe that Ag thin film produces Ag nanoparticles at high temperatures and enhances the reaction between oxygen and gallium, contributing to denser and longer Ga2O3 nanowires compared to those grown without silver catalyst. This process can be optimized for large-scale production of high-quality, dense, and long nanowires. [ABSTRACT FROM AUTHOR]