β‐Ga 2 O 3 is an ultrawide‐bandgap semiconductor whose thermal stability makes it a potentially interesting candidate for high‐temperature thermoelectric applications. In this work, the experimental relation between the room‐temperature (RT) Hall electron density ( n ) and the RT Seebeck coefficient ( S ) for β‐Ga 2 O 3 ( n from 8 × 10 16 to 4 × 10 19 cm −3 ) is provided. The use of high‐structural‐quality 1) melt‐grown single‐crystal wafers and 2) Si‐doped homoepitaxial layers allows to minimize the effect of structural defects. The experimental Seebeck coefficients are located between the modeled S ( n ) relation for ionized impurity scattering and polar optical phonon scattering mechanism. The S ( n ) data reported in this work can serve as a “calibration curve” for the estimation of volume electron concentrations from simple Seebeck coefficient measurements. Combining our data with the measured electrical conductivity and thermal conductivity values from the literature, the thermoelectric power factor and figure of merit ZT are calculated. These results suggest the maximum of ZT to be found at electron concentrations above 4 × 10 19 cm −3 at RT.

The Schottky barriers of Ti, Mo, Co, Ni, Pd, and Au on (100) β-Ga2O3 substrates were analyzed using a combination of current-voltage (J-V), capacitance-voltage (C-V), and current-voltage-temperature (J-V-T) measurements. Near-ideal, average ideality factors for Ti, Mo, Co, and Ni were 1.05–1.15, whereas higher ideality factors (∼1.3) were observed for Pd and Au contacts. Barrier heights ranging from 0.60 to 1.20 eV were calculated from J-V measurements for the metals with low ideality factors. C-V measurements of all Schottky metals were conducted and yielded average barrier heights ranging from 0.78 to 1.98 eV. J-V-T measurements of Ti and Co diodes yielded barrier heights of 0.81 and 1.35 eV, respectively. The results reveal a strong positive correlation between the calculated Schottky barrier heights and the metal work functions: the index of interface behavior, S = 0.70, 0.97, and 0.81 for J-V, C-V, and J-V-T data, respectively.

Due to its large bandgap (~4.6-4.8 eV), wide range of n-type doping, and single-crystal wafer availability, β-Ga2O3 holds promise for high-efficiency power devices. R&D of β-Ga2O3 has been conducted in Japan for more than a decade; however, funded research programs on this semiconductor material began in the U.S. only five years ago, highlighting its early stage of development. Device development of a semiconductor technology requires the ability to produce suitable ohmic and rectifying (Schottky) contacts. Whereas ohmic and Schottky contacts have been demonstrated on n-type β-Ga2O3, certain characteristics of contacts to β-Ga2O3 differ from contacts to other wide bandgap semiconductors. For example, we found that morphology was problem with many ohmic contact metals, and it was concluded that metal work function is not a dominant predictive factor for forming an ohmic contact to β-Ga2O3 [1]. Furthermore, thermal instability of the standard ohmic contact metallization (Ti/Au) is a concern for operation of devices at elevated temperatures. Schottky contacts to β-Ga2O3 have shown dependence on the orientation, growth method, and choice of metal. For example, on (-201) β-Ga2O3 Schottky barrier heights calculated from I-V measurements were typically between 0.9 and 1.3 eV and displayed little dependence on the metal work function [2]. Our preliminary measurements of selected metals on (100) β-Ga2O3 substrates yielded lower ideality factors and a correlation between barrier height and metal workfunction. The results for Schottky contacts on the (100) surface show some similarities to those previously reported on the (010) surface. In this presentation specific examples of ohmic and Schottky contact metallizations to β-Ga2O3 will provide a platform for more detailed discussion. [1] Y. Yao, R.F. Davis, and L.M. Porter, J. Electron. Mater. 46(4), 2053-2060 (2016). [2] Y. Yao, R. Gangireddy, J. Kim, K. Das, R.F. Davis, and L.M. Porter, J. Vac. Sci. Technol. B 35(3), 03D113.1-7 (2017).

We measure the low-frequency noise in epitaxial β-Ga2O3 grown by MOVPE. Both 1/f and generation-recombination noise components are well resolved. The Hooge parameters characterizing the 1/f noise are 3 × 10–4 at room temperature and 2 × 10–5 at temperatures near 200 K. Mid bandgap trap states result in generation-recombination noise that is analyzed using temperature dependent low-frequency deep-level noise spectroscopy. Trap levels with energies of 165, 127, and 37 meV below the conduction band minimum are characterized in terms of density and activation energy.

The temperature dependence of the Seebeck coefficient of homoepitaxial metal organic vapor phase grown, silicon doped β-Ga2O3 thin films was measured relative to aluminum. For room temperature, we found the relative Seebeck coefficient of Sβ-Ga2O3-Al=(−300±20) µV/K. At high bath temperatures T > 240 K, the scattering is determined by electron-phonon-interaction. At lower bath temperatures between T = 100 K and T = 300 K, an increase in the magnitude of the Seebeck coefficient is explained in the frame of Stratton’s formula. The influence of different scattering mechanisms on the magnitude of the Seebeck coefficient is discussed and compared with Hall measurement results.

We report the first demonstration of self-aligned gate (SAG) beta-Ga2O3 metal-oxide-semiconductor field-effect transistors (MOSFETs) as a path toward eliminating source access resistance for low-loss power applications. The SAG process is implemented with a subtractively defined and etched refractory metal, such as Tungsten, combined with ion-implantation. We report experimental and modeled DC performance of a representative SAG device that achieved a maximum transconductance of 35 mS mm(-1) and an on-resistance of similar to 30 Omega mm with a 2.5 mu m gate length. These results highlight the advantage of implant technology for SAG beta-Ga2O3 MOSFETs enabling future power switching and RF devices with low parasitic resistance.

Lateral beta-Ga2O3 MOSFET for power switching applications with a 1.8 kV breakdown voltage and a record power figure of merit of 155 MW/cm(2) are demonstrated. Sub-mu m gate length combined with gate recess was used to achieve low ON-state resistances with reasonable threshold voltages above -24 V. The combination of compensation-doped high-quality crystals, implantation-based inter-device isolation, and SiNx-passivation yielded in consistently high average breakdown field strengths of 1.8-2.2 MV/cm for gate-drain spacings between 2 and 10 mu m. These values outperform the results of more established wide-bandgap device technologies, such as SiC or GaN, and the major Ga2O3 material promise-a higher breakdown strength-is well demonstrated.

The influence of substrate miscut on the surface morphology and Hall mobility at optimized growth conditions of $\beta$ -Ga 2 O 3 layers was investigated. The results demonstrate that the surface morphology and the Hall mobility are impacted by the miscut angle of $\beta$ -Ga 2 O 3 substrates. For substrates with miscut angle of 6° and a growth rate of about 4 nm/min, a morphology with step bunches was observed and a Hall mobility of the layer of $115\pm 10$ cm2/(Vs) was obtained. Using substrates with 4° miscut, step-flow growth mode and an enhancement of the Hall mobility about $135\pm 10 \mathrm{cm}^{2}/(\mathrm{Vs})$ was achieved.

In this study, we report on the application of multiple energy nitrogen ion implantation for the electrical isolation of electronic devices on monoclinic β-Ga2O3. By the introduction of uniformly distributed midgap damage-related levels in the Ga2O3 crystal lattice, we are able to increase the sheet resistances by more than 9 orders of magnitude to ≥1013 Ω/sq which remains stable up to annealing temperatures of 600 °C carried out for 60 s under a nitrogen atmosphere. At higher annealing temperatures, the damage-related trap levels are being removed causing a significant drop of the sheet resistance down to 4 × 105 Ω/sq for annealing temperatures of 800 °C. This effect is preceded by a structural recovery of the implantation damages via the recrystallization of the crystal lattice at already 400 °C as verified by x-ray diffraction measurements. The extracted activation energies of the deep states controlling the high resistivity of Ga2O3 after implantation are in the range of 0.7 eV, showing a strong correlation with the annealing temperature dependence of the sheet resistance and thus supporting the theory of a damage-induced isolation mechanism.

We demonstrate a beta-Ga2O3 MOSFET with record-high transconductance (g(m)) of 21 mS/mm and extrinsic cutoff frequency (f(T)) and maximum oscillating frequency (f(max)) of 3.3 and 12.9 GHz, respectively, enabled by implementing a new highly doped ohmic cap layer with a sub-micron gate recess process. RF performance was further verified by CW Class-A power measurements with passive source and load tuning at 800 MHz, resulting in P-OUT, power gain, and power-added efficiency of 0.23 W/mm, 5.1 dB, and 6.3%, respectively. These preliminary results indicate potential for monolithic or heterogeneous integration of power switch and RF devices using beta-Ga2O3.

Monoclinic β-Ga2O3 (bandgap = 4.85eV) is a transparent semiconducting oxide with very promising perspectives especially in solar blind UV photodetectors and high power device applications. In particular, for high power switching β-Ga2O3 is predicted to outperform the leading technology based on SiC and GaN, due to a much higher calculated critical field strength. Another significant advantage of β-Ga2O3, especially in view of large-scale production, is that large bulk crystals can be grown from the melt by standard techniques, making available reasonably-priced native substrates for the growth of high quality homoepitaxial layers, which is essential for the fabrication of high performance power devices. This review article focuses on the growth of bulk and homoepitaxial β-Ga2O3, summarizing the research work carried out in this field and pointing out the strengths and the main challenges of different growth techniques. The impressive material development already achieved for both bulk and epitaxial β-Ga2O3 crystals has enabled the fabrication of prototype devices with very promising performances, demonstrating the outstanding potential of β-Ga2O3 for power electronics applications.

A Sn-doped (100) β-Ga2O3 epitaxial layer was grown via metal-organic vapor phase epitaxy onto a single-crystal, Mg-doped semi-insulating (100) β-Ga2O3 substrate. Ga2O3-based metal-oxide-semiconductor field-effect transistors with a 2-μm gate length (LG), 3.4-μm source-drain spacing (LSD), and 0.6-μm gate-drain spacing (LGD) were fabricated and characterized. Devices were observed to hold a gate-to-drain voltage of 230 V in the OFF-state. The gate-to-drain electric field corresponds to 3.8 MV/cm, which is the highest reported for any transistor and surpassing bulk GaN and SiC theoretical limits. Further performance projections are made based on layout, process, and material optimizations to be considered in future iterations.

Heteroepitaxial Ga2O3 was grown on c-plane sapphire by molecular beam epitaxy, pulsed-laser deposition, and metalorganic Chemical vapor deposition. Investigation by scanning transmission electron microscopy (STEM) revealed the presence of a three-monolayer-thick pseudomorphically grown layer of trigonal alpha-Ga2O3 at the interface between the c-plane sapphire substrate and the beta-Ga2O3 independent of the growth method. On top of this pseudomorphically grown layer, plastically relaxed monoclinic beta-Ga2O3 grew in the form of rotational domains. We rationalize the stable growth of the high-pressure trigonal alpha-phase of Ga2O3 in terms of the stabilization of the alpha-Ga2O3 phase by the latticemismatch-induced strain. (C) 2015 The Japan Society of Applied Physics

Epitaxial β‐Ga2O3 layers have been grown on β‐Ga2O3 (100) substrates using metal‐organic vapor phase epitaxy. Trimethylgallium and pure oxygen or water were used as precursors for gallium and oxygen, respectively. By using pure oxygen as oxidant, we obtained nano‐crystals in form of wires or agglomerates although the growth parameters were varied in wide range. With water as an oxidant, smooth homoepitaxial β‐Ga2O3 layers were obtained under suitable conditions. Based on thermodynamical considerations of the gas phase and published ab initio data on the catalytic action of the (100) surface of β‐Ga2O3 we discuss the adsorption and incorporation processes that promote epitaxial layer growth. The structural properties of the β‐Ga2O3 epitaxial layers were characterized by X‐ray diffraction pattern and high resolution transmission electron microscopy. As‐grown layers exhibited sharp peaks that were assigned to the monocline gallium oxide phase and odd reflections that could be assigned to stacking faults and twin boundaries, also confirmed by TEM. Shifts of the layer peak towards smaller 2θ values with respect to the Bragg reflection for the bulk peaks have been observed. After post growth thermal treatment in oxygen‐containing atmosphere the reflections of the layers do shift back to the position of the bulk β‐Ga2O3 peaks, which was attributed to significant reduction of lattice defects in the grown layers after thermal treatment.