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An MOS transistor fabricated on (001) β-Ga 2 O 3 exfoliated from a commercial (−201) β-Ga 2 O 3 substrate is reported. A maximum drain current of 11.1 mA/mm was measured, and a non-destructive breakdown was reached around 80 V in the off state. Threshold voltage of +2.9 V was extracted at 0.1 V drain bias, and peak transconductance of 0.18 mS/mm was measured at V DS = 1 V, corresponding to a field effect mobility of 0.17 cm 2 /Vs. Hall effect and electron spin resonance data suggested that electron conductivity was due primarily to O vacancy donors (V O + ) with an estimated density of 2. The single-crystal monoclinic (β) phase of Ga 2 O 3 is an advantageous material for high-power, high-temperature electronic device applications due to its high energy gap (4.8-4.9 eV) and high breakdown field (8 MV/cm), yielding a nearly ten-fold higher Baliga figure of merit than that of 4H-SiC (BFOM Ga 2 O 3 = 3444, BFOM 4H-SiC = 300).1 Commercially available β-Ga 2 O 3 substrates enable the epitaxial growth of low defect density epitaxial β-Ga 2 O 3 by a number of methods, including chemical vapor deposition, hydride vapor phase epitaxy (HVPE), and molecular beam epitaxy (MBE), among others.2-6 Schottky barrier diodes (SBDs) based on Ga 2 O 3 have exhibited very low turn on voltage and reverse leakage current, suggesting that unintentionally doped Ga 2 O 3 has extremely low generation/recombination rates and thus a high photoconductive gain.7 Advances in doping control have enabled exceptional early reports of metal-and metal-insulatorgated field effect transistors (MOSFETs). Wong et al. demonstrated a field-plated β-Ga 2 O 3 MOSFET with a breakdown voltage of over 750 V using a Si-implanted channel.8 Most recently, Green and coworkers have reported a Ga 2 O 3 MOSFET with a Sn-doped channel and a 0.6 μm gate-drain spacing to operate at 200 V drain bias, experimentally demonstrating gate-drain fields in excess of 3 MV/cm. 9This excellent progress has positioned Ga 2 O 3 as a viable candidate for next generation material for power applications. However, no demonstration of normally-off operation, a key requirement for fail-safe operation of power switches, has been achieved or proposed to-date.From a practical perspective, development of Ga 2 O 3 transistors has been limited by the availability of device-quality epitaxial films. For this reason, early reports have exploited the relatively large a-plane lattice constant of β-Ga 2 O 3 (1.2 nm) in order to mechanically exfoliate thin films from the (001) plane of a substrate using the scotch tape method to fabricate back-gated devices. 10,11 We employed a similar method to transfer a thin (∼300 nm) Ga 2 O 3 flake onto a SiO 2 /Si substrate, 12 and performed a standard top-side insulated-gate process to fabricate a three-terminal device. We also utilized a high-k HfO 2 gate dielectric process, as only SiO 2 and Al 2 O 3 have been reported to-date. 13,14Experimental A thin sliver of Ga 2 O 3 was cleaved along the (001) face of an on-axis (−201), non-intentionally n-type doped (∼3 ×...

Gallium oxide (β-Ga2O3) is an emerging semiconductor with relevant properties for power electronics, solar-blind photodetectors, and some sensor applications due to its ultra-wide bandgap and developing technology base for high quality, melt-based substrate growth and thick, low-doped homoepitaxial layers. Of critical importance for the commercialization of this potentially important material is understanding of doping mechanisms in the monoclinic lattice, where two types of Ga sites and three types of O sites have been identified. A critical literature review of doping and defects of the monoclinic β-phase of gallium oxide is provided in this work. Theoretical fundamentals of both donor and acceptor doping in Ga2O3 are reviewed. Advances in doping of epitaxial Ga2O3 with a focus on molecular beam epitaxy and ion implantation are critically examined. As doping is fundamentally related to defects, particularly in this material, a review of defect characterization by optical and electrical spectroscopic methods is provided as well. P-type doping, one of the fundamental challenges for Ga2O3, is discussed in terms of first-principles calculations and ion implantation of known acceptors such as Mg and N.

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Ga 2 O 3 has emerged as a promising material for next-generation power electronics. Beyond the most stable and studied β phase, metastable α-, ε-, and κ-Ga 2 O 3 have unique characteristics such as larger bandgaps, potential alloying for dopant and band engineering, and polarization, all of which can be leveraged in electronic device applications. Plasma-enhanced atomic layer deposition (PEALD) is a conformal, energy-enhanced synthesis method with many advantages including reduced growth temperatures, access to metastable phases, and improved crystallinity. In this study, PEALD was employed to deposit highly resistive, crystalline Ga 2 O 3 films from 265 to 475 °C on cplane sapphire substrates. Crystallinity, atypical at these low growth temperatures, was presumably due to the high flux of energetic ions to the growth surface independent of other growth parameters. Phase selectivity of β, α, ε(κ)-Ga 2 O 3 was demonstrated as a function of plasma gas composition, gas flow and pressure during the plasma pulse, as well as growth temperature. Factors such as atomic oxygen generation and the flux of energetic ions were found to have a significant impact on the ability to attain metastable phases. Optimum films of each phase were fully characterized to determine the feasibility of PEALD Ga 2 O 3 films. While both highquality, single-phase βand α-Ga 2 O 3 films were achieved, ε-Ga 2 O 3 films were not able to be completely isolated and even under the best conditions contained components of βand κ-Ga 2 O 3 as identified by transmission electron microscopy. Trends suggest that this could be a limitation of the underlying substrate or reactor configuration.