(Invited) MOCVD-Grown Ga2O3 Field Effect Transistors on Sapphire
Authors: Razeghi Manijeh , Park Ji-Hyeon , McClintock Ryan 
Published: February 2020

Gallium oxide (Ga2O3) materials have bandgaps in the range of ~4.8 eV, competitive mobility (~100 cm2/Vs), and high breakdown fields in the range of ~8-9 MV/cm. This breakdown is significantly higher than GaN or SiC, which are commonly used in today’s power transistors. As a consequence, Ga2O3
has an exceptionally large Baliga’s figure of merit[1] of 3214 vs only 846 for GaN or 317 for SiC.[2] This makes Ga2O3
a leading candidate to address the ultra-high power market, >1 kW.
The simplest approach is to grow Ga2O3 on a native Ga2O3 substrate; however, these substrates are still expensive and limited in wafer size. There is a desire to develop heteroepitaxy of Ga2O3 with sapphire being one of the most common substrate choices.[3] One the factors complicating this work has been the many polymorphs possible for Ga2O3: α-, β-, γ-, δ-, ε- and κ- phase have all been reported. Most of the heteroepitaxial devices reported to date have been based on a-Ga2O3 which undergoes a phase transition at higher temperatures that limits high temperature operation—a major issue for a power transistors.
In this work we report the metal organic chemical vapor deposition (MOCVD) growth of n-type and p-type Ga2O3:Si on c-plane sapphire substrates. We use a commercial Axitron 200/4 RF horizontal-flow reactor with trimethylgallium (TMGa) as the gallium precursor, high purity deionized water as the oxygen precursor, and SiH4 gas as the dopant. Both p- and n-type doping can be achived via MOCVD using silicon as the primary dopant, but by varryign the doping conditions alternately p- or n-type material can be achieved. MOCVD is very favorable or growing Ga2O3 thin films at low cost with high reproducibility for eventual industrial scale production. The use of low cost sapphire substrates also lends itself to a potential cost as existing industrial-scale MOCVD reacords currently used to grow GaN for solid-state lighting could easily be adapted to grow Ga2O3.
We have previously studied our MOCVD-grown films via transmission electron microscopy (TEM) and reported them to be κ-Ga2O3 (an analog of orthorhombic κ-Al2O3 with the space group of Pna21).[4] This phase is unique among MOCVD heteroepitaxial material in that high temperature annealing of these films show that these films are stable when annealed in a nitrogen and water vapor environment at temperatures up to 900 °C—only a slight broadening of the x-ray full-with a half-maximum (FWHM) occurs, with no phase transition. This is important for the operation of high power desity transistors.
These layers are then fabricated into various metal oxide semiconductor field effect transistors (MOSFET). We report devices with both p- and n-channels. We tested the best n-channel devices at high temperature found them to be thermally stable at temperatures from room temperature all the way up to 250 °C. The best MOSFETS exhibited complete pinch-off at a gate voltage of −10V with an on-off ratio of 108 at a drain source voltage (VDS) of 100 V. The three-terminal breakdown voltage (Vbr) at the off state is as high as 390 V at a gate voltage (VG) of −30 V. The drain current (ID) and on/off ratio slightly decrease at high operating temperatures; however, the devices maintains an on/off ratio of >106 even at 250 °C. Furthermore, the maximum ID was maintained even after thermal cycling the device 10 times from 250 °C to room temperature, and it was observed that the on/off ratio of 108 at RT was fully recovered. These thermally-stable MOSFETs fabricated from Ga2O3:Si grown by MOCVD on c-plane sapphire demonstrate the potential of Ga2O3 for next generation high power device applications.
[1] B. J. Baliga, "Power semiconductor device figure of merit for high-frequency applications," in IEEE Electron Device Letters, vol. 10, no. 10, pp. 455-457, Oct. 1989.
[2] Michael A. Mastro,a Akito Kuramata,b,c Jacob Calkins,d Jihyun Kim,e Fan Ren,f,∗ and S. J. Peartong, “Opportunities and Future Directions for Ga2O3,” ECS Journal of Solid State Science and Technology, 6 (5) P356-P359 (2017).
[3] Manijeh Razeghi, Ji-Hyeon Park, Ryan McClintock, Dimitris Pavlidis, Ferechteh H. Teherani, David J. Rogers, Brenden A. Magill, Giti A. Khodaparast, Yaobin Xu, Jinsong Wu, Vinayak P. Dravid, "A review of the growth, doping, and applications of Beta-Ga2O3 thin films," Proc. SPIE 10533, Oxide-based Materials and Devices IX, 105330R (14 March 2018)
[4] Y. Xu, J. Park, Z. Yao, C. Wolverton, M. Razeghi, J. Wu, and V. P. Dravid, “Strain-Induced Metastable Phase Stabilization in Ga2O3 Thin Films,” ACS Appl. Mater. Interfaces, vol. 11, p. acsami.8b17731, 2019.

Figure 1