Phase Composition and Microstructure of κ-Ga2O3 Heteroepitaxial Films Grown by MOCVD

Abstract

Gallium oxide (Ga2o3.) is an ultra-wide bandgap semiconductor (4.85-5.3 eV) with emerging applications in power electronics because of its high thermal stability, large breakdown voltage and high dielectric constant. Ga, 03 occurs in the a, B, y and k(€) polymorphs having trigonal, monoclinic, cubic normal spinel and orthorhombic structures, respectively. B- “Ga, 03 is thermodynamically stable and produced in melt-grown substrates. Recently, k- -Ga,0. has accrued increased interest because it has demonstrated stability at temperatures 2 700 °c! and its large spontaneous polarization could be exploited in heterostructures” for high electron mobility transistors. In this study, metal organic chemical vapor deposition (MOCVD) was used to grow epitaxial Ga,0. films under different growth conditions including temperature, growth rate, and diluent gas flow rate. The growth experiments were conducted in a vertical, low-pressure, cold-wall MOCVD reactor using triethylgallium (TEGa) and 0, precursors and commercially available 0.15 off-axis (toward m-plane) c-plane sapphire. No was used as both the carrier gas for TEGa and the diluent gas. The chamber pressure was held at 20 Torr. The growth rate was controlled by the TEGa flow rate which ranged from 0.29 sccm to 2.1 sccm. The diluent gas flow rate was varied from 4 slm to 8 sim. The O flow rate was maintained at 500 sccm. The temperature was constant for each growth but varied from 470°C to 570 C. Phase composition and microstructural evolution of the films were investigated using different analytical tools including the X-ray diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM) and scanning transmission electron microscopy (STEM). XRD and SEM revealed that the top layer varied in phase composition between ~100% k-Ga,0, and ~100% B-Ga,0.; the surface microstructure ranged from poorly coalesced to completely coalesced grains as a function of growth temperature, growth rate, or diluent gas flow rate. In general, it was found that the k-phase tends to grow at lower temperatures and higher growth rates (higher TEGa flow rates). The growth of nominally single- phase k-Ga 90 within the top layer was observed in a temperature range between 500° C and 530° C. Below 470° C, only amorphous Ga 90,was obtained; above 570° Cc only the B-phase was deposited. HRTEM and STEM of a film grown at 530° C revealed the initial pseudomorphic growth of 3-4 monolayers of a-Ga,0., a 20-60 nm transition layer that contained both B- and y-Ga,0., and a top ~700 nm thick layer of phase-pure k-Ga,0,. H.Y. Playford, A.C. Hannon, E.R. Barney, and R.|. Walton, Chem. Eur. J. 19, 2803 (2013). R. Roy, V.G. Hill, and E.F. Osborn, J. Am. Chem. Soc. 74, 719 (1952). Y. Oshima, E.G. Villora, Y. Matsushita, S. Yamamoto, and K. Shimamura, J. Appl. Phys. 118, 085301 (2015). J. Lee, H. Kim, L. Gautam, K. He, X. Hu, V. Dravid, and M. Razeghi, Photonics 8, 17 (2021). P. Ranga, S.B. Cho, R. Mishra, and S. Krishnamoorthy, Appl. Phys. Express 13, 061009 (2020). On K. Jiang, J. Tang, M. J. Cabral, A. Park, L. Gu, R. F. Davis and L. M. Porter, J. Appl. Phys. (2022)