Direct growth of 𝜷-Ga2O3 on sapphire substrate by mist chemical vapor deposition method for Schottky diode application

Abstract

Alpha and beta-phase gallium oxide (Ga2O3), has drawn significant attention in the field of high power semiconductor devices due to their ultra-wideband gap of 4.4 to 5.3 eV, high critical breakdown voltage, and higher Baliga figure of merit. Hot-wall mist chem-ical vapor deposition (mist-CVD) is a low-cost, solution-processed, low power con-sumption method often used to grow α phase rather than β-Ga2O3 on the c-plane sap-phire substrate. The lattice misfit between the c-sapphire and α-Ga2O3 is 3.5 %. C plane sapphire substrate is widely used as a substrate for the growth of high-quality hetero-epitaxy α-Ga2O3 using mist CVD method for commercial power device applications. Meanwhile, developing a very uniform, high-quality β phase on the sapphire substrate is challenging due to the considerable lattice mismatch between β-Ga2O3 and the sap-phire substrate of 6.6 %. This study aims to understand the temperature and mist flow profile during the growth stage using the COMSOL simulation tool finite element method and to validate with experimental studies. The growth of the α/β mixed-phase to β phases Ga2O3 epilayer on the sapphire substrate was performed by controlling the growth parameters to obtain uniform and smooth surface thin films. In the theoretical COMSOL analysis, it was shown that growth temperature was influenced by mist flow velocity and applying low mist velocity leads to a lower surface roughness thinfilm. In this work, we grow and characterize mixed and pure β phase on the sapphire substrate and discusse the thin-film crystallinity, optical and surface morphological properties by X-ray diffraction (XRD), atomic force microscopy (AFM), and high-resolution trans-mission electron microscopy (HR-TEM), respectively. The X-ray diffraction (XRD) analysis indicated that polycrystalline (α & β) mixed-phase Ga2O3 thin films formed on the α-Al2O3 substrate at 470 - 600 °C with high dislocation density >1010 cm−2. Above 600 °C temperature, pure monoclinic β-phase formes on the c plane (0001) orientation of the α-Al2O3 substrate. AFM results show thin-film surface roughness is 2-3 nm and uniform surface morphology at the grown temperature of 600 and 700 oC respectively. UV-Vis transmission spectrum showed Ga2O3 thin film transparency of 80 % with an optical bandgap between 4.8 to 5.2 eV. TEM analysis further confirmed the mixed-phase to pure β phase film is of ultrathin nature of 5 and 20 nm at growth temperatures of 600 and 700 oC respectively. X-ray photoelectron spectroscopy (XPS) confirmed the presence of the thin film's expected surface elements Ga and O. Finally, we fabricated mist CVD grown β-Ga2O3 Schottky Barrier Diode by following the photolithography process and IV measurement was obtained. The IV results revealed that a very small current μA, flowed through the material while applying forward bias voltage. The study results contribute to the understanding of the direct growth mechanism by controlling mist flow rate and physical properties of β-phase gallium oxide thin films, which can help in the development of efficient β gallium oxide Schottky bar-rier diode devices for various applications.