【Domestic Papers】Sun Yat-Sen University --- Growth of Ga₂O₃ thin films on Si(111) substrates by metal-organic chemical vapor deposition

      Researchers from the Sun Yat-Sen University have published a dissertation titled "Growth of Ga₂O₃ thin films on Si(111) substrates by metal-organic chemical vapor deposition" in Vacuum.

Project support

      This work was supported by the Natural Science Foundation of China (Grant No.  62074167), the Research and Development Plan Project of Guangdong Province (No. 2021B0101300005).

Background

      Ga₂O₃ is an attractive semiconductor for applications in solar-blind ultra-violet detectors, heterojunction PN diodes, Schottky barrier diodes (SBDs), and field effect transistors (FETs) due to its ultra-wide bandgap (E_g = 4.4–5.3 eV), high breakdown electric field strength (E_br = 3.8–8 MV/cm) , and large dielectric constant (ε_r = 10–32) . The Ga₂O₃ family includes five polymorphs: α, β, γ, δ, and ε (also referred to as κ). Among these Ga₂O₃ polymorphs, β-Ga₂O₃ is the most thermodynamically stable phase, and it has been the focus of intensive investigation. However, its relatively high cost, low thermal conductivity, and anisotropy properties severely hinder the industrialization of β-Ga₂O₃-based applications. The other four polymorphs exhibit a tendency for thermostability in the order of ε > α > δ > γ, and they can ultimately be transformed into the β-phase structure as the annealing temperature increases. The α-Ga₂O₃ phase, which belongs to the trigonal corundum structure and possesses the largest bandgap (E_g = 5.3 eV), can be converted to β-Ga₂O₃ when the growth temperature exceeds 550 °C. The cubic bixbyite structure δ-Ga₂O₃, characterized by a lattice constant of 0.9225 nm, was successfully epitaxially grown on β-Fe₂O₃ buffer layers for the first time in 2023. Furthermore, γ-Ga₂O₃ is characterized by a defective cubic spinel structure with a lattice parameter of a = 0.822 nm and exhibits lower growth temperatures (<500 °C) .

Abstract

      Low-cost and large-scale Ga₂O₃ epitaxial films are crucial for commercial applications based on Ga₂O₃. Traditional homoepitaxial β-Ga₂O₃ faces significant challenges in terms of cost-effectiveness. Therefore, the realization of heteroepitaxy of ε-Ga₂O₃ thin films on commercially mature silicon substrate is important. In this study, we conducted systematic investigations into the growth of Ga₂O₃ films on Si(111) substrates to explore the growth mechanism. Based on a comprehensive analysis of X-ray diffraction (XRD), optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM), it is evident that the presence of amorphous SiO_x and Ga₂O₃, the existence of β-nuclei, and numerous inclined Ga₂O₃ islands significantly degrade the crystal quality and surface morphology. Furthermore, the disordered atomic arrangement and the lack of coalescence of the epilayer can primarily be attributed to the oxidation of the silicon face, as observed through high-resolution TEM measurements. This study elucidates the evolution process of Ga₂O₃ nuclei and offers profound insights into the growth mechanisms during the early growth stage on the silicon substrate, which are essential for obtaining high-quality silicon-based ε-Ga₂O₃ films.

Highlights

      • The growth investigation of Ga₂O₃ thin films on Si(111) substrates via MOCVD was conducted.

      • The presence of amorphous SiO_x on the silicon surface adversely affects the crystalline quality of Ga₂O₃.

      • The main growth issues of silicon-based Ga₂O₃ arise from the amorphous layer, mixed phases, and grain distortion.

Conclusions

      In summary, systematic investigations on the heteroepitaxy of silicon-based ε-Ga₂O₃ thin films were conducted using various methods including XRD, OM, SEM, TEM and EDS. Based on characterizations from XRD and OM, it is evident that optimizing the growth parameters, particularly the nucleation temperature (<600 °C), enhances the crystalline quality and coalescence degree of Ga₂O₃ films grown via MOCVD. Furthermore, the rough surface morphology and lack of coalescence observed in the Ga₂O₃ epilayers can be attributed to the presence of amorphous SiO_x, the coexistence of β-nuclei, ε-nuclei and amorphous Ga₂O₃, as well as numerous inclined Ga₂O₃ islands, as revealed by SEM and TEM investigations. HRTEM and EDS measurements underscore the importance of suppressing silicon surface oxidation and β-Ga₂O₃ nucleation to achieve a high-quality ε-Ga₂O₃ film on a silicon substrate. This study elucidates the origin of the disordered Ga₂O₃ layers directly grown on Si(111) substrates and provides guidance for solving this problem for the MOCVD growth of ε-Ga₂O₃ thin films.