Stabilization and Characterization of Epitaxial Ga2O3 Polymorphs Grown by Chemical Vapor Deposition

 Ga₂O₃ is an emerging ultra-wide-bandgap semiconductor material for applications in high-power electronic devices. It is characterized by four commonly accepted polymorphs: α (trigonal), β (monoclinic), γ (cubic-defective spinel), and κ(ε) (orthorhombic). Only β-Ga₂O₃ is thermodynamically stable to its melting point of ~2000 K. However, the monoclinic symmetry of this polymorph results in anisotropic optical, electronic, and thermal properties. As such, investigations of the metastable polymorphs have been recently conducted, including the research detailed in this dissertation, due to their unique physical properties, crystal structures, and symmetries. The overarching objective of my thesis is the growth and characterization of each Ga₂O₃ polymorph via metal-organic chemical vapor deposition (MOCVD) and halide vapor phase epitaxy (HVPE) by controlling select growth parameters including temperature, growth rate, and substrate material. The thermal stability of each of the metastable polymorphs to phase transformation to β-Ga₂O₃ was also investigated via in-situ high temperature X-ray-based annealing experiments. A suite of characterization tools was used to investigate the crystallinity of the deposited films, the epitaxial relationships between the films and the substrates, as well as the defects present inside the films. 

Nominally phase pure (001) oriented κ(ε)-Ga₂O₃ and (100) oriented γ-Ga₂O₃ were grown on c-plane sapphire and (100) MgAl₂O₄ via MOCVD at temperatures of 530 ℃ and 470 ℃, respectively. Higher growth temperatures resulted in the formation of (-201) oriented β-Ga₂O₃ as well as (100) oriented β-Ga₂O₃. Grown of these phases via HVPE at 650 ℃ resulted in enhanced phase purity and crystallinity as indicated by the absence of β peak shoulders and narrower full-width-at-half-maxima (FWHM) of the rocking curves compared to the films grown by MOCVD. Additionally, (11-20) and (10-10) oriented α-Ga₂O₃ were grown on a- and m- plane sapphire substrates via HVPE during the same growth run mentioned above, which indicated the advantages of HVPE in growing metastable Ga₂O₃ polymorphs. High temperature X-ray diffraction (HT-XRD) performed in air, N₂ and vacuum indicated that α-Ga₂O₃, κ(ε)-Ga₂O₃ and γ-Ga₂O₃ films exhibit thermal stability within the range of ~490 ℃-525 ℃, ~818 ℃-825 ℃, and ~490 ℃-575 ℃, respectively, before converting to β-Ga₂O₃. 

The results of my research provide valuable insights regarding the requirements necessary to achieve the epitaxial stabilization of each of the phase pure Ga₂O₃ polymorphs and their maximum application temperatures for future device fabrication and employment.