Investigation of Epitaxial Growth of Gallium-Oxide2 (Ga2O3）Polymorphs for High Efficiency Power Electronics by Chemical Vapor Deposition

The emerging ultra-wide bandgap semiconductor material of gallium oxide (Ga₂O₃) with a bandgap ~ 4.8 eV has attracted significant interest in terms of materials research as well as the design and fabrication of electronic devices for high-power applications. It occurs in the trigonal 𝛼 , monoclinic 𝛽 , cubic 𝛾 and orthorhombic 𝜅 polymorphs. The thermodynamic stable phase of β-Ga₂O₃ has been the primary focus for future power electronic devices. However, the other metastable phases also have unique properties. For example (1) 𝛼-Ga₂O₃ can be combined with Al₂O₃ to form (Al_xGa_1-x)₂O₃ alloys and used to fabricate GaN/(Al_xGa_1-x)₂O₃ heterostructures for high electron mobility transistors. The bandgap of these alloys can be also tuned from 5.1eV to 7.8 eV. (2) 𝜅-Ga₂O₃ is predicted to have ferroelectric properties for potential applications in ferroelectric field effect transistors and for devices applicable for neuromorphic computing. (3) 𝛾-Ga₂O₃ has been demonstrated theoretically to have potential for ferromagnetic and photocatalytic applications. The coexistence of the 𝛾 -phase with other Ga₂O₃ polymorphs during the growth of epitaxial thin films has been reported in many studies. 

The overarching scope of this thesis is the investigation of the growth of thin films of different metastable Ga₂O₃ polymorphs via metal-organic chemical vapor deposition (MOCVD) and the characterization of these films via a suite of analytical tools. I have investigated the phase and microstructural evolution of gallium oxide (Ga₂O₃) films grown on vicinal (0001) sapphire (α-Al₂O₃) substrates. Scanning/transmission electron microscopy (S/TEM) analysis of a film grown at 530°C revealed the initial pseudomorphic growth of 3-4 monolayers of α-Ga₂O₃. Additionally, a transition layer with a thickness of 20-60 nm was identified, consisting of both β- and γ-Ga₂O₃ phases. The top layer, approximately 700 nm thick, exhibited a textured structure and consisted exclusively of κ-Ga₂O₃. Further growths of Ga₂O₃ films, accompanied by X-ray diffraction (XRD) and scanning electron microscopy (SEM) investigations, demonstrated variations in the phase composition of the top layer. It ranged from around 100% κ-Ga₂O₃ to 100% β-Ga₂O₃. The surface microstructure displayed a range of characteristics, from poorly coalesced to completely coalesced grains, depending on the growth temperature, growth rate, or diluent gas flow rate. In general, the κ-phase was found to be favored at lower growth temperatures and higher triethylgallium (TEGa) flow rates (low VI/III ratios). The growth of predominantly single-phase κ-Ga₂O₃ within the top layer was observed within a narrow temperature range of 500°C to 530°C. Below 470°C, only amorphous Ga₂O₃ was obtained, while above 570°C, only the β-phase was deposited. 

Another crucial aspect of my research involved the electrical characterization of κ-Ga₂O₃ films to determine their potential piezoelectric/ferroelectric properties. To achieve this, a (111) Pt/(001) κ-Ga₂O₃/(111) Pt (metal/insulator/metal) device structure was fabricated on a platinized c-plane sapphire substrate using DC sputtering for the Pt metal layers and MOCVD for the κ-Ga₂O₃ epitaxial layer. Electrical analysis of the κ-Ga₂O₃ films was performed using various techniques, including positive-up-negative-down (PUND) characterization, polarization-electrical field (P-E) hysteresis loop characterization, and piezoresponse force microscopy (PFM) measurements. The 700 nm thick κ-Ga₂O₃ film exhibited a relative permittivity of ~ 18 and a loss tangent of 1% at 10 kHz, along with a breakdown field of ~ 1100 KV/cm. PFM measurements on the κ-Ga₂O₃ films indicated a piezoelectric response ranging between 2.9 pm/V and 3.0 pm/V, confirming the piezoelectric nature of κ-Ga₂O₃. However, no fully saturated P-E loop was observed in the P-E hysteresis loop characterization prior to the device breakdown. These results suggest that the higher leakage currents in the κ-Ga₂O₃ films hindered accurate polarization measurements. This investigation determined that κ-Ga₂O₃ exhibits piezoelectric properties with a moderate permittivity value, but no definitive evidence of ferroelectric switching was observed for electric fields up to 1100 kV/cm. 

To investigate the presence of 𝛾-phase inclusions observed in different epitaxial growths of 𝛽-Ga₂O₃ alloyed with Al and Mg, I employed atomic resolution scanning/transmission electron microscopy (S/TEM) with energy-dispersive X-ray (EDX) analysis. This allowed me to investigate the microstructure of phase pure 𝛽-Ga₂O₃ grown on (100) MgAl₂O₄ via metal-organic vapor phase epitaxy and the phase transformation of this 𝛽 -Ga₂O₃ to γ-Ga₂O₃ solid solutions via chemical interdiffusion of Mg and Al from the (100) MgAl₂O₄ substrate under high temperature thermal treatments from 800℃ to 1000℃ in ambient air. My observations revealed that the transformation from β-Ga₂O₃ to the γ-Ga₂O₃ solid solutions occurred at the interface between the film and the substrate, accompanied by the interdiffusion of Al and Mg from the substrate. I determined that a critical amount of ~ 4.6 at% of Al + Mg is required to trigger this phase transformation. It is worth noting that in the spinel structure, Al and Mg exhibit a preference for occupying octahedral sites, while Ga prefers tetrahedral sites. It is thus suggested that the elements, such as Al and Mg, that have an octahedral site preference in spinels stabilize the 𝛾 -phase in 𝛽 -Ga₂O₃. This indicates that the Al and Mg alloyed γ-Ga₂O₃ solid solutions may be a thermodynamically favored phase relative to pure 𝛽-Ga₂O₃.