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 (AlxGa1-x)2O3 alloys and used to fabricate GaN/(AlxGa1-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.
- Materials Science and Engineering
- Doctor of Philosophy (PhD)