Date of Award

Summer 2023

Document Type

Open Access Dissertation


Electrical Engineering

First Advisor

Iftikhar Ahmad


Ultra-wide bandgap (UWBG) gallium oxide (Ga2O3), and aluminum gallium oxide (AlxGa1-x)2O3 materials with bandgap EG ≥ 4.8 eV are promising for heterostructure field effect transistors (HFETs) with high breakdown voltage and operating at high temperatures. Despite some initial breakthroughs in good quality β-Ga2O3 thin films growth on (001), (100), (010), and (201) β-Ga2O3 substrates, and promising results on β- (AlxGa1-x)2O3/β-Ga2O3 HFETs fabricated on (010) β-Ga2O3 substrates, heat dissipation in Ga2O3 materials and devices remain as a critical issue. Developing β-Ga2O3 heteroepitaxy on thermally conductive materials like AlN/sapphire template, bulk AlN or 4H-SiC or 6H-SiC or to a less extent sapphire substrates can improve heat dissipation where β-Ga2O3 grows in (201) direction. In the (2̅01) growth direction, the adatoms have limited diffusion length, thin films have many twins, and stacking faults, making it difficult to grow thick β-Ga2O3 due to high surface roughness. This work deals with the understanding of Ga2O3 materials and their defects properties in various growth environments through theoretical calculations, the development of a reduced roughness, over 0.5 µm thick β-Ga2O3 on sapphire as well as AlN for HFET channel layer and reporting the crucial surface properties of (AlxGa1-x)2O3 needed for ohmic/Schottky contacts, and dielectric deposition for HFETs.

A comprehensive study based on density functional theory (DFT), using the generalized gradient approximation (GGA): Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional, of point defects in corundum (α), monoclinic (β), and orthorhombic (ε) phases of Ga2O3 is presented. Defect formation energies, charge transition energy levels, and defect concentrations variation with temperature are listed/presented under both gallium-rich (Ga-rich) and oxygen-rich (O-rich) growth conditions.

The theoretical analysis gives us the understanding to develop a >0.5 µm thick β-Ga2O3 thin films using the metal-organic chemical vapor deposition (MOCVD) growth process on sapphire and AlN templates using silicon-oxygen bonding (SiOx) as a phase stabilizer. X-ray diffraction reveals a pure-monoclinic phase in the presence of SiOx. The creation SiOx with silane (SiH4) flow shows a significant change in β-Ga2O3 surface morphology in both AlN and sapphire where roughness reduces from 16.2 nm to 4.2 nm. AlN is expected to improve heat dissipation in HFETs. The reduction of surface roughness should increase the mobility of HFET devices. To realize the heterostructure for HFETs, (AlxGa1-x)2O3 films with aluminum composition ranging from x=0 to x= 36% were grown and analyzed for structural and surface properties. The Al composition increase resulted in a phase transition from a monoclinic to an amorphous structure, showing the limit for Al incorporation in β-(AlxGa1-x)2O3.

It is imperative to have information about the work function of the semiconductor layers to make contacts for the devices. We report the surface work function of (AlxGa1- x)2O3 decrease from 6.13 to 5.62 eV as aluminum composition increased from 0 to 36% using the scanning Kevin probe microscopy (SKPM). Direct measurements of epitaxial surface potential in (AlxGa1-x)2O3 thin films show downward band bending ranging from 1.19 to 0.65 eV. The density of surface states is found to be ~2×1012 to ~4×1013 cm-2 . These properties are required to select metals for ohmic/Schottky contacts, dielectric deposition, and understanding the passivation mechanisms in β-(AlxGa1-x)2O3/β-Ga2O3 HFETs.

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