Authors: Ming‐Hsun Lee
Source: Deep Blue (University of Michigan) (article, 2021-01-01)
Field: Materials Science — Electronic, Optical and Magnetic Materials

Investigation of Ohmic Contact Interfaces and Crystalline Defects for Wide-Bandgap Beta-Phase Gallium Oxide and its Alloys Over 10 percent of the electricity generated today in the U.S., worth more than $40 billion, is lost due to inefficient power conversion using contemporary Si-based modules. To address this problem, wide bandgap (WBG) semiconductor devices have been developed for efficient high-voltage power control and management. Beta-phase gallium oxide (Ga2O3) features an ultra-WBG of approximately 4.8 eV, a high breakdown strength, and low-cost substrates, making it an ideal candidate for next-generation power electronic applications. However, to deliver the full potential of Ga2O3, several essential materials issues need close examination. In particular, the stability of electrical contacts, the nature and presence of defects in the crystalline solid, and doping for alloyed systems are of key importance. In this thesis, we aim to investigate the ohmic contact interfaces and crystalline defects for beta-phase Ga2O3 and Al-alloyed Ga2O3 (AlxGa2-xO3) using a process-microstructure-property framework. We first examined the interfacial reaction of the (010) beta-phase Ga2O3 – Ti/Au ohmic contact, its kinetic evolution, and quantitatively assessed its electrical performance. We found that a Ti-TiOx layer formed in situ acts as a diffusion barrier to facilitate a low-resistance ohmic contact that is stable upon accelerated aging for more than 100 hours at 300 degrees Celsius. The dominant charge transport mechanisms across the metal-semiconductor junctions are analyzed and are found to be modulated by the ion implantation doping process. We further assessed the impact of crystalline anisotropy of gallium oxide on ohmic contacts. We demonstrated a reduction in Ti/Au contact resistance of approximately two orders of magnitude for (100)-oriented beta-phase Ga2O3 compared with (010)-oriented junctions. Microscopy revealed that the interfacial reactions vary with substrate orientation: the superior electrical characteristics of the (100) Ga2O3 contact are attributed to the formation of a thinner Ti-TiOx interfacial layer. In sum, we found that the rapid interfacial reaction between Ti/Au metal and gallium oxide is responsible for stable ohmic contact formation and that substrate orientation can drastically influence electrical performance. Shifting focus to the bulk substrate, we investigated intrinsic defects in beta-phase Ga2O3. Using high spatial and temporal resolution electron microscopy, the diffusion of intrinsic point defects was directly observed in real-time. By comparing experimental images with image simulation, statistical analysis, and ab initio calculations, we identified the divacancy defect structures and their ~0.8 eV migration energy barriers in the crystal. This work combines atomistic diffusion models with direct imaging, providing new insights on the energy landscape of point defect migration in oxide semiconductors. To enable high-performance Ga2O3-heterostructure-based devices, the alloyed system of AlxGa2-xO3, with its wider bandgap, is promising. Here, we studied doping, defects, and ohmic contact formation for this Al-alloyed semiconductor. We started by examining MOCVD-grown in situ doped beta-phase AlxGa2-xO3 and found the presence of various extended defects which degrade the crystalline quality. To resolve this doping challenge, we developed an ex situ doping technique for AlxGa2-xO3 via Si ion implantation followed by post-implant anneal. Correlated with the improved electrical performance, implant-induced gamma-phase-like structural defects are found to be annihilated during the anneal for lattice recovery. We further demonstrated ohmic contact formation on heavily implanted doped Al0.4Ga1.6O3 using a Ti/Au bilayer metal stack, achieving a ~2E-3 Ωcm2 low-resistance contact. This thesis provides essential knowledge to enable development of the next generation of power electronics. Gallium oxide ; Ohmic contact ; Materials science ; Gallium ; Oxide ; BETA (programming language) ; Phase (matter) ; Band gap ; Wide-bandgap semiconductor ; Optoelectronics ; Nanotechnology ; Metallurgy ; Chemistry ; Computer science ; Layer (electronics) Ga2O3 and related materials ; GaN-based semiconductor devices and materials ; ZnO doping and properties