Date of Award

Spring 2022

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Electrical Engineering (ENAS)

First Advisor

Han, Jung

Abstract

Gallium Nitride (GaN) has a great potential in high-power and high-frequency applications due to its wide energy gap and good transport property. GaN-based high electron mobility transistors (HEMTs) have been deployed for microwave power amplification for both commercial and military applications. So far all of the commercial GaN optoelectronic and electronic devices have planar junctions and heterostructures prepared by epitaxy, especially metalorganic chemical vapor deposition (MOCVD). In order to take the advantage of the merits of GaN material properties, more sophisticated device configurations such as current-aperture vertical electron transistors (CAVETs), junction field-effect transistors (JFETs), and superjunction (SJ) devices require the ability to form in-plane, lateral junctions by selective area doping (SAD). Unlike silicon (Si) or silicon carbide (SiC), in which lateral junctions can be achieved by ion-implantation and dopant diffusion processes, GaN cannot be easily doped by these two techniques. In this thesis, we explored an alternative approach of realizing SAD through selective-area etching (SAE) followed by selective-area growth (SAG). In this process, selective-area etching of GaN is considered to be the most challenging and critical step. The commonly-used inductively-coupled plasma (ICP) etching is known to produce damages and introduce impurities to the as-etched surface or interface. Once placed near the active region of the power devices, these imperfections are detrimental to the device performance. It is desired to find an alternative etching method for GaN to achieve a pristine interface for regrowth. We introduced tertiarybutyl-chloride (TBCl) into metal-organic chemical vapor phase deposition (MOCVD) system, which is the first time applied to GaN. In-situ etching and selective-area etching of GaN by TBCl were demonstrated. Electrical and material characterizations confirmed that TBCl etching is an intrinsically clean and low-damage etching process. More importantly, TBCl is shown to remove lattice defects. At the same time, we also observed limitations in TBCl etching in removing impurities. By minimizing impurities originated extrinsically, we believe TBCl etching is still a promising etching method for high-power applications. Non-planar selective area growth of GaN was also investigated towards the formation of lateral p-n junction devices. The growth evolution in this process was studied and explained by the kinetic Wulff diagram. With the help of atom probe tomography (APT), non-uniform Mg doping was observed in the selectively grown p-GaN region, and the local Mg doping concentration was found to vary inversely with the local growth rate. In addition, scanning spreading resistance microscope (SSRM) revealed a high concentration of silicon and oxygen in regrown regions, mainly originated from the SiO2 mask. To find an intrinsically-clean masking material for the selective-area growth of GaN, low-temperature AlN mask was employed and demonstrated to provide great selectivity. In addition to using TBCl for in-situ etching, we also demonstrated that co-flowing TBCl during the MOCVD growth of GaN could effectively suppress parasitic reaction and enabled a very high growth rate of GaN. This study was motivated by the need for producing thick drift layers (> 50 m) in vertical power devices cost-effectively. In comparison, the growth rate of conventional MOCVD growth of GaN is subject to severe gas-phase reaction with a maximum rate of ~ 8 m/hr.

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