Date of Award

8-1-2023

Language

English

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

College/School/Department

Department of Nanoscale Science and Engineering

Dissertation/Thesis Chair

Fatemeh Shahedipour-Sandvik

Committee Members

Travis Anderson, Michael Reshchikov, Carl Ventrice, Mengbing Huang

Keywords

diffusion, gallium nitride, III-Nitride, ion implantation, microwave annealing, photoluminescence

Subject Categories

Nanoscience and Nanotechnology

Abstract

The material properties of the (Al)GaN system, including its high breakdown electric field and high electron mobility, make it a powerful candidate for next-generation high-power and high-frequency electronic devices. The success of this technology is dependent on the ability to form selective-area doped (SAD) p-type regions, which ideally may be accomplished by a combination of straightforward, low-cost ion implantation with Mg, the prevailing p-type dopant, and post-implant annealing. Obtaining high-conductivity p-type GaN following Mg-implantation remains a significant challenge and an area of ongoing research interest. Ion implantation induces lattice damage and creates charged point defects, including the donor-like VN, which compensate activated Mg and requires high temperatures (1300–1500 °C) to remove or agglomerate into clusters. However, GaN is unstable at these temperatures under near-atmospheric N2 overpressure, potentially undergoing catastrophic degradation. As a result, specialized annealing techniques that either heat very rapidly or use ultra-high pressure conditions are required to obtain high-conductivity p-type material. We subjected implanted GaN to microwave using a gyrotron source to investigate the process of defect creation and annihilation, and dopant redistribution in response to ultra high-temperature annealing of 1350–1450 °C. The optical signature indicating the presence of large numbers of VN in GaN is the GL2 band observed in photoluminescence, and the presence of an intense GL2 band can suggest that the material under study is highly resistive. The origin of this band after ion implantation and nonequilibrium annealing is shown to originate not from degradation induced by the anneal process, but primarily from the ion implant process. By annealing at high temperature (~1350 °C), the relative intensity of the GL2 is significantly suppressed, and the population of compensating defects reduced sufficiently to obtain p-type GaN. We obtain a further reduction in the concentration of acceptor-compensating VN is found to result from co-implantation N with Mg. N co-implant, when followed by high-temperature annealing, can enhance p-type conductivity by providing an above-stoichiometric source of implant-region N to occupy VN sites during the annealing process. The CN-related YL photoluminescence band is similar in shape to the GL2 and is ubiquitous in GaN grown by metalorganic chemical vapor deposition (MOCVD). While generally observed to be absent in implant-annealed MOCVD GaN, we find that the combination of Mg+N co-implantation and gyrotron microwave annealing suppresses the GL2 to such an extent that the YL becomes visible. This approach to removal of VN accompanies an increase in the free hole concentration by a factor 102 relative to a Mg-only implant. The temperatures required for efficient p-type activation of implanted GaN also promote mobility of the implanted species. These species are made even more mobile by the presence of high point defect concentrations, visible as distortion of the GaN lattice through x-ray diffractometry. In power electronics with carefully designed selective-area doped (SAD) regions, deformation of these regions by implanted dopant out-diffusion during the activation-anneal process can severely impact device performance. While damage is largely removed at 1000 °C, significant Mg diffusion and attainment of p-GaN do not occur at annealing temperatures <1300 °C. We induce suppression of the defect-mediated diffusion pathway by performing a 1000 °C anneal prior to ultra-high-temperature gyrotron microwave annealing is found to suppress Mg diffusion rate by a factor of 10, greatly improving the fidelity of the original implant profile, opening a pathway to well-defined doped regions of high p-type conductivity.

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