Selective area growth of gallium nitride y plasma-assisted molecular beam epitaxy or wide and ultra-wide bandgap power device applications

Landi, Matthew M

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https://hdl.handle.net/2142/127360 Copy

Description

Title

Selective area growth of gallium nitride y plasma-assisted molecular beam epitaxy or wide and ultra-wide bandgap power device applications

Author(s)

Landi, Matthew M

Issue Date

2024-11-26

Director of Research (if dissertation) or Advisor (if thesis)

Kim, Kyekyoon

Doctoral Committee Chair(s)

Kim, Kyekyoon

Committee Member(s)

• Bayram, Can
• Dallesasse, John
• Lee, Minjoo
• Sardela, Mauro

Department of Study

Electrical & Computer Eng

Discipline

Electrical & Computer Engr

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

GaN, PAMBE, Epitaxy, Characterization, Ga2O3, Diffraction, Spectroscopy, AFM

Abstract

GaN is a wide-bandgap semiconducting material which possesses notable electronic and chemical properties making it a versatile option for high-mobility transistors and high-efficiency power diodes. Use of Plasma-Assisted Molecular Beam Epitaxy (PAMBE) for epitaxial growth of GaN enables high-quality device fabrication owing to the ultra-high vacuum growth chamber free of contaminants, and high-purity elemental source materials. Nitrogen plasma is used over traditional ammonia, giving the name plasma-assisted, to further remove unnecessary contamination. For this reason, PAMBE facilitates high activation efficiency for conventionally difficult p-type growth, and low background carrier densities in undoped films. Selective area growth of Gallium Nitride films is investigated here-in as an alternative to traditional vertical epitaxial processing routes. Crystal damage from conventional processing (i.e. inductively coupled plasma reactive ion etching, and ion-implantation) currently bottle-necks GaN microelectronics performance, resulting in reverse leakage current and contact resistivity. To date, selective area growth of GaN by MOCVD results in impurity incorporation from the mask, and variable growth modes due to flux variation along the perimeter of the growth window. By MBE selective area growth, accumulation of poly-crystalline GaN prevents the generation of thick drift regions required for power devices, and is thus only suitable for thin contact regions. To enable the growth of thick drift regions, silicon-nitride shadowed, selective area growth, or SNS-SAG, is developed as a methodology which is compatible with PAMBE GaN growth and prevents the intersection of poly-GaN with the device area, thus obviating conventional processing. First-generation SNS-SAG masks were observed to suffer from tensile channeling failure. Mask failure results in deleterious defects. Modulation of the mask deposition conditions was shown to control the residual strain and eliminate stress cracking within the GaN film and substrate. Optimization was enabled by iterative characterization by SEM and Raman spectroscopy. Impurity diffusion was investigated by XPS and CL, establishing that PAMBE SNS-SAG is a high-purity SAG technique with no observable impurity incorporation originating within the mask. AFM measurement reveals the step-flow growth mode at the mask edge, resulting in a beveled edge with steps of height n*d002. Retaining the step-flow growth mode through-out the window prevents the formation of semi-polar growth modes which are susceptible to impurity incorporation. The cleanliness is attributed in part to the growth mode, and in part to the low growth temperature in comparison to MOCVD SAG growth. The defect density along the sidewall was investigated by conductive-AFM. Utilizing a custom tilted stage, the sample is lofted such that the sidewall is perpendicular to the probe tip. In this configuration, highly conductive threading dislocation clusters are identified. The defect density along the sidewall is shown to be comparable to the high-quality substrate material, indicating SNS-SAG enables regrowth without introducing crystalline damage. Beveled edge termination structures were then fabricated to produce p-n diodes. TCAD Sentaurus was used to simulate the device geometry to inform doping windows and bevel angle optimization. The bevel structures were fabricated by SNS-SAG with a single mask and growth step. The implementation of a UID inclination layer enabled by SNS-SAG improved device performance over traditional bevel design. Device performance is currently limited by the nucleation of threading dislocation clusters which lead to leakage current through the bulk of the device area. GaN deposition on Ga2O3 facilitated by plasma nitridation, was explored for ultra-wide bandgap heterojunction diodes for next-generation power systems applications. Analysis of the nitridation kinetics under various conditions suggest rapid nucleation dominates the surface phase transformation at high temperature (T approaching 700 ◦C), while slow grain growth is dominant at low temperatures (T approaching 500 ◦C). This is leveraged to optimize the nitridation process, enabling the growth of high-quality p-type and UID GaN films. Various orientations of Ga2O3 are available, providing additional tools for optimization of GaN deposition on Ga2O3. Differences in nitridation and growth on (100), (001), and (201) are investigated by XPS, AFM, and XRD. Heterojunction devices were fabricated and tested, revealing the dominant leakage mechanisms. In conjuncture with the kinetic analysis, nitridation schemes are proposed and experimentally investigated to reduce leakage current, resulting in p-n diodes with record rectification ratio of 7.2x104.

Graduation Semester

2024-12

Type of Resource

Thesis

Handle URL

https://hdl.handle.net/2142/127360

Copyright and License Information

Copyright 2024 Matthew Landi

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