Date

2018

Document Type

Dissertation

Degree

Doctor of Philosophy

Department

Mechanical Engineering

First Adviser

Brandon A. Krick

Abstract

The optoelectronic properties of gallium nitride (GaN) has been extensively studied for decades, which has facilitated its application in many different areas, cementing it as one of the most important semiconductor materials in the world. However, in comparison to the study of its optoelectronic properties, there are few studies of its mechanical properties - especially the tribological performance. Knowing the tribological properties of GaN, such as friction and wear, is crucial for understanding its machinability, the implementation of GaN in MEMS, solar cells, and other devices, as well as the wear performance of these GaN-based devices when working under harsh environments. In our study, we reveal that GaN has an ultralow wear nature, and that its wear rate can approach that of diamond. We also discover that the wear rate of GaN is affected by its crystallographic orientation, humidity, and composition.For the crystallographic orientation dependence, we look into the physics by both experimental and computational methods. We demonstrate that both the friction coefficient and wear rate of GaN exhibits a 60° periodicity. We conclude that these periodic variations of wear rate and friction coefficient are the results of a periodic variation of the energy barrier.The moisture dependent wear mechanism of GaN has been investigated under dry, low humidity, and high humidity environments. The results show that the wear rate of GaN perfectly follows an increasing of the humidity which spans over two orders of magnitude when the testing environment switches from dry nitrogen to humid lab air. On the contrary, the friction coefficient gave a contrary response, i.e., the lowest friction coefficient was found under low humidity environment, dry nitrogen had the highest friction coefficient, and the humid environment had its friction in the middle. Various characterization techniques, including SEM/EDS, AFM and TEM were employed to interrogate the worn surfaces under each condition. Based on the results, we hypothesize that the wear under dry nitrogen environment is adhesive in nature whereas grooving abrasive wear dominates the wear behavior of GaN under a humid environment.The compositional study of GaN wear revealed that by alloying different elements into the GaN system, one can not only tune the bandgap, but also modify the wear rate. This finding can be useful for applications and design that require suitable electronic properties while keep the wear rate within an acceptable range.Furthermore, during the investigation of the GaN wear mechanism, we discovered that the tribological sliding can also alter the surface band bending of this material. Our results demonstrate that the environment, number of sliding cycles, and normal loads can effectively tune the surface band bending of GaN. This finding shows the capability of mechanical dynamic contact for surface electronic property modification, which can be used in various applications, such as gas sensing, photocatalysis, and photochemistry.Understanding of the wear mechanism of GaN as well as the shear-induced band bending on GaN can remarkably promote the applications of GaN in various fields other than the optoelectronic area. This also reinforces the important message that tribology is not only a discipline that focuses on investigation of protective coating and lubrication but also can be used in device design and fabrication.

Available for download on Wednesday, February 05, 2020

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