RESEARCH GROUP: Brian Thoms,  Yong Yang, Joon-Sang Lee

RESEARCH

The research goal of this program is to obtain a fundamental understanding of the physics and chemistry of wide bandgap semiconductor surfaces. Wide bandgap semiconductors (WBS) offer many potential electronics and optics applications. For example, the use of WBS in solar-blind detectors, blue lasers and LEDs, and optical windows and coatings are among the motivations for the recent work in this area. Besides their large bandgap, these materials often possess other useful properties such as hardness, corrosion resistance, high thermal conductivity, negative electron affinity, and high carrier mobilities which make them useful in applications such as high power, high frequency devices, field emitters, heat sinks, and protective coatings. Ironically, it is the extreme properties of these materials which also make their use in technological applications challenging.

Currently, our research is focussed on the surfaces of gallium nitride. The first objective of our GaN research is to use surface science techniques to determine the composition, structure, electronic properties, and chemical environment which exists under growth or processing conditions. This requires an understanding of the structure of both bare and hydrogenated GaN(0001) as well as determinations of the mechanisms and kinetics of processes which add or remove hydrogen at the surface. The second objective of this research is to understand the reactivity of the GaN surface. In particular, we seek to understand the reactions of species important to growth and processing. We also seek to understand the role in these reactions of various important parameters, such as temperature, surface coverage, and presence of surface hydrogen. Some of the adsorbates of interest are nitrogen sources such as ammonia and atomic nitrogen, hydrocarbons molecules and radicals generated by growth or doping precursors, dopants such as Mg and Si, and etchants such as Cl and F.

LABORATORY

Experiments on structure, bonding, chemical reactivity, and electronic properties of wide bandgap semiconductor surfaces are conducted in the Department's surface science laboratory. The laboratory is equipped with an ultra-high vacuum surface science apparatus with capabilities for sample preparation and surface modification as well as multiple surface sensitive techniques. The ultra-high vacuum system is comprised of three interconnected vacuum chambers in which samples can be transferred without exposure to the ambient environment. In each chamber, the sample can be positioned, heated to 1300 K, or cooled to near liquid nitrogen temperatures. One of these chambers is dedicated to sample preparation (surface cleaning) by heating in a controlled environment, ion bombardment, or exposure to a flux of excited species (evaporated metal or hydrogen atoms). The remaining two chambers are used for surface characterization using various techniques including temperature programmed desorption (TPD), low energy electron diffraction (LEED), Auger electron spectroscopy (AES), secondary electron emission (SEE), energy loss spectroscopy (ELS), and high resolution electron energy loss spectroscopy (HREELS). LEED images are photographed off of a fluorescent screen using a digital camera. All other data are acquired under computer control using GPIB, CAMAC, or serial interfaces to microcomputers.

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