Determine the Atomic Structure of a Surface with Mixed Structure Phases by Using Leed Patterson Function

This dissertation, "Determine the Atomic Structure of a Surface With Mixed Structure Phases by Using LEED Patterson Function" by Wai-kan, Tsang, 曾衛勤, was obtained from The University of Hong Kong (Pokfulam, Hong Kong) and is being sold pursuant to Creative Commons: Attribution 3.0 Hong Kong License. The content of this dissertation has not been altered in any way. We have altered the formatting in order to facilitate the ease of printing and reading of the dissertation. All rights not granted by the above license are retained by the author. Abstract: Abstract of thesis entitled DETERMINE THE ATOMIC STRUCTURE OF A SURFACE WITH MIXED STRUCTURE PHASES BY USING LEED PATTERSON FUNCTION Submitted by Tsang, Wai Kan for the degree of Master of Philosophy at The University of Hong Kong in November 2004 Low-Energy Electron Diffraction (LEED) is widely used to determine the atomic structure of a surface. However, for surfaces with mixed structure phases, LEED cannot provide direct evidence for the mixture surface model. It is also very difficult and time consuming to obtain promising result using model guessing. Instead, the recent discovered technique, the Patterson Function calculated from LEED I-V Curves at multiple incident angles, can serve as a tool for surface structure model construction. By comparing the Patterson Function from experimental LEED I-V Curves and that from calculated LEED I-V Curves, the surface model proposed can be fine-tuned. In this work, GaN grown on a clean (33)SiC surface was used to obtain experimental LEED I-V Curves. From its corresponding Patterson Function, Ga(top)/GaN surface cubic structure for the sample crystal were proposed by observing the spot positions on the Patterson Function Map. It was found that the Patterson Function calculated from theoretical LEED I-V Curves for pure Ga(top)/GaN surface structure shows little similarity with that calculatd from experimental LEED I-V Curves. And yet, the Patterson Function calculated from theoretical LEED I-V Curves for mixture surface structure of Ga(top)/GaN and GaN gives excellent similarity. The Patterson Functions calculated from the theoretical LEED I-V Curves at different incident angles were then investigated. It was found that the trend of ratio between relative spot intensity of the spot from Ga-Ga pairs and that from Ga-N pairs P in the experimental Patterson Function with the EXP increase in polar incident angles is much greater than its counterpart trend P in the SIM theoretical Patterson Function. This result suggests that the mixture surface model is not complete. We then proposed that, the disorder layer on top of Ga(top)/GaN surface is thicker than that on top of GaN surface. LEED I-V Curves for the modified model were recalculated so that the Patterson Functions were obtained. We demonstrated that, with the correct model proposed, P has similar value comparing with P and area ratio of two surface SIM EXP structure Ga(top)/GaN and GaN can be suggested. DOI: 10.5353/th_b3049711 Subjects: Surfaces (Physics) Atomic structure Low energy electron diffraction Patterson function