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Title: A numerical study of spatially controlled impurity-free vacancy enhanced GaAs/AlGaAs quantum well intermixing
Other Titles: Liang zi jing shen hun zhi yan jiu
Authors: Wong, Yue-hing (黃裕慶)
Department: Dept. of Electronic Engineering
Degree: Master of Philosophy
Issue Date: 1999
Publisher: Dept. of Electronic Engineering, City University of Hong Kong
Subjects: Optoelectronic devices
Quantum wells
Notes: CityU Call Number: TA1750.W66 1999
Includes bibliographical references (leaves 87-96)
Thesis (M.Phil.)--City University of Hong Kong, 1999
x, 96 leaves : ill. ; 30 cm.
Type: Thesis
Abstract: In recent years, great effort has been put in the research of impurity-free vacancy-enhanced intermixing using defect diffusion with a SiO2 cap as the vacancies source to enhance intermixing of the quantum well. As part of an effort to understand the Ga vacancy diffusion in SiO2 layer and the behaviour of Al-Ga intermixing in the quantum well, the distribution of the Ga vacancy is studied with the affect of the thermal stress taken into account. Presently the study involved numerical analysis of the spatially-controlled quantum well intermixing and the fabrication of quantum wire structures using vacancy enhanced intermixing of quantum wells. First, we considered different vacancy diffusion coefficient in the SiO2 layer and investigated the distribution of Al-Ga in the quantum well layer. By varying the coverage of the SrF2 mask, the degree of the energy shift in the quantum well was investigated. Secondly, we used a phenomenological equation to describe the effects of stress and strain on vacancy diffusion. In addition, we applied a new approach called "the stress gradient model" which assumed the atoms diffusion not only dependent on the concentration gradient, but also to the stress gradient. The quantum wire confinement potentials were studied as function of the trench widths and the separation &stances from the SiO2/GaAs interface. Results showed that Ga vacancy diffusion coefficient in the SiO2 layer was less than 10¯¹²cm²/s, the normalized vacancy concentration in the SiO2/GaAs interface was about 140 and the Al-Ga interdiffusion coefficient changed with time for the first 10s of annealing. The degree of the energy shift was proportional to the area of SiO2 in direct contact with the sample. However, we also found that the degree of the energy shift was proportional to the volume fraction of SiO2 in direct contact to the sample for some cases. The lateral confinement energy was not a monotonic function of the trench width. The behaviour of the energy reaches a maximum then it decreases with increasing width. When the separation distance was reduced, the confinement potential was changed from a non-square profile to a square-well profile. This study may lead to a better understanding of controlling the vacancy diffusion and the effect of thermal stress on the vacancy diffusion.
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