| In this thesis, I present the results of three surface science studies on (1) II-VI semiconductor surface preparation, (2) low-temperature chemically driven atomic layer epitaxy, and (3) investigations of surface chemistry for material deposition with chemical precursors.; First, in Chapter 3, I present results of using combination of atomic hydrogen and oxygen plasma to removal surface contaminants such as O, C, S, Cl, etc. from CdTe and Cd1−xZnxTe surfaces at room temperature. XPS, AES, and LEED measurements showed that this method is very effective in removing common surface contaminants via low temperature surface reactions. The surface damage cause by the reactions is much lower than high temperature thermal annealing and ion sputtering. In addition, in situ photoluminescence results shows about three orders of magnitude increase of near band-gap feature intensity after the sample went through H cleaning and annealing to a proper temperature.; Second, in Chapter 4, an in situ molecular-level study of material growth using a binary reaction sequence of hydride and metalorganic precursors is presented. The study used a model material system of CdS/ZnSe (100) and focused on the material chemistry of heteroepitaxy growth. In the growth process, dimethylcadmium and H2S precursors were sequentially dosed onto a c(2x2) ZnSe (100) substrate under high-vacuum conditions. At temperatures of ∼300K, saturated chernisorption of a Cd and a S monolayer occurred during each cycle of the binary reaction sequence. Characterization of the growth surface was accomplished in the growth chamber using AES, XPS and LEIS for probing surface chemical composition and LEED for determining surface order. These measurements showed layer-by-layer growth at a substrate temperature of ∼300 K, yielding an ordered stoichiometric US film. Strong variations in the composition of the grown surface layer were observed at different substrate temperatures; these variations were found to be related to the temperature dependence of the precursor reactions with the growth surfaces.; Finally, in Chapter 5, I will discuss the detail surface chemical investigation using TPD and NEXAFS for atomic layer epitaxy of CdS on ZnSe(100) surface. Both TPD and NEXAFS results show that either a monolayer of surface methyl or hydrogen termination exists after the growth surface is dosed with DMCd or H2S. Both precursors adsorb on the surface dissociatively. The terminating groups (CH3 and H) are thermally stable on the surface in a certain temperature range, thus passivate the growth surfaces preventing further uptake of materials after monolayer-coverage is reached. However, surface CH3 or H is reactive to the alternative precursor, so the other constituent material can be deposited subsequently. This methyl or hydrogen passivation provides a satisfactory self-limiting material deposition mechanism to allow the growth proceed in a layer-by-layer manner. TPD and NEXAFS results obtained after different reaction steps are presented separately in details. |
