Influence of Thermal Treatments on the Chemistry and Self-Assembly of Ge Nanoparticles on SiO 2 Surfaces
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Influence of Thermal Treatments on the Chemistry and Self-Assembly of Ge Nanoparticles on SiO2 Surfaces Scott K. Stanley, Shawn S. Coffee, and John G. Ekerdt Department of Chemical Engineering, The University of Texas at Austin Austin, Texas USA ABSTRACT GeH4 is thermally cracked over a hot filament depositing 0.7-15 ML Ge onto 2-7 nm SiO2/Si(100) at substrate temperatures of 300-970 K. Ge, GeHx, GeO, and GeO2 desorption is monitored through temperature programmed desorption in the temperature range 300-1000 K. Ge bonding changes are analyzed during annealing from 300-1000 K with X-ray photoelectron spectroscopy (XPS). Low temperature desorption features are attributed to GeO and GeH4. No GeO2 desorption is observed, but GeO2 decomposition to Ge through high temperature pathways is seen above 700 K. Germanium oxidization results from Ge etching of the oxide substrate, which is demonstrated through XPS. Ge nanoparticle formation on SiO2 is demonstrated using the agglomeration process. With these results, explanations for the difficulties of conventional chemical vapor deposition to produce Ge nanocrystals on SiO2 surfaces are proposed. INTRODUCTION Germanium and SiO2 interactions are important to a host of next generation electronic devices such as single electron transistors, optical waveguides, and flash memories. Specifically, germanium nanocrystals on insulator substrates, which have higher carrier mobility and lower band edge than Si nanocrystals, are now being extensively studied as the charge storage elements in flash memory devices [1-4] and Ge doped silica has applications in optical waveguides [5, 6]. The Ge/SiO2 material system is, however, not well characterized and questions remain concerning the difficulties of chemical vapor deposition (CVD) to produce Ge nanocrystals. CVD is a robust, efficient, and industry-compatible method to deposit clean semiconductor films and nanocrystals. The successful CVD of Si nanocrystals on oxide substrates is well known in the literature. Additionally, CVD of Ge onto Si surfaces has been studied thoroughly. However, CVD of pure Ge nanocrystals on clean SiO2 surfaces has not yet been demonstrated in the literature and other fabrication methods have been used, such as, agglomeration of Ge containing films [7-9], Ge ion implantation in silica [10-12], molecular beam epitaxy (MBE) [13], and annealing of Ge doped SiO2 films [14, 15]. Other recent approaches towards CVD of Ge onto SiO2 involve first creating Si nuclei [16, 17], or a Si rich surface layer [18] and subsequently depositing Ge onto the Si nuclei but not directly depositing Ge onto the SiO2 surface. In this work, different thermal treatments are used to investigate the interactions of Ge atoms on SiO2 surfaces in relation to Ge nanoparticle growth through agglomeration and chemical vapor deposition (CVD). EXPERIMENT Samples with 9 nm thermal SiO2 on Si(100) provided by Motorola, Inc. are cleaned, slightly etched, loaded into an ultra-high-vacuum (UHV) CVD system, all described previously [19], and
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