Epitaxial growth of Si nanowires by a modified VLS method using molten Ga as growth assistant
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Epitaxial growth of Si nanowires by a modified VLS method using molten Ga as growth assistant Annika Gewalt1, Bodo Kalkofen1, Marco Lisker1, and Edmund P. Burte1 1 Faculty of Electrical Engineering and Information Technology, Institute of Micro and Sensor Systems, Otto-von-Guericke University, Magdeburg, Germany ABSTRACT In this paper the deposition and morphological characterization of gallium island structures on silicon and first results of silicon wire growth assisted by the created gallium droplets are presented. The islands and wires were grown on (111)-oriented single crystalline pdoped silicon substrates by microwave plasma enhanced chemical vapor deposition (MW PECVD) using trimethylgallium (TMGa) and silane (SiH4) as precursors for island and wire growth, respectively. The samples were investigated by SEM, EDS, XPS, and AFM. INTRODUCTION The epitaxial growth of Si nanowires (NWs) is one of the main subjects among the active research work in semiconductor technology. This is due to the increase of potential applications, and the steadily shrinking of electronic devices according to Moore`s law [1]. However, economical utilization of Si NWs in electronic and optoelectronic devices will require the control of the synthesis as well as the reproducible batch growth. In the past decades various methods have been developed for the synthesis of Si NWs. Beside the simply heating of Si wafers to high temperature [2] and the evaporation of Si powders in presence of gold or transition metals as catalysts [3], even several methods for thin film growth have been tested to grow Si whiskers. Under these thin film techniques MBE [4], laser ablation [5], and MOVPE [6] are often mentioned. Most of these techniques are based on the vapor liquid solid (VLS) mechanism, firstly reported by Wagner and Ellis in 1964 [3]. Independent from the process methods used for synthesis, NW growth is very complex. Various possibilities of wire formation have been reported for each technique. Even a slight change of a single process parameter leads to a completely different morphology of the product [7]. Therefore, the particular interests should be the examination of the wires grown under different conditions to reach more knowledge about the growth behavior and the dependencies of the process parameters. Following this ambition, we investigated the synthesis of Si NWs with the assistance of the low melting point metal Ga. In contrast to an Au or Al based VLS mechanism Ga does not work as catalyst, since it does not chemically assist the dissociation of SiH4, the Si precursor, far below its thermal decomposition point. Gallium and other low melting point metals like indium can only act as a solvent [8, 9]. The decomposition must be enhanced by a plasma treatment [10]. For both, the initial experiments creating the Ga droplets as well as the first attempts in growing Si whiskers, we used MW PECVD. Although PECVD has become one of the most important methods among the various thin film deposition techniques, especially for coating
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