SiGe Nanowires Grown by LPCVD: Morphological and Structural Analysis
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SiGe Nanowires Grown by LPCVD: Morphological and Structural Analysis A. Rodríguez 1, J. Sangrador 1, T. Rodríguez 1, C. Ballesteros 2, Á. C. Prieto 3, J. Jiménez 3 1 Tecnología Electrónica, Universidad Politécnica de Madrid, E.T.S.I.T., 28040 Madrid, Spain 2 Física, Universidad Carlos III, 28911 Leganés (Madrid), Spain 3 Física de la Materia Condensada, Univ. de Valladolid, E.T.S.I.I., 47011 Valladolid, Spain
ABSTRACT SiGe nanowires were grown by the vapor-liquid-solid method using a low pressure chemical vapor deposition reactor and different flows of the GeH4 and Si2H6 gas precursors. The morphology of the nanowires was studied by field emission scanning electron microscopy, and the length, diameter and density of nanowires were determined. Their structure and crystallinity were analyzed by transmission electron microscopy and its related techniques. Energy dispersive X-ray emission of individual nanowires as well a Raman spectroscopy were used to determine their composition and to analyze its homogeneity. INTRODUCTION Semiconductor nanowires (NWs) have attracted a great deal of attention because of their potential applications in electronics, optoelectronics, photovoltaics, and sensing [1]. Group IV NWs, either Si or Ge, have been extensively analyzed because of their compatibility with current device fabrication technologies. SiGe nanowires are of interest since their electronic properties can be tailored over a wide range by varying the composition, although their growth still presents some challenges [2, 3]. Therefore, studies of the main morphologic, structural and compositional features of the NWs are interesting to feedback the growth processes. In this work, a study of the morphology, structure and composition of SiGe nanowires grown by the vapor-liquid-solid (VLS) method and a low pressure chemical vapor deposition (LPCVD) reactor is presented. EXPERIMENTAL The substrates used were (100) Si wafers with 3 nm thick Au films deposited on top by thermal evaporation. An annealing step at 500 ºC for 1 hour was carried out in H2 atmosphere (400 mTorr, 100 sccm) inside the LPCVD reactor to break the Au film and allow Au and Si to alloy, forming a distribution of Au-Si droplets on the surface of the Si substrate. During the subsequent growth process, carried out at 400 ºC for 30 minutes, Si2H6 and GeH4 were used as precursor gases for Si and Ge respectively with flows ranging from 2 to 20 sccm for GeH4 and from 8 to 2 sccm for Si2H6, in such way that the GeH4:Si2H6 flow ratio (R) was varied from 0.25 to 10 (see Table I). The flow of H2 was changed from run to run to achieve a total flow of 100 sccm and the total pressure was kept constant at 400 mTorr. In some cases either the growth temperature or the total pressure were varied from the indicated values, keeping constant the flow ratio at R = 1, to explore the influence of those variables on the main results.
The average length and diameter, the density of NWs and other morphological features of the NWs were studied using a high resolution field e
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