SiGe Nanowires Grown by LPCVD using Ga-Au Catalysts
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SiGe Nanowires Grown by LPCVD using Ga-Au Catalysts M. Monasterio 1, A. Rodríguez 1, T. Rodríguez 1, C. Ballesteros 2 1 Tecnología Electrónica, Universidad Politécnica de Madrid, E.T.S.I.T., 28040 Madrid, Spain 2 Física, Universidad Carlos III, 28911 Leganés (Madrid), Spain ABSTRACT The use of Ga-Au alloys as metal catalysts for the growth of SiGe nanowires has been investigated. The grown nanowires are cylindrical and straight, with a defect-free crystalline structure, sharp nanowire-droplet interfaces and an almost constant Ge atomic fraction throughout all their length. These features represent significant improvements over the results obtained using pure Au. INTRODUCTION Group IV (Si, Ge or SiGe) nanowires (NWs) have been extensively analyzed because of their potential applications in electronics, optoelectronics, photovoltaics and sensing, as well as due to their compatibility with current device fabrication technologies [1, 2]. The most commonly used catalyst metal for the growth of semiconductor nanowires by the Vapour-LiquidSolid (VLS) method is Au. However, among other drawbacks, the interfaces in Si-SiGe NWs heterostructures show a compositional interfacial broadening of the order of the NW diameter [3]. The VLS growth of Si NWs using other metals, like Ga [4, 5] and In [6], has been considered. These metals show very low solubilities of Si and Ge, an effect that could be advantageous to improve the abruptness of interfaces in heterostructures. However, they are not catalytic, so conventional chemical vapor deposition (CVD) techniques cannot be used and plasma assisted CVD techniques are required. The use of alloys of catalytic and non-catalytic metals, like Ga-Au, appears as an alternative to take advantage of the properties of both species using a conventional CVD process. Ga-Au alloys have been used for the growth of Si NWs on Si by CVD [7]. In this work, the growth of SiGe NWs by CVD using Ga-Au alloys of different compositions as catalysts is investigated. The morphological, structural and compositional properties of the grown NWs are compared with those of the NWs grown using Au. EXPERIMENTAL SiGe NWs of different compositions were grown by the VLS method using a Low Pressure CVD reactor. Bare (100) and (111) as well as thermally oxidized Si wafers were used as substrates and metal films of Ga and Au (Ga evaporated first) were deposited on top of them by evaporation. The Ga content was varied from 0 (pure Au) to 100% (pure Ga) by changing the expected thickness to be achieved from each evaporation process keeping a constant total thickness of around 10 nm. The intermediate values of the nominal Ga content were 50%, 75% and 90%. Prior to NW growth, the samples were annealed at 500 ºC in hydrogen atmosphere
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inside the reactor. During the subsequent growth process, Si2H6 and GeH4 were used as precursor gases for Si and Ge with a GeH4:Si2H6 flow ratio (R) varying from 0 to 2. The total pressure was kept constant at 400 mTorr, the temperature was varied from 400 to 470ºC depending on the value of R
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