Abstract
Semiconductor nanowires are promising material systems for coming-of-age nanotechnology. The usage of the vapor–solid–solid (VSS) route, where the catalyst used for promoting axial growth of nanowires is a solid, offers certain advantages compared to the common vapor–liquid–solid (VLS) route (using a liquid catalyst). The VSS growth of group-IV elemental nanowires has been investigated by other groups in situ during growth in a transmission electron microscope (TEM). Though it is known that compound nanowire growth has different dynamics compared to elemental semiconductors, the layer growth dynamics of VSS growth of compound nanowires have not been studied yet. Here we investigate for the first time controlled VSS growth of compound nanowires by in situ microscopy, using Au-seeded GaAs as a model system. The ledge-flow growth kinetics and dynamics at the wire–catalyst interface are studied and compared for liquid and solid catalysts under similar growth conditions. Here the temperature and thermal history of the system are manipulated to control the catalyst phase. In the first experiment discussed here we reduce the growth temperature in steps to solidify the initially liquid catalyst, and compare the dynamics between VLS and VSS growth observed at slightly different temperatures. In the second experiment we exploit thermal hysteresis of the system to obtain both VLS and VSS at the same temperature. The VSS growth rate is comparable or slightly slower than the VLS growth rate. Unlike in the VLS case, during VSS growth we frequently observe that a new layer starts before the previous layer is completely grown, i.e., ‘multilayer growth’. Understanding the VSS growth mode enables better control of nanowire properties by widening the range of usable nanowire growth parameters.
Highlights
Controlling the electronic, mechanical and optical properties of semiconductor nanowires by tuning their crystal structure, composition and morphology enable their application in, for instance, nano-electronic, optoelectronic and energy harvesting devices.[1,2,3,4] Precise understanding and control of the dynamics of the crystal growth process are in turn key to tuning these important parameters
Vapor–solid–solid growth of GaAs nanowires is compared with the vapor–liquid–solid growth by in situ investigation in a transmission electron microscope (TEM)
The VSS growth rate was found to be slightly slower than the VLS growth rate, but comparable, rather than substantially slower as observed in some other systems
Summary
Controlling the electronic, mechanical and optical properties of semiconductor nanowires by tuning their crystal structure, composition and morphology enable their application in, for instance, nano-electronic, optoelectronic and energy harvesting devices.[1,2,3,4] Precise understanding and control of the dynamics of the crystal growth process are in turn key to tuning these important parameters. Nanowires are most o en grown using a foreign liquid metal to promote the anisotropic onedimensional growth. This process occurs by the ‘vapor– liquid–solid’ (VLS) mechanism;[5,6] the atomic species constituting the semiconductor dissolve in the seed particle, form a supersaturated metallic liquid alloy (‘catalyst’), One major advantage of VSS growth compared to VLS is that in some material systems it can enable fast switching of materials in axial nanowire heterostructures.[22] During VLS growth the liquid catalyst acts as a reservoir of the nanowire species, and the switch from one composition to another is gradual.[27,28,29] On the other hand, the solubility of nanowire
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