Nanosize effect in Germanium Nanowire Growth with Binary Metal Alloys
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Nanosize effect in Germanium Nanowire Growth with Binary Metal Alloys Subhajit Biswas 1, Colm O’Regan1, Michael A. Morris1,2 and Justin D. Holmes1,2 1
Materials Chemistry & Analysis Group, Department of Chemistry and the Tyndall National Institute, University College Cork, Cork, Ireland. 2 Centre for Research on Adaptive Nanostructures and Nanodevices (CRANN), Trinity College Dublin, Dublin 2, Ireland. *
ABSTRACT This article describes feasible and improved ways towards enhanced nanowire growth kinetics by reducing the equilibrium solute concentration in the liquid collector phase in a vapor-liquidsolid (VLS) like growth model. Use of bi-metallic alloy seeds (AuxAg1-x) influences the germanium supersaturation for a faster nucleation and growth kinetics. Nanowire growth with ternary eutectic alloys shows Gibbs-Thompson effect with diameter dependent growth rate. Insitu transmission electron microscopy (TEM) annealing experiments directly confirms the role of equilibrium concentration in nanowire growth kinetics and was used to correlate the equilibrium content of metastable alloys with the growth kinetics of Ge nanowires. The shape and geometry of the heterogeneous interfaces between the liquid eutectic and solid Ge nanowires were found to vary as a function of nanowire diameter and eutectic alloy composition.
INTRODUCTION A popular route for growing high aspect ratio one-dimensional (1D) nanostructures is to use a metallic growth promoter in a liquid state, typically gold, as catalytic seed via a VLS mechanism [1, 2]. According to classical crystal growth theory, in VLS growth, growth velocity (v) is proportional to (∆μ/kT)2 , where T is the synthesis temperature and Δμ is a thermodynamic quantity called supersaturation, which is the chemical potential difference between adatoms of growth species in the vapor phase and the solid crystal phase [3]. Supersaturation (Δμ) is the driving force in a layer-by-layer crystal growth process [4]. In reference to the classical nucleation theory for 2D-island formation, the nucleation rate and the lateral growth rate of islands combine to contribute to the normal growth rate of crystals which have a direct dependence on supersaturation [5]. Altering the supersaturation will readily influence the growth behavior of nanowires, favoring faster crystallization rates at high Δμ values. The composition of the liquid metal-semiconductor binary eutectic alloy (Au-Ge), governs the supersaturation, or driving force [3] and hence nanowire VLS-growth kinetics. A shift in the liquidus curve, i.e. the equilibrium composition of the eutectic alloy, alters the supersaturation of the catalyst seeds which subsequently influences the kinetics of nanowire growth [6, 7]. In the synthesis of Group 14 nanowires, via VLS growth, Au is the popular growth promoter due to its low eutectic melting temperature and the high solubility of Group 14 growth species in Au. A shift in the liquidus of Au-Ge (or Au-Si) binary alloy can be achieved by incorporating an external metal (foreign) element into the meta
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