Kinetics and Statistics of Vapor-Liquid-Solid Growth of III-V Nanowires
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Kinetics and Statistics of Vapor-Liquid-Solid Growth of III-V Nanowires
Jean-Christophe Harmand, Frank Glas, Gilles Patriarche, Fauzia Jabeen and Mohamed Réda Ramdani Laboratoire de Photonique et de Nanostructures, CNRS, Route de Nozay, 91460 Marcoussis, France
ABSTRACT The use of semiconductor nanowires as new material building blocks for developing original devices is conditioned by the controllability of their growth. An important challenge is to form nanowires which include heterostructures of predictable dimensions. This objective requires a precise knowledge of the growth kinetics which appears much more complex for nanowires than for standard two-dimensional layers. Here, we present a method which provides detailed information on nanowire formation. The method is implemented with InP1-xAsx nanowires grown by Au-catalyzed molecular beam epitaxy. Controlled and periodic modulations of the incident vapor phase are generated. Due to these modulations, the nanowires show small and short oscillations of composition along their growth axis. These oscillations furnish a time scale which is recorded in the nanowire solid phase. The instantaneous growth rate and the total length of individual nanowires at any time of the growth are accessible. Moreover, the distribution of the oscillation lengths contains the nucleation statistics. This statistics is shown to be strongly sub-Poissonian, which indicates that some regulation mechanism operates. The rapid depletion of group V atoms in the catalyst drop which follows the growth of each ML could explain the self-regulation of nucleation events. INTRODUCTION A method commonly used to elaborate semiconductor nanowires (NWs) is the directed growth catalyzed by foreign metallic particles deposited on a crystalline substrate. Most often, these particles become liquid after alloying with the semiconductor constituents and growth is sustained by a vapor phase. Incoming atoms first dissolve in the catalyst drop and then incorporate into the solid phase. This is known as the vapor-liquid-solid (VLS) [1] growth mode. This method has been intensively studied in the past decade, but the experimental exploration of NW growth kinetics remains scarce [2-4]. The mechanisms underlying nanowire formation contrast significantly with those operating in standard epitaxial growth of two-dimensional (2D) thin layers. First, in VLS, the catalyst particle acts as a finite reservoir of variable composition, even at constant incoming vapor phase composition [5,6]. This generates non linear and transient growth rates. In particular, transients occur at the beginning or end of growth when the drop departs from or returns to equilibrium with the solid [7]. Heterostructure formation is also prone to transient regimes because the drop composition does not change instantaneously [8].
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Moreover, the supersaturation of the nanowire constituents in the liquid phase is dependent on the drop size via the Gibbs Thomson effect [9]. Second, at least two different surface orientations coexist during nanowire
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