We started studying the growth of semiconductor nanowires (NWs) in 2004. Our activity now extends from the in depth study of growth mechanisms to the fabrication of NWs for applications (for which, see III-V nitride nanowiresand Nanowires for photovoltaics).
The growth mechanisms of semiconductor NWs constitute a rich domain of basic physical investigations, the understanding of which also conditions the controlled fabrication of NWs with predefined morphology, size, crystal structure, local composition (and possibly mutual arrangement) that are essential for applications. Rather uniquely, Elphyse is able to combine growth proper, and analyses, and modelling.
Most of our studies deal with the Vapor-Liquid-Solid (VLS) growth of NWs of III-V materials. In this case, new monolayers (MLs) form one by one at the interface between the already grown solid NW stem and an apical liquid catalyst nanodroplet, using atoms transferred from the latter (for non-VLS growth, see III-V nitride nanowires).
A striking feature of the NWs of P, As or Sb-based III-V compounds is that they frequently adopt the hexagonal wurtzite (WZ) crystal structure whereas, under any other form (be it bulk, 2D or 0D), these materials invariably adopt the cubic zinc blende (ZB) structure. Early on, we proposed a kinetic explanation of WZ formation, based on the idea of nucleation at the VLS triple phase line. Although our model has become the standard model in the field and is now confirmed experimentally, NW polytypism remains a hot topic, which precise control would allow the routine fabrication of complex crystal phase heterostructures. Our current research, based on in situ studies and thermodynamic and kinetic modelling, aims at clarifying the subtle interplay between droplet and interface geometry and ML nucleation, depending on growth conditions.
NWs are a choice system for the study of nucleation and growth from a nanosize supersaturated mother phase, which presents many specific features as compared with a bulk medium. Experimentally, we initially developed post growth studies of NW ensembles and measurements of the chronology of the VLS growth of single NWs. These are now superseded by the in situ investigation of growth kinetics at the sub-monolayer spatial and temporal levels.
Our theoretical studies are based on the calculation of the thermodynamic functions of the often complex liquid phases involved. As regards kinetics and statistics, for III-V NWs, the key parameter is the amount of the volatile group V element(s) present in the liquid droplet, which depends on system size and growth regime. If this amount is larger than the content of a ML, the MLs form very rapidly and we may develop predictive kinetic calculations based on a continuum description of the interplay between the various fluxes that feed the systems and the nucleation of new MLs. Even in this case, the group V element remains scarce so that the formation of even a single ML may significantly deplete the liquid droplet and decrease its chemical potential. This makes a new nucleation less likely after a first one than before and induces an anticorrelation of the nucleation events, which distribution becomes sub-Poissonian. After having discovered this nucleation antibunching, we studied it in depth experimentally, numerically and analytically.
Quite frequently however, the amount of group V in the liquid at nucleation is less, and possibly much less, than a ML, so that only fractional MLs form at nucleation. This regime is the main focus of our current research. Based on our recent in situ studies of the nucleation and propagation of individual MLs, we currently develop and test thermodynamic and kinetic models that account atom by atom for the exchanges between the various phases in presence (external vapor, liquid droplet, solid NW). We are in particular very interested in the emergence of a quasi-deterministic regime whereby, although of course nucleation remains stochastic, the ML formation cycle would become periodic, with the prospect of growing perfectly controlled and regular structures.
NWs offer many prospects for growing material combinations inaccessible in other systems, such as joint axial and radial heterostructures or single-material crystal phase heterostructures. Despite the freedom granted by the NW geometry, the formation of high quality heterostructures in NWs raises issues broadly similar to those encountered in planar structures.
In VLS-grown NWs, the sharpness of compositional heterostructures may be limited by the accumulation of the currently grown element in the liquid (reservoir effect). We develop and compare thermodynamic and kinetic models of the formation of such heterostructures. This provides guidance to the choice of materials, catalysts and growth sequence and allows one ultimately to achieve interface widths of only a few MLs.
Stain relaxation in mismatched heterostructures is another issue. Our model predicting quantitatively the critical dimensions for axial heterostructures (below which no extended defect should form) has also become standard and is widely used by growers. The literature about core-shell heterostructures being quite confusing, we currently investigate this case via numerical and analytical modelling.
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