Frank Glas

The formation of nanowires of semiconductors and of their heterostructures offers several choice examples of crystal growth in a confined medium of nanometric size. I shall discuss different studies carried out recently at LPN in the domain of III-V nanowires, concentrating on the intimate coupling between the modeling of epitaxial growth and its experimental study. This will give me the opportunity to recall how nanowires grow, without hiding that the understanding of the growth mechanisms, and sometimes even their very identification, are far from being stabilized yet.

Nanowires commonly grow in the vapor-liquid-solid (VLS) mode, from a liquid droplet whose volume is of the order of a zeptoliter. The multiplicity of material pathways via which this droplet (which is continuously consumed) may be refilled, produces original growth kinetics [1]. The depletion of the droplet that occurs when a new nanowire monolayer is formed induces very original nucleation statistics and confers a self-regulated character to growth [2].

The formation of highly mismatched heterostructures is much easier in nanowires than in the planar geometry, due to very efficient stress relaxation at the free lateral surfaces. Somewhat unexpectedly, the coherent deposition of a mismatched material on the top facet of a nanowire may however lead to the formation of islands occupying only a fraction of this facet, an effect reminiscent of the standard Volmer-Weber or Stranski-Krastanow growth modes. Our modeling of this effect[3] seems confirmed by recent experiments carried out by others on the catalyst-free growth of nanowires of group III nitrides (GaN and related materials).

Finally, I will present very recent work on the modeling of self-catalyzed nanowire growth. In this variant of the VLS mode, which has become very popular in the last few years, the metal of the catalyst droplet is the group III element of the nanowire. We have developed an « As-only » model of this type of growth for GaAs. Not only does it reproduce all the salient features of our recent detailed experimental study[[4] but, since it involves only a small number of parameters that we have all determined, it also allows one to model a priori the growth of an arbitrary nanowire whose apical droplet has a given geometry. The predictive character of this model is truly original in the field of nanowire growth[5].

[1] J.-C. Harmand, F. Glas, G. Patriarche, Phys. Rev. B 81, 235436 (2010).
[2] F. Glas, J.-C. Harmand, G. Patriarche, Phys. Rev. Lett. 104, 135501 (2010).
[3] F. Glas and B. Daudin, Phys. Rev. B 86, 174112 (2012).
[4] M. R. Ramdani, J. C. Harmand, F. Glas, G. Patriarche, L. Travers, Cryst. Growth Des. 13, 91 (2013).
[5] F. Glas, M. R. Ramdani, G. Patriarche J. C. Harmand, soumis à Phys. Rev. B