Growth in the form of nanowires instead of planar films offers the advantage that the interface between adsorbate and substrate is very small and the aspect ratio of the structure is high. Hence, strain induced by mismatch in lattice constant or thermal expansion coefficient can elastically relax at the free sidewalls, and dissimilar materials can be combined in high crystal quality. This advantage is particularly relevant for group-III-nitrides because bulk substrates are still not readily available for this material class.

Already in the late nineties it was discovered that GaN forms nanowires when grown by molecular beam epitaxy (MBE) under suitable conditions [1], i.e. at fairly high substrate temperature (>750°C, higher than for the growth of planar films) and under excess of nitrogen. These nanowires are of excellent structural as well as optical quality [2], and thus very attractive both for fundamental studies and applications such as solid state lighting, sensing, and energy harvesting. Very interestingly, the formation of these nanowires does not require any guidance by a mask or catalyst and is not mediated by droplets or particles at the tips of the nanowires. Also, they can be grown on a number of different substrates including silicon. The aim of this talk is to provide an overview of what we know about the self-induced nucleation and subsequent growth of GaN nanowires by MBE with a focus on recent insights.

In particular, recently we showed that the nucleation of such nanowires can occur spontaneously, i.e. does not require any structural defects or irregularities in substrate morphology [3]. Also, even on an AlN buffer layer of Al-polarity that should lead to the growth of Ga-polar GaN nanowires, we found only N-polar nanowires. Thus, this type of polarity must play an important role for the microscopic growth mechanisms. In addition, a kinetic growth model will be presented that explains why nanowire growth requires effectively N-rich conditions and how the nanowire radius depends on the V/III ratio.

 [1] M. Yoshizawa, A. Kikuchi, M. Mori, N. Fujita, and K. Kishino, Japanese Journal of Applied Physics 36, L459 (1997); M.A. Sanchez-García, E. Calleja, E. Monroy, F.J. Sanchez, F. Calle, E. Muñoz, and R. Beresford, Journal of Crystal Growth 183, 23 (1998).

[2] L. Geelhaar, C. Chèze, B. Jenichen, O. Brandt, C. Pfüller, S. Münch, R. Rothemund, S. Reitzenstein, A. Forchel, T. Kehagias, P. Komninou, G.P. Dimitrakopulos, T. Karakostas, L. Lari, P.R. Chalker, M.H. Gass, and H. Riechert, IEEE Journal of Selected Topics in Quantum Electronics 17, 878 (2011).

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