In recent years, the self-organized growth of III-nitride based nanocolumns attracted considerable interest since it allows for the preparation of defect-free single-crystalline epitaxial structures for various applications. We focus on a straightforward catalyst- and mask-free deposition on r-plane sapphire [1].

Prior to the growth, a nitridation of the substrate is performed in a MOVPE reactor using NH3, which leads to the formation of AlN islands of approx. 30nm diameter on the Al₂O₃surface. These substrates are transferred into the MBE chamber where growth of the GaN takes place. Almost immediately, the RHEED diffraction pattern indicates the development of two different crystal phases, namely a compact two-dimensional a-plane GaN layer and an ensemble of nanocolumns with diameters of 50-200nm, which are oriented with an average angle of 62° towards the surface plane. The nanocolumns show a hexagonal cross-section, they are highly ordered and well aligned. The length of the nanocolumns depends nearly linearly on the deposition time, while their formation is very sensitive to the growth temperature (range of ΔT=10K) but insensitive to the III/V-ratio.The crystallographic orientations of both phases were analyzed by HRXRD polar-figures and the polarity of the rods was determined by convergent beam electron diffraction measurements to be N polar. TEM darkfield measurements indicate a very low defect density in the rods. This is in good agreement with µ-PL measurements, which show no yellow or stacking-fault related luminescence. The spectrum of a single GaN column is dominated by a very sharp D⁰X line with a FWHM of 1.2meV. Doping of the rods with silicon (n-type) and magnesium (p-type) was carried out. Depening on the concentration an increase of the D⁰X intensity was observed. Furthermore, first results concerning nanocolumn-based heterostructures, e.g. InGaN-QWs and AlN-interlayers will be discussed.

[1] T. Aschenbrenner et al. Nanotechnology 20, 75604 (2009)

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