InN Nanostructures: Strain and Morphology
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InN Nanostructures: Strain and Morphology Francois Demangeot1, Jean Frandon1, Claire Pinquier1, Michel Caumont1, Olivier Briot2, Benedicte Maleyre2, Sandra Clur-Ruffenach2 and Bernard Gil2 1 Laboratoire de Physique des Solides IRSAMC, UMR 5477 CNRS, 118 Route de Narbonne Universite Paul Sabatier, 31062 Toulouse Cedex 04, France 2 Groupe d’Etudes des Semiconducteurs, UMR 5650 CNRS, Place Eugene Bataillon, Universite Montpellier II, 34095 Montpellier, France
ABSTRACT We presented an experimental work on InN nanostructures grown on a GaN buffer layer deposited on sapphire (0001) by Metal Organic Vapor Phase Epitaxy. InN islands of controlled sizes were fabricated by using specific growth conditions and taking advantage of self-organization that results from Stranski-Krastanov growth mode. Then nanometric islands as small as 25 nm were characterized by using atomic force microscopy (AFM) and microRaman spectroscopy. AFM measurements revealed that the current shape of islands correspond to truncated hexagons. In-plane residual strain field was deduced from the E2 phonon frequency shift in micro-Raman spectra recorded on islands of various sizes. Careful analysis of these data made clear that the key parameter in determining the strain magnitude was the height of the islands: this was not surprising, keeping in mind that the shape was roughly independent of the size. Nevertheless, the dislocation density was believed to increase as function of the island thickness, leading to various degrees of strained relaxation, as probed by the present micro-Raman study. This conclusion was reinforced by the strain variation on the facets of single islands with respect to its value at the centre.
INTRODUCTION In this paper, we report on the first investigation of single InN islands by inelastic light scattering. Their deformation has been estimated and heterogeneities in the strain field across the island has been evidenced. Intensive work has been recently focused on InN, especially in the concern of a re-evaluation of its electronic band gap energy [1, 2]. An accepted value of this energy would be around 0.7 eV, although recent works tend to demonstrate that this value would be closer to 1.2 eV [3]. In both cases, this energy range opens the way to potential applications for infrared emission and detection. Nevertheless, many questions need to be addressed, such as the emission energy and its efficiency in such structures as a function of the growth characteristics. InN islands can be fabricated by using the three dimensional Stranski-Krastanov growth mode (formation of dislocated islands on the surface) [4,5]. This mode of growth takes place after a transition from a 2D growth mode as a result of strain relaxation in the film, induced by the large lattice mismatch (~ 11%) between InN and the GaN template. Data concerning electronic properties are still scarce and fundamental properties as the electron-phonon interaction should be studied in InN islands, since it is well known that electrons and phonons are strongly couple
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