Today, the challenge for InN is the growth of epitaxial materials with low defect density and high electron mobility. MBE achieved first good layers in 2003 (Shaff et al.), but recently also MOVPE has shown encouraging results (Briot et al., 2004). But even then, the necessity of a material with uniform properties is reflected in the high dispersion of InN bandgap values proposed up to now. Good crystal quality is also the prerequisite for the fabrication of promising InN based electronic devices. Sapphire is presently the most used substrate for both epitaxial techniques, due to the experience gained in GaN epitaxy during the last 15 years. It is well known, for both GaN and InN, that sapphire nitridation is a necessary step to improve the layer quality. Since InN peculiarities require a careful and dedicated study, the effects of different sapphire nitridation procedures on the properties of InN epitaxial layer will be the focus of this presentation.
In-situ spectroscopic ellipsometry (SE) is used here for the first time to reveal and control the nitridation dynamics of sapphire in MOVPE. Sapphire nitridation is performed at 1050C with ammonia using nitrogen as carrier gas. SE identifies two distinct process-steps occurring during sapphire nitridation: (i) a fast surface reaction occurring within the first 30s and thereafter (ii) a slow nitridation reaction into the sapphire bulk with a simultaneous roughening of the surface. A set of InN samples was then prepared using the same growth procedure but with different nitridation times for the sapphire substrates. Our growth procedure involves high temperatures (580C) and low V/III ratio (NH₃/TMIn: 10000) resulting in growth rates as high as 350 nm/hour and in layers with specular surfaces. The InN samples were characterised ex-situ with respect to their electronic and crystallographic properties. The optimal nitridation times coincide with the completion of the fast surface reaction (30s).

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