Cubic InN on r-plane sapphire

InN has been grown directly on r-plane sapphire substrates by plasma-induced molecular beam epitaxy. X-ray diffraction and transmission electron microscopy investigations have shown that the InN layers consist of a predominant zinc-blende (cubic) structure along with a fraction of the wurtzite (hexagonal) phase whose content increases with proceeding growth. The InN layer is defect rich with a high number of stacking faults and twins. As a consequence a very high residual doping of n∼1019cm−3 was estimated. The lattice constant for the zinc-blende phase of InN was found to be a=4.986 Å. The optical investigations were strongly affected by a high number of defects, but nevertheless indicated an absorption edge below 0.6 eV. For this unusual growth of the metastable cubic phase on a noncubic substrate an epitaxial relationship was proposed, where the metastable zinc-blende phase grows directly on the r-plane of sapphire.