Structural and optical properties of indium nitride grown by plasma-assisted molecular beam epitaxy

Abstract

The bandgap of indium nitride has long been accepted to be 1.9 eV. However, recent results have cast doubt over this as modern epitaxy techniques have allowed experimental studies of high quality material. Single crystalline and polycrystalline indium nitride films have been grown on (0001) sapphire and silica glass using plasma assisted molecular beam epitaxy (PAMBE). Optical measurements on the films revealed a luminescence feature in the vicinity of 0.8 eV for all films, both on sapphire and glass. No feature around 1.9 eV could be identified above the background noise. To our knowledge this is the first report of polycrystalline InN exhibiting the 0.8 eV feature. Ion beam analysis of the material could find no measurable oxygen contamination in the bulk of the films. These results along with recent reports of blue shifting of the absorption onset of InN films with increasing oxygen content appear to point towards oxygen contamination as being the source of the previously reported higher bandgap. Like other groups we observed a small anomalous blue shifting of the luminescence with increasing temperature when using a germanium detector. We have verified that this is a real feature by measuring the temperature dependent PL with a lead sulphide detector. Two distinct growth regimes were identified. High In:N flux ratios lead to spotty RHEED accompanied by a morphology of flat plateaus separated by narrow valleys. Low In:N flux ratios lead to rough films consisting of facets largely disjoint from each other. Surprisingly, this regime gave streaky RHEED, suggesting high levels of crystal alignment between facets and high crystal quality within facets.