Visible emission near 1.9-2.2eV from hexagonal InN films grown by electron cyclotron resonance plasma-assisted molecular-beam epitaxy

We have investigated the influence of residual oxygen (O) atoms and In 2O3 grains included in 200-nm-thick hexagonal (α)-InN crystalline films grown on Si (1 1 1) substrates. (1 1 1)-oriented cubic (β)-In2O3 crystal grains were formed from O atoms in In droplets adhering to the film surface, and from residual O atoms in the film when the samples were annealed at 500°C in a N2 atmosphere after growth. The samples exhibited relatively strong and uniform orange photoluminescence (PL) emissions on the whole surface when using a 325nm He-Cd laser excitation. The emission consisted of two broad bands at approximately 1.9 and 2.0eV, however no PL emissions were observed when using a 514.5nm Ar+ ion laser. β-In2O3, formed from residual O atoms in the film, was also detected following post-growth annealing after In droplets were removed by HCl etching. It is shown that the residual O concentration in the as-grown film can be qualitatively estimated from the orange peaks in the PL spectra. It is suggested that the origin of the localized yellow emissions observed at 2.2eV under a 514.5nm Ar+ ion laser excitation for as-grown InN films, as reported previously [Phys. Stat. Sol. B 228 (2001) 21; Appl. Phys. Lett. 80 (2002) 968], is not β-In 2O3 crystal grains although it is closely related to the residual O concentration.