On the crystalline structure, stoichiometry and band gap of InN thin films

Detailed transmission electron microscopy, x-ray diffraction (XRD), and optical characterization of a variety of InN thin films grown by molecular-beam epitaxy under both optimized and nonoptimized conditions is reported. Optical characterization by absorption and photoluminescence confirms that the bandgap of single-crystalline and polycrystalline wurtzite InN is 0.70±0.05eV. Films grown under optimized conditions with an AlN nucleation layer and a GaN buffer layer are stoichiometric, single-crystalline wurtzite structure with dislocation densities not exceeding mid-1010cm−2. Nonoptimal films can be polycrystalline and display an XRD diffraction feature at 2θ≈33°; this feature has been attributed by others to the presence of metallic In clusters. Careful indexing of wide-angle XRD scans and selected area diffraction patterns shows that this peak is in fact due to the presence of polycrystalline InN grains; no evidence of metallic In clusters was found in any of the studied samples.