Electron accumulation is found to occur at the surface of wurtzite ͑1120͒, ͑0001͒, and ͑0001͒ and zinc-blende ͑001͒ InN using x-ray photoemission spectroscopy. The accumulation is shown to be a universal feature of InN surfaces. This is due to the low ⌫-point conduction band minimum lying significantly below the charge neutrality level.

Cubic InN layers were grown by plasma assisted molecular beam epitaxy on 3C-SiC (001) substrates at growth temperatures from 419to490°C. X-ray diffraction investigations show that the layers have zinc blende structure with only a small fraction of wurtzite phase inclusions on the (111) facets of the cubic layer. The full width at half maximum of the c-InN (002) x-ray rocking curve is less than 50arcmin. The lattice constant is 5.01±0.01Å. Low temperature photoluminescence measurements yield a c-InN band gap of 0.61eV. At room temperature the band gap is about 0.56eV and the free electron concentration is about n∼1.7×1019cm−3.

A detailed discussion of the optical properties of Al-rich Al 1−x In x N alloy films is presented. The (0001)-oriented layers with In contents between x = 0.143 and x = 0.242 were grown by metal-organic vapor phase epitaxy on thick GaN buffers. Sapphire or Si(111) served as the substrate. High-resolution X-ray diffraction revealed pseudomorphic growth of the nearly lattice-matched alloys; the data analysis yielded the composition as well as the in-plain strain. The complex dielectric function (DF) between 1 and 10 eV was determined from spectroscopic ellipsometry measurements. The sharp onset of the imaginary part of the DF defines the direct absorption edge, while clearly visible features in the high-photon energy range of the DF, attributed to critical points of the band structure, indicate promising crystalline quality of the AlInN layers. It is demonstrated that the experimental data can be well reproduced by an analytical DF model. The extracted characteristic transition energies are used to determine the bowing parameters for all critical points of the band structure. In particular, strain and the high exciton binding energies for the Al-rich alloys are taken into account in order to assess the splitting between the valence band with Γ v 9 symmetry and the Γ c 7 conduction band at the center of the Brillouin zone. Finally, the compositional dependence of the high-frequency dielectric constants is reported.

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A detailed analysis of the dielectric function (DF) for wurtzite InN as well as for In‐rich InAlN alloys is presented. The experimental data, covering the energy range from 0.72 up to 9.5 eV, were obtained by ellipsometric studies of an (11&2macr;0) a‐plane InN film and low carrier density (0001) c‐plane films. Model calculations of the imaginary part of the DF around the band gap provide direct insight how to determine the energetic position of the Fermi energy from the experimental results. Then, taking into account both, band gap renormalization and Burstein–Moss shift, the values of the gaps at zero carrier density are calculated. The dependence of the InAlN band gap on the alloy composition is described by a bowing parameter of 4.0 eV. The a‐plane film exhibits a characteristic optical anisotropy below 1 eV which is attributed to the polarization dependence of transition probabilities from the three valence bands at the Γ point of the Brillouin zone into the conduction band. The splitting of 25 meV between the absorption edges for the two polarization directions can be well explained by a crystal field energy of 19 (24) meV if a calculated spin–orbit energy of 13 (5) meV is assumed. All results emphasize a band gap value of wurtzite InN of about 0.68 eV. By fitting the third derivatives of the dielectric function up to 9.5 eV we determine the compositional dependences of the transition energies for at least three critical points of the band structure. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)