The bulk and surface electronic structure of In 2 O 3 has proved controversial, prompting the current combined experimental and theoretical investigation. The band gap of single-crystalline In 2 O 3 is determined as 2.93Ϯ 0.15 and 3.02Ϯ 0.15 eV for the cubic bixbyite and rhombohedral polymorphs, respectively. The valence-band density of states is investigated from x-ray photoemission spectroscopy measurements and density-functional theory calculations. These show excellent agreement, supporting the absence of any significant indirect nature of the In 2 O 3 band gap. Clear experimental evidence for an s-d coupling between In 4d and O 2s derived states is also observed. Electron accumulation, recently reported at the ͑001͒ surface of bixbyite material, is also shown to be present at the bixbyite ͑111͒ surface and the ͑0001͒ surface of rhombohedral In 2 O 3 .

With the increasing requirements for microelectromechanical systems (MEMS) regarding stability, miniaturization and integration, novel materials such as wide band gap semiconductors are attracting more attention. Polycrystalline SiC has first been implemented into Si micromachining techniques, mainly as etch stop and protective layers. However, the outstanding properties of wide band gap semiconductors offer many more possibilities for the implementation of new functionalities. Now, a variety of technologies for SiC and group III nitrides exist to fabricate fully wide band gap semiconductor based MEMS. In this paper we first review the basic technology (deposition and etching) for group III nitrides and SiC with a special focus on the fabrication of three-dimensional microstructures relevant for MEMS. The basic operation principle for MEMS with wide band gap semiconductors is described. Finally, the first applications of SiC based MEMS are demonstrated, and innovative MEMS and NEMS devices are reviewed.

This work presents a study of intrinsic zinc oxide thin film as ozone sensor based on the ultraviolet ͑UV͒ photoreduction and subsequent ozone re oxidation of zinc oxide as a fully reversible process performed at room temperature. The films analyzed were produced by spray pyrolysis, dc and rf magnetron sputtering. The dc resistivity of the films produced by rf magnetron sputtering and constituted by nanocrystallites changes more than eight orders of magnitude when exposed to an UV dose of 4 mW/ cm 2 . On the other hand, porous and textured zinc oxide films produced by spray pyrolysis at low substrate temperature exhibit an excellent ac impedance response where the reactance changes by more than seven orders of magnitude when exposed to the same UV dose, with a response frequency above 15 kHz, thus showing improved ozone ac sensing discrimination.

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We report on the phase stabilization of rhombohedral (rh-) In 2 O 3 films on sapphire substrate deposited by metal organic chemical vapor deposition. With the help of a high-temperature nucleation layer and evolutionary structural selection of rhombohedral phase during the growth process, stable epitaxial growth of single crystalline rh-In 2 O 3 is achieved. The mechanism of phase selective epitaxial growth is studied by means of high-resolution X-ray diffraction and transmission electron microscopy measurements. Furthermore, Raman spectroscopy measurements are carried out to investigate the phonon properties of rh-In 2 O 3 . Raman-active phonon modes of rh-In 2 O 3 are first identified.

High-quality InN nanocolumns grown by molecular beam epitaxy on n-type Si(111) have been electrically characterized by atomic force microscopy. Current-voltage characteristics were measured on InN nanocolumns with similar heights but different diameters. The conductivity scales the nanocolumns reciprocal diameter, pointing to the nanocolumn lateral surface as the main conduction path. These results, opposing those found in undoped GaN nanocolumns where the conductivity is rather independent of the diameter (conduction through the volume), agree well with a model that predicts electron accumulation by Fermi level pinning within the conduction band on nonpolar (m plane) InN surfaces reconstructed under In-rich conditions.

Phase selective growth of rhombohedral and cubic indium oxide polytypes was studied. The selective growth of different polytypes was achieved by adjusting substrate temperature and trimethylindium flow rate during metal organic chemical vapor deposition on c-plane sapphire. The optical band gaps of the cubic and rhombohedral phases were determined to be ∼3.7 and ∼3.0eV, respectively. On the basis of the performed structural investigations, a phenomenological model of the nucleation and growth of highly textured cubic In2O3 on Al2O3 (0001) is proposed.

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