Atmospheric barrier-torch discharge deposited ZnO films: optical properties, doping and grain size effects.

Technology aspects, structure and characteristic properties of the atmospheric barrier torch discharge technique grown nominally pure and Al (0.1-5%) and Mn (0.05-0.5%) doped ZnO nanocrystalline thin films deposited on fused quartz substrates are reported. The set of films with various thicknesses in the range from 70 nm to 1000 nm were prepared for each composition. XRD and AFM analysis show well formed [001] preferably oriented hexagonal high quality films witch could be operatively fabricated in air atmosphere with controlled grain size. All films under study revealed n-type conductivity except some Mn doped films, which revealed p-type. Spectroscopic optical ellipsometry and luminescence spectroscopy studies evidenced luminescence properties typical for well formed ZnO and appreciable influence of Al doping and grain size on the optical absorption edge. Al doping shifts strongly absorption edge to the higher energies. The grain size decreasing (from 100 to 15 nm) leads to distinct diffusing of the absorption edge accompanied by increase of absorption in the near band edge region. For the smallest grains the pronounced Urbach-type absorption tail can be observed manifesting films inhomogeneity development.

SrTiO(3):Cr nanocrystalline powders: size effects and optical properties.

The crystal structure, optical absorption, and photoluminescence of chromium impurity centers were studied in nanocrystalline SrTiO(3):Cr (0.1 mol%) powders with average particle size within the range 13-100 nm prepared by the Pechini-type polymeric sol-gel method. Only the presence of a cubic perovskite phase of O(h)(1) symmetry was proved for the powders at room temperature, by means of x-ray diffraction. The lattice constant a = 3.910 Å, larger than that of bulk SrTiO(3) crystals (a = 3.905 Å), was found for nanoparticles with the size about 20 nm. The optical absorption edge and the zero-phonon R-line ([Formula: see text]) of luminescence of the octahedral Cr(3+) centers shifted to higher energies with decreasing nanoparticle size. These size effects were regarded as intrinsic to SrTiO(3). An unusual and large temperature shift of the R-line position very similar to the 'dielectric related' one of the bulk crystals was observed for all powders, evidencing their quantum paraelectric behavior. However, the powders with the average particle size about 13 and 20 nm did not reveal completely reproducible behavior of the R-line position at low temperatures. This instability was considered a possible manifestation of a low-temperature phase transition in small enough SrTiO(3) nanoparticles.