The density-of-state effective masses of impurity doped polycrystalline ZnO thin films were measured by the method of four coefficients technique. By applying the first-order non-parabolicity approximation, the polaron effective mass and the bare band mass at the conduction band minimum, together with the corresponding non-parabolicity parameters, were analysed successfully. The determined perpendicular polaron mass of 0.29 me and the bare band mass of 0.247 me at the conduction band minimum corresponded very well to the previous results obtained for ZnO single crystals. The non-parabolicity parameter of 0.457 eV−1 derived for the polaron effective mass was larger than 0.33 eV−1 which was obtained for the bare band mass due to the increasing function of the Fröhlich coupling constant with respect to the bare band mass in polycrystalline ZnO films.
The optical properties of impurity doped ZnO thin films were analyzed by taking into account the nonparabolicity in the conduction-band and the optically determined carrier concentration and mobility were correlated with those measured by Hall measurement. The Drude parameters obtained by applying a simple Drude model combined with the Lorentz oscillator model for the optical transmittance and reflectance spectrum were analyzed by using the carrier density dependent bare band effective mass determined by the first-order nonparabolicity approximation. The squared plasma energy multiplied by the carrier density dependent effective mass yielded fairly linear relationship with respect to the carrier concentration in wide carrier density range of 1019 − 1021 cm−3, verifying the applicability of the nonparabolicity parameter for various types of impurity doped ZnO thin films. The correlation between the optical and Hall analyses was examined by taking the ratios of optical to Hall measurements for carrier density, mobility, and resistivity by introducing a parameter, Rdl, which represents the ratio of the resistances to electron transport from the inside of the lattice and from the crystallographic defects. For both the carrier concentration and mobility, the ratios of optical to Hall measurements were shown to exhibit a monotonically decreasing function of Rdl, indicating that the parameter Rdl could be used as a yardstick in correlating the optically determined carrier density and mobility with those measured by Hall analysis.
To improve the stability of sputter-deposited ZnO:Al (AZO) films at high temperature above 300 • C, an amorphous Zn-Sn-O (ZTO) film was deposited on the top of AZO films as an protective layer by co-sputtering of pure ZnO and SnO 2 targets. Amorphous ZTO films had resistivity in the range from 10 −2 to 10 −3 cm and were stable up to temperature of 400 • C. Heat treatments of bare AZO films in the atmosphere at 400 • C resulted in a dramatic increase in the resistivity accompanied by substantial decrease in carrier concentration and Hall mobility. The AZO films covered with the ZTO film showed remarkable improvement in thermal stability for subsequent heat treatments in the temperature range from 200 to 400 • C in the atmosphere as well as chemical stability in weak acidic solution. X-ray photoelectron spectroscopy analysis showed that the improvement was attained by ZTO layer acting as diffusion barrier of oxygens and/or water vapors.
Four Au : SiO 2 composite films with varying Au particle size at fixed volume fraction were fabricated by alternate sputtering, and their 3rd order nonlinear optical properties were investigated by the z-scan technique using picosecond pulse laser. TEM analysis was used to estimate the particle size for samples with varying Au nominal thickness, and size dependent linear absorption spectra were examined. Particle size dependence on the imaginary part of 3rd order susceptibility at surface plasmon wavelength together with the dispersion of the real and the imaginary components around surface plasmon wavelength were resolved and compared with the theoretical values.
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