No abstract

Perovskite-structured Ba 1−x La x SnO 3 (BLSO, x = 0− 0.10) films have been directly deposited on (0001) sapphire substrates by a sol−gel method. The effects of La substitution on the microstructure, morphology, and carrier density of the BLSO films have been investigated by X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), and Halleffect measurement. XRD analysis shows that all of the films are of a single perovskite phase and the calculated lattice constants agree well with the theoretical lattice parameters. The AFM and SEM pictures show that the BLSO films do not contain cracks with the root-meansquare surface roughness of 2 nm. Temperature dependent conductivity behavior suggests that BLSO films do have a metalliclike conduction, but the resistivity in poly samples is dominated by grain boundary scattering and ionized-dopant scattering. The films exhibit high transparency with more than 80% in the visible region, which is due to the prohibition of electron transition from the conduction band minimum to the second conduction band. Dielectric functions in photon energy of 0.5−6.5 eV can be uniquely extracted by fitting transmittance spectra with the double Tauc-Lorentz and Drude model. It was found that there are two interband transitions (about 4 and 6 eV) and the peak energies increase linearly with increasing La concentration. Farinfrared reflectance spectra was measured in the frequency range of 50−700 cm −1 and three main reflection bands can be observed. The phonon strength of BLSO films decreases with increasing La concentration.

Temperature-dependent interband transitions and exciton excitations of sol-gel derived CuCr 1−x Mg x O 2 (2% x 8%) films have been investigated by transmittance spectra (8-300 K) and photoluminescence (PL) spectra (77-300 K). An abnormal dependence of the optical band gap with the temperature has been found for the films with x = 0.02 and 0.04. At the low-temperature region, the gap energy shows a redshift trend with decreasing temperature. It is due to the strong Cr 3d-O 2p-Cu 3d interaction in the upper part of the valence band, which can be weaken by heavily Mg doping. The spin-orbit interactions of Cr 3+ ions in an octahedral environment make the 3d states more disperse, which can contribute to the relatively high conductivity. A well-defined low-energy absorption has been assigned to the spin-allowed 3d → 3d transition. Moreover, a strong exciton excitation around 1.8 eV has been observed due to the naturally low-dimensional structure of the delafossite, which can be modulated by temperature and hole concentration.

No abstract

Highly transparent CuCr1−xMgxO2 (0 ≤ x ≤ 12%) films were prepared on (001) sapphire substrates by sol-gel method. The microstructure, phonon modes, optical band gap, and electrical transport properties have been systematically discussed. It was found that Mg-doping improved the crystal quality and enhanced the (00l) preferred orientation. The spectral transmittance of films approaches about 70%–75% in the visible-near-infrared wavelength region. With increasing Mg-composition, the optical band gap first declines and climbs up due to the band gap renormalization and Burstein-Moss effect. The direct and indirect band gaps of CuCr0.94Mg0.06O2 film are 3.00 and 2.56 eV, respectively. In addition, it shows a crossover from the thermal activation behavior to that of three-dimensional variable range hopping from temperature-dependent electrical conductivity. The crossover temperature decreases with increasing Mg-doping composition, which can be ascribed to the change of spin-charge coupling between the hole and the local spin at Cr site. It should be noted that the electrical conductivity of CuCr1−xMgxO2 films becomes larger with increasing x value. The highest electrical conductivity of 3.85 S cm−1 at room temperature for x = 12% is four-order magnitude larger than that (8.81 × 10−4 S cm−1) for pure CuCrO2 film. The high spectral transmittance and larger conductivity indicate that Mg-doped CuCrO2 films are promising for optoelectronic device applications.