Soft x-ray spectroscopy of Ca-doped BiCuOSe thin films grown by pulsed laser deposition

Thin films of Ca-doped BiCuOSe were grown on (001) MgO and SrTiO$_{3}$ substrates via pulsed laser deposition. X-ray absorption and emission spectroscopy were used to analyze the O $K$-edge and Cu $L_{3,2}$-edge in order to determine the bulk electronic structure of BiCuOSe. Analysis of the O $K$-edge XAS/XES spectra yield a band gap of $\sim $1 eV, consistent with optical measurements on thin films. Optical measurements on single crystals show a band gap of $\sim $0.83 eV. XAS/XES results show the presence of strongly hybridized Bi 6$s$ - O 2$p$ orbitals in undoped samples. This hybridization is diminished in doped samples, providing strong evidence of Ca$^{2+}$ on the Bi$^{3+}$ site. X-ray diffraction measurements show that the films are highly oriented, with rocking curves around the (003) peak having a FWHM of 1\r{ }. Expansion of the c-axis is observed as Ca concentration is increased. All films show $p$-type conductivity and develop more metallic character as calcium doping increased. Ca concentration was determined by EPMA, which shows non-stoichiometric transfer of Ca from the target into the films.