Observation and enhancement through alkali metal doping of p-type conductivity in the layered oxyselenides SrZnOCuSe and BaZn O CuSe.

The optoelectronic properties of two layered copper oxyselenide compounds, with nominal composition SrZnOCuSe and BaZnOCuSe, have been investigated to determine their suitability as p-type conductors. The structure, band gaps and electrical conductivity of pristine and alkali-metal-doped samples have been determined. We find that the strontium-containing compound, SrZnOCuSe, adopts the expected tetragonal structure with 4/ symmetry, and has a band gap of 2.16 eV, and a room temperature conductivity of 4.8 × 10 S cm. The conductivity of the compound could be increased to 4.2 S cm when sodium doped to a nominal composition of NaSrZnOCuSe. In contrast, the barium containing material was found to have a small zinc oxide deficiency, with a sample dependent compositional range of BaZn O CuSe where 0.01 < < 0.06, as determined by single crystal X-ray diffraction and powder neutron diffraction. The barium-containing structure could also be modelled using the tetragonal 4/ structure, but significant elongation of the oxygen displacement ellipsoid along the Zn-O bonds in the average structure was observed. This indicated that the oxide ion position was better modelled as a disordered split site with a displacement to change the local zinc coordination from square planar to linear. Electron diffraction data confirmed that the oxide site in BaZn O CuSe does not adopt a long range ordered arrangement, but also that the idealised 4/ structure with an unsplit oxide site was not consistent with the extra reflections observed in the electron diffractograms. The band gap and conductivity of BaZn O CuSe were determined to be 2.22 eV and 2.0 × 10 S cm respectively. The conductivity could be increased to 1.5 × 10 S cm with potassium doping in KBaZn O CuSe. Hall measurements confirmed that both materials were p-type conductors with holes as the dominant charge carriers.