Observation and enhancement through alkali metal doping of p-type conductivity in the layered oxyselenides Sr2ZnO2Cu2Se2 and Ba2Zn1-x O2-x Cu2Se2.

The optoelectronic properties of two layered copper oxyselenide compounds, with nominal composition Sr2ZnO2Cu2Se2 and Ba2ZnO2Cu2Se2, 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, Sr2ZnO2Cu2Se2, adopts the expected tetragonal Sr 2 Mn 3 SbO 2 structure with I4/mmm symmetry, and has a band gap of 2.16 eV, and a room temperature conductivity of 4.8 × 10-1 S cm-1. The conductivity of the compound could be increased to 4.2 S cm-1 when sodium doped to a nominal composition of Na0.1Sr1.9ZnO2Cu2Se2. In contrast, the barium containing material was found to have a small zinc oxide deficiency, with a sample dependent compositional range of Ba2Zn1-x O2-x Cu2Se2 where 0.01 < x < 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 I4/mmm 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 Ba2Zn1-x O2-x Cu2Se2 does not adopt a long range ordered arrangement, but also that the idealised I4/mmm structure with an unsplit oxide site was not consistent with the extra reflections observed in the electron diffractograms. The band gap and conductivity of Ba2Zn1-x O2-x Cu2Se2 were determined to be 2.22 eV and 2.0 × 10-3 S cm-1 respectively. The conductivity could be increased to 1.5 × 10-1 S cm-1 with potassium doping in K0.1Ba1.9Zn1-x O2-x Cu2Se2. Hall measurements confirmed that both materials were p-type conductors with holes as the dominant charge carriers.