Observation and enhancement through alkali metal doping of p-type conductivity in the layered oxyselenides Sr₂ZnO₂Cu₂Se₂ and Ba₂Zn_1-xO_2-xCu₂Se₂

Abstract

The optoelectronic properties of two layered copper oxyselenide compounds, with nominal composition Sr ₂ZnO ₂Cu ₂Se ₂ and Ba ₂ZnO ₂Cu ₂Se ₂, 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, Sr ₂ZnO ₂Cu ₂Se ₂, 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 Na _0.1Sr _1.9ZnO ₂Cu ₂Se ₂. In contrast, the barium containing material was found to have a small zinc oxide deficiency, with a sample dependent compositional range of Ba ₂Zn 1_{- x} O _{2- x} Cu ₂Se ₂ 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 Ba ₂Zn _{1- x} O _{2- x} Cu ₂Se ₂ 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 Ba ₂Zn _{1- x} O 2- x Cu ₂Se ₂ 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 K _0.1Ba 1.9Zn _{1- x} O _{2- x} Cu ₂Se ₂. Hall measurements confirmed that both materials were p-type conductors with holes as the dominant charge carriers.

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