Characterisation of p-type ZnS:Cu transparent conducting films fabricated by high-temperature pulsed laser deposition

TL;DR

This study reports on the fabrication and detailed opto-electrical characterization of p-type ZnS:Cu thin films, demonstrating promising conductivity and transparency for transparent electronics, with insights into effects of annealing and growth conditions.

Contribution

It introduces a high-temperature pulsed laser deposition method for ZnS:Cu films and provides comprehensive opto-electrical data, including effects of annealing and substrate motion.

Findings

Peak conductivity of 49.0 S/cm at 33% Cu content

Transparency of 71.8% at 100 nm thickness

Band gaps ranging from 3.22 to 3.52 eV

Abstract

Copper-doped zinc sulphide (ZnS:Cu) thin films were synthesized through pulsed laser ablation in an inert background gas on stationary and rotating substrates, and a comprehensive opto-electrical characterisation is presented. The Cu$_x$Zn$_{1-x}$S films demonstrated comparable conductivity and transparency to other leading p-type transparent conducting materials, with a peak conductivity of 49.0 Scm$^{-1}$ and a hole mobility of 1.22 cm$^2$V$^{-1}$s$^{-1}$ for films alloyed with an x = 0.33 copper content. The most conducting films displayed a transparency of 71.8$\%$ over the visible range at a thickness of 100 nm, and band gaps were found in the range 3.22-3.52 eV, which showed a strong negative correlation with copper content. The effects of sulphur-rich rapid thermal annealing on the synthesized compound are reported, with films reliably displaying an increase in conductivity and carrier mobility. Films grown using a stationary substrate possessed large spatial thickness distributions and displayed sub-band gap absorption, which is discussed with respect to inhomogeneous copper substitution. Films deposited at 450$^\circ$C were found to be in the zincblende phase before and after annealing, with no occurrence of a phase change to wurtzite structure.