P-Type Transparent Cu-Alloyed ZnS Deposited at Room Temperature

All transparent conducting materials (TCMs) of technological practicality are n-type; the inferior conductivity of p-type TCMs has limited their adoption. In addition, many relatively high-performing p-type TCMs require synthesis temperatures >400 °C. Here, room-temperature pulsed laser deposition of copper-alloyed zinc sulfide (Cu_xZn_{1- x}S) thin films (0 ≤ x ≤ 0.75) is reported. For 0.09 ≤ x ≤ 0.35, Cu_xZn_{1- x}S has high p-type conductivity, up to 42 S cm⁻¹ at x = 0.30, with an optical band gap tunable from ≈3.0–3.3 eV and transparency, averaged over the visible, of 50%–71% for 200–250 nm thick films. In this range, synchrotron X-ray and electron diffraction reveal a nanocrystalline ZnS structure. Secondary crystalline Cu_yS phases are not observed, and at higher Cu concentrations, x > 0.45, films are amorphous and poorly conducting. Within the TCM regime, the conductivity is temperature independent, indicating degenerate hole conduction. A decrease in lattice parameter with Cu content suggests that the hole conduction is due to substitutional incorporation of Cu onto Zn sites. This hole-conducting phase is embedded in a less conducting amorphous Cu_yS, which dominates at higher Cu concentrations. The combination of high hole conductivity and optical transparency for the peak conductivity Cu_xZn_{1- x}S films is among the best reported to date for a room temperature deposited p-type TCM.