Synthesis and characterization of RF-sputtered ZnTe/Cu_{2- x} Te thin films for solar cell applications

The incorporation of copper and oxygen into zinc telluride (ZnTe) thin films deposited by radio-frequency magnetron sputtering from a single ZnTe-CuO composite target was investigated. The nominal Cu and O concentrations ranged from 3 to 13 atom %, and films were grown at substrate temperatures of 300 and 350 °C. Energy-dispersive X-ray spectroscopy confirmed controlled compositional transfer from the target to the films. X-ray diffraction analysis revealed that all films are polycrystalline, exhibiting a coexistence of zinc blende and wurtzite ZnTe phases. Low dopant concentrations produced only minor lattice modifications, while higher Cu and O contents promoted the formation of Cu_{2- x} Te secondary phases, as confirmed by Raman spectroscopy, grazing-incidence X-ray diffraction and scanning electron microscopy. Optical measurements showed a significant reduction in infrared transmittance with increasing Cu concentration, attributed to the metallic-like absorption of Cu_{2- x} Te phases. Electrical characterization revealed a transition from semiconducting to highly conductive behavior with resistivity decreasing from approximately 10² to 10^-2 Ω·cm for films grown at 300 °C and from 10¹ to 10^-3 Ω·cm for films grown at 350 °C. Simultaneously, the carrier concentration increased from approximately 10¹⁷ to 10²¹ cm^-3 and the mobility from 10^-1 to 10¹ cm²·V^-1·s^-1. These results indicate that structural, optical, and electrical properties of ZnTe are primarily influenced by Cu incorporation and the formation of conductive Cu-rich secondary phases, yielding a biphasic system composed of semiconducting ZnTe and conductive Cu-rich telluride phases. These material properties suggest potential relevance for future studies of back-contact materials in CdTe-based solar cells.