Impact of sulfurization parameters on properties of CZTS thin films grown using quaternary target

Abstract

In this study, CZTS thin films were grown by annealing of sputtered films using quaternary single target employing various annealing parameters. The effects of the post-sulfurization treatment, reaction temperature (500, 525, 550 and 575 °C) and sulfurization time (60, 90, 120 and 150 s) on the properties of CZTS thin films were analyzed. The optimization of reaction temperature for 60 s dwell time was examined by annealing the precursor films with/without sulfur atmosphere. It was shown that annealing of the films under the sulfur atmosphere prevents Zn-loss in the samples for higher annealing temperatures (550 and 575 °C) and hindering the formation of secondary phases such as Cu2−xS, Cu2SnS3 (CTS). The FWHM values of the sulfurized samples revealed that the sulfurization temperature of 550 °C is preferable for the fabrication of CZTS samples. Further optimization was performed at 550 °C for various sulfurization times. It was seen that all the samples have Cu-poor and Zn-rich composition. The XRD pattern of CZTS samples displayed formation of kesterite CZTS phase but SnS2 phase formations were also observed for longer sulfurization time (> 120 s). It was also observed that the sulfurization time has more significant contribution on the crystallite size of the samples with respect to sulfurization temperature. The Raman spectra of the CZTS samples confirmed the formation of kesterite structures for all the films and appearance of secondary phase for films prepared using longer sulfurization time (> 120 s). All the samples displayed a dense and polycrystalline surface morphology, but the sulfurized sample for 120 s displayed more homogenous and prominent morphology. The room temperature PL measurements demonstrated a broad band which peaked at about 1.36–1.37 eV, which is very close to the band gap of kesterite CZTS structure. The electrical characterization of the samples showed that all the samples have p-type conductivity and the CZTS-S-550-120 sample has a more promising result considering both resistivity and carrier concentration.