Growth characteristics, material properties, and optical properties of zinc oxysulfide films deposited by atomic layer deposition

Abstract

Zinc oxysulfide—Zn(O,S)—is a wide bandgap semiconductor with tunable electronic and optical properties, making it of potential interest as a buffer layer for thin film photovoltaics. Atomic layer deposition (ALD) of ZnS, ZnO, and Zn(O,S) films from dimethylzinc, H2O, and H2S was performed, and the deposited films were characterized by means of x-ray diffraction, x-ray photoelectron spectroscopy, and spectroscopic ellipsometry. With focus on the investigation of Zn(O,S) film growth characteristics and material properties, the ZnO/(ZnO + ZnS) ALD cycle ratios were systematically varied from 0 (ZnS ALD) to 1 (ZnO ALD). Notably, a strong effect ofthematerial properties on the optical characteristics is confirmed for the ternary films. The Zn(O,S) ALD growth and crystal structure resemble those of ZnS up to a 0.6 cycle ratio, at whichpoint XPS indicates 10% oxygen is incorporated into the film. For higher cycle ratios thefilm structure becomes amorphous, which is confirmed with XRD patterns and also reflected inthe optical constants as determined by spectroscopic ellipsometry; in particular, the optical bandgap transforms from direct type for the (cubic) ZnS like phase to a more narrow bandgap withamorphous characteristics, causing bandgap bowing. A direct bandgap is recovered atyethigherZnO/(ZnO + ZnS) cycle ratios, whereproperties converge toward ZnO ALD in termsof film growth rate, crystallinity, and composition.