Structural properties and enhanced bandgap tunability of quaternary CdZnOS epitaxial films grown by pulsed laser deposition

Abstract

Bandgap engineering of ZnO was attempted with isovalent cation and anion co-substitution of Cd for Zn and S for O, respectively. Growth of quaternary CdxZn1−xO1−ySy (CdZnOS) films was realized by pulsed laser deposition using a self-made Cd0.2Zn0.8O0.11S0.89 ceramic target with various O2 pressures. Single-phase CdxZn1−xO1−ySy alloy films having a wurtzite structure were epitaxially grown on c-plane sapphire substrates with different S contents (0 ≤ ys ≤ 0.26) and a nearly constant Cd concentration of 5%. Upon decreasing O2 pressure from 6 to 1.5 Pa, i.e., increasing S content from 0 to 0.26, the optical bandgap of CdxZn1−xO1−ySy films was tuned downward from 3.26 to 2.58 eV. With simultaneous incorporation of certain amount of Cd, the quaternary CdZnOS films exhibit significantly narrower bandgaps than the ternary ZnOS films with the same S content. The synergetic co-substitution of Cd and S leads to considerably extended solid solubility in addition to cumulative bandgap adjustment, eventually resulting in an enhanced bandgap tunability of quaternary CdZnOS.