Investigation of Zn_1−xCd_xO films bandgap and Zn_1−xCd_xO/ZnO heterojunctions band offset by x-ray photoelectron spectroscopy

A series of Zn${}_{1-x}$Cd${}_x$O thin films have been fabricated on sapphire by pulsed-laser deposition (PLD), successfully. To investigate the effect of Cd concentration on structural and optical properties of Zn${}_{1-x}$Cd${}_x$O films, x-ray diffraction (XRD), ultraviolet-visible spectroscopy (UV-vis), and x-ray photoelectron spectroscopy (XPS) are employed to characterize the films in detail. The XRD pattern indicates that the Zn${}_{1-x}$Cd${}_x$O thin films have high single-orientation of the c axis. The energy bandgap values of ZnCdO thin films decrease from 3.26 eV to 2.98 eV with the increasing Cd concentration (x) according to the ${\left(\alpha h\nu \right)}^2$−$h\nu$ curve. Furthermore, the band offsets of Zn${}_{1-x}$Cd${}_x$O/ZnO heterojunctions are determinated by XPS, indicating that a type-I alignment takes place at the interface and the value of band offset could be tuned by adjusting the Cd concentration. (paper)