Impact of Synthesis Parameters upon the Electronic Structure in PVD-Deposited Cd _x Zn_1-x O Composite Thin Films: An XPS-XANES Investigation

The impact of different synthesis parameters, such as thickness, postsynthesis annealing temperature, and oxygen gas flow rate, upon the electronic structure is discussed in detail in the present experimental investigation. X-ray photoelectron spectroscopy (XPS) and X-ray absorption near-edge structure (XANES) spectroscopy techniques are used to evaluate the surface electronic properties along with the presence and stability of the CdO2 surface oxide in CdxZn1-xO (x = 0.4) composite thin films. The thin films were synthesized with varying thicknesses using a Cd0.4Zn0.6O (CZO) ceramic and Cd0.4Zn0.6 (CZ) metallic targets and oxygen gas flow rates during magnetron sputtering. The Zn L-3,L-2 edge and O K edge XANES spectra are affected by the oxygen gas flow rate. For the zero rate, an increase in intensity is observed in the Zn L-3,L-2 edge, and notable changes occur in the overall spectral features of the O K edge. In the films synthesized in the presence of oxygen, highly probable O 2p -> antibonding Zn 3d electronic transitions decrease the probability of the Zn 2p(1/2) -> antibonding Zn 3d electronic transition by filling the vacant antibonding Zn 3d states, leading to the reduction in overall intensity in the Zn L-3,L-2 edge. Scanning electron microscopy reveals grain growth with increasing annealing temperature. The annealing induces orbital hybridization, generating new electronic states with higher transition probabilities and intensity enhancement in both Zn L-3,L-2 and O K edges. The presence of the CdO2 surface phase is confirmed by analyzing the Cd 3d(5/2) and O 1s XPS core levels. The CdO2 surface phase is observed in the films synthesized using the CZO target for all thicknesses, while the CZ target is only observed for higher thicknesses. Further postsynthesis annealing treatment results in the disappearance of the CdO2 phase. The CdO2 surface phase can be controlled by varying the film thickness and postsynthesis annealing temperature.