Influence of Cd doping on structural and optical properties of (Cd, Al)-codoped ZnO powders synthesized via sol-gel method

The effect of Cd doping on structural and optical properties of 1 at.% Al-doped Zn1−xCdxO (x = 0-8%) powders prepared by sol-gel method was systematically investigated. X-ray diffraction (XRD) patterns revealed the powders retained a hexagonal wurtzite structure of ZnO below x = 6%, while cubic rocksalt CdO appeared when x = 8%. The bandgap and near band emission (NBE) energies determined from ultraviolet-visible (UV-vis) absorbance and photoluminescence (PL) spectra, respectively, decreased linearly with increasing Cd content up to x = 6%, it seemed that the Cd worked effectively on ZnO bandgap engineering. However, the Stokes shift unexpectedly decreased from 136 meV to 33 meV with increasing Cd content. According to the fluctuation of integrated intensity of Cd 3d5/2 X-ray photoelectron spectroscopy (XPS), and redshift for x ⩽ 4% and then blueshift for x > 4% of the broadening A1(LO)/E1(LO) Raman mode, it was concluded that CdO impurity might also exist in the sample of x = 6% at least. The redshift of bandgap and NBE could not be simply ascribed to bandgap shrinkage of Zn1−xCdxO powders for x > 4%, but also due to the existence of CdO impurity with narrower bandgap of 2.3 eV.