Structural, Electronic Band Transition and Optoelectronic Properties of p-Type Transparent Conductive CuCr1-xNixO2 Semiconductor Films

The transparent conductive CuCr1-xNixO2 (0 <= x <= 8%) films were prepared by sol-gel method with two-step annealing process. The structure, morphology, phonon modes, chemical composition, optical bandgap, and electrical properties were systematically studied. It was found that the crystal quality improves first and then decreases with increasing the Ni content, and the inflection point occurs when the doping content is 2% and all samples have the (00l) preferred orientation. In the visible region, the transmittances of these samples range from 75% to 55% as the Ni dopant increases. On the basis of the theoretical calculation, we are inclined to the view that there are two ways of coexisting electronic transitions. The direct band gap decreases from 3.13 to 3.02 eV, and the indirect band gap decreases from 2.78 to 2.49 eV with increasing Ni content. Transmittance spectra of the CuCrO2 film at the temperatures from 323 to 453 K present the fact that the absorption edge shows a slight redshift, which is due to the lattice thermal expansion and the electron-phonon interaction. Raman spectroscopy shows that Ni doping can substitute the Cr sites and further affect Cu-O bonds, which is caused by oxygen deficiency or a relatively smaller crystallite size. Moreover, it suggested that the nonlinear temperature dependence of phonon frequency and line width is a sign of nonharmonic lattice dynamics. The inherent anharmonic force from the crystal results in energy exchange between harmonic phonon normal modes, thereby promoting thermal balance, and in the lattice. The Cu cations have been confirmed to exist in the valence state of Cu1+, and all doped Ni cations exist in a positive divalent state by XPS. Hall effect measurement shows that the conductivities and carrier concentrations of the doped CuCr1-xNixO2 films can be improved by several orders of magnitude larger than that of undoped CuCrO2. The present work suggests that these well crystallized films without a secondary phase can be applied in future p-type optoelectronic devices.