Effects of the d-donor level of vanadium on the properties of Zn_1-xV_xO films

We report the effect of d-levels of vanadium atoms on the electronic band structure of ZnO. Polycrystalline layers of Zn_1-xV_xO with 0 ≤ x ≤ 0.08 were synthesized using magnetron sputtering technique. Electrical measurements show that electron concentration increases with vanadium up to x = 0.04 and then decreases and films become insulating for x > 0.06. Optical characterization reveals that the absorption edge shifts to higher energy, while the photoluminescence (PL) peak shows a shift to lower energy with increasing vanadium content. This unusual optical behavior can be explained by an anticrossing interaction between the vanadium d-levels and the conduction band (CB) of ZnO. The interaction results in an upward shift of unoccupied CB (E+) and the downward shift of the fully occupied E- band derived from the vanadium d-levels. The composition dependence of optical absorption edge (E+) and PL peak (E-) can be fitted using the Band Anticrossing model with the vanadium d-level located at 0.13 eV below CB of ZnO and a coupling constant of 0.65 eV.