Absorption edge shift in ZnO thin films at high carrier densities

The optical absorption edge has been measured as a function of carrier concentration for thin films of zinc oxide prepared by organometallic chemical vapour deposition and reactive R.F. magnetron sputtering. Large shifts of the absorption edge have been observed which are only a function of the carrier concentration. Below n = 3 × 10¹⁹ cm^-3 the shifts are well described by the Burstein-Moss model. For carrier concentrations between 3–5 ×10¹⁹cm^-3, the shift decreases very rapidly, finally increasing again with further increases in the carrier density. These effects are consistent with a merging of the donor band with the conduction band following a semiconductor-metal transition.