Spectroscopic ellipsometry study of optical transitions in Zn1-xCoxO alloys

Zn1-xCoxO (xless than or equal to0.22) films were prepared on (0001)-oriented Al2O3 substrates by rf magnetron sputtering. The alloys show wurtzite crystal structure with the c-axis lattice constant increasing with increasing x. The optical properties of the samples were measured by spectroscopic ellipsometry at room temperature in the 1.5-5 eV photon energy region. As x increases, the optical band gap absorption edge (E-0) of the alloys shows a redshift from that of pure ZnO, reaching 350 meV for x=0.22. The excitonic character of the E-0 edge is gradually reduced as x increases and is replaced by the three-dimensional critical-point shape. Optical absorption structures are also observed below the E-0 edge near 2 eV and interpreted as due to the transitions between the crystal-field-split 3d levels of tetrahedral Co2+ ions substituting Zn2+ ions. (C) 2002 American Institute of Physics.