On the blueshift in Sn1-xCoxO2-delta transparent ferromagnetic semiconductor thin films

摘要

Optical properties of spray pyrolysed Sn1-xCoxO2-delta (0 <= x <= 0.15) thin films exhibiting room temperature ferromagnetism have been investigated in the energy range 1 - 6 eV. The onset of optical absorption is blueshifted by as much as 220 meV for a cobalt concentration of 10a/o (x = 0.10). Our detailed analysis indicates that the most probable cause of the blueshift is the Burstein Moss effect. The optical conductivity calculations reveal that Co does not introduce any mid-gap states. Also the small change in the slope of alpha(2) versus h. indicates that the O and Sn band edges are affected on Co addition. This band-edge shift also implies the presence of strong Coulomb interaction between Co, Sn and O sites, as a plausible cause of the observed ferromagnetism in these samples. The Burstein - Moss effect seems to be masking the effect of band structure modification introduced by Co addition. The high (similar to 75%) optical transparency of these films in the visible region remains practically unaffected on Co addition.