Effect of thickness on the optical absorption edge of sputtered vanadium oxide films

A study of the optical absorption edge of dc magnetron sputtered vanadium oxide films is reported. It is shown that the absorption edge is strongly dependent on the thickness of the films. From the spectral transmittance characteristics and optical absorption edge behaviour it is demonstrated that there is a critical thickness (250 nm) above which the films become completely stoichiometric V2O5. At thickness greater than 250 nm the absorption edge is 2.24 eV corresponding to stoichiometric V2O5 and below that the values range between 2.4 and 2.5 eV indicating the possible existence of non-stoichiometry in thinner films. The most significant result of the present study is to show that thickness of the films in conjunction with oxygen partial pressure can be used to modulate optical band gap of vanadium oxide films.