AVS 46th International Symposium
    Thin Films Division Thursday Sessions
       Session TF-ThM

Paper TF-ThM10
Correlation Between Phase Constituency and Near Ultraviolet Optical Absorption in Nanophase Titania Films

Thursday, October 28, 1999, 11:20 am, Room 615

Session: Nanophase Thin Films
Presenter: C.R. Aita, University of Wisconsin, Milwaukee
Authors: J.D. DeLoach, University of Wisconsin, Milwaukee
G. Scarel, University of Wisconsin, Milwaukee
C.R. Aita, University of Wisconsin, Milwaukee
Correspondent: Click to Email
Titania forms coexisting nanocrystalline and vitreous structures in films grown at room temperature. This study's goal is to correlate fundamental optical absorption edge characteristics with nanophase constituency of titania films. Films with coexistent rutile, anatase, and vitreous constituents were sputter deposited onto fused silica, and post-deposition air-annealed at 700 and 1000 @super o@C to affect phase changes. X-ray diffraction was used for phase identification, and UV spectrophotometry was used to determine the optical absorption coefficient at the onset of interband transitions. The absorption coefficient was modeled using the coherent potential approximation, with Gaussian site disorder introduced into the valence and conduction bands of a perfect virtual crystal. Two parameters of the disordered crystal were defined: the optical band gap, E@sub x@, and the slope of absorption edge, E@sub o@. The results are discussed in terms of two extreme cases: (1) Films containing a large rutile volume fraction (0.70-1) share a rutile virtual crystal, with E@sub g@=3.22 eV. Data for these films were combined with single crystal data to develop an expression interrelating E@sub g@, E@sub x@, and E@sub o@. This expression is applicable to any structure with a rutile virtual crystal. The relationship between structural disorder (i.e., volume fraction of vitreous material) and electronic disorder (i.e., E@sub o@), is consistent with the CPA model. (2) Films with a small rutile volume fraction (0.02-0.17), and hence a large anatase+vitreous component, share a non-rutile virtual crystal, with E@sub g@=3.41 eV. The effect of increasing the structural disorder (i.e., rutile volume fraction), in these films is to shift E@sub x@ to lower values, which is consistent with the CPA model. Furthermore, anatase and vitreous components were modeled using the same non-rutile virtual crystal, indicating these structures have a common short-range order in these films.