AVS 49th International Symposium
    Nanometer Structures Tuesday Sessions
       Session NS-TuP

Paper NS-TuP5
Substrate Temperature Dependence of Electrical Conduction in Nanocrystalline CdTe:TiO@sub 2@ Sputtered Films

Tuesday, November 5, 2002, 5:30 pm, Room Exhibit Hall B2

Session: Nanometer Structures A
Presenter: S.N. Sharma, National Physical Laboratory, India
Authors: S.N. Sharma, National Physical Laboratory, India
S. Kohli, Colorado State University
S.M Shivaprasad, National Physical Laboratory, India
A.C. Rastogi, University of Massachusetts
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TiO@sub 2@ thin films with high volume fraction (~ 50-60 %) of CdTe nanoparticles were prepared by rf magnetron sputtering from a composite TiO@sub 2@:CdTe target. Variations in the substrate temperature, T@sub S@ (room temperature-RT and 373 K), produces two distinct structural regimes with different electrical properties in thermally treated TiO@sub 2@ thin films: (i) a metallic regime for low T@sub S@ films where the CdTe/Cd grains touch each other and form a metallic continuum. Here, homogeneous, ordered and electrically-continuous structure was obtained. Here, due to the presence of large-scale coalescent islands of CdTe/Cd, three-dimensional network of crystallites could be realized. Such films exhibited positive temperature coefficient of resistance. For metallic regime films, electrical conduction is essentially due to electrical percolation through CdTe/Cd crystallites embedded in an amorphous TiO@sub 2@ matrix. XPS studies indicated the segregation of metallic Cd upon thermal treatment which were responsible for metallic-type of conduction exhibited by these films. (ii) a non-metallic regime for high Ts films in which small isolated particles of CdTe/Cd are dispersed in an amorphous matrix. Here, highly disordered and electrically-discontinuous structure was obtained and thus formation of CdTe network could not be realized. The formation of oxides on the surface of the CdTe/Cd crystallites acts as an electrical insulation and thus, the conduction is thermally activated. The electrical conduction in high T@sub S@ films is essentially by hopping mechanism thus indicating transfer of charge carriers via thermally activated tunneling. Such films exhibited negative temperature coefficient of resistance.