Pure Appl. Chem., Vol. 74, No. 9, pp. 1739–1749, 2002. © 2002 IUPAC 1739 Substrate temperature dependence of electrical conduction in nanocrystalline CdTe:TiO 2 sputtered films* S. N. Sharma 1,‡ , S. M. Shivaprasad 2 , Sandeep Kohli 3 , and A. C. Rastogi 1 1 Materials Division, 2 Surface Physics Group, National Physical Laboratory, Dr. K. S. Krishnan Marg, New Delhi 110012, India; 3 Department of Chemistry, Colorado State University, Fort Collins, CO 80523, USA Abstract: TiO 2 thin films with high volume fraction (~50–70 %) of CdTe nanoparticles were prepared by radio frequency (rf) magnetron sputtering from a composite TiO 2 :CdTe target. With increase in substrate temperature T s from room temperature (RT ~ 300 K) to 373 K, a transition from an ordered structure exhibiting metallic-type conduction to a dis- ordered structure exhibiting nonmetallic-type conduction was observed for annealed nanocrystalline CdTe:TiO 2 films. The annealed RT-deposited films showed a large coales- cence of distinct islands (size ~0.3–0.7 µm) mainly of Cd and CdTe, and as result, a 3D network was realized. For metallic regime films, electrical conduction is essentially due to electrical percolation through Cd/CdTe crystallites embedded in an amorphous TiO 2 matrix. However, the annealed high T s films consisted of noncoalescent, small islands (size ~0.15–0.3 µm) of Cd and CdTe embedded in amorphous TiO 2 matrix. Here, the conduc- tion is essentially by hopping mechanism via thermally activated tunneling. INTRODUCTION Nanocomposite films of semiconductors and insulators have been the subject of intensive studies from both fundamental, experimental, and applied interests [1]. CdS, CdSe, and CdTe are among the most studied cases [2]. CdTe, owing to its large exciton Bohr diameter (15 nm), is a strong contender for the future low-dimensional optoelectronic devices [3]. Although the optical properties of nanocomposite films have been studied intensively, the electrical transport properties of these films are not fully under- stood. Therefore, it is imperative to understand the electron transport process in these low-dimensional structures having different sets of grain size and inter-grain size. Thus, by varying the substrate tem- perature (T s ) during deposition, a series of films with different grain sizes and inter-grain separations could be obtained. Among the various physical vapor deposition (PVD) techniques for the preparation of CdTe nanocrystalline composites in TiO 2 matrix, rf sputtering has been the most versatile and proven one, since it enables the controlled distribution of nanoparticle size and volume over a wide range [4]. Moreover, using rf magnetron sputtering, the processing at relatively low temperatures can avoid the possible reaction between the matrix and the semiconductor nanoparticles [4]. The present work reports detailed electrical studies of 0.5 µm rf magnetron co-sputtered nanocrystalline CdTe:TiO 2 composite thin films deposited at low and high T s (RT and 373 K), respec- *Pure Appl. Chem. 74, 1489–1783 (2002). An issue of reviews and research papers based on lectures presented at the 2 nd IUPAC Workshop on Advanced Materials (WAM II), Bangalore, India, 13–16 February 2002, on the theme of nanostructured advanced materials. ‡ Corresponding author: Present address: Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana, IN 46556, USA; Tel.: 574-631-5407; Fax: 574-631-8068; E-mail: sharma@hertz.rad.nd.edu