Optical properties of CdSe and CdTe nanoparticles embedded in SiO 2 films
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Optical properties of CdSe and CdTe nanoparticles embedded in SiO2 films P. Babu Dayal *1, B. R. Mehta 1, and P. D. Paulson 2 Department of Physics, Indian Institute of Technology Delhi, New Delhi-16, India. 2 Institute of Energy Conversion, University of Delaware, Newark, Delaware-19716, USA. 1
Abstract CdSe and CdTe nanoparticles dispersed in silicon dioxide films have been grown by magnetron sputtering technique followed by thermal annealing. Effect of thermal annealing conditions on the structural and optical properties of CdTe and CdSe nanoparticles has been studied. Glancing angle x-ray diffraction (GAXRD), optical absorption (OA), transmission electron microscope (TEM) and spectroscopic ellipsometry (SE) techniques have been used to study the crystal structure, particle size and dielectric function spectra of nanoparticles. ε2 spectra of CdTe nanoparticles show a decrease in dielectric constant values in comparison to bulk CdTe. ε2" spectrum of CdTe nanoparticles show four critical points in the electronic band structure of CdTe. Optical absorption studies in the case of CdTe and CdSe nanoparticles clearly show the blue shift of fundamental absorption edge due to quantum confinement effect. Introduction: Semiconductor nanoparticles dispersed in glass matrix have attracted significant research attention due to the unique properties of optical gain, bi-stability and ultra-fast relaxation time due to quantum confinement of charge carriers [1-5]. Electronic and optical properties of the semiconductor nanoparticles can be controlled by varying size, shape and surface layers [6-9]. SiO2 matrix is an ideal medium for studying the optical properties of nanoparticles, as it provides a protective environment to the nanoparticle surface right from its early stages of growth and its transparent nature makes optical characterization easy [10-14].This is advantageous in comparison to chemically capped or nanoparticles dispersed in other matrices [15]. Even though size-dependent properties of semiconductor nanoparticles of I-VII, II-VI, III-V and IV group semiconductors in glass matrix have been widely reported [15], the non-stoichiometry of nanoparticles that effecting structural and optical properties, remains a problem to be sorted out. The control of nanoparticle size with varying the stoichiometry and surface structure can be achieved by using elemental Cd, Se and Te along with SiO2 targets. This has been done by keeping the area of elemental targets in proportion to the sputtering yield. This method of making semiconductor nanoparticles using elemental targets by magnetron sputtering seems to be superior in comparison to conventional melting, quenching techniques and sputtering technique using compound targets [16]. Although ‘Optical properties of II-VI semiconductors’ is probably the most widely reported nanoparticle characteristic, experimental studies are limited to size-induced shift of fundamental absorption edge [12-14,17-19]. No efforts were there to understand the dielectric function and the subsequen
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