Optical properties of polycrystalline CdSe films
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The optical properties of polycrystalline CdSe films prepared using electroless deposition techniques are discussed. Refractive index measurements are reported for the wavelength region 400-2000 nm. Extinction and absorption coefficient measurements are reported for the strong absorption regime (450-750 nm). UV reflectance measurements are also discussed.
I. INTRODUCTION The lattice structure of CdSe can be found in two different forms: zinc blende (cubic) and wurtzite (hexagonal).1"5 The zinc blende structure with lattice parameter aQ = 6.05 A is the stable form found in bulk crystals at atmospheric pressure.6 The wurtzite structure of CdSe has been observed in films prepared by various techniques including sublimation of the compound,7 oblique deposition of the compound in vacuum deposition,8"10 and sputtering.11'12 The zinc blende structure is formed by precipitation from solution at room temperature, and partial conversion takes place upon heating at 130 °C. 13 In this paper, the optical properties of a series of CdSe films prepared by electroless deposition techniques are discussed. Refractive index measurements are reported for the wavelength range 400-2000 nm. Many of the layers studied exhibited new structure in their UV reflectance which is attributed to band structure changes associated with the presence of the hexagonal phase. Extinction coefficient and absorption coefficient measurements are reported for the strong absorption regime (450-750 nm). Grain growth of the polycrystalline CdSe films deposited by electroless deposition techniques has been studied by the scanning electron microscope. II. EXPERIMENTAL A. Film preparation and structural studies All chemicals used in the present electroless deposition techniques were reagent grade. Corning 7059 glass substrates were prepared by cleaning in different reagents. The cleaning procedure has been described elsewhere.14 The deposition bath was prepared by mixing 25 ml of 0.12 M Na 2 SeSO 3 reagent, 1.0 ml of 6.5 M NH 4 OH, and 3.0 ml of 0.5 M Cd (CH 3 COO) 2 , in that order. The sodium seleno-sulfate (Na2SeSO3) reagent was freshly prepared by refluxing Na 2 SO 3 and selenium powder. After mixing, the deposition bath turned yellow in color. The cleaned glass substrates to be used for CdSe deposition were suspended vertically in the solution J. Mater. Res., Vol. 7, No. 5, May 1992 http://journals.cambridge.org
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contained in a beaker. The beaker was immersed in a 60 °C constant temperature bath. After 1-2 h of deposition, the CdSe film appeared as a transparent red coating on the surface exposed to the bath. The substrates were then removed from the bath and rinsed in deionized (DI) water. All samples were annealed at 250 °C for 3 h, in an inert atmosphere. The thicknesses of the films were measured with a Sloan Dektak surface profile instrument. Compositional analyses of CdSe samples were conducted using a Nicolet 1211 series analytical x-ray instrument. The instrument used a photomultiplier and was interfaced with a Microvax for p
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