JOURNAL OF MATERIALS SCIENCE LETTERS 11 (1992) 1097-1099 Studies on thin films of ZnSe prepared by a chemical deposition method G. N. CHAUDHARI, S. D. SATHAYE, PRABHAT SlNGH, V. J. RAO* Physical and Structural Chemistry Division, National Chemical Laboratory, Pune 411007, India V. MANORAMA, R. S. BHIDE, S. V. BHORASKAR Physics Department, University of Poona, Pune 411007, India Zinc selenide (ZnSe) has several properties which make it an attractive material for optoelectronic device applications; it has a direct bandgap, is transparent over a wide range of the visible spectrum and has a relatively large non-linear optical coeffi- cient. Some devices which take advantage of these properties include blue-light-emitting diodes, elec- troluminescent display, second-harmonic genera- tors, frequency mixers and optical modulation. Also, it is a prospective material for GaAs passiva- tion, as there is a very small lattice mismatch. For this reason these materials have been extensively studied as single crystals and also as epitaxial and polycrystalline thin films [1-6]. Polycrystalline thin films are of particular importance in the preparation of electroluminescent structures compatible with large-area displays and integrated circuits. Several deposition techniques have been used to produce polycrystalline ZnSe films. The most com- mon are based on sputtering [7] and electron-beam evaporation [8]. Recently techniques have been developed for depositing polycrystalline films on non-crystalline substrates [4, 7-9]. These latter tech- niques have distinct advantages with respect to controlling impurity and the orientation of the films. This letter describes our successful attempt at depositing polycrystalline ZnSe films by a simple technique known as the liquid-gas interface reaction (LGIR) technique, which yields good-quality films at higher bath temperature. The technique is an extension of the one reported earlier [10], where the details are given. Structural properties were studied by X-ray diffraction (XRD; Philips PW 1730), trans- mission electron microscopy (TEM; Jeol 1200EX) and the compositions of these films were estimated by X-ray fluorescence (XRF; Rigacu 3070). Thin films of ZnSe were grown by a simple technique based on a double-decomposition reac- tion. The reactants were an aqueous solution of zinc, e.g. zinc acetate (pH of solution about 5) and hydrogen selenide, which was generated in situ by the reaction of a metallic selenide such as cadmium selenide with concentrated hydrochloric acid. The reactions were CdSe + 2HC1 ~ H2Se + CdC12 (1) *Present address: Indian Institute of Chemical Technology, Hyderabad 500 007, India. 0261-8028 © 1992 Chapman & Hall Zn(CH3COO)2 + HzSe --->ZnSe(film) + 2CH3COOH (2) Hydrogen selenide gas generated in Reaction 1 reacted with zinc acetate solution and formed a thin film of ZnSe at the gas-solution interface. The experimental set-up consisted of an airtight desicca- tor which enclosed the reactants. The reaction was allowed to take place for about 2 min. Once the film is formed the reaction stops, because there is no more zinc acetate exposed to the hydrogen selenide. The ZnSe films were then picked with the help of an inverted watch glass and allowed to float on deionized water. This was to remove any residual zinc acetate or other soluble contaminated impuri- ties. The films then could be picked on to any desired substrate. The films picked on clear glass slides were allowed to dry. The average film thickness was in the range 0.05-0.1 #m. XRD data reveal that the films prepared at different bath temperatures show that ZnSe depo- sited at 2 °C and room temperature were amorphous and the films deposited at the higher bath tem- perature of 80 °C showed a polycrystalline nature. The observed d-values are in good agreement with standard d-value taken from the American Society for Tesing and Materials diffraction data file. The structure of the films grown at room tem- perature was totally amorphous, as no electron diffraction patterns could be obtained. However, the crystallinity of the films grown at 80 °C was good. The corresponding electron diffraction pattern be- came sharper and well-defined. Gold was taken as a standard to obtain the spectrometer constant. Table I gives the observed d-values and the literature values for a cubic polycrystalline ZnSe film deposited on a Cu grid at 80 °C. Fig. 1 shows the TABLE I d-Value data (dealt, calculated and dlit, literature values) of ZnSe from the electron diffraction pattern d-Value (nm) Ring of radius (cm) d~lc d~t h k l 0.57 0.327 72 0.3273 111 0.93 0.200 63 0.2003 2 2 0 1.15 0.163 82 0.1635 2 2 2 1.45 0.13023 0.1299 33 1 1.60 0.11775 0.11561 422 1097