Optical Response of As 2 S 3 : Melt Quenched Versus Spin Casted Materials
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OPTICAL RESPONSE OF As2 S3 : MELT QUENCHED VERSUS SPIN CASTED MATERIALS M. HANWAM AND J.J. SANTIAGO University of Pennsylvania, Center for Sensors Technology, Moore School of Electrical Engineering, Philadelphia, PA 19104 ABSTRACT The transmittance spectra of amorphous As2S3 films prepared by two different techniques, namely spin coating and quenched from the melt were
investigated. All measurements were done at room temperature in the energy range from 1.2 to 3.8 eV. The spin casted material was prepared by dissolving powdered stoichiometric arsenic trisulfide in amines and amides, the resulting solution spinned on an oxide glass substrate and baked to remove the volatiles. A typical resulting film was a few micrometers thick and
microstructure free as characterized under an optical microscope. The quenched material was prepared by melting a stoichiometric mixture of the reactants in an evacuated and sealed ampoule and alloying at 1100'K for 15 hrs. The melt was subsequently quenched in air to obtain vitreous materials. The transmittance spectra of both types of samples was measured and from the spectra the refractive index dispersion, film thickness and absorption coefficient were obtained. A considerable difference between the optical constants of the spin casted samples and thermally evaporated ones was found. This was attributed to the incorporation of carbon and hydrogen in the amine salt alkyl group formed when dissolving the chalcogenide material. INTRODUCTION The optical properties of the amorphous chalcogenides have shown a wide range of values depending on preparation, purity of precursors and processing methods [1,2]. The nominal method for the growing of bulk samples is air quenching from the melt [3]. This usually results in stoichiometric samples of high purity, morphology and free of microstructure. The spin casting method was developed for the use of these materials as photoresist for high resolution lithography [4] and for its compatibility with planar technology. The spin casted material results in thin films with near optimal properties as compared with the melt material except that it is impossible to remove all the organic solvent. This results in the formation of chalcogenide-amine salts. One of the purposes of this study is to monitor the effect of this organic residue on some optical parameters as compared to the melt grown material. EXPERIMENTAL RESULTS AND DISCUSSION The solvent cast technique involves the dissolution of stoichiometric chalcogenide powders obtained commercially, in primary amines as a first step to get the amine salt of the chalcogenide. Then an amide solvent was used as a second step to obtain the chalcogen solution. In this case As2 S3 powder (Servofrax) was diluted in n-butylamine and n,n-dimethylacetamide. The clear liquid is spin-coated at 2,000 to 3,000 rpm for 30 seconds on a flat transparent substrate. The samples so obtained were pre-baked at 90°C for 20 min and then baked at 120%C for another 20 min to remove the volatiles. A typical film thickness as measured
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