Chemical Spray Pyrolysis of Complex Thin Solid Films
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CHEMICAL SPRAY PYROLYSIS OF COMPLEX THIN SOLID FILMS CLAYTON W. BATES, JR.*, ELIZABETH B. VARNER* SVETLANA ALSHVANG* AND KEITH SUMMERS** *Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305 **Tuskegee University, Department of Electrical Engineering, Tuskegee, AL 36088
ABSTRACT Chemical spray pyrolysis (CSP) is a technique in which compounds of the constituents of the thin film to be fabricated are dissolved in an aqueous solution which is subsequently sprayed onto a heated substrate using nitrogen as the atomizing gas. At relatively low substrate temperatures (150-400'C) chemical reactions take place in which film formation of the desired compound occurs concomitantly with the release of volatile chemical reactants. The technique has advantages that include: (1) simplicity, (2) low cost and simple equipment, (3) the ability to prepare films over large areas with various shapes with relative ease and (4) the possibility of varying the physical properties through chemical means in known ways at modest temperatures. Results on the preparation of CuInSe2 which is a defect dominated semiconductor of technological interest are presented. Both n- and p- type materials were prepared with resistivities varying from 10-2-104 ohm-cm illustrating the use of the control of solution chemistry to produce films with reproducibly controlled properties.
I. Introduction Traditional methods for fabricating thin films of materials (10A to several microns thick) for scientific studies of a fundamental nature or technological applications include vacuum evaporation, sputtering, molecular beam, vapor and liquid phase epitaxy. These techniques involve very pure starting materials which have been grown or prepared as a source for use in a given technique and can be particularly expensive if multiple sources are required. They also require relatively large expenditures of energy using fairly sophisticated equipment. And few of these methods may be easily scaled up for producing large area devices such as solar cells employed in terrestrial applications. Chemical spray pyrolysis (CSP), a technique that until fairly recently had been used almost exclusively for applying the transparent electrical conductor tin oxide to glass 1 , offers in principle the possibility of preparing complex thin solid films consisting of three or more components as readily as more simple two component systems. In chemical spray pyrolysis (CSP) a solution containing the film constituents is sprayed onto a heated substrate. In the ideal case a chemical reaction takes place in which film formation of the desired compound occurs concomitantly with the release of volatile chemical reactants. The work by Chamberlin and Skarman in 19662,3 on CdS/Cu2-xS solar cells first brought the technique to the attention of those interested in producing thin films. Since their efforts the technique has been used for producing a number of semiconducting compounds for various applications with varying degrees of success. These include CdS/Cu2-xS, C
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