Chemical Bath Deposition of Zinc Sulfide from Acidic Solutions
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Chemical Bath Deposition of Zinc Sulfide from Acidic Solutions Iain P. O’Hare1, Kuvasani Govender1, Paul O’Brien1 and David Smyth-Boyle2 The University of Manchester, Department of Chemistry and the Materials Science Centre, Oxford Road, Manchester, M13 9PL, U.K. 2 The University of Durham, Department of Physics, Rochester Building, Science Laboratories, South Road, Durham, DH1 3LE, U.K. 1
ABSTRACT
At present, the soft processing of materials attracts considerable interest. Chemical bath deposition (CBD) effects film formation by means of a controlled chemical reaction. Adherent, uniform and reproducible films of zinc sulfide (ZnS) have been deposited, upon low iron-content microscope slides, under acidic conditions, from a solution containing zinc chloride, urea and thioacetamide. Scanning electron micrographs of the deposited layers suggest that, as the reaction proceeds, uniform film growth is associated with increasing particle size, rather than continuous nucleation and deposition of new primary particles. Energy Dispersive Analysis by X-Rays (EDAX) spectra are typical of CBD-deposited films of ZnS; signals attributable to elements within the glass substrate are also detected, an observation consistent with the thin nature of the films. Grain size distributions have been investigated using computational image analysis, and an average increase in the diameter of the deposited particles of 33.7 nanometres per hour has been calculated.
INTRODUCTION
Zinc sulfide (ZnS) is a wide, direct band gap semiconductor, which exists in both cubic and hexagonal forms. A diverse range of solar device applications exists for thin films of this material, including as filters, mirrors, heterojunction devices and within anti-reflection coatings. Application of the material within photovoltaic devices is another area receiving attention, but one which remains to be fully developed. Various techniques have been employed in the preparation of thin films of ZnS, such as r.f. sputtering, metal organic chemical vapour deposition (MOCVD), molecular beam epitaxy (MBE) and atomic layer epitaxy (ALE) [1,2]. Such vapour-phase deposition methods demand the use of vacuum conditions and sophisticated equipment. In contrast, chemical bath deposition (CBD) [3,4], also known as electroless deposition, represents an alternative, and relatively simple, method, which is both convenient and inexpensive. Films may be deposited at low temperatures, upon a variety of substrates, and the thickness of the deposited layers may be readily controlled by variation of the length of the deposition time. Moreover, the process may be easily adapted to large area processing at low fabrication cost. Thin films deposited by this method may be of high quality, and studies of the deposition of ZnS by CBD form the subject of the present work. The CBD method has been successfully employed to deposit a broad range of semiconductor materials, in the form of thin films, including binary materials, such as cadmium sulfide (CdS) [3,5-8], together with ternary phases, such
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