CBD-In(OH) x S y Thin Films: An Approach to the Growth Mechanism
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CBD-In(OH)x Sy Thin Films: An Approach to the Growth Mechanism
Rocío Bayón and José Herrero Departamento de Energías Renovables. (CIEMAT) Avd. Complutense 22, E-28040 Madrid (Spain) Electronic mail: [email protected]
ABSTRACT The surface morphology of chemical bath deposited In(OH)xSy thin films has been studied using SEM and TEM. Different surface morphologies have been obtained depending on the deposition conditions (reactant concentrations, pH and bath temperature) as a consequence of the variation of both the homogeneous nucleation and particle-growth rates with them. The In(OH)3 colloids present in the bulk solution and on the substrate surface play an important role in the growth mechanism since they promote the In(OH)xSy formation. It has been found that the growth mechanism of the films is the socalled particle-by-particle or colloidal growth.
INTRODUCTION One of the latest issues in the R&D activities of chalcopyrite-based thin film solar cells has been the study of new compounds for replacing the CdS buffer-layer. The technique of chemical bath deposition (CBD) has been the most widely used for preparing such alternative thin film semiconductors. Although CBD is said to be a simple method, many of its aspects such as the growth kinetics and mechanisms are still not fully understood. One compound that has proven to be a good candidate to replace the CdS is the In(OH)xSy. In previous papers we have reported that film properties of this chalcogenide and the kinetics of the reactions taking place during the deposition process strongly depend on the chemical conditions (reactant concentrations, deposition time, bath temperature, etc) [1, 2, 3, 4]. As a new contribution to the understanding of the CBD process, in this paper we have undertaken a thorough study of the surface morphology of CDB-In(OH)xSy thin films in order to propose a descriptive model for the growth mechanism, which, in principle, may be extended to other CBD-semiconductors. EXPERIMENTAL CDB-In(OH)xSy thin films were prepared from a bath containing InCl3, thioacetamide (TA) and acetic acid (AcOH) following the procedure described elsewhere [1]. The thioacetamide concentration was varied from 0.05 to 0.5 M and the acetic acid concentration from 0 to 0.3 M. The InCl3 concentration remained constant at 0.025 M, the deposition time used in the experiments was 25 min and the bath temperature was varied from 65o to 85oC. The surface morphology was studied by means of scanning and transmission electron microscopies (SEM and TEM). Samples observed by SEM were previously covered by a 200-Å-gold-film and the images were recorded by using either a JEOL JMS-
H8.3.1
5800 or a HITACHI S-2500 apparatus. All the samples for TEM investigation were directly deposited from the bath onto nickel-grids covered by a cellulose-carbon thin film and the micrographs were recorded using a JEOL JEM 2010 ELECTRON MICROSCOPE with a LaB6 filament, 200 kV as electron-beam accelerating voltage and 10-5 Pa pressure. RESULTS AND DISCUSSION In Figure 1 SEM micrographs hav
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