Substrate spacing and thin-film yield in chemical bath deposition of semiconductor thin films

Abstract

Thin-film yield in the chemical bath deposition technique is studied as a function of separation between substrates in batch production. Based on a mathematical model, it is proposed and experimentally verified in the case of CdS thin films that the film thickness reaches an asymptotic maximum with increase in substrate separation. It is shown that at a separation less than 1 mm between substrates the yield, i.e. percentage in moles of a soluble cadmium salt deposited as a thin film of CdS, can exceed 50%. This behaviour is explained on the basis of the existence of a critical layer of solution near the substrate, within which the relevant ionic species have a higher probability of interacting with the thin-film layer than of contributing to precipitate formation. The critical layer depends on the solution composition and the temperature of the bath as well as the duration of deposition. An effective value for the critical layer thickness has been defined as half the substrate separation at which 90% of the maximum film thickness for the particular bath composition, bath temperature and duration of deposition is obtained. In the case of CdS thin films studied as an example, the critical layer is found to extend from 0.5 to 2.5 mm from the substrate surface, depending on the deposition conditions.

Morphology reliance of cobalt sulfide thin films: A chemo-thermo-mechanical perception

“…Increased temperature leads to the increased dissociation of the complex and the reaction proceeds in a direction that opposes the change in a reaction parameter [1]. Thus concentration of the metal ions in the reaction bath depends on the forces between the constituent ions, and the temperature, via the thermodynamic driving forces of the system [13,14]. Besides, thermal energy also enhances the kinetic energy of the ions leading to the enhanced interaction between the reacting species and the substrate surface.…”

“…Broad absorption edge was observed in as-deposited thin film samples and the absorption coefficient (a) of the deposited films is high (10 À4 cm À1 ). Optical band gap (E g ) of thin films can be calculated on the basis of the optical absorption spectra using the Tauc relation [7][8][9]14,16].…”

“…For direct transitions n = 1/2, 3/2, 2, 3, for allowed direct, forbidden direct, allowed indirect and forbidden indirect transitions, respectively [7][8][9]14,16]. The best fit of the experimental curves were obtained for n = 1/2.…”

“…No significant change is observed in the band gap values of deposited thin films. The minor change may be attributed to the morphology amendments and dependence of band gap on the terminal layer thickness[7][8][9]11,14,16].…”

Preparation of Low-Resistivity Ga-Doped ZnO Epitaxial Films from Aqueous Solution Using Flow Reactor

“…However, using such methods means that the chemical reaction occurs in the bulk solution, resulting in not only film growth on the substrate, but also precipitation in the solution. 14,15 Furthermore, any precipitate formed can potentially settle on the growing film, thereby reducing its crystallinity and surface flatness. Any change in the concentration or pH of the solution as the reaction progresses also makes it difficult to obtain a film with high uniformity in the growth direction, or to correlate the reaction conditions with the growth mechanism and properties of the resulting film.…”