Quartz crystal microbalance study of the growth of indium(III) sulphide films from a chemical solution

The growth of indium(III) sulphide thin films from aqueous thioacetamide (TA)-In(III) solution has been studied with a quartz crystal microbalance (QCM). It is found that the growth of the film consists in the parallel deposition of In₂S₃ and In₂O₃. Both processes are induced by sulphide anions (S²⁻) produced after decomposition of thioacetamide. In₂S₃ is deposited by precipitation of crystallites formed in the bulk solution. On stirred solutions this reaction is hindered due to the disruption of the nucleation centres. On the other hand, In₂O₃ is deposited by electrochemical reduction of naturally dissolved oxygen (O₂) by the S²⁻ anions, followed by chemical reaction with In³⁺. This process is of electroless-chemical nature and has important consequences on the properties of the films. Both reaction mechanisms, chemical and electroless-chemical, compete under different experimental conditions: temperature, solution composition, stirring. For instances, the deposit of In₂O₃ is favoured at low bath temperature, in aerated solutions, giving rise to films with higher oxide proportion. Additives like hydrochloric acid and acetic acid also favour In₂O₃ deposition. The system has easy possibility to tune chemical and physical properties of the films, like composition, transparency and absorption edge, of interest for photovoltaic applications.