Electrochemical quartz crystal microbalance (EQCM) studies of compound semiconductor formation by EC-ALE
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Electrochemical atomic layer epitaxy (EC-ALE) has been developed in this research group to electrochemically prepare thin films of compound semiconductors. It is an electrochemical analog of atomic layer epitaxy (ALE), where a compound is grown in layer-bylayer fashion. A surface limited reaction, underpotential deposition (UPD), is used to deposit constituent elements one monolayer at a time. The EC-ALE promotes two-dimensional growth, resulting in better control of composition and structure in deposited film than conventional electrochemical co-deposition. The composition of films grown using EC-ALE is initially approximated from the charges passed during depositions. Faraday’s law is used to calculate the amounts of deposited elements. In general, the coulometric results indicate the films are off stoichiometric. However, electron microprobe analysis (EPMA) suggests otherwise, that the films are quite stoichiometric. The differences may correspond to side reactions, such as hydrogen evolution, oxygen reduction, and contaminants, which result in extra charges and errors in the composition calculations. An electrochemical quartz crystal microbalance (EQCM) is capable of measuring small mass changes on its surface. A flow cell EQCM was constructed to study the formation of various metal chalcogenides using EC-ALE. The detailed description of the flow cell EQCM is described. The design of the flow cell currently used in this laboratory was modified to accommodate the quartz crystal oscillator. Two pumping methods (He pressure and peristaltic pump) were used to flow solutions with acceptable noise level in ?f measurements. Higher solution flow rates were obtained by using the peristaltic pump. When solution flow was stopped, shifts in ?f values were observed, associated with changes in pressure. The flow cell EQCM was used to grow CdSe and PbTe. Cyclic voltammetry was used to determine UPD and bulk potentials for each element. A series of depositions was carried out with various deposition programs using the UPD potentials. Both current and ?f were measured as a function of deposition time and used to calculate coverages of deposited elements. With these results, it was determined that extra charges corresponded to the reduction of oxygen dissolved in deposition solutions. The optimized deposition programs for CdSe and PbTe were also suggested.