Electrochemical quartz crystal microbalance (EQCM) studies of compound semiconductor and metal thin flms formation by electrochemical atomic layer deposition (ALD)
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Electrochemical atomic layer deposition (ALD) has been developed in this research group to grow thin films of compound semiconductors. It is an electrochemical analog of atomic layer epitaxy (ALE) and atomic layer deposition (ALD), all of which are based on growth of material a monolayer at a time. Under potential deposition (UPD), electrochemical surface limited reactions are used to form the desired materials in a series of steps to form one monolayer at a time. Electrochemical ALD promotes two-dimensional growth, resulting in better control of composition and structure than conventional electrodeposition method. Electrochemical Atomic Layer Deposition (ALD) has recently been used to grow thin films of metals (Cu, Pt on Au) using Surface Limited Redox Replacement (SLRR) reaction. In SLRR, an atomic layer of a less noble metal is replaced by an atomic layer of a more noble metal at Open Circuit. Electrochemical quartz crystal microbalance (EQCM) is an in-situ mass sensitive device which can measure small changes on its surface. A flow cell EQCM was constructed in our laboratory to study the formation of various compound semiconductors and metal thin films via electrochemical Atomic Layer Deposition (ALD). The regular flow cell was modified to accommodate quartz crystal. Solutions were pushed in to cell either by He pressure or peristaltic pump. Solutions are flowed continiously in to the cell to avoid shifts in [delta]f values observed when the flow is stopped. Stoichiometry of thin films based on charge and from the electron probe micro analysis (EPMA) doesn’t correspond with each other. The extra charges correspond to the side reactions, such as hydrogen evolution, oxygen reduction, and contaminants. EQCM is used to calculate the mass changes during deposition and to better understand the deposition process. Automated flow cell EQCM was used to grow CdSe, CdTe, HgTe, CdTe/HgTe, Cu and Pt thin films. LabView program is used to run the flow cell. Current and ûf were measured simultaneously as a function of deposition time. The coverage in then calculated based on the charge and mass of deposited elements.