Structure Control in Electrochemical Atomic Layer Eptiaxy

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The long term objective of this project is to gain fundamental understanding of compound semiconductor electrodeposition and to achieve deposition of compound semiconductor thin film structures with the quality and control necessary to form optoelectronic devices. The methodology proposed for these studies is electrochemical atomic layer epitaxy (ECALE), the electrochemical analog of atomic layer epitaxy (ALE), where surface limited reactions are used to form a material, one atomic layer at a time. Underpotential deposition, upd, is another name for a surface limited electrochemical reaction. It is a phenomenon where an atomic layer of one element is deposited on a second at a potential prior to that needed to deposit the first on itself. The driving force is formation of a surface compound, and the free energy of compound formation. Using upd in an ALE cycle, deposits are formed at or near equilibrium, a monolayer at a time. An atomic layer of each element is deposited in turn, in a cycle, to form a monolayer of the compound. The number of cycles determines the thickness of the deposit. A major benefit of ECALE is that it breaks compound electrodeposition into a series of fundamental steps, allowing their independent investigation and control. ECALE studies are the most direct route to the mechanisms of compound electrodeposition. Questions to be addressed include: How much control can be obtained in the electrochemical formation of compound semiconductor device structures? What types of electrochemical reactions can be used to create an ALE cycle? Can the deposit habit be altered by using different substrate or by changing the deposition parameters? Can ternary compounds be formed? Can programs be developed to form graded deposits as buffer layers, or to help with lattice matching issues. What types of electrochemical reactions can be used to create an ALE cycle? Can doping be controlled in compound electrodeposition? Can both p and n type materials be formed? An automated flow deposition system is used to form deposits using ECALE, where potentials and solution changes are controlled by computer. Methodologies for characterization include in-situ electrochemical scanning tunneling microscopy (ECSTM), electrochemical quartz crystal microbalance (EQCM), ultrahigh vacuum electrochemistry (UHV-EC) for studies of deposit surfaces, as well as reflection, absorption, luminescence, photoelectrochemical, photoconductivity, and Hall measurements for studies of physical and electronic properties. %%% An important feature of the project is in education and human resource development through the integration of research and education. The project involves graduate and undergraduate students with participants at all levels interacting with each other and the principal investigator (PI) on a regular basis. Two of the five students working in the PI's group are from under-represented groups, and will be supported by this award. Students are actively involved in publishing papers and presenting talks and posters at professional society meetings. Most of the PI's students have over five presentations on their vitae before they graduate. It is also intended to construct a web page on ECALE, where students design and build various sections, with the intent of forming a page where the present status of ECALE can be found. The page will include descriptions of how to form deposits using ECALE, listings of publications by all groups working in the area, methodologies, instrument designs, as well as programs written in lab view for controlling deposition, accessible by anyone interested in the method. ***