Surface Chemistry Studies During Thin-Film Growth Using Electrochemical Atomic Layer Epitaxy (EC-ALE)

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The goal of this project is greater understanding and control of electrochemical epitaxial processing of compound semiconductors. The focus of the project is atomic layer epitaxy (ALE), where deposits are formed an atomic layer at a time. In ALE, surface limited reactions are used to form each atomic layer. Surface limited electrochemical reactions generally occur at underpotentials, potentials below those needed to deposit the element on itself. The approach makes use of an automated electrochemical flow-cell, which facilitates growth of films thick enough for analysis by X-ray diffraction (XRD), electron microprobe analysis (EPMA), infrared (IR), and optical spectroscopies. Previous EC-ALE studies have focused either on the first few atomic layers or the structure, composition, and morphology of completed films. This project will address surface chemistry of the EC-ALE cycle as the deposit is being formed. It is thought that optimal conditions (potentials) change as the deposit grows, and some form of feedback is needed to better control the process. However, currents measured during various cycle steps do not provide an accurate picture of the deposition process. It is proposed to study the surface chemistry after various numbers of cycles, and at different points in the cycle, while deposits are forming, that is, to follow the surface chemistry during the 2nd , 5th , 10th , 25th , -..200th , cycles. Surface sensitive probes will be used to follow the EC-ALE cycle chemistry during film growth. A unique electrochemical STM flow-cell will be used to monitor surface structure and morphology during deposition. This apparatus allows atomic scale imaging in a controlled environment where solutions are easily exchanged and EC-ALE deposits can be formed. The mass of the deposits will be monitored at each step in the EC-ALE cycle using an electrochemical quartz crystal microbalance (EC-QCM) system. A microbalance crystal will be used as a substrate in a flow-cell. The mass of the deposit at each step will be compared with observed currents to elucidate interfacial processes and current efficiencies. An electrochemical flow-cell deposition system will be constructed for use in the antechamber of a UHV surface system, so that the composition of the surface can be monitored after any number of cycles and after any cycle step. Deposits will be transferred periodically from the flow cell directly to the analysis chamber for examination with AES, XPS, LEED, STM, and LEIS. Improved understanding of the surface chemistry, leading to better control over deposit structure, composition and morphology is expected. InAs and InSb are being grown using EC-ALE and work on the formation of III-V compounds in general will continue. CdSe/CdTe and InAs/InSb superlattices have been formed, and will continue to be studied.%%%The project addresses basic research issues in a topical area of materials science with high technological relevance. New, innovative experimental techniques such as electrochemical atomic layer epitaxy can now be characterized more fully leading to greater understanding and control of elementary chemical and diffusion processes which will allow advances in fundamental materials science and technology. The basic knowledge and understanding gained from the research is expected to contribute to improving the ability to efficiently deposit high crystal quality semiconductor films for electronic and photonic applications. An important feature of the program is the integration of research and education through the training of students in a fundamentally and technologically significant area.***