Current Research

    Semiconductors have optical properties that are closely tied to their electronic and structural properties. Optical measurements can be made without contacts to the sample, in air, vacuum, or liquid, and in real time as the material is being grown or etched. Hence optical probes are a very powerful technique for studying the properties of semiconductors, as well as other materials. Spectroscopic ellipsometry (SE) is an optical technique that is very sensitive to both the optical properties and thickness of a thin film.
Research at UNL has demonstrated the usefulness of SE in studying and characterizing thin layered materials such as heterostructures, quantum wells, strained layers, and low temperature grown epilayers. Information obtained includes layer thicknesses (from monolayers to microns), compositions in ternary alloys such as AlGaAs and InGaAs, and quantum and strain effects in layers as thin as 2 nm. Real time studies of plasma etching of III-V semiconductors (with Professor Ianno's group) have shown SE to be effective in monitoring surface conditions and controlling the remaining layer thickness. Recent work has included real time monitoring during etching of patterned Si wafers and GaAs-AlGaAs heterostructures. Accurate real time control of etch depth using SE was demonstrated. With patterning it is also possible to monitor etching into the bulk substrate, which is not possible in the unpatterned case.
Many other thin film materials and processes have been investigated, all with optical and/or microelectronic applications. Recent studies include photochromics and nano-scale patterned structures. Recent work has been funded by the Air Force Office of Scientific Research and the Office of Naval Research.

Recent Key Publications

S.-J. Cho, P. G. Snyder, C. M. Herzinger, B. Johs, “Etch Depth Control in Bulk GaAs using Patterning and Real Time Spectroscopic Ellipsometry”, J. Vac. Sci. Technol. B 20, 197-202 (2002).

Y.-G. Mo, R. O. Dillon, P. G. Snyder, T. E. Tiwald, “Optical Properties of Photochromic Organic-inorganic Composites”, Thin Solid Films 355-356, 1-5 (1999).

S.-J. Cho, P. G. Snyder, “Real Time Monitoring and Control of Wet Etching of GaAs/ AlGaAs using Real Time Spectroscopic Ellipsometry”, J. Vac. Sci. Technol. B 17, 2045-2049 (1999).

P. G. Snyder, T. E. Tiwald, D. W. Thompson, N. J. Ianno, J. A. Woollam, M. G. Mauk and Z. A. Shellenbarger, “Infrared Free Carrier Response of In0.15Ga0.85As0.17Sb0.83 Epilayers on GaSb”, Thin Solid Films 313-314, 667-670 (1998). Also presented at 2nd International Conference on Spectroscopic Ellipsometry (ICSE-2), Charleston, 1997.

Recent Graduates and Current Affiliations

· Sang-Jun Cho, Ph.D. 2002, now on a post-doctoral appointment at Virginia Commonwealth University, Department of Electrical Engineering, with Professor Hadis Morkoç.