Faculty
Stephen G. Bishop
Associate Head for Administrative and Instructional Affairs
Instructional and Graduate Affairs
153 Everitt Lab, MC-702
1406 W. Green
Urbana, Illinois 61801
(217) 333-2302
Professor
311 Coordinated Science Lab, MC-228
1308 W. Main St.
Urbana, Illinois 61801
(217) 244-6372
Ph.D. Physics Brown University 1965
Research Statement:
We are investigating the fundamental optical, electronic, and structural properties of the Ge2Sb2Te5 (GST) phase-change alloy that is used as the active medium in rewritable optical memory disks, and in current-switched phase-change random access memories now under development. Of particular interest is the micro- or nano-structural basis for the rapidly reversible, thermally-induced atomic scale rearrangements that underlie the phase change phenomenon. Optical absorption spectroscopy, spectroscopic ellipsometry (SE), temperature dependent electrical conductivity, photoconductivity, and fluctuation electron microscopy are among the techniques we are applying to the study of thin films of GST fabricated at collaborating institutions. We have made an unambiguous determination of the optical band gaps of amorphous, fcc and hexagonal phases of GST by proper analyses of optical data (SE, IR absorption spectra, and photoconductivity). The implications of these studies for inferred structural differences between amorphous GST and other chalcogenide glasses are being explored. Similarly, the results of our investigation of the temperature dependence of the electrical conductivity and photoconductivity of GST below room temperature will be compared with the corresponding properties of conventional chalcogenide glasses. The technique of fluctuation electron microscopy (FEM), which is carried out in the TEM, has been used to probe the existence of nanometer-scale structural order in the amorphous phase. FEM is directly sensitive to three- and four-atom position correlations within the coherent probe of 1-4 nm diameter, whereas diffraction is only a two-atom correlation. We find that all as-deposited Ge2Sb2Te5 films contain nanoscale order, in some cases crystallites of 2-4 nm diameter and in other cases finer-scale order. Using a scanning TEM to perform FEM, we statistically filter out the regions that contain nanocrystallites and extract the characteristic size of the order in the matrix.
Research Interests:
- Optical and Electrical Characterization of Crystalline and Amorphous Semiconductors and Semiconductor Nanostructures; Compound Semiconductors: GaAs, InP, AlGaAs, ZnSe, SiC; Defects in Semiconductors; Isoelectronic Defects; Rare Earth-doped Chalcogenide Glasses and GaN. Experimental Techniques: Photoluminescence, Nuclear Magnetic Resonance, Electron Spin Resonance, Magneto-Optics, Photoemission, Infrared Spectroscopy.
Honors, Recognition, and Outstanding Achievements
- Fellow, Optical Society of America, 1999
- Fellow, American Association for the Advancement of Science, 1996
- Tau Beta Pi, Honorary Member and Eminent Engineer, UIUC, 1993
- Board of Trustees, Gettysburg College, 1992 - 2006
- Distinguished Alumni Award, Gettysburg College, 1990
- Fellow, American Physical Society, 1982
- DoD-Ministry of Defense (U.K.) Exchange Scientist, 1978-1979
- Sigma Xi Pure Science Award, NRL, 1977
- NRL Sabbatical Study Program, 1973-1974
- NAS-NRC Post-Doctoral Research Associate at NRL, 1966-1968
Honors, Recognition, and Outstanding Achievements for Teaching
- College of Engineering 2007 Outstanding Advisors List, UIUC.
Honors, Recognition, and Outstanding Achievements for Research
- Fellow, Optical Society of America, 1999
- Fellow, American Association for the Advancement of Science, 1996
- Eight Patent Application Awards and Seven Patent Issuance Awards, NRL
- Fellow, American Physical Society, 1982
- Pure Science Award, NRL Chapter of Sigma Xi, 1977
- Alan Berman Research Publication Award, NRL, 1975
Honors, Recognition, and Outstanding Achievements for Public Service
- Congregational Council, Saint Matthew Lutheran Church, Urbana, IL 1997-2001
- Board of Trustees, Gettysburg College - 1992-2006