Theme: Photons in Applied Materials
Example Project: Persistent photoconductivity
The proposed REU project will investigate large, room-temperature persistent photoconductivity (PPC) in oxide semiconductors. The project follows NSF-supported work that discovered PPC in strontium titanate (SrTiO3) single crystals, elucidated the physical mechanism behind the effect, and used it to write a conductive pathway. Upon exposure to sub-band-gap photons > 2.9 eV, the electrical resistance decreases by three orders of magnitude. After the light is turned off, the heightened conductivity persists “forever” at room temperature (Figure *).
Our observation of PPC in SrTiO3 crystals is unique because it is large, very persistent, and occurs at room temperature. This opens up possibilities for data storage, reconfigurable electronics, and surface modification (e.g., adhesion of cells to a substrate). The ability to define conductive regions on an insulating substrate without using a mask has the potential to reduce cost and enable reusability of the substrate.
Working closely with the faculty and graduate-student mentors, the undergraduate student will investigate PPC in barium titanate (BaTiO3), a ferroelectric material that could exhibit larger conductivity changes than SrTiO3. The kinetics of the process, relevant energy-barrier heights, and fundamental defect properties will be measured. Electronic transport properties, investigated over a range of temperatures, will provide insight into scattering mechanisms. The project has a “gee whiz” aspect to it and illustrates the connection between light and electrical conductivity.
Prof. McCluskey has involved a total of 16 undergraduate students (10 female, 6 male) in semiconductor and high-pressure research. Undergraduate students gained valuable hands-on experience and made contributions in IR spectroscopy of materials and materials under pressure. Leah Snyder, a WSU physics major, is currently performing research on the physics of polymers under high pressures.
Resistance of light-exposed sample in the dark. The solid line is a fit to two exponentials with time constants of 17 days and 800 years.2
 M.C. Tarun, F.A. Selim, and M.D. McCluskey, “Persistent photoconductivity in SrTiO3,” Phys. Rev. Lett. 111, 187403:1-5 (2013). 10.1103/PhysRevLett.111.187403
 V.M. Poole, J. Huso, and M.D. McCluskey, “The role of hydrogen and oxygen in the persistent photoconductivity of strontium titanate,” J. Appl. Phys. 123, 161545:1-5 (2018). 10.1063/1.5009596
 V.M. Poole, S.J. Jokela, and M.D. McCluskey, “Using persistent photoconductivity to write a low-resistance path in SrTiO3,” Scientific Reports 7, 6659:1-6 (2017). 10.1038/s41598-017-07090-2
 P.J. Snyder, R. Kirste, R. Collazo, and A. Ivanisevic, “Persistent Photoconductivity, Nanoscale Topography, and Chemical Functionalization Can Collectively Influence the Behavior of PC12 Cells on Wide Bandgap Semiconductor Surfaces,” Small 13, 1700481:1-6. 10.1002/smll.201700481
 McCluskey, M.D., Grover, D.I., and Zhuravlev, K.K., “Infrared spectroscopy of bis(4-nitrophenyl) disulfide grown on a Pb layer,” Chem. Lett., 2002, No. 11, 1138-9. 10.1246/cl.2002.1138
 URL: http://www.youtube.com/watch?v=VLuQoNhVv3U