Joel W. Ager III is a Staff Scientist in the Materials Sciences Division of Lawrence Berkeley National Laboratory. He is a Principal Investigator in the Electronic Materials Program and a Project Leader in the Joint Center for Artificial Photosynthesis (JCAP). He graduated from Harvard College in 1982 with an A.B. in Chemistry and from the University of Colorado in 1986 with a Ph.D. in Chemical Physics. After a post-doctoral fellowship at the University of Heidelberg, he joined Lawrence Berkeley National Laboratory in 1989. His research interests include materials development and fundamental electronic and transport characteristics of photovoltaic materials, development of new photoanodes and photocathodes based on abundant elements, and the development of new oxide and sulfide based transparent conductors, and recently, he has organized energy related symposia at the Materials Research Society and American Physical Society annual meetings and has served on program committees for the International Symposium on Compound Semiconductors and the Electronic Materials Conference. Dr. Ager has published over 230 papers in refereed journals.
Recent EMAT publications:• J. W. Ager III, N. Miller, R. E. Jones, K. M. Yu, J. Wu, W. J. Schaff, and W. Walukiewicz, “ Mg-doped InN and InGaN – photoluminescence, capacitance-voltage and thermopower measurements,” phys. stat. sol. (b) 245, 873–877 (2008).
• J. J. L. Morton, A. M. Tyryshkin, R. M. Brown, S. Shankar, B. W. Lovett, A. Ardavan, T. Schenkel, E. E. Haller, J. W. Ager, and S. A. Lyon, “Solid state quantum memory using the 31P nuclear spin,” Nature 455, 1085 (2008).
• N. Miller, J. W. Ager III, H. M. Smith III, M. A. Mayer, K. M. Yu, E. E. Haller, W. Walukiewicz, W. J. Schaff, C. Gallinat, G. Koblmüller, and J. S. Speck, “Hole transport and
photoluminescence in Mg-doped InN,” J. Appl. Phys 107, 113712 (2010).
• K. Wang, N. Miller, R. Iwamoto, T. Yamaguchi, M. A. Mayer, T. Araki, Y. Nanishi, K. M. Yu, E. E. Haller, W. Walukiewicz, and J. W. Ager III, “Mg doped InN and confirmation of free holes in InN,” Appl. Phys. Lett. 98, 042104 (2011).
• N. Miller, E. E. Haller, G. Koblmüller,
C. Gallinat, J. S. Speck, W. J. Schaff,
M. E. Hawkridge, K. M. Yu, and J. W. Ager III, Effect of charged dislocation
scattering on electrical and electrothermal transport in n-type InN, Phys. Rev. B 84, 075315 (2011).
• E. Alarcón-Lladó, M. A. Mayer, B. W. Boudouris, R. A. Segalman, N. Miller, T. Yamaguchi, K. Wang, Y. Nanishi, E. E. Haller, and J. W. Ager, “PN junction rectification in electrolyte gated Mg-doped InN,” Appl. Phys. Lett. 99, 102106 (2011).
Recent publications from other LBNL efforts:• Zhiyong Fan, Haleh Razavi, Jae-won Do, Aimee Moriwaki, Onur Ergen, Yu-Lun Chueh, Paul W. Leu, Johnny C. Ho, Toshitake Takahashi, Lothar A. Reichertz, Gregory F. Brown, Steven Neale, Kyoungsik Yu, Ming Wu, Junqiao Wu, Joel W. Ager, and Ali Javey, “Three dimensional nanopillar array photovoltaics on low cost and flexible substrates,” Nature Materials 8, 648- 653 (2009).
• L. A. Reichertz, I. Gherasoiu, K. M. Yu, V. M. Kao, W. Walukiewicz, and J. W. Ager III, “Demonstration of a III-nitride/silicon tandem solar cell,” Appl. Phys. Express 2 122202 (2009).
• S. Y. Yang, J. Seidel, S. J. Byrnes, P. Shafer, C.-H. Yang, M. D. Rossell, P. Yu, Y.-H. Chu, G. Catalan, J. F. Scott, J. W. Ager III, L. W. Martin, and R. Ramesh, “Above-bandgap voltages from ferroelectric photovoltaic devices,” Nature Nanotechnology 5, 143-147 (2010).
• J. Seidel, D. Fu, S.-Y. Yang, E. Alarcón-Lladó, J. Wu, R. Ramesh, and J. W. Ager III, “Efficient photovoltaic current generation at ferroelectric domain walls,” Phys. Rev. Lett. 107, 126805 (2011).
The semiconductor indium nitride (InN) has a narrow direct bandgap of 0.65 eV and high electron mobility (>2000 cm2 V-1 s-1), suggesting considerable promise for opto-electronic applications. However, InN also exhibits strong surface electron accumulation due to its large electron affinity, the largest of any III-V semiconductor. This surface electron layer had been a major impediment both in understanding carrier transport, as it forms a shorting path in any top-contacted device. Moreover, while prior EMAT research pioneered the achievement of p-type transport, the surface electron layer had prevented the demonstration of a rectifying pn junction in this material.
Here, we take advantage of the dielectric properties of an ionic liquid similar to those used in batteries and organic electronics to control the surface electron accumulation. We fabricated a device similar to a field effect transistor in which the ionic liquid serves as a gate dielectric. Under the appropriate gate bias conditions, we can reduce the electron accumulation at certain areas in the surface of p-type InN and thus force the carriers to flow through the pn junction created between the n-type surface layer and the p-type bulk. We observe the characteristic signature of junction rectification in good agreement with the expected equivalent circuit. Observation of a pn homojunction is a breakthrough for the development of InN-based optoelectronic devices and can further lead to a better understanding of the electrical connection between p- and n-type InN.