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RESEARCH

The wide-bandgap semiconductors of GaN and ZnO have gained unprecedented attention due to their unique applications in blue laser diodes and LEDs, optical detectors, high-power amplifiers, and chemical/gas sensing. Progress, however, is still challenged by the high density of defects in GaN and the lack of reliable p-type doping for ZnO. Another important issue, although sometimes neglected, is the role played by surfaces and interfaces in the electrical and optical properties of these semiconductors. Such surface-related effects can result in the reduced efficiency of emitters, shorter laser operation lifetimes, and earlier degradation of electronic devices. In particular, it is possible that the irreproducible and unstable p-type conductivity of ZnO is related to peculiarities of the surface conductivity in this material.
  
For this NSF-supported project, we are conducting an in-depth investigation of the processes at and near the surface for both GaN and ZnO. The electrical and optical properties of these systems are being probed under different ambient conditions, temperatures, and illumination. Band bending near the surface and its variation under illumination (photovoltage) are being studied using two unique methods: 1) a Kelvin probe combined with an optical cryostat; and 2) a microscopy technique combining local charge injection with subsequent imaging of the surface charge. As a result of these studies, the underlying mechanisms for surface band bending will be related to sample preparation, temperature, and ambient environment. Effective passivation schemes will also be explored to improve the performance of optical and electronic devices based on these wide-bandgap semiconductors.

Selected publications:

• "Study of the electronic behavior of the Zn- and O-polar ZnO surfaces using conductive atomic force microscopy," J.C. Moore, S.M. Kenny (Longwood student), C.S. Baird (Longwood student), H. Morkoç, A.A. Baski, J. Appl. Phys. 105, 116102 (2009). 
• "Investigation of charge trapping at the oxide/semiconductor interface for MBE-grown GaN films," J.C. Moore, M.A. Reshchikov, J.E. Ortiz, J. Xie, H. Morkoç, A.A. Baski, Proc. of SPIE, 6894, 68940B (2008).
• "Local electronic and optical behavior of a-plane GaN grown via epitaxial lateral overgrowth," J.C. Moore, V. Kasliwal, X. Ni, Ü. Özgür, H. Morkoç, A.A. Baski, Appl. Phy. Lett. 90, 011913 (2007).

Growth and characterization of ZnO thin films and nanowires
We have experience growing ZnO thin-films using rf magnetron sputtering of ZnO, dc magnetron sputtering of Zn-metal in a reactive oxygen environment, and via thermal oxidation of Zn-metal films. The first two techniques are well characterized in the literature and our results are similar. However, the latter technique requires further study, specifically with regards to variability due to Zn film thickness, annealing temperature, and doping. We have recently begun using thermal oxidation of Zn films in air ambient to fabricate ZnO thin films, and we are currently relating morphology and structure to initial film thickness, annealing temperature and initial substrate conditions. X-ray diffraction spectra indicate that after annealing, the resulting ZnO films possess a polycrystalline hexagonal wurtzite structure without a preferred orientation. At annealing temperatures of 300°C, Zn peaks are still observed. At 600°C, no Zn peaks are observed, indicating that the film is completely ZnO.

Preliminary characterization via AFM has shown significant differences in surface morphology for various temperatures, substrate temperatures during Zn metal growth, and between two-step and single step processes. We intend to determine whether the (002) orientation can be enhanced on glass and sapphire substrates via manipulation of annealing temperature, Zn-film texture, substrate temperature during growth, sputter gas, and adhesion-enhancing layers. The (002) orientation is preferred for electrooptic and piezoelectric properties, and glass substrates are preferred for solar cell applications.

Doping ZnO with aluminum during sputter deposition has been shown to increase film conductivity. We will also begin an exploratory investigation into whether a two-step Al/Zn sputter process with subsequent thermal oxidation can lead to doped ZnO film growth with similar conductivity gains. 

In this research program, we are working to gain a comprehensive understanding of the electrical behavior of ZnO nanowire systems. Quasi-one-dimensional semiconductor nanostructures have attracted a great deal of attention primarily due to their potential applications as gas sensors, and as electronic and light-emitting nanodevices. In particular, ZnO nanowires are especially promising for the realization of novel nano-scale light emitting devices in the blue-ultraviolet spectral range. Although the optical properties of ZnO nanowires are beginning to receive attention, there is very little discussion in the literature concerning their electrical properties.

Using conductive atomic force microscopy (CAFM), we are investigating the single-wire I-V behavior and photo-response of electrically isolated nanowires grown in porous anodic aluminum oxide (AAO) templates. We are also investigating the nature of the electronic states at the interface and charge transfer mechanisms using a novel technique that we have developed called electronic pump-probe AFM. These investigations should provide a fundamental understanding of the role of the ambient environment in the photo-response, which could lead to higher reliability, longer lifetime, and higher efficiency bright light sources, UV detectors, and gas sensors. Aspects of this research are also being incorporated in Longwood University’s Modern Physics and Electronics labs, such as photoluminescence studies of quantum-confined systems, and fabrication and characterization of simple p-n junctions.

Selected publications:

• “Carrier relaxation and stimulated emission in ZnO nanorods grown by catalyst-assisted vapor transport on various substrates,” V. Avrutin, Ü. Özgür, N. Izyumskaya, S. Chevtchenko, J. Leach, J.C. Moore, A.A. Baski, H.O. Everitt, K.T. Tsen, P. Ruterana, H. Morkoç, Zinc Oxide Materials and Devices II, (Ed. F.H. Teherani, C.W. Litton), Proc. of SPIE 64741M (2007).
• “Morphology and optical properties of ZnO nanorods grown by catalyst-assisted vapor transport on various substrates,” V. Avrutin, Ü. Özgür, N. Izyumskaya, S. Chevtchenko, J. Leach, J.C. Moore, A.A. Baski, H.O. Everitt, K.T. Tsen, P. Ruterana, H. Morkoç, MRS Symp. Proc., 963E (Fall 2006).

Physics education research
We are experimenting with a new pedagogical model for the University Physics I and II courses. This model is based on Tutorials in Introductory Physics by Lillian McDermott. Specifically, we are comparing learning gains for traditionally-taught courses and modified courses. Preliminary results from the current PHYS 202 course suggest that significant learning gains are achieved with the modified curriculum in comparison to traditionally-taught courses. We intend to continue this research over the next two years with the involvement of 1-2 undergraduate “learning assistants” each semester. This work fills a hole in the PER literature, since we are using the tutorials completely within lecture periods, effectively eliminating half of the lecture time. Tutorials are typically administered during recitation sections, but conversations with colleagues at other institutions suggest that many schools (Longwood included) do not have a recitation period.

Several students and I have developed a simple means for determining range without resorting to trigonometric arguments as well as a corresponding laboratory exercise. The intention is to directly confront common student misconceptions in projectile motion for courses having limited math pre-requisites. To gage efficacy, pre- and post-instruction examinations will be developed to query student understanding before and after instruction. These exams and the corresponding curriculum will be used in PHYS 103 being taught in the Fall 2009 semester.

A highly inquiry-based curriculum is being used in the PHYS 103 course during the Fall 2009 semester. Lawson's Classroom Test or Scientific Reasoning will be given both pre- and post-instruction to determine if significant gains in scientific reasoning can be achieved via this inquiry-based approach in comparison to traditionally-taught courses.

Selected publications:

• “A lecture-based implementation of Tutorials in Introductory Physics,” J.C. Moore, Phys. Educ. (in prep).
• “Graphical method for determining projectile trajectory,” J.C. Moore, J.C. Baker, L. Franzel, D. McMahon and D. Songer, Phys. Teach. (submitted, May 2009). (Underlined names indicate Longwood students.)
• “Two tracks demonstrate average speed,” J.C. Moore, Phys. Educ. 44, 456-457 (2009).