Technical. This project addresses the role played by surfaces and interfaces in the electrical and optical properties of GaN and ZnO. It thought that surface-related effects may be responsible for reduced efficiency of emitters, shorter laser operation lifetimes, and earlier degradation of elec-tronic devices. Additionally, the irreproducible and unstable p-type conductivity of ZnO may be related to peculiarities of the surface conductivity in this material. This project aims for an in-depth investigation to gain a comprehensive understanding of the processes at and near the sur-face for both GaN and ZnO. Electrical and optical properties will be probed under different am-bient conditions, temperatures, and illumination. Band bending near the surface and its variation under illumination (photovoltage) will be studied using two methods: 1) a Kelvin probe com-bined with an optical cryostat; and 2) a microscopy technique combining local charge injection with subsequent imaging of the surface charge. 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. Non-Technical. The project addresses fundamental research issues in a topical area of elec-tronic/photonic materials science having technological relevance. This effort may lead to higher reliability, longer lifetime, and higher efficiency bright light sources based on GaN and ZnO. The proposed research program will leverage strong collaborations between research groups in the Departments of Physics and Electrical Engineering at Virginia Commonwealth University (VCU), as well as initiate a collaboration with the Department of Chemistry and Physics at Longwood University, a nearby undergraduate institution. The project provides support for graduate students at VCU and undergraduate students at both VCU and Longwood, where all of the students will be involved in collaborative efforts between multiple research groups. Women and underrepresented minority groups will be actively recruited and supported within this pro-ject. VCU is a growing urban university with a diverse student body (18% African American, 22% other minorities), and the PI?s have a record of mentoring students from underrepresented groups in their laboratories. An international collaboration is also planned with the Ioffe Physical Technical Institute of the Russian Academy of Science.
PI – A. A. Baski, CoPI – M. A. Reshchikov, both from Virginia Commonwealth University (VCU), CoPI - J. Moore, Longwood University/Coastal Carolina. The grant produced 12 refereed publications, 10 conference proceedings, 30 conference presentations, one PhD dissertation (M. Foussekis) and four M.S. Theses (M. Foussekis, J. Ferguson, I. Ruchala, and A. Olsen). This research program was a collaboration between a research group at VCU (A. Baski & M. Reshchikov) and Longwood University/Coastal Carolina (J. Moore). Funds have been used to support five graduate students at VCU (includes three women) and several undergraduate students at VCU, Longwood University, and Coastal Carolina. The undergraduates have included two women at VCU and three underrepresented minority students at Longwood. Figure 1 shows one of our physics majors who is now in the M.S. program, Joy McNamara, presenting her research on GaN at a local AVS meeting. In addition to undergraduate research opportunities supported by this program, we have sponsored "nano-day" events at VCU in summer 2008, 2010, 2011, and 2012 with the Richmond area Mathematics & Science Innovation Center (MSIC). Students from area middle schools came to the VCU Physics Department to learn about nanoscience and tour our laboratory facilities. The main focus of this research program was to elucidate the mechanisms of surface band bending in gallium nitride (GaN) and zinc oxide ( ZnO), and to study the effect of this band bending on electrical and optical properties of these two semiconductors. Band bending refers to the local change in energy of electrons near the semiconductor surface due to space charge effects. We have found that, similar to other semiconductors, there is an upward band bending in n-type GaN (due to accumulated negative charge at the surface) and a downward band bending in p-type GaN (due to accumulated positive charge at the surface). The absolute value of the band bending decreased when the surface was illuminated with ultraviolet (UV) light. This effect was explained by a separation of photo-generated electron-hole pairs in the near-surface depletion region due to strong electric field caused by the bend bending and accumulation of holes (in n-type) or electrons (in p-type) at the surface. The change in the surface potential (or band bending) due to illumination is called the surface photovoltage (SPV). The magnitude and direction of the band bending change was measured with a Kelvin probe – a device that measures a potential difference between a metal probe and the semiconductor surface. Our Kelvin probe set-up includes a cryostat, a HeCd laser or a Xenon lamp with a spectrometer (as sources of incident light), an ultra-high-vacuum Kelvin probe and related electronics. We could change temperature from 80 to 600 K, light wavelength and ambient (air, vacuum, pure oxygen or nitrogen). One of the most important findings was that UV light causes desorption of oxygen from the surface (the main source of negative charge at the surface) when a sample is in vacuum, and causes oxygen adsorption or even growth of oxide on GaN surface when the sample is kept in air or pure oxygen. In n-type GaN, these two phenomena could be observed as a slow decrease of the SPV signal in air ambient and slow increase of the SPV in vacuum (Fig. 2). After the light was switched off, the surface potential slowly restored to its dark value what was observed as a slow (logarithmic in time) decrease of the SPV signal (Fig. 2). In p-type GaN, UV light caused negative SPV signal, which also depended on ambient. To explain the results, we have developed a phenomenological model of SPV based on the rate equations for electrons in a semiconductor with a depletion region near the surface. For the first time the effect of charge variation in a surface oxide was also taken into account. Similar effects have been observed for the ZnO surfaces. In addition to SPV measurements using a Kelvin probe, we studied the roles of the surface on photoluminescence (PL). PL is spontaneous emission of light induced by exposure to light. An extremely weak PL signal was observed from the GaN surface that was mechanically polished. However, after removal of a defective 700 nm-thick layer by reactive ion etching, the PL intensity increased by four orders of magnitude. We explained these changes by a significant contribution of surface states to the recombination of charge carriers created by a laser. This work has improved our basic understanding of semiconductor surfaces and mechanisms of radiative and nonradiative recombination and hopefully it will lead to significant improvements in device performance for solid-state lighting, biosensors, communications, and other applications. A number of graduate and undergraduate students were associated with this project, benefiting from the atmosphere of a strong collaborations between research groups at Physics department (VCU), Electrical and Computer Engineering (VCU), and Longwood University.
