Technical: This project addresses materials science synthesis/processing research of m-plane GaN for use in GaN-based devices. GaN-based materials are higly polar on c-plane which when strain is present or a compositional gradient is introduced resulting in polarization charge. The ensuing Stark shift reduces radiative recombination efficiency and can cause carrier leakage in light emitting diodes (LEDs) reducing their efficiency. The research emphasis will be on synthesis of epitaxial m-plane GaN and investigation of optical processes. An accompanying aim is to explore growth of well-aligned, m-plane GaN films on relatively low cost patterned sapphire and Si substrates to understand the fundamentals of growth mechanisms involved. A basic investigation of non polar m-plane GaN synthesis on inexpensive and readily available, but appropriately patterned substrates will be carried out in conjunction with requisite characterization to gain further understanding on radiative recombination processes in such layers. The orientation and morphology of the films, as well as extended and point defects will be investigated using structural, electrical, and optical characterization techniques. The objective is to explore the science underlying the synthesis of m-plane GaN on low-cost sapphire and Si substrates with etched facets. Additionally, the enhancement of optical matrix elements, p-type conductivity in m-plane oriented epitaxial layers, and basic investigation of quantum confinement effects and their influence on the optical efficiency in the absence of polarization fields will be studied.
This project addresses basic research issues in a topical area of materials science with high technological relevance. The research outcomes could pave the way for more efficient light emitters such as LEDs with acceptable color rendering index and efficacy for solid state lighting. These investigations have the potential to produce sufficiently bright light sources for solid state lighting applications, as well as lasers and detectors for consumer and military applications. The research is important in ensuring that the US maintains its current status as a leader in the exploration and implementation of technologies that will lead to energy savings and concomitant environmental benefits. The project also integrates research and education. The PIs have established a collaborative and interdisciplinary research environment for graduate and undergraduate students from electrical engineering and physics fields, and this project will provide graduate and undergraduate students opportunities for valuable cross-disciplinary learning, for interactions with international scientists, for development of industrial connections, in addition to assisting their education and understanding of fundamentals of semiconductor technology and physics. Students are also involved in the PIs' efforts toward reduced green house gas emission by helping to achieve semiconductor based efficient white light generation. Since VCU is a growing urban university with a diverse student body (18% African American, 22% other minorities), the PIs recruit from a relatively large pool of students from underrepresented groups. Consequently, they have established minority students in their laboratories and will continue to address diversity.
This research program has been devoted to the exploration of growth and optical properties of the semiconductor material gallium nitride (GaN) of nonpolar and semipolar crystallographic orientations on low-cost sapphire and Si substrates to develop the framework for high efficiency light emitters. GaN is becoming the dominant semiconductor for light emitting diodes (LEDs) and laser diodes for near ultra-violet to blue/green spectral ranges, which represent a rapidly growing market in general lighting and many technological applications (some 13% growth in year 2014). Currently, the major work horse of industrial development is the "polar" orientation of GaN, which in the presence of strain or compositional gradient (both unavoidable in devices) possesses polarization charge that leads to internal electric field along the growth direction. The ensuing separation of opposite charges reduces the radiative recombination, i.e. the light emission rate and also causes current leakage in LEDs, reducing their efficiency. Nonpolar and semipolar orientations make it possible to eliminate or substantially reduce the electric field in the vertical direction thus improving the overall device efficiency and reducing the associated energy consumption and carbon emission endemic to energy production. To exploit the potential of the nonpolar and semipolar GaN to its fullest, this research program focused on the development of the growth technologies and an understanding of processes governing radiative recombination. The results obtained would help pave the way for high efficiency LEDs for solid state lighting as well as lasers and detectors for consumer applications. During this program, we accomplished the synthesis of nonpolar m-plane and semipolar, specifically (1-101) and (11-22) plane, GaN thin films on patterned Si and planar m-plane sapphire substrates. The crystallographic orientation and morphology of the films as well as the distribution and interplay of structural defects were investigated using a variety of structural, electrical, and optical characterization techniques, which revealed pathways to improved material quality. Growth methodologies were developed to produce high quality material with regions essentially free from structural defects, which are detrimental for device operation. In particular, semipolar (1-101) GaN on patterned Si was found to exhibit high optical quality, approaching the state-of-the-art c-plane GaN grown using an in situ nanonetwork method, and thus, shows great promise for light emitting devices. In addition to exploring the limitations on p-type conductivity, this program also shed light on the anisotropy of optical properties and quantum confinement effects and their influence on the optical efficiency in structures with reduced or no polarization fields. LED structures produced on semipolar GaN templates had performance comparable to those on highly optimized polar c-plane counterparts featured by a high internal quantum efficiency of about 80%. The growth methodologies developed under this program have potential applications for a wider range of technologically important materials, including semiconductors and multifunctional oxides. Another significant outcome is the development of the state-of-the-art characterization methods making it possible to evaluate the optical and structural quality of materials from macro- down to nano-scale which would enable optimization of growth and fabrication techniques for a large variety of functional materials for electronics and optoelectronics. In regard to training, this program facilitated a multidisciplinary environment with unique opportunities for the professional development of undergraduate and graduate students. All the team members have been cross-trained in advanced thin-film growth techniques as well as in structural and optical characterization methods. In particular, the students focused on optical characterization were also involved in the cutting-edge growth and fabrication techniques, while the team members, whose primary responsibilities were centered on growth and fabrication, learned how powerful modern optical characterization techniques, such as time- and polarization-resolved PL, near-field scanning optical microscopy, and spatially and spectrally resolved CL spectroscopy, can unveil the relationship between the growth procedure and formation of structural defects and the impacts of various defects and defect distributions on the optical characteristics of the material. The students involved in this project also benefited from the invaluable educational opportunity provided by extensive national and international collaborations with research groups that are world leaders in their respective fields: Arizona State University, Otto von Guericke University of Magdeburg (Germany), University of Linköping (Sweden), University of Montpellier-2 (France), University of Vilnius (Lithuania), and Kyma Technologies (Raleigh, NC). The major scientific findings of this effort have been widely disseminated through publications in peer-reviewed journals, and presentations at international conferences and workshops. In total, 16 peer-reviewed articles have been published and 18 conference presentations and talks have been delivered. Selected research outcomes related to growth procedures and characterization of nonpolar and semipolar GaN have been incorporated into graduate level courses in Advanced Semiconductor Devices and Characterization of Semiconductor Materials and Devices. Finally, this research effort has facilitated four PhD dissertations at VCU.
