Technical: Although nitride semiconductor materials are being used in some electronic and photonic applications including heterostructure field-effect transistors and blue light emitters, defects, impurities, and surface issues are hampering new applications such as the use of increasingly higher aluminum content AlGaN alloys for ultraviolet light emitting diodes and lasers and the development of InN-based transistors for terahertz emitters. This project aims to address these materials science issues through a tightly coupled experimental and computational effort. High quality AlN, InN, and their alloys are grow epitaxially and the high quality samples enable detailed characterization of surfaces, point defects, and impurities using structural, electrical, and optical techniques. Electron accumulation layers on the surface are investigated and controlled. The limits of n-type and p-type doping will be pushed. The experimental effort is directly tied to state-of-the-art first-principles calculations based on density functional theory; the "band-gap problem" will be addressed through the use of techniques that have recently successfully been implemented, such as quasi-particle calculations and hybrid functionals. Deep (localized) and shallow (extended) levels of defects and impurities are investigated, as well as surface reconstructions and surface states. Closing the loop between theoretical and experimental results is expected to provide deep understanding of fundamental atomic-level mechanisms and phenomena associated with synthesis and processing of these novel materials.
This project addresses basic research issues in a topical area of materials science with high technological relevance. The wide-band-gap semiconductors that are the focus of the investigation are recognized as the prime materials for light emitters throughout the visible and into the ultraviolet, and the transition to solid-state lighting will have tremendous impact both in the developed as well as the developing world. Nitrides are also starting to gain ground in the area of high-frequency devices, with applications in telecommunications and radar. Another area of high potential impact is for photovoltaics, where the nitride materials systems can span the range from ultraviolet to infrared. This project has significant educational value: the graduate students are involved in a tight collaboration between theory and experiment. In addition, the computational project will make use of the brand new Allosphere facility at the California Nanosystems Institute at UCSB. The Allosphere is a 3-story-high spherical space in which a fully immersive and interactive virtual environment can be experienced. It will be helpful with the visualization and understanding of wave functions and bonding environments in the various materials, as well as with the dissemination of results to a broad audience.
Wide-band-gap group-III nitrides, namely gallium nitride, aluminum nitride, and indium nitride, are the enablers of the revolution in solid state lighting and are the key materials for future high efficiency power conversion devices. Even in their immature state of materials understanding, wide-band-gap materials such as gallium nitride offer the potential to reduce electricity consumption by a total of ~20% through high efficiency lighting and power conversion. Despite the technological impact of group III nitrides, basic materials properties are poorly understood – these properties include surface energies, fracture toughness, and defect formation energies – all necessary fundamental parameters for improving crystal growth, material quality and ultimately for improving the efficiency of group-III nitride-based devices such as light emitting diodes (LEDs) for solid state lighting and power transistors for power conversion. In this project, significant progress was made towards achieving a more complete understanding of basic materials properties through a combination of high quality growth, detailed characterization, and first-principles computations. A fundamental property of single crystals, such as semiconductor single crystals used as substrates ("wafers") for the growth of thin layers, i.e., epitaxial growth, is the equilibrium shape. The equilibrium crystal is usually determined through a construction referred to as the Wulff plot. In the Wulff plot, the energy of a crystal face is plotted parallel to the face normal. The equilibrium crystal shape is determined by the inner tangent surface to the Wulff plot. In this NSF-funded research program, the crystal shape of high quality gallium nitride was experimentally determined by the near-equilibrium growth technique of hydride vapor phase epitaxy. Image 1 shows an example of a high quality single crystal island demonstrating the equilibrium crystal planes for gallium nitride. The three inclined facet planes on the left of the image are semipolar {10-11} planes, three nonpolar {10-10} m planes, and a singular N-face (000-1) plane. Polar planes such as Ga-face (0001) plane and N-face (000-1) plane can be readily produced on low cost large area substrates, such as silicon or sapphire and are favored for power electronic devices. For LEDs, however, growth on semipolar planes and nonpolar planes minimizes or altogether avoids fundamental issues related to high internal electric fields in the quantum wells used for light emission. LEDs on nonpolar and semipolar planes provide the most viable solution to the efficiency droop problem that limits efficiency at high electrical currents. The results of this program have been essential in verifying that the nonpolar m plane is indeed an equilibrium crystal plane for GaN and thus a preferred plane for device development. Our computational work has addressed the relative and absolute energies of these gallium nitride surfaces, and explored the consequences for cracking of films that are grown under tensile strain. These studies were carried out with cutting-edge computational techniques that provide accurate and reliable descriptions of electronic and atomic structure as well as energetics. The approach was also applied to the study of defects and impurities. Highly relevant results were obtained for acceptor dopants such as carbon and magnesium. Carbon was found to be a deep acceptor, meaning that it cannot lead to p-type material, and gives rise to below-band-gap luminescence; in this case the frequently observed "yellow luminescence" [Image 2]. Magnesium exhibits an unexpected dual character: it has a small enough ionization energy to act as a shallow acceptor and provide p-type doping; but optically it behaves as a deep acceptor. Such fundamental insights are crucial, since magnesium is the only acceptor that can produce the gallium nitride p-type layers that are essential for LEDs.
