Growth and Characterization of Nitride Based Nanowire Heterostructures

Harvard University, Cambridge MA

Investigators

Abstract

Major advances in the physics and applications of semiconductors have been made through structures with reduced dimensionality. Until recently, most of the nanostructures studied relied on quantum wells and quantum dots, structures with one and three-dimensional carrier confinement, respectively. Recent demonstrations of new materials science and applications turned to the less explored nanowires, structures with two-dimensional confinement. The field of semiconductor nanowires appears to hold great promise of achieving functional material structures at the nanoscale. Indeed, the ability to synthesize high quality materials and our ability to determine their properties are improving very fast. However, the electrical properties of nanowires are not yet well understood and there is a great need for characterization methods capable of evaluating single nanowires. One of the semiconductors of interest of the last few years has been GaN, a large bandgap material that promises to extend the range of optical devices well into the visible. The possibility of combining GaN, InN, and AlN into ternary and quaternary alloys allows for the design and implementation of high performance heterostructure devices. The absence of lattice matched substrates for the nitrides and the large lattice constant differences between the binary compounds result in high density of dislocations that impair performance of many devices. The presence of dislocations also makes it difficult to determine fundamental properties of these materials. Novel methods of preparing GaN and related heterostructures are clearly needed. The vapor-liquid-solid growth offers a radically different route to the formation of highly perfect nanowires of these semiconductors and their heterostructures. In this proposal we plan to synthesize coaxial and longitudinal heterostructure nanowires based on GaN by vapor-liquid-solid growth, and to use advanced scanning probe methods developed in the P.I.'s lab to carry out electrical measurements on ensembles of nanowires and single nanowires. Intellectual Merit The possibility of modifying and adjusting properties of materials at a very fundamental level makes nanotechnology attractive across a range of fields, from physics and chemistry to biology. High quality nanostructures and nano-scale methods of evaluating them are needed to determine and realize the full scope of this field. It is also beginning to be clear that heterostructures, a key element in any device applications of nanowires, must be synthesized and characterized with high level of precision in order to understand their properties and device applications. This proposal addresses the two key problems of the field of nanostructures; advanced synthesis and sophisticated measurements. Broader Impact This proposal addresses scientific and educational issues of current interest to many disciplines. The project is interdisciplinary in nature and combines fundamental physics and materials science with advanced engineering. The combination is expected to provide excellent training for graduate students and a post-doctoral associate. The PI has a strong commitment to enhancing undergraduate education through research experiences and undergraduate students will be encouraged to participate through a REU program. He currently has 2 undergraduates working in the area and a second year female graduate student will be working on the project.