The aim of this project is to explore the formation, growth, and physical properties of micro- and nano-columns. Arrays of high aspect ratio silicon nanocolumns will be grown on the surface of a silicon wafer by using a pulsed excimer laser to generate "active tips" where the deposition of silicon from the vapor phase is dramatically augmented. Mechanisms of initial formation and subsequent growth of nanocolumn arrays will be studied, and electronic and optoelectronic properties of silicon nano-columns measured. Controlled growth of silicon micro- and nano-column structures could find significant applications in solar cells, field emission tips and sensors. Prior work indicates a connection between silicon microcolumn growth during pulsed-laser irradiation and earlier studies of silicon whisker growth by the vapor-liquid-solid (VLS) method. This link opens new avenues for greater understanding of both methods and provides new tools for the production of semiconductor structures with reduced dimensionality. In the first stage of research the tips of microcolumns will be melted to activate nanocolumn growth. In a second research stage, melting will be avoided in order to grow nanometer scale tips using primarily photolytic activation of growth, based on established knowledge of laser-assisted CVD. In both stages the nanocolumns are grown using previously grown microcolumns as a template. Nanocolumns are expected to grow straight, maintaining the crystal orientation of the microcolumns that serve as their pedestals, and will be directly connected to the wafer beneath. This makes possible control and characterization of nanocolumn structure and properties. With straight, interconnected nanocolumns a short carrier path and short propagation/response time to electrical signals can be achieved. Pulsed-laser growth also permits control of the dopant-atom content as well as simple formation of a passivation layer from the gas phase. The effects of growth conditions on the length, diameter, and evolution of micro- and nano-columns will be investigated. In situ, nanosecond resolution diagnostics including ion probe measurements and emission and absorption spectroscopy will be used to determine the species present close to the silicon surface during ablation and their temporal behavior. Morphological, compositional, and structural features of the individual columns and arrays, and their growth kinetics, will be determined using HRSEM, HRTEM, and AFM microscopies. Computer modeling also will be used to guide understanding of the column formation and growth processes. %%% The project addresses basic research issues in a topical area of materials science having potential technological relevance. The basic knowledge and understanding gained from the research is expected to contribute to improving the perform-ance of current and new electronic/photonic applications by providing a fundamental understanding and a basis for designing and producing improved nanostructured materials. A variety of fundamental issues are to be addressed in these investigations. Central to the project is the training of graduate and undergraduate students to conduct research using state-of-the-art equipment and appropriate scientific methodology as they advance basic understanding of materials growth phenomena using materials having interesting and important applications. Thus the program integrates research and education through the training of students in a fundamentally and technologically significant area. ***
