This project aims for greater understanding and control of defects associated with growth mechanisms of bulk aluminum nitride crystals produced by sublimation. Defects such as dislocations, stacking faults, grain boundaries, polytypoids and inversion domains will be correlated to impurities in crystals, growth conditions, and thermal and mechanical stresses. The types, densities, and the spatial distribution of defects will be measured by x-ray topography, transmission electron microscopy, defect selective etching, and decorative oxidation. Specific types of defects caused by oxygen and carbon will be determined. These impurities are typically present at relatively high concentrations (~1019 cm-3) in AlN crystals as a result of their high concentrations in the original source AlN powder, graphite heating elements, and furnace fixtures. The tendency for these elements to cause stacking faults, precipitates, and other defects will be investigated by comparing crystals with a broad range of impurity concentrations. Differences due to the crystal orientation and polarity will also be determined. The impact of temperature and nitrogen pressure on anisotropic growth rates and defects subsequently present in the crystals will be resolved. A series of crystals produced at different temperatures and nitrogen pressures will be examined to determine which defects are present, and their resulting influence on surface steps. Effects of sample mounting and other physical constraints on stress and subsequent spatial defect distribution will be examined. Residual stress in crystals will be measured and modeled. Surface and subsurface damage caused by mechanical action of polishing the AlN crystals will also be measured. Feedback from this information on defects in AlN is expected to allow the crystal growth process to be modified in a knowledgeable way so that crystal quality can be improved. %%% The project addresses fundamental research issues associated with electronic/photonic materials having technological relevance, and integrates research and educational activities. The project provides graduate students with unique opportunities to learn fundamental aspects of crystal growth and defect characterization. Through the established collaborations, students will be able to study research issues in greater depth than would be possible under separate investigations. Students will visit national laboratories, gain valuable experience working with state-of-the-art equipment and interacting with experienced scientists. The results of the research will be broadly disseminated by publication in high quality scientific journals and by organizing a materials research society symposium. Students will be actively involved by presenting their findings at professional society meetings. Workshops will be arranged for women and under represented group high school students to learn about AlN crystal growth and characterization. Principal investigators and their students will also visit middle school and high school to discuss this project, and to provide special encouragement to women and minority students to join science and engineering. ***
