| Summary: | Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1998. === Includes bibliographical references (leaves 137-147). === Silicon germanium layers on silicon substrates (SiGe/Si) are useful for a variety of microelectronics applications. The most successful technique for growing low defect density relaxed SiGe/Si layers is relaxed compositional grading. However, with increasing Ge content in the graded SiGe layers, the following materials concerns need to be addressed-{i) a high surface roughness, (ii) the formation of dislocation pile-ups, (iii) an increase in the threading dislocation density, (iv) tensile strains and micro-cracking due to thermal mismatch, and (v) particulate contamination from germane gas phase nucleation. We have grown relaxed graded SiGe/Si layers using ultra-high vacuum chemical vapor deposition (UHVCVD) at growth temperatures ranging between 500°-900° C and pressures between 30-500 millitorr. The SiGe growth rates at different temperature and Ge content regimes agree with previously proposed theories. By applying both a dislocation blocking criterion and surface roughness effects to graded SiGe/Si structures, we have proposed a model to explain and predict the formation of dislocation pile-ups in graded structures. We have discovered that there is a substantial improvement in the surface roughness and dislocation pile-up density in graded SiGe layers by growing on miscut Si(00l) substrates. It was found that the array of 60° dislocations that usually forms to relieve the misfit stress could transform into a novel lower energy hexagonal dislocation network consisting of all edge dislocations. High resolution X-ray diffraction measurements indicate that there is a decrease in the rate of epilayer tilting in the Ge-rich layers of the graded buffer in agreement with the observed dislocation structure. We have designed an optimized relaxed buffer (ORB) process that allows us to grow high quality Ge layers on Si substrates. By employing a chemical mechanical polishing (CMP) and regrowth step within the epitaxial structure, we have minimized the formation of dislocation pile-ups. Compressive strain has been incorporated into the graded layers to overcome the thermal mismatch problem. The ORB process eliminates dislocation pile-ups, decreases gas-phase nucleation of particles, and eliminates the increase in threading dislocation density. Germanium p-n photodiodes were fabricated to assess the diectronic quality and prove the feasibility of a high quality infrared detector on a Si substrate. The dark current in the diodes was at least two orders of magnitude lower than any previously reported value for Ge photodiodes on Si substrates. Capacitance measurements indicate that the devices are capable of high speed operation. Dislocation filtering experiments were conducted to reduce threading dislocation densities on pattered mesas. It was found that at the strain levels from small compressive mismatch, dislocation nucleation from the mesa edges dominates over dislocation filtering. === by Srikanth B. Samavedam. === Ph.D.
|
|---|
