| Germanium offers many features such as high carrier mobilities and efficient optical absorption, that make it an attractive material for the formation of semiconductor devices. However, silicon substrates are the standard material in the semiconductor device field. Graded-alloy epitaxy can be used to form Ge-based devices on Si substrates, allowing not only the use of cheaper, larger, stronger Si (relative to Ge) as the substrate, but also offering the opportunity for the co-integration of Si and Ge based devices. Here is presented an approach to the integration of Ge MOSFETs onto a Si substrate: the use of a graded-alloy buffer layer to confine the dislocations associated with the approximate 4% lattice mismatch between Si and Ge to well below the semiconductor surface. A device-quality Ge cap is then epitaxially deposited on the buffer layer. Conventional device processing is then used to form the transistors on the cap. First, the theory and design of the graded-alloy layer is presented. Then, different methods of material analysis of Ge-Si alloys are described, with novel analytic methods presented for Rutherford Backscattering Spectroscopy and Optical Reflectance Spectroscopy. Results are applied to the refinement of the graded-alloy layer process. Diodes, capacitors, and transistors made in the Ge cap are then described and analyzed, with considerable attention given to the design of the field-effect transistor. Finally, extensions to the present technology are discussed. |
