High quality germanium photodiodes on silicon substrates using an intermediate chemical mechanical polishing step

Germanium (Ge) photodiodes are capable of high quantum yields and can operate at gigahertz frequencies in the 1-1.6 mu m wavelength regime. The compatibility of SiGe alloys with Si substrates makes Ge a natural choice for photodetectors in Si-based optoelectronics applications. The large lattice mismatch (approximate to 4%) between Si and Ge, however, leads to the formation of a high density of misfit and associated threading dislocations when uniform Ge layers are grown on Si substrates. High quality Ge layers were grown on relaxed graded SiGe/Si layers by ultra-high vacuum chemical vapor deposition (UHVCVD). Typically, as the Ge concentration in the graded layers increases, strain fields from underlying misfit dislocations result in increased surface roughness and the formation of dislocation pile-ups. The generation of pile-ups increases the threading dislocation density in the relaxed layers. In this study the pileup formation was minimized by growing on miscut (001) substrates employing a chemical mechanical polishing (CMP) step within the epitaxial structure. Other problems such as the thermal mismatch between Si and Ge, results in unwanted residual tensile stresses and surface microcracks when the substrates are cooled from the growth temperature. Compressive strain has been incorporated into the graded layers to overcome the thermal mismatch problem, resulting in crack-free relaxed cubic Ge on Si at room temperature. The overall result of the CMP step and the growth modifications have eliminated dislocation pile-ups, decreased gas-phase nucleation of particles, and eliminated the increase in threading dislocation density that occurs when grading to Ge concentrations greater than 70% Ge. The threading dislocation density in the Ge layers determined through plan view transmission electron microscopy (TEM) and etch pit density (EPD) was found to be in the range of 2 x 10(6)/cm(2). Ge p-n diodes were fabricated to assess the electronic quality and prove the feasibility of high quality photodetectors on Si substrates.