SiGe waveguides offer fiberoptic solutions
W. Conard Holton
Silicon-compatible optoelectronic de vices continue to lure researchers with the promise of ease of fabrication, low cost, and integration with silicon complementary metal-oxide electronic components. One of the most promising new materials for this purpose has been silicon germanium (SiGe), which re searchers at the Institute for Micro structural Sciences at the National Research Council of Canada (Ottawa) are incorporating into active and passive waveguide components.
Within the past decade, the commercial growth of silicon germanium devices has been made feasible by the development of ultrahigh vacuum chemical-vapor deposition technology. Despite the potential advantages of SiGe, some major drawbacks--principally the lattice mismatch that occurs between Si and Ge, which has a 4% larger lattice constant--have prevented development of practical devices. The resulting strained epilayer relaxes if it exceeds the critical thickness, forming dislocations between the Si and Ge layers that reduce the refractive index and produce current leakage paths in electronic devices.
According to researcher Siegfried Janz, the team has adapted several approaches toward its goals of fabricating waveguide components such as star couplers, arrayed waveguide grating (AWG) demultiplexers, and photodetectors for 850, 1300, and 1550 nm. He says its study of the index of refraction for pseudomorphic Si1-xGex epilayers using a waveguide mode-profiling technique shows that SiGe waveguides have an intrinsic material birefringence that must be taken into account when designing devices.
Based on the characterization of SiGe properties, the team has created waveguides with a total SiGe layer thickness well beyond the measured critical thickness for lattice relaxation--and stabilized them by inserting Si capping layers during growth. Single-mode curved-ridge waveguides formed in this way show no obvious bend losses for radii of curvature as small as 4 mm. An optical star coupler and the first prototype of a SiGe/Si AWG demultiplexer also were created. Janp says that the AWG demultiplexers would be very inexpensive to manufacture and could find use in fiber-to-the-home networks.
As for a SiGe photodiode, the re searchers used a silicon-on-insulator substrate and developed a waveguide configuration that provides a long absorption region for photocarrier generation and collection and can be monolithically integrated with other optoelectronic components (see figure). Rather than growing planar SiGe multiple quantum wells on Si substrates, the re searchers grew strained SiGe quantum wells with periodic thickness variations along the surface plane.
These "coherent wave quantum wells" reduced quantum confinement at the wave crest, where the germanium tended to migrate, and in turn led to significantly lower bandgaps than typical planar quantum wells. The resulting SiGe photodetector had a bandgap below 800 meV for operation at 1.55 µm and initially demonstrated responsivity of approximately 0.3 A/W at 1.3 µm and 0.05 A/W at 1.55 µm--the key telecommunications windows.