The rapid expansion in the use of the Internet has resulted in a demand for high speed communications links and devices, including optical links and devices. Optical links using fiber optics have many advantages compared to electrical links: large bandwidth, high noise immunity, reduced power dissipation and minimal crosstalk. Optoelectronic integrated circuits made of silicon are highly desirable since they could be fabricated in the same foundries used to make VLSI integrated circuits. Optical communications technology is typically operating in the 1.3 μm and 1.55 μm infrared wavelength bands. The optical properties of silicon are well suited for the transmission of optical signals, due to its transparency in the infrared wavelength bands of 1.3 μm and 1.55 μm and its high refractive index. As a result, low loss planar silicon optical waveguides have been successfully built.
A silicon based waveguide is just one of many components needed to make an integrated optoelectronic circuit. An optical signal received by an optoelectronic circuit has in many cases to be converted to an electronic signal for further processing by electronic circuits. Conversion of optical signals to electronic signals can be achieved by a photodetector. Silicon, due to its bandgap of 1.12 eV, cannot be used to make photodetectors for infrared band signals, because it is transparent to light at wavelengths above 1.1 μm. Silicon's transparency to infrared light makes it ideal for use as a planar waveguide on an integrated circuit, but eliminates it from use as an infrared photodetector.
Hybrid and integrated optoelectronics have been built with photodetectors made of III-V type semiconductors such as InGaAs, but these devices are difficult to integrate into a silicon chip. SiGe alloys have potential as infrared photodetectors, but primarily in the 1.3 μm wavelength band. Germanium in bulk form has been used as a photodetector in the infrared band, due to its bandgap of 0.66 eV. Making a germanium on silicon photodetector has been difficult, due to the large lattice mismatch, of about 4% for pure germanium on silicon.
After conversion of an optical signal to an electronic signal, the low level electrical signal needs to be amplified and processed by associated electronic circuits. The electronic circuits are typically built on a silicon chip. Due to the lack of useable silicon based IR photodetectors, such optoelectronic conversion is typically performed by hybrid circuits, which are much more expensive than monolithic integrated circuits.