A compact pin Ge photodetector is integrated in submicron SOI rib waveguide. The detector length is reduced down to 15 microm using butt coupling configuration which is sufficient to totally absorb light at the wavelength of 1.55 microm. A -3 dB bandwidth of 42 GHz has been measured at a 4V reverse bias with a responsivity as high as 1 A/W at the wavelength of 1.55 microm and a low dark current density of 60 mA/cm(2). At a wavelength of 1.52 microm, a responsivity of 1 A/W is obtained under -0.5 V bias. The process is fully compatible with CMOS technology.

Ge-based photodetectors operating in the low loss windows (1.3–1.6 μm) of silica fibers are highly desirable for the development of optical interconnections on silicon-on-insulator substrates. We have therefore investigated the structural and optical properties of Ge thick films grown directly onto Si(001) substrates using a production-compatible reduced pressure chemical vapor deposition system. We have first of all evidenced a Ge growth regime which is akin to a supply-limited one in the 400–750 °C temperature range (Ea=6.9 kcal mol−1). The thick Ge layers grown using a low-temperature/high-temperature approach are in a definite tensile-strain configuration, with a threading dislocation density for as-grown layers of the order of 9×108 cm−2 (annealed: <2×108 cm−2). The surface of those Ge thick layers is rather smooth, especially when considering the large lattice mismatch between Ge and Si. The root-mean-square roughness is indeed of the order of 0.6 nm (2 nm) only for as-grown (annealed) layers. A chemical mechanical polishing step followed by some Ge re-epitaxy can help in bringing the surface roughness of annealed layers down, however (0.5 nm). The Ge layers produced are of high optical quality. An absorption coefficient alpha equal to 4300 cm−1 (3400 cm−1) has indeed been found at room temperature and for a 1.55-μm wavelength for as-grown (annealed) layers. A 20-meV band-gap shrinkage with respect to bulk Ge (0.78 eV⇔0.80 eV) is observed as well in those tensile-strained Ge epilayers.

We report the experimental demonstration of a germanium metal-semiconductor-metal (MSM) photodetector integrated in a SOI rib waveguide. Femtosecond pulse and frequency experiments have been used to characterize those MSM Ge photodetectors. The measured bandwidth under 6V bias is about 25 GHz at 1.55 microm wavelength with a responsivity as high as 1 A/W. The used technological processes are compatible with complementary-metal-oxide-semiconductor (CMOS) technology.

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An experimental characterization of the grating couplers for sub-micrometer silicon-on-insulator (SOI) waveguides is presented. The grating couplers have been designed, realized, and characterized for the +1 diffraction order at an operating wavelength of 1.31 µm for TE polarization. At the resonant angle, a coupling efficiency higher than 55% has been measured. The angular coupling range and the wavelength tolerance have been evaluated to 3 • and 20 nm, respectively. The grating coupler is followed by a taper, and about 50% of the input power at 1.31 µm is coupled into sub-micrometer rib and strip SOI waveguides. The ration between light power decoupled toward the cladding and light power decoupled toward the substrate is about three.

Rib microwaveguides are demonstrated on silicon-on-insulator substrates with Si film thickness of either 380 or 200 nm and a width of 1 microm. Corner mirrors that allow compact 90 degrees turns between two perpendicular waveguides are characterized. Measured propagation losses are approximately 0.4 dB/cm and approximately 0.5 dB/cm for 380-nm and 200-nm Si film, respectively, and mirror losses are approximately 1 dB. This allows the development of applications such as optical interconnects in integrated circuits over propagation distances larger than several centimeters.

A high speed and low loss silicon optical modulator based on carrier depletion has been made using an original structure consisting of a p-doped slit embedded in the intrinsic region of a lateral pin diode. This design allows a good overlap between the optical mode and carrier density variations. Insertion loss of 5 dB has been measured with a contrast ratio of 14 dB for a 3 dB bandwidth of 10 GHz.

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