This paper reports theoretical and experimental investigations of germanium photodetectors integrated in silicon on
insulator waveguides for metal-semiconductor-metal (MSM) photodetectors integrated in a slightly etched rib
waveguide. Experimental characteristics of germanium on silicon photodetectors have been obtained using time
measurements with femtosecond pulses and opto-RF experiments. For MSM structure with 1μm electrode spacing, the
measured bandwidth under 6V bias is 25 GHz at 1.55 μm wavelength with a responsivity as high as 1 A/W and the
bandwidth reaches 30GHz for 0.7μm electrode spacing under 1V bias.
This paper reports on fabrication and characterization of two kinds of photodetectors: interdigited metal-germanium on
silicon-metal photodetectors (Metal-Semiconductor-Metal or MSM) and pin germanium photodiodes for operation at
optical telecommunication wavelengths. For both 1.31 micron and 1.55 micron wavelengths, the measured -3dB bandwidth of
interdigited MSM photodetectors is 35 GHz under 2V bias for electrode spacing equal to 0.5 micron. For pin diodes at
1.55 micron wavelength, the measured -3dB bandwidth under -3V bias ranges from 9 to 29 GHz for mesa diameters from 20
to 7 microns, respectively.
SOI microwaveguides and associated devices (splitters, turns,...) are used for light distribution. Rib SOI geometries obtained by shallow etching of the silicon film offer definite advantages for the integration of active devices while fulfilling efficiency and compactness. Propagation losses of such waveguides are one order of magnitude smaller than for single mode strip waveguides. Rib-based compact and low loss optical signal distribution from one input to up to 1024 output points has been demonstrated. Light injection in submicron SOI waveguides is discussed. The indirect bandgap of silicon is not in favor of light emission and modulation. Realization of silicon sources and efficient high speed silicon-based modulators is a real challenge. For light detection, germanium can be grown on silicon and Ge photodetectors with -3dB bandwidths up to 30 GHz have been demonstrated.
CMOS technology downscaling is expected to encounter a metallic interconnect bottleneck in the near future due to increasing delays of global on-chip interconnects, signal distortion, and timing uncertainty (skew and jitter), as well as power consumption. Silicon-compatible integration of optical on-chip interconnects is presented as an alternative solution. Integrated optics using silicon-on-insulator single-mode waveguides, Si-based modulators, and Ge photodetectors offers a feasible way to distribute the global clock over the whole chip. Taking into account the photodetectors' characteristics, with a CMOS-inverter-based transimpedance front-end amplifier and additional gain stages to ensure sufficient voltage swing, the performance of optical global clock distributions is compared with that of their metallic counterparts. The main advantages brought by optics include signal propagation with negligible distortion over centimeter-long distances (pulse spreading <1% of the clock period at a frequency of 100 GHz), reduction of skew and jitter in comparison with electrical repeater lines (optical skew/jitter <3% to 4% at a frequency of 50 GHz), lower sensitivity to temperature variations (optical delay variation <1.8% for a 100-K variation), and reduction of the total chip power consumption (negligible power consumption for the global clock distribution: <70 mW).
The main characteristics of germanium photodetectors integrated in silicon-on-insulator optical waveguides for intrachip optical interconnects are presented. The epitaxial Ge layers are grown on Si(001) by reduced-pressure chemical vapor deposition. The optical absorption of Ge layers is recorded from 1.2 to 1.7 µm and linked to the layer strain. The responsivity of an interdigitated metal-semiconductor-metal Ge photodetector has been measured. Light coupling from a slightly etched submicron rib silicon-on-insulator waveguide to a Ge photodetector is studied for two configurations: butt coupling and vertical coupling.
This paper presents the potential characteristics of germanium photodetectors integrated in silicon-on-insulator optical waveguides for intra-chip optical interconnects. Experimental results on the optical absorption, from 1.1 μm to 1.7 μm of Ge layers epitaxially grown on Si are reported, as well as the measured responsivity of an interdigited MSM Ge photodetector. Light coupling from a rib SOI waveguide to a Ge photodetector is studied for two possible configurations: butt coupling or Ge deposition on top of the waveguide. Comparisons between MSM and PIN Ge detectors are carried out by estimating the dark current, capacitance and time response.
Downscaling the CMOS technology is expected to encounter a metallic interconnect bottleneck in the near future due to the increasing delays of global on-chip interconnects, problems of signal integrity and timing uncertainty (skew and jitter) as well as power consumption. The possible silicon-compatible monolithic integration of optical on-chip interconnects is described as an alternative solution. It is shown that integrated optics using SOI single-mode waveguides, Si-based modulators, and Ge photodetectors offers a feasible way to distribute global signals such as the global clock across a chip. Taking into account the photodetectors followed by a CMOS-inverter-based transimpedance front-end amplifier with additional gain stages to ensure sufficient voltage swing, optical interconnects characteristics are compared with the performances of future metallic global interconnects recently published in the literature. The main advantages brought by optics include signal propagation with negligible distortion over cm-long distances, reduction of total chip power consumption, reduced delay, skew and jitter if compared with electrical repeated lines, and a lower sensitivity to temperature variations.