This paper reports on fabrication and characterization of waveguide integrated vertical PIN Ge/Si photodetectors for operation at optical telecommunication wavelengths. The measured -3dB bandwidth of waveguide integrated photodetectors at 1.53 μm wavelength under 4 V reverse bias is 42 GHz.
A vertically illuminated photodetector based on Ge-on-SOI, which operates at 8 Gb/s for reverse bias as low as 0.5 V, is
presented. The integrated photodetector also features low dark current and good photogenerated carrier collection
efficiency.
This paper reports on fabrication and characterization of two kinds of photodetectors: surface illuminated and waveguide integrated vertical PIN Ge/Si photodetectors for operation at optical telecommunication wavelengths. The measured -3dB bandwidth of surface illuminated photodetectors is 40 GHz under 5 V reverse bias at 1.53 µm wavelength. For waveguide integrated photodetectors, the measured -3dB bandwidth at 1.53 µm wavelength under 4 V reverse bias is 42 GHz.
A high speed and low loss silicon optical modulator based on carrier depletion has been made. It is based on carrier
depletion and consists of a p-doped slit embedded in the intrinsic region of a lateral pin diode. This device has
advantages such as a low capacitance and low optical insertion loss. Experimental results are reported. Using a Mach-Zehnder interferometer with 4 mm-long phase shifter, contrast ratio of 14 dB has been obtained with insertion loss of
5 dB. A 3 dB-bandwidth of 10 GHz has been measured at λ = 1.55μm. Driving electrical power is evaluated. For a
5 mm-long active region, driving power is 100 mW at a frequency of 10 GHz. A large contribution of the dissipated
power comes from the 50 Ω load at the end of the device. By integrating the modulator and its driver on CMOS chip, the
load value could be varied and driving power could be reduced to a few tens of mW.
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.
A structure based on the free-carrier-induced electrorefractive effect in Si/SiGe modulation-doped quantum wells, placed in the intrinsic region of a PIN diode has been proposed. Effective index variation produced by carrier depletion under a reverse bias leads to a phase modulation of a guided wave. The measured variation of the effective index is typically 2.10-4 for a 0V to 6V variation of the reverse bias voltage. This study is focused on the integration of modulation doped SiGe/Si quantum-well optical modulator in SOI submicron rib waveguides with optical losses lower than 0.4dB/cm. The influence of the geometrical parameters, of layer doping and of the metallic contacts has been determined through numerical simulations and optimized modulation structures are defined. The obtained factor of merit LχVχ is then of 1.26 V.cm which can be favorably compared with the best published results obtained with other optimized modulators.
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).
A SiGe/Si optical modulator based on the free-carrier plasma dispersion effect is presented. A SiGe/Si multilayer structure is included in a p-i-n diode and integrated in a submicrometric silicon-on-insulator (SOI) rib waveguide. A P+ Si layer inserted in the Si barriers provide holes that are confined in the SiGe wells at the equilibrium and can be depleted by applying a reverse bias. This structure offers high-speed phase modulation properties. Numerical simulations are used to design the optical modulator. An optimal structure is defined comprising three 10-nm-thick SiGe layers and four 5-nm-thick P+ Si layers. The predicted refractive index variation is 1.7×10–4 under a –6-V bias voltage, and the associated absorption variation is 3 dB/cm. Intensity modulation is obtained by embedding the active region inside a microinterferometric structure. Using resonant cavities a few hundred micrometers long, a modulation depth larger than 60% is achieved with insertion losses of 12 dB, whereas Mach-Zehnder interferometers of a few millimeters length lead to modulation depths larger than 95% with insertion losses lower than 8 dB.
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.
A SiGe/Si Modulation-Doped Multiple Quantum Well modulator, integrated in a rib waveguide is presented. Refractive index variation is achieved by depletion of free carriers initially present in the wells. Numerical simulations are used to design the modulator. An optimal structure is defined: it is formed by 3 SiGe QW, 10 nm-thick, and 4 Si P+ layers, 5 nm-thick. The predicted electrorefractive effect is 1.7.10-4 under a -6V bias voltage, and the associate absorption variation is 0.7 cm-1. Intensity modulation is obtained by including this active region inside an interferometer structure. Using resonant cavities of a few hundred of µm long, modulation depth of more than 60% is achieved with losses in the ON state below 12 dB. Mach-Zehnder interferometer of 3.8 mm-long leads to modulation depth of more than 95%, with losses in the ON state lower than 8 dB.
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.
Using a rigorous differential theory of diffraction gratings, we have investigated the optical properties of some peculiar bandgap structures. We have dealt wit devices made of two stacks of equidistant rod gratings separated by a central planar defect which leads to the occurrence of a transmission peal inside the wavelength bandgap. We have studied how the stability of the effective bandgaps copes with fluctuations of various experimental parameters. For various bandgaps and polarizations in structures including a defect we have studied the resulting filter characteristics and its possibilities as optical switch when the defect is made of nonlinear materials. As long as plastics have convenient refractive indices, optical losses and nonlinearities, the devices we describe could be manufactured using such materials.
In this paper we report on progress made in optical logic planes consisting of hybrid electrically assisted thermo-optic resonators fabricated with standard Silicon technology. As a starting point, the features and shortcomings of a first generation of these bistable elements, based on double-sided polished Silicon wafers, are reviewed. Consecutively we demonstrate, both with modeling and experimental results, that some of the shortcomings of the first generation can be remedied using a Silicon on Sapphire based device. Finally we propose a novel structure, based on Silicon Implanted Oxide substrates, that combines the advantages of the two pervious structures. We show via a heat transfer model that these devices can display bistability at multi-microwatt optical bias power and that, when adequately dimensioned, crosstalk can be avoided such that they can be packed into dense arrays. Both these characteristics and their stability, wavelength flexibility, hybrid mode of operation, and the potentialities for microsecond switching times, can turn these low-cost devices into work- horses for prototype digital optical parallel computing circuitry.
Nonlinear semiconductor waveguides are investigated with respect to their use for optical bistability and temporal optical switching with results given for silicon-on-sapphire devices. The performance of the waveguides is assessed with comparisons to nonlinear Fabry-Perot cavities, and the conditions for optical bistability are theoretically shown to be possible by combining a waveguide and a diffraction-grating coupler. Structures experimentally investigated include semiconductor layers epitaxially grown on substrates having low refractive indices. Optical switching is reported on reflected or transmitted beams and on the guided-wave intensity by means of excitation pulses in the nanosecond or picosecond range. The data confirm the usefulness of nonlinear waveguides as alternative devices for temporal optical switching and logic-gate development in part due to their good switching times.
These photoconductive devices operate with c.w. Nd-YAG laser and use Joule effect enhanced
nonlinearity, associated with the sharp angular resonance corresponding to the TEo guided mode excitation in
a SOS (Silicon On Sapphire) film. Transfer characteristics and spatial uniformity of the transmitted beam for
devices of various geometries have been measured and contrasted characteristics have been observed
locally. Potential interest of these devices is discussed.
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