In this paper we present a separate-absorption-charge-multiplication Ge/Si avalanche photodiode, which has a
high gain-bandwidth product (e.g., >860GHz at a wavelength of 1310nm). Such a high gain-bandwidth product is
attributed to the peak enhancement of the frequency response at the high frequency range. From a small signal analysis,
we establish an equivalent circuit model which includes a capacitance parallel connected with an inductance due to the
avalanche process. When the APD operates at high bias voltages, the LC circuit provides a resonance in the avalanche,
which introduces a peak enhancement.
Avalanche Photodiodes (APDs) are widely used in fiber-optic communications as well as imaging and sensing
applications where high sensitivities are needed. Traditional InP-based APD receivers typically offer a 10 dB
improvement in sensitivity up to 10 Gb/s when compared to standard p-i-n based detector counterparts. As the data rates
increase, however, a limited gain-bandwidth product (~100GHz) results in degraded receiver sensitivity. An increasing
amount of research is now focusing on alternative multiplication materials for APDs to overcome this limitation, and one
of the most promising is silicon. The difficulty in realizing a silicon-based APD device at near infrared wavelengths is
that a compatible absorbing material is difficult to find. Research on germanium-on-silicon p-i-n detectors has shown
acceptable responsivity at wavelengths as long as 1550 nm, and this work extends the approach to the more complicated
APD structure. We are reporting here a germanium-on-silicon Separate Absorption Charge and Multiplication (SACM)
APD which operates at 1310 nm, with a responsivity of 0.55A/W at unity gain with long dark current densities. The
measured gain bandwidth product of this device is much higher than that of a typical III-V APD. Other device
performances, like reliability, sensitivity and thermal stability, will also be discussed in this talk. This basic
demonstration of a new silicon photonic device is an important step towards practical APD devices operating at 40 Gb/s,
as well as for new applications which require low cost, high volume receivers with high sensitivity such as imaging and
sensing.
Research and development on silicon-based optoelectronic devices is increasing as the need for integrated optical
devices is becoming more apparent. One component which has seen rapid performance improvement over the last five
years has been a Ge-on-Si photodetector which can operate between 850 and 1600 nm with high quantum efficiencies
and bandwidths. We have reported on three types of these detectors; normal incident illuminated p-i-n detectors,
waveguide p-i-n detectors, and avalanche photodetectors (APDs). The former has achieved -14.5 dBm sensitivity at 10
Gb/s and 850 nm, which is comparable to similarly commercially packaged GaAs devices. Waveguide photodetectors
have achieved bandwidths of approximately 30 GHz at 1550 nm with internal quantum efficiencies of 90%. Normal
incident avalanche photodetectors operating at 1310 nm have achieved a primary responsivity of 0.54 A/W with a 3-dB
bandwidth of 9GHz at a gain of 17.
Silicon photonics, especially that based on silicon-on-insulator (SOI), has recently attracted a great deal of attention. The mature industrial infrastructure of CMOS fabrication offers an opportunity for low cost silicon based opto-electronic solutions for applications ranging from telecommunications to chip-to-chip interconnects. The high volume and high performance manufacturing disciplines are advantageous to electro-optics application development and fabrication. However, many technical hurdles still need to be addressed. This paper will give an overview of these opportunities as well as discuss some practical issues and challenges concerning processing silicon photonic devices in a high volume CMOS manufacturing environment.
High density integrated optics on the scale of VLSI is of interest as it allows complicated optical interconnect circuitry to be mass produced. In this paper we present micron-sized high Q resonant cavity structures based on silicon on insulator devices. These resonant cavities may be used in channel dropping filters and modulators. Because of their small size, they have high packing densities on the order of one million devices per square centimeter. This technology has the added advantage in that it can utilize the embedded VLSI electronics manufacturing capacity. In previous work, we studied silicon on oxide photonic band gap (PBG) devices and demonstrated devices with a 400 nm stop band and with a defect which had a Q of 265 centered at a wavelength of 1560 nm. In addition, we fabricated 3 to 5 micrometer radii micro-rings with Qs of approximately 250 and free spectral widths of over 20 nm. In this work, we report results on micro-racetracks, which are oval shaped resonators, with resonances that are approximately 16 nm apart and Qs of about 1000. These racetracks incorporate a vertical coupling technology in which the bus waveguides and the ring are on separate planes. This vertical coupling scheme allows for independent control of the Q of the ring via the distance between the ring and the bus. We demonstrate higher order multi-resonator filters with similar Q and free spectral range to the single resonator filters. The individual resonators in each filter have slightly different resonant frequencies from each other resulting in multi-peaked resonances and lower drop efficiencies. Finally, we show that it is possible to thermally tune the resonances by 1 nm leading to a 10:1 contrast ratio.
Optical interconnects offer advantages over electrical interconnects in terms of clock skew, crosstalk, and RC delay for ULSI (Ultra Large Scale Integrated-Circuit) silicon technology. Optical interconnects are also applicable in optical communications where compact optical devices are fabricated and incorporated in an on-chip integrated optical system. Polycrystalline silicon (polySi)/SiO2 is an attractive waveguiding system that offers significant advantages in both applications with its compact size and compatibility with multilevel CMOS processing. Based on the process optimization that led to a low-loss polySi material, we have fabricated compact waveguide bends and splitters that were microns in size. To study the modal behavior in bending and splitting, we compared multi-mode and single-mode waveguides that were used in fabricating bends and splitters. Two waveguide cross-section dimensions, 0.5 micron X 0.2 micron and 2 microns X 0.2 microns, were used for single- mode waveguide and multi-mode waveguide, respectively. Micron- sized bending was realized with a low loss of a few dBs. Single-mode bends showed less than 3 dB loss for a bending radius of 3 microns, which was lower than that for multi-mode bends. Two different types of splitters, single-mode Y- splitters and multi-mode Y-splitters were fabricated and characterized in terms of their splitting uniformity. One X four and 1 X 16 optical power distribution systems were built based on different splitting schemes and their output power uniformity was compared. Due to the high dielectric contrast of our polySi/SiO2 waveguide system, the smallest 1 X 16 optical power distribution was realized in an area smaller than 0.0001 cm2.
Erbium {tris[bis(trimethylsilyl)]amide} has been employed as a dopant material for incorporation of the rare earth element erbium into a silicon host lattice for fabrication of monochromatic and temperature-independent optoelectronic devices. Electronic characteristics, including room temperature electroluminescence and glancing angle X-ray diffraction, have been obtained from deposited Si:Er films. The utilized erbium amide precursor has been compared to alternative erbium sources as hydrocarbon and fluorocarbon containing (beta) -diketonates (tmhd and fod).
Research in erbium-doped silicon (Si:Er) is discussed in light of our effort to improve the luminescence performance of our LEDs and to demonstrate an integration scheme for a microphotonic clock distribution system. Excitation from Si:Er can occur int ow ways: (1) direct excitation of an Er ion by high energy electrons or (2) energy transfer from an injected electron-hole pair to an Er ion in the lattice. In an LED the first excitation mechanism corresponds to operation in reverse bias, and the latter corresponds to operation in forward bias. We have studied the forward bias case, and we use an energy pathway model to describe the excitation and de-excitation processes. The competing, nonradiative processes against excitation and spontaneous emission are discussed. Maximization of light output can be approached in three ways: (1) decreasing the number of nonradiative energy pathways, (2) enhancing the probability of the radiative pathway, or (3) simply increasing the concentration of active Er sties. We report specific methods that address these issues, and we discuss more device structures that can be used as emitters, optical waveguides, and optical switches in a fully integrated microphotonic system.
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