A series of on-chip optical links of 50-μm length, utilizing 650 to 850 nm propagation wavelength, with Si avalanche-mode optical sources, silicon nitride-based waveguides, and Si Ge detectors, have been designed and realized, with a 0.35-μm SiGe radio frequency bipolar integrated circuit process. The optical coupling between the optical source and the detectors was realized by a set of dedicated designed optical waveguides, which were all fabricated with components of the SiGe radio frequency process. All components were fully integrated on the same silicon chip. The Si avalanche-mode light-emitting diodes (Si AMLEDs) emitted in the 650- to 850-nm wavelength regime. Correspondingly, small microdimensioned detectors utilize SiGe detector technology with detection efficiencies of up to 0.85 in the same wavelength regime and with a transition frequency of up to 20 GHz. Best performances for the optical links as realized show optical coupling of up to 5 GHz with a total optical link budget loss of −40 dB. A set of link results are presented and several interpretations are given on current realizations. The technology is particularly suitable for realization of low-cost on-chip optical signal processing, optical interconnects, and various types of on-chip microsensors.
A novel silicon SiGe edge light emitting diode (SiGe ELED) was realized in standard RF bipolar SiGe technology process that uses a p-n junction either in reverse and forward bias mode configurations. A vertical cubical columnar SiGe/Si HBT like structure was used. The light emitting process in the reverse bias mode is by means of an avalanche breakdown process. The reverse biased device emits light in the wavelength range of 450–650 nm, with operating voltage and current of 1.3 V and 8 mA respectively, while the forward biased mode emitted at about 850nm. In the forward biased mode, it operates in a two junction mode with the first n+p junction emitting low energy electrons into a lowly doped p region. The SiGe ELED is intended to be implemented in an optical interconnect with an external detector via a lateral optical waveguide coupling. Because the LED emit in a broad spectrum, localizing the emission source point is of paramount importance. Two techniques were used to attempt to realise this objective. Optical Probe measurement and Optical Power Meter Mapping technique. Localization of the emission source point process was performed through scanning a lensed fiber coupled to an optical power meter, over the edge surface profile of the diced device. The side edge of the diced device structure was interface with the lens fibre in other to ascertain the maximum emission point and the nature of light emitting process. This was done using a smooth dicing of the LED device close to its emitting edge and scanning its edge surface through a multimode lensed fiber coupled to an optical power meter. The mapping area scanned was 40μm x 40μm and 60μ x 60μ to localize the emitting source point region. A total emitted optical power as measured from the Led was 2.86nW as measured by the optical power meter connected to the lensed optical fibre. This was confirmed with a light-current-voltage (LIV) characteristics power measurement curve obtained from the device by means of the edge mapping techniques. A rough estimated localization of the source point was approximately 0.3mW optical power with a current of 8mA was realized with this technique. These results can be used to design accurate electro-optical conversions in integrated photonic circuitry as well as designing well coupled optical interconnects from the chip to the environment.
A two-junction micro p+np+ silicon avalanche-mode light-emitting device (Si AMLED) is analyzed for its dispersion characteristics, which generally resulted in different wavelengths of light (colors) being emitted at different angles from the surface of the device. The SiAMLED is integrated into on-chip bipolar radio frequency-integrated circuitry at micron dimensions. LEDs have high-frequency modulation frequencies reaching into the GHz range. Such devices, which are of micron dimension, operate at 8 to 10 V, 1 μA to 2 mA. The emission wavelength is in the 450- to 850-nm range, emission spot sizes are about 1 μm2, and emission intensities are up to 200 nW . μm − 2. The observed geometrical-chromatic dispersion characteristics range from 0.01 deg / nm wavelength for green radiation at a 5 deg exit angle to the normal of the device to 0.16 deg / nm wavelength for blue radiation at a 60 deg exit angle to the normal of the surface of the device. The high dispersion characteristics of the emitted radiation are attributed to the positioning of the optical source ∼1 μm subsurface to the silicon–silicon oxide interface, as well as to the high-refractive index differences between silicon and the surrounding lower refractive index silicon oxide layers. It is believed that the identified dispersion characteristics will have interesting and futuristic on-chip electro-optic applications for on-chip micro-optical wavelength dispersers, futuristic optical communication demultiplexers, along with on-chip microgas and biosensor applications.
Optical emission probabilities from silicon were analyzed by appropriate modelling, taking the silicon energy band structure, available carrier energy spread, and available carrier momentum spreads that can be realized with typical device design and operating conditions as available in current silicon technologies. The analyses showed that creation of micron-dimensioned conduction channels as made possible by using a RF bipolar fabrication process, appropriate doping and variations in the channel utilizing Boron, Phosphorous and Germanium doping, and using reversed biased junctions to energize specifically electrons, appropriately controlling carrier energy and carrier density, and control over carrier momentum through appropriate impurity scattering technology; particularly, 280nm, 650nm and 850nm emissions can be stimulated. Particularly, using p+nn and p+np+ device designs with appropriate control over carrier energy, carrier type balancing and implementing enhanced impurity scattering in some device regions, show the greatest potential to enhance these emissions. First iteration empirically conducted device realizations results show interesting peaking features and nonuniform high intensity behaviors. Particularly, it was succeeded to increase the emissions at 650nm with about two orders of magnitude. Internal electrical- to- optical conversion efficiencies of up to 10-4 and intensity emissions of up to 200 nW μm2 are derived, with further prospects to increase emissions further. The attained results compare extremely favorable, and in some cases exceeds, results as published by Venter et al, Kuindersma et al and Du Plessis et al using related technologies.
Si Av LEDs are easily integrated in on-chip integrated circuitry. They have high modulation frequencies into the GHz range and can be fabricated to sub-micron dimensions. Due to subsurface light generation in the silicon device itself, and the high refractive index differences between silicon and the device environment, the exiting light radiation has interesting dispersion characteristics. Three junction micro p+-np+ Silicon Avalanche based Light Emitting Devices (Si Av LEDs) have been analyzed in terms of dispersion characteristics, generally resulting in different wavelengths of light (colors) being emitted at different angles and solid angles from the surfaces of these devices. The emission wavelength is in the 450 - 850 nm range. The devices are of micron dimension and operate at 8 - 10V, 1μA - 2mA. The emission spot sizes are about 1 micron square. Emission intensities are up to 500 nW.μm-2. The observed dispersion characteristics range from 0.05 degrees per nm per degree at emission angle of 5 degrees, to 0.15 degrees per nm at emission angles of 30 degrees. It is believed that the dispersion characteristics can find interesting and futuristic on-chip electro-optic applications involving particularly a ranging from on chip micro optical wavelength dispersers, communication de-multiplexers, and novel bio-sensor applications. All of these could penetrate into the nanoscale dimensions.
This paper demonstrates the experimental study of edge and top illuminated SiGe phototransistors (HPT) implemented using the existing industrial SiGe2RF Telefunken GmbH BiCMOS technology for opto-microwave (OM) applications using 850nm Multi-Mode Fibers (MMF). Its technology and structure are described. Two different optical window size HPTs with top illumination (5x5μm2, 10x10μm2) and an edge illuminated HPTs having 5μm x5μm size are presented and compared. A two-step post fabrication process was used to create an optical access on the edge of the HPT for lateral illumination with a lensed MMF through simple polishing and dicing techniques. We perform Opto-microwave Scanning Near-field Optical Microscopy (OM-SNOM) analysis on edge and top illuminated HPTs in order to observe the fastest and the highest sensitive regions of the HPTs. This analysis also allows understanding the parasitic effect from the substrate, and thus draws a conclusion on the design aspect of SiGe/Si HPT. A low frequency OM responsivity of 0.45A/W and a cutoff frequency, f-3dB, of 890MHz were measured for edge illuminated HPT. Compared to the top illuminated HPT of the same size, the edge illuminated HPT improves the f-3dB by a factor of more than two and also improves the low frequency responsivity by a factor of more than four. These results demonstrate that a simple etched HPT is still enough to achieve performance improvements compared to the top illuminated HPT without requiring a complex coupling structure. Indeed, it also proves the potential of edge coupled SiGe HPT in the ultra-low-cost silicon based optoelectronics circuits with a new approach of the optical packaging and system integration to 850nm MMF.
Silicon Photonics is an emerging field of research and technology, where nano-silicon can play a fundamental role. Visible light emitted from reverse-biased p-n junctions at highly localized regions, where avalanche breakdown occurs, can be used to realize a visible electro-optical sources in silicon by means of light-emitting diodes (Si-LEDs) is reviewed by characterizing the spectral distribution. Regarding applications, a monolithic optoelectronic integrated circuit (OEIC) for on-chip optical interconnection based on standard CMOS technology is discussed. Although there are some of the present challenges with regard to the realization of suitable electro-optical elements for diverse integrated circuit applications, the type of silicon light source can be further developed into be a Si-based optical short-distance on-chip optical interconnect applications.
Si Avalanche based LEDs technology has been developed in the 650 -850nm wavelength regime [1, 2]. Correspondingly, small micro-dimensioned detectors with pW/μm2 sensitivity have been developed for the same wavelength range utilizing Si-Ge detector technology with detection efficiencies of up to 0.85, and with a transition frequencies of up to 80 GHz [3] A series of on-chip optical links of 50 micron length, utilizing 650 – 850 nm propagation wavelength have been designed and realized, utilizing a Si Ge radio frequency bipolar process. Micron dimensioned optical sources, waveguides and detectors were all integrated on the same chip to form a complete optical link on-chip. Avalanche based Si LEDs (Si Av LEDs), Schottky contacting, TEOS densification strategies, silicon nitride based waveguides, and state of the art Si-Ge bipolar detector technologies were used as key design strategies. Best performances show optical coupling from source to detector of up to 10GHz and - 40dBm total optical link budget loss with a potential transition frequency coupling of up to 40GHz utilizing Si Ge based LEDs. The technology is particularly suitable for application as on-chip optical links, optical MEMS and MOEMS, as well as for optical interconnects utilizing low loss, side surface, waveguide- to-optical fiber coupling. Most particularly is one of our designed waveguide which have a good core axis alignment with the optical source and yield 10GHz -30dB on-chip micro-optical links as shown in Fig 9 (c). The technology as developed has been appropriately IP protected.
Graded junction, carrier energy and momentum engineering concepts have been utilized to realize a high intensity 100 nW 5GHz Silicon Avalanche based LED (Si Av LED). A silicon 0.35 micron RF bi-polar process was used as design and processing technology. Particularly, the carrier momentum and energy distributions were modeled in graded junction Silicon p+-i-n structures, and utilized to increase optical yield. Best performance are up to 750nW emission in a 7 micron square active area at 10 V and 1mA. The device show up to 5 GHz modulation bandwidth. The spectral range is from 450 nm to 850 nm with an emphasized components in the white spectral region. The process is greatly CMOS compatible. The technology is particularly suitable for application in futuristic on- chip micro-photonic systems, lab-on chip systems, silicon- based micro display systems, on chip optical links, and optical inter-connects systems.
Micron dimensioned on-chip optical links of 50 micron length, utilizing 650 – 850 nm propagation wavelength, have been realized in a Si Ge bipolar process. Key design strategies is the utilization of high speed avalanche based Si light emitting devices (Si Av LEds) in combination with silicon nitride based wave guides and high speeds Si Ge based optical detectors. The optical source, waveguide and detector were all integrated on the same chip. TEOS densification strategies and state of the art Si-Ge bipolar technology were further used as key design strategies. Best performances show up to 25 GHz RF carrier modulation and - 40dBm total optical link budget loss with a power consumption of only 0.1 mW per GHz bandwidth. Improvement possibilities still exist. The process used is in regular production. The technology is particularly suitable for application as optical interconnects utilizing low loss, side surface, waveguide to optical fibre coupling.
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