Optical circuits based on low-loss glass waveguide on silicon are a practical and promising approach to integrate different functional components. Fiber attachment to planar waveguide provides a practical application for optical communications. Microwave Plasma Assisted Chemical Vapor Deposition (MPACVD) produces superior quality, low birefringence, low-loss, planar waveguides for integrated optical devices. Microwave plasma initiates the chemical vapor of SiCl4, GeCl4 and oxygen. A Ge-doped silica layer is thus deposited with a compatible high growth rate (i.e. 0.4 - 0.5 micrometer/min). Film properties are based on various parameters, such as chemical flow rates, chamber pressure and temperature, power level and injector design. The resultant refractive index can be varied between 1.46 (i.e. pure silica) and 1.60 (i.e. pure germania). Waveguides can be fabricated with any desired refractive index profile. Standard photolithography defines the waveguide pattern on a mask layer. The core layer is removed by plasma dry etch which has been investigated by both reactive ion etch (RIE) and inductively coupled plasma (ICP) etch. Etch rates of 3000 - 4000 angstrom/min have been achieved using ICP compared to typical etch rates of 200 - 300 angstrom/min using conventional RIE. Planar waveguides offer good mode matching to optical fiber. A polished fiber end can be glued to the end facet of waveguide with a very low optical coupling loss. In addition, anisotropic etching of silicon V- grooves provides a passive alignment capability. Epoxy and solder were used to fix the fiber within the guiding groove. Several designs of waveguide-fiber attachment will be discussed.
Microsystems are presented in which a SiON based optical waveguiding system is monolithically integrated with photodiodes, which are implemented in the Si substrate. Coupling structures of various type enable to transfer whether (part of) the power of one selected mode or the power of all modi propagating through the waveguide, to the photodiode. Here we focus on coupling structures for use in integrated optical absorption sensor systems, where information can be obtained from both the TE0 and TM0 mode, propagating simultaneously through the waveguide system. The coupling into the photodiodes is achieved by thinning down the thickness of the core layer in the region above the photodiode, which results in a mode specific modewidth expansion of the propagating modi. It will be shown that in asymmetrical layer systems, within a certain interaction length all TM0 power can be absorbed by the Si detector, while the TE0 mode shows only a negligible attenuation. The selectivity of the coupling can be strongly enhanced by implementing an additional substrate layer, having a refractive index in between that of the TE0 and TM0 mode. Both theoretical and experimental results will be presented.
The realization of monolithic optical interconnects by integration of III-V materials with conventional Si circuitry has long been hindered by materials incompatibilities (i.e. lattice mismatch and heterovalent interface) and practical processing constraints. We have demonstrated successful integration of hetero-epitaxially grown InGaAs/Si diodes with an n-well CMOS process on (001) Si offcut 6 degrees towards . The In0.15Ga0.85As/InxGa1- xAs/GaAs/Si diodes were grown by atmospheric pressure organo-metallic chemical vapor deposition (OMCVD) and features a room temperature R0A product of 20,000 ohm-cm2. No degradation of PMOS or NMOS transistor characteristics was detected upon integration of the III-V devices. Further improvement of III-V/Si device characteristics are anticipated in future efforts by incorporating relaxed, compositionally- graded Ge/GexSi1-x/Si with low threading dislocation densities (approximately 2 X 106/cm2) to bridge the gap in lattice constants between Si and GaAs. Recent progress towards this end includes the suppression of antiphase disorder during GaAs growth on Ge/GexSi1-x/Si by OMCVD and strong room temperature photoluminescence from In0.20Ga0.80As QW test structures on GaAs/GexSi1- x/Si at 920 nm.
The simulation and performance of dual-wavelength demultiplexers fabricated in SiGe are presented. The device design is a symmetric directional coupler optimized for 1.3/1.55 micrometer demultiplexing. Modeling results using the Beam Propagation Method are presented as a means of examining the fabrication tolerances and design considerations of the devices. Initially the duplexers were fabricated with a Si.97Ge.03 core using chemical etching, and displayed crosstalk of -19 dB and -15 dB in the 1.3 and 1.55 micrometer wavelength channels, respectively. This performance was enhanced by thermal tuning, resulting in isolation of -21 dB and -18 dB. The high degree of strain in pseudomorphic SiGe layers results in highly birefringent waveguides. This characteristic restricts effective duplexer operation to a single polarization, but suggests that the same devices can be modified to act as polarization splitters for a chosen wavelength. Modeling of the devices in this configuration is also presented. The simple device design used for demultiplexers and splitters was chosen to evaluate the potential for fabrication of SiGe-based optoelectronic devices using standard VLSI processing relying on the local oxidation of silicon (LOCOS). A second set of devices was successfully fabricated using LOCOS and effectively separated 1.15 and 1.3 micrometer wavelength signals. This performance is compared to similar devices that were wet-etched.
Operation and management of Wavelength Division Multiplexing (WDM) systems require the monitoring of optical channel frequency and power. In this paper we propose a simple and low-cost solution for tracking the frequency of WDM channels, based on the thermo-optic tuning of single or coupled-cavities Fabry-Perot silicon optical filters. The fabricated structures are single-cavity filters exhibiting, thanks to a suitable coating stack on both cavity sides, high finesse and narrow bandwidth. Moreover, the free spectral range is large compared to the channel spacing, allowing the monitoring of one carrier frequency at a time. By means of a heater we change the cavity refractive index and move the transmission peaks, thus scanning the WDM frequency set. In particular, we demonstrate the possibility to resolve up to seven 50-GHz-spaced channels, with a crosstalk of -10 dB, at wavelengths around 1550 nm. Better performances, in terms of resolvable channels and cross-talk can be obtained by using two coupled-cavities, having a common resonance peak and different free spectral ranges. The global optical transfer function of such a cascade shows only one transmission peak in a frequency range of about 30 nm, and can be thermally tuned in this range.
For Si-based photonic integrated circuits (PICs), photodiodes with good responsivity at 1.3 micrometer and 1.55 micrometer wavelengths made of Si-based materials are highly desirable. Previously, work has been reported using epitaxial SiGe planar multiple quantum wells (MQWs) on Si substrates. Since the high lattice mismatch limits the maximum Ge concentration and SiGe layer thickness, responsivity at 1.55 micrometer was limited. Under appropriate growth conditions, strained SiGe QW's grow with periodic thickness variations along the surface plane. Ge tends to migrate towards the thickness maxima. This increase in local Ge concentration and the reduced quantum confinement at the coherent wave crest produces strained QW's with significantly lower band-gaps compared to planar QW's with the same nominal composition. In this paper, we report the first MSM SiGe waveguide photodetectors fabricated using coherent wave growth mode with a band gap below 800 meV. The heterostructures were grown on a SOI substrate by an ultra- high vacuum chemical vapor deposition (UHVCVD) system. The 2 micrometer thick Si/SiGe/Si on oxide structure provides waveguiding for the detector structures and permits effective fiber coupling. Preliminary measurements have demonstrated internal responsivities of approximately 1 A/W at 1.3 micrometer wavelength and 0.1 A/W at 1.55 micrometer wavelength for a 240 micrometer long device.
We report on a novel silicon-based resonant cavity photodiode with a buried silicon dioxide layer as the bottom reflector. The buried oxide is created by using a separation by implantation of oxygen technique. The device shows large Fabry-Perot oscillations. Resonant peaks and anti-resonant troughs are observed as a function of the wavelength, with a peak responsivity of about 50 mA/W at 650 nm and 709 nm. The leakage current density is 85 pA/mm2 at -5 V, and the average zero-bias capacitance is 12 pF/mm2. We also demonstrate that the buried oxide prevents carriers generated deep within the substrate from reaching the top contacts, thus removing any slow carrier diffusion tail from the impulse response.
We report on the fabrication of a detector array for the near infrared on silicon substrate. Thermally evaporated polycrystalline germanium is used as the active layer in the device which consists of 16 pixel with dot-pitch of about 100 micron; the single pixel has a metal-semiconductor-metal structure. We demonstrate a responsivity of 16 mA/W at 1.3 micron and extending down to 1.55 micron. At the same wavelength an operation speed in the nanoseconds range is demonstrated. The overall fabrication process, including substrate cleaning and preparation, requires temperatures lower than 350 degrees Celsius being fully compatible with silicon electronics.
Highly p-doped silicon/silicon-germanium (Si/SiGe) quantum well (QW) structures have been grown by molecular beam epitaxy (MBE) on <100> Si substrates for mid-infrared (3(mu) - 5(mu) ; 8(mu) - 12(mu) ) detection. These detectors operate at 77 K and are based on hetero-internal photoemission (HIP) of holes from a highly p-doped SiGe quantum well into a undoped silicon layer. They are grown on weakly-doped (50(Omega) cm), double-sided polished Si substrate on which the highly p-doped Si1-xGex wells (p++ approximately 5 1020cm-3) are deposited. The Ge content, doping level and the well width determine the operating wavelength. The samples have been characterized by secondary ion mass spectroscopy (SIMS), X-ray diffraction, and absorption measurements. Single mesa detectors as well as large area focal plane arrays (FPA) with 256 X 256 pixels have been fabricated from these samples using standard Si integrated processing techniques. Dark- and photocurrents have been measured at 77 K and up to 225 K. Broad photoresponse curves with peak external quantum efficiencies up to 0.5% have been measured at 77 K and 4$u. Detectivities in excess of 2 1010 cm(root)Hz/W have been obtained. We demonstrate that by varying the well design of the SiGe HIP detectors by means of Ge-content, doping and Ge gradients in double and stacked multi-wells the photoresponse peak and the shape of the spectrum can be tuned over a wide wavelength range. The fabricated 256 X 256 Si/SiGe FPA array has a pixel size of 21 X 21(mu) 2 and a pitch of 24(mu) . The mesa detectors diameters range from 1 mm down to 125(mu) . The epitaxial versatility and the compatibility of the Si/SiGe array fabrication compared to the existing silicide fabrication process demonstrates the advantages of the SiGe system in comparison over commercially used silicide detectors.
Long wavelength Si0.8Ge0.2/Si quantum well infrared photodetectors (QWIPs) grown by low pressure CVD have been fabricated both as discrete devices and integrated onto a CMOS readout circuit to produce a monolithic Si-based sensor circuit for detection of thermal radiation. The peak photoresponse of the detectors near 8 micrometer is dominated by transitions to unbound final states associated with the spin-orbit split-off valence band. These optical transitions are allowed by symmetry reduction in the quantum wells, which is also evident in the electrical properties. The electrical noise is nearly ideal for temperatures up to 70 K, with no excess low frequency flicker noise. The capture probability for photoexcited holes into the quantum wells is approximately 0.55 at low temperature. The external 500 K black body responsivities for both the discrete and the monolithically integrated QWIPs are approximately 1.8 mA W-1 at 1 V bias, corresponding to a single pass of the radiation through the detectors. There is no degradation of either the CMOS transistors or the QWIPS caused by the integration process to create the monolithic sensor circuit.
Mid-infrared absorption in boron-doped and modulation boron- doped self-assembled Ge quantum dots grown on (001) oriented Si substrates is reported for the first time in this paper. The structures, which were grown by a solid source molecular beam epitaxy system, are composed of 20 or 30 periods of Ge dot layers and Si spacer films. The structural properties of the multilayers and some uncapped Ge dots on sample surface were tested by cross-sectional transmission electron and atomic force microscopes, respectively. Through use of Fourier transform infrared and Raman spectrometers, infrared absorption signals peaking in the mid-infrared range were observed. The absorption is strongly polarized along the growth axis of the samples. Experimental and theoretical analysis suggests that the mid-infrared response be attributed to intraband transitions in the valence band of the Ge quantum dots. This study demonstrates the application potential of these kinds of Ge/Si quantum dot multilayer structures for developing mid-infrared photodetectors.
Silicon has been proven to be a viable material for passive and active optoelectronic applications in the infrared region ((lambda) greater than 1.2 micrometer) of the electromagnetic spectrum. To date, light has been guided, modulated and switched in silicon. In this paper, novel silicon modulators incorporated into a rib waveguide will be discussed. The modulators are based upon transverse p-i-n structures, utilizing the plasma dispersion effect to produce the desired refractive index change in an optical rib waveguide. Although the devices measure several microns in cross sectional dimensions, they support a single optical mode, thereby simplifying fabrication and allowing efficient coupling to other single mode devices such as optical fibers. The modulators have been modeled extensively using the SILVACO semiconductor device simulator. SILVACO has been employed to optimize the overlap between the injected free carriers and the propagating optical mode. Both the dc and switching characteristics of the modulators have been evaluated. The device performance is encouraging. One of the devices studied requires a driving current of 2.8 mA to achieve a (pi) radian phase shift, corresponding to a current density of 112 A/cm2. Additionally the 10 - 90% rise and fall times are 29 ns and 4 ns respectively. We believe this to be the lowest predicted driving current or current density for modulators in silicon-on-insulator (SOI), although clearly experimental confirmation is required. Potential applications for silicon based optoelectronic devices include optical transmitters, optical receivers and optical sensor devices.
We have designed waveguide modulators using (beta) -SiC-on- insulator waveguides and the Pockels effect. A 2D semiconductor device simulator was used to determine the electric field configuration in a double-Schottky diode structure. This allowed us to evaluate the local modulation of the refractive index as a function of applied external bias and to determine the effective index modulation of the guided mode. The optical simulations were performed using the Spectral Index and the Effective Index methods. Different 2D geometries are analyzed and the material parameters needed for fabricating such a device are determined. Application to Mach- Zehnder intensity modulators is described. Such devices have potential for high-speed Si-based photonic devices compatible with silicon technology.
We have considered effects of spatial confinement of acoustic phonons on silicon thermal conductivity and thermal management of ultra-thin silicon-on-insulator (SOI) structures. It has been shown that modification of the phonon modes in thin silicon layers (10 nm - 100 nm) sandwiched between two layers of silicon dioxide leads to a significant increase of the phonon relaxation rates and corresponding drop of lateral lattice thermal conductivity. The latter may bring about additional degradation in the electrostatic discharge (ESD) failure voltage for ultra-thin SOI devices. Obtained results help to realize the importance of proper thermal management of ultra-thin SOI based devices. Our theoretical and numerical results are consistent with recent experimental measurements of lateral thermal conductivity.
Porous silicon (PSi) multilayer structures are used in Si- based optoelectronic devices which exhibit interesting optical and electrical properties. Integration of these structure can be achieved either passively or actively. Passive elements include high reflectivity mirrors and optical filters with a maximum reflectivity peak approximately 100%. The benefit of adding a multilayer mirror below a luminescent PSi film is to reduce the amount of light absorbed by the silicon substrate and increase the light output. Placing two multilayer mirrors in between a highly luminescent PSi film creates an active microcavity resonator structure in which a significant photoluminescence and electroluminescence line narrowing (FWHM less than or equal to 20 nm) is observed. A detailed study on the passive and active roles of PSi multilayer structures is presented in a device configuration.
It was shown that Ge+-implanted SiO2 thermally grown on crystalline Si is one of the most promising materials for Si-based light emission. This material exhibits strong violet photo-(PL) and electroluminescence (EL) at room temperature, which is well visible by the naked eye at ambient light. In detail, the PL spectra of Ge-rich layers peaking around 400 nm reach a maximum after annealing at 500 degrees Celsius to 800 degrees Celsius for Ge concentrations in the range of 0.3 to 2 at%. Based on PL excitation (PLE) spectra we tentatively interpret the violet PL as due to the neutral oxygen vacancy typical for Si-rich SiO2 and similar Ge- related defects in Ge+-implanted silicon dioxide. EL and electrical measurements were carried out at MOS capacitors using a transparent ITO and a thick Al front contact, respectively. The I-V-dependence exhibiting the typical behavior of Fowler-Nordheim tunneling shows an increase of the breakdown voltage and the tunneling current for Ge-rich oxide in comparison to the unimplanted material. The EL spectrum of the Ge-implanted oxide correlates very well with the PL one. The EL intensity shows a linear dependence on the injected current over three orders of magnitude. EL efficiencies up to 5 X 10-4 for Ge+-implanted silicon dioxide were found.
The electroluminescence of PIN diodes with either strained SiGe/Si or Ge islands in the i-region have been investigated. For diodes with strained Si0.80Ge0.20 simulations have shown that the emission at 300 K can be improved if the thickness of the SiGe layer is increased. To overcome the thickness limitation due to the plastic relaxation selective epitaxy was used to deposit strained layers in small holes on patterned wafers. Indeed, for samples four times thicker than the critical thickness emission was observed to persist up to 300 K in contrast to thinner SiGe layers. Light emitting diodes with SiGe islands were shown previously to emit more light at low injection currents than diodes with strained SiGe layers due to the localization of carriers in the islands, while at higher injection levels the emission efficiency was comparable. Here it will be shown that the emission efficiency of diodes with islands could be increased, however, it is still lower than from diodes with thick SiGe layers.
Molecular beam epitaxy (MBE) has been used to deposit quantum structures in the material system Si-Ge-C in order to evaluate the possibilities for Si based opto-electronics. In particular the growth of Si/SiGeC quantum wells, the growth of quantum structures on pre-patterned Si substrate and the self- organized growth of Ge and C-induced Ge dots have been investigated. Studying the photoluminescence (PL) response of strained SiGeC quantum wells of various compositions and well widths by MBE the band discontinuities for compressively strained and lattice matched SiGeC/Si heterostructures have been determined. The data indicate a type I bandalignment in Si/SiGeC quantum well structures. By modifying the morphology, the chemistry or the strain of Si surfaces the formation of Ge quantum dots can be triggered. The growth of strained SiGe alloys on a small mesa leads to plastic relaxation of the strained film. The degree of relaxation depends on the thickness, the size, and the crystallographic orientation of the mesa. Phonon resolved PL spectra were obtained from the type II transition between the strained Si and the relaxed SiGe grown on small mesa structures. In addition, the self organized growth of Ge dots on bare and on C covered Si (100) surfaces has been studied. The deposition of 2 - 4 monolayers of Ge on these surfaces leads to the formation of small, irregularly shaped islands without facets. Intense photoluminescence is observed from samples containing multiple C-induced Ge island layers.
In this paper we propose an optoelectronic Y-switch based on a three terminal active device and on the mode-mixing principle. It consists of a 1400 micron long asymmetrical rib waveguide designed to sustain only the fundamental and the first propagating mode; along this waveguide the difference in effective indices between these modes cumulates to 2(pi) and the light emerges on the same side where it has been injected. In the ON state, the presence of an electron-hole plasma injected and spatially controlled with a Bipolar Mode Field Effect Transistor device (BMFET) forces the fundamental mode and the first propagating mode to be shifted by an additional -(pi) thus allowing the light to emerge from the other side of the waveguide. Numerical simulation of losses, cross-talk and switching speed are presented.
An accelerometer based on optical intensity modulation has been designed and fabricated using thick-polysilicon (12 micrometer) surface-micromachining technology. A layer of polysilicon is surface micromachined to form a grating connected to springs, which are anchored on the silicon substrate. A layer of metal is evaporated on the top of the entire device making the suspended grating opaque as well as the regions on the substrate that are exposed by the grating. This leaves the areas directly beneath the grating shadowed from incident light. In the unperturbed state, the light that shines on the device is completely reflected by the metal on top of the entire device. When the grating moves under the influence of acceleration, the areas which are not covered by the metal are then exposed and the amount of light passing through the substrate increases. The optical intensity variation translates to the acceleration experienced by the grating proof mass. Twelve micrometer thick polysilicon surface micromachining was developed to improve the performance of the device. The film's mechanical qualities, internal stain, stress gradient and surface roughness have been characterized. This optical accelerometer has a sensitivity of 70 milli-g with 0.5-micron movement.
This paper addresses the topic of the simulation analysis of arrayed waveguide grating demultiplexers. These are very important devices used for WDM communications and, being quite a complex structure, they need accurate and efficient simulation tools to evaluate their transmission spectral characteristics. The objective of this paper is to present a new formulation that has the advantage of allowing a faster yet rigorous analysis. Comparisons between the new simulation technique and other currently used are presented and discussed.
This paper reports a low cost method of converting SiO2 or silica based glass into crystalline silicon by simple thermal annealing of Ti/Al coated SiO2 or silica glass slides. The studies were performed using Raman scattering and x-ray diffraction. The results suggest that it is possible to extract reasonably good quality crystalline silicon from SiO2 based substrates. This technique may have a wide range of applications in Si-based optoelectronics and other industry.
The gate length dependencies of the optical response characteristics in the optically controlled MOSFET have measured. This device was the integrated structure of absorption region and MOSFET region by using direct wafer bonding technique. By reducing the gate length of MOSFET region, the transconductance of FET channel was increased, and we obtained high current modulation and responsivity by irradiation of 1.5 micrometer wavelength light.
In this report, we show that a thin interfacial silicon dioxide layer placed between the Schottky metal and the silicon substrate reduces the leakage current of a metal- semiconductor-metal photodetector. We find the optimal interfacial oxide layer to be about 3 or 4 nm thick and is made by dry furnace oxidation of the silicon substrate at 800 degrees Celsius, taken from a 0.18 micrometer CMOS process normally used to make the gate oxide. As compared to a metal- semiconductor-metal detector without this oxide layer, we measure a factor 5 reduction in leakage current density to 18 (mu) A/cm2 at 5 V, a weaker increase in dark current with bias, and a factor 3 improvement in photoresponsivity to 0.35 A/W at 635 nm wavelength. Additionally, we compare Schottky barrier height, effective Richardson constant, and capacitance measurements between photodetectors made with and without this interfacial oxide.
We have developed a new MBE growth technique by using low- temperature-Si (LT-Si) or LT-GeSi buffer layers. Even if the Ge fraction up to 90%, the total thickness of fully relaxed GexSi1-x buffers can be reduced to 1.7 micrometer with dislocation density lower than 5 X 106 cm-2. The roughness is no more than 6 nm. According to the analysis of X-ray diffraction, the crystal quality of the top layer is very good, and the strain relaxation is quite inhomogeneous from the beginning of relaxation. By using high resolution cross section TEM, we observed that stacking faults are induced and form the misfit dislocations in the interface of GeSi/LT-Si. We propose that the formation of the stacking faults is due to the aggregation of the large amount of the vacant defects in the LT-Si layer.
In this work planar planar (beta) -SiC-on-insulator waveguides were investigated. The waveguides were fabricated by two different methods. In the first a technological process similar to that of SIMOX was used, and therefore a buried SiO2 layer was formed by high energy ion implantation of oxygen in SiC/Si wafers. For the second type of waveguides we used heteroepitaxy of SiC on SOI(SIMOX). The losses have been measured at 0.633, 1.3 and 1.55 micrometer in both TE and TM polarization. A detailed analysis of the different loss mechanisms is presented. These types of waveguides have potential for high-speed silicon-based photonic devices compatible with silicon technology.
We report on the development of building blocks of Si/Si1- xGex/Si planar waveguide WDM circuits -- an optical splitter/combiner and the first demonstration of an Si/Si1- xGex/Si arrayed waveguide grating (AWG) demultiplexer. The prototype AWG is a four channel device with a 400 GHz ((Delta) (lambda) equals 2.3 nm) channel spacing, and operates in the 1300 nm wavelength range. Strain and refractive index in the Si1-xGex epilayers play a critical role in device performance. The AWG waveguide heterostructure has total Si1-xGex layer thickness well beyond the measured critical thickness for lattice relaxation, but is stabilized against dislocation formation by the insertion of Si capping layers during growth. Single mode curved ridge waveguides formed from this material show no obvious bend losses for radii of curvature as small as 4 mm.