Conducting applications research and exploiting the full potential of integrated optical circuits (IOC) could substantially benefit from the availability of a production method along the lines of microelectronic processes such as CMOS or bipolar. This article describes a process known as Active Silicon integrated Optical Circuits (ASOC^TM) which addresses this issue and which may provide a generic technique to allow researchers to explore more efficiently the full potential of IOCs.

We present design concepts for fast, VLSI-compatible optical modulators based on an SOl materials system. Difficulties associated with corrugated Bragg switches can, in principle, be overcome by the use of optically-resonant periodic electrode (ORPEL) structures. We present a design for a fast, VLSI-compatible optoelectronic switch based on MSM technology.

A theoretical study aimed at the optimum design of thermo- optic guided-wave switches realized in silicon-on-oxide technology is reported. The heat transfer processes occurring inside such devices are analyzed by means of a 3D finite element thermal simulator. In particular, in rib waveguiding structures, we demonstrate that the optimum values of the oxide layer thickness and of the outer rib height are a function of the heating pulse power used to drive the device. The theoretical results carried out are consistent with the experiments reported in literature.

Integrated Optics in SOI need a concept for passive optical couplers, which is highly insensitive to fabrication tolerances. For this purpose directional couplers as well as couplers based on Multi-Mode-Interference (MMI) have been investigated, both theoretically and experimentally. BESOI material with a top layer thickness of 11 micrometers is optimally adjusted to single-mode-fibers resulting in a field mismatch loss of 0.17 dB. For fabrication tolerances of 5%, only MMI- couplers showed an insensitive behavior with additional losses of 0.4 dB and a crosstalk (CRT) of -30 dB at a wavelength of 1.3 micrometers . A particular interest was paid to the additional coupling in the access region of the coupler, since there is no possibility to its compensation in this concept. The potential application of couplers in combination with phase shifters was studied theoretically as well. Realized MMI-based cross-couplers showed additional losses of 1 dB and a CRT of -17 dB, the best values for SOI-based couplers reported so far.

In this paper we propose a novel silicon optical amplitude- phase modulator integrated into a Silicon-On-Insulator waveguide and based on a three terminal electronic structure, which gives rise to definite advantages in comparison with classical p-i-n diode based modulator. The proposed device utilizes the free carrier dispersion effect to produce the desired complex refractive index variations. The MEDICI device simulator has been employed to analyze the electrical operation, with reference to the injected free carriers concentration into the optical channel, its uniformity and the required current density and electrical power. The optical investigation was carried out by means of Finite Difference Method, Effective Index Method and Beam Propagation Method tools, giving rise to a complete evaluation of the properties of our device. We report the results for both the amplitude and phase modulators, paying attention to the static and the dynamic behavior. In particular, a modulation depth of 20%, with an injection power expense of about 126 mW, and a switching time of 5.6 ns is achieved. Furthermore, as a phase modulator, the device exhibits a very high figure of merit, predicting an induced phase shift per volt per millimeter of about 215 degree(s)/V(DOT)mm, for a injection power of about 43 mW, and a switching time shorter than 3.5 ns. The most attractive characteristic of the proposed device is the new bias operation mode which is based on the drift of the plasma injected into the optical channel. Respect to the p-i-n based modulator, based on the free carriers injection and depletion, the switching speed is almost one order of magnitude smaller.

Silicon-germanium (SiGe) alloys are attractive for the monolithic integration of Si photonics with mainstream VLSI technology. The addition of Ge extends the wavelength range of silicon and SiGe/Si multi quantum wells (MQW's) can be epitaxially grown coherent with the Si substrate which allows an additional degree of freedom in bandgap engineering. In this work, SiGe quantum wells were grown at 525 degree(s)C using a commercially available, ultra-high vacuum chemical vapor deposition system. A strong blue shift is observed in photoluminescence on annealing as-grown MQW's. Shifts in photoluminescence line energies, which are directly related to the changes in SiGe QW shape during annealing, are monitored. SiGe MQW's annealed using a two- step process in which strain and Ge peak concentration remain unchanged after the first (low temperature) step, show a much lower rate of interdiffusion during the second step. It is argued that strain and Ge incorporation alone cannot explain the enhanced initial interdiffusion, which is attributed to grown-in, non-equilibrium point defects. Further confirmation was obtained using Si ion implantation, which only increased the interdiffusion during the first seconds of the anneal, without changing the `steady state' diffusion coefficient. This opens up the possibility of point defect mediated local tailoring of the bandgap, since enhanced interdiffusion and its associated blue shift can be laterally controlled by lithographic masking of implanted regions. The intriguing prospect of low loss (band-gap shifted) waveguides coupled with wavelength tunable SiGe/Si MQW photodetectors, fully compatible with CMOS, is discussed.

Silicon-on-insulator offers the chance to produce integrated optical `circuits' with properties which are appropriate even for demanding applications. Developments in SOI waveguide technology have been combined with the well- developed micro-engineering properties of silicon for use in fields such as telecommunication and sensors. An integrated optical transceiver is selected as an example with which to describe the features of the technology. The design will be used to illustrate the benefits brought by the use of SOI waveguide elements. These functional `building blocks' include alignment features, integrated mode-matching waveguide tapers, tap-off couplers and low back-reflection interfaces. Further possible integrated elements are described, including WDMs, as relevant to optical transceiver technology. The economic and technical drivers and difficulties surrounding the convergence of electrical, CMOS-like and optical SOI technologies are also considered. There is a spreading acceptance that low-cost motherboard technology is needed, to realize volume production of optical transceivers. A range of materials solutions have been reported. The relative merits of SOI technology are discussed. Motherboard techniques provide a platform for precise optical alignment between components. The SOI approach can deliver self-aligned waveguide and hybridization features--such as fiber attach or laser diode connections--and includes the ability to adapt to laser diode and optical fiber near-field characteristics.

We present results on epitaxial SiGeC alloy layers grown on Si substrates for optical receiver applications in the 1.3 - 1.55 micrometers wavelength range. The active absorbing layer of the pin photodiodes consist of a strained SiGeC alloy. The SiGeC alloy has a Ge content of 55% and 40%, with a thickness of 80 nm and 200 nm, respectively. TEM images review a high quality epitaxial film with a sharp interface between the SiGeC layer and the Si substrate. The devices show a breakdown voltage of about 6 V. Both surface normal and waveguide structures have been fabricated, and optical response extending to 1.55 micrometers has been demonstrated. At normal incidence the external quantum efficiency of the device with a 55%-Ge content is close to 1% at 1.3 micrometers . For a waveguide structure of 400-micrometers length the external quantum efficiency is 8% at 1.3 micrometers , and limited by the fiber coupling coefficient.

Silicon based photonic circuits are an attractive option for future generations of microprocessors, if standard VLSI electronics can be coupled with on chip optical interconnects and photodetectors for information transfer and clock distribution. A silicon, VLSI compatible, integrated waveguide-photodetector technology for operation at (lambda) equals 1.3 micrometers is presented. Functionality at 1.3 micrometers permits the use of Si/SiO2 waveguides and offers compatibility with short-haul silica fiber optic systems. These waveguides have a large index contrast ((Delta) n equals 2) thus offering superior optical confinement in strip waveguides with dimensions as small as 0.5 micrometers by 0.2 micrometers . The strong confinement and these small dimensions allow high interconnect line densities without cross-talk or RC delay concerns. We measure optical losses in polysilicon waveguides as low as 13 dB/cm at (lambda) equals 1.3 micrometers using an optical cutback technique. A completely relaxed Si_0.5Ge_0.5 buffer with low threading dislocation density (approximately 10⁶ cm^-2) is used as an epitaxial template for a P-I-N photodetector. The relaxed buffer is grown at 815 degree(s)C with ultra high vacuum chemical vapor deposition using a composition graded layer technique with a grading rate of 10% Ge/micrometers . We measure carrier collection efficiencies of 50% and responsivity of 3 mA/W. A beam propagation model is used to determine an effective absorption length less than 2 micrometers in photodetectors butt- coupled to polySi waveguides.

Recently, resonantly enhanced photoresponse in the > 7 micrometers range has been demonstrated for long wavelength SiGe/Si multi-quantum well infrared photodetectors using reflection from a thick buried SiO₂ layer. The SiGe/Si detector structures were grown epitaxially on bond-and-etch- back silicon-on-insulator substrates, with the separation of the reflecting oxide and detector surface determining the wavelength of resonant detection. Difficulties were, however, encountered in producing the desired cavity width. In this paper we show the origin to be a thickness-dependent error in the pyrometer measurement of wafer temperature caused by interference in the cavity of radiation to which the pyrometer is sensitive. Judicious choice of substrate oxide thickness is shown to reduce the effect. In-situ real- time monitoring of epitaxial growth rate and thickness using spectroscopic ellipsometry (SE) is demonstrated to be a more flexible solution. Thickness dependent oscillations in the SE spectra allow accurate position of the MQW and end- pointing of the cavity width to give optimum resonant enhancement effect. Use of surface sensitive regions of the SE spectra also allow monitoring of the repeatability of the individual MQW periods. Detectors grown using SE exhibit superior peak responsivities within 0.1 micrometers of the design wavelength.

Researchers at the Honeywell Technology Center are currently developing a new type of high-performance optical sensor which is a `hybrid' micromachined silicon/fiber optic sensor that utilizes the best attributes of each technology. Fiber optics provides a nearly noise free method to read out the sensor without electrical power required at the measurement point. The micromachined silicon STORM (Sensor Transduction by Optically Resonant Microbeams) devices provide a method to design many different types of sensors such as temperature, pressure, acceleration, or magnetic field, and report the sensor data using FM encoding methods. Testing results have demonstrated relaxed alignment tolerance in packaging these devices with excellent SNR. Networks of 16 or more sensors are currently being developed.

Hughes Electronics has developed a novel surface micromachining process for fabricating extremely low cost integrated tunneling sensors for a wide variety of military and commercial applications. Previously fabricated bulk micromachined tunneling devices have demonstrated the high displacement sensitivity (approximately 4.0 X 10^-5 nm/(root)Hz at 500 Hz) obtainable with tunneling transduction. However, these early devices were fabricated with processes that yielded fairly large multi- wafer sensors which are difficult to integrate with the control electronics and package, thus limiting commercial development. This paper describes our surface micromachining techniques, their compatibility with large volume production, accelerometer device performance, and the system applications for this new technology.

We report the demonstration of planar phased-array wavelength division multiplexers which incorporate two self- imaging multimode interference (MMI) couplers interconnected by an array of monomode waveguides. The MMI couplers operate as power splitters/combiners, and the waveguide array is the dispersive element. The excellent characteristics of MMI couplers offer the possibility of designing small-size devices with low loss and with high uniformity among different channels. Two variations of a 5-channel device with 2-nm channel spacing at 1550 nm have been designed and fabricated in a SiO₂-SiON rib waveguide system. Both simulated and experimental performance of these multiplexers are presented. It is shown that the first device can function as a 5 X 5 wavelength selective interconnecting component. Also, it is demonstrated, through using a 1 X 5 MMI nonuniform power splitter in the second device, that sidelobes in the multiplexer spectral response can be suppressed by appropriately weighting the power samples in the array waveguides.

The silicon/silicon dioxide (Si/SiO₂) materials system provides a high index contrast waveguide platform compatible with existing monolithic microelectronic fabrication processes. The large index difference between the Si and SiO₂ ((Delta) n approximately equals 2.0) allows the miniaturization of waveguide cross-sectional dimensions: single-mode strip waveguides with 0.2 X 0.5 micrometers cross-sections are possible. Additionally, right angle waveguide bends with radii of 2.0 micrometers can be fabricated with insertion loss of less than 1.0 dB. Bend radii of 250 micrometers or more are required to achieve the same performance in less confined waveguide systems such as GaAs/AlGaAs. The high confinement of the Si/SiO₂ system also allows Y-branch power splitters with splitting angles greater than 20 degree(s) to operate with low loss. The combination of small cross- section, small bend radius, and large splitting angle provides a highly compact light guiding technology. Calculations of the loss due to 90 degree(s) bends in these waveguides and preliminary loss measurements for bends from 2.0 to 100.0 micrometers in radius are reported. Y-branch power splitters are analyzed and measurements of branches from 2 degree(s) to 40 degree(s) are presented.

We are developing a completely new technology for the low cost fabrication of integrated optic devices (IODs) in silica glasses. This technology is based on direct photolithographic writing of IODs onto photoactive sol-gel glasses. Waveguide patterns are formed by straightforward photolithographic techniques. Depending on the choice of photosensitive glass, these waveguide patterns can be passive (simply having a higher index of refraction than the surrounding `host' glass) or active (with optical properties that can be influenced by the application of electric or magnetic fields). The simplicity of this process makes it very promising for the fabrication of commercially viable integrated optic devices.

This paper describes the maturation of silicon on insulator technology in integrated optics. Results are presented of optical elements, showing their viability in such fields as telecommunications. Low cost packaging schemes are presented, allowing passive alignment of optical fibers in etched V grooves to the single mode ridge waveguides with an excess loss < 0.5 dB. We then discuss device applications for this technology.

Crosslinked polymers offer the promise of great long term temporal stability and chemical resistance. However, a crosslinkable nonlinear optical (NLO) polymer is more difficult to be processed into high optical quality thin films than other type of NLO polymers such as side-chain, main-chain or guest-host polymers. The crosslinking process imposes more stringent requirements on the solvents. The process of searching for a compatible solvent for a crosslinkable NLO polymer is described. Two detrimental phenomena during the fabrication of electrooptic waveguides from crosslinkable polymers are reported. One is the crystallization of the crosslinker. The other is the formation of wavy surfaces when a commercially available optical adhesive is used to prepare the cladding. By anchoring the small molecules to the long chain NLO polymer and precuring the optical adhesive, good optical quality polymer waveguides are prepared.

The sharp luminescence at a wavelength 1.54 micrometers from the 4 f shell of the Er³⁺ ion in the Si:Er system makes it an excellent candidate for a silicon-based optoelectronic source. To design efficient optoelectronic emitters, it is crucial to understand the excitation and relaxation processes of Si:Er³⁺. Cz silicon substrates were co- implanted with Er and O at Er beam energies of 320 keV, 400 keV and 4.5 MeV. The implanted substrates were subsequently annealed at temperatures between 600 degree(s)C and 1000 degree(s)C for 30 minutes. Spreading resistance profile and capacitance-voltage measurements show that Er and O implantation into Si generates donors. The donor concentration decreases as annealing temperature increases from 800 degree(s)C to 1000 degree(s)C, and the donor profile moves toward the surface due to oxygen outdiffusion. The number of donors shows a linear correspondence with the luminescence intensity. Hall effect and temperature dependent capacitance measurements reveal the presence of energy levels at 40 meV and 160 meV below the conduction band edge. These observations suggest that the donors are likely to be [Si:ErO_x]^0/+ complexes and that they provide the gateway to Si:Er³⁺ excitation. The Si:Er photoluminescence intensity (PL) decreases with increasing temperature in two distinct regimes: the PL intensity is weakly dependent on temperature from 4 K to 100 K and, it shows an activation energy of 160 meV above 100 K. We demonstrate that the first regime is governed by the impurity Auger effect and the second regime is dominated by a phonon-mediated back transfer mechanism through charge transfer at [Si:ErO_x]^0/+ related traps in the silicon bandgap.

Ge_1-yC_y alloys are meta-stable and challenging to produce due to the large disparity between the atomic sizes of Ge and C. However, this same disparity results in an alloy system that potentially spans a wide range of bandgaps, refraction indices, and lattice constants. As such, it has potential as a Si lattice matched material for use in Si based waveguides, detectors, modulators, and other devices. The performance of these devices, however, depends on the refractive indices, which are not well known in these alloys. We present the results of comprehensive measurements of refractive index, energy bandgap, and free carrier absorption versus doping level. In situ B and P doped 550 nm Ge_1-yC_y alloy films were grown on Si (100) substrates by molecular beam epitaxy. The infrared optical transmission spectra were measured at room temperature. The indices of refraction were obtained from the amplitude of the interference fringes at sub-bandgap photon energies. Hall effect measurements were employed to measure the carrier concentrations. The refractive index of our Ge- 1-y)C_y alloys was nominally 4.01, and decreased with increasing B and P concentration.

We report on the fabrication and characterization of a photodiode made from a heterojunction of epitaxial p-type Ge_1-xC_x on an n-type Si substrate. Epitaxial Ge_1-xC_x layers with carbon percentages of 0.2, 0.8, 1.4 and 2% were grown on (100) Si substrates by solid source molecular beam epitaxy. The p-GeC/n-Si junction exhibits diode rectification with low reverse saturation current (2 at -1 volt) and high reverse breakdown voltage in excess of -40 volts. Despite the large number of dislocations and defects at the heterojunction, photoresponsivity was observed from the p- Ge_1-xC_x/n-Si diodes using laser excitation at a wavelength of 1.3 micrometers . External quantum efficiency was measured between 1.2 and 2.3%.

Arrays of 40 - 50 nm quantum dots were fabricated from Si- Si_1-xGe_x single quantum well and superlattice structures grown by molecular beam epitaxy. The dots showing strong luminescence were studied by synchrotron source x-ray diffraction. Some of the dots were coated with SiN_x films containing different build-in stresses. It was found that the luminescence intensity depends strongly on the amount of stress in the SiN_x coating, being strongest when the stress was close to zero. A strain symmetrization process occurred in the bright dots. Although an exact physical origin is not yet available, it is exciting that these quantum dot diodes work at room temperature and that the whole fabrication procedure is compatible with Si- technology.

The design of manufacturable Si/Si_1-xGe_x waveguide WDM components involves several unique materials and fabrication issues which must be confronted and resolved. Accurate data for the refractive indices of the waveguide materials are essential. Furthermore, the waveguide design is tightly constrained by the requirement that Si/Si_1-xGe_x layer thickness is within the pseudomorphic growth limit. By combining refractive indices determined from mode profile measurements of MBE and CVD grown waveguides, and epilayer thickness constraints set by the pseudomorphic growth limits, we have determined a set of design criteria for Si/Si_1-xGe_x waveguides for WDM and optical signal routing applications. Optically smooth and vertical Si/Si_1-xGe_x waveguide facets are critical in permitting highly efficient coupling between the fiber and the Si chip. Since Si has an equal probability of cleaving alone either the <110> or <111> planes, producing such high quality facets consistently is extremely challenging. We have demonstrated that high quality facets can be obtained consistently by cleaving, with and without a dielectric layer on Si substrates.