Optical Code-Division Multiple Access technology enables simultaneous, asynchronous, multi-rate users to transmit and receive information on a single fiber and has the full compatibility with multiple protocol network solutions. In this paper, we review our development of the technology and our effort to replace the bulky optics with Silicon-On- Insulator based photonic devices, such as arrayed waveguide gratings, thermo-optical switches as well as modulators. The future market prospective of the technology will also be discussed.

This paper presents theoretical and experimental results detailing the design and performance of arrayed waveguide grating (AWG) demultiplexers fabricated in silicon-on- insulator (SOI). The SOI waveguide is inherently multimode because of the high refractive index difference between Si and SiO₂, although appropriate tailoring of the rib width to height ratio can be used to make single mode rib waveguides. This single mode condition cannot be met in the input and output combiner sections, which can therefore support many higher order modes. Modeling results demonstrate that coupling from a single mode ridge waveguide to the fundamental slab mode is typically two orders of magnitude larger than the coupling to higher modes. Hence the effect of multimode combiners on performance should be minimal. We also present calculations of bending losses which indicate that with a Si thickness of 1.5 micrometers , single mode rib waveguides can be made with radii of curvature as low as 200 micrometers . Such waveguides can also be made with zero birefringence. AWG devices were fabricated with 8 channels centered around (lambda) equals 1550 nm, and chip sizes less than 5 X 5 mm. The performance of these devices is compared with our modeling results.

Porous silicon (PS) has been known for quite a long time for its photoluminescence and for its usage as a sensing element. However, only in recent years this material has been proposed as a substrate for integrated optoelectronic devices and, despite the low fabrication costs and the possibility to tailor the refractive index varying the material porosity, its usage is still at the very beginning. In this paper we present the fabrication of integrated waveguides in PS and we describe our efforts to reduce the propagation losses. Different fabrication approaches have been studied: the first one uses selective anodization to obtain layers with different porosity and thus different refractive index. Another one exploits the different oxidation grades of the various porous layers to fabricate dense oxidized porous silicon waveguides. A detailed characterization of the obtained waveguides is reported. In particular, propagation losses as low as 7 dB/cm have been obtained in simple non-optimized multimode planar waveguides at the optical communication wavelength of 1.55 micrometers . This encouraging result paves the way to the next realization of porous silicon-based integrated optical devices for communication and sensing purposes. Finally, the results concerning a completely new approach, based on a laser ablation technique, to define the rib structure of porous silicon channel waveguides is presented.

The realization of single-mode rib waveguides in standard epitaxial silicon layer on lightly-doped silicon substrate, using ion-implantation to form the lower cladding, is reported. We exploited a standard microelectronic process step, followed by a calibrated thermal treatment in order to activate and drive-in the implanted impurities, so obtaining a spatially confined lower cladding. The implanted buffer layer enhances the vertical confinement and improves the propagation characteristics. The waveguides were designed with a cross-section comparable in size to the mode-field- diameter of standard single-mode optical fiber, so reducing the fiber-waveguide coupling losses. Propagation losses of about 1.2 dB/cm, for (lambda) equals 1.3 micrometers , in the single mode regime, have been measured. This attenuation is about one order of magnitude lower respect to similar standard all-silicon waveguides. This is the best value of attenuation, to our knowledge, for all-silicon single-mode small-cross-section waveguides reported in literature. A numerical analysis has been performed to evaluate the theoretical attenuation and the transverse optical field profiles, both for (lambda) equals 1.3 micrometers and (lambda) equals 1.55 micrometers . As a result of the presence of the ion implanted buffer layer, a strong reduction of propagation losses and an increase of the fundamental mode confinement have been shown. This results in a great enhancement of the coupling efficiency with standard single-mode optical fibers. Moreover, the proposed technique is low-cost, fully compatible with standard VLSI processes, and allows a great flexibility in the integration of guided-wave devices and electronic circuits. Finally, the very high thermal conductivity characterizing these waveguides makes them attractive host-structures for electrically and thermally- controlled active optical devices.

A series of light emitting devices were designed and realized with a standard 2 micron CMOS technology, 1.2 micron CMOS technology and 0.8 micron Bi-CMOS integrated circuit fabrication technology. The devices operated in the reverse breakdown avalanche mode, at voltage levels of 8 - 20 V and in the current range 80 (mu) A - 10 mA. The devices emit visible light in the 450 - 750 nm wavelength region at intensity levels of up to 1 nWmicrometers ^-2 (10 mW.cm^-2). A series of optimized optical detectors were developed using the same technologies in order to detect lateral and glancing incidence visible and infrared radiation optimally. A series of waveguiding structures of up to 100 micron in length were designed and realized with CMOS technologies by utilizing the field oxide, the inter- metallic oxides and the aluminum metal layers as construction elements. Signal levels ranging from 60 nA to 1 micro-amperes could be detected at the detectors of waveguiding structures of up to 100 micron in length. Finally, a complete optoelectronic integrated circuit was designed and simulated with 0.8 micron Bi-CMOS technology with some of the developed light sources, detectors, waveguiding structures and added driving and amplification circuitry. In particular a very powerful high gain wide- bandwidth MOSFET signal amplifiers was developed that could be successfully integrated in the optoelectronic integrated circuit. The developed technologies show potential for application of optoelectronic circuits in next generation silicon CMOS integrated circuits.

Silicon is the most important semiconductor material for electronics industry. However, its indirect bandgap makes it hardly emit light, so its applications in optoelectronics are limited. Many efforts had been devoted to converting silicon to light-emitting materials, including porous silicon-based devices, nanocrystalline Si, and so on. In this work, we report electroluminescence on silicon with simple metal-oxide-semiconductor (MOS) structure. The thin oxide is grown by well-controlled rapid thermal oxidation. With extremely thin oxide, significant tunneling current flows through the MOS structure as the metal is properly biased. The tunneled electrons could then occupy the upper energy levels more than the thermal-equilibrium situation. Then luminescence occurs when they have radiative transition to lower energy states. For low biased voltages, the emission occurs around 1150 nm, approximately corresponding to the Si bandgap energy. For large applied voltages, the emission shifts to longer wavelengths and becomes voltage- dependent. MOS structures fabricated on both p-type and n- type silicon exhibit electroluminescence. This is significant because the fabrication of those MOS structures is compatible with CMOS electronics. Therefore, the MOS EL devices provide a particular advantage over other types of luminescence on silicon. The details of the electroluminescence and its physical reason are reported and discussed.

Attempts to obtain electroluminescence from silicon-based devices have been largely frustrated by the indirect bandgap of the semiconductor. One approach, described here, is to fabricate a direct bandgap material which is compatible with silicon processing and which can then be excited via standard carrier injection across p-n junctions. We have used ion implantation of iron, typically at an energy of 180 keV and a dose of 1.5 X 10¹⁶ cm^-2, conditions which are easily achievable in modern commercial implanters, to form precipitates of (beta) -iron disilicide, which has a direct bandgap of 0.8 eV. At 80 K and under forward bias conditions, the devices emit light at 1.5 micrometers with an external quantum efficiency of 5 X 10^-3, and emission at room temperature has been observed. The emission lifetime has been placed at shorter than 60 ns, as expected of a direct bandgap material. Results will be presented showing how the electroluminescence properties change with the dose of implanted iron.

In the past ten years, a number of miniature spectrometers covering the visible and near infrared wavelengths out to 2.5 microns wavelength have been developed and are now commercially available. These small but high performance instruments have taken advantage of continuing advances in high sensitivity detectors--both CCD's and diode arrays, improvements in holographic gratings, and the availability of low-loss optical materials both in bulk and fiber form that transmit at these wavelengths and that can readily be formed into monolithic shapes for complex optical structures. More recently, a number of researchers have addressed the more intractable problems of extending these miniaturization innovations to spectrometers capable of operation in the mid-infrared wavelengths from 3 microns to 12 microns and beyond. Key enabling technologies for this effort include the recent development of high D*, uncooled thermopile and micro-bolometer detector arrays, new low- mass, high-efficiency pulsed infrared sources, and the design and fabrication of novel monolithic optical structures and waveguides using high index infrared optical materials. This paper reviews the development of these innovative infrared spectrometers and, in particular, the development of the `wedge' spectrometer by Foster-Miller, Inc. and the MicroSpec^TM, a MEMS-based solid state spectrograph, by Ion Optics, Inc.

We report on a novel solid state wavelength meter in the near infrared. The device is an array of six photodetectors based on polycrystalline germanium film evaporated on a silicon substrate and each element is a wavelength sensitive detector. We describe the design, the fabrication and the characterization of such device and we demonstrate its capability in the measurement of the wavelength of quasi- monochromatic light beams.

We have measured avalanche multiplication and noise in Si p- i-n diodes with avalanche widths, w, of 0.12 micrometers , 0.18 micrometers and 0.32 micrometers , both for pure electron and mixed carrier injection. Multiplication and excess noise measurements were also performed with hole injection on a n⁺-i-p⁺ diode with w equals 0.84 micrometers . Pure electron initiated avalanche noise results were found to be almost indistinguishable in all three layers. The excess noise factor increases dramatically with increasing w when the injection is mixed.

We report on the growth of InAs Quantum dot stacks of various periods on silicon grown by molecular beam epitaxy. Quantum dot layers of InAs, separated by a very thin GaAs spacer layer, are grown directly on hydrogen terminated (100) Si surface. The dependence of dimensional distribution on the growth parameters like temperature and monolayer coverage is studied by atomic force microscopy. The effects of rapid thermal annealing on the stability of stacked structures are investigated by Raman scattering experiments. The morphological changes are characterized in terms of shifts in the longitudinal optic and transverse optic phonon modes of InAs and GaAs forming the structure. Post growth annealing has been found to lead to significant alloying of InAs and GaAs in the successive layers leading to the transformation of 3D quantum dot structure to a 2D In_xGa_1-xAs like compositional alloy layer.

The careful design of active and passive silicon-based optoelectronic devices requires the precise knowledge of the refractive index and its modification with temperature. The thermo-optic coefficient ((partial)n/(partial)T) of silicon has been therefore measured in the temperature range from 30 to 300 degree(s)C at the important wavelength of 1.5 micrometers for fiber-optic communication applications. The adopted technique is very simple and reliable, and is based on the measurement of the modal shift induced by temperature variations in a Fabry-Perot resonant cavity made of the material to be characterized, namely silicon. As an interferometric measure scheme, the technique provides high precision and sensitivity. Various samples of silicon were characterized, differing in doping type, doping level, and crystal plane orientation. The reported data allows to rule out the temperature dependence of the thermo-optic coefficient of silicon on the considered technological parameters.

With the great demand for WDM (Wavelength Division Multiplexer) optical communications, optical switches are expected to become essential components in future networks. A micromachined optical device has been developed for optical communications due to its high reliability, low power, low crosstalk, and low insertion loss. In this paper, we present two types of new lateral actuators for optical switches and tunable filters. The microactuator for an optical switch utilized triple-folded springs with higher compliance for low voltage operation, and electrostatic comb driver for large stroke with low power, respectively. For higher resolution of tunable filter, the microactuator employed a stroke reduction mechanism with meander-type springs. In order to verify the effectiveness of a proposed microactuators, we fabricated the prototypes of polysilicon microactuators for optical switch and tunable filter. The lateral microactuator consists of a polysilicon of 6.5 micrometers thickness as a structural layer and thermal oxide of 2 micrometers thickness as a sacrificial layer. The structures of silicon microactuators are patterned by RIE (Reactive Ion Etching), and finally released by using newly developed HF GPE (Gas- Phase Etching) process with virtually no stiction. We showed the theoretical and experimental driving characteristics of the fabricated microactuators and also discussed the optical properties of a designed optical switch with a focusing mirror.

Silicon oxide films are deposited on silicon wafers by PECVD from SiH₄ and N₂O at different values of the deposition parameters. The refractive index is found to vary with gas flow ratio (R), making these films suitable for a silicon-based integrated optics technology. The films are submitted to annealing processes on inert atmosphere in order to facilitate the impurity effusion, thus reducing the light transmission losses. However, relying upon deposition conditions, this process significantly changes the film stress and often affects the film integrity. In this work a study of the evolution of mechanical stress under different annealing conditions is carried out for silicon oxide PECVD- films. All as-deposited samples exhibit compressive stress. During thermal cycles up to 300 degree(s)C a different behavior of the mechanical stress is obtained depending on the deposition parameters. For R at about 20 a significant stress hysteresis is observed at low deposition temperatures. For R at about 5 no hysteresis is observed at any deposition temperature. After a subsequent RTA a drastic and opposite variation of the stress is observed for the two kind of films and a very stable material from the mechanical point of view, is obtained. An insight into the physical causes of these behaviors is presented.

Photoreactions for polysilane copolymers and terpolymers have been investigated and the resultant photochemical properties evaluated using deep-UV irradiation, differential photocalorimeter (DPC), FTIR and GPC. When exposed in the solid state to UV light, an increase of the molecular weights of the polymers was observed by GPC analysis of the soluble fractions. The reaction is highly exothermic as shown in the DPC spectra and this indicates that bond formation, or photocrosslinking is predominant during UV exposure in air. The photooxidation of the hydropolysilane thin films at high fluences of UV irradiation is limited by oxygen diffusion into the film. The UV photosensitivity of the polymers was evaluated by UV/VIS spectroscopy, on films that were exposed to varying amounts of UV energy. The examination of the photobleaching effect as a function of exposure energy reveals that the photobleaching rate increases with increasing Si-H content in the linear copolymers, and that the photobleaching rate also increases with increasing branch site content in the branched copolymers at a constant amount of Si-H component. Also, the alkyl-substituted polymer derivatives possess a higher photobleaching rate than the aryl-substituted polymers. The thermal sensitivity and degradation of a number of hydropolysilanes have been investigated by thermogravimetric analysis and FTIR spectroscopy in both air and oxygen free inert gas, such as helium or purified nitrogen, at temperatures up to 900 degree(s)C. The pyrolysis of hydropolysilanes in air is mainly attributed to a thermooxidative reaction and the products are predominantly SiOx, especially at higher temperatures. The polymers pyrolyzed under nitrogen yielded SiC-like products. The amount of SiC in the final products increases with increasing pyrolytic temperature as well as heating rate.

We report on a new type of a high quantum efficiency diode photodetector based on ultra-thin low temperature-grown InGaAs-on-Si heteroepitaxial layer. The device was characterized optically and electrically using low- temperature photoluminescence, capacitance-voltage, current- voltage, DLTS and spectral responsivity measurements. We study characteristics of the photodetectors with various In contents in InGaAs film. Photosensitivity spectral characteristics shift to the longer wavelengths with In/Ga flux ratio increase during the ultra-thing film growth. This dependence proves that photosensitivity of the InGaAs/Si heterostructure is a result of photons absorption and carrier separation in thin epitaxial layer of the polar semiconductor (2 - 20 nm). DLTS measurements underscore the role of a trap with activation energy of 0.59 eV in the photodetector properties. Using capacity-voltage measurements we demonstrate that the trap has delta-like spatial distribution and is localized at InGaAs/Si interface. According to our estimations the photosensitivity of the photodetectors exceeds 1.5 A/W at wavelength of 632.8 nm. We also reveal that our InGaAs-on-Si photodetectors have high external quantum efficiency in ultraviolet region, which exceeds one in the visible region. The high responsivity of the photodetector is explained by effective carriers separation in InGaAs layer and the carrier multiplication effect at InGaAs/Si interface.

We report the fabrication of fast heterojunction Ge/Si photodetectors which, to the best of our knowledge, exhibit the highest near infrared responsivity at normal incidence reported to date. Such performances are related to the quality of the epitaxial Ge film grown by a two-step UHV-CVD process followed by cyclic thermal annealing. We have measured a fast (FWHM equals 850 ps at 1.3 micrometers ) and efficient (R equals 0.55 A/W at 1.3 micrometers and 0.25 A/W at 1.55 micrometers ) photoresponse. Our technology makes these devices suitable for integration with other electronic and optoelectronic components on Si chips. In the paper we discuss processing technology, material quality, device fabrication and performance measurements.