The potential of silicon-based designs for various optoelectronic functions is discussed. Sibased light detectors are the most advanced, especially in the form of metal-Si-metal (MSM) photodetectors. They use Si band to band absorption for visible light and Schottky-barrier emission for the infrared (IR). The different light sources show rapid progress, but still face challenges to reach a quantum efficiency of 10−2. In addition to the intrinsic silicon based designs, some new Si breadboard concepts are shown: especially for waveguides, modulators and all-optical amplifiers, it may be advantageous to add entirely different materials (polymers, glasses, BaTiO3 or Al2O3) onto the Si wafer.

Impressive advances have been made over the last few years in teaching silicon how to emit light. Recently, light-emitting devices made of porous silicon and other forms of nanoscale silicon have been demonstrated with specifications that start to make them attractive for commercial applications. This paper reviews the state-of-the-art in the materials science and device properties of nanoscale silicon-based LEDs, including their integration with microelectronic circuits.

This paper reviews various integrated optics techniques and devices in the silicon material system. Different approaches for waveguides, passive and active components in silicon and the performance of such devices will be discussed.

The path for silicon materials development has been charted. By the year 2010 we will have fabricated integrated circuit chips containing 109 transistors with 40 Å thick gate oxides and 1000 Å minimum feature sizes running at 4 GHz clock speeds. It is conceivable that incremental advances on the current chip architecture will satisfy the required materials and process improvements. The interconnection problem is the only challenge without a proposed solution. The signal propagation delay between devices is now longer than the individual device gate delay. The resistance and capacitance associated with fine line Al interconnects limit speed and increase power consumption and crosstalk. High power line drivers are limited by the reliability constraint of electromigration. There is no current paradigm for 4 GHz, electronic clock distribution. Optical interconnection can remove the electronic transmission bandwidth limit. The main challenge is development of a silicon-compatible, microphotonic technology.

Rare earth doping has provided a means of sharp-line electroluminescence from silicon at λ = 1.54 μm. Silicon's high index of refraction and low absorption in the near infrared yield an ideal optical waveguide. As with microelectronics, the silicon / silicon-dioxide materials system allows high levels of integration and functionality. The applications of silicon materials to light emission (Si:Er), optical waveguides (Si/SiO2), photonic switching (Si/SiO2) and photon detection (SiGe) are reviewed. These developments are discussed in the context of systems applications to communications and computation.

Active Silicon integrated Optical Circuits (ASOC™) is a technology based on single-mode rib waveguides formed on silicon-on-insulator that is being used to manufacture commercial integrated optics components. Silicon waveguides have excellent properties for many applications in the 1.3 and 1.55 micron telecommunications bands including relatively low loss. An important aspect of ASOC™ technology is the development of a set of waveguide-based elements that can be assembled into practical integrated optics devices. The fundamental waveguide elements include bends, couplers and fiber-waveguide interfaces, and additional elements include doped structures and waveguide gratings. Discrete lasers and photodetectors are also incorporated into ASOC™ technology to form hybrid devices. The technology is being used to manufacture devices for applications in telecomunications and optical sensing, the first major product being a two-wavelength single-fiber bi-directional optical transceiver.

The roadmap for silicon device technology has been drawn, extending to the year 2010, and featuring a CMOS transistor with a gate length of 0.07 μm [1]. Beyond this point, silicon heterojunctions could provide a means to further device scaling. Silicon heterojunctions could also bring new devices to the silicon substrate including light emitters and detectors, and resonant tunneling diodes (RTDs). Today SiGe/Si and SiGeC/Si heterojunctions are receiving the greatest attention, but heterojunctions now being developed to realize silicon RTDs are increasing the heterojunction options for silicon-based quantum-well and optical devices. Here we outline the fundamental device requirements for silicon optical and tunneling devices and describe progress on silicon heterojunction development towards demonstration of silicon-based RTDs. Materials now under study include, ZnS, crystalline oxides and nitrides; new processes could provide methods for forming crystalline materials over amorphous barriers.

We report here efforts to advance 850 nm VCSELs designed specifically for integration to Si- CMOS. Our VCSEL design is based on an oxidized current aperture and oxidized top mirror with the intent that the device operate as a bottom emitter after integration. Our design also employs intercavity contacts and the first non-alloyed buried contact have been realized. The devices produce resonantly enhanced light emission with record output of 70 μW/st.rad.

Photonic band gap (PBG) structures are multidimensional lattices, formed by a strong modulation of dielectric constant which prevent the propagation of photons over a forbidden energy range1. Since the first demonstration of a PBG at microwave frequencies2-7, it has proved technologically difficult to create a PBG in the optical region of the spectrum8-11 since the band gap centre wavelength is of the order of twice the lattice pitch. Recently, we have succeeded in fabricating novel waveguide designs which incorporate a TE polarised visible photonic band gap12,13 By reducing the band gap to visible wavelengths, we greatly simplify the experimental investigation of the behaviour of these complicated structures.

In this paper, we present the fabrication and initial optical testing of devices with band gaps in the visible wavelength range 545nm-632.8nm. These devices demonstrate unusual optical effects dependent upon the number of lattice periods such as: broadband wavelength filtering, wavelength dependent beam splitting, and broadband polarisation selective routing. In contrast to the well known active applications of PBGs, we suggest a host of new passive applications for PBGs in optical computing and wavelength division multiplexed (WDM) communications.

Photoelastic waveguides in bulk Si and SiGe/Si heterostructures have been modelled and characterised. The calculated transverse strain profiles of photoelastic waveguide structures in SiGe/Si heterostructures and bulk silicon are in good agreement with those obtained by microRaman experiments. The waveguide characteristics are also found to be in good agreement with those obtained from the strain modelling and demonstrate that low loss waveguides can be fabricated using these structures

The index of refraction of pseudomorphic layers grown on Si has been determined at wavelengths λ=1330 nm and λ= 1550 nm, for Ge concentrations between x=0.01 and x=0.1. The refractive index values were obtained from mode profile measurements on a series of Si1−xGex waveguides. The index of refraction is significantly larger for light polarized parallel to the growth direction than for light polarized in the plane of the epilayer. This birefringence is consistent with the anisotropic index change predicted using photoelelastic theory, given the biaxial strain present in the pseudomorphic Si1−xGex layers.

To verify the availability of a porous silicon (PS) technology for applications to photonic devices, fundamental chracteristics of PS waveguides are investigated. An edge-emitting device with an optical waveguide is fabricated by current modulation technique during anodization. The net internal loss for prepared samples is measured by varying the length of excitation beam. The experimental results show that the internal loss increases with decreasing the operation wavelength, possibly due to self-absorption and Rayleigh scattering. Besides the step-index PS waveguides, buried-type waveguides can be fabricated based on a doping modulation technique. A theoretical analysis about a bending loss is also carried out. The results of experimental and theoretical analyses suggest that a curveture radius capable of guiding lightwave can be significantly minimized, owing to a difinitely different refractive indedices between the core and cladding region.

This paper reports about the fabrication and experimental test of an interferometric light intensity modulator integrated in a low loss (0.7 dB/cm), amorphous silicon based waveguide. It measures approximately 1 mm in length, while its cross section is 30-μm-wide and 3-μm-high. The device, which exploits the strong thermo-optic effect in thin film a-Si for its operation, is designed for application at the infrared wavelengths of 1.3 and 1.55 μm. The measured maximum operating on-off switching frequency of the device is 600 kHz. The very simple fabrication technology involves maximum process temperatures of 230 °C, and is therefore compatible with the standard microelectronic technology. This offers a new opportunity for the integration of optical and electronic functions on the same substrate.

Reflection spectroscopy was applied to evaluate the optical homogeneity and the refractive index of thin porous silicon (PS) layer. Variation in PS layer optical thickness was evaluated by measuring and mapping the reflectance over the surface area. For the circular anodization area of 17 mm diameter, the measured variation was less than 5 % on an area of 11 mm diameter. The anodization electrode position was found to have little influence on the homogeneity. A method to derive refractive index from a reflection spectrum is studied. The wavelength dependence of PS index measured by the method showed fair coincidence with the calculation based on effective medium approximation. In the two layer PS formation, optical thickness was found to change whether high porosity or low porosity layer is formed first.

A detailed investigation of the Auger non radiative de-excitation processes, which compete with the radiative emission of Er in Si, will be presented. This process, in which the energy released by the Er de-excitation is transferred to free carriers, is demonstrated to be extremely efficient and characterized by an Auger coefficient CA˜4.4×l0−13 cm3/s. This Auger process and an efficient incorporation method have been used to improve the performances of Er implanted light emitting diodes. It will be shown that by exciting Er within the depletion region of reverse biased p+-n+ Si diodes in the breakdown regime, it is possible to avoid Auger quenching and to achieve high efficiency. Moreover, at the switch off of the diode, when the depletion region shrinks, the excited Er ions become suddenly embedded within the neutral heavily doped region. In this region Auger de-excitation with free carriers sets in allowing to modulate the light signal at frequencies as high as a few MHz.

By using a low temperature growth process, Er doping in Si during MBE using Er2O3 or ErF3 as dopant sources has achieved a level of 5 × 1019 cm−3 without precipitation and generation of other extended defects. Luminescence properties of these Er-doped MBE Si structures have been extensively studied using both photon and hot electron impact excitation at a wide range of temperatures (2–300 K). It has been found that by incorporating C into Er/O doped layers, the room temperature EL emission with a FWHM value of 14 meV was ten times more intense than that with lower C doping. Post thermal annealing gave a strong effect on Er/F doped layers, leading to a 7-fold increase of the highest peak intensity while the peak line width reduced to 0.12 meV, which is very important for laser applications.

Electroluminescence (EL) characteristics of avalanching diodes fabricated by Er and O co-implantation and subsequent annealing have been studied. Distribution of Er3+-related EL at 1.538 μm was found to be uniform over the device area at 300 K. Saturation of the Er-related EL intensity is achieved under the avalanche regime at current density one order of magnitude lower than that under the tunnel regime. An EL decay is less than 10 μs (being time response of our detector) after the diode turned off the avalanche regime.

Porous silicon (PSi) was doped by Er using electromigration from a solution and converted to Er-doped silicon-rich silicon oxide (SRSO:Er) by partial thermal oxidation at 600–950°C following densification at 1100°C in an inert atmosphere. Room-temperature photoluminescence (PL) at ∼1.5 μm is intense and decreases by less than 20% from 12 K to 300 K. The PL spectrum of SRSO:Er reveals no luminescence bands related to Si-bandedgerecombination, point defects or dislocations, and shows that the Er3+ centers are the most efficient radiative recombination centers. A light-emitting diode (LED) with an active layer made of SRSO:Er was manufactured using a pre-oxidation cleaning step to increase the quality of the interface between SRSO:Er and the top electrode. Room temperature electroluminescence at ∼1.5 μm was demonstrated.

It is shown that porous polycrystalline Si (PPS) diodes operate as light-emitting diodes (LEDs) with efficiencies comparable to conventional porous silicon (PS) diodes fabricated on singlecrystalline substrates. Judging from the electroluminescence (EL) characteristics such as emission spectra, diode current dependence of EL intensity, and its temperature dependence, the common EL mechanism seems to exist in PPS and PS diodes. Observed similarity in the photoluminescence (PL) properties between PPS and PS is consistent with this hypothesis. The capability of a poly-Si thin film transistor as a driver of PPS-LED is also demonstrated.

Electroluminescence (EL) of Si bulk materials has been studied using lowly doped substrate with two Al-Si Schottky contacts. By applying a forward bias on the structure, the intense light emissions at 1.094 eV due to the TO-phonon assisted recombination was obtained at 40 K while other TA- and 2TO-associated transitions were also observed. The Si-TO EL peak persists up to RT with a radiative decay of ∼ 5 μs. EL emission mechanisms of these Si Schottky diodes are discussed based on temperature dependent- and injection current-dependent EL measurements.

Previous studies on electroluminescence in porous silicon were based on crystalline wafers. In this paper, we shall report the characteristics of a LED based on porous effects in a cast polycrystalline silicon substrate. A layer of porous region was first formed on a cast polycrystalline silicon substrate by anodization, followed by the deposition of a semitransparent Au layer. Under forward bias, the LED emits stable yellowish white light (with the presence of bright spots) for currents above 20 mA/cm2. From the electroluminescence spectra measured, we suggest that the emission is due to the recombination of electron-hole pairs in a microplasma region. We propose a model where the microplasma is present in the depletion region of the heterojunction formed between the bulk polysilicon and the surface porous polysilicon. The defects and grain boundaries in a polycrystalline material facilitate the formation of such microplasma. The heterojunction model will also be used to explain the current characteristics of the LED. The effect on the LED characteristics due to indium coating on the porous substrate prior to Au deposition was studied, and the results agree with the heterojunction model. Our work shows that cast polycrystalline silicon substrates have potential for LED fabrication in cheap and large area applications.