Showing papers by "Philippe M. Fauchet published in 2004"

Macroporous silicon electrical sensor for DNA hybridization detection.

Abstract: Macroporous silicon (pore diameter 1–2 μm) was used in an electrical sensor for real time, label free detection of DNA hybridization. Electrical contacts were made exclusively on the back side of the substrate, which allowed complete exposure of the porous layer to DNA. Hybridization of a DNA probe with its complementary sequence produced a reduction in the impedance and a shift in the phase angle resulting from a change in dielectric constant inside the porous matrix and a modification of the depletion layer width in the crystalline silicon structure. The effect of the DNA charge on the response was corroborated using peptide nucleic acid (PNA), an uncharged analog of DNA. The sensitivity and selectivity of the device were characterized and the sensing properties of the porous layer alone were investigated using self-supporting macroporous silicon membranes.

Polarization surface-charge density of single semiconductor quantum rods

Abstract: Electrostatic force microscopy was used to determine that single CdSe quantum rods (QRs) have a permanent polarization surface-charge density, an unexpected observation for supposedly well-shaped particles. The surface charge results from a slight angle between the QR sides and the direction of internal electric polarization. By contrast, despite the large dipole moment expected for CdSe QRs, none was observed. The unavoidable presence of permanently charged surfaces on CdSe QRs has the potential to impede the development of novel devices incorporating these materials.

Effect of oxidation on charge localization and transport in a single layer of silicon nanocrystals

Abstract: The effect of oxidation on charge transport and retention within a sheet of silicon (Si) nanocrystals was investigated with an electrostatic force microscope. Single layers of nanocrystals with smooth and abrupt Si/SiO2 interfaces were prepared by thermal crystallization of thin amorphous Si layers, followed by an oxidation treatment for isolating the nanocrystals. Controlled amounts of charge were injected into the nanocrystals and their in-plane diffusion was monitored in real time. Rapid transport of the injected charge occurred for the nonoxidized nanocrystals. Oxidation of the nanocrystal layer resulted in suppression of lateral transport. The nanocrystals oxidized for 30 min retained the injected charge in a well-defined, localized region with retention times of the order of several days. These long-term charge retention characteristics indicate that nanocrystals prepared by this process could be attractive candidates for nonvolatile memory applications.

Biosensing with one-dimensional photonic bandgap structure

Abstract: The optical properties of photonic bandgap (PBG) structures are highly sensitive to the geometry and refractive index. This makes PBG structures a good host for sensor applications. The binding of target species inside the PBG structure changes the refractive index of the material, which can be detected by monitoring the optical response of the device. One-dimensional PBG biosensors based on porous silicon (PSi) have been fabricated. The device is a microcavity, made of a symmetry breaking PSi layer (defect layer) inserted between two PSi Bragg mirrors. Narrow resonances are introduced in the photoluminescence and reflectance spectra. The large internal surface of the sensor is functionalized for the capture of target biological materials. When the sensor is exposed to the target, binding to the internal surface increases the effective optical thickness of the microcavity and thus causes a red shift of the optical spectrum. The sensor's sensitivity is determined by the morphology and geometry of the device. We will present the details of the materials science, sensor fabrication and optimization, and also describe experiments performed with biological targets.

The development of nanocrystalline silicon for emerging microelectronic and nanoelectronic applications

Abstract: Films consisting of nanometer-scale silicon crystals with narrow size distribution can be fabricated using a variety of techniques and are of technological interest for nonvolatile semiconductor memory applications One fabrication technique based on the crystallization of thin amorphous silicon films also shows potential for large-scale production of single isolated nanocrystals for future single-electron transistor and memory applications A review of the most promising nanocrystalline silicon fabrication techniques and a discussion of current research in the area of crystallized thin-film amorphous silicon are presented in this article

Method for controlling one or more temperature dependent optical properties of a structure and a system and product thereof

Abstract: A method for controlling one or more temperature dependent optical properties of a structure in accordance with embodiments of the present invention includes heating at least a portion of a photonic band-gap structure and oxidizing the portion of the photonic band-gap structure during the heating to alter at least one temperature dependent optical property of the stack.

Dynamically tunable 1D and 2D photonic bandgap structures for optical interconnect applications

Abstract: Optical interconnects have begun replacing electrical wires in long distance, backplane applications. As their switching speed and efficiency improves, optical interconnects will penetrate deeper into the device architecture for inter- and intra-chip communications where direct integration with silicon microelectronics is a necessity. Tunable 1D and 2D silicon-based photonic bandgap (PBG) structures are viable building blocks for optical interconnects because they have the capability to redirect light both in- and out-of-plane. In this work, we report on external modulation of the optical properties of 1D and 2D porous silicon PBG structures infiltrated with liquid crystals. This class of eletrooptic modulators offers an inexpensive and versatile way of integrating optical interconnects with standard microelectronic circuits.

Integrated sensor arrays with a configurable network interface for chemical and biological detection

Abstract: Sensors based on macroporous silicon (M-PSI) have demonstrated the ability to detect the presence of certain chemical and biological materials. The devices utilize silicon sensing membranes with deep trench structures (macropores) formed by electrochemical etching to depths up to 100μm. The sensor structure is unique in that it exploits the vertical dimension of the planar silicon substrate, utilizing the large internal surface area of the membrane as the active sensing region. Upon exposure to organic solvents (i.e. ethanol, acetone, benzene) the devices exhibit a characteristic impedance signature. Discrimination is achieved by recognizing a specific response characteristic, or by placing appropriate probe materials to provide an electrically detectable signal upon exposure to the target substance. M-PSi sensing devices have demonstrated an electrical response to DNA hybridization and shown discrimination between binding and non-binding events. The size of the pores in the sensing elements can host larger molecules such as proteins, which extends the use of the devices to other fields of biotechnology. The sensors have been designed and fabricated in array configurations. A flexible electronics interface platform has been developed to accommodate the use of the sensors for a variety of applications.

Control and elimination of the effect of ambient temperature fluctuations on photonic bandgap device operation

Abstract: As photonic bandgap (PBG) technology matures and practical devices are realized, the effects of environmental factors, such as ambient temperature, on PBG device operation must be considered. The position of a PBG is determined by the geometry and refractive index of the constituent materials. Therefore, a thermally induced material expansion or refractive index change will alter the location of the PBG and affect the operation of PBG devices. In order to achieve faster switching times for PBG optical interconnects, enhanced sensitivity for PBG sensors, and smaller channel spacing for PBG-based wavelength division multiplexing, increasingly narrow PBG resonances are required. The drawback for the improved device operation is increased sensitivity to small changes in environmental conditions. A method to control and eliminate thermally induced drifts of silicon-based PBG structures has been developed based on a simple oxidation treatment. Oxide coverage of the silicon matrix provides a counterforce to the effect of the temperature dependent silicon refractive index. Depending on the degree of oxidation achieved, a redshift, no shift, or a blueshift of the PBG resonance results when the silicon-based PBG structure is heated. Control over the effects of thermal fluctuations has been demonstrated for two different PBG structure designs. Extensive reflectance and x-ray diffraction measurements have been performed to understand the mechanism behind this oxidation procedure as it relates to one-dimensional silicon-based PBG microcavities.

Temperature stability for photonic crystal devices

Abstract: Ageneral method has been developed to allow photonic crystal devices to operate in a variable temperature environment. Recent work in the photonic crystal area has focused on the achievement of high Q-factor cavities in which light is confined within a very small wavelength range.1 While this is an advantage for achieving low threshold lasing, high sensitivity detection of chemical and biological species, and wavelength division multiplexed components for optical interconnects and optical communication systems, there is a price to be paid in terms of practical device operation and reliability in changeable ambient conditions. The higher the Q of the cavity, the more sensitive the device will be to small changes in environmental conditions. The key to the performance of photonic crystals is a periodic dielectric function, which introduces a wavelength range over which light is forbidden to propagate. This range of zero transmission is known as the photonic bandgap (PBG). Light confinement within the PBG is achieved by introducing a defect, or break, in the periodicity of the dielectric function. As a result, a resonance, in which light can propagate, emerges in the PBG. Any changes to the dielectric function of the photonic crystal will change the resonance wavelength. Consequently, for photonic crystals with Q-factors above 10,000, even a few degrees C change in temperature could cause a 10 dB change in transmission at the resonance wavelength. A new method has been developed to create temperature insensitive siliconbased photonic crystals. The temperature dependence of the silicon refractive index [dn/dT ~ 2-4 10-4 K-1 for the visible and the near infrared (IR) regions] causes the resonance wavelength of siliconbased photonic crystals to red shift upon heating. We have shown that there exists a proper thermal oxidation condition (e.g., temperature and ambient oxygen content) which counterbalances this effect and leads to temperature insensitive photonic crystal resonances.2 During oxidation, a thin layer of silicon is converted into silicon oxide. Because silicon oxide’s thermal expansion coefficient is one-fifth that of silicon, as the operating temperature of a PBG device increases, silicon’s thermal expansion is impeded, thus creating a compressive stress within the silicon. The dependence of silicon’s refractive index on this resulting pressure (dn/dP ~ -10-5 MPa-1) is of the opposite sign to that of the temperature dependence on the refractive index. The oxidation method has been demonstrated on one-dimensional porous silicon PBG microcavities (see Fig. 1). Scaling the oxidation method to other silicon-based photonic crystal structures will simply involve finding the proper oxide thickness for a given silicon feature size. The method could also be generalized to other pairs of materials.

Harvesting Betavoltaic and Photovoltaic Energy with Three Dimensional Porous Silicon Diodes

Abstract: Conventional two-dimensional p-n diodes can be used for betavoltaic and photovoltaic energy conversion, but the device efficiency is limited by the planar geometry. We propose and demonstrate a novel three-dimensional diode geometry based on porous silicon. The 3D pore array provides two very important features: (1) the storage of the radioisotope energy source and (2) its extreme proximity to the p-n junction on each pore wall. The particle energy losses are thereby minimized prior to entering the conversion layer. In betavoltaics, our 3D betavoltaic device efficiency is 10 times that of a similar planar device. In photovoltaics, photons play the role of beta particles and photon trapping inside the pores enhances the conversion efficiency. Further fabrication and geometry optimization can result in practical, high performance devices.

Charge Transport in Silicon Nanocrystal Arrays

Abstract: Single layers of isolated, size-controlled silicon nanocrystals were prepared by thermal crystallization of a thin amorphous silicon layer sandwiched between silicon dioxide layers. A subsequent oxidation treatment ensured controlled increase in their lateral separation. The size of the nanocrystals, separation of the nanocrystals (from < 1 nm to ~ 4 nm), stoichiometry of the resulting oxide and surface morphology were monitored with transmission electron microscopy, scanning transmission electron microscopy, atomic force microscopy, and x-ray photoelectron spectroscopy. Mesoscopic charge transport studies performed with an electrostatic force microscope (EFM) revealed rapid lateral transport of charges when the nanocrystals were tightly packed (< 1 nm average separation) and interconnected. As the inter-nanocrystal separation was increased, lateral charge transport was rapidly suppressed. Nanocrystals separated by up to 3.6 nm retained the injected charges in a well-defined localized region (~ 62 nm diameter region) for a time of the order of several days. The ability to switch from a very short to a very long retention time using the same structure by simply changing the post-growth processing conditions is attractive for various applications involving charge transport and localization.

Silicon-based photonic bandgap modulators

Abstract: Silicon-based modulators are the missing link necessary for practical optical interconnects. Electrically and thermally tunable silicon-based photonic bandgap structures are demonstrated as building blocks for compact and low power out-of-plane and in-plane optical modulation.

Photonic structures: lateral thinking with photonic crystal fibers

Abstract: The introduction of photonic crystal fibers (PCFs) has enabled researchers to redefine the concept of optical fibers and extend their functionality beyond the realm of traditional optical transportation. The regular lattice of holes that characterizes PCFs gives rise to unique waveguiding properties, along with fascinating effects such as photonic bandgap (PBG) guidance, “endlessly single-mode” guidance and supercontinuum generation.1 Until recently, however, the periodic nature of PCFs has been used only to enhance the fibers’ longitudinal waveguiding characteristics. We have introduced another application of PCFs: the manipulation of light propagating transversely across the fiber.2 In other words we exploit the fact that the fiber’s cross-sectional profile is essentially a two-dimensional (2D) photonic crystal (PC). Figure 1(a) illustrates such a geometry, with a scanning electron micrograph of a typical microstructure. The light enters the PCF from the side, interacts with the periodic microstructure and emerges on the far side as it would in a planar PC.3 By studying its transmission and reflection characteristics both experimentally and through modeling, we have demonstrated that such a transverse fiber can indeed behave as a 2D PC. For instance, light emerges at a series of angles to form diffraction spots [Fig. 1(b)], and the transmission of certain wavelengths is suppressed by the PBG. The existence of this transverse PCF geometry leads to the possibility that a range of planar-device concepts can be designed into the fiber microstructure. In this context, the simple yet flexible fabrication of PCFs gives the transverse fiber an advantage in many respects over the variety of existing PCs. These include: smooth walls which suppress scattering; a potentially arbitrary microstructure which adds flexibility to device concepts; and tunability.

Birefringence in porous silicon structures

Abstract: We present a method for treating birefringent effects in layered media, and apply it to porous silicon structures. The approach is to characterize the fields in terms of s- and p-polarized amplitudes in each layer. The calculations then naturally employ Fresnel reflection and transmission coefficients for the anisotropic media.

Biosensing Platform Using Porous Silicon Microcavities

Abstract: A label free optical biosensor consisting of a macroporous silicon microcavity has been demonstrated. This new biosensing platform allows the infiltration of large biological targets inside the sensor and significantly enhances the biosensing performance in protein-to-protein recognition and bacterial cells detection.

Tunable 2D photonic crystals for switching applications

Abstract: Reversible switching in 2D silicon-based photonic crystal structures is demonstrated using nematic liquid crystals. These tunable devices can be used as a platform for a number of ultracompact active optical elements, such as modulators and routers. The flexible and inexpensive fabrication processes allow for integration with existing microelectronic technology.