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Systematic studies of the relaxation processes in nm-thick Er-doped SiO2/nc-Si multilayers have been conducted using cw photoluminescence and time-resolved photoluminescence. The size of the Si nanocrystals is determined by thickness of the Si layer. The distance between the Si nanocrystals and the Er ions in the SiO2 layers is fixed between 0 and 2.2 nm. An efficient energy transfer from amorphous and crystalline Si nanostructures to Er is observed and leads to a strong Er photoluminescence at 1535 nm, which increases as the Si layer thickness decreases. The fast Er photoluminescence (μs lifetime) results from energy transfer from a-Si nanostructures and the slow Er emission (ms lifetime) is caused by energy transfer from confined states in nc-Si. The transfer time was estimated to be 20 ± 5 ns by a direct time-resolved photoluminescence experiment. A model is proposed to explain these results.

We have developed electroosmotic pumps (EOPs) fabricated from 15-nm-thick porous nanocrystalline silicon (pnc-Si) membranes. Ultrathin pnc-Si membranes enable high electroosmotic flow per unit voltage. We demonstrate that electroosmosis theory compares well with the observed pnc-Si flow rates. We attribute the high flow rates to high electrical fields present across the 15-nm span of the membrane. Surface modifications, such as plasma oxidation or silanization, can influence the electroosmotic flow rates through pnc-Si membranes by alteration of the zeta potential of the material. A prototype EOP that uses pnc-Si membranes and Ag/AgCl electrodes was shown to pump microliter per minute-range flow through a 0.5-mm-diameter capillary tubing with as low as 250 mV of applied voltage. This silicon-based platform enables straightforward integration of low-voltage, on-chip EOPs into portable microfluidic devices with low back pressures.

Infrared photoluminescence (PL) and electroluminescence (EL) near 1.5 μm from Er-doped SiO2/nc-Si multilayers are reported. A novel, effective method of lateral carrier injection is demonstrated in forward bias. The in-plane geometry increases the current density over those typically reported from Er doped SiO2 layers containing Si nanocrystals under vertical carrier injection. The observed strong PL under off-resonance excitation for Er in SiO2 and EL under forward bias are very promising for Si-based light sources – the missing link in an all-silicon on-chip optical interconnection system.

A photonic crystal (PhC) waveguide based optical biosensor capable of label-free and error-corrected sensing was investigated in this study. The detection principle of the biosensor involved shifts in the resonant mode wavelength of nanocavities coupled to the silicon PhC waveguide due to changes in ambient refractive index. The optical characteristics of the nanocavity structure were predicted by FDTD theoretical methods. The device was fabricated using standard nanolithography and reactive-ion-etching techniques. Experimental results showed that the structure had a refractive index sensitivity of 10−2 RIU. The biosensing capability of the nanocavity sensor was tested by detecting human IgG molecules. The device sensitivity was found to be 2.3 ± 0.24 × 105 nm/M with an achievable lowest detection limit of 1.5 fg for human IgG molecules. Additionally, experimental results demonstrated that the PhC devices were specific in IgG detection and provided concentration-dependent responses consistent with Langmuir behavior. The PhC devices manifest outstanding potential as microscale label-free error-correcting sensors, and may have future utility as ultrasensitive multiplex devices.

In this work we study the role of fluorine for the selective suppression of porous silicon in localized SOI applications. The formation of porous silicon can be done selectively by controlling the Fermi level in areas to be etched or not etched, which is typically done by adjusting the level of doping. Implanted fluorine in silicon has demonstrated a donor effect upon annealing at low temperature (600 °C), which is reversible as the fluorine outdiffuses during higher temperature annealing (1100 °C). The investigated technique has been used to form crystalline silicon active regions with thickness less than 200 nm completely surrounded by oxidized porous silicon. The electronic integrity of the active area has also been established through nMOS transistor fabrication and characterization (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A novel method of selecting a subset of Bloch modes in silicon-based photonic crystal microring resonators (PhCR)s is demonstrated. Bloch modes in the PhCR are calculated, and their intensity beating patterns are analyzed. Based on the different spatial intensity distribution for each resonance, a subset of resonances is out-coupled using an output coupler waveguide (CWG) which is positioned at an angle θ=90° with respect to the input CWG. As shown in theory and experiment, resonances with an even mode number are selected, while resonances with an odd mode number are rejected. The highest contrast between mode selection and mode rejection is ∼9 dB in the experiments. This approach opens another design freedom for ring resonator-based devices and could potentially reduce the footprint of microring resonator-based multiplexers and add-drop filters.

We report on the fabrication of a macroporous silicon diode that successfully operates in a photovoltaic mode of energy conversion. Typical device structures are fabricated on 4″, Cz grown, , p-type, 5 – 30 Ω-cm, Si substrates and make use of random or pre-patterned macroporous silicon films up to 100 μm thick with pores ∼1 μm in diameter and a center-to-center pore spacing of 2.5 μm. Phosphorous dopants are introduced throughout the porous silicon region with a newly adapted technique based on proximity rapid thermal diffusion. This development effort has enhanced the uniformity of dopant diffusion into the pores and subsequently improved device performance. The resulting p-n diode within the porous silicon is of a three-dimensional nature and exhibits an extremely high internal surface area up to ∼6690 cm2/cm3. To our knowledge, this is the first use of such a technique to fabricate conformal, three-dimensional p-n junctions for photovoltaic applications. The large internal surface area of this diode makes it applicable to next generation photovoltaics. Moreover, the porous silicon within this device is not simply an antireflection coating; rather it is a light trapping medium that has the ability to become a large surface area diode and can serve as a host matrix for photoactive compounds. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Interconnect has become a primary bottleneck in integrated circuit design. As CMOS technology is scaled, it will become increasingly difficult for conventional copper interconnect to satisfy the design requirements of delay, power, bandwidth, and noise. On-chip optical interconnect has been considered as a potential substitute for electrical interconnect in the past two decades. In this paper, predictions of the performance of CMOS compatible optical devices are made based on current state-of-art optical technologies. Electrical and optical interconnects are compared for various design criteria based on these predictions. The critical dimensions beyond which optical interconnect becomes advantageous over electrical interconnect are shown to be approximately one tenth of the chip edge length at the 22 nm technology node.

A three-dimensional p-n diode structure is presented for the generation of energy via photovoltaic and betavoltaic modes of operation. Macroporous Silicon (MPS) has a large degree of internal surface area and its vertically oriented pores, which extend deep into the bulk of the Si substrate, allow for the creation of three-dimensional structures. In this device the MPS will not only serve as a means for creating 3D diode structures, it will also serve as a host matrix for a tritium isotope which emits energetic beta particles. By varying electrochemical etching conditions and using a prepatterning technique, 1.1 μm diameter pores with a spacing of 2.5 μm were achieved. The p-n junction was created using a rapid thermal process (RTP) which relies on the diffusion from an n-type solid source into the MPS. To ensure the quality of the diode structure, devices were tested using a light source which resulted in a photovoltaic response. Finally, betavoltaic operation was demonstrated by exposing devices to a tritium gas source. The average energy conversion efficiency of the first generation 3D diode was one order of magnitude higher than that of a similar planar device.

Photochemical etching of silicon assisted by two-photon absorption (TPA) in the presence of hydrofluoric acid (HF) is demonstrated using a below-bandgap femtosecond laser source. We investigate the morphology of the etched silicon as a function of the laser power, exposure time, as well as the substrate doping level. Self-organized periodic silicon trenches with ∼150 nm spacing were observed at the etch front as the initial stage of the TPA photochemical etching process. Since the etching front can be precisely controlled by the focal point, this technique can be used for writing silicon microfluidic systems and for 3D micromachining.

Integration of electrical and fluidic systems for the design and fabrication of a system-on-chip (SOC) capable of sensing various liquid phase solvents is reported. A monolithic integration strategy makes use of macroporous silicon (MPS) as a gateway to interface the electrical and fluidic domains. In this application, the MPS material, acting as a sensing membrane, is used in a flow-through structure to transport an analyte from fluidic channels on one side of the chip to sensing electrodes on the other. A fluid-oxide-semiconductor interface results in the modulation of a space charge region in the semiconductor where real-time measurements are used to detect and distinguish between the presences of various solvents. The fluidic system has delivered sample volumes as small as 2 mul. Selected test solvents (i.e. acetone, ethanol, isopropyl alcohol, methanol, and toluene) have generated a measured change in capacitance up to 11%. A practical application of this sensor was demonstrated by monitoring various concentrations of isopropyl alcohol in a water supply. Undiluted samples provide characteristic responses that can be used for signature identification. The sensing device has a high degree of reusability and does not require heating or other solvent removal methods often necessitated in other sensing devices

Intrachip optical interconnects (OIs) have the potential to outperform electrical wires and to ultimately solve the communication bottleneck in high-performance integrated circuits. Performance targets and critical directions for ICs progress are yet to be fully explored. In this paper, the International Technology Roadmap for Semiconductors (ITRS) is used as a reference to explore the requirements that silicon-based ICs must satisfy to successfully outperform copper electrical interconnects (IEs). Considering the state-of-the-art devices, these requirements are extended to specific IC components

Porous silicon one-dimensional (1-D) photonic crystal microcavities are fabricated as building blocks for optical modulators. The microcavities consist of porous silicon layers with alternating low and high porosity. Since the pore size is much smaller than the incident wavelength of light, each porous silicon layer is treated as an effective medium with a specific effective refractive index that corresponds to the given porosity. Active tuning of the microcavity resonance is accomplished through electrical actuation of liquid crystals infiltrated into the pores. As the refractive index of the liquid crystals change in response to the external stimulus, the effective refractive index of the porous silicon layers changes and the resonance wavelength shifts accordingly. The resonance wavelength shift creates an optical signal intensity modulation at a particular wavelength. Extinction ratios greater than 10 dB are achievable for these devices. The performance characteristics of the porous silicon photonic crystal microcavities can be tailored to specific applications based on the choice of infiltrated optically active material and the quality factor of the device

Electrical tuning of high refractive index-contrast photonic bandgap (PBG) structures is required for a majority of PBG applications, particularly for integrated optics. When the host material is a semiconductor with poor electro-optical properties, tuning can be achieved by infiltrating the structure with an active optical material. In this paper we analyze the switching of electro-optic material in such structures, and suggest design rules to help achieve electrical tuning. In particular, a design concept that eliminates the electric field screening effects is proposed. The developed rules and concepts are demonstrated by the electrical tuning of liquid crystals inside two-dimensional porous silicon PBG structures. This approach can be generalized to different combinations of semiconductor PBG structures and active optical materials

New nanocomposite materials with large optical nonlinearity are important for the development and miniaturizing of active components and integrated circuits for all-optical signal processing and computing. To enable on-chip integration with silicon microelectronics nonlinear signal processing components compatible with silicon nanophotonics need to be developed. In this paper, we study the nonlinear optical response of silicon-based photonic bandgap (PBG) structures filled with PbSe quantum dot (QD) materials and experimentally demonstrate the feasibility of a new class of nonlinear nanophotonic devices for all-optical signal processing based on such microstructures. Active tuning is demonstrated for both out-of-plane components, with one-dimensional porous silicon PBG microcavities and in-plane components, using one-dimensional PBG waveguide-based microresonators.

Macroporous silicon microcavities for detection of large biological molecules have been fabricated from highly doped n-type silicon. Well-defined controllable pore sizes up to 120 nm have been obtained by systematically optimizing the etching parameters. The dependence of the sensor sensitivity on pore size is discussed. Excellent infiltration inside these macroporous silicon microcavities is demonstrated using 60 nm diameter latex spheres and rabbit IgG (150 kDa; 1Da = 1 g mol–1). The sensing performance of the device is tested using a biotin/streptavidin couple, and protein concentration down to 1–2 μM (equivalent to 0.3 ng mm–2) could be detected. Simulations show that the sensitivity of the technique is currently approximately 1–2 % of a protein monolayer.

Photonic bandgap (PBG) materials offer an attractive and compact way of integrating the building blocks for optical interconnects and optoelectronic circuits. In this work we demonstrate 2-D macroporous silicon PBG structures fabricated using interferometric lithography and electrochemical etching of silicon. Optical measurements match well with simulations. We achieve tuning of the PBG using liquid crystals (LCs) infiltrated inside the porous silicon. Surface treatment methods for controlling the initial LC molecule orientation inside the pores are also demonstrated. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Thermal tuning of silicon-based one-dimensional photonic bandgap microcavities is demonstrated. Thermally induced spectral shifts are caused by both the host silicon matrix and the optically active material infiltrated inside the photonic bandgap structures. The active material leads to the dominant thermal tuning contribution but the effect of the silicon matrix cannot be neglected. The interaction of the temperature dependence of the host matrix with that of the active material is explored. The general trends revealed by the characterization should be relevant for two-dimensional silicon-based photonic bandgap structures as well as other photonic bandgap materials systems. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

As continued progress towards faster, low power consuming microelectronic devices becomes increasing difficult due to the scaling challenges of electrical interconnects, it becomes even more critical to explore alternative technologies. Tunable porous silicon photonic bandgap structures are viable building blocks for optical interconnects, which present a possible long term solution to the interconnect problem. Forming the structures on a silicon platform provides the advantage of easier integration with current semiconductor processing techniques. In this work, tuning of the optical properties is controlled by liquid crystals (LCs) that are infiltrated into the silicon matrix. Active tuning is demonstrated both out-of-plane, with one-dimensional porous silicon photonic bandgap microcavities, and in-plane, using two-dimensional porous silicon photonic bandgap structures.

An extensive experimental study of optical gain in silicon nanocrystals is under way. Different types of samples have been tested (e.g., nanocrystalline silicon superlattices prepared in Rochester, ion implanted silicon dioxide prepared in Canberra) using different measurement techniques (e.g., variable stripe length method in Trento and in Rochester, prism coupling in Canberra) and different pump laser sources (from femtosecond to cw). All the results presented here have been reproduced in at least two different laboratories, making it unlikely that experimental artifacts play a role. So far, we have observed nsec-duration gain in the visible/near infrared range in nanocrystalline silicon superlattices under high photoinjection conditions with short laser pulses. For other conditions (e.g., lower photoinjection, ion implanted samples with a lower concentration of quantum dots), we have not observed gain.

Si quantum dots (QDs) as small as ∼2 nm in diameter have been synthesized by a variety of techniques. Because of quantum confinement and the elimination of bulk or surface defects, these dots can emit light from the near infrared throughout the visible with quantum efficiencies in excess of 10%. The luminescence wavelength range has been extended to longer wavelengths by the addition of light-emitting rare earths such as erbium (Er). Light-emitting devices (LEDs) have been fabricated and their performances are starting to approach those of direct band gap semiconductor or organic LEDs. A search for a Si QD-based laser is even under way. The state-of-the-art in the materials science, physics, and device development of luminescent Si QDs is reviewed and areas of future research are pointed out.

The objective of this work is to use porous silicon (PSi) layers as substrate material to transduce hybridization of DNA into a change in conductance. A coplanar configuration of the contact electrodes allows a complete exposure of the porous silicon layer to the sensing species without the need of top contacts or electrolyte solutions. Synthetic oligonucleotides were infiltrated into the porous silicon layers to sense the hybridization event. Hybridization of a single base sequence to its complementary strand was used as a model to demonstrate the response of the layer to the binding of complex sequences. We hypothesize that the observed response involves the activity of surface states present at the silicon-silicon dioxide (Si-SiO 2 ) interface and sensitive to the change in charge at the surface of the layer.

If porous silicon (PSi) is going to be seriously considered for antireflection (AR) applications in the solar cell industry, it must not only offer superior AR properties, but must also prove minimally disruptive to the current manufacturing process. This limits its application to the final (backend) fabrication steps, after p-n junction formation. Since typical commercial junction depths are on the order of 350 nm, the PSi etching process must only consume a portion of this emitter layer if the electrical properties of these devices are to be maintained. In this study, we have focused on the development of a dynamic etching process to form thin (

We demonstrate tunable mirrors that consist of a porous silicon microcavity infiltrated with liquid crystal molecules. We show theoretically that by utilizing the electro-optic properties of liquid crystals and the sensitivity of the microcavity resonance position to small changes in optical thickness, the porous silicon active mirror can be switched on (high reflectance) and off (low reflectance) by simply applying a voltage. We discuss the issues of obtaining uniform infiltration of liquid crystal molecules in the constricted geometry of the porous silicon microcavity and determining the necessary change in the liquid crystal orientation to achieve a high reflectance contrast. We also present preliminary experimental results showing a greater than 40% change in the reflectance of our active mirror with the application of voltage.

We report photocarrier time-of-flight measurements in diode structures made of highly porous crystalline silicon. The corresponding electron and hole drift mobilities are very small ð

A photonic model for freshly anodized porous silicon multilayer structures is presented. The photonic structures are composed of alternating high and low dielectric function porous silicon layers. The model takes into account the presence of silicon dioxide and its lattice expansion in the porous structure. We work with oxidized structures and our results fit completely the experimentally measured optical shift.

We have observed self-organization in Si nanocrystals fabricated by high temperature annealing of nanometer-thick layers of amorphous Si confined between thin SiO 2 layers. Silicon nanoparticles larger than ∼8 nm spontaneously form brick-shaped crystallites oriented along the [1 1 1] direction while smaller nanocrystals adopt a more spherical geometry. The observed ordering and self-organization are important steps towards reproducible maskless Si nanofabrication and the consequent construction of Si/SiO 2 quantum devices.

Plasmonic nanoparticles have unique optical properties and these properties are affected by any surrounding structures, or lack thereof. Nanoparticles are often added to a device without fully assessing the effect that each interface will have on the nanoparticle’s response. In this work, we simulate and fabricate devices utilizing hemispherical nanoparticles integrated into the back reflector of an amorphous silicon solar cell. 3D finite difference time domain simulations were used to calculate the optical absorption of a 300nm amorphous silicon layer as a function of the size of the nanoparticles, the distance between the nanoparticles and the active layer, and the distance between the nanoparticles and the mirror. Two transparent conducting oxides, aluminum doped zinc oxide and indium tin oxide, are investigated to determine the importance of the material properties between the nanoparticles and mirror. Silver hemispherical nanoparticles with a diameter of 150nm placed directly on the a-Si:H and a 60nm aluminum doped zinc oxide layer between the nanoparticles and the mirror lead to a maximum absorption increase of 7.2% in the 500nm to 800nm wavelength range. Experimental devices confirmed the trends predicted by theory but did not achieve enhancement, likely due to fabrication challenges. Fabricating a solar cell with the simulated design requires a high quality transparent conductive oxide and high control over the nanoparticle size distribution.

A label-free optical biosensor based on a one-dimensional photonic crystal microring resonator with enhanced light-matter interaction is demonstrated. More than a 2-fold improvement in volumetric and surface sensing sensitivity is achieved compared to conventional microring sensors. The experimental bulk detection sensitivity is ~248nm/RIU and label-free detection of DNA and proteins is reported at the nanomolar scale. With a minimum feature size greater than 100nm, the photonic crystal microring resonator biosensor can be fabricated with the same standard lithographic techniques used to mass fabricate conventional microring resonators.

We demonstrate evanescent coupling between a photonic crystal (PhC) waveguide and a PhC embedded microring resonator on the silicon-on-insulator platform. The mode converter comprises 6 linearly chirped air holes that adiabatically couple the light between the silicon waveguide mode and the PhC mode. Three-dimensional finitedifference time-domain simulations reveal a coupling bandwidth of >100nm. From our experiment, the optical spectra show a photonic bandgap located below ~1590nm. At the resonances in the slow-light regime, a loaded quality factor as high as ~2500 was measured and a group index of ~16 in the PhC embedded microring resonator was estimated from the non-uniform free spectral ranges.

Experimental studies on the chemical detection sensitivity of high surface area photonic crystal microring resonators are presented. We report a detection sensitivity of ~170nm/RIU for slow-light resonance modes close to the band edge.

This paper presents a successful unification of standard lithographic approaches (top down), anisotropic etching of atomically smooth surfaces, and controlled crystallization of silicon quantum dots (bottom up) to produce silicon nanoclusters at desired locations. These results, in combination with our previous demonstration of Si nanocrystal uniformity in size, shape, and crystallographic orientation, show strong potential for the application of silicon nanocrystals in electronic devices.

We report the design and testing of a versatile biosensor exclusively using silicon. The device structure consists of a microcavity resonator made of various porous silicon layers. Porous silicon contains silicon nanocrystals that can luminesce efficiently in the visible, depending on the size and passivation conditions. When a luminescent porous silicon layer is inserted between two Bragg reflectors (also made of porous silicon), the broad luminescence band is altered and multiple and very narrow peaks are detected. The position of these peaks is extremely sensitive to a small change in refractive index, such as that obtained when a biological object is attached to the large internal surface of porous silicon. We have demonstrated a DNA sensor that displays appropriate sensitivity, selectivity and response speed. The device fabrication procedure and the results of extensive testing are presented. An extension of the DNA biosensor has been made to include the detection of viral DNA. This work will lead to the development of an all-silicon sensor array for the detection of biomacromolecules.

We report the optical and electrical properties of all-silicon microcavity light emitting devices (LEDs) made of a thin, high-porosity porous silicon active layer sandwiched between two Bragg reflectors made of several pairs of low index/high index of refraction porous silicon layers of lower porosities. The design and passivation of the structures are discussed. The electroluminescent properties are examined in detail, including the strong narrowing of the spectrum, the widely tunable peak position, the emission directionality, the power efficiency, and the stability. The possible uses of these silicon microcavity LEDs are considered and the challenges that need to be met in terms of materials science and device design are examined.

We demonstrate stable room-temperature electroluminescence at 1.54 μm under both forward and reverse bias conditions from erbium-doped silicon nanocomposites. We also show enhanced and tunable emission from erbium when incorporated in porous silicon based microcavities. Erbium is infiltrated in the pores by cathodic electrochemical migration of the ions followed by high temperature annealing (600–1100°C) to produce a composite material made of silicon nanocrystals and silicon dioxide. The devices exhibit an exponential electroluminescence dependence in both bias conditions as a function of the driving current and driving voltage. In reverse bias, the external quantum efficiency reaches 0.01%. The devices show a large temperature dependence of the electroluminescence intensity. The electroluminescence intensity decreases by a factor of 24 in reverse bias and 2.6 in forward bias when the temperature increases from 240 to 300 K. The photoluminescence from the erbium-doped microcavity resonators is enhanced by more than one order of magnitude and tuned to emit in areas where the natural erbium emission is very weak.

Porous silicon microcavity resonators possess the unique characteristics of line narrowing and luminescence enhancement. The emission peak position is completely tunable by changing the properties of the central layer. Increasing the thickness of the central active layer introduces multiple narrow peaks in the luminescence spectrum. Narrow and visible luminescence peaks are observed with a full width at half the maximum (FWHM) value of 3 nm. The microcavity structure is highly sensitive and any slight change in the effective optical thickness modifies the reflectivity spectra, causing a spectral shift in the interference peaks. This makes porous silicon microcavities an ideal host for sensor applications. We demonstrate the usefulness of this microcavity resonator structure as a biosensor. A DNA biosensor has been fabricated based on a porous silicon multiple peak microcavity structure. Shifts in the luminescence spectra are observed and detected for various DNA concentrations. When exposed to a non-complementary DNA strand, no shifts are observed. An extension of the DNA biosensor has been made to include the detection of viral DNA. Both sensor fabrication and results of extensive testing are presented in this report.

We have fabricated erbium-doped porous silicon (PSi) devices and demonstrated stable room-temperature electroluminescence (EL) at 1.54 μm under both forward and reverse bias conditions. Erbium was infiltrated into the nanostructured matrix of PSi (≤ 1019 cm—3) by cathodic electrochemical migration of the ions followed by high temperature annealing (950–1100 °C). The devices exhibit an exponential EL dependence in both bias conditions as a function of driving current and voltage. External quantum efficiencies of 0.01% have been obtained. The EL intensity in reverse bias and the transport properties of the devices show large temperature dependences. From the differences in temperature, electrical, and EL characteristics in forward and reverse biases, we believe that different excitation mechanisms are responsible for the EL.

The fabrication of size-controlled Si nanocrystals encapsulated by SiO2 is complicated by the amorphous nature of SiO2 and the mismatch in their thermal expansion coefficients. A preferred crystallographic orientation is especially important for Si quantum dots because of the Si indirect-gap band structure and a strong anisotropy in the energy-momentum dispersion. We report self-organization in Si nanocrystals fabricated by the crystallization of nanometer-thick layers of amorphous Si confined between SiO2 layers. Si nanoparticles larger than 8 nm spontaneously form brick-shaped crystallites oriented along the [111] direction. The preferred orientation and distinct shape result in a relatively narrow luminescence. The observed ordering and self-organization is an important step toward reproducible maskless Si nanofabrication and the consequent construction of Si/SiO2 quantum devices., American Physical Society, Annual March Meeting, 12-16 March 2001, Seattle, Washington, USA

We have measured the crystallite sizes, the bandgap energies, and the photoluminescence (PL) energies in porous silicon (PSi) samples having a wide range of porosities and kept in different ambient conditions. The dependence of the bandgap energy on the crystallite size agrees with theory. For PSi samples exposed to air and containing crystallites smaller than 5 nm, the PL intensity increases by several orders of magnitude and the PL peak energy shifts from the near infrared to the red, in agreement with the quantum confinement model for the PL. For crystallites smaller than 3 nm, there is a Stokes shift between the excitonic bandgap and PL energies, which increases to several hundreds of meV for sizes ∼2 nm, indicating that, in PSi exposed to air, the PL is not due to free excitons. Before exposure to air, very high porosity PSi samples emit at shorter wavelengths than after exposure to air, suggesting that the Stokes shift depends on the surface chemistry.

A nanocrystalline silicon superlattice (nc-Si SLs) is a structure consisting of Si nanocrystal layers separated by nanometer-thick SiO2. A long range order in the nc-Si SL is obtained along the direction of growth by periodically alternating layers of Si nanocrystals and SiO2. A number of characterization techniques such as transmission electron microscopy (TEM) and atomic force microscopy (AFM), Auger elemental microanalysis. X-ray diffraction and X-ray small angle reflection have proved that the nc-Si SL exhibits a very narrow nanocrystal size distribution (less than 5% in average) and very abrupt and flat nc-Si/SiO2 interfaces with a roughness of

The stability, efficiency, speed, and spectral range of light-emitting devices (LEDs) made of nanoscale porous silicon is improving. The first part of this paper discusses the preparation and properties of nanoscale silicon, with emphasis on porous silicon. In the second part, the properties of LEDs made of porous silicon are reviewed. In the third part, the integration of PSi LEDs with silicon microelectronic circuits is discussed. The paper ends with a short discussion of the prospects for realistic optoelectronic devices.

In this report we demonstrate control over porous silicon (PSi) emission properties by changing the dielectric environment surrounding the silicon crystallites, as well as provide information on the effects of pore infiltration of PSi. This is achieved by making PSi–polymer nanocomposites by diffusing or polymerizing a range of varying dielectric constant polymers into the pores. The degree of modification in photoluminescence (PL) depends on the dielectric constant of the polymers. By increasing the dielectric constant of the environment surrounding the crystallites, a blue shift in PL as high as 222 meV has been observed. The blue shift is attributed to the high dielectric constant of the polymers relative to PSi, which causes a partial screening of the excitons allowing the excitonic levels to shift closer to the bandgap. The shift in excitonic levels increases when the dielectric constant of the polymer increases. PSi–polymer nanocomposites also exhibit an increase in PL intensity, which suggest that the inert infiltrated polymers are able to passivate existing nonradiative channels.

Partially oxidized free-standing porous silicon films show a strongly superlinear increase in photoluminescence (PL) intensity above a threshold cw excitation intensity of ∼10 W/cm2. The PL-intensity increase can be expressed by a power law with n∼9 as a function of the excitation intensity. The PL-peak wavelength of this emission is slightly redshifted from that at low-excitation levels. These changes are fully reversible and reproducible, but not observed in samples on substrate. We attribute this behavior to thermal reexcitation of carriers trapped at the dangling bond states in initially nonluminescent Si nanocrystallites.

Ge and GeO2 nanocrystals are prepared in a-SiOx matrix by DC-magnetron sputtering and thermal annealing. An increased infrared absorption at 860 cm−1 is found in films containing GeO2 clusters. Under ultra violet excitation, both types of films luminesce near 3.1 eV, with a characteristic blue PL decay time around 1 ns, independent of the chemistry and size. The strongest PL intensity is found for the films containing GeO2 crystals and for the films containing the largest Ge crystals. The room temperature PL excitation spectra are independent of the crystal size. Although the blue luminescence is correlated with the formation of the crystals, it is not due to the radiative recombination of confined excitons. We suggest that the origin for the blue PL band is a Ge/O related defect located in the inhomogeneous strain field of the crystal surfaces.

Rapid thermal annealing of amorphous Si/SiO 2 superlattices deposited by rf-sputtering and plasma oxidation forms Si nanocrystals while maintaining the superlattice structure. The crystal size is controlled by the thickness of the Si sublayers and can be varied from 12 nm to ∼2.5 nm. The strain in the c-Si/SiO 2 structure is released by annealing at 1050°C using a slow temperature ramp. After wet oxidation which reduces the size and completes the passivation of the crystals by SiO 2 , the room-temperature band-edge photoluminescence reaches an external quantum efficiency >0.1%.

We report the fabrication of photovoltaic devices by the anodization of microcrystalline silicon films on single-crystal silicon substrates. The porosity of the films was varied from 20% to 60% by changing the anodization conditions. The influence of the porosity on the series resistance ( R s ), the reflectance, and the spectral response of the devices was studied. In order to determine R s , the current–voltage characteristics were analyzed, both in the dark and under illumination. We observed that the value of R s increased from 3 to 100 Ω and the value of the reflectance decreased from 24% to 7% when the porosity increased from 20% to 60%. The optimum device performance (fill factor of 0.53 and efficiency of 7.2%), which was achieved for a porosity of 40% and can be improved further, resulted from a trade-off between good electrical and optical properties. From optical beam-induced current images, the homogeneity of the devices was found to be excellent and no defects were detected. Our results indicate that the fabrication of commercial solar cells based on porous microcrystalline Si (PMSi) is possible.

The optical properties of silicon nanocrystallites of known sizes, present in supercritically dried porous silicon films of porosities as high as 92%, have been measured by a variety of techniques. The bandgap and luminescence energies have been measured as a function of size for the first time. The bandgap increases by more than 1 eV due to quantum confinement. The peak luminescence energy which also shifts to the blue is increasingly Stokes shifted with respect to the bandgap, as the size decreases. The measured bandgap is in agreement with realistic theories and the Stokes-shift between bandgap and luminescence energies coincides with the exciton binding energy predicted by these theories. These results demonstrate unambiguously and quantitatively the role of quantum confinement in the optical properties of this indirect gap semiconductor.