Showing papers on "Silicon published in 2000"

Optical gain in silicon nanocrystals

Abstract: Adding optical functionality to a silicon microelectronic chip is one of the most challenging problems of materials research. Silicon is an indirect-bandgap semiconductor and so is an inefficient emitter of light. For this reason, integration of optically functional elements with silicon microelectronic circuitry has largely been achieved through the use of direct-bandgap compound semiconductors. For optoelectronic applications, the key device is the light source--a laser. Compound semiconductor lasers exploit low-dimensional electronic systems, such as quantum wells and quantum dots, as the active optical amplifying medium. Here we demonstrate that light amplification is possible using silicon itself, in the form of quantum dots dispersed in a silicon dioxide matrix. Net optical gain is seen in both waveguide and transmission configurations, with the material gain being of the same order as that of direct-bandgap quantum dots. We explain the observations using a model based on population inversion of radiative states associated with the Si/SiO2 interface. These findings open a route to the fabrication of a silicon laser.

Control of Thickness and Orientation of Solution-Grown Silicon Nanowires

Abstract: Bulk quantities of defect-free silicon (Si) nanowires with nearly uniform diameters ranging from 40 to 50 angstroms were grown to a length of several micrometers with a supercritical fluid solution-phase approach. Alkanethiol-coated gold nanocrystals (25 angstroms in diameter) were used as uniform seeds to direct one-dimensional Si crystallization in a solvent heated and pressurized above its critical point. The orientation of the Si nanowires produced with this method could be controlled with reaction pressure. Visible photoluminescence due to quantum confinement effects was observed, as were discrete optical transitions in the ultraviolet-visible absorbance spectra.

Large-scale synthesis of a silicon photonic crystal with a complete three-dimensional bandgap near 1.5 micrometres

Abstract: Photonic technology, using light instead of electrons as the information carrier, is increasingly replacing electronics in communication and information management systems. Microscopic light manipulation, for this purpose, is achievable through photonic bandgap materials1,2, a special class of photonic crystals in which three-dimensional, periodic dielectric constant variations controllably prohibit electromagnetic propagation throughout a specified frequency band. This can result in the localization of photons3,4,5,6, thus providing a mechanism for controlling and inhibiting spontaneous light emission that can be exploited for photonic device fabrication. In fact, carefully engineered line defects could act as waveguides connecting photonic devices in all-optical microchips7, and infiltration of the photonic material with suitable liquid crystals might produce photonic bandgap structures (and hence light-flow patterns) fully tunable by an externally applied voltage8,9,10. However, the realization of this technology requires a strategy for the efficient synthesis of high-quality, large-scale photonic crystals with photonic bandgaps at micrometre and sub-micrometre wavelengths, and with rationally designed line and point defects for optical circuitry. Here we describe single crystals of silicon inverse opal with a complete three-dimensional photonic bandgap centred on 1.46 µm, produced by growing silicon inside the voids of an opal template of close-packed silica spheres that are connected by small ‘necks’ formed during sintering, followed by removal of the silica template. The synthesis method is simple and inexpensive, yielding photonic crystals of pure silicon that are easily integrated with existing silicon-based microelectronics.

Two dimensional electron gases induced by spontaneous and piezoelectric polarization in undoped and doped AlGaN/GaN heterostructures

Abstract: Two dimensional electron gases in Al x Ga 12x N/GaN based heterostructures, suitable for high electron mobility transistors, are induced by strong polarization effects. The sheet carrier concentration and the confinement of the two dimensional electron gases located close to the AlGaN/GaN interface are sensitive to a large number of different physical properties such as polarity, alloy composition, strain, thickness, and doping of the AlGaN barrier. We have investigated these physical properties for undoped and silicon doped transistor structures by a combination of high resolution x-ray diffraction, atomic force microscopy, Hall effect, and capacitance‐voltage profiling measurements. The polarization induced sheet charge bound at the AlGaN/GaN interfaces was calculated from different sets of piezoelectric constants available in the literature. The sheet carrier concentration induced by polarization charges was determined

Metal-assisted chemical etching in HF/H2O2 produces porous silicon

Abstract: A simple and effective method is presented for producing light-emitting porous silicon (PSi) A thin (d<10 nm) layer of Au, Pt, or Au/Pd is deposited on the (100) Si surface prior to immersion in a solution of HF and H2O2 Depending on the type of metal deposited and Si doping type and doping level, PSi with different morphologies and light-emitting properties is produced PSi production occurs on the time scale of seconds, without electrical current, in the dark, on both p- and n-type Si Thin metal coatings facilitate the etching in HF and H2O2, and of the metals investigated, Pt yields the fastest etch rates and produces PSi with the most intense luminescence A reaction scheme involving local coupling of redox reactions with the metal is proposed to explain the metal-assisted etching process The observation that some metal remains on the PSi surface after etching raises the possibility of fabricating in situ PSi contacts

Silicon metabolism in diatoms: implications for growth

Abstract: Diatoms are the world's largest contributors to biosilicification and are one of the predominant contributors to global carbon fixation. Silicon is a major limiting nutrient for diatom growth and hence is a controlling factor in primary productivity. Because our understanding of the cellular metabolism of silicon is limited, we are not fully knowledgeable about intracellular factors that may affect diatom productivity in the oceans. The goal of this review is to present an overview of silicon metabolism in diatoms and to identify areas for future research. Numerous studies have characterized parameters of silicic acid uptake by diatoms, and molecular characterization of transport has begun with the isolation of genes encoding the transporter proteins. Multiple types of silicic acid transporter gene have been identified in a single diatom species, and multiple types appear to be present in all diatom species. The controlled expression and perhaps localization of the transporters in the cell may be factors in the overall regulation of silicic acid uptake. Transport can also be regulated by the rate of silica incorporation into the cell wall, suggesting that an intracellular sensing and control mechanism couples transport with incorporation. Sizable intracellular pools of soluble silicon have been identified in diatoms, at levels well above saturation for silica solubility, yet the mechanism for maintenance of supersaturated levels has not been determined. The mechanism of intracellular transport of silicon is also unknown, but this must be an important part of the silicification process because of the close coupling between silica incorporation and uptake. Although detailed ultrastructural analyses of silica deposition have been reported, we know little about the molecular details of this process. However, proteins occluded within silica that promote silicification in vitro have recently been characterized, and the application of molecular techniques holds the promise of great advances in this area. Cellular energy for silicification and transport comes from aerobic respiration without any direct involvement of photosynthetic energy. As such, diatom silicon metabolism differs from that of other major limiting nutrients such as nitrogen and phosphorous, which are closely linked to photosynthetic metabolism. Cell wall silicification and silicic acid transport are tightly coupled to the cell cycle, which results in a dependency in the extent of silicification on growth rate. Silica dissolution is an important part of diatom cellular silicon metabolism, because dissolution must be prevented in the living cell, and because much of the raw material for mineralization in natural assemblages is supplied by dissolution of dead cells. Perhaps part of the reason for the ecological success of diatoms is due to their use of a silicified cell wall, which has been calculated to impart a substantial energy savings to organisms that have them. However, the growth of diatoms and other siliceous organisms has depleted the oceans of silicon, such that silicon availability is now a major factor in the control of primary productivity. Much new progress in understanding silicon metabolism in diatoms is expected because of the application of molecular approaches and sophisticated analytical techniques. Such insight is likely to lead to a greater understanding of the role of silicon in controlling diatom growth, and hence primary productivity, and of the mechanisms involved in the formation of the intricate silicified structures of the diatom cell wall.

Surface passivation of crystalline silicon solar cells: a review

Abstract: In the 1980s, advances in the passivation of both cell surfaces led to the first crystalline silicon solar cells with conversion efficiencies above 20%. With today's industry trend towards thinner wafers and higher cell efficiency, the passivation of the front and rear surfaces is now also becoming vitally important for commercial silicon cells. This paper presents a review of the surface passivation methods used since the 1970s, both on laboratory-type as well as industrial cells. Given the trend towards lower-cost (but also lower-quality) Si materials such as block-cast multicrystalline Si, ribbon Si or thin-film polycrystalline Si, the most promising surface passivation methods identified to date are the fabrication of a p–n junction and the subsequent passivation of the resulting silicon surface with plasma silicon nitride as this material, besides reducing surface recombination and reflection losses, additionally provides a very efficient passivation of bulk defects. Copyright © 2000 John Wiley & Sons, Ltd.

Self-Assembly Is Not the Only Reaction Possible between Alkyltrichlorosilanes and Surfaces: Monomolecular and Oligomeric Covalently Attached Layers of Dichloro- and Trichloroalkylsilanes on Silicon

Abstract: Silicon-supported alkylsiloxane layers were prepared by reaction of alkylmethyldichlorosilanes and alkyltrichlorosilanes with silicon wafers under two conditions: (1) in the vapor phase and (2) in toluene in the presence of ethyldiisopropylamine. Covalent attachment of di- and trichlorosilanes to the surface of silicon/silicon oxide through SiS−O−Si bonds occurs for the amine-catalyzed reactions. This sets apart this reaction from the self-assembly process that occurs in the reaction between certain trichlorosilanes and hydrated silica with no amine present. The thickness of the layers formed from dichloro- and trichlorosilanes (as assessed by ellipsometry) is on the order of the single molecule sizes and increases gradually with alkyl chain length. The thickness values are considerably smaller (by a factor of ∼0.75) than the length of the fully stretched alkyl chain, which argues for disordered structures of the monolayers. Dynamic advancing and receding contact angles for water, methylene iodide, and h...

Atomic Layer Deposition of Oxide Thin Films with Metal Alkoxides as Oxygen Sources

Abstract: A chemical approach to atomic layer deposition (ALD) of oxide thin films is reported here. Instead of using water or other compounds for an oxygen source, oxygen is obtained from a metal alkoxide, which serves as both an oxygen and a metal source when it reacts with another metal compound such as a metal chloride or a metal alkyl. These reactions generally enable deposition of oxides of many metals. With this approach, an alumina film has been deposited on silicon without creating an interfacial silicon oxide layer that otherwise forms easily. This finding adds to the other benefits of the ALD method, especially the atomic-level thickness control and excellent uniformity, and takes a major step toward the scientifically challenging and technologically important task of replacing silica as the gate dielectric in the future generations of metal oxide semiconductor field effect transistors.

Self-directed growth of molecular nanostructures on silicon

Abstract: Advances in techniques for the nanoscale manipulation of matter are important for the realization of molecule-based miniature devices with new or advanced functions. A particularly promising approach involves the construction of hybrid organic-molecule/silicon devices. But challenges remain--both in the formation of nanostructures that will constitute the active parts of future devices, and in the construction of commensurately small connecting wires. Atom-by-atom crafting of structures with scanning tunnelling microscopes, although essential to fundamental advances, is too slow for any practical fabrication process; self-assembly approaches may permit rapid fabrication, but lack the ability to control growth location and shape. Furthermore, molecular diffusion on silicon is greatly inhibited, thereby presenting a problem for self-assembly techniques. Here we report an approach for fabricating nanoscale organic structures on silicon surfaces, employing minimal intervention by the tip of a scanning tunnelling microscope and a spontaneous self-directed chemical growth process. We demonstrate growth of straight molecular styrene lines--each composed of many organic molecules--and the crystalline silicon substrate determines both the orientation of the lines and the molecular spacing within these lines. This process should, in principle, allow parallel fabrication of identical complex functional structures.

Generation of dense electron-hole plasmas in silicon

Abstract: Generation of dense electron-hole plasmas in silicon with intense 100-fs laser pulses is studied by time-resolved measurements of the optical reflectivity at 625 nm. For fluences F between $10 {\mathrm{m}\mathrm{J}/\mathrm{c}\mathrm{m}}^{2}lFl400 {\mathrm{m}\mathrm{J}/\mathrm{c}\mathrm{m}}^{2},$ plasma generation is dominated by strong two-photon absorption, and possibly higher-order nonlinearities, which lead to very steep spatial carrier distributions. The maximum carrier densities at the sample surface are in excess of ${10}^{22} {\mathrm{cm}}^{\ensuremath{-}3},$ and therefore, the reflectivity shows a mainly Drude-like free-carrier response. Within the Drude model, limits for the optical effective mass and the damping time are determined.

Silicon nanowire devices

Abstract: Transport measurements were carried out on 15–35 nm diameter silicon nanowires grown using SiH4 chemical vapor deposition via Au or Zn particle-nucleated vapor-liquid-solid growth at 440°C. Both Al and Ti/Au contacts to the wires were investigated. The wires, as produced, were essentially intrinsic, although Au nucleated wires exhibited a slightly higher conductance. Thermal treatment of the fabricated devices resulted in better electrical contacts, as well as diffusion of dopant atoms into the nanowires, and increased the nanowire conductance by as much as 10^4. Three terminal devices indicate that the doping of the wires is p type.

Fabrication and analysis of deep submicron strained-Si n-MOSFET's

Abstract: Deep submicron strained-Si n-MOSFETs were fabricated on strained Si/relaxed Si/sub 0.8/Ge/sub 0.2/ heterostructures. Epitaxial layer structures were designed to yield well-matched channel doping profiles after processing, allowing comparison of strained and unstrained Si surface channel devices. In spite of the high substrate doping and high vertical fields, the MOSFET mobility of the strained-Si devices is enhanced by 75% compared to that of the unstrained-Si control devices and the state-of-the-art universal MOSFET mobility. Although the strained and unstrained-Si MOSFETs exhibit very similar short-channel effects, the intrinsic transconductance of the strained Si devices is enhanced by roughly 60% for the entire channel length range investigated (1 to 0.1 /spl mu/m) when self-heating is reduced by an ac measurement technique. Comparison of the measured transconductance to hydrodynamic device simulations indicates that in addition to the increased low-field mobility, improved high-field transport in strained Si is necessary to explain the observed performance improvement. Reduced carrier-phonon scattering for electrons with average energies less than a few hundred meV accounts for the enhanced high-field electron transport in strained Si. Since strained Si provides device performance enhancements through changes in material properties rather than changes in device geometry and doping, strained Si is a promising candidate for improving the performance of Si CMOS technology without compromising the control of short channel effects.

Current status of environmental barrier coatings for Si-Based ceramics

Abstract: Silicon-based ceramics are the leading candidates for high temperature structural components in next generation gas turbine engines. One key drawback of silicon-based ceramics for such an application is volatilization of the protective silica scale in water vapor and the resulting rapid ceramic recession. Therefore, the realization of Si-based ceramics components in advanced gas turbine engines depends on the development of protection schemes from water vapor attack. Currently, plasma-sprayed external environmental barrier coatings (EBCs) are the most promising approach. In the late 1980s and early 1990s a wide range of refractory oxide materials were tested as coatings on Si-based ceramics to provide protection from hot corrosion. After the discovery of silica volatilization in water vapor in the early 1990s, the focus of EBC development research has been shifted towards the protection from water vapor attack. Experience learned form the earlier coating developmental effort provided the foundation upon which more complex and advanced EBC coatings have been developed. This paper will discuss the brief history and the current status of EBC development for Si-based ceramics with the main focus on water vapor protection.

Extraction current transients: new method of study of charge transport in microcrystalline silicon

Abstract: The transport properties of microcrystalline silicon, namely, mobility and conductivity, are investigated by a new method, for which the simple theory as well as numerical modeling is presented. The basic idea of the new method is verified on amorphous hydrogenated silicon by comparison with the widely used time-of-flight method. Contrary to time of flight, the new method can be used even for relatively conductive materials. Preliminary results on microcrystalline silicon clearly indicate the critical role of amorphouslike tissue in transport in microcrystalline silicon.

High-resolution depth profiling in ultrathin Al2O3 films on Si

Abstract: A combination of two complementary depth profiling techniques with sub-nm depth resolution, nuclear resonance profiling and medium energy ion scattering, and cross-sectional high-resolution transmission electron microscopy were used to study compositional and microstructural aspects of ultrathin (sub-10 nm) Al2O3 films on silicon. All three techniques demonstrate uniform continuous films of stoichiometric Al2O3 with abrupt interfaces. These film properties lead to the ability of making metal-oxide semiconductor devices with Al2O3 gate dielectric with equivalent electrical thickness in the sub-2 nm range.

Pulsed laser deposition of thin films

Abstract: The pulsed laser deposition is a new technique for the growth of thin films,which has been attended generally by people recently The physical principle, unique characteristics and the proceeding of the study were introduced briefly In addation, the result of the PtSi nanometer thin film based on silicon prepared by the pulsed laser deposition was describedPULS

Waveguiding in Planar Photonic Crystals

Abstract: Photonic crystal planar circuits designed and fabricated in silicon on silicon dioxide are demonstrated Our structures are based on two-dimensional confinement by photonic crystals in the plane of propagation, and total internal reflection to achieve confinement in the third dimension These circuits are shown to guide light at 1550 nm around sharp corners where the radius of curvature is similar to the wavelength of light

Method for fabricating a semiconductor structure including a metal oxide interface with silicon

Abstract: A method of fabricating a semiconductor structure including the steps of providing a silicon substrate (10) having a surface (12), forming on the surface (12) of the silicon substrate (10), by atomic layer deposition (ALD), a seed layer (20;20') characterised by a silicate material and forming, by atomic layer deposition (ALD) one or more layers of a high dielectric constant oxide (40) on the seed layer (20;20').

On the morphology and the electrochemical formation mechanism of mesoporous silicon

Abstract: Electrochemical pore formation in silicon electrodes is a well-known phenomenon. While micropore formation is commonly understood as due to quantum size effects, the formation of larger pores is dominated by the electric field of the space charge region. In contrast to the macropore regime which is well understood, little is known about the morphology and formation mechanism of mesopores. In this report mesopore morphology and its dependence on formation parameters, such as HF concentration, current density, bias, and substrate doping density, is investigated in detail. In addition, a simulation of the breakdown conditions at the pore tip is performed which shows that mesopore formation is dominated by charge carrier tunneling, while avalanche breakdown is found to be responsible for the formation of large etchpits.

Analytic description of short-channel effects in fully-depleted double-gate and cylindrical, surrounding-gate MOSFETs

Abstract: Short-channel effects in fully-depleted double-gate (DG) and cylindrical, surrounding-gate (Cyl) MOSFETs are governed by the electrostatic potential as confined by the gates, and thus by the device dimensions. The simple but powerful evanescent-mode analysis shows that the length /spl lambda/, over which the source and drain perturb the channel potential, is 1//spl pi/ of the effective device thickness in the double-gate case, and 1/4.810 of the effective diameter in the cylindrical case, in excellent agreement with PADRE device simulations. Thus for equivalent silicon and gate oxide thicknesses, evanescent-mode analysis indicates that Cyl-MOSFETs can be scaled to 35% shorter channel lengths than DG-MOSFETs.

Self-ordered pore structure of anodized aluminum on silicon and pattern transfer

Abstract: A practical approach of transferring a hexagonal array of nanosized pores produced in porous alumina into silicon and other substrates is discussed. The alumina pores have dimensions of 25–250 nm pore diameters and 50–300 nm pore spacings depending on the anodization conditions used. The characteristics of the alumina pores and the alumina–silicon interface are studied for different substrate materials and anodizing conditions. The unique structure of the barrier layer allows for the alumina to be directly used as an etch mask for pattern transfer into the silicon substrate.

Effect of phase transformations on the shape of the unloading curve in the nanoindentation of silicon

Abstract: Silicon wafers subject to depth-sensing indentation tests have been studied using Raman microspectroscopy. We report a strong correlation between the shape of the load-displacement curve and the phase transformations occurring within a nanoindentation. The results of Raman microanalysis of nanoindentations in silicon suggest that sudden volume change in the unloading part of the load-displacement curve (“pop-out” or “kink-back” effect) corresponds to the formation of Si–XII and Si–III phases, whereas the gradual slope change of the unloading curve (“elbow”) is due to the amorphization of silicon on pressure release. The transformation pressures obtained in nanoindentation tests are in agreement with the results of high pressure cell experiments.

Elucidation of the layer exchange mechanism in the formation of polycrystalline silicon by aluminum-induced crystallization

Abstract: Aluminum-induced crystallization of amorphous silicon is studied as a promising low-temperature alternative to solid-phase and laser crystallization. Its advantages for the formation of polycrystalline silicon on foreign substrates are the possible usage of simple techniques, such as thermal evaporation and dc magnetron sputtering deposition, and relatively short processing times in the range of 1 h. The overall process of the Al and Si layer exchange during annealing at temperatures below the eutectic temperature of 577 °C is investigated by various microscopy techniques. It is shown that the ratio of the Al and a-Si layer thicknesses is vitally important for the formation of continuous polycrystalline silicon films on glass substrates. The grain size of these films is dependent on the annealing temperature and evidence is given that grain sizes of 20 μm and more can be achieved. The poly-Si films are described as solid solutions containing 3×1019 cm−3 Al atoms as solute. Only a fraction of the solute is...

Stable zirconium silicate gate dielectrics deposited directly on silicon

Abstract: Zirconium silicate (ZrSixOy) gate dielectric films with ∼3–5 at. % Zr exhibit excellent electrical properties and high thermal stability in direct contact with Si. We demonstrate an equivalent oxide thickness of about 21 A for a 50 A ZrSixOy film sputter-deposited directly on a Si substrate, as measured by capacitance–voltage techniques, with a hysteresis shift less than 10 mV. Leakage currents for these films are very low, approximately 1×10−6 A/cm2 at 1.0 V bias in accumulation. Films ramped to hard breakdown exhibit breakdown fields Ebd ∼10 MV/cm. Excellent electrical properties are obtained with Au electrodes, in particular.

Subatomic Features on the Silicon (111)-(7×7) Surface Observed by Atomic Force Microscopy

Abstract: The atomic force microscope images surfaces by sensing the forces between a sharp tip and a sample. If the tip-sample interaction is dominated by short-range forces due to the formation of covalent bonds, the image of an individual atom should reflect the angular symmetry of the interaction. Here, we report on a distinct substructure in the images of individual adatoms on silicon (111)-(7x7), two crescents with a spherical envelope. The crescents are interpreted as images of two atomic orbitals of the front atom of the tip. Key for the observation of these subatomic features is a force-detection scheme with superior noise performance and enhanced sensitivity to short-range forces.

Iron contamination in silicon technology

Abstract: This article continues the review of fundamental physical properties of iron and its complexes in silicon (Appl. Phys. A 69, 13 (1999)), and is focused on ongoing applied research of iron in silicon technology. The first section of this article presents an analysis of the effect of iron on devices, including integrated circuits, power devices, and solar cells. Then, sources of unintentional iron contamination and reaction paths of iron during device manufacturing are discussed. Experimental techniques to measure trace contamination levels of iron in silicon, such as minority carrier lifetime techniques (SPV, μ-PCD, and ELYMAT), deep-level transient spectroscopy (DLTS), total X-ray fluorescence (TXRF) and vapor-phase decomposition TXRF (VPD-TXRF), atomic absorption spectroscopy (AAS), mass spectrometry and its modifications (SIMS, SNMS, ICP-MS), and neutron activation analysis (NAA) are reviewed in the second section of the article. Prospective analytical tools, such as heavy-ion backscattering spectroscopy (HIBS) and synchrotron-based X-ray microprobe techniques (XPS, XANES, XRF) are briefly discussed. The third section includes a discussion of the present achievements and challenges of the electrochemistry and physics of cleaning of silicon wafers, with an emphasis on removal of iron contamination from the wafers. Finally, the techniques for gettering of iron are presented.