Showing papers on "Amorphous silicon published in 2007"

Reversible Cycling of Crystalline Silicon Powder

Abstract: A method is described in which crystalline silicon can be used as a practical anode material for lithium-ion batteries. Commercial lithium-ion cells are typically charged at a constant current to a fixed voltage and then are held by the charger at constant voltage until the current decreases to a certain value (also known as constant current/constant voltage or CCCV charging). It is first shown that CCCV charging can be used to reversibly cycle crystalline silicon and limit its capacity. A cycling method is then demonstrated in which crystalline silicon is first partially converted to amorphous silicon, in situ, during conditioning cycles. After the conditioning cycles the silicon can be cycled normally, using CCCV cycling limits, with good coulombic efficiency and little overlithiation during the first cycle.

Silicon nanowire solar cells

Abstract: Silicon nanowire-based solar cells on metal foil are described. The key benefits of such devices are discussed, followed by optical reflectance, current-voltage, and external quantum efficiency data for a cell design employing a thin amorphous silicon layer deposited on the nanowire array to form the p-n junction. A promising current density of ∼1.6mA∕cm2 for 1.8cm2 cells was obtained, and a broad external quantum efficiency was measured with a maximum value of ∼12% at 690nm. The optical reflectance of the silicon nanowire solar cells is reduced by one to two orders of magnitude compared to planar cells.

Method and system for improving dielectric film quality for void free gap fill

Abstract: A method of forming a silicon oxide layer on a substrate. The method includes providing a substrate and forming a first silicon oxide layer overlying at least a portion of the substrate, the first silicon oxide layer including residual water, hydroxyl groups, and carbon species. The method further includes exposing the first silicon oxide layer to a plurality of silicon-containing species to form a plurality of amorphous silicon components being partially intermixed with the first silicon oxide layer. Additionally, the method includes annealing the first silicon oxide layer partially intermixed with the plurality of amorphous silicon components in an oxidative environment to form a second silicon oxide layer on the substrate. At least a portion of amorphous silicon components are oxidized to become part of the second silicon oxide layer and unreacted residual hydroxyl groups and carbon species in the second silicon oxide layer are substantially removed.

Interaction potential for silicon carbide: A molecular dynamics study of elastic constants and vibrational density of states for crystalline and amorphous silicon carbide

Abstract: An effective interatomic interaction potential for SiC is proposed. The potential consists of two-body and three-body covalent interactions. The two-body potential includes steric repulsions due to atomic sizes, Coulomb interactions resulting from charge transfer between atoms, charge-induced dipole-interactions due to the electronic polarizability of ions, and induced dipole-dipole (van der Waals) interactions. The covalent characters of the Si–C–Si and C–Si–C bonds are described by the three-body potential. The proposed three-body interaction potential is a modification of the Stillinger-Weber form proposed to describe Si. Using the molecular dynamics method, the interaction potential is used to study structural, elastic, and dynamical properties of crystalline (3C), amorphous, and liquid states of SiC for several densities and temperatures. The structural energy for cubic (3C) structure has the lowest energy, followed by the wurtzite (2H) and rock-salt (RS) structures. The pressure for the structural transformation from 3C-to-RS from the common tangent is found to be 90 GPa. For 3C-SiC, our computed elastic constants (C11, C12, and C44), melting temperature, vibrational density-of-states, and specific heat agree well with the experiments. Predictions are made for the elastic constant as a function of density for the crystalline and amorphous phase. Structural correlations, such as pair distribution function and neutron and x-ray static structure factors are calculated for the amorphous and liquid state.

Organic small molecule field-effect transistors with Cytop™ gate dielectric: Eliminating gate bias stress effects

Abstract: Organic field-effect transistors with unprecedented resistance against gate bias stress are described. The single crystal and thin-film transistors employ the organic gate dielectric Cytop™. This fluoropolymer is highly water repellent and shows a remarkable electrical breakdown strength. The single crystal transistors are consistently of very high electrical quality: near zero onset, very steep subthreshold swing [average: 1.3nFV∕(decadecm2)] and negligible current hysteresis. Furthermore, extended gate bias stress only leads to marginal changes in the transfer characteristics. It appears that there is no conceptual limitation for the stability of organic semiconductors in contrast to hydrogenated amorphous silicon.

A Low-Temperature-Grown Oxide Diode as a New Switch Element for High-Density, Nonvolatile Memories†

Abstract: A one-bit cell of a general nonvolatile memory consists of a memory element and a switch element. Several memory elements have been tried given that any bistable states, that is, two charging states, two spin states, or two resistance states, can be used for a memory element. On the other hand, silicon-based transistors have been the most popularly used switch element. However, silicon-based transistors do not conform to high-density, nonvolatile memories with three-dimensional (3D) stack structures due to their high processing temperatures and the difficulty of growing high-quality epitaxial silicon over metals. Here, we show a low-temperaturegrown oxide diode, Pt/p-NiOx/n-TiOx/Pt, applied as a switch element for high-density, nonvolatile memories. The diode exhibits good rectifying characteristics at room temperature: a rectifying ratio of 10 at ± 3 V, a forward current density of up to ∼ 5×10 A cm, an ideality factor of 4.3, and a turn-on voltage of 2 V. Furthermore, we verify its ability to allow and deny access to the Pt/NiO/Pt memory element with two stable resistance states. Under the forward-bias condition, we could access the memory element and change the resistance state, although access was denied under the reverse bias condition. This one-diode/one-resistor (1D/1R) structure could be a promising building block for high-density, nonvolatile random-access memories with 3D stack structures. A p–n diode, like a transistor, is a fundamental circuit element for thin-film electronics. Until now, epitaxial silicon was most frequently used to fabricate p–n diodes in electronic devices with planar structures. However, to increase device density further, we require p–n diodes that are applicable to devices with 3D stack structures. Epitaxial silicon-based p–n diodes cannot be fabricated with stack structures as it is difficult to grow on a metal layer and high processing temperatures are required. On the other hand, although amorphous silicon allows for lower processing temperatures, it does not provide the required semiconducting performance. Therefore, to realize high-density electronic devices with 3D stack structures, we need new p–n diodes composed of semiconducting materials with low processing temperatures and high performance. In particular, new p–n diodes with low processing temperatures and high performance are indispensable to high-density, nonvolatile random-access memory devices. By replacing a transistor with a simpler diode as a switch element, there exists the possibility of producing memory cells with cross-point structures composed of bit lines and word lines perpendicular to each other, with a memory element lying between them. Theoretically, by utilizing this cross-point structure, the cell size can be scaled down to 4F (F: feature size used for patterning the cell), which is the smallest cell size attainable in nonvolatile memories with planar structures. Furthermore, by fabricating 3D stacks of the cross-point structure, the effective cell size can be scaled down to 2F, 1F, and so on. A common issue in realizing a cross-point structure is the availability of a thin-film diode with the high rectifying ratio and current density required for the switch element to access the memory element. Oxide based p–n diodes are good candidates to provide solutions to the issues associated with Si-based diodes. Most oxides, such as TiO2, [4] ZrO2, [5] ZnO, and indium tin oxide (ITO), are well-known n-type semiconductors that are characterized by the electron-transport properties of oxygen vacancies. As NiOx is a well-known p-type semiconductor beC O M M U N IC A IO N

Photovoltaic and photoelectrochemical conversion of solar energy.

Abstract: The Sun provides approximately 100,000 terawatts to the Earth which is about 10000 times more than the present rate of the world's present energy consumption. Photovoltaic cells are being increasingly used to tap into this huge resource and will play a key role in future sustainable energy systems. So far, solid-state junction devices, usually made of silicon, crystalline or amorphous, and profiting from the experience and material availability resulting from the semiconductor industry, have dominated photovoltaic solar energy converters. These systems have by now attained a mature state serving a rapidly growing market, expected to rise to 300 GW by 2030. However, the cost of photovoltaic electricity production is still too high to be competitive with nuclear or fossil energy. Thin film photovoltaic cells made of CuInSe or CdTe are being increasingly employed along with amorphous silicon. The recently discovered cells based on mesoscopic inorganic or organic semiconductors commonly referred to as 'bulk' junctions due to their three-dimensional structure are very attractive alternatives which offer the prospect of very low cost fabrication. The prototype of this family of devices is the dye-sensitized solar cell (DSC), which accomplishes the optical absorption and the charge separation processes by the association of a sensitizer as light-absorbing material with a wide band gap semiconductor of mesoporous or nanocrystalline morphology. Research is booming also in the area of third generation photovoltaic cells where multi-junction devices and a recent breakthrough concerning multiple carrier generation in quantum dot absorbers offer promising perspectives.

Dynamics of Threshold Voltage Shifts in Organic and Amorphous Silicon Field‐Effect Transistors

Abstract: Progress in environmental stability and processability, and the increase of the field-effect mobility of organic semiconductors has triggered their use as the active element in microelectronic devices. The advantages of their application are the easy processing, for example, spin-coating and ink-jet printing, without a temperature hierarchy, and their mechanical flexibility. Applications are foreseen in the field of large-area electronics where numerous devices are integrated on low-cost substrates such as plastics. The first flexible, even rollable, quarter video graphics array (QVGA) active matrix displays based on organic semiconductors have already been reported.[1]

Abruptness of a-Si :H/c-Si interface revealed by carrier lifetime measurements

Abstract: Intrinsic hydrogenated amorphous silicon films can yield outstanding electronic surface passivation of crystalline silicon wafers. In this letter the authors confirm that this is strongly determined by the abruptness of the interface. For completely amorphous films the passivation quality improves by annealing at temperatures up to 260°C, most likely by film relaxation. This is different when an epitaxial layer has been grown at the interface during film deposition. Annealing is in such a case detrimental for the passivation. Consequently, the authors argue that annealing followed by carrier lifetime measurements allows determining whether the interface is abrupt.

Organic small molecule field-effect transistors with Cytop(TM) gate dielectric: eliminating gate bias stress effects

Abstract: We report on organic field-effect transistors with unprecedented resistance against gate bias stress. The single crystal and thin-film transistors employ the organic gate dielectric Cytop(TM). This fluoropolymer is highly water repellent and shows a remarkable electrical breakdown strength. The single crystal transistors are consistently of very high electrical quality: near zero onset, very steep subthreshold swing (average: 1.3 nF V/(dec cm2)) and negligible current hysteresis. Furthermore, extended gate bias stress only leads to marginal changes in the transfer characteristics. It appears that there is no conceptual limitation for the stability of organic semiconductors in contrast to hydrogenated amorphous silicon.

Impact of epitaxial growth at the heterointerface of a-Si:H∕c-Si solar cells

Abstract: The authors have demonstrated that interface structures of heterojunction solar cells consisting of hydrogenated amorphous silicon (a-Si:H) and crystalline silicon (c-Si) have quite large impact on the solar cell performance. In particular, unintentional epitaxial growth was found to occur during an intended a-Si:H i-layer growth on c-Si in plasma-enhanced chemical vapor deposition (PECVD). By the formation of the epitaxial layer at the interface, the solar cell efficiency decreases significantly. Their result shows that the epitaxial layer is formed rather easily in PECVD, even without the presence of H2 gas, and may have affected many previous studies on a-Si:H∕c-Si solar cells seriously.

Effects of a‐Si:H layer thicknesses on the performance of a‐Si:H∕c‐Si heterojunction solar cells

Abstract: We have fabricated hydrogenated amorphous silicon (a‐Si:H)∕crystalline silicon (c‐Si) heterojunction solar cells with different a‐Si:H layer thicknesses, in order to determine effects of a‐Si:H layer thicknesses on the performance of a‐Si:H∕c‐Si solar cells The thicknesses of a‐Si:H p‐i layers formed on a n-type c‐Si substrate were controlled accurately on the atomic scale by applying real-time spectroscopic ellipsometry during the a‐Si:H growth With increasing a‐Si:H p‐i layer thicknesses, the open-circuit voltage (Voc) and fill factor increase drastically up to 40A (i layer) and 30A (p layer), whereas the short-circuit current density (Jsc) reduces gradually By using optimum a‐Si:H layer thicknesses (i∕p=40∕30A), we obtained a solar cell efficiency of 161% without incorporating surface texture and a back-surface field structure Quite interestingly, the optimum a‐Si:H i-layer thickness (40A) shows good correlation with a SiH2-rich interface structure formed at the a‐Si:H∕c‐Si heterointerface, sugges

Thin-film silicon solar cells with efficient periodic light trapping texture

Abstract: For solar cells based on thin-film microcrystalline (μc-Si:H) or amorphous silicon (a-Si:H) with absorber layers in the micrometer range, highly effective light trapping and an optimal incoupling of the entire sun spectrum are essential. To investigate and optimize both effects the wave propagation in thin-film silicon solar cells is modeled in three dimensions (3D) solving the Maxwell equations rigorously. A periodic nanostructured texture is investigated as an alternative to the common randomly rough texture. Inverted 3D pyramids with a periodicity of 850nm and structure height of 400nm show promising high quantum efficiencies close to the Tiedje limit.

In Situ Conductivity, Impedance Spectroscopy, and Ex Situ Raman Spectra of Amorphous Silicon during the Insertion/Extraction of Lithium

Abstract: The electrical conductivity and the impedance behavior of thin layers of amorphous silicon (a-Si), which are promising anode materials for lithium-ion batteries, were monitored in situ during the insertion/extraction of lithium in 1 M of a LiBOB (Li-bioxalato borate) propylene carbonate solution. In addition, Raman spectra of the same electrodes were recorded in situ and ex situ during lithiation/delithiation processes in the above-mentioned solutions. The conductivity of the a-Si electrode was increased by about 3.5 orders of magnitude during the course of lithium insertion. While the impedance response of these electrodes is complicated and cannot be resolved unambiguously, it is clear that the electrical conductivity influences strongly the electrodes' impedance: a similar dependence of the electrical conductivity and the impedance of these electrodes on the potential are measured. The intensity of the Raman signal dropped significantly upon lithiation and recovered at a potential of 0.523 V vs Li/Li+...

Amorphous IZO TTFTs with saturation mobilities exceeding 100 cm2/Vs

Abstract: In this paper we demonstrate the use of amorphous binary In2O3–ZnO oxides simultaneously as active channel layer and as source/drain regions in transparent thin film transistor (TTFT), processed at room temperature by rf sputtering. The TTFTs operate in the enhancement mode and their performances are thickness dependent. The best TTFTs exhibit saturation mobilities higher than 102 cm2/Vs, threshold voltages lower than 6 V, gate voltage swing of 0.8 V/dec and an on/off current ratio of 107. This mobility is at least two orders of magnitude higher than that of conventional amorphous silicon TFTs and comparable to or even better than other polycrystalline semiconductors. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

High Ambipolar Mobility in a Highly Ordered Smectic Phase of a Dialkylphenylterthiophene Derivative That Can Be Applied to Solution‐Processed Organic Field‐Effect Transistors

Abstract: Since the 1990s, we have witnessed remarkable progress in organic semiconductor technology. [1] In particular, reasonably high carrier mobilities, exceeding those of amorphous silicon, were observed in thin-film transistors fabricated from a single crystal of rubrene. [2] In general, it is difficult to fabricate single crystals of aromatic compounds; therefore, zone-melt and Bridgeman crystal-growth [3] or vacuum crystal-growth techniques [4] are indispensable. Polycrystalline thin films are relatively easy to fabricate and suitable for practical devices. High carrier mobilities—of the order of 1 cm 2 V –1 s –1 —have been observed in field-effect transistor (FET) devices based on polycrystalline pentacene thin films. [5] However, defects and domain boundaries affect the carrier transport in aromatic polycrystalline thin films; therefore, the crystal growth under the vacuum process is rigorously controlled. [6] Device fabrication with a more practical solution process has been investigated. As well as conjugated polymers, [7] precursor methods in which thin films fabricated using soluble precursors are transformed to polycrystalline thin films by thermal treatment, [8] and solution-processable pentacene and anthradithiophene derivatives, which form polycrystalline thin films using a spin-coat method, have been investigated. [9] The field-effect mobilities in these studies are of the order of 10 –2 cm 2 , and the carrier mobility is increased up to 0.1 ≈ 1c m 2 V –1 s –1 by thermal treatment. [8,9] The optimum mobility is lower than those of the FET devices fabricated using vacuum deposition; the device characteristics strongly depend upon the film morphology, because the organic semiconductor thin films fabricated by the solution process have many defects and exhibit low carrier mobility.

Interdigitated back contact silicon heterojunction solar cell and the effect of front surface passivation

Abstract: This letter reports interdigitated back contact silicon heterojunction (IBC-SHJ) solar cells which combine the performance benefits of both back contact and heterojunction technologies while reducing their limitations. Low temperature (<200°C) deposited p- and n-type amorphous silicon used to form interdigitated heteroemitter and contacts in the rear preserves substrate lifetime while minimizes optical losses in the front. The IBC-SHJ structure is ideal for diagnosing surface passivation quality, which is analyzed and measured by internal quantum efficiency and minority carrier lifetime measurements. Initial cells have independently confirmed efficiency of 11.8% under AM1.5 illumination. Simulations indicate efficiencies greater than 20% after optimization.

Solar cell having doped semiconductor heterojunction contacts

Abstract: A silicon solar cell has doped amorphous silicon contacts formed on a tunnel silicon oxide layer on a surface of a silicon substrate. High temperature processing is unnecessary in fabricating the solar cell.

Ultralong single-crystal metallic Ni2Si nanowires with low resistivity.

Abstract: Ultralong, single-crystal Ni2Si nanowires sheathed with amorphous silicon oxide were synthesized on a large scale by a chemical vapor transport (CVT) method, using iodine as the transport reagent and Ni2Si powder as the source material. Structural characterization using powder X-ray diffraction, electron microscopy, and energy-dispersive spectroscopy shows that the nanowires have Ni2Si−SiOx core−shell structure with single-crystal Ni2Si core and amorphous silicon oxide shell. The oxide shell is electrically insulating and can be removed by HF etching. Four-terminal electrical measurements show that the single-crystal nanowire has extremely low resistivity of 21 μΩ·cm and is capable of supporting remarkably high failure current density >108 A/cm2. These unique Ni2Si nanowires are very attractive nanoscale building blocks for interconnects and fully silicided (FUSI) gate applications in nanoelectronics.

Application of hydrogenated amorphous silicon oxide layers to c-Si heterojunction solar cells

Abstract: Wide-gap hydrogenated amorphous silicon oxide (a-SiO:H), fabricated by plasma process using SiH4 and CO2 gas mixture, has been applied to crystalline silicon (c-Si) heterojunction solar cells It has been demonstrated that incorporation of an a-SiO:H p layer, instead of a hydrogenated amorphous silicon (a-Si:H) p layer, improves the conversion efficiency slightly Moreover, when an a-SiO:H i layer is formed on the c-Si substrate, Si epitaxial growth that occurs at an a-Si:H∕c-Si heterointerface at high deposition temperatures can be prevented entirely Accordingly, high-efficiency solar cells are fabricated more easily by applying a-SiO:H p-i layers to n-type c-Si heterojunction solar cells

Fast Thin-Film Transistor Circuits Based on Amorphous Oxide Semiconductor

Abstract: Five-stage ring oscillators (ROs) composed of amorphous In/Ga/Zn/O (a-IGZO) channel thin-film transistors (TFTs) with the channel lengths of 10 mum were fabricated on a glass substrate. The a-IGZO layer was deposited by RF magnetron sputtering onto the unheated substrate. The RO operated at 410 kHz (the propagation delay of 0.24 mus/stage), when supplied with an external voltage of +18 V. This is the fastest integrated circuit based on oxide-semiconductor channel TFTs to date that operates faster than the ROs using conventional hydrogenated amorphous silicon TFTs and organic TFTs

Preparation of superhydrophobic silicon oxide nanowire surfaces.

Abstract: The paper reports on the preparation of superhydrophobic amorphous silicon oxide nanowires (a-SiONWs) on silicon substrates with a contact angle greater than 150° by means of surface roughness and self-assembly. Nanowires with an average mean diameter in the range 20−150 nm and 15−20 μm in length were obtained by the so-called solid−liquid−solid (SLS) technique. The porous nature and the high roughness of the resulting surfaces were confirmed by AFM imaging. The superhydrophobicity resulted from the combined effects of surface roughness and chemical modification with fluorodecyl trichlorosilane.

Thin-Film Encapsulation of Organic Light-Emitting Devices

Abstract: This project seeks to extend the lifetime of organic light-emitting devices (OLEDs) by blocking the transmission of ambient oxygen and moisture into the delicate emissive region of the device. Our approach is to encapsulate the organic device with hybrid thin-film stacks of both organic and inorganic materials. It is projected that, for commercial applications, such an encapsulation scheme will need to reduce the water vapor transmission to 1 x10 -6 g/m 2 -day and the oxygen transmission to 1 x10 -5 cm 3 (STP)/m 2 -day. We have developed a low-temperature (100oC) PECVD (ltPECVD) process for both amorphous silicon nitride and amorphous silicon oxide layers which yield optically transparent layers (>85% transmission) with low stress (-1.6 x10 9

Microwave Annealing of Polymer Photovoltaic Devices

Abstract: Organic photovoltaic (OPV) devices are receiving increasing attention because of their potential application for solar energy conversion. The advantages of using OPV devices over inorganic systems are mechanical flexibility, light weight, low cost, and fabrication at low temperature. Since the discovery of ultra-fast photoinduced charge separation between conjugated polymers and fullerenes, organic solar cells prepared from polymer semiconductors have been studied extensively. The so-called “bulk heterojunction” structure is commonly used on account of its simple device structure and thin-film processability. Recently, the external quantum efficiency (EQE) of OPV devices prepared from poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C-61-butyric acid methyl ester (PCBM) was improved through treatment thermally and/or under an electric field. In addition, Li et al. reported a solvent annealing method that achieved a remarkable efficiency (4.4 %). Meanwhile, Ma et al. improved the efficiency to 5 % through post-thermal annealing. Apparently, the annealing process is a key step toward obtaining high power conversion efficiency (PCE). Although thermal annealing is conventionally implemented through thermal conduction methods, such as the use of hotplates or thermal ovens, the energy loss and low efficiency of energy usage during such processes can be problematic. Because the degree of microwave absorption depends on the rotation of the dipoles of a material, microwave annealing, which can be used to heat materials selectively, is a potential approach toward enhancing the efficiency of energy usage. Moreover, microwave heating is a non-contact, rapid heating process; for example, the heating rate for amorphous carbon powders smaller than 1 lm can reach 1258 °C min at room temperature under microwave irradiation at 2.45 GHz. In addition, microwave-assisted annealing and sintering processes have been applied to improve the crystallization of amorphous silicon. Only a few studies, however, have focused on the behavior of conjugated polymers under microwave irradiation. In this paper, we describe the application of microwave annealing to enhance the efficiency of polymer OPV devices (Fig. 1). The unique selectivity and short annealing times might make this method suitable for the efficient industrial production of OPV devices.

Optically actuated thermocapillary movement of gas bubbles on an absorbing substrate

Abstract: The authors demonstrate an optical manipulation mechanism of gas bubbles for microfluidic applications. Air bubbles in a silicone oil medium are manipulated via thermocapillary forces generated by the absorption of a laser in an amorphous silicon thin film. In contrast to previous demonstrations of optically controlled thermally driven bubble movement, transparent liquids can be used, as the thermal gradient is formed from laser absorption in the amorphous silicon substrate, and not in the liquid. A variety of bubbles with volumes ranging from 19 pl to 23 nl was transported at measured velocities of up to 1.5 mm/s.