Showing papers on "Amorphous silicon published in 2005"

Advances in organic field-effect transistors

Abstract: Since organic field-effect transistors (OFETs) were first described in 1987, they have undergone great progress, especially in the last several years. Nowadays, the performance of OFETs is similar to that of amorphous silicon (a-Si : H) devices and they have become one of the most important components of organic electronics. This feature article introduces briefly the operating principles, fabrication techniques of the transistors, and in particular highlights the recent progress, not only including materials and fabrication techniques, but also involving organic single crystal FETs and organic light-emitting FETs, which have been reported recently. Finally, the prospects and problems of OFETs that exist are discussed.

Mask material conversion

Abstract: The dimensions of mask patterns, such as pitch-multiplied spacers, are controlled by controlled growth of features in the patterns after they are formed. To form a pattern of pitch-multiplied spacers, a pattern of mandrels is first formed overlying a semiconductor substrate. Spacers are then formed on sidewalls of the mandrels by depositing a blanket layer of material over the mandrels and preferentially removing spacer material from horizontal surfaces. The mandrels are then selectively removed, leaving behind a pattern of freestanding spacers. The spacers comprise a material, such as polysilicon and amorphous silicon, known to increase in size upon being oxidized. The spacers are oxidized to grow them to a desired width. After reaching the desired width, the spacers can be used as a mask to pattern underlying layers and the substrate. Advantageously, because the spacers are grown by oxidation, thinner blanket layers can be deposited over the mandrels, thereby allowing the deposition of more conformal blanket layers and widening the process window for spacer formation.

The ultrasmoothness of diamond-like carbon surfaces.

Abstract: The ultrasmoothness of diamond-like carbon coatings is explained by an atomistic/continuum multiscale model. At the atomic scale, carbon ion impacts induce downhill currents in the top layer of a growing film. At the continuum scale, these currents cause a rapid smoothing of initially rough substrates by erosion of hills into neighboring hollows. The predicted surface evolution is in excellent agreement with atomic force microscopy measurements. This mechanism is general, as shown by similar simulations for amorphous silicon. It explains the recently reported smoothing of multilayers and amorphous transition metal oxide films and underlines the general importance of impact-induced downhill currents for ion deposition, polishing, and nanopattering.

A density-driven phase transition between semiconducting and metallic polyamorphs of silicon

Abstract: Amorphous and crystalline forms of silicon are well-known, tetrahedrally coordinated semiconductors. High-pressure studies have revealed extensive polymorphism among various metallic crystal structures containing atoms in six-, eight- and 12-fold coordination1,2. Melting silicon at ambient or high pressure results in a conducting liquid, in which the average coordination is greater than four (ref. 3). This liquid cannot normally be quenched to a glass, because of rapid crystallization to the diamond-structured semiconductor4. Solid amorphous silicon is obtained by synthesis routes such as chemical or physical vapour deposition that result in a tetrahedrally bonded semiconducting state. It has long been speculated that the amorphous solid and the liquid could represent two polymorphic forms of the amorphous state that are linked by density- or entropy-driven transformations5,6,7,8. Such polyamorphic transitions are recognized to occur among several different types of liquid and glassy systems9,10,11,12,13,14. Here we present experimental evidence for the occurrence of a density-driven polyamorphic transition between semiconducting and metallic forms of solid amorphous silicon. The experiments are combined with molecular dynamics simulations that map the behaviour of the amorphous solid on to that of the liquid state.

High-Performance Nanowire Electronics and Photonics and Nanoscale Patterning on Flexible Plastic Substrates

Abstract: The introduction of an ambient-temperature route for integrating high-mobility semiconductors on flexible substrates could enable the development of novel electronic and photonic devices with the potential to impact a broad spectrum of applications. Here we review our recent studies demonstrating that high-quality single-crystal nanowires (NWs) can be assembled onto flexible plastic substrates under ambient conditions to create FETs and light-emitting diodes. We also show that polymer substrates can be patterned through the use of a room temperature nanoimprint lithography technique for the general fabrication of hundred-nanometer scale features, which can be hierarchically patterned to the millimeter scale and integrated with semiconductor NWs to make high-performance FETs. The key to our approach is the separation of the high-temperature synthesis of single-crystal NWs from room temperature solution-based assembly, thus enabling fabrication of single-crystal devices on virtually any substrate. Silicon NW FETs on plastic substrates display mobilities of 200 cm/sup 2/-V/sup -1/-s/sup -1/, rivaling those of single-crystal silicon and exceeding those of state-of-the-art amorphous silicon and organic transistors currently used for flexible electronics. Furthermore, the generality of this bottom-up assembly approach suggests the integration of diverse nanoscale building blocks on a variety of substrates, potentially enabling far-reaching advances in lightweight display, mobile computing, and information storage applications.

Surface nanostructuring by nano-/femtosecond laser-assisted scanning force microscopy

Abstract: Surface nanostructuring with lateral resolutions beyond the capabilities of conventional optical lithography techniques was demonstrated in this study. Various nanoscopic surface features, such as grids, craters, and curves, were produced on thin metal and semiconductor films and bulk silicon by using the enhanced electric field underneath a proximity scanning probe tip irradiated with a laser beam. Nanoscale melting and crystallization of amorphous silicon films illustrates the capacity of the present scheme to provide an effective nanolaser source. Numerical simulations yield insight into the spatial distribution of the enhanced field intensity underneath the tip and associated physical phenomena. Calculations of the temperature distribution in the microprobe tip and possible tip expansion show that the main reason for the highly localized nanostructuring achieved with this technique is the enhancement of the electric field in the tip–sample gap. Possible applications of the developed nanostructuring process are anticipated in various nanotechnology fields.

20 1%-efficient Crystalline Silicon Solar Cell with Amorphous Silicon Rear-surface Passivation

Abstract: We have developed a crystalline silicon solar cell with amorphous silicon (a-Si:H) rear-surface passivation based on a simple process. The a-Si:H layer is deposited at 225°C by plasma-enhanced chemical vapor deposition. An aluminum grid is evaporated onto the a-Si:H-passivated rear. The base contacts are formed by COSIMA (contact formation to a-Si:H passivated wafers by means of annealing) when subsequently depositing the front silicon nitride layer at 325°C. The a-Si:H underneath the aluminum fingers dissolves completely within the aluminum and an ohmic contact to the base is formed. This contacting scheme results in a very low contact resistance of 3.5 ±0.2 mΩ cm2 on low-resistivity (0.5 Ω cm) p-type silicon, which is below that obtained for conventional Al/Si contacts. We achieve an independently confirmed energy conversion efficiency of 20.1% under one-sun standard testing conditions for a 4 cm2 large cell. Measurements of the internal quantum efficiency show an improved rear surface passivation compared with reference cells with a silicon nitride rear passivation. Copyright © 2005 John Wiley & Sons, Ltd.

Deformation mechanisms of silicon during nanoscratching

Abstract: PACS 62.20Fe, 61.50Ks, 61.43Dq, 63.20Mt, 61.72Ff, 68.37Hk The deformation mechanisms of silicon {001} surfaces during nanoscratching were found to depend strongly on the loading conditions. Nanoscratches with increasing load were performed at 2 µm/s (low ve-locity) and 100 µm/s (high velocity). The load-penetration-distance curves acquired during the scratching process at low velocity suggests that two deformation regimes can be defined, an elasto-plastic regime at low loads and a fully plastic regime at high loads. High resolution scanning electron microscopy of the damaged location shows that the residual scratch morphologies are strongly influenced by the scratch ve-locity and the applied load. Micro-Raman spectroscopy shows that after pressure release, the deformed volume inside the nanoscratch is mainly composed of amorphous silicon and Si-XII at low scratch speeds and of amorphous silicon at high speeds. Transmission electron microscopy shows that Si nanocrystals are embedded in an amorphous matrix at low speeds, whereas at high speeds the transformed zone is com-pletely amorphous. Furthermore, the extend of the transformed zone is almost independent of the scratch-ing speed and is delimited by a dislocation rich area that extends about as deep as the contact radius into the surface. To explain the observed phase and defect distribution a contact mechanics based decompres-sion model that takes into account the load, the velocity, the materials properties and the contact radius in scratching is proposed. It shows that the decompression rate is higher at low penetration depth, which is consistent with the observation of amorphous silicon in this case. The stress field under the tip is com-puted using an elastic contact mechanics model based on Hertz’s theory. The model explains the observed shape of the transformed zone and suggests that during load increase, phase transformation takes place prior to dislocation nucleation.

Flexible membrane pressure sensor

Abstract: A new flexible low-pressure sensor design with convention architectures of n-type doped hydrogenated amorphous silicon with metal-on-amorphous silicon contacts on flexible substrate is fabricated. The sensing elements are wired according to a full Wheatstone-bridge layout, to reduce any temperature effects. These low-pressure sensors are subjected to repetitive strains/pressure testing. The experiment demonstrates a linear pressure relationship in the 0–2.0 psi range with a sensitivity of 1.953 ± 0.020 mV / psi . The measurements observed are in good agreement with the analytical solution.

Real-time monitoring and process control in amorphous∕crystalline silicon heterojunction solar cells by spectroscopic ellipsometry and infrared spectroscopy

Abstract: In amorphous∕crystalline silicon heterojunction solar cells, we have performed real-time thickness control of hydrogenated amorphous silicon (a‐Si:H) layers with a precision better than ±1A by applying spectroscopic ellipsometry (SE). A heterojunction solar cell fabricated by this process shows a relatively high conversion efficiency of 14.5%. At the amorphous∕crystalline interface, however, infrared attenuated total reflection spectroscopy (ATR) revealed the formation of a porous a‐Si:H layer with a large SiH2-hydrogen content of 27 at. %. Based on SE and ATR results, we discuss the growth processes and structures of a‐Si:H in heterojunction solar cells.

Large open-circuit voltage improvement by rapid thermal annealing of evaporated solid-phase-crystallized thin-film silicon solar cells on glass

Abstract: In this letter, we investigate the impact of rapid thermal annealing (RTA) on thin-film polycrystalline silicon (pc-Si) solar cells on glass made by evaporation of amorphous silicon (a-Si) and subsequent solid-phase crystallization (SPC). These devices have the potential to deliver low-cost photovoltaic electricity and are named EVA cells (SPC of EVAporated a-Si). The RTA is used to perform a high-temperature (>700°C) process for point defect annealing and dopant activation. RTA processes have predominantly been developed for wafer-based devices yet also have great potential for low-temperature devices such as thin-film pc-Si on glass solar cells. Parameter variations are performed on EVA solar cells to determine optimum values for point defect removal and dopant activation while minimizing dopant diffusion causing junction smearing. The 1-Sun open-circuit voltage, Voc, of the as-crystallized pc-Si devices is rather modest (135mV). However, after RTA and subsequent hydrogen passivation in a rf PECVD plasm...

Threshold voltage instability of amorphous silicon thin-film transistors under constant current stress

Abstract: We investigate the time-dependent shift in the threshold voltage of amorphous silicon thin-film transistor stressed with constant drain current. We observe a nonsaturating power-law time dependence, which is in contrast to the conventional stretched exponential that saturates at prolonged stress time. The result is consistent with the carrier-induced defect creation model and corroborates the nonlinear dependence of the rate of defect creation on the band-tail carrier density.

Conducting polymer and hydrogenated amorphous silicon hybrid solar cells

Abstract: An organic-inorganic hybrid solar cell with a p-i-n stack structure has been investigated. The p-layer was a spin coated film of PEDOT:PSS [poly(3,4-ethylenedioxythiophene) poly (styrenesulfonate)]. The i-layer was hydrogenated amorphous silicon (a-Si:H), and the n-layer was microcrystalline silicon (μc-Si). The inorganic layers were deposited on top of the organic layer by the hot-wire chemical vapor deposition technique at 200°C. These hybrid devices exhibited open circuit voltages (VOC) as large as 0.88V and solar conversion efficiencies as large as 2.1%. Comparison of these devices with those incorporating a-SiC:H:B p-layers indicates that the organic layer is acting as an electrically ideal p-layer.

Thickness dependence of properties of excimer laser crystallized nano-polycrystalline silicon

Abstract: Excimer laser crystallization is used to produce layered nanocrystalline silicon from hydrogenated amorphous silicon, using a partial melting process. Three types of hydrogenated amorphous silicon samples, 100, 300, and 500 nm thick, were laser treated in order to investigate the changes to the structural, optical, and electrical properties as a function of amorphous silicon thickness with excimer laser crystallization. The resulting nanocrystalline thin films were characterized using Raman spectroscopy, optical absorption measurements, atomic force microscopy, forward recoil spectrometry, and current–voltage measurements. The relationship of crystalline volume and laser energy density was established, along with the behavior of the optical gap and its relationship to hydrogen content. Surface roughness effects are discussed in the context of photovoltaic applications. The effect of increased mobility on photoconductivity after excimer laser crystallization is also examined.

Microprocessing of ITO and a-Si thin films using ns laser sources

Abstract: Selective ablation of thin films for the development of new photovoltaic panels and sensoring devices based on amorphous silicon (a-Si) is an emerging field, in which laser micromachining systems appear as appropriate tools for process development and device fabrication. In particular, a promising application is the development of purely photovoltaic position sensors. Standard p–i–n or Schottky configurations using transparent conductive oxides (TCO), a-Si and metals are especially well suited for these applications, appearing selective laser ablation as an ideal process for controlled material patterning and isolation. In this work a detailed study of laser ablation of a widely used TCO, indium-tin-oxide (ITO), and a-Si thin films of different thicknesses is presented, with special emphasis on the morphological analysis of the generated grooves. Excimer (KrF, λ = 248 nm) and DPSS lasers (λ = 355 and λ = 1064 nm) with nanosecond pulse duration have been used for material patterning. Confocal laser scanning microscopy (CLSM) and scanning electron microscopy (SEM) techniques have been applied for the characterization of the ablated grooves. Additionally, process parametric windows have been determined in order to assess this technology as potentially competitive to standard photolithographic processes. The encouraging results obtained, with well-defined ablation grooves having thicknesses in the order of 10 µm both in ITO and in a-Si, open up the possibility of developing a high-performance double Schottky photovoltaic matrix position sensor.

The role of carbon in unexpected visco(an)elastic behavior of amorphous silicon oxycarbide above 1273 K

Abstract: Polymer-derived silicon oxycarbide is ostensibly amorphous, yet we show that it exhibits high temperature anelasticity that has so far been seen only in crystalline structures. The time dependency, and the magnitude of strain recovery upon unloading, are non-linear. Indeed the pattern is reminiscent of polymers where viscoelasticity arises from the time dependent but recoverable movement of one-dimensional carbon chains that are intertwined and cross-linked to some degree. In analogy we propose that viscoelasticity in this polymer-derived ceramic arises from an intertwined network of two-dimensional graphene sheets .

A method for the analysis of multiphase bonding structures in amorphous SiOxNy films

Abstract: A tetrahedral model is presented to explain the bonding properties of nonstoichiometric amorphous silicon oxynitride (a-SiOxNy) alloys, grown under highly nonequilibrium conditions, whose structures obey neither the random bonding model nor the random mixture model Based on our approach, a numerical procedure is proposed to obtain the relative atomic percentages of each component structural phase from the deconvolution of the high-resolution x-ray photoelectron spectroscopy (XPS) spectra in the Si 2p3∕2 region The tetrahedral model is then used to study the bonding properties of a-SiOxNy films grown by electron-cyclotron resonance plasma-enhanced chemical-vapor deposition, having relatively low values of the O/Si atomic ratio (⩽037) incorporated in their networks The experimental results show that five tetrahedral phases (tetrahedrons Si–Si4, Si–Si2ON, Si–N4, Si–Si3O, and Si–O4) are present in a-SiOxNy films with low N/Si atomic ratios (⩽093), while only three phases (Si–SiON2, Si–N4, and Si–O2N2) ar

Diamond-silicon carbide composite

Abstract: Fully dense, diamond-silicon carbide composites are prepared from ball-milled microcrystalline diamond/amorphous silicon powder mixture. The ball-milled powder is sintered (P=5-8 GPa, T=1400K-2300K) to form composites having high fracture toughness. A composite made at 5 GPa/1673K had a measured fracture toughness of 12 MPa.m 1/2 . By contrast, liquid infiltration of silicon into diamond powder at 5 GPa/1673K produces a composite with higher hardness but lower fracture toughness. X-ray diffraction patterns and Raman spectra indicate that amorphous silicon is partially transformed into nanocrystalline silicon at 5 GPa/873K, and nanocrystalline silicon carbide forms at higher temperatures.

Study of low-temperature crystallization of amorphous Si films obtained using ferritin with Ni nanoparticles

Abstract: A polycrystalline silicon thin film with a high crystallinity was obtained using ferritin with a Ni core (7nm), which enabled us to precisely control the density and position of the nucleus for crystal growth. The core density of ferritin adsorbed on the amorphous silicon surface was controlled in the range from 109cm−2to1011cm−2. Crystal growth was performed at 550°C in N2. Crystallinity or grain size strongly depended on Ni core density. Polycrystalline silicon film with the average grain size of 3μm and a high crystallinity was obtained at a low Ni atom density of 1012cm−2.

Printing Methods and Materials for Large-Area Electronic Devices

Abstract: Two digital printing methods for the fabrication of active matrix thin-film transistor (AM-TFT) backplanes for displays are described. A process using printed resists layers, referred to as digital lithography, was used to fabricate arrays of hydrogenated amorphous silicon TFTs. TFTs were also fabricated using a combination of digital lithography to pattern metals and inkjet printing to pattern and deposit a polymeric semiconducting layer. The relative performance of amorphous silicon and polymer TFTs were evaluated. The utility of digital lithographic processing was demonstrated by the fabrication of prototype reflective displays using electrophoretic media.

Stress control in masking layers for deep wet micromachining of Pyrex glass

Abstract: In this paper, we report three etching masking technologies of Cr/Au, Cr/Cu and PECVD amorphous silicon developed for Pyrex glass micromachining in hydrofluoric acid solution. Our study reveals that the residual stress, especially the tensile stress, in the mask layers is responsible for the pinholes on the glass surface and notch defects on the edge formed at the etch edges of glass due to the breakage of highly stressed mask layers during the etching process. The Cr/Au metal mask can achieve a glass etch depth up to 100 μm, along with a number of pinholes and notch defects on the edge due to the high tensile residual stress in the Cr/Au layer. The Cr/Cu metal masking layer improves the glass etch quality by the reduced residual stress. Detailed studies have been done using the amorphous silicon film as a glass etch mask. The PECVD process and the subsequent annealing process have been optimized to reduce the compressive residual stress in the amorphous silicon layer. The maximum etch depth in the glass can be as high as 200 μm and almost without pinholes and notch defects on the edge. To our knowledge, this is the best result reported in the literature so far.