Thin Amorphous Silicon Research Articles

Electrodeposition of amorphous silicon anode for lithium ion batteries

Amorphous silicon has been successfully electrodeposited on copper using a SiCl4 based organic electrolyte under galvanostatic conditions. The electrodeposited silicon films were characterized for their composition, morphology and structural characteristics using glancing angle X-ray diffraction (GAXRD), scanning electron microscopy (SEM), and Raman spectroscopy. GAXRD and Raman analyses clearly confirm the amorphous state of the deposited silicon film. The deposited films were tested for possible application as anodes for Li-ion battery. The results indicate that this binder free amorphous silicon anode exhibits a reversible capacity of ∼1300 mAh g−1 with a columbic efficiency of >99.5% up to 100 cycles. Impedance measurements at the end of each charge cycle show a non-variable charge transfer resistance which contributes to the excellent cyclability over 100 cycles observed for the films. This approach of developing thin amorphous silicon films directly on copper eliminates the use of binders and conducting additives, rendering the process simple, facile and easily amenable for large scale manufacturing.

Preface: Phys. Status Solidi C 6/2012

AbstractThis proceedings issue is a selective collection of papers presented at Symposium B “Amorphous Nanostructured Materials” which was conducted during the E‐MRS Fall Meeting 2011, Warsaw, Poland. The symposium program covered various aspects of amorphous materials science and technology, including the preparation of materials and structures, the characterization and study of their properties (structural, optical, electrical, magnetic), the performance of devices. The objective of the symposium was to put together leading scientists working in the field and to update the research achievements in the area.Among the most substantial issues, discussions at the symposium highlighted the importance of structures and materials that combine amorphous and crystalline phases or that allow phase transitions between such phases. These structures have new physical properties and open the way to new potential applications in the fields of information and energy, in particular for information storage, light emission, and solar cells. Great emphasis was put on silicon‐based materials and structures, either from low temperature (amorphous, polymorphous, microcrystalline silicon) or high temperature processes (silicon nanoparticles and nanocrystals). Recent progress has been achieved in understanding the doping and defect properties of amorphous silicon, as well as the behavior of heterostructures comprising thin amorphous silicon films. Light emission properties of silicon nanoclusters and silicon‐based materials like rare‐earth implanted amorphous silicon dioxide and silicon oxicarbide layers were also underlined. A carrier multiplication effect in silicon nanocrystals embedded in an amorphous silicon oxide matrix was reported, implying the relaxation of photogenerated hot carriers not only at photon absorption sites but also through energy transfer into neighboring nanocrystals with creation of additional electron–hole pairs. Utilization of this effect should improve the efficiency of silicon‐based solar cells. Future trends in materials physics include the study of silicon–germanium nanocrystals and the use for hybrid silicon–organic electronics. In particular, silicon nanocrystals could be used in conjunction with organic semiconductors for hybrid bulk heterojunction solar cells. Studies of other nanostructured silicon‐based devices such as silicon heterojunction and silicon nanowires solar cells were also presented.Considerable attention was paid to the fabrication and properties of metallic nanostructures. In particular, a new method was reported allowing production of amorphous metal nanoparticles that, contrary to their crystalline counterparts, do not coagulate when approaching each other. This unusual property permitted fabrication of thin films consisting of closely packed but still individual nanoparticles. When made of magnetic materials, the characteristics of such films were shown to be determined by magnetic exchange interaction between the particles combined with particle self‐organization. As a result, the blocking temperature of the films was found to be close to room temperature, which is a property being important in magnetic storage application.Related to the above‐mentioned subjects, the issue content is organized as follows. First four articles are devoted to different aspects of light emission from silicon‐based materials (silicon oxicarbide, silicon and metal oxide clusters). The second group of three articles deals with correlation between structure and electrical properties of the materials based on amorphous silicon. Next, four articles consider technology and structure formation mechanisms of amorphous silicon. The following three articles touch upon the problem of silicon‐based solar cells. The last group of four articles deals with fabrication and properties of promising amorphous chalcogenides, metals, metal oxides, and carbon materials.All contributing authors are greatly acknowledged for their input to this issue.The symposium was sponsored by INCATTECH LLC (the company is located in St. Petersburg, Russia, www.incattech.com).

Structural investigations of silicon nanostructures grown by self-organized island formation for photovoltaic applications

The self-organized growth of crystalline silicon nanodots and their structural characteristics are investigated. For the nanodot synthesis, thin amorphous silicon (a-Si) lay- ers with different thicknesses have been deposited onto the ultrathin (2 nm) oxidized (111) surface of Si wafers by elec- tron beam evaporation under ultrahigh vacuum conditions. The solid phase crystallization of the initial layer is induced by a subsequent in situ annealing step at 700 °C, which leads to the dewetting of the initial a-Si layer. This process results in the self-organized formation of highly crystalline Si nan- odot islands. Scanning electron microscopy confirms that size, shape, and planar distribution of the nanodots depend on the thickness of the initial a-Si layer. Cross-sectional in- vestigations reveal a single-crystalline structure of the nan- odots. This characteristic is observed as long as the thick-

CW laser induced crystallization of thin amorphous silicon films deposited by EBE and PECVD

This work presents the Continuous Wave (CW) laser crystallization of thin amorphous silicon (a-Si) films deposited by Plasma Enhanced Chemical Vapor Deposition (PECVD) and by Electron Beam Evaporation (EBE) on low cost glass substrate. The films are characterized by Elastic Recoil Detection Analysis (ERDA) and by Fourier-Transform Infrared (FTIR) spectroscopy to evaluate the hydrogen content. Analysis shows that the PECVD films contain a high hydrogen concentration (∼10at.%) while the EBE films are almost hydrogen-free. It is found that the hydrogen is in a bonding configuration with the a-Si network and in a free form, requiring a long thermal annealing for exodiffusion before the laser treatment to avoid explosive effusion. The CW laser crystallization process of the amorphous silicon films was operated in liquid phase regime. We show by Electron Backscatter Diffraction (EBSD) that polysilicon films with large grains can be obtained with EBE as well as for the PECVD amorphous silicon provided that for the latest the hydrogen content is lower than 2at.%.

Energy and exergy analysis of a building integrated semitransparent photovoltaic thermal (BISPVT) system

In this paper, a study has been carried out to evaluate the energy and exergy performance of a building integrated semitransparent photovoltaic thermal (BISPVT) system integrated to the roof of a room. Comparisons have been carried out on the basis of energy and exergy by considering six different photovoltaic (PV) modules. It is observed that maximum annual electrical energy is 810kWh for heterojunction comprised of a thin amorphous silicon (a-Si) PV cell on top of a crystalline silicon (c-Si) cell (also known as HIT) and the maximum annual thermal energy is 464kWh for a-Si. It is also concluded that HIT PV module is suitable for producing electrical power whereas a-Si is suitable for space heating. However, an annual overall thermal energy (2497kWh) and exergy (834kWh) is maximum for HIT PV module.

Highly Scaled Poly-Silicon Channel Vertical SONOS Cell for Ultra High Density NAND Technology

Highly scaled vertical cylindrical cell with 22nm diameter bi-layer poly-silicon channel for 3D NAND Flash memory is successfully developed. A thin amorphous silicon layer along with the oxide-nitride-oxide (ONO) gate stack inside the memory hole was introduced. This additional silicon layer protects the tunnel oxide during opening of the gate stack at the bottom of the memory hole. The smallest working cells have been fabricated with feature size F down to 45 nm corresponding to an equivalent 11nm planar cell technology node for the case of 16 stacked cells. We also explore, what is the equivalent feature size (FEQ) that can be reached by layer stacking, taking the inevitable process restrictions into consideration.

High-efficiency Silicon Heterojunction Solar Cells: A Review

Abstract Silicon heterojunction solar cells consist of thin amorphous silicon layers deposited on crystalline silicon wafers. This design enables energy conversion efficiencies above 20% at the industrial production level. The key feature of this technology is that the metal contacts, which are highly recombination active in traditional, diffused-junction cells, are electronically separated from the absorber by insertion of a wider bandgap layer. This enables the record open-circuit voltages typically associated with heterojunction devices without the need for expensive patterning techniques. This article reviews the salient points of this technology. First, we briefly elucidate device characteristics. This is followed by a discussion of each processing step, device operation, and device stability and industrial upscaling, including the fabrication of solar cells with energy-conversion efficiencies over 21%. Finally, future trends are pointed out.

Investigation of the corrosion behaviour of steel coated with amorphous silicon carbon alloys

Thin amorphous silicon (a-Si:H) and carbonated silicon (a-Si1−xCx:H) layers were deposited on stainless steel substrates using plasma-enhanced chemical vapour deposition (PECVD) in a “low-power” regime. The carbon content of the carbonated silicon (a-Si1−xCx:H) alloys was varied between 0.1 and 0.37at.%. The performance of these interfaces as effective corrosion barriers in 3% sodium chloride aqueous solutions was evaluated. Potentiodynamic polarisation curves and electrochemical impedance spectroscopy (EIS) were used next to X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) to investigate the protection efficiency of the different barriers. The a-Si:H coated showed better corrosion resistance as compared to the carbonated silicon alloys. No degradation was observed after 14days immersion of the steel substrate coated with a-Si:H in 3% sodium chloride aqueous solution, making this coating an attractive candidate as corrosion barrier.

Preparation of Large Size Pyramidal Texture on N-Type Monocrystalline Silicon Using TMAH Solution for Heterojunction Solar Cells

Pyramidal texture is one traditional method to realize antireflection for c-Si solar cells, due to its low cost and simplicity. As one high efficiency silicon solar cell, amorphous/crystalline silicon heterojunction (SHJ) solar cell has attracted much attention all over the world. The heterojunction interface with very low defects and interface states is critical to the SHJ solar cell performance. In order to obtain high quality interface passivation by depositing a very thin intrinsic amorphous silicon layer on the textured Si conformally, large size pyramidal texture with no metal ion contamination is required. In this work, we utilized tetra-methyl ammonium hydroxide (TMAH) instead of NaOH in the alkaline etching to prepare pyramidal texture on N-type monocrystalline silicon to avoid the possible Na+ contamination. By optimizing the etching conditions, uniform large size pyramidal texture with pyramid size of about 10 μm was fabricated successfully. Furthermore, excellent antireflection performance was demonstrated on such textured Si surface. The average reflectance was lower than 10% in the visible and near infrared spectrum range. Such pyramidally textured Si wafers will be very suitable for SHJ solar cells.

Optimisation of p-doped μc-Si:H Emitter Layers in Crystalline-amorphous Silicon Heterojunction Solar Cells

Heterojunction silicon wafer solar cells, using a microcrystalline silicon (μc-Si:H) thin-film emitter and a very thin intrinsic amorphous silicon (a-Si:H) passivation layer between the crystalline silicon wafer and the emitter layer, have been reported to exhibit stable performance and high efficiency. Desired properties for the emitter layer include wide bandgap, low surface and interface recombination, and good doping efficiency. In this study, we report on the thin-film properties of p-doped μc-Si:H emitter layers deposited using RF (13.56MHz) PECVD, at different SiH4/H2 gas flow ratios, pressures, and temperatures at the same RF power. Trends relating deposition conditions to relevant film characteristics such as thickness, crystalline fraction and conductivity are discussed. Finally, device relevant symmetrical p+/i/c-Si/i/p+ heterojunction lifetime test structures are investigated, using the optimised parameters for p-doped μc-Si:H layers (discussed in this paper) and for intrinsic a-Si:H layers (discussed in a companion paper [1]). These exhibit promising effective lifetimes of up to 2.4ms at an injection level of 1015cm-3.

Synthesis, Characterization and Kinetics of Epitaxial-Oriented Silicon Nanowire Arrays on Si Substrates

Abstract The fabrication of vertical-oriented, high aspect ratio silicon nanowires (SiNWs) with controllable density and length is of interest for the development of nanowire-based electronics and photovoltaic devices. Here we reported a both simple and economical method for synthesizing large-area epitaxial-oriented SiNW arrays, which was achieved on the Si (111) substrates by Au catalyzed vapor-liquid-solid mechanism using the conventional chemical vapor deposition furnace system. Their morphologies and microstructures were investigated with scanning electron microscopy and transmission electron microscopy, respectively. The results showed that most of nanowires were vertically grown on substrates, their density and length can be well controlled. As-grown SiNW is composed of a single crystalline silicon core and a thin amorphous silicon oxide coating layer. Furthermore, their growth kinetics was discussed in detail. It indicates that there are both the substrate-nanowire Si adatom surface diffusion and the slight radial growth during the upgrowth of nanowire, and besides, the migration of Au on the sidewall of nanowire was also found for such epitaxial-oriental SiNWs.

Investigation of the Superconducting Properties of Nb Filmscovered by PECVD a-Si:H Layers for Superconducting Qubit Application

Abstract Hydrogenated amorphous silicon (a-Si:H) grown by PECVD has a lower loss tangent (tan _) among conventional dielectrics (such as SiO2 and SiNx) and hence is considered as the best amorphous dielectric material for superconducting qubit application. The incorporation of PECVD a-Si:H into the Nb technology requires attention due to the possible degradation of the superconductivity of the Nb films. Superconducting transition temperature (Tc) and residual resistivity (_0) of 20 nm, 50 nm and 100 nm thick Nb films were measured before and after a-Si:H deposition. The penetration of oxygen and hydrogen inside the Nb films was evaluated from the variation of the lattice parameter obtained by X-ray di_raction. The high process temperature (250_ C) and the presence of energetic hydrogen ions during the a-Si:H layer growth caused a decrease of Tc and increase of _0 through two physical processes: 1) oxygen di_usion from the surface Nb oxides and 2) hydrogen di_usion inside the Nb films. The degradation of Tc was reduced with the increase of the film thickness. Nitridation of Nb films and deposition of a sputtered thin amorphous silicon layer (a-Si) on the Nb films (in both cases made in situ after the Nb film deposition) were investigated a s surface treatments to protect the Nb filmsduring PECVD. It was demonstrated that both methods markedly reduce oxygen and hydrogen di_usion into Nb films during a-Si:H deposition, but the a-Si layer was more e_ective to protect the Nb films.

Comparison of growth methods for Si/SiO2 nanostructures as nanodot hetero-emitters for photovoltaic applications

Two different growth mechanisms are compared for the fabrication of Si/SiO2 nanostructures on crystalline silicon (c-Si) to be used as hetero-emitter in high-efficiency solar cells: (1) The decomposition of substoichiometric amorphous SiOx (a-SiOx) films with 0<x<1.3 and (2) the dewetting of thin amorphous silicon (a-Si) layers.The grown layers are investigated with regard to their structural properties, their passivation quality for c-Si wafer substrates and their electrical properties in order to evaluate their suitability as a nanodot hetero-emitter. While by layer decomposition, no passivating nanodots could be formed, the dewetting process allows fabricating nanodot passivation layers at temperatures as low as 600°C. The series resistance through Ag/[Si-nanodots in SiO2]/c-Si/Al structures for dewetting is similar to nanostructured silicon rich SiOx films. Still, a nanodot hetero-emitter which exhibits both a satisfying passivation of the substrate and induces a high band bending by doping at the same time could not be fabricated yet.

Highly Scaled Vertical Cylindrical SONOS Cell With Bilayer Polysilicon Channel for 3-D nand Flash Memory

A vertical cylindrical SONOS cell with a novel bilayer polysilicon channel down to 22-nm diameter for 3-D NAND Flash memory is successfully developed. We introduce a thin amorphous silicon layer along with the oxide-nitride-oxide (ONO) gate stack inside the memory hole. This silicon layer protects the tunnel oxide during opening of the gate stack at the bottom of the memory hole, after which it serves as the first layer of the bilayer polysilicon channel. This approach enables the 3-D architecture to achieve minimum cell area (4 <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">F</i> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , with <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">F</i> being the feature size) without the need for the so-called pipeline connections. The smallest functional cells have the memory hole diameter <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">F</i> = 45 nm, resulting in 22-nm channel diameter. In case 16 cells are stacked, <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">F</i> = 45 nm would correspond to an equivalent 11-nm planar cell technology node. Excellent program/erase and retention obtained with the all-deposited ONO stack are demonstrated.

Improved amorphous/crystalline silicon interface passivation by hydrogen plasma treatment

Silicon heterojunction solar cells have high open-circuit voltages thanks to excellent passivation of the wafer surfaces by thin intrinsic amorphous silicon (a-Si:H) layers deposited by plasma-enhanced chemical vapor deposition. We show a dramatic improvement in passivation when H2 plasma treatments are used during film deposition. Although the bulk of the a-Si:H layers is slightly more disordered after H2 treatment, the hydrogenation of the wafer/film interface is nevertheless improved with as-deposited layers. Employing H2 treatments, 4 cm2 heterojunction solar cells were produced with industry-compatible processes, yielding open-circuit voltages up to 725 mV and aperture area efficiencies up to 21%.

Sub-nm equivalent oxide thickness on Si-passivated GaAs capacitors with low Dit

A thin amorphous silicon interlayer, inserted between the III-V semiconductor and the gate dielectric is expected to prevent III-V oxidation, as required for high-mobility channel transistors. We demonstrate that the addition of a thin Al2O3 barrier layer between the a-Si and the high-k HfO2, together with optimized post-metallization annealing, is the key to reduce the a-Si consumption and to achieve a highly scaled gate stack with equivalent oxide thickness of ∼0.8 nm. The evolution of the interfaces during growth and the quality of the stack are investigated by in-situ X-ray photoelectron spectroscopy and electrical measurements on metal-oxide-semiconductors capacitors.

Oxidation of the surface of a thin amorphous silicon film

The oxidation of clean crystalline silicon surfaces is self-limiting at moderate oxygen pressures (10 − 5 Pa) and temperatures (500 °C), forming 0.7–0.8 nm thick oxide layers. This study looks at the oxidation of a surface of a thin amorphous silicon film to establish if a similar mechanism is active in this case. We have devised a special experimental procedure to check the oxidation mechanism of thin amorphous silicon films. For the spectroscopic investigations we used photoemission with synchrotron radiation with the highest possible surface sensitivity and resolution. This permits a detailed decomposition of the Si 2p spectral details, using a mathematical decomposition procedure. The results clearly show that the oxidation mechanism of the surface of an amorphous silicon film under similar conditions is severely hindered compared to cases of crystalline substrates, indicating less reactivity at the surface and less transport of oxygen into the amorphous material.

A comparative study on different textured surfaces passivated with amorphous silicon

AbstractIn this work it is studied the influence of different texturizations on minority carrier lifetime of n‐type Fz silicon wafers passivated with thin amorphous silicon layers (a‐Si:H). For amorphous‐crystalline silicon based heterojunction solar cells a good light trapping is essential. Therefore a front side texturization is needed as it can increase the photocurrent due to its low reflectance. The interface between the amorphous emitter and the crystalline base plays an important role for two reasons: firstly, the micromorphic surface topology can generate different deposition regimes that results in different lifetimes, and secondly, the nanomorphic structure also show influences on the lifetime. We compare two different types of wet chemical texturizations by different carbonate solutions and a plasma texturization subsequently covered by a thin layer of a‐Si:H by means of plasma‐enhanced chemical vapor deposition. The surface morphology produced by the texturization on the silicon wafers is analyzed by scanning electron microscopy and atomic force microscopy. The quality of the passivation of the textured silicon surface by the a‐Si:H deposition is evaluated by measuring minority carrier lifetimes with a microwave photo‐conductance decay and quasi steady state photo‐conductance devices. (© 2011 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Modeling of CW laser diode irradiation of amorphous silicon films

The purpose of this work is to determine the optimal parameters required to crystallize thin amorphous silicon films on glass substrate with a continuous wave (CW) laser diode ( λ = 808 nm), using a numerical model developed in COMSOL Multiphysics. The numerical simulation of the laser crystallization process takes into account the solid–liquid phase change and the difference between the melting temperature of amorphous (Tm a-Si = 1420 K) and that of crystalline silicon (Tm c-Si = 1690 K). We have varied the main parameters controlling the crystallization process, namely the power and the scan speed of the laser beam. Furthermore the initial temperature as well as the thickness of the a-Si:H layer were also taken as a parameter to optimize the process. We have determined the melting, crystallization and ablation energy threshold versus the different operational parameters.

The silane depletion fraction as an indicator for the amorphous/crystalline silicon interface passivation quality

In silicon heterojunction solar cells, thin amorphous silicon layers passivate the crystalline silicon wafer surfaces. By using in situ diagnostics during plasma-enhanced chemical vapor deposition (PECVD), the authors report how the passivation quality of such layers directly relate to the plasma conditions. Good interface passivation is obtained from highly depleted silane plasmas. Based upon this finding, layers deposited in a large-area very high frequency (40.68 MHz) PECVD reactor were optimized for heterojunction solar cells, yielding aperture efficiencies up to 20.3% on 4 cm2 cells.