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Studies the metastability under high-intensity illumination of device quality a-Si:H thin films deposited over a wide range of conditions. Formation of metastable mobile hydrogen; Deep trapping of mobile hydrogen; Mechanism proposed for the formation of the bonds.

Understanding and controlling effectively the behavior of metal catalyst droplets during the Vapor-Liquid-Solid growth of nanowires are crucial for their applications. In this work, silicon nanowires are produced by plasma-assisted Chemical Vapor Deposition using gold as a catalyst. The influence of hydrogen plasma on nanowire growth is investigated experimentally and theoretically. Interestingly, in contrast to conventional chemical vapor deposition, the growth rate of silicon nanowires shows a decrease as a function of their diameters, which is consistent with the incorporation of silicon via sidewall diffusion. We show that Ostwald ripening of catalyst droplets during nanowire growth is inhibited in the presence of a hydrogen plasma. However, when the plasma is off, the diffusion of Au atoms on the nanowire sidewall can take place. Based on this observation, we have developed a convenient method to grow silicon nanotrees. [ABSTRACT FROM AUTHOR]

Examines the optical and electronic properties of intrinsic and doped hydrogenated amorphous silicon films. Comparison of the film properties in the as-deposited and annealed states; Effects of post-deposition annealing on the growth of hydrogenated silicon; Decrease of the metastable defect density in the intrinsic films.

In this review, we report several rational designs of nanowire-based solar cells from single nanowire to nanowire arrays. Two methods of nanowires fabrication: via ‘top-down’ and ‘bottom-up’, and two types of configurations including axial and radial junction are presented for nanowire-based solar cells. To enhance absorption, several photon management schemes are shown in detail, including anti-reflection coating, diffractive grating, and plasmonics. Considering the rational design of nanowire arrays, we summarize a total of seven solar cell structures including axial junctions, radial junctions, substrate interfacial junctions, planar junctions, conductors, junctionless and tandem. Each type is supported by examples which are presented and discussed. Finally, a general comparison between bulk and nanowire solar cell efficiencies is given. [ABSTRACT FROM AUTHOR]

Heterojunction solar cells based on crystalline silicon (c-Si) passivated by hydrogenated amorphous silicon (a-Si:H) thin films are one of the most promising architectures for high energy conversion efficiency. Indeed, a-Si:H thin films can passivate both p-type and n-type wafers and can be deposited at low temperature (<200°C) using PECVD. However, such passivation layers, in particular ptype a-Si:H, show a dramatic degradation in passivation quality above 200°C. Yet, annealing at 300 - 400°C the TCO layer and metallic contacts is highly desirable to reduce the contact resistance as well as the TCO optical absorption. In this work, we show that as expected, ion implantation (5 - 30 keV) introduces defects at the c-Si/a-Si:H interface which strongly degrade the effective lifetime, down to a few micro-seconds. However, the passivation quality can be restored and lifetime values can be improved up to 2 ms over the initial value with annealing. We show here that effective lifetimes above 1 ms can be maintained up to 380°C, opening up the possibility for higher process temperatures in silicon heterojunction device fabrication. [ABSTRACT FROM AUTHOR]

Cells are subjected to multiple mechanical inputs throughout their lives. Their ability to detect these environmental cues is called mechanosensing, a process in which integrins play an important role. During cellular mechanosensing, plasma membrane (PM) tension is adjusted to mechanical stress through the buffering action of caveolae; however, little is known about the role of caveolae in early integrin mechanosensing regulation. Here, we show that Cav1KO fibroblasts increase adhesion to FN-coated beads when pulled with magnetic tweezers, as compared to wild type fibroblasts. This phenotype is Rho-independent and mainly derived from increased active β1-integrin content on the surface of Cav1KO fibroblasts. Florescence recovery after photobleaching analysis and endocytosis/recycling assays revealed that active β1-integrin is mostly endocytosed through the clathrin independent carrier/glycosylphosphatidyl inositol (GPI)-enriched endocytic compartment pathway and is more rapidly recycled to the PM in Cav1KO fibroblasts, in a Rab4 and PM tension-dependent manner. Moreover, the threshold for PM tension-driven β1-integrin activation is lower in Cav1KO mouse embryonic fibroblasts (MEFs) than in wild type MEFs, through a mechanism dependent on talin activity. Our findings suggest that caveolae couple mechanical stress to integrin cycling and activation, thereby regulating the early steps of the cellular mechanosensing response. [ABSTRACT FROM AUTHOR]

Hydrogenated polymorphous silicon (pm-Si:H) is a nanostructured material consisting of silicon nanocrystals embedded in an amorphous silicon matrix. Its use as the intrinsic layer in thin film p-i-n solar cells has led to good cell properties in terms of stability and efficiency. Here, we have been able to assess directly the concentration and distribution of nanocrystals and impurities (dopants) in p-i-n solar cells, by using femtosecond laser-assisted atom probe tomography (APT). An effective sample preparation method for APT characterization is developed. Based on the difference in atomic density between hydrogenated amorphous and crystalline silicon, we are able to distinguish the nanocrystals from the amorphous matrix by using APT. Moreover, thanks to the three-dimensional reconstruction, we demonstrate that Si nanocrystals are homogeneously distributed in the entire intrinsic layer of the solar cell. The influence of the process pressure on the incorporation of nanocrystals and their distribution is also investigated. Thanks to APT we could determine crystalline fractions as low as 4.2% in the pm-Si:H films, which is very difficult to determine by standard techniques, such as X-ray diffraction, Raman spectroscopy, and spectroscopic ellipsometry. Moreover, we also demonstrate a sharp p/i interface in our solar cells. [ABSTRACT FROM AUTHOR]

• Silicon nanowires (NWs) grown on various substrates via Vapor–Liquid–Solid method. • Best crystallized SiNWs obtained using tin catalyst on silicon germanium substrate. • Morphology and crystalline nature strongly depend on the junction substrate-catalyst. • Substrate surface energy affects growth by regulating catalyst repartition. The plasma-assisted Vapor-Liquid-Solid technique has been used to grow Silicon nanowires (SiNWs) on different substrates coated with 1 nm of tin and indium, to study the impact of the surface energy of the substrates on the morphology and structure of the NWs. The characteristics of the prepared Si NWs were analyzed by field emission scanning electron microscopy, X-ray diffraction (XRD) and Raman spectroscopy. The morphological characteristics differ in the density, diameter and length of the synthesized SiNWs. The mean diameters of these SiNWs range from 36 to 51 nm at the bottom and from 15 nm to 34 nm at the tip, while their average length varies from 0.2 μm up to 0.54 μm depending on the substrate and catalyst. The presence of strong peaks in all XRD patterns indicates that the as-grown SiNWs are highly crystalline. The XRD and Raman characteristics of the Sn and In catalyzed SiNWs formed on different substrates show that the crystalline grain size is influenced by the substrate nature and catalyst material, which can be related to the substrate surface energy. In particular shorter NWs with improved crystallinity (larger grain size) are obtained when using Sn as a catalyst on an amorphous silicon-germanium (a-SiGe) substrates, while the small grains were obtained on the same substrate when indium was used as a catalyst. [ABSTRACT FROM AUTHOR]

In and Sn are the type of catalysts which do not introduce deep level electrical defects within the bandgap of germanium (Ge). However, Ge nanowires produced using these catalysts usually have a large diameter, a tapered morphology, and mixed crystalline and amorphous phases. In this study, we show that plasma-assisted vaporâ€"liquidâ€"solid (PA-VLS) method can be used to synthesize Ge nanowires. Moreover, at certain parameter domains, the sidewall deposition issues of this synthesis method can be avoided and long, thin tapering-free monocrystalline Ge nanowires can be obtained with In and Sn catalysts. We find two quite different parameter domains where Ge nanowire growth can occur via PA-VLS using In and Sn catalysts: (i) a low temperature-low pressure domain, below ∼235 °C at a GeH4 partial pressure of ∼6 mTorr, where supersaturation in the catalyst occurs thanks to the low solubility of Ge in the catalysts, and (ii) a high temperature-high pressure domain, at ∼400 °C and a GeH4 partial pressure above ∼20 mTorr, where supersaturation occurs thanks to the high GeH4 concentration. While growth at 235 °C results in tapered short wires, operating at 400 °C enables cylindrical nanowire growth. With the increase of growth temperature, the crystalline structure of the nanowires changes from multi-crystalline to mono-crystalline and their growth rate increases from ∼0.3 nm sâˆ'1 to 5 nm sâˆ'1. The cylindrical Ge nanowires grown at 400°C usually have a length of few microns and a radius of around 10 nm, which is well below the Bohr exciton radius in bulk Ge (24.3 nm). To explain the growth mechanism, a detailed growth model based on the key chemical reactions is provided. [ABSTRACT FROM AUTHOR]

Contemporary externally bonded structural upgrading schemes for masonry structures employ Fiber Reinforced Polymer (FRP) systems (a technique that was extended from concrete to masonry structures) and technical textiles (structural fiber grids) embedded in inorganic matrices. The latter account for a multitude of systems depending on the type of grid - fiber material, bundle treatment (dry, coated or even impregnated), grid geometry, manufacturing method etc. - and matrix - binder (e.g. cement or lime), rheology etc. - resulting in the derivation of many different acronyms (FRCM, TRM, CMG, IMG or other - see for definitions below). The mechanical behavior of such systems and their interaction with different substrates may vary significantly (e.g. dry vs. impregnated fiber grids embedded in mortars). This paper aims to summarize all reported efforts to increase the load-carrying and/or deformation capacity of unreinforced masonry walls against in-plane loading and second-order phenomena (eccentric compressive loading). [ABSTRACT FROM AUTHOR]

The effect of wettability on theundesirable bundling of siliconnanowire (SiNW) arrays fabricated by metal-assisted chemical etching(MACE) method is investigated. This paper reports a simple and low-costapproach to achieve dense SiNW arrays with excellent lateral separation.A hydrophilic pretreatment on the initial wafer substrate prior tothe etching procedure, followed by a hydrophobic post-treatment ofthe fabricated SiNWs, allows the fabrication of large and dense arraysof SiNWs with no agglomeration. These results are discussed withinthe framework of the detailed balance of forces acting on the nanowires. [ABSTRACT FROM AUTHOR]

Abstract: Nano-structured silicon thin films have been grown on tin-doped indium oxide (ITO) by Plasma-Enhanced Chemical Vapor Deposition (PECVD) at temperatures lower than 200 °C. Nanometer-scaled aggregates of metal (copper or gold) obtained from evaporated layers were necessary to initiate the nano-structuring growth. Different deposition conditions have been investigated. The highest aspect ratio was obtained with copper and high-pressure plasmas with SiH4 diluted in H2. The metals help dissociating silane so the deposition starts faster on the aggregates than around them, which leads to nano-structuration. It is likely that the metal remains confined at the interface between ITO and silicon and do not diffuse in the silicon layer. [Copyright &y& Elsevier]

Abstract: Bottom gate microcrystalline silicon thin film transistors (μc-Si:H TFT) have been fabricated at three different deposition temperatures (150, 200 and 250°C) for the μc-Si layer. We found that the linear field effect mobility increases from 0.1 to 0.44cm2/Vs by decreasing the temperature from 250°C to 150°C, and that the leakage current of TFTs with μc-Si deposited at 150°C is lower than that of μc-Si:H deposited at 250°C. Moreover, there is no influence of the deposition temperature on the stability of μc-Si:H TFTs. The improvement of the electrical characteristics at lower deposition temperatures is discussed in terms of a lower concentration of donor active oxygen atoms at lower temperature. [Copyright &y& Elsevier]

Abstract: Polymorphous silicon carbon alloys (pm-Si1−x C x :H) have been prepared from the decomposition of SiH4–CH4–H2 mixtures at 200°C in a standard capacitively coupled RF glow discharge reactor at high pressure, under conditions where silicon clusters, nanocrystals and their agglomerates are produced in the plasma and contribute to the deposition. Visible photoluminescence (PL) was observed at room temperature in these materials in the spectral region from red to orange. The PL intensity and peak position show a strong dependence on the deposition pressure. Moreover, electroluminescent (EL) diodes with p-i-n structure were realized using the optimized materials as the i-layer. EL spectra, comparable to the PL ones, were obtained by applying a DC voltage above 8V. EL is stable for several hours of continuous operation which makes these devices interesting for applications. [Copyright &y& Elsevier]

Alloying Ge with Sn is one of the promising ways for achieving Si compatible optoelectronics. Here, GeSn nanowires (NWs) are realized via nano-crystallization of a hydrogenated amorphous Ge (a-Ge:H) layer with the help of metal Sn droplets. The full process consists of three steps: (1) SnO2 nanoparticle (NP) reduction in a hydrogen plasma to produce Sn catalyst; (2) a-Ge:H deposition at 120 °C and (3) annealing. GeSn alloys with rich morphologies such as discrete nanocrystals (NCs), random, and straight NWs were successfully synthesized by changing process conditions. We show that annealing under Ar plasma favors the elaboration of straight GeSn NWs in contrast to the conventional random GeSn NWs obtained when annealing is performed under a H2 atmosphere. Interestingly, GeSn in the form of discrete NCs can be fabricated during the deposition of a-Ge:H at 180 °C. Even more, the synthesis of out-of-plane GeSn NWs has been demonstrated by reversing the deposition sequence of SnO2 NPs and a-Ge:H layer. [ABSTRACT FROM AUTHOR]

A 1D fluid model including a detailed chemistry for silane-hydrogen discharges as well as surface reactions to account for deposition and etching processes has been implemented to study the effects of gas pressure (1 to 3.5 Torr) and silane concentration (2 to 10%) on the deposition rate of silicon thin films in a standard RF-PECVD reactor. The thickness of the films and their deposition rate as functions of the process conditions were determined from optical modelling of UV-visible spectroscopic ellipsometry measurements. The experimental values of the deposition rate were compared with results from the 1D fluid model. SiH3 radicals are found to be the main contributor to the computed deposition rates, while H3+ ions play the main role in the etching process. The study reveals that etching by hydrogen ions must be taken into account to reproduce properly the experimental deposition rate. In particular etching by H3+ ions must be taken into account to achieve a good agreement between the experimental and modelled values of the deposition rate as a function of the total gas pressure and the silane fraction in the discharge. [ABSTRACT FROM AUTHOR]

When atomic hydrogen interacts with hydrogenated amorphous silicon (a-Si:H), the induced modifications are of crucial importance during a-Si:H based devices manufacturing or processing. In the case of hydrogen plasma, the depth of the modified zone depends not only on the plasma processing parameters but also on the material. In this work, we exposed a-Si:H thin films to H² plasma just after their deposition. In situ UV-visible spectroscopic ellipsometry measurements were performed to track the H-induced changes in the material. The competition between hydrogen insertion and silicon etching leads to first order kinetics in the time-evolution of the thickness of the H-modified zone. We analyzed the correlation between the steady state structural parameters of the H-modified layer and the main levers that control the plasma-surface interaction. In comparison with a simple doped layer, exposure of a-Si:H based junctions to the same plasma treatment leads to a thinner H-rich subsurface layer, suggesting a possible charged state of hydrogen diffusing. [ABSTRACT FROM AUTHOR]

We report on the growth of microcrystalline silicon films from the dissociation of SiF4/H2/Ar gas mixtures. For this growth chemistry, the formation of HF molecules provides a clear signature of the amorphous to microcrystalline growth transition. Depositing films from silicon tetrafluoride requires the removal of F produced by SiF4 dissociation, and this removal is promoted by the addition of H2 which strongly reacts with F to form HF molecules. At low H2 flow rates, the films grow amorphous as all the available hydrogen is consumed to form HF. Above a critical flow rate, corresponding to the full removal of F, microcrystalline films are produced as there is an excess of atomic hydrogen in the plasma. A simple yet accurate phenomenological model is proposed to explain the SiF4/H2 plasma chemistry in accordance with experimental data. This model provides some rules of thumb to achieve high deposition rates for microcrystalline silicon, namely, that increased RF power must be balanced by an increased H2 flow rate. [ABSTRACT FROM AUTHOR]

We report here a strong and stable electroluminescence (EL) from Si-rich hydrogenated polymorphous silicon carbon thin films (pm-Si1-xCx : H) fabricated in a plasma-enhanced chemical vapor deposition system. We investigate an unusual forming process in the pm-Si1-xCx : H thin films, during initial EL stressing, and propose a current-stress-induced phase separation process for the formation of new Si nanoclusters, which give rise to strongly enhanced emissions in both visible and near infrared ranges at 1.8-2.1 eV and 0.8-1.2 eV, respectively. The sub-crystalline-Si-bandgap emission is particularly attractive to realize a Si-based multi-band light source for optical interconnection and telecommunication. [ABSTRACT FROM AUTHOR]

In this paper, we use a fluid model to simulate the excitation of a hydrogen radio-frequency discharge, and employ tailored voltage waveforms to assess the effect of charged species transport properties. Results of the fluid simulation are compared with experimental data and previous results obtained with a hybrid model. Several expressions for electron and ion transport coefficients are compared, and their impact on the self-bias potential is studied. The self-bias is shown to be insensitive to the choice of electron transport coefficients, while remarkably sensitive to variations in ion mobility. Besides, our results show that fluid models can be competitive with hybrid models, provided self-consistent ion transport models and rate constants are used. [ABSTRACT FROM AUTHOR]

Structure and crystal growth of nickel oxide thin films (10–300 nm) prepared by low-temperature sputtering have been investigated by scanning electron microscopy (SEM), X-ray diffraction, and spectroscopic ellipsometry. Very thin films are compact and homogeneous and are made of almost randomly oriented crystals. A preferential growth direction is then observed following the (111), (220) and (311) planes to the detriment of the (222) and (200) planes, inducing a growth of the materials in columns perpendicularly to the substrate. An optical model able to account for this particular structure has been created from the spectroscopic ellipsometry measurements, and correlates well with the structure observed by SEM. Moreover, it enables an accurate estimation of the thickness without damage to the substrate. [ABSTRACT FROM AUTHOR]

We present our systematic work on the in situ generation of In nanoparticles (NPs) from the reduction of ITO thin films by hydrogen (H2) plasma exposure. In contrast to NP deposition from the vapor phase (i.e., evaporation), the ITO surface can be considered to be a solid reservoir of In atoms thanks to H2 plasma reduction. On one hand, below the In melting temperature, solid In NP formation is governed by the island-growth mode, which is a self-limiting process because the H2 plasma/ITO interaction will be gradually eliminated by the growing In NPs that cover the ITO surface. On the other hand, we show that above the melting temperature In droplets prefer to grow along the grain boundaries on the ITO surface and dramatic coalescence occurs when the growing NPs connect with each other. This growth-connection-coalescence behavior is even strengthened on In/ITO bilayers, where In particles larger than 10 μm can be formed, which are made of evaporated In atoms and in situ released ones. Thanks to this understanding, we manage to disperse dense evaporated In NPs under H2 plasma exposure when inserting an ITO layer between them and substrate like c-Si wafer or glass by modifying the substrate surface chemistry. Further studies are needed for more precise control of this self-assembling method. We expect that our findings are not limited to ITO thin films but could be applicable to various metal NPs generation from the corresponding metal oxide thin films. [ABSTRACT FROM AUTHOR]

Fourier Transform Photocurrent Spectroscopy (FTPS) has been used to investigate on the absorption coefficient versus photon energy ( α( hν)) spectroscopy of thin film semiconductors. We have developed an FTPS experiment to study the α( hν) spectrum of hydrogenated amorphous silicon (a-Si:H) thin films, to subsequently deduce information on the density of states in the sub bandgap region. In this work we studied whether the same α( hν) spectroscopy could be found both for coplanar a-Si:H films deposited on glass and for NIP diodes incorporating the same material as the intrinsic layer. The back and front contacts of the diodes are made of transparent conductive oxides, either ITO or ZnO. The results show that the Urbach tails of both coplanar and NIP diodes are very similar but that α at low photon energy is slightly lower for the diodes than for the coplanar samples. In addition, we noticed that the diode with ZnO contacts presents a rapid increase of α below 0.95 eV. Transmission-reflection measurements performed on the diodes show that the diode with ZnO contacts exhibits a weaker transmission in the same wavelength range where we observed an increase of α, though the reflection with both types of contacts remains the same, indicating a parasitic absorption of the back ZnO layer. These results show that FTPS measurements performed on diodes must be taken very cautiously and that one has to take into account the optical properties (e.g., absorption) of the contacts to properly deduce reliable α( hν) spectroscopy. [ABSTRACT FROM AUTHOR]

Controlling the doping profile in solar cells emitter and front/back surface field is mandatory to reach high efficiencies. In the current state of the art, these doped layers are made by dopant diffusion at around 900°C, which implies potential temperature induced damages in the c-Si absorber and for which a precise control of doping is difficult. An alternative solution based on boron-doped epitaxial silicon layers grown by plasma-enhanced chemical vapor deposition (PECVD) from 200°C using SiF4/H2/Ar/B2H6 chemistry is reported. The structural properties of the doped and undoped epitaxial layers were assessed by spectroscopic ellipsometry (SE), high resolution transmission electron microscopy (HR-TEM) and X-ray diffraction (XRD). The incorporation of boron has been studied via plasma profiling time of flight mass spectrometry (PP-TOFMS) and secondary ion mass spectrometry (SIMS) measurements. The boron-doped epitaxial layers revealed excellent structural and electrical properties even for high carrier concentrations (>1019cm-3). Sheet resistances between 100 and 130 Ω/sq can been obtained depending on the thickness and the doping concentration, which is within the range of targeted values for emitters in c-Si solar cells. Electrochemical capacitance voltage (ECV) revealed a uniform doping profile around 3.1019 cm-3 and by comparing with SIMS measurement a doping efficiency around 50% has been found. [ABSTRACT FROM AUTHOR]

The heteroepitaxial growth of crystal silicon thin films on sapphire, usually referred to as SoS, has been a key technology for high-speed mixed-signal integrated circuits and processors. Here, we report a novel nanoscale SoS heteroepitaxial growth that resembles the in-plane writing of self-aligned silicon nanowires (SiNWs) on R-plane sapphire. During a low-temperature growth at <350 °C, compared to that required for conventional SoS fabrication at >900 °C, the bottom heterointerface cultivates crystalline Si pyramid seeds within the catalyst droplet, while the vertical SiNW/catalyst interface subsequently threads the seeds into continuous nanowires, producing self-oriented in-plane SiNWs that follow a set of crystallographic directions of the sapphire substrate. Despite the low-temperature fabrication process, the field effect transistors built on the SoS-SiNWs demonstrate a high on/off ratio of >5 × 104 and a peak hole mobility of >50 cm2/V·s. These results indicate the novel potential of deploying in-plane SoS nanowire channels in places that require high-performance nanoelectronics and optoelectronics with a drastically reduced thermal budget and a simplified manufacturing procedure. [ABSTRACT FROM AUTHOR]

We evaluate the use of p-type silicon oxide (p-SiO x) dielectric layers as a boron diffusion source for n-type crystalline silicon (c-Si) substrates. The p-SiO x layers grown on n-type c-Si substrates by plasma enhanced chemical vapor deposition using a gas mixture of He/hexamethyldisiloxane/CO2/B2H6 are thermally stable and do not peel off during annealing up to 1050 °C, making them effective diffusion sources. The layers were examined before and after annealing with characterization techniques including spectroscopic ellipsometry and secondary ion mass spectrometry. We observe that there is a reduction in the thickness of the p-SiO x layer after annealing by about 50%, and that boron diffuses into the n-type c-Si substrate, forming a p+ layer, limited by the formation of a carbon-rich layer above the c-Si surface. The concentration of holes in the diffused region was measured by the electrochemical capacitance-voltage technique, and it was found that essentially all the boron that diffused into the n-type c-Si was active, unaffected by the presence of carbon and oxygen atoms. The concentration of carriers can be controlled by the initial thickness of the p-SiO x layers and the depth of the p+/n junction can be controlled by the time of annealing. A surface carrier concentration of 3 × 1019 at cm−3 and a sheet resistance of the order of 120 Ω sq−1 was obtained upon annealing at 1050 °C for 30 min. [ABSTRACT FROM AUTHOR]

Low-melting point metals such as bismuth (Bi) and tin (Sn) are ideal choices for mediating a low temperature growth of silicon nanowires (SiNWs) for radial junction thin film solar cells. The incorporation of Bi catalyst atoms leads to sufficient n-type doping in the SiNWs core that exempts the use of hazardous dopant gases, while an easy morphology control with pure Bi catalyst has never been demonstrated so far. We here propose a Bi-Sn alloy catalyst strategy to achieve both a beneficial catalyst-doping and an ideal SiNW morphology control. In addition to a potential of further growth temperature reduction, we show that the alloy catalyst can remain quite stable during a vapor-liquid-solid growth, while providing still sufficient n-type catalyst-doping to the SiNWs. Radial junction solar cells constructed over the alloy-catalyzed SiNWs have demonstrated a strongly enhanced photocurrent generation, thanks to optimized nanowire morphology, and largely improved performance compared to the reference samples based on the pure Bi or Sn-catalyzed SiNWs. [ABSTRACT FROM AUTHOR]

Abstract: We explore the influence of the sputtering deposition conditions on the outcome of an excimer laser anneal and chemical etching process with the goal of producing highly textured substrates for thin film silicon solar cells. Aluminum-doped zinc oxide (ZnO:Al) thin films were prepared on glass substrates by radio frequency magnetron sputtering from a ceramic target at room temperature. The effects of the process pressure (0.11–1.2Pa) and oxygen flow (0–2sccm) on the optical and electrical properties of ZnO:Al thin films have been studied both before and after an excimer laser annealing treatment followed by a dilute HCl chemical etch. The as-deposited films varied from completely opaque to yellowish. Thin film laser annealing dramatically improves the optical properties of the most opaque thin films. After laser annealing at the optimum fluence, the average transmittance in the visible wavelength range was around 80% for most films, and reasonable electrical performance was obtained for the films deposited at lower pressures and without oxygen flux (7Ω/□ for films of 1μm). After etching, all films displayed a dramatic improvement in haze, but only the low pressure, low oxygen films retained acceptable electrical properties (<11Ω/□). [Copyright &y& Elsevier]

The radial junction (RJ) architecture has proven beneficial for the design of a new generation of high performance thin film photovoltaics. We herein carry out a comprehensive modeling of the light in-coupling, propagation and absorption profile within RJ thin film cells based on an accurate set of material properties extracted from spectroscopic ellipsometry measurements. This has enabled us to understand and evaluate the impact of varying several key parameters on the light harvesting in radially formed thin film solar cells. We found that the resonance mode absorption and antenna-like light in-coupling behavior in the RJ cell cavity can lead to a unique absorption distribution in the absorber that is very different from the situation expected in a planar thin film cell, and that has to be taken into account in the design of high performance RJ thin film solar cells. When compared to the experimental EQE response of real RJ solar cells, this modeling also provides an insightful and powerful tool to resolve the wavelength-dependent contributions arising from individual RJ units and/or from strong light trapping due to the presence of the RJ cell array. [ABSTRACT FROM AUTHOR]

The radial junction (RJ) architecture has proven beneficial for the design of a new generation of high performance thin film photovoltaics. We herein carry out a comprehensive modeling of the light in-coupling, propagation and absorption profile within RJ thin film cells based on an accurate set of material properties extracted from spectroscopic ellipsometry measurements. This has enabled us to understand and evaluate the impact of varying several key parameters on the light harvesting in radially formed thin film solar cells. We found that the resonance mode absorption and antenna-like light in-coupling behavior in the RJ cell cavity can lead to a unique absorption distribution in the absorber that is very different from the situation expected in a planar thin film cell, and that has to be taken into account in the design of high performance RJ thin film solar cells. When compared to the experimental EQE response of real RJ solar cells, this modeling also provides an insightful and powerful tool to resolve the wavelength-dependent contributions arising from individual RJ units and/or from strong light trapping due to the presence of the RJ cell array. [ABSTRACT FROM AUTHOR]

We investigate the specific optical regime occurring at short wavelengths, in the high absorption regime, in silicon thin-films patterned by periodically arranged nano-holes. Near-field scanning optical microscopy indicates that the incoming light is coupled to vertically channelling modes. Optical modelling and simulations show that the light, travelling inside the low-index regions, is absorbed at the direct vicinity of the nano-holes sidewalls. This channelling regime should be taken into account for light management in optoelectronic devices. [ABSTRACT FROM AUTHOR]

In heterojunction solar cells, a-Si:H/c-Si heterointerface is of significant importance, since the heterointerface characteristics directly affect junction properties and thus solar cell efficiency. In this study, we have performed time resolved microwave conductivity (TRMC) measurements on n-type c-Si wafers passivated on both sides with intrinsic and doped a-Si:H layers in order to investigate electrical property and passivation quality of the a-Si:H/c-Si heterojunctions. It was found that the TRMC decay time and decay curve shape varied with the laser wavelength and power intensity and also depended on sample structures. By using 1064 nm laser pulse with high excitation, differences in the decay curve shape between samples with and without p-n junction were observed. The samples containing p-n junction(s) had unique slow decay mode, after the initial fast decay, which we ascribed to the release of carriers from the low-mobility amorphous layer into the high-mobility crystalline wafer as the built-in field of the junction was restored. Experimental results suggest that the TRMC is useful nondestructive technique which is suitable for primary check of the a-Si:H/c-Si heterojunctions during the solar cell fabrication process. [ABSTRACT FROM AUTHOR]

Silicon nanowires (SiNWs) grown on low-cost substrates provide an ideal framework for the monolithic fabrication of radial junction photovoltaics. However, the quality of junction formation over a random matrix of SiNWs, fabricated via a vapor-liquid-solid (VLS) mechanism, has never been assessed in a realistic context. To address this, we probe the current response of individual radial junction solar cells under electron-beam and optical-beam excitations. Excellent current generation from the radial junction units, compared to their planar counterparts, has been recorded, indicating a high junction quality and effective doping in the ultra-thin SiNWs with diameters thinner than 20 nm. Interestingly, we found that the formation of radial junctions by plasma deposition can be quite robust against geometrical disorder and even the crossings of neighboring cell units. These results provide a strong support to the feasibility of building high-quality radial junction solar cells over high-throughput VLS-grown SiNWs on low-cost substrates. [ABSTRACT FROM AUTHOR]

We report on the use of an innovative optical characterization technique--real-time Mueller polarimetric imaging in transmission--for the characterization of thin-film silicon solar cells. In this work, we used this technique to monitor the evolution of optical retardance induced by the mechanical stresses in hydrogenated amorphous and polymorphous silicon (a-Si :H and pm-Si :H) p-i-n (PIN) solar cells. Under current injection of 200mAcm-2, the retardance of the pm-Si :H PIN solar cells decreased, while that of the a-Si :H PIN solar cells showed no significant change. After the current injection, the pm-Si :H PIN solar cells showed dramatic macroscopic changes on a scale of tens of micrometres, such as local peel-off and delamination from the substrate. Our results demonstrate that current injection introduces local stress relaxation, which can be efficiently monitored prior to irreversible damage from a decrease in the retardance of the pm-Si :H PIN solar cells. [ABSTRACT FROM AUTHOR]

Unlike typical Au used as a catalyst for the synthesis of silicon nanowires via the vapour-liquid-solid mechanism, Cu has been found to induce a synthesis process governed by the vapour-solid-solid mechanism. Moreover, the temperature window for obtaining high-quality wires with Cu has been found to be relatively smaller than that shown by the Au: from 600 to 650?°C. However, high-resolution transmission electron microscopy analysis reveals significant new properties of the nanowires obtained. They have the peculiarity of successively switching the silicon structure from diamond to the wurtzite phase along the growth direction. This change of the crystalline structure implies that it has an important impact on the transport properties and characteristics of electronic devices. The results will be important for the future integration and application of silicon, where electrical and thermal transport properties play a significant role. [ABSTRACT FROM AUTHOR]

We report on a guided growth of silicon nanowires (SiNWs) based on an in-plane solid-liquid-solid mechanism, which provides a general strategy to deploy SiNWs precisely into desired circuits. During a reacting-gas-free annealing process, the SiNWs are activated to grow and be guided into predefined patterns by effective controlling the movement of the catalyst drops. We demonstrate three different approaches to achieve a guided growth of SiNWs, which are as follows: (1) by an a-Si:H channel, (2) by a step edge, and (3) by an a-Si:H edge. These results provide a design principle for future SiNWs-based nanodevices. [ABSTRACT FROM AUTHOR]

Germanium quantum dots (GeQDs), addressed by self-aligned and epitaxial silicon nanowires (SiNWs) as electrodes, represent the most fundamental and the smallest units that can be integrated into Si optoelectronics for 1550 nm wavelength detection. In this work, individual GeQD photodetectors have been fabricated based on a low temperature self-condensation of uniform amorphous Si (a-Si)/a-Ge bilayers at 300 °C, led by rolling indium (In) droplets. Remarkably, the diameter of the GeQD nodes can be independently controlled to achieve wider GeQDs for maximizing infrared absorption with narrower SiNW electrodes to ensure a high quality Ge/Si hetero-epitaxial connection. Importantly, these hetero GeQD/SiNW photodetectors can be deployed into predesigned locations for scalable device fabrication. The photodetectors demonstrate a responsivity of 1.5 mA W−1 and a photoconductive gain exceeding 102 to the communication wavelength signals, which are related to the beneficial type-II Ge/Si alignment, gradient Ge/Si epitaxial transition and a larger QD/NW diameter ratio. These results indicate a new approach to batch-fabricate and integrate GeQDs for ultra-compact Si-compatible photodetection and imaging applications. [ABSTRACT FROM AUTHOR]

Plasma-generated silicon nanocrystals have been selectively trapped on a cooled substrate to yield nanocrystalline films. We here present experimental evidence that the contribution of positively charged nanocrystals largely dominates the film deposition. As a direct application, we illustrate how the use of a simple substrate bias voltage allows us to “toggle switch” between 100% nanocrystalline and 100% amorphous layers. Moreover, we demonstrate that the applied bias voltage can be used to “tune” the photoluminescence of the nanocrystals between 630 and 730 nm. [ABSTRACT FROM AUTHOR]

Three-dimensional (3D) construction of radial junction hydrogenated amorphous silicon (a-Si:H) thin film solar cells on standing silicon nanowires (SiNWs) is a promising strategy to maximize the light harvesting performance and improve the photocarrier collection in an optimized junction configuration. The unique light in-coupling and absorption behaviour in the antenna-like 3D photonic structures also necessitates a set of new theoretical models and simulation tools to design, predict and optimize the photovoltaic performance of radial junction solar cells, which can be rather different from planar junction solar cells. Recently, the performance of radial junction a-Si:H thin film solar cells has progressed steadily to a level comparable or even superior to that of their planar counterparts, with plenty of room for further improvement. This review will first address the growth strategy and critical parameter control of SiNWs produced via a plasma-assisted low-temperature vapour–liquid–solid procedure using low-melting-point metals as the catalyst. Then, the construction of high-performance radial junction thin film solar cells over the standing SiNW matrix, as well as their optimal structural designs, will be introduced. At the end, the new applications of 3D radial junction units will be summarized, which include, for example, the construction of very flexible, low-cost and efficient a-Si:H solar cells with the highest power-to-weight ratio, the demonstration of highly sensitive solar-blind photodetectors operating at the ultraviolet wavelength spectrum and the development of novel biomimetic radial tandem junction photodetectors with an intrinsic red–green–blue (RGB) colour distinguishing capability. [ABSTRACT FROM AUTHOR]

The fabrication of arrays of silicon nanowires (Si NWs) with well-defined surface coverage using the vapor–liquid–solid process requires a good control of the density and size distribution for the metal catalyst. We report on a cost-effective bottom-up approach to produce Si NWs by a low-temperature deposition technology using plasma-enhanced chemical vapor deposition and tin dioxide (SnO2) nanoparticles as the source of tin catalyst. This strategy offers a straightforward method to select specific particle sizes by conventional colloidal techniques, and to tune the surface coverage using a polyelectrolyte layer to efficiently immobilize the particles on the substrate by electrostatic grafting. After a further step of reduction into tin metal droplets using hydrogen plasma treatment, the catalyst particles are used for the growth of Si NWs. This approach allows the prodcution of controlled Si NWs arrays which can be used as a template for radial junction thin film solar cells. [ABSTRACT FROM AUTHOR]

Crystalline silicon thin films are promising candidates for low cost and flexible photovoltaics. Among various synthesis techniques, epitaxial growth via low temperature plasma-enhanced chemical vapor deposition is an interesting choice because of two low temperature related benefits: low thermal budget and better doping profile control. However, increasing the growth rate is a tricky issue because the agglomeration of clusters required for epitaxy leads to powder formation in the plasma. In this work, we have measured precisely the time evolution of the self-bias voltage in silane/hydrogen plasmas at millisecond time scale, for different values of the direct-current bias voltage applied to the radio frequency (RF) electrode and growth temperatures. We demonstrate that the decisive factor to increase the epitaxial growth rate, i.e. the inhibition of the agglomeration of plasma-born clusters, can be obtained by decreasing the RF OFF time or increasing the growth temperature. The influence of these two parameters on the growth rate and epitaxial film quality is also presented. [ABSTRACT FROM AUTHOR]

Solar cells based on epitaxial silicon layers as the absorber attract increasing attention because of the potential cost reduction. In this work, we studied the influence of the deposition rate on the structural properties of epitaxial silicon layers produced by plasma-enhanced chemical vapor deposition (epi-PECVD) using silane as a precursor and hydrogen as a carrier gas. We found that the crystalline quality of epi-PECVD layers depends on their thickness and deposition rate. Moreover, increasing the deposition rate may lead to epitaxy breakdown. In that case, we observe the formation of embedded amorphous silicon cones in the epi-PECVD layer. To explain this phenomenon, we develop a model based on the coupling of hydrogen and built-in strain. By optimizing the deposition conditions to avoid epitaxy breakdown, including substrate temperatures and plasma potential, we have been able to synthesize epi-PECVD layers up to a deposition rate of 8.3 Å/s. In such case, we found that the incorporation of hydrogen in the hydrogenated crystalline silicon can reach 4 at. % at a substrate temperature of 350 °C. [ABSTRACT FROM AUTHOR]

Incorporation of nanostructures is a recent trend in the photovoltaic community, aimed at improving light absorption and consequently cell efficiency. In this regard, semiconductor nanowires provide an attractive research platform for a new generation of cost-effective and efficient solar cells. Thanks to their unique geometry, silicon nanowires enhance light trapping and anti-reflection effects by means of multiple scattering between individual nanowires, and by coupling the light into confined eigenmodes over a broad range of the solar spectrum. Moreover, radial junction solar cells built around nanowires decouple the light absorption and carrier collection directions, which allows for a higher internal field and better carrier collection. Thus, arrays of radial junction solar cells bring advantages of high efficiency with reduced material amount. This is particularly attractive for devices based on hydrogenated amorphous and microcrystalline silicon thin films. In this paper, after reviewing different approaches to fabricate silicon nanowires, we focus on nanowires grown using the plasma-assisted vapour–liquid–solid method because of the simplicity and compatibility with current silicon thin-film technology. Their application to a-Si : H based radial junction solar cells has already resulted in ∼8% of stable devices with an absorber layer thickness of only 100 nm. Moreover, current challenges and perspectives such as the use of a microcrystalline silicon absorber are also reviewed. [ABSTRACT FROM AUTHOR]

Plasma-enhanced low temperature growth (<300 degC) of silicon nanowires (SiNWs) and hierarchical structures via a vapor-liquid-solid (VLS) mechanism are investigated. The SiNWs were grown using tin and indium as catalysts prepared by in situ H2 plasma reduction of SnO2 and ITO substrates, respectively. Effective growth of SiNWs at temperatures as low as 240 degC have been achieved, while tin is found to be more ideal than indium in achieving a better size and density control of the SiNWs. Ultra-thin (4-8 nm) silica nanowires, sprouting from the dendritic nucleation patterns on the catalyst's surface, were also observed to form during the cooling process. A kinetic growth model has been proposed to account for their formation mechanism. This hierarchical structure combines the advantages of the size and position controllability from the catalyst-on-top VLS-SiNWs and the ultra-thin size from the catalyst-on-bottom VLS-ScNWs. [ABSTRACT FROM AUTHOR]