Nanocrystalline Si Research Articles

Synthesis of High Performance Si Nanoflakes and Nanorod Anode Morphologies Using Water Soluble Recyclable Templates for Lithium Ion Batteries

Silicon based anode systems have attracted considerable attention as alternative materials to replace graphite as a high energy density anode for lithium ion batteries due to silicon’s high theoretical capacity (~4200mAh/g). However, the material suffers from colossal volumetric expansion (~400%) associated with the formation of Li4.4Si phase during the lithiation reaction. Over the past few years, different approaches have been developed to address this issue which causes mechanical degradation and results in rapid loss in capacity of silicon anodes. These approaches involve the use of silicon nanoparticles [1], composite structures of amorphous silicon [2], active-inactive matrices [3], electrodeposited thin films and VACNT-Si heterostructures [4-6]. Nanostructures of silicon have been shown to withstand the mechanical pulverization by providing stress – strain relaxation mechanisms thus, showing improved cycling stability. However, most of the synthesis methods used to generate these Si nanostructures involve high processing costs, complex machinery/equipment, utilization of expensive templates and large number of processing steps. Additionally these materials show higher first cycle irreversible (FIR) loss and have poor areal loading densities. In the present work, Si nanostructures are generated using a simple two-step procedure employing a low-cost recyclable template. The template was generated using a high throughput high energy mechanical milling process (HEMM) utilizing an abundant, cheap and water soluble material. Si nanoflakes and Si nanorods (deposited using low pressure chemical vapor deposition) were obtained by varying the template synthesis conditions. The template is then recovered and reused by washing in water making the procedure amenable for commercialization. Slurry based electrodes of these nanostructures mixed with binder and conductive additives were then fabricated and tested in a half cell configuration against lithium foil between the voltage range 0.01V – 1V vs. Li/Li+ in 1M LiPF6(dissolved in EC:DEC:FEC=45:45:10) electrolyte. Fig. 1a shows the XRD patterns of the two different morphologies of silicon obtained using this simple two-step procedure. It can be seen therein that the procedure can be tailored to obtain either amorphous (Si flakes) or nanocrystalline (Si rods) silicon materials. The aim of this study is to understand the effect of the synthesis conditions on the morphology, crystal structure and subsequent electrochemical performance of low-cost template-derived silicon nanostructures. It can be seen in Fig. 1b that both morphology and crystallinity affect the silicon capacity and cycling behavior of both silicon morphologies. Though Si rods show a superior first cycle discharge capacity (~2790 mAh/g at current rate of 50mA/g; FIR loss ~12%-13%) as compared to Si flakes (~2930 mAh/g at current rate of 50mA/g; FIR loss ~17%-20%), the long term cycling behavior is in stark contrast, with Si nanoflakes and nanorods showing a fade rate of ~0.03% and ~0.4% loss per cycle, respectively, due to the differences in morphology and crystallinity. Additionally, electrodes with areal loading density of ~1.3 – 1.5mg/cm2were obtained independent of the morphology of the silicon nanostructures. Furthermore, Si nanoflakes and nanorods showed a specific discharge capacity of ~1150mAh/g and ~1025mAh/g, respectively (at a current rate of 1A/g), at the end of 150 cycles. Results of these studies will be presented and discussed. Acknowledgement: The authors gratefully acknowledge financial support of DOE contract administered through PNNL and NSF-CBET 1511390. The authors also acknowledge the Edward R. Weidlein Chair Professorship funds and the Center for Complex Engineered Multifunctional Materials (CCEMM) for support of this research.

Microstructure and carrier‐transport behaviors of nanocrystalline silicon thin films annealed at various temperatures

Hydrogenated amorphous Si films were prepared by a plasma‐enhanced chemical vapor deposition technique. As‐deposited samples were then thermally annealed at various temperatures to obtain nanocrystalline silicon films. The microstructures and carrier‐transport characteristics were investigated during the transition process from amorphous to nanocrystalline phase. It was found that the Hall mobility was improved while the dark conductivity was increased by over two orders of magnitude after annealing. Temperature‐dependent Hall measurements were performed in the temperature range from 310 to 575 K. Hall mobility first increases with rising temperature to 400 K and then decreases with further increasing temperature. The different temperature dependence of Hall mobility shows that the microscopic mechanisms governing charge transport are different for nanocrystalline Si films in different temperature regions.

Quantum confinement in semiconductor nanofilms: Optical spectra and multiple exciton generation

We report optical absorption and photoluminescence (PL) spectra of Si and SnO2 nanocrystalline films in the UV–vis–NIR range, featuring discrete bands resulting from transverse quantum confinement, observed in the optical spectra of nanofilms for the first time ever. The film thickness ranged from 3.9 to 12.2nm, depending on the material. The results are interpreted within the particle-in-a-box model, with infinite walls. The calculated values of the effective electron mass are independent on the film thickness and equal to 0.17mo (Si) and 0.21mo (SnO2), with mo the mass of the free electron. The second calculated model parameter, the quantum number n of the HOMO (valence band), was also thickness-independent: 8.00 (Si) and 7.00 (SnO2). The transitions observed in absorption all start at the level n and correspond to Δn=1, 2, 3,…. The photoluminescence bands exhibit large Stokes shifts, shifting to higher energies with increased excitation energy. In effect, nanolayers of Si, an indirect-gap semiconductor, behave as a direct-gap semiconductor, as regards the transverse-quantized level system. A prototype Si–SnO2 nanofilm photovoltaic cell demonstrated photoelectron quantum yields achieving 2.5, showing clear evidence of multiple exciton generation, for the first time ever in a working nanofilm device.

The photoluminescence of multicolor silicon

ABSTRACTThe present paper deals with the photoluminescence (PL) of the multicolor silicon (MC-Si) – a nanocrystalline layer produced by the surface structure chemical transfer (SSCT) method on the p-type Si. The PL behaviours recorded at room temperature are compared with ∼500-nm-thick porous Si prepared electrochemically in the HF + methanol solution on the same Si substrate. The PL spectra of porous Si are shifted to higher energy in comparison with MC-Si. The PL records were fitted by Gaussian curves. We attribute the PL band of porous Si with maximum 2.3 eV to the own luminescence of ∼2.5-nm-thick nanocrystalline Si grains observed due to quantum confinement. This PL band is missing in the PL spectra of MC-Si structures due to the absence of Si nanocrystals of similar size. The PL bands of both MC-Si and porous Si between 1.89 and 2.05 eV are connected with SiO(x) compounds covering the surfaces of Si nanocrystals. Additional two PL bands of all MC-Si structures at 1.69–1.70 eV and 1.80–1.84 eV we rel...

Performance and retention characteristics of nanocrystalline Si floating gate memory with an Al2O3 tunnel layer fabricated by plasma-enhanced atomic layer deposition

The down-scaling of nanocrystal Si (nc-Si) floating gate memory must overcome the challenge of leakage current induced by the conventional ultra-thin tunnel layer. We demonstrate that an improved memory performance based on the Al/SiNx/nc-Si/Al2O3/Si structure can be achieved by adopting the Al2O3 tunnel layer fabricated by plasma-enhanced atomic layer deposition. A larger memory window of 7.9 V and better retention characteristics of 4.7 V after 105 s can be obtained compared with the devices containing a conventional SiO2 tunnel layer of equivalent thickness. The capacitance-voltage characteristic reveals that the Al2O3 tunnel layer has a smaller electron barrier height, which ensures that more electrons are injected into the nc-Si dots through the Al2O3/Si interface. The analysis of the conductance-voltage and high-resolution cross-section transmission microscopy reveals that the smaller nc-Si dots dominate in the charge injection in the nc-Si floating gate MOS device with an Al2O3 tunnel layer. With an increase of the nc-Si size, both nc-Si and the interface contribute to the charge storage capacity and retention. The introduction of the Al2O3 tunnel layer in nc-Si floating gate memory provides a method to achieve an improved performance of nc-Si floating gate memory.

Surface modification of Al–Si alloy by excimer laser pulse processing

The laser irradiation on Al-Si alloy sample is carried out by excimer laser in ambient conditions for 30 or 45 pulses. Microstructural investigation of laser treated sample is done by OM, SEM and TEM and the surface hardness is evaluated by Vickers micro indentation. Laser treated, samples suggested the dissolution of coarse primary Si and β-AlFeSi particle in α-Al matrix. The SEM/EDS study shows the enhancement of retained Si in α-Al matrix. The interface analysis of laser treated sample suggested the effected modified depth is ∼6 μm. TEM investigation shows the formation of nanocrystalline Si in size ∼2–15 nm. The cellular structures of size range ∼30–50 nm are observed after 45 pulses. The α-Al cells and Si precipitates sizes were considerably refined at higher number of pulses. The fine Si precipitates are found to be dispersed in the intercellular boundaries. An improvement in surface hardness from ∼1.6 to 1.8 is observed 30 and 45 pulse treatment, respectively. The mechanism involves for improvement in surface properties are non-equilibrium solidification, metastable phase formation and microstructural refinement.

Preparation and properties of Si/SiCxOy multilayer films containing Si quantum dots

Si/SiCxOy multilayers have been prepared by magnetron sputtering technique. Effects of annealing temperature (T a ) on the microstructural and electrical properties of Si/SiCxOy multilayer films were systematically investigated using high resolution transmission electron microscope, grazing incident X-ray diffraction, Raman spectroscopy, UV–Vis–NIR spectroscopy, and Hall measurements. The T a has great effects on the optical properties of Si/SiCxOy multilayer thin films. It is found that the onset of nanocrystalline Si phase (Si quantum dots) formation occurs at 800 °C. SiC crystalline phases are not formed in Si/SiCxOy multilayer film even at the T a 1000 °C. It is indicated that the method of depositing a Si/SiCxOy multilayer followed by thermal induced crystallization gives flexibility to suppress the crystallization of an a-SiC phase. With increasing T a the grain size, dot density, and optical bandgap increase, the conductivity also increase. These results demonstrate that high quality Si QDs embedded in Si/SiCxOy multilayer films deposited by magnetron sputtering can be obtained by properly controlling the annealing condition.

Tailoring of microstructure in hydrogenated nanocrystalline Si thin films by ICP-assisted RF magnetron sputtering

Utilizing plasma-assisted deposition by combining an RF magnetron and an inductively coupled plasma (ICP) source it is possible to fabricate highly crystallized nc-Si:H films at a relatively low substrate temperature (300 °C). Microstructural analysis reveals enhancement in crystallinity along with (2 2 0) preferential orientation throughout the depth of the film. The possible mechanism of crystallinity enhancement and preferential orientation is presented on the basis of plasma diagnostics using optical emission spectroscopy and various film analysis tools. This work also reports the effectiveness of the ICP source and elevated temperature for the control of film microstructure and crystallinity.

Development of ballistic hot electron emitter and its applications to parallel processing: active-matrix massive direct-write lithography in vacuum and thin-film deposition in solutions

Making the best use of the characteristic features in nanocrystalline Si (nc-Si) ballistic hot electron source, an alternative lithographic technology is presented based on two approaches: physical excitation in vacuum and chemical reduction in solutions. The nc-Si cold cathode is composed of a thin metal film, an nc-Si layer, an n+-Si substrate, and an ohmic back contact. Under a biased condition, energetic electrons are uniformly and directionally emitted through the thin surface electrodes. In vacuum, this emitter is available for active-matrix drive massive parallel lithography. Arrayed 100×100 emitters (each emitting area: 10×10 μm2) are fabricated on a silicon substrate by a conventional planar process, and then every emitter is bonded with the integrated driver using through-silicon-via interconnect technology. Another application is the use of this emitter as an active electrode supplying highly reducing electrons into solutions. A very small amount of metal-salt solutions is dripped onto the nc-Si emitter surface, and the emitter is driven without using any counter electrodes. After the emitter operation, thin metal and elemental semiconductors (Si and Ge) films are uniformly deposited on the emitting surface. Spectroscopic surface and compositional analyses indicate that there are no significant contaminations in deposited thin films.

Improved electrochemical performance of boron-doped SiO negative electrode materials in lithium-ion batteries

We introduce a one-step process that consists of thermal disproportionation and impurity doping to enhance the reversible capacity and electrical conductivity of silicon monoxide (SiO)-based negative electrode materials in Li-ion batteries. Transmission electron microscope (TEM) results reveal that thermally treated SiO at 900 °C (H–SiO) consists of uniformly dispersed nano-crystalline Si (nc-Si) in an amorphous silicon oxide (SiOx) matrix. Compared to that of prinstine SiO, the electrochemical performance of H–SiO shows improved specific capacity, due mainly to the increased reversible capacity by nc-Si and to the reduced volume expansion by thermally disproportionated SiOx matrix. Further electrochemical improvements can be obtained by boron-doping on SiO (HB–SiO) using solution dopant during thermal disproportionation. HB–SiO electrode without carbon coating exhibits significantly enhanced specific capacity superior to that of undoped H–SiO electrode, having 947 mAh g−1 at 0.5C rate and excellent capacity retention of 93.3% over 100 cycles. Electrochemical impedance spectroscopy (EIS) measurement reveals that the internal resistance of the HB–SiO electrode is significantly reduced by boron doping.

Effect Of Xe Dilution On Structural, Electrical And Optical Properties Of Nanocrystalline Si Films Deposited By HW-CVD Method

We investigated the effect of Xe dilution of silane on structural, optical and electrical properties of nanocrystalline Si films deposited by HW-CVD. With increase in Xe dilution of silane nanocrystalline-to-amorphous transition or amorphization has been observed in nanocrystalline Si films. The amorphization has been confirmed from dark and photoconductivity measurement, Raman spectroscopy, low angle XRD and atomic force microscopy analysis. The FTIR spectroscopy analysis showed that with increase in Xe dilution of silane, in addition to di-hydrogen [Si-H2] and poly-hydrogen [(Si-H2)n] complexes, hydrogen incorporated in these films in mono-hydrogen [Si-H] bonded species. The hydrogen content was found < 5 at. % over the entire range of Xe dilution of silane studied and it increases with increase in Xe dilution of silane. On the other hand, ETauc and E04 show decreasing trend with increasing Xe dilution of silane. The ETauc decreases from 2.4eV to 1.9eV whereas E04 decreases from 2.9eV to 2.3eV. The optical band gap values estimated from E04 method are found higher than ETauc values calculated from Tauc’s method. Finally, it has been concluded that Xe dilution of silane in HW-CVD enhances the deposition rate but has adverse effect on the crystallinity of nanocrystalline Si films. Copyright © 2015 VBRI Press.

Impact of the Film Storage Conditions on the Performance of Si-Based Anodes for Li-Ion Batteries

For several years, great attention has been paid to silicon as negative electrode material for Li-ion batteries, due to its very high gravimetric capacity (3579 mAh g-1) in comparison to that of graphite (372 mAh g-1). However, Si electrodes suffer from poor cyclability due to the large volumetric expansion (up to 300%) of Si upon its lithiation, resulting in the electrode architecture disintegration, and in the instability of the solid electrolyte interphase (SEI). We have recently shown that low-cost and high-performance Si-based electrodes can be obtained by combining (i) the use of ball-milled (nanocrystalline) Si powder resulting in a smoother phase transition; (ii) the processing of the electrode at pH 3 with carboxymethylcellulose (CMC) binder favoring the covalent grafting of the CMC to the Si particles; (iii) the use of fluoroethylene and vinylene carbonates (FEC/VC) electrolyte additives resulting in a more stable SEI.1 In the present study, it is shown for the first time that the storage conditions of the Si-based film before assembling in the electrochemical cell has also a major impact on the electrode performance. This is illustrated in Fig. 1 which compares the evolution of the discharge capacities with cycling* of ball-milled Si-based electrodes prepared after storage at room temperature of the composite film** (a) for 1 day and 55 days in air and (b) for 6 days in air (10-15% relative humidity) and in humid air (70% relative humidity). It clearly appears that the film storage in air has a very positive impact on the electrode cycle life, which is accentuated under humid atmosphere. A possible explanation is that during the film storage, water molecules from air react with the film and modify the chemical links between the Si particles and the CMC binder. This tends to be confirmed by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) analysis of the composite electrodes***. Indeed, as shown in Fig. 2, a significant increase of the peak centered at 1630 cm− 1 assigned to the stretching band of the carboxyl group of CMC is observed for the film stored in humid air, while the peak centered at 1750 cm-1assigned to a covalent bond between Si and CMC is decreased. This suggests an increase of the hydrogen bonds between the Si particles and CMC binder (Fig. 3), which could increase the deformability of the electrode, favorable to its mechanical stability with cycling. We believe that an optimum covalent / hydrogen bond ratio is required to obtain a film sufficiently rigid to maintain electronic connections but flexible enough to vary in volume without breaking/peeling off during cycling. * Electrodes were cycled in Swagelok-type cells with Li as counter/ref. electrode at full capacity between 1 and 0.005 V at a current density of 480 mA g-1 of Si. The electrolyte was LP30 + 10wt%FEC. ** (Si + CB + CMC+ pH3 buffer salts) in a weight ratio of 73.1/11.0/7.3/8.6. The Si loading was 1 mg/cm2. *** Electrodes were dried at 100°C under vacuum prior to ATR-FTIR analysis (same procedure as prior electrode cycling). Reference 1. M. Gauthier, D. Mazouzi, D. Reyter, B. Lestriez, P. Moreau, B. Lestriez, D. Guyomard, L. Roué. A low-cost and high-performance Si-based electrode for Li-ion batteries. Energy Environ. Sci. 6 (2013) 2145–2155. Figure 1

(Invited) Electronic and Acoustic Applications of Anodized Nano-Crystalline Silicon

Electrochemically anodized nano-crystalline Si (nc-Si) behaves as a functional material with a confined structure. In the electron transport through a chain of nc-Si dots, phonon scattering losses are minimized due to discrete energy levels in the conduction band, and then ballistic electrons are generated at the early stage of injection [1]. This induces efficient ballistic hot electron emission from an nc-Si diode. It is possible to use this emitter not only in vacuum, but also in atmospheric-pressure gases and solutions [2]. Extremely lowered thermal conductivity and heat capacity of the nc-Si layer [3], on the other hand, significantly enhances thermo-acoustic effect. In contrast to the conventional piezoelectric transducer, the thermally induced acoustic emission exhibits no resonant peaks in the frequency response. Here we discuss topics in the studies on these subjects. The nc-Si emitter is a kind of MIS diode consisting of a thin film surface electrode, an nc-Si layer (~1 μm thick), a crystalline silicon wafer substrate, and a back contact. The nc-Si layer includes chains of nc-Si dots (~3 nm in mean diameter) interconnected with tunnel oxides. When a positive voltage is applied to the surface electrode, electrons are accelerated in the nc-Si layer toward the outer surface, and then some of them are emitted through the surface electrode as ballistic hot electrons with a small angle dispersion (±10°with respect to the normal). At applied voltages of 12-20 V, the mean energy of output electrons reaches 3-7 eV [2]. One possible application of the nc-Si emitter is the use as an exposure source of multibeam parallel lithography. The emitter array is fabricated on a Si substrate by planar processes. Each emitter with an active area of 10 μm☐ is connected to an active matrix driving circuit through a TSV technique. The evaluation was performed with the 1:1 exposure test system, in which an EB-resist coated target wafer was placed at about 4 mm distance from the emitter surface. The wafer was grounded, and emitter surface was set at -5 kV for accelerating the electron. It has been demonstrated that the selected emitter patterns is delineated well corresponding to the activated emitters [4]. The integrated nc-Si emitter array is compatible with the active-matrix drive for multi-beam massive parallel lithography. Another application is thin film deposition using the reducing activity of ballistic hot electrons. When the emitter is driven in metal-salt solutions such as CuSO4, CuCl2, NiCl2, SiCl4, and GeCl4, injected electrons directly reduce Cu2+, Si4+ and Ge4+ ions, and then thin Cu films [5] and amorphous Si and Ge films [6,7] are grown on the emitting area with neither contaminations nor byproducts. Based on this observation, a printing scheme has been employed, in which a target substrate is separately located in close proximity to the emitter. A gap between the nc-Si emitter and the target substrate is controlled by a piezoelectric actuator depending the ambient (N2 gas) pressure. Emitted electrons hit metal-salt solutions on the substrate leading to the reducing reaction. The first printing experiment was sought for deposition of thin metal film, where an extremely small amount of CuSO4 or CuCl2 solution was coated on a single-crystalline Si wafer and used as a target substrate. It has been observed that thin Cu films are deposited on the irradiated area. The broad-band flat emissivity of the nc-Si thermo-acoustic device is useful for developing low-distortion digital speaker [8] and reproducing complicated ultrasonic communication calls between mice [9].

Electronic properties and charge storage effect of amorphous SiN passivated nanocrystalline silicon

Nanocrystalline Si (nc-Si) with mean size of about 4 nm embedded in amorphous SiN film was prepared by annealing Si-rich amorphous SiN film. The film compositions and microstructures were revealed by x-ray photoelectron spectroscopy, Raman spectroscopy, and transmission electron microscopy. It was found the room temperature conductivity is increased from 7 × 10−9 to 1 × 10−5 S/cm due to the formation of nc-Si. The carrier transport process of nc-Si embedded in amorphous SiN matrix is dominated by trap-assisted tunneling mechanism. Moreover, by forming a-SiN0.81/nc-Si(SiN)/a-SiN0.81 sandwiched floating gate structures, both electron and hole can be injected and stored in nc-Si by controlling the applied bias polarity. A large memory window up to about 7 V was observed, and the stored carrier density was about 1012 cm−2. Our experimental results suggested that the interface states of nc-Si can be well passivated by the amorphous SiN matrix, which results in the good charge storage effect.

High aspect ratio Si micro-holes formed by wet etching using Pt needles

Si micro-holes with a high aspect ratio can be formed by use of catalytic activity of Pt needles, i.e., contact of Pt needles with Si wafers immersed in an HF plus H2O2 solution. In the case of p-Si with ∼10 Ωcm resistivity, the shape of the Pt needle is accurately transferred to the Si substrate, and the aspect ratio of 8 is achieved. High-resolution TEM observations of the sidewall of the Si micro-holes show that they are formed through formation of a nanocrystalline Si layer followed by its dissolution. By comparing micro-holes formed in n-Si and in p-Si, it becomes obvious that diffusion of injected holes, which form the nanocrystalline Si layer, affects the shape of micro-holes. In the case of n-Si, injected holes diffuse to the Si/solution interface due to the Si band-bending resulting from the difference between the redox potential of the solution and the Si Fermi level, and cause an Si dissolution reaction at non-contact regions with the Pt needle.

Aerosol assisted atmospheric pressure chemical vapor deposition of silicon thin films using liquid cyclic hydrosilanes

Silicon (Si) thin films were produced using an aerosol assisted atmospheric pressure chemical vapor deposition technique with liquid hydrosilane precursors cyclopentasilane (CPS, Si5H10) and cyclohexasilane (CHS, Si6H12). Thin films were deposited at temperatures between 300 and 500°C, with maximum observed deposition rates of 55 and 47nm/s for CPS and CHS, respectively, at 500°C. Atomic force microscopic analyses of the films depict smooth surfaces with roughness of 4–8nm. Raman spectroscopic analysis indicates that the Si films deposited at 300°C and 350°C consist of a hydrogenated amorphous Si (a-Si:H) phase while the films deposited at 400, 450, and 500°C are comprised predominantly of a hydrogenated nanocrystalline Si (nc-Si:H) phase. The wide optical bandgaps of 2–2.28eV for films deposited at 350–400°C and 1.7–1.8eV for those deposited at 450–500°C support the Raman data and depict a transition from a-Si:H to nc-Si:H. Films deposited at 450 oC possess the highest photosensitivity of 102–103 under AM 1.5G illumination. Based on the growth model developed for other silanes, we suggest a mechanism that governs the film growth using CPS and CHS.

Utility of dual frequency hybrid source for plasma and radical generation in plasma enhanced chemical vapor deposition process

Looking into the aspect of material processing, this work evaluates alternative plasma concepts in SiH4/H2 plasmas to investigate the radical and plasma generation in the plasma enhanced chemical vapor deposition (PECVD) synthesis of nanocrystalline Si (nc-Si:H). Simultaneous measurements by vacuum ultraviolet absorption spectroscopy (VUVAS), optical emission spectroscopy (OES), and radio frequency (RF) compensated Langmuir probe (LP) reveal that RF/ultrahigh frequency (UHF) hybrid source can efficiently produce H radicals and plasmas that are accountable for nc-Si:H film synthesis. The efficacy of hybrid plasmas is also discussed.

Ultralow reflectivity surfaces by formation of nanocrystalline Si layer for crystalline Si solar cells

We have developed a simple method to form a nanocrystalline Si layer, which simply involves contact of Pt catalysts with Si wafers immersed in an H2O2 plus HF solution. The reflectivity becomes less than 3% after the formation of the nanocrystalline Si layer of ~150nm thickness. High quality pn-junction can be produced on the nanocrystalline Si/crystalline Si structure. With surface passivation using the deposition method, p-type single crystalline Si-based solar cells with the nanocrystalline Si layer generate a high photocurrent density of 39.2mA/cm2 under the standard test condition (STD) even without anti-reflection coating and the conversion efficiency of 18.2% is achieved. The passivation method using deposition of phosphosilicate glass on the nanocrystalline Si layer followed by annealing in forming gas improves the quantum efficiency in the short-wavelength region ranging between 300 and 600nm.

Efficiency enhancement for SiN-based light emitting device through introduction of Si nanocones in emitting layer

Silicon nitride-based light-emitting devices were fabricated with a SiNx emitting layer grown on annealed Si film of dense nano-crystalline cones. Comparative studies revealed that the patterned SiNx emitting layer, with embedding nanocrystalline Si cones and a rough surface morphology of its own, manifests a much enhanced, even doubled at sufficiently large injected current density, electroluminescence efficiency. Both the increased light-extraction capability and the effective hole-blocking by the presence of Si nanocones, the latter is favorable for the balance of carrier injection in emitting layer, are responsible for this remarkable efficiency enhancement. The current work established an alternative approach toward the fabrication of more efficient SiN-based light-emitting devices.

Prospects of low-dimensional and nanostructured silicon-based thermoelectric materials: findings from theory and simulation

Silicon based low-dimensional materials receive significant attention as new generation thermoelectric materials after they have demonstrated record low thermal conductivities. Very few works to-date, however, report significant advances with regards to the power factor. In this review we examine possibilities of power factor enhancement in: (i) low-dimensional Si channels and (ii) nanocrystalline Si materials. For low-dimensional channels we use atomistic simulations and consider ultra-narrow Si nanowires and ultra-thin Si layers of feature sizes below 15 nm. Room temperature is exclusively considered. We show that, in general, low-dimensionality does not offer possibilities for power factor improvement, because although the Seebeck coefficient could slightly increase, the conductivity inevitably degrades at a much larger extend. The power factor in these channels, however, can be optimized by proper choice of geometrical parameters such as the transport orientation, confinement orientation, and confinement length scale. Our simulations show that in the case where room temperature thermal conductivities as low as κ l = 2 W/mK are achieved, the ZT figure of merit of an optimized Si low-dimensional channel could reach values around unity. For the second case of materials, we show that by making effective use of energy filtering, and taking advantage of the inhomogeneity within the nanocrystalline geometry, the underlying potential profile and dopant distribution large improvements in the thermoelectric power factor can be achieved. The paper is intended to be a review of the main findings with regards to the thermoelectric performance of nanoscale Si through our simulation work as well as through recent experimental observations.