Silica Buffer Layer Research Articles

Fabrication of Aluminum Oxide Thin-Film Devices Based on Atomic Layer Deposition and Pulsed Discrete Feed Method

This study demonstrates the low-temperature (<100 °C) process for growing a thin silica buffer layer and aluminum oxide by atomic layer deposition (ALD) in the same reaction chamber. Heterogeneous multilayer thin films are prepared by a dual-mode equipment based on atomic layer deposition and plasma-enhanced chemical vapor deposition (PECVD) techniques. The pulse discrete feeding method (DFM) was used to divide the precursor purging steps into smaller intervals and generate discrete feeds, which improved the saturated distribution of gas precursors, film density and deposition selectivity. The experimental results show that the process method produces a uniform microstructure and that the best film uniformity is ±2.3% and growth rate is 0.69 Å/cycle. The thickness of aluminum oxide film has a linear relationship with the cyclic growth number from 360 to 1800 cycles. Meanwhile, the structural and mechanical stress properties of aluminum oxide thin films were also verified to meet the requirements of advanced thin-film devices.

Performance improvement of blue TADF top-emission OLEDs by tuning hole injection barriers using a nickel-doped silicon dioxide buffer layer

The severe exciton self-quenching and twisted structure of typical thermally activated delayed fluorescence (TADF) emitters result in a low external quantum efficiency (EQE) and broad emission spectrum, especially in blue organic light-emitting diodes (OLEDs). These challenges have been overcome by employing complex device fabrication steps and complex molecular structures requiring fine stoichiometric adjustments. However, only a few attempts have been made to improve the performance of OLEDs through device structure engineering, particularly in top-emission OLEDs (TEOLEDs). Herein, we report blue TADF TEOLEDs fabricated by employing a second-order microcavity structure. These TEOLEDs simultaneously exhibit a high EQE (∼20.2 %) and narrow full width at half maximum (29 nm) owing to the incorporation of a Ni-doped SiO2 buffer layer that balances the injection charges. The wide bandgap of SiO2 prevents rapid hole injection from the anode and modifies the anode–organic layer interface, while tuning of the Ni doping concentration reduces the turn-on voltage of the device through co-sputtering. The charge balance mechanism is elucidated in detail by analyzing tunneling effects across the buffer layer and the bonding states of Ni atoms in the SiO2 film. This study can promote the development of TADF top-emission devices with high efficiency and high color purity.

Performance Improvement of Blue Tadf Top-Emission Oleds by Tuning Hole Injection Barriers Using a Nickel-Doped Silicon Dioxide Buffer Layer

Performance Improvement of Blue Tadf Top-Emission Oleds by Tuning Hole Injection Barriers Using a Nickel-Doped Silicon Dioxide Buffer Layer

Field effect transistor with thin AlOxNy film as gate dielectric

PurposeThe purpose of this study is to directly measure and determine the Si/SiO2/AlOxNy interface state density on metal insulator semiconductor field effect transistor (MISFET) structures. The primary advantage of using aluminum oxynitride (AlOxNy) is the perfectly controlled variability of the properties of these layers depending on their stoichiometry, which can be easily controlled by the parameters of the magnetron sputtering process. Therefore, a continuous spectrum of properties can be achieved from the specific values for oxide to the specific ones for nitride, thus opening a wide range of applications in high power, high temperature and high frequency electronics, optics and sensors and even acoustic devices.Design/methodology/approachThe basic subject of this study is n-channel transistors manufactured using silicon with 50-nm-thick AlOxNy films deposited on a silicon dioxide buffer layer via magnetron sputtering in which the gate dielectric was etched with wet solutions and/or dry plasma mixtures. Furthermore, the output, transfer and charge pumping (CP) characteristics were measured and compared for all modifications of the etching process.FindingsAn electrical measurement of MISFETs with AlOxNy gate dielectrics was conducted to plot the current-voltage and CP characteristics and examine the influence of the etching method on MISFET parameters.Originality/valueIn this report, a flat band and threshold voltage and the density of interface traps were determined to evaluate and improve an AlOxNy-based MISFET performance toward highly sensitive field effect transistors for hydrogen detection by applying a Pd-based nanocrystalline layer. The sensitivity of the detectors was highly correlated with the quality of the etching process of the gate dielectrics.

Electronic structure of molybdenum-involved amorphous silica buffer layer in MoOx/n-Si heterojunction

An amorphous silica layer containing Mo component (a-SiOx(Mo) ∼ 3.5–4.0 nm) is naturally formed in the process of evaporating MoO3 power onto n-type silicon, which plays a significant role on the non-equilibrium carriers’ transport of MoOx/n-Si heterojunction photovoltaic device. The electronic structure and the charge transition levels of the amorphous silicon oxide doped with Mo are derived from density functional theory. The density of states show that the five local states are existed in the band gap of amorphous silicon oxide, caused by the hybridization of Mo 4d orbital and O 2p orbital. The charge transition level of ε(+1/0) is located at 3.59 eV above the valance band maximum of a-SiO2, which could be a passageway of holes produced in the opto-electric conversion. Defects level-assisted quantum transport mechanism is put forwarded to explain hole transport phenomenon in the MoOx/SiOx(Mo)/n-Si PV device.

GaAs Solar Cells on Nanopatterned Si Substrates

Integrating III–Vs onto Si is a promising route toward tandem photovoltaics and cost mitigation of III–V substrates. While many III–V/Si photovoltaic integration approaches have been studied, epitaxial growth on Si allows for fewer processing steps compared to other approaches. However, current epitaxial pathways utilize expensive techniques, such as thermal cycle annealing or thick buffer layers to control defect densities, undermining the low-cost goal of integrating III–Vs with Si. Here, we present single-junction GaAs solar cells grown directly on Si using selective area growth as an alternative low-cost technique to control material quality with a much thinner buffer. We demonstrate a 10.4%-efficient GaAs device grown on a V-grooved Si substrate, which achieved an antiphase domain-free III–V/Si interface and a threading dislocation density of 2 × 107 cm –2, despite the lack of a graded buffer. We compare this growth on V-grooved Si to solar cells grown on polished Si with and without a patterned silica buffer layer, which demonstrate efficiencies of 6.5% and 6.8%, respectively; the polished Si was patterned using nanoimprint lithography, which is a low-cost patterning technique compatible with III–V selective area growth. Cracking is found to be a critical challenge that hinders solar cell performance and is exacerbated by the V-grooved Si surface topography. The results in this paper provide a promising pathway toward high-efficiency, low-cost III–V/Si tandem photovoltaics.

Diminished band discontinuity at the p/i interface of narrow-gap a-SiGe:H solar cell by hydrogenated amorphous silicon oxide buffer layer

We optimized hydrogenated amorphous silicon oxide (a-SiOx:H) as a buffer layer at the p/i interface of hydrogenated amorphous silicon germanium (a-SiGe:H) solar cells. This buffer layer was intended to effectively diminish the band discontinuity between the high-bandgap p-window layer and low-gap a-SiGe:H absorption layer, thereby enhancing the cell performance. The open-circuit voltage (Voc) increased by 3.7% as the [CO2]/[SiH4] gas ratio increased from 0 to 0.06. In addition, the short-circuit current density (Jsc) gradually increased with the gas ratio. Using the optimized a-SiOx:H buffer layer, a high performance of 9.6% was recorded for narrow-gap a-SiGe:H thin film solar cells. a-SiGe:H solar cells with optimized a-SiOx:H buffer layers, used as middle subcells, are expected to enhance the total Voc of triple-junction configuration solar cells.

Recent Advances in Low-Temperature Deposition Methods of Transparent, Photocatalytic TiO2 Coatings on Polymers

In this paper, we present an overview as well as current advances in the low-temperature deposition of highly crystalline suspensions of titania nanoparticles on polymers for photocatalytic applications. The presence of preformed titania nanoparticles yields the possibility of producing photocatalytically active coatings at reduced temperatures. Transparent and photocatalytically active TiO2 coatings that degrade organic matter, have been widely applied to bestow self-cleaning properties onto surfaces. This low-temperature deposition method and its transition to polymers would open an entire array of possible self-cleaning applications. During this research, incorporation of a silica buffer layer was applied to improve the compatibility of the inorganic coating on a substrate, such as polymethylmethacrylate (PMMA) and polyphenylsulphone (PPSU). The photocatalytic activity of the obtained coating was analyzed for its photocatalytic abilities by evaluating the color removal of a dye solution (methylene blue, MB) under UV irradiation and compared with commercial Pilkington Activ® self-cleaning glass. Our results indicate that the titania-coated silica-polymer systems yield a higher photocatalytic activity towards the degradation of organic pollutants. This method proves that the deposition of highly crystalline anatase suspensions on silica buffer layers is a viable method to produce photocatalytic coatings on heat-sensitive substrates.

The role of hydrogenated amorphous silicon oxide buffer layer on improving the performance of hydrogenated amorphous silicon germanium single-junction solar cells

Hydrogenated amorphous silicon oxide (a-Si1−xOx:H) film was used as a buffer layer at the p-layer (μc-Si1−xOx:H)/i-layer (a-Si1−xGex:H) interface for a narrow band gap hydrogenated amorphous silicon germanium (a-Si1−xGex:H) single-junction solar cell. The a-Si1−xOx:H film was deposited by plasma enhanced chemical vapor deposition (PECVD) at 40 MHz in a same processing chamber as depositing the p-type layer. An optimization of the thickness of the a-Si1−xOx:H buffer layer and the CO2/SiH4 ratio was performed in the fabrication of the a-Si1−xGex:H single junction solar cells. By using the wide band gap a-Si1−xOx:H buffer layer with optimum thickness and CO2/SiH4 ratio, the solar cells showed an improvement in the open-circuit voltage (Voc), fill factor (FF), and short circuit current density (Jsc), compared with the solar cells fabricated using the conventional a-Si:H buffer layer. The experimental results indicated the excellent potential of the wide-gap a-Si1−xOx:H buffer layers for narrow band gap a-Si1−xGex:H single junction solar cells.

고효율 실리콘 박막태양전지를 위한 신규 수소저감형 비정질실리콘 산화막 버퍼층 개발

We propose a novel hydrogen-reduced p-type amorphous silicon oxide buffer layer between TiO₂ antireflection layer and p-type silicon window layer of silicon thin film solar cells. This new buffer layer can protect underlying the TiO₂ by suppressing hydrogen plasma, which could be made by excluding H2 gas introduction during plasma deposition. Amorphous silicon oxide thin film solar cells with employing the new buffer layer exhibited better conversion efficiency (8.10 %) compared with the standard cell (7.88 %) without the buffer layer. This new buffer layer can be processed in the same p-chamber with in-situ mode before depositing main p-type amorphous silicon oxide window layer. Comparing with state-of-the-art buffer layer of AZO/p-nc-SiOx:H, our new buffer layer can be processed with cost-effective, much simple process based on similar device performances.

Synergy Between Plasma‐Assisted ALD and Roll‐to‐Roll Atmospheric Pressure PE‐CVD Processing of Moisture Barrier Films on Polymers

The synergy between fast (1600 nm · min−1), roll‐to‐roll plasma‐enhanced chemical vapor deposited (PE‐CVD) SiO2 layers and plasma‐assisted atomic layer deposited (PA‐ALD) ultra‐thin Al2O3 films has been investigated in terms of moisture permeation barrier properties. The effective and intrinsic water vapor transmission rates (WVTR) were studied as a function of the number of ALD cycles. It was demonstrated that a synergistic combination of a silica buffer layer deposited on polymer with an ultra‐thin (≤ 2 nm) alumina barrier film can provide excellent intrinsic (10−5–10−6 g · m−2 · day−1) and good effective (∼10−3 g · m−2 · day−1) WVTR values, whereas both single layers individually exhibit poor barrier performances with effective WVTR values of ≥ 1.0 g · m−2 · day−1.

Improved performance of microcrystalline silicon solar cell with graded-band-gap silicon oxide buffer layer**Project supported by the Key Basic Research Project of Hebei Province, China (Grant Nos. 12963930D and 12963929D), the Natural Science Foundation of Hebei Province, China (Grant Nos. F2013201250 and E2012201059), and the Science and Technology Research Projects of the Education Department of Hebei Province, China (Grant No. ZH2012030).

Microcrystalline silicon (μc-Si:H) solar cell with graded band gap microcrystalline silicon oxide (μc-SiOx:H) buffer layer is prepared by plasma enhanced chemical vapor deposition and exhibits improved performance compared with the cell without it. The buffer layer moderates the band gap mismatch by reducing the barrier of the p/i interface, which promotes the nucleation of the i-layer and effectively eliminates the incubation layer, and then enhances the collection efficiency of the cell in the short wavelength region of the spectrum. The p/i interface defect density also decreases from 2.2 × 1012 cm−2 to 5.0 × 1011 cm−2. This graded buffer layer allows to simplify the deposition process for the μc-Si:H solar cell application.

Hierarchical hybrid carbon nano-structures as robust and reusable adsorbents: Kinetic studies with model dye compound

This study demonstrates the potential of using multi-scale hierarchical carbon structures as robust, reusable solids suitable for removal of aqueous pollutants such as dye molecules from wastewater. Carpet-like vertical arrays of carbon nanotubes (CNT) were attached on surfaces of porous carbon foams by pre-coating with silica buffer layer followed by chemical vapor deposition (CVD). The surface morphology and specific surface areas were varied by controlling the buffer layer thickness and CVD deposition times. Surface characteristics have been correlated with adsorption thermodynamics and kinetics by investigating the removal rates of methylene blue (MB) dye in simulated water. The results show that MB adsorption capacity correlates well with total CNT surface area in the carpet, indicating full utilization of the nanotube surfaces. Adsorption rates fit well with pseudo second order kinetics model. Maximum MB adsorption capacity of the CNT surfaces in this structure was estimated by extrapolating equilibrium adsorption amounts at different dye concentrations using the Langmuir isotherm. This was found to be about 43.5mg/g, which compares favorably with adsorption capacity of isolated nanotubes. It must be noted that whereas isolated nanotubes can disperse in the liquid and pose environmental threats, the hierarchical solids as demonstrated in this study can be repeatedly agitated in water without the loss of CNT or performance deterioration. These results indicate promising application potential for these types of hybrid materials in environmental purification applications.

Thermoelectric systems: Ion beam enhanced thermoelectric properties

Abstract A thermoelectric system based on the interaction of composites of two metal nanocrystals in silica substrate prepared by a multilayer structure having alternate layers of metal/material mixture is discussed in detail in this paper. The alternate layers component of this work has two metal content, gold and silver. The layered structure irradiated with ionizing radiation to produce nanoclusters in these layers one on the top of each metal nanocrystal containing nanolayer with no silica buffer layer. The differing metal content serves to quench the nanoclusters to isolate nanoclusters along the 5.0 MeV Si beam irradiation track, as described in our past 16 years publications [1] , [2] , [3] , [4] , [5] . The result is a thermoelectric material with high electrical conductivity, low thermal conductivity, high Seebeck coefficient thus a high figure of merit as high as 3.1, as described in this work.

An Ultraviolent-Enhanced Response Solar Cell and Tunneling Effect

A sort of ITO/SiO2/n+p configuration of solar grade silicon - based SINP device has been successfully fabricated by coupling high quality of indium tin oxide (ITO) thin film and ultra-thin silicon dioxide buffer layer on the p-n junction, which showed a potential to improve the short-circuit current density (JSC) as well as the conversion efficiency. The progress to avoid the use of last two processes with PECVD and high temperature firing in the normal production for the crystal silicon solar cell may be a way toward the low cost and the high efficiency. In addition, the tunneling hypostasis of carriers is present in the device.

Silicon oxide buffer layer at the p–i interface in amorphous and microcrystalline silicon solar cells

The use of intrinsic silicon oxide as a buffer layer at the p–i interface of thin-film silicon solar cells is shown to provide significant advantages. For microcrystalline silicon solar cells, when associated with highly crystalline i-layers deposited at high rates, all electrical parameters are improved. Larger efficiency gains are achieved with substrates of increased roughness. For cells with an improved i-layer material quality, there is mainly a gain in short-circuit current density. An improvement in carrier collection in the blue region of the spectrum is systematically observed on all the cells. The presence of a silicon oxide buffer layer also promotes the nucleation of the subsequent intrinsic microcrystalline silicon layer. In amorphous silicon solar cells, the silicon oxide buffer layer is proven to act as an efficient barrier to boron cross-contamination, eliminating the need for additional processing steps (e.g. water vapor flush), while providing a wide bandgap material at the interface. The implementation of silicon oxide buffer layers for both types of cells thus provides a decisive improvement, as it allows extremely fast deposition of the full p–i–n stack of layers of the cell in a single-chamber configuration while providing a high-quality substrate-resilient p–i interface.

Optimized amorphous silicon oxide buffer layers for silicon heterojunction solar cells with microcrystalline silicon oxide contact layers

We report on the systematic optimization of the intrinsic amorphous silicon oxide buffer layer in interplay with doped microcrystalline silicon oxide contact layers for silicon heterojunction solar cells using all silicon oxide based functional layers on flat p-type float-zone wafers. While the surface passivation quality is comparably good within a wide range of low oxygen contents, the optical band gap increases and the dark conductivity decreases with increasing oxygen content, giving rise to an inevitable trade-off between optical transparency and electrical conductivity. On the cell level, fill factor FF and short circuit current density Jsc losses compete with the open circuit voltage Voc gains resulting from a thickness increase of the front buffer layers, whereas Jsc and Voc gains compete with FF losses resulting from increasing thickness of the rear buffer layers. We obtained the highest active area efficiency of ηact = 18.5% with Voc = 664 mV, Jsc = 35.7 mA/cm2, and FF = 78.0% using 4 nm front and 8 nm rear buffer layer with an oxygen content of 5%.

Metal-coated hollow nanowires for low-loss transportation of plasmonic modes with nanoscale mode confinement

Two types of plasmonic waveguiding structures based on hollow dielectric nanowires are proposed and their modal properties are investigated numerically at a wavelength of 1550 nm. The first type of waveguide consists of a high-index hollow nanowire covered directly by a thin metallic film. Depending on the size of the hollow nanowire, such a waveguide could support a plasmonic mode with lower propagation loss than the metal-coated nanowire structures without a hollow core. To further reduce the propagation loss, a second type of waveguide is proposed, which includes an additional low-index silica buffer layer between the metal layer and the hollow nanowire. Simulations reveal that the additional low-index buffer could enable strong hybridization between the dielectric mode and the plasmonic mode, which leads to even lower propagation loss while maintaining nanoscale confinement similar to that of the first type of waveguide. Both of the proposed waveguides are feasible using modern fabrication methods and could facilitate potential applications in integrated photonic components and circuits.

Compact Polarization Rotator Based on Surface Plasmon Polariton With Low Insertion Loss

An ultracompact mode-evolution-based polarization rotator is proposed. The polarization rotator consists of silicon wire waveguide covered by silica buffer layer and linearly tapered thin aluminum. Numerical simulations using 3-D finite element method demonstrate that it is possible to obtain an 11-μm-long polarization rotator with about -20-dB extinction ratio and less than 4.7-dB loss in C-band for both polarization conversions from TM-like mode to TE-like mode and from TE-like mode to TM-like mode.

Diffractive Backside Structures via Nanoimprint Lithography

For decreasing thicknesses of wafer based silicon solar cells, photon management structures to maintain high quantum efficiencies will gain importance. Diffractive gratings on the wafer back side can be designed to achieve very high path length enhancements, especially for weakly absorbed infrared radiation. This technologically demanding concept has to be realised using processes with upscaling potential. Therefore, we present a fabrication process for producing photonic structures in silicon based on interference lithography and nanoimprint lithography (NIL).We realised linear as well as crossed gratings of different depths, which were etched into the wafer back side. Polarisation dependent reflection measurements were made to get information about potential absorption enhancement as well as the occurrence of parasitic absorption in the metal reflector. This is conducted for a PECVD silicon oxide buffer layer between grating and reflector as well as a spin coated silicon oxide layer. Besides these optical characterisations, we further investigated the electrical properties of the back surface, where we applied a concept in which electrical and optical properties are decoupled. This is realised by a layer stack on the wafer back side, consisting of a thin Al2O3 passivation and a doped amorphous silicon layer.