Thin SiOx Layer Research Articles

Study of SiOx Thickness Effects on Aluminum-induced Crystallization

The aluminum-induced crystallization (AIC) used SiOx was studied layer as an intermediate layer of aluminum and silicon. SiOx layer thickness was varied from 2 nm to 20 nm, and affected crystallization process of the AIC. In the case of thin SiOx layer, crystallized silicon morphology showed kinetic-limited aggregation. On the other hand, crystallized silicon processed with thick SiOx layer showed diffusion-limited aggregation due to slow silicon diffusion velocity. The kinetic-limited aggregation showed large grain. The crystallized silicon grain size relatively increased at kinetic-limited aggregation. The schematic crystallization model was used to describe a relationship between crystallization and grain size in this paper.

The application of poly-Si/SiOx contacts as passivated top/rear contacts in Si solar cells

Passivated contacts based on low-pressure chemical vapor deposited (LPCVD) heavily-doped poly-Si and a thin SiOx layer are explored for the application in an interdigitated back contact (IBC) solar cell. The poly-Si/SiOx contacts are realized by applying wet-chemically grown SiOx tunnel layers and amorphous Si (a-Si) layers doped via ion implantation that are subsequently transformed into poly-Si/SiOx contacts by a high temperature step. The impact of doping species, ion dose, and poly-Si thickness on the surface passivation of such contacts is studied. Excellent J0 values down to 4.5fA/cm2 were measured for n+-poly-Si contacts, while J0 values as low as 22fA/cm2 were obtained for p+-poly-Si contacts. Solar cells with top/rear poly-Si contacts were processed and Voc values up to 709mV and FF values above 81% were measured. Furthermore, the upper bound for the parasitic absorption losses in 10–40nm thick poly-Si films was quantified to be about 0.5mA/cm2 per 10nm poly-Si layer thickness.

Passivation analysis of silicon surfaces via sol—gel derived Al-rich ZnO film

Electronic recombination losses can be reduced via passivation of silicon surfaces. Most techniques available in the literature are either not cost effective or not applicable for solar cell applications. We investigate low cost sol–gel derived Al-rich zinc oxide (ZnO:Al) film and its effective passivation of p-type silicon surfaces. Herein, we present the elemental composition of the film and interfacial structure of ZnO:Al/Si using FTIR, XPS, TEM, and SIMS characterizations. ZnO:Al is polycrystalline and contains some very small amorphous regions of Al2O3. At the ZnO:Al/c-Si interface, a thin SiOx layer with a thickness of ∼6 nm is formed. The XPS analyses reveal that the Al/Zn molar ratio in the ZnO:Al increases from ∼10% at the surface to ∼80% at the ZnO:Al/c-Si interface. The hydrogen content also gradually increases from the surface to the interface. The FTIR absorption area corresponding to the Si–H bonding is ∼2.89 au. The obtained hydrogen concentration is ∼3.93 × 1022 atoms cm−3. A fixed negative charge is created by ZnO:Al on ZnO//SiOx interface. The thermal equilibrium was established between Si and ZnO:Al through SiOx by electron tunneling current. Here, the c-Si may be passivated for two reasons: (i) Al creates defects on the ZnO:Al/c-Si interface and H is attached to the defects (dangling bonds) and (ii) due to the field effect passivation via the negative charged ZnO:Al film.

Epitaxial growth of silver nanoislands on the surface of silicon nanowires in ambient air

We study the epitaxial growth of Ag nanoislands on the surface of silicon nanowires (SiNWs) in ambient air by annealing SiNWs fabricated by metal-assisted chemical etching using Ag particles as the catalysts. We demonstrate that the nucleation of Ag on the surface of SiNWs is realized either by the direct decomposition of AgmO or by the reaction between AgmO and Si depending on the annealing temperature; AgmO is an adspecies with a lower effective detachment barrier, commonly introduced in oxygen-induced Ag surface migration systems. Along with the formation of Ag nanoislands, a thin layer of SiOx is formed on the outside surface of Ag nanoislands. The ambient-air epitaxial growth of Ag islands on the surface of SiNWs offers great flexibility in designing ideal metal contacts and Schottky barrier formation, and studying the electronic, magnetic and optical properties of the Si/Ag system. We also believe that SiNWs decorated by Ag nanoislands have many potential applications in plasmonic photovoltaic cells, surface-enhanced Raman spectroscopy detection and electronic devices.

Improved passivation effect at the amorphous/crystalline silicon interface due to ultrathin SiOx layers pre-formed in chemical solutions

To improve the passivation effect at a-Si:H/c-Si interface in heterojunction (HJ) solar cells, ultrathin SiOx layers with a thickness of approximately 2 nm were pre-formed on c-Si surfaces in chemical solutions. It was demonstrated that the SiOx layers pre-formed in hot de-ionized water and hydrochloric acid solutions improve effective carrier lifetime, and it is further enhanced through a post annealing process. When the thin SiOx layers were applied to HJ solar cells, increase in both Voc and Jsc was achieved, implying the improved interface quality for these HJ solar cells, as compared with the reference without the SiOx layer.

Interactions of gaseous HNO3 and water with individual and mixed alkyl self-assembled monolayers at room temperature.

The major removal processes for gaseous nitric acid (HNO3) in the atmosphere are dry and wet deposition onto various surfaces. The surface in the boundary layer is often covered with organic films, but the interaction of gaseous HNO3 with them is not well understood. To better understand the factors controlling the uptake of gaseous nitric acid and its dissociation in organic films, studies were carried out using single component and mixtures of C8 and C18 alkyl self-assembled monolayers (SAMs) attached to a germanium (Ge) attenuated total reflectance (ATR) crystal upon which a thin layer of SiOx had been deposited. For comparison, diffuse reflectance infrared Fourier transform spectrometry (DRIFTS) studies were also carried out using a C18 SAM attached to the native oxide layer on the surface of silicon powder. These studies show that the alkyl chain length and order/disorder of the SAMs does not significantly affect the uptake or dissociation/recombination of molecular HNO3. Thus, independent of the nature of the SAM, molecular HNO3 is observed up to 70-90% relative humidity. After dissociation, molecular HNO3 is regenerated on all SAM surfaces when water is removed. Results of molecular dynamics simulations are consistent with experiments and show that defects and pores on the surfaces control the uptake, dissociation and recombination of molecular HNO3. Organic films on surfaces in the boundary layer will certainly be more irregular and less ordered than SAMs studied here, therefore undissociated HNO3 may be present on surfaces in the boundary layer to a greater extent than previously thought. The combination of this observation with the results of recent studies showing enhanced photolysis of nitric acid on surfaces suggests that renoxification of deposited nitric acid may need to be taken into account in atmospheric models.

Silicon-Catalyzed Growth of Amorphous SiOx Nanowires by Laser Vaporization of Si and Si/SiO2

Products from the laser vaporization of targets of four types—Si, (1:1) Si/SiO2, (1:9) Si/SiO2, and SiO2—without the addition of any metal catalysts in the presence of high-pressure (0.9 MPa) Ar gas were characterized by transmission electron microscopy (TEM), high-resolution TEM, energy-filtered TEM, transmission electron diffractometry, energy-dispersive X-ray spectroscopy, electron energy loss spectroscopy, and powder X-ray diffractometry. Amorphous SiOx (0.8 ≤ x ≤ 2.0) nanowires (NWs) (10–40 nm in diameter and up to 1 μm long), having crystalline Si nanoparticles (NPs) covered with thin amorphous SiOx layers at the NW tips, were observed for the products from the Si and Si/SiO2 composite targets, while only amorphous SiOx NPs were produced from the SiO2 target. The O content relative to that of the Si in SiOx NWs was also increased with increasing the content of SiO2 in the targets. We propose a novel mechanism for the growth of amorphous SiOx NWs, in which nuclei of the NWs are formed on the Si-rich ...

Surface modification of poly(dimethylsiloxane) by atmospheric pressure high temperature plasma torch to prepare high-performance gas separation membranes

In this study, an atmospheric pressure high temperature plasma torch (APHTPT) was used to modify poly(dimethylsiloxane) (PDMS) membranes in order to form a thin SiOx layer on the membrane surface. The chemical properties of the PDMS and APHTPT-treated membranes, as well as the conversion of polysiloxane, were determined by X-ray photoelectron spectroscopy (XPS) and energy-dispersive X-ray (EDX) microanalysis. The microstructure of the membranes as a function of depth was obtained with a variable monoenergy slow positron beam (VMSPB) spectroscopy. Results of XPS and EDX indicated that the conversion of polysiloxane to silica was favored with increasing plasma power. VMSPB data demonstrated that the treated membrane exhibited an asymmetric organic–inorganic hybrid structure. Based on the analysis of the treated membrane, the free-volume size increased with the depth and showed a bi-modal distribution. The gas permeation properties of O2, N2, and CO2 were tested. The change in the applied plasma power was found to have a great effect on the membrane gas separation performance. The membrane selectivity for O2/N2 and CO2/N2 increased from 2.11 to 4.93 and from 9.54 to 17.19, respectively, after treatment at 10kW. The APHTPT-treated PDMS membrane was found to have the advantages of both organic and inorganic membranes, leading to outstanding gas separation performance.

Self-Assembled Incorporation of Modulated Block Copolymer Nanostructures in Phase-Change Memory for Switching Power Reduction

Phase change memory (PCM), which exploits the phase change behavior of chalcogenide materials, affords tremendous advantages over conventional solid-state memory due to its nonvolatility, high speed, and scalability. However, high power consumption of PCM poses a critical challenge and has been the most significant obstacle to its widespread commercialization. Here, we present a novel approach based on the self-assembly of a block copolymer (BCP) to form a thin nanostructured SiOx layer that locally blocks the contact between a heater electrode and a phase change material. The writing current is decreased 5-fold (corresponding to a power reduction by 1/20) as the occupying area fraction of SiOx nanostructures is increased from a fill factor of 9.1% to 63.6%. Simulation results theoretically explain the current reduction mechanism by localized switching of BCP-blocked phase change materials.

Effects of interface modification by H2O2 treatment on the electrical properties of n-type ZnO/p-type Si diodes

The fabrication and detailed electrical properties of heterojunction diodes based on n-type ZnO and p-type Si were reported. The effect of interface modification by H2O2 treatment on the electrical properties of n-type ZnO/p-type Si diodes was investigated. The n-type ZnO/p-type Si diode without H2O2 treatment showed a poor rectifying behavior with an ideality factor (n) of 2.5 and high leakage, indicating that the interfacial ZnSixOy layer influenced the electronic conduction through the device. However, the n-type ZnO/p-type Si diode with H2O2 treatment showed a good rectifying behavior with n of 1.3 and low leakage. This is because the thin SiOx layer acts as a thermodynamically stable buffer layer to suppress interfacial reaction between ZnO and Si. In addition, the enhanced photo-responsivity can be interpreted by the device rectifying performance and interface passivation.

The work function of n-ZnO deduced from heterojunctions with Si prepared by ALD

Highly doped n-type ZnO films have been grown on n-type and p-type Si substrates by atomic layer deposition (ALD). Transmission electron microscopy shows columnar growth of the ZnO films with randomly oriented grains and a very thin interfacial layer of SiOx(x ⩽ 2) with a thickness below 0.4 nm to the Si substrate. Current–voltage and capacitance–voltage measurements performed at temperatures from 50 to 300 K reveal a strong rectifying behaviour on both types of substrates with an ideality factor close to unity between 180 and 280 K. Using the classical approach of thermionic emission, the barrier heights of the ZnO/n-Si and ZnO/p-Si junctions have been deduced and consistent values are obtained yielding a work function of n-type ZnO close to 4.65 eV.

Preparation and Characteristics of SiOx Coated Carbon Nanotubes with High Surface Area.

An easy method to synthesize SiOx coated carbon nanotubes (SiOx-CNT) through thermal decomposition of polycarbomethylsilane adsorbed on the surface of CNTs is reported. Physical properties of SiOx-CNT samples depending on various Si contents and synthesis conditions are examined by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), nitrogen isotherm, scanning electron microscope (SEM), and transmission electron microscope (TEM). Morphology of the SiOx-CNT appears to be perfectly identical to that of the pristine CNT. It is confirmed that SiOx is formed in a thin layer of approximately 1 nm thickness over the surface of CNTs. The specific surface area is significantly increased by the coating, because thin layer of SiOx is highly porous. The surface properties such as porosity and thickness of SiOx layers are found to be controlled by SiOx contents and heat treatment conditions. The preparation method in this study is to provide useful nano-hybrid composite materials with multi-functional surface properties.

Fabrication of a n-ZnO/p-Si heterojunction diode by ultra-high vacuum magnetron sputtering

Heterojunction diodes of n-type ZnO were fabricated on a p-type Si(100) substrate using an ultra-high vacuum radio frequency magnetron sputtering method at room temperature. A short-time post-annealing process was performed to prevent inter-diffusion of Zn, dopants, and Si atoms. The post-annealing process at 600°C enhanced the crystallinity of ZnO films and produced a high forward to reverse current ratio of the heterojunction diode with a barrier height of approximately 0.336eV. A thin SiOx layer at the interface of the ZnO film and Si substrate appeared distinctly at the 600°C annealing, however the post-annealing at 700°C showed an a-(Zn2xSi1−xO2) structure caused by diffusion of silicon into the ZnO film. In the n-ZnO/p-Si sample annealed at 700°C, a rapid change in the barrier height was considered due to the effect of the dopant segregation from the substrate and deformation of the a-SiOx structure.

Carbon nanotube bundles self-assembled in double helix microstructures

Double helix microstructures consisting of two parallel strands, each composed of hundreds of multiwalled carbon nanotubes (MWCNTs) are synthesised by chemical vapour deposition (CVD) of ferrocene/toluene vapours on thermochemically treated metal substrates, such as steel, Cu, Al and W. The thermochemical treatment produces a thin and brittle layer of SiOx. During the CVD process, carbon nanotubes (CNT) grow adhered to this layer, and as growth progresses, small SiOx microparticles detach from the substrate, directing the helical development of the growing MWCNTs double strands. This growth model for the helical microstructures is compared in the manuscript with models previously reported for coiled carbon fibres grown in the gas phase. A unique aspect of these double helices when they are composed of carbon nanotubes is that they grow on top of a forest of aligned CNTs.

Photoluminescence study of the structural evolution of amorphous and crystalline silicon nanoclusters during the thermal annealing of silicon suboxide films with different stoichiometry

The effect of the stoichiometry of thin silicon suboxide films on the processes of the formation and evolution of silicon nanoclusters during thermal annealing is studied by photoluminescence measurements. The samples are produced by the thermal sputtering of a SiO powder in an oxygen atmosphere, with the subsequent deposition of a 500 nm-thick SiOx layer onto a Si substrate. The morphological properties and size of Si nanoclusters are explored by analyzing the photoluminescence spectra and kinetics. A comparative study of the luminescence properties of thin SiOx layers with different stoichiometric parameters, x = 1.10, 1.29, 1.56, and 1.68, is accomplished for samples annealed at different temperatures in the range 850 to 1200°C. The dependences of the photoluminescence decay time on the annealing temperature, the stoichiometric parameter of the initial silicon suboxide film, and the nanocluster size are studied.

The effect of MgO(111) interlayer on the interface phase stability and structure of BaFe12O19/SiC(0001)

We present a study on the effect of an interlayer of thin MgO(111) film on SiC(0001) on the interface phase stability and structure of the BaFe12O19 (BaM). The 10 nm MgO(111) interlayer followed by the BaM film were grown by molecular beam epitaxy on 6H-SiC. Cross-sectional transmission electron microscopy shows the formation of a magnesium ferrite spinel phase at the interface, and after 25 nm, a well structured BaM film was observed. In addition to the two main phases (Mg-ferrite and BaM), a thin layer of SiOx (2–3 nm) is formed at the SiC interface. In spite of the formation of this amorphous layer, the diffraction studies show that the BaM film is epitaxially grown and it has a single crystal structure. The energy dispersive x-ray analysis from the interface region shows that the MgO layer prevents significant outdiffusion of the Si into the film. Total energy calculations by density functional theory were used to investigate the stability of the various phases and to explain the observed interfacial phases in the studied system.

Large-scale: Synthesis, microstructure, and FT-IR property of SiC nanowires

Large-scale SiC nanowires have been synthesized via reacting silicon powder with carbon powder using Ni2O3 as catalyst by simple chemical vapor deposition method. The mixed powders were heated in an alumina horizontal tube furnace up to 1350 °C for 1 h in an argon atmosphere. Characterization by scanning electron microscope, transmission electron microscope, and X-ray diffractometer indicated that the nanowires covered with a thin layer of amorphous SiOx. The 3C–SiC nanowires were formed by vapor–liquid–solid growth mechanism and they have diameters of about 20–90 nm. The result of FT-IR spectrum shows the nanowires were composed of SiC and amorphous of silicon oxide.

Structural and photoluminescence investigation on the hot-wire assisted plasma enhanced chemical vapor deposition growth silicon nanowires

High density of silicon nanowires (SiNWs) were synthesized by a hot-wire assisted plasma enhanced chemical vapor deposition technique. The structural and optical properties of the as-grown SiNWs prepared at different rf power of 40 and 80W were analyzed in this study. The SiNWs prepared at rf power of 40W exhibited highly crystalline structure with a high crystal volume fraction, XC of ∼82% and are surrounded by a thin layer of SiOx. The NWs show high absorption in the high energy region (E>1.8eV) and strong photoluminescence at 1.73 to 2.05eV (red–orange region) with a weak shoulder at 1.65 to 1.73eV (near IR region). An increase in rf power to 80W reduced the XC to ∼65% and led to the formation of nanocrystalline Si structures with a crystallite size of <4nm within the SiNWs. These NWs are covered by a mixture of uncatalyzed amorphous Si layer. The SiNWs prepared at 80W exhibited a high optical absorption ability above 99% in the broadband range between 220 and ∼1500nm and red emission between 1.65 and 1.95eV. The interesting light absorption and photoluminescence properties from both SiNWs are discussed in the text.

A 3D porous architecture of Si/graphene nanocomposite as high-performance anode materials for Li-ion batteries

A 3D porous architecture of Si/graphene nanocomposite has been rationally designed and constructed through a series of controlled chemical processes. In contrast to random mixture of Si nanoparticles and graphene nanosheets, the porous nanoarchitectured composite has superior electrochemical stability because the Si nanoparticles are firmly riveted on the graphene nanosheets through a thin SiOx layer. The 3D graphene network enhances electrical conductivity, and improves rate performance, demonstrating a superior rate capability over the 2D nanostructure. This 3D porous architecture can deliver a reversible capacity of ∼900 mA h g−1 with very little fading when the charge rates change from 100 mA g−1 to 1 A g−1. Furthermore, the 3D nanoarchitechture of Si/graphene can be cycled at extremely high Li+ extraction rates, such as 5 A g−1 and 10 A g−1, for over than 100 times. Both the highly conductive graphene network and porous architecture are considered to contribute to the remarkable rate capability and cycling stability, thereby pointing to a new synthesis route to improving the electrochemical performances of the Si-based anode materials for advanced Li-ion batteries.

Controlling the fixed charge and passivation properties of Si(100)/Al2O3 interfaces using ultrathin SiO2 interlayers synthesized by atomic layer deposition

Al2O3 synthesized by atomic layer deposition (ALD) on H-terminated Si(100) exhibits a very thin (∼1 nm) interfacial SiOx layer. At this interface, a high fixed negative charge density, Qf, is present after annealing which contributes to ultralow surface recombination velocities &amp;lt;2 cm/s. Here, we identify the thickness of the interfacial SiO2 layer as a key parameter determining Qf. The SiO2 thickness was controlled by intentionally growing ultrathin SiO2 interlayers (0.7−30 nm) by ALD. Optical second-harmonic generation spectroscopy revealed a marked decrease in Qf for increasing SiO2 thickness between 0 and 5 nm. This phenomenon is consistent with charge injection across the interfacial layer during annealing. For thicker SiO2 interlayers (&amp;gt;∼5 nm), the polarity of the effective charge density changed from negative to positive. The observed changes in Qf and the associated field-effect passivation had a significant influence on the injection-level-dependent minority carrier lifetime of Si.