Excess Si Content Research Articles

Effect of Si on the Oxidation Behaviors of Ti3 Al1-x Six C2 at 1000 °C.

In this work, Ti3 Al1-x Six C2 (x=0, 0.2, 0.4, and 0.6) with Al/Si solid solution structure are synthesized, and the effects of Si on their oxidation behaviors at 1000 °C are evaluated. The addition of Si not only contributes to the formation of Ti5 Si3 impurity but also affects the composition of the oxide scale. Particularly, the incorporation of Si in the TiO2 lattice is demonstrated, which alters the formation energy of the (110) plane in TiO2 , thus leading to the preferential growth of Si-doped TiO2 to dendritic congeries. Moreover, the Si addition is believed to affect mass transportation during the oxidation process, which accelerates the formation of a continuous Al2 O3 layer in the oxide scale. With an optimized Si content, the oxidation of Ti3 Al1-x Six C2 is restrained. However, with excess Si content, the continuity of the resulting Al2 O3 layer is destroyed, thus the oxidation rate rises again.

Comparison of C-, N-, and O-Incorporated Non-blistering PECVD Si Films for Application in SiOx-Based Passivating Contacts for Si Solar Cells

For application in high-efficiency silicon solar cells bearing SiOx-based passivating contacts, we have developed 100 nm thick, PECVD Si-rich amorphous films, capable of being processed in industrial furnaces without blistering, by incorporating adequate amounts of each of C, N, and O in the films during PECVD. We found that non-blistering of the films is a result of the low intrinsic stress (compressive) in the films and not due to a low H content, which, in fact, is much higher in the non-blistering films than in the C-, N-, and O-free films. Among the three types of non-blistering Si-rich amorphous films, we conclude that the O-incorporated one is the most suitable for application in SiOx-based passivating contacts owing to the film’s high excess Si content and ability to crystallize.

A case study of machinability evaluation of a gravity cast hypereutectic Al-Si binary MMC in dry turning

Hard Si particles when dispersed widely throughout a eutectic α-Al matrix forms hypereutectic Al-Si binary alloy which is actually a simple in situ metal matrix composite (MMC). Hence, like other Al MMCs, they offer amalgamation of high strength-to-weight ratio and excellent wear resistance properties with high specific elastic modulus, exceptional heat-resistance and low thermal expansion and hence, are used in many industries. Machining of such materials like all other Al alloys is a challenging task mostly because of formation of built-up edges (BUEs) and long continuous chips. A case study of machinability evaluation of such a hypereutectic material with excess Si contents manufactured by gravity cast is presented. The study has been accomplished through investigation and analysis while carrying out turning experiments on a lathe by coated carbide tool inserts. The Taguchi orthogonal array is used for design and Response Surface Methodology has been employed for regression analysis and optimization.

Microstructure characterization of as-cast Al–Mg–Si alloys with high content Li element addition

The effects of adding a large quantity of Li on the microstructure and solidified precipitation behavior of an as-cast Al–Mg–Si alloy was investigated by thermodynamic calculation and microstructural characterization. The addition of Li substantially changed microstructural features and solidified precipitate phases of the Al–Mg–Si alloy. According to thermodynamic calculations, in addition to the Mg2Si phase, the second phases of Li-containing alloy also included δ-AlLi and Al2LiMg precipitates, in contrast to the base Al–Mg–Si alloy. Unusually, the Al2LiMg phase was found even when the content of Mg was below 2 wt%. The content, quantity, and distribution of the second phases in different alloys were revealed by optical microscopy, scanning electron microscopy and x-ray diffraction analysis synthetically. AlLiSi ternary compound was only found in the alloy with excess Si content. In addition, transmission electron microscopy observations were used to characterize crystal structures of the Li-containing phases. The results showed that a δ′-Al3Li metastable phase, instead of a δ-AlLi phase, was precipitated because of the occurrence of non-equilibrium solidification.

Investigation of Quench Sensitivity in 6xxx Aluminum Alloys

Quench trials were performed on AA6005A and AA6016 alloys to assess the sensitivity of their tensile properties as well as bendability to quench after solution heat treatment. Results indicate that the tensile properties in T4 and in the paint-baked state (2% pre-strain + 185 °C/20 min) are hardly affected by quench rate as long as the exit temperature (Texit) is sufficiently low. The bendability however, appears to be more sensitive to quench rate, and the sensitivity depends on the chemical composition of the alloy. The alloy with a higher excess Si content exhibits higher sensitivity to natural aging which in turn affects the bending and hemming performance of the material. Therefore, it is not only the quench rate which affects the bendability but also the temperature of the material at the end of quench. DSC analysis revealed how cluster formation proceeding the solution heat-treatment (SH) and quench provokes the quench sensitivity.

Origin of authigenic quartz in organic-rich shales of the Wufeng and Longmaxi Formations in the Sichuan Basin, South China: Implications for pore evolution

Primary sedimentary composition of shales and subsequent diagenetic modifications are key factors in controlling the pore development and preservation in shale gas reservoirs. Thin section examination, X-ray diffraction analysis, scanning electron microscopy (SEM) determination with cathodoluminescence and geochemical analyses were combined to investigate Wufeng and Longmaxi shales of the Sichuan Basin. These experimental methods allowed us to focus on the origin of authigenic quartz in organic-rich shales and evaluate its importance in the evolution of shale gas reservoirs. The siliceous shales in Wufeng and Longmaxi Formations contain up to 60% microcrystalline aggregates of quartz, which are more abundant than silt-size detrital quartz to suggest an authigenic origin. Most authigenic quartz originates from biogenic siliceous dissolution and re-precipitate. Therefore, high excess silica concentration indicates a higher surface productivity with more organic matter (OM) deposition. The abundance of OM pores is positively correlated with the excess-Si content (R2 = 0.77). We suggest that the migrated OM filled in authigenic microcrystalline quartz aggregates is the principal matrix for OM pore development. Authigenic microcrystalline quartz aggregates filled in primary pore space and destroyed primary interparticle porosity. However these authigenic quartz can restrain compaction and preserve the internal pore structure as a rigid framework. Pore space is abundant enough in microcrystalline quartz aggregates for oil and bitumen filling during the oil window. Migrated OM filled the associated pore network in authigenic microcrystalline quartz aggregates would more likely to generate a better three-dimensional OM pore network through secondary cracking and provides favorable reservoir spaces for shale gas.

Structural, optical, and electrical properties of silicon nanocrystals fabricated by high silicon content silicon-rich oxide and silicon dioxide bilayers

Intrinsic, boron (B)-doped, and phosphorus (P)-doped silicon nanocrystals (Si NCs) formed from an excess Si concentration of 40 at. % were investigated to study their structural, optical, and electrical properties. Atom probe tomography (APT) revealed that the size and arrangement of Si NCs were different in each sample. A strong blue shift in photoluminescence spectra for the intrinsic and B-doped Si NCs was correlated with the volume fraction of small Si NCs. The lower resistivity of the B-doped sample than the P-doped one was explained by the percolation of Si NCs through the film.

Extension of silicon emission model to silicon pillar oxidation

Missing Si in the oxidation of Si pillar structures is investigated by extending the Si emission model to the oxidation of planar structures. The original Si emission model [H. Kageshima et al., Jpn. J. Appl. Phys. 38, L971 (1999)] assumes the emission of excess Si from the interface into the oxide during the oxidation process, the diffusion of the excess Si through the oxide, and the control of the oxidation rate by the concentration of remaining excess Si around the interface. By assuming the sublimation of the excess Si from the oxide surface in addition to the assumptions of the original Si emission model, the origin of the missing Si is consistently explained. It is suggested that the amount of the missing Si is enhanced by the geometrical effect of the pillar structure because the concentration of excess Si is inversely proportional to the radial position. This also suggests that the missing Si is inevitable for the thin pillar structures. Careful approaches to the oxidation process are recommended for pillar structures.

Second-order nonlinear optical behavior of amorphous SiOx thin films grown by sputtering

Second harmonic generation from amorphous SiOx thin films prepared by sputtering was investigated under a nanosecond pulse excitation at 1064 nm. The investigation over a wide range of preparation conditions, i.e. excess Si concentration (CexSi) and annealing temperature, revealed that the effective second-order nonlinear optical coefficient (deff) reaches up to 0.59 pm V−1, which is comparable to that of α-quartz. The origin was discussed based on the results of x-ray photoelectron spectroscopy, electron spin resonance spectroscopy, and photoluminescence spectroscopy. A correlation was found between the ratio of deff to CexSi and the peak energy of the photoluminescence, suggesting that Si clusters are the most probable origin of the second-order nonlinearity.

Structure and light emission of Si-rich Al2O3 and Si-rich-SiO2 nanocomposites

This work presents the comparative investigation of Six(Al2O3)1−x and Six(SiO2)1−x films with different excess Si content, x, grown by RF magnetron sputtering. Their properties were investigated by means of Raman scattering, X-ray diffraction, Electron paramagnetic resonance and photoluminescence methods. As-deposited films with the x⩾0.3 were found to be two-phase systems that contained an amorphous Si phase. Contrary to Six(SiO2)1−x films, tensile stresses were observed for Six(Al2O3)1−x samples due to lattice mismatch between the film and quartz substrate. The Si nanocrystals (Si-ncs) were formed upon annealing at 1150°C for 30min in nitrogen flow in both types of samples. Along with this, for the films with the x>0.3, amorphous Si phase was also detected, but its contribution was smaller in the Six(Al2O3)1−x films. Besides, the Si-ncs embedded in Al2O3 host remained under tensile stresses after annealing. For the films with the same x values, the Si-ncs in Al2O3 were found to be larger than those embedded in SiO2. Photoluminescence spectra showed that the main radiative channel in Six(SiO2)1−x films is exciton recombination in Si-ncs, while in Six(Al2O3)1−x films the defect related emission prevails due to higher amount of interface defects in the Six(Al2O3)1−x. The nature of these defects is discussed.

Nutrient leakage from the North Pacific to the Bering Sea (IODP Site U1341) following the onset of Northern Hemispheric Glaciation?

Intergrated Ocean Drilling Program Expedition 323 recovered a sediment record covering the last ~4.3 Ma from the Bering Sea (Integrated Ocean Drilling Program Site U1341, Bowers Ridge, 2177 m water depth). To resolve Pliocene‐Pleistocene paleoenvironmental changes in this marginal basin, ~190 sediment samples were analyzed for their bulk element composition. Aluminium contents in Bowers Ridge sediments are variable but overall higher toward younger sediments, probably related to the intensification of the Northern Hemispheric Glaciation and increasing sea ice transport in the Bering Sea. The gradual increase of terrigenous input is mirrored by decreasing SiO2 and excess Si (Sixs) contents, but the overall Si enrichment of the deposits reflects continuous opal deposition since the Pliocene at Bowers Ridge. Unlike in the North Pacific, the Sixs record at Site U1341 does not support a dramatic decrease in opal export following the onset of the Northern Hemispheric Glaciation around 2.7 Ma, but SiO2xs have higher accumulation rates (up to ~8 g/cm2/ka) between ~2.6 and ~1.8 Ma BP. During this period, the major oceanic opal deposition centers shifted globally from open marine high latitude regions to upwelling areas. We here discuss how the onset of North Pacific stratification at ~2.7 Ma BP may have caused leakage of nutrient‐rich deep/intermediate North Pacific water into the Bering Sea via the deep Kamtchatka Strait, leading to increased opal deposition—and likely reactive organic matter export—at Bowers Ridge. As a result, magnetic and geochemical records were overprinted by intensified diagenesis, significantly affecting their applications as paleoceanographic proxies.

Structural factors impacting carrier transport and electroluminescence from Si nanocluster-sensitized Er ions

We present an analysis of factors influencing carrier transport and electroluminescence (EL) at 1.5 µm from erbium-doped silicon-rich silica (SiOx) layers. The effects of both the active layer thickness and the Si-excess content on the electrical excitation of erbium are studied. We demonstrate that when the thickness is decreased from a few hundred to tens of nanometers the conductivity is greatly enhanced. Carrier transport is well described in all cases by a Poole-Frenkel mechanism, while the thickness-dependent current density suggests an evolution of both density and distribution of trapping states induced by Si nanoinclusions. We ascribe this observation to stress-induced effects prevailing in thin films, which inhibit the agglomeration of Si atoms, resulting in a high density of sub-nm Si inclusions that induce traps much shallower than those generated by Si nanoclusters (Si-ncs) formed in thicker films. There is no direct correlation between high conductivity and optimized EL intensity at 1.5 µm. Our results suggest that the main excitation mechanism governing the EL signal is impact excitation, which gradually becomes more efficient as film thickness increases, thanks to the increased segregation of Si-ncs, which in turn allows more efficient injection of hot electrons into the oxide matrix. Optimization of the EL signal is thus found to be a compromise between conductivity and both number and degree of segregation of Si-ncs, all of which are governed by a combination of excess Si content and sample thickness. This material study has strong implications for many electrically-driven devices using Si-ncs or Si-excess mediated EL.

Structural analysis and depth profiling of nanometric SiO2/SRO multilayers

Multilayers (MLs) consisting of alternating nanometer‐thick silicon‐rich oxide (SRO) and SiO2 layers are attractive structures to produce Si nanocrystals embedded in SiO2 matrix, with controlled size and distribution. The resulting materials are of great interest for applications in silicon light emitting diodes, memory devices and solar cells. The electro‐optical properties depend strongly on the phase separation process and crystallization of the silicon nanocrystals. In addition to furnace annealing (FA), rapid thermal annealing (RTA) can be used to induce the phase separation process. RTA reduces the thermal budget considerably with respect to FA. Here, we present a study on SiO2/SRO MLs annealed either with RTA or FA. Dynamic SIMS depth profiles were acquired to analyze how the excess silicon distribution is influenced by the annealing and to confirm that the ML structure is maintained during the annealing process. In addition, we used photoelectron spectroscopy (XPS) to analyze the composition of the films and to monitor the phase separation process. The SIMS profiles indicate that the layered structure is stable at all temperatures for both FA and RTA annealing. During annealing the concentration of excess Si increases at the centre of the SRO‐rich layer due to formation of Si‐rich dots. Also, the XPS measurements indicate that 10 s of annealing at 1000 or 1100 °C induces substantial phase separation. Copyright © 2012 John Wiley & Sons, Ltd.

Er3+ INFRARED PHOTO- AND ELECTRO-LUMINESCENCE FROM Er-DOPED Si-RICH SILICON NITRIDE FILMS WITH VARYING Si AND Er CONTENTS

Both Er -doped Si -rich silicon nitride ( Er : Si 3(1+x) N 4) films and electroluminescence (EL) devices with structure of indium tin oxide/ Er : Si 3(1+x) N 4/ p-Si with varying Si and Er contents have been fabricated by reactive magnetron sputtering technique. We have investigated the effects of the excess Si content in a wide range (x from 0 to 2.1) on the Er3+ 1.54 μm photoluminescence (PL), EL and the electrical transport of the Er : Si 3(1+x) N 4 films. It is found that the Er 3+ 1.54 μm PL intensity of the Er : Si 3(1+x) N 4 films and the EL power efficiency of the Er : Si 3(1+x) N 4 devices exhibit strong dependences on x and are enhanced by one to two orders of magnitude around x = 0.27 and x = 0.53, respectively. The Er concentration also has a great influence on both Er 3+ 1.54 μm PL and EL of the Er:Si 3(1+x) N 4 films. The optimum Er concentrations for PL and EL in the Er:Si 3(1+x) N 4 films with x = 0.53 are both ~3 at.%.

Precipitation of silicon nanoclusters by laser direct-write

The ability to use a laser to direct-write tracks of localized emission enhancement in PECVD-deposited Silicon rich oxide (SRO) films is demonstrated. For this purpose, 400 nm thick SRO films with varying excess Si content were irradiated with loosely focused 355 nm, 12 ps pulses at 80 MHz while being translated at 2mm/s. Mapping of areas irradiated with energies between 4.7 nJ and 5.5 nJ/pulse exhibits regions with the largest emission enhancement. Raman and photoluminescence (PL) measurements suggest precipitation of amorphous and crystalline Si nanoclusters. In the most emissive regions, the PL efficiency of the laser-annealed films was ~70% of that obtained by standard oven-annealing processes. Stress in Si crystals in some areas is identified as leading to quenching of the PL and is hypothesized to be caused by the densification of SRO matrix.

Photoluminescence responses of Si nanocrystal to differing pumping conditions

Under a CW Ar+ laser beam excitation with wavelength of 496 nm, the relationship of photoluminescence (PL) intensity versus pumping power density was investigated for both multilayered Si nanocrystal doped SiO2/SiO2 sample (or Si-nc:SiO2/SiO2) and single-layered Si-nc:SiO2 one. The threshold of pump power density for the transition from linear to superlinear relation of PL intensity versus pump power was ∼ 86 W cm−2 for the multilayered sample, while such a transition was not found for the single-layered one that possessed the same excess Si content within the power density range of this work. Using a pulsed Ti:sapphire laser (wavelength = 400 nm), a net optical gain coefficient was measured by means of variable slit length for the multilayered sample, which was 114 cm−1 at the pump fluence of 200 mJ cm−2, but only optical absorption was found for the single-layered sample. We attributed our results to the relatively high PL efficiency of the multilayered Si-nc:SiO2/SiO2.

Photoluminescence of Si from Si nanocrystal-doped SiO 2/Si multilayered sample

A multilayered Si nanocrystal-doped SiO 2/Si (or Si-nc:SiO 2/Si) sample structure is studied to acquire strong photoluminescence (PL) emission of Si via modulating excess Si concentration. The Si-nc:SiO 2 results from SiO thin film after thermal annealing. The total thickness of SiO layer remains 150 nm, and is partitioned equally into a number of sublayers ( N = 3, 5, 10, or 30) by Si interlayers. For each N-layered sample, a maximal PL intensity of Si can be obtained via optimizing the thickness of Si interlayer (or d Si). This maximal PL intensity varies with N, but the ratio of Si to O is nearly a constant. The brightest sample is found to be that of N = 10 and d Si = 1 nm, whose PL intensity is ∼5 times that of N = 1 without additional Si doping, and ∼2.5 times that of Si-nc:SiO 2 prepared by co-evaporating of SiO and Si at the same optimized ratio of Si to O. Discussions are made based on PL, TEM, EDX and reflectance measurements.

Influence of implantation parameters on the depth distribution of emitting Si-nc

The photoluminescence (PL) spectra of silicon nanocrystals (Si-nc) produced by Si + implantation in SiO 2 are modulated by interference effects associated with the thickness of the SiO 2 layer and the depth where the Si-nc are situated. A model based on the Fresnel equation is used to determine the depth profile of emitting centers corresponding to the best fit between the calculated and measured integrated emission intensity of Si-nc for several thicknesses of SiO 2 [1]. A parametric study has been performed for a range of implantation energy and excess Si concentration. The emitter depth profiles obtained with an excitation at 405 nm have greater amplitude and are generally wider than those pumped at 488 nm; this has been attributed to the excitation of larger band gap Si-nc. The shape of the emitter depth profiles generally agrees with those of implanted Si +, Si-nc and extinction coefficient. However, measurements performed using the 405 nm laser suggest that the emitter depth profiles produced with the large ion fluence of 1 × 10 17 cm −2 are deeper, a possible consequence of the strong damage incurred during implantation. Finally, when the local excess Si concentration exceeds the threshold required for the formation of large Si-nc (∼7 × 10 21 cm −3), saturation and, probably a decrease in emitter density is observed.

Influence of implantation dose on electroluminescence from Si-implanted silicon nitride thin films

Strong visible electroluminescence (EL) with electrically tunable colors from violet to white has been observed from Si-implanted silicon nitride thin films. Influence of the implanted Si ion dose on both the current conduction and EL properties has been studied. With a larger excess Si concentration, the carrier transport is enhanced leading to a higher EL intensity, but the light emission efficiency is reduced. On the other hand, the increase of the excess Si concentration causes redshifts in the major EL bands and improves the transition of the EL colors with increasing current. The excess Si concentration is also found to have a significant influence on the EL degradation. These findings are important to the application of the Si-implanted thin films in light emitting devices.

Structure and Optical Properties of Magnetron-Sputtered SiO<sub>x</sub> Layers with Silicon Nanoparticles

The properties of SiOx layer prepared by magnetron sputtering is studied by photoluminescence Auger and SIMS methods. The depth distribution of emission characteristics and chemical composition is obtained. It is shown that as-sputtered SiOx layers are non-emitted and characterized by homogeneous enough chemical composition. High-temperature annealing in nitrogen atmosphere stimulates not only the Si nanocrystal formation but also the redistribution of silicon and the appearance of Si depleted region near layer-substrate interface. The last process is found to be dependent on excess Si content. It is found that decrease of silicon content in the depth of annealed layers is followed by the decrease of particle sizes that is proved by the blue shift of photoluminescence maximum. The possible reasons of the appearance of Si depleted region are discussed.