Characterization of oxygen precipitates in Czochralski silicon by imaging SIMS

This paper presents a three-dimensional characterization of oxygen defects in Czochralski-silicon (CZ) by 3D-SIMS using a camera based imaging system. Different manufacturing processes yielding a variation in size and distribution of oxygen precipitates are monitored by imaging SIMS and conventional depth profiling. Emphasis is laid on the characterization of the precipitation behavior governed by different annealing processes. The limitations of 3D SIMS for oxygen defect imaging are studied. Sputter induced microroughening of the crater bottom, investigated by SEM, is shown to be strongly correlated with defect densities caused by oxygen precipitates.     

Time‐of‐flight secondary ion mass spectroscopic characterization of the nitrogen profile of shallow gate oxynitride thermally grown on a silicon substrate

Silicon oxynitride has been used as a shallow gate oxide material for microelectronics and its thickness has been reduced over the years to only a few tens of angstroms due to device size scaling. The nitride distribution and density characteristic in the gate oxide thus becomes imperative for the devices. The shallow depth profiling capability using time‐of‐flight secondary ion mass spectrometry (TOF‐SIMS) has huge potential for the nitrogen characterization of the shallow gate oxide film. In this article, both positive and negative spectra of TOF‐SIMS on silicon oxynitride have been extensively studied and it was found that the silicon nitride clusters Si_xN^? (x = 1–4) are able to represent the nitrogen profiles because their ion yields are high enough, especially for the low‐level nitride doping in the oxide, which is formed by the annealing of nitric oxide on SiO₂/Si. The gate oxide thickness measured by the TOF‐SIMS profiling method using ¹⁸O or CsO profile calibration was found to correlate very well with transmission electron microscope measurement. The nitrogen concentration in the gate oxide measured using the TOF‐SIMS method was consistent with the results obtained using the dynamic SIMS method, which is currently applied to relatively thicker oxynitride films. Copyright © 2012 John Wiley & Sons, Ltd.     

Elaboration and characterisation of yttria psz coatings deposited by RF sputtering on silicon

Yttria stabilised Zirconia coatings on polycrystalline silicon wafers have been performed using physical vapor deposition with negative bias applied to the substrate. The thicknesses determined by ellipsometry were between 45 and 70 nm and the deposition rate was about 0.25 nm per min. Secondary ion mass spectroscopy analyses revealed no interdiffusion of metallic elements between the zirconia coating and silicon substrate. Grazing incidence X-ray diffraction indicates that the coatings are crystallized, but it can not permit to identify all the crystalline phases. The disparity between the experimental and the k-ratios (Pouchou and Pichoir simulation of electron probe microanalysis analyses) showed the usefulness of others techniques to determine coating and interface compositions. Cross sectional transmission electron microscopy observations demonstrate an amorphous layer between the zirconia coating and the substrate with a thickness of 30–120 nm. If it is considered to be an amorphous silicon layer, the PAP simulation based on the ellipsometric thicknesses and an yttria stabilised zirconia density of 4 g/cm³ fits correctly the experimental values.     

Evaluation of exchange‐correlation functionals in comparison to B3LYP for the description of silicon and Cu‐doped silicon clusters

Several developed exchange‐correlation functionals in density functional theory have been systematically applied to describe the geometries and electronic properties of small silicon (Si_n+1, n < 5) and doped silicon (CuSi_n) clusters. The performance of the various approaches is done with their critical comparison with B3LYP and available high level wave function methods. Our calculations indicate that all functional give reasonable results. Further, OLYP/6‐311+G* approach generally agrees with B3LYP results. The good performance of OLYP is of significant interest knowing that the hybrid functionals are computationally more demanding than nonhybrid schemes. So, we recommend OLYP/6‐311+G* approach for studying the doped silicon clusters and understanding the electronic properties of silicon by the presence of doped metal impurities. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

Direct phase identification with SIMS by evaluation of atomic and cluster ion intensities: A Study of calcium sulfide formation in hard metals during sintering

A SIMS method for the direct identification of micro and nanophases based on the quantitative evaluation of atomic and cluster ion intensities is described and applied to the study of the formation of CaS phases in hard metals.     

Analysis of thin films and molecular orientation using cluster SIMS

A study of phenylalanine films of different thicknesses from submonolayer to 55 nm on Si wafers has been made using Bi_n⁺ and C₆₀⁺ cluster primary ions in static SIMS. This shows that the effect of film thickness on ion yield is very similar for all primary ions, with an enhanced molecular yield at approximately 1 monolayer attributed to substrate backscattering. The static SIMS ion yields of phenylalanine at different thicknesses are, in principle, the equivalent of a static SIMS depth profile, without the complication of ion beam damage and roughness resulting from sputtering to the relevant thickness. Analyzing thin films of phenylalanine of different thicknesses allows an interpretation of molecular bonding to, and orientation on, the silicon substrate that is confirmed by XPS. The large crater size for cluster ions has interesting effects on the secondary ion intensities of both the overlayer and the substrate for monolayer and submonolayer quantities. This study expands the capability of SIMS for identification of the chemical structure of molecules at surfaces. © Crown copyright 2010.     

Lithium insertion into silicon films produced by magnetron sputtering

Using the method of magnetron-plasma sputtering of polycrystalline silicon target, amorphous silicon films 32–214 nm thick were produced on various (copper and titanium, polished and rough) substrates. A study of their charge-discharge characteristics under the galvanostatic conditions showed that all thin-filmed electrodes are capable of reversible lithium insertion. The amount of lithium inserted in the first cycles is close to the theoretical one. An analysis of composition and morphology of surface layer and also the behavior of reversible and irreversible capacities during cycling showed that the degradation of capacity is caused by the exfoliation of films from the substrate (the effect is more pronounced for the specimens with polished substrates) and somewhat breaking (cracking) of films. The thicker are the films, the severer is the disruption of silicon films in the cycling. The adhesion of films to the substrate surface is favored by the film roughness. At sufficiently high adhesion of films, their electrochemical properties only slightly depend on the nature (copper or titanium) of substrate.     

Efficiency of random search procedures along the silicon cluster series: Si(n) (n=5-10, 15, and 20)

The efficiency of the simplest isomeric search procedure consisting in random generation of sets of atomic coordinates followed by density functional theory geometry optimization is tested on the silicon cluster series (Si(5-10, 15, 20)). Criteria such as yield, isomer distributions and recurrences are used to clearly establish the performance of the approach with respect to increasing cluster size. The elimination of unphysical candidate structures and the use of distinct box shapes and theoretical levels are also investigated. For the smaller Si(n) (n=5-10) clusters, the generation of random coordinates within a spherical box is found to offer a reasonable alternative to more complex algorithms by allowing straightforward identification of every known low-lying local minima. The simple stochastic search of larger clusters (i.e. Si(15) and Si(20)) is however complicated by the exponentially increasing number of both low- and high-lying minima leading to rather arbitrary and non-comprehensive results.     

Significant nonlinear‐optical switching capacity in atomic clusters built from silicon and lithium: A combined ab initio and density functional study

Starting from a hypothetical but fundamental charge/discharge sequence, the topic of nonlinear optical switching in atomic clusters built from silicon and alkali metals is opened up. The outcomes presented in this work, obtained with ab initio methods of exceptional predictive capabilities, offer strong evidences that sizable hyperpolarizability contrasts between neutral and charged alkali metal doped cluster forms might be simultaneously accomplished. The observed switching procedure involves redox polyatomic clusters formed by Si atoms. These centers function as electron acceptors at the ground state and as electron donors at the excited states facilitating low energy charge transfer transitions upon electronic excitation. © 2014 Wiley Periodicals, Inc.     

Simulation of the structure of some silicon carbide clusters by the MNDO method

Simulations of the geometric and electronic structure of C₄₄, C₄₅, Si₄₅, C₄₀Si₅, and C₄₄Si clusters were performed by the MNDO method. The geometries of the filled clusters, calculated by the MM2 method, were used as initial approximations. It was found that the filled clusters C₄₅ and C₄₄Si are transformed into endohedral clusters X@C₄₄ (X-C or Si, respectively) after energy optimization. The highest occupied energy level of the HOMO of the filled tetrahedral cluster Si₄₅ ofT symmetry is triply degenerate and is only occupied by four electrons. The structure of Si₄₅ ²⁻ dianion ofT symmetry was calculated. Two filled structures for the C₄₀Si₅ cluster were found. The coordination numbers of the central Si atom in these structures are 4 and 3, respectively. Translated fromIzvestiya Akademii Nauk, Seriya Khimicheskaya, No. 1, pp. 54–56, January, 1997.     

Structural and static electric response properties of highly symmetric lithiated silicon cages: Theoretical predictions

It is shown by density functional theory calculations that high symmetry silicon cages can be designed by coating with Li atoms. The resulting highly symmetric lithiated silicon cages (up to D_5d symmetry) are low‐lying true minima of the energy hypersurface with binding energies of the order of 4.6 eV per Si atom and moderate highest occupied molecular orbital–lowest unoccupied molecular orbital gaps. Moreover, relying on a systematic study of the electric response properties obtained by ab initio (Hartree–Fock, MP2, and configuration interaction singles (CIS)) and density functional (B3LYP, B2PLYP, and CAM‐B3LYP) methods, it is shown that lithium coating has a large impact on the magnitude of their second hyperpolarizabilities resulting to highly hyperpolarizable species. Such hyperpolarizable character is directly connected to the increase in the density of the low‐lying excited states triggered by the interaction between the Si cage and the surrounding Li atoms. © 2012 Wiley Periodicals, Inc.     

Mechanisms of surface processes in silicon etching

A unified view on the mechanism allowing one to explain the experimental features governing spontaneous silicon etching by atomic fluorine is presented. Analysis of the phenomenological equation of adsorption shows a significant difference between etching mechanisms at high and low heat of adsorption on the surface being etched. As follows from the parameter estimates, one or another case can be realized under different experimental conditions. At steady-state the etching is argued to be determined only by the processes taking place on the SiF. film surface. To describe the process, it is necessary to understand the mechanism of overcoming the surface barrier for fluorine penetration into the film. At low heat of fluorine adsorption the barrier is overcome by thermal activation. In the opposite case the etching mechanism includes electron tunneling from silicon to adatoms and creation of a surface electric field. The field lowers the high energetic barrier for fluorine penetration. Based on the kinetic equations describing the electronic and atomic processes on the surface, the equation of the field strength is obtained. This equation is analyzed in different limit cases. The observed features are shown to be reproduced at some conditions on the parameters. Definite predictions on the temperature dependence of the etch rate are made.     

Multi-element characterization of silicon nitride powders by atomic and mass spectrometric solution methods

A sample pretreatment technique for silicon nitride involving digestion and matrix/traces separation was developed by means of radiotracers and applied to analysis of this material by inductively coupled plasma atomic emission spectrometry, inductively coupled plasma mass spectrometry and graphite furnace atomic absorption spectrometry. The results obtained for a high purity silicon nitride material by these methods are compared each with the other and with those obtained by neutron activation analysis. The limits of detection and the capabilities of the methods are compared and discussed.     

Charge transfer on porous silicon membranes studied by current-sensing atomic force microscopy

A visible rectification effect on the current-voltage curves of metal/porous silicon/p-silicon has been observed by current-sensing atomic force microscopy. The current-voltage curves of porous silicon membranes with different porosities, prepared through variation of etching current density for a constant time, indicate that a higher porosity results in a higher resistance and thus a lower rectification, until the current reaches a threshold at a porosity 〉55%. We propose that the conductance mode in the porous silicon membrane with porosities 〉55% is mainly a hopping mechanism between nano-crystallites and an inverse static electric field between the porous silicon and p-Si interface blocks the electron injection from porous silicon to p-Si, but with porosities ≤55%, electron flows through a direct continuous channel between nano-crystallites.     

电感耦合等离子体发射光谱法测定高纯铝中硅

建立电感耦合等离子体发射光谱(ICP-AES)法测定高纯铝中硅元素的分析方法。样品采用碱溶法进行预处理,以氢氧化钠溶液溶解,再加入盐酸、硝酸酸化。选取了硅251.611 nm分析谱线进行分析。硅的质量分数在0.001%~0.01%范围内与光谱强度具有良好的线性关系,线性相关系数为0.9991。该方法检出限为0.00012%,测定结果的相对标准偏差不大于10.2%(n=8),加标回收率为93.2%~104.2%。该方法操作简便,测定结果准确、可靠,适用于快速检测高纯铝中硅元素,具有推广价值。     

第一性原理模拟氢气分子在氧化硅团簇上的吸附

基于密度泛函理论,采用广义梯度近似(GGA)分析了H2分子吸附在氧化硅团簇上的几何结构、电子性质以及吸附能.结果发现:H2分子与Si3O4团簇相互作用时,H2分子被分解,游离的H原子优先吸附在末端Si原子上,表明Si3O4团簇体系对氢气的存储主要依赖于末端存在悬挂键的Si原子,接着H2分子才以分子的形式以较小吸附能吸附在Si3O4H4团簇上.氢气分子主要引起与其邻近的原子电荷的重新分布.该团簇体系的红外、拉曼光谱图有效地鉴定了H2分子的吸附状态,为理论上确定团簇的稳定结构和实验上对观测结果的分析提供有力的途径.     

Silicon determination by inductively coupled plasma atomic emission spectrometry after generation of volatile silicon tetrafluoride

A method for the determination of silicon by inductively coupled plasma atomic emission spectrometry (ICP-AES) is described. The procedure is based on a discontinuous generation of volatile silicon tetrafluoride in concentrated sulphuric acid medium after injecting 125 μl of 0.1%, w/v sodium fluoride solution into 100 μl of the sample. The gaseous silicon tetrafluoride is fed directly into the ICP torch by a flow of 250 ml min⁻¹ Ar carrier gas. The calibration curve was linear up to at least 100 μg ml⁻¹ of Si(IV) and the absolute detection limit was 9.8 ng working with a solution volume of 100 μl. The relative standard deviation for six measurements of 10 μg ml⁻¹ of Si(IV) was 2.32%. The method was applied to the determination of silicon in water and iron ores.