Relative enhancement of photoluminescence intensity of passivated silicon nanocrystals in a silicon dioxide matrix

Photoluminescence (PL) intensity of passivated silicon nanocrystals (Si NCs) embedded in a SiO 2 matrix is compared with that of unpassivated Si NCs. We investigate the relative enhancement of PL intensity (I R ) as a function of annealing temperature and implanted Si ion dose. The I R increases simultaneously with the annealing temperature. This demonstrates an increase in the number of dangling bonds (DBs) with the degree of Si crystallization varying via the annealing temperature. The increase in I R with implanted Si ion dose is also observed. We believe that the near-field interaction between DBs and neighboring Si NCs is an additional factor that reduces the PL efficiency of unpassivated Si NCs.     

The potential curve of the He-α-quartz surface interaction

One of the fundamental problems for gas-surface interaction physics is to relate the pairwise gas atom-surface atom interaction characteristics to that of the aggregate collision complex. One method of studying the problem on a microscopic scale is to model the surface lattice structure and calculate the interaction characteristics using the Monte Carlo technique. We describe the calculation of the He-α-quartz (SiO₂) interaction potential applying this method assuming pairwise He-surface atom Lennard-Jones (12, 6) potentials. The results show that the interaction potential follows a (13.5, 3.8) shape function in the gas-surface interaction range below 8 Å, and an asymptotic dependence with 3.8 < m < 6 for the region beyond 8 Å. This potential well shape function differs substantially from the (9, 3) function suggested by earlier work. The calculated potential curve shows a weak dependence on surface and gas temperature. The potential well depth of the gas-surface interaction shows a strong nonlinear relationship to the equilibrium internuclear distance of the HeSi pair potential applied to the calculation. We find that the empirically determined parameters for the HeSi pair potential provide resultant gas-surface potentials with well depth remarkably close to those required by the measured heat of adsorption.     

Mechanism and enhancement of photoluminescence from silicon nanocrystals implanted in SiO2 matrix

Photoluminescence (PL) spectra of Si nanocrystals (NCs) prepared by 130 keV Si ions implantation onto SiO2 matrix were investigated as a function of annealing temperature and implanted ion dose. PL spectra consist of two PL peaks, originated from smaller Si NCs due to quantum confinement effect (QCE) and the interface states located at the surface of larger Si NCs. The evolution of number of dangling bonds (DBs) on Si NCs was also investigated. For hydrogen-passivated samples, a monotonic increase in PL peak intensity with the dose of implanted Si ions up to 3×1017 ions /cm2 is observed. The number of DBs on individual Si NC, the interaction between DBs at the surface of neighbouring Si NCs and their effects on the efficiency of PL are discussed.     

Relative enhancement of photoluminescence intensity of passivated silicon nanocrystals in silicon dioxide matrix

Photoluminescence (PL) intensity of passivated silicon nanocrystals (Si NCs) embeded in an SiO₂ matrix is compared with that of unpassivated ones. We investigate the relative enhancement of PL intensity (I_R) as a function of annealing temperature and implanted Si ion dose. The I_R increases simultaneously with the annealing temperature. This demonstrates an increase in the number of dangling bonds (DBs) with the degree of Si crystallization via varying the annealing temperature. The increase in I_R with implanted Si ion dose is also observed. We believe that the near-field interaction between DBs and neighboring Si NCs is an additional factor that reduces the PL efficiency of unpassivated Si NCs.     

Influence of boron on the initial stages of Si molecular beam epitaxy on Si(1 1 1) studied by reflection high-energy electron diffraction

The influence of boron as a function of coverage on molecular beam epitaxial Si growth on Si(1 1 1) surface was systematically studied by reflection high-energy electron diffraction. At boron coverage above 0.3 monolayer regular oscillations occur with a period typically for two-bilayers, whereas at lower boron coverage a transient behaviour with irregular intensity oscillations was observed in the initial growth stages. This behaviour can be attributed to a superposition of a bilayer and a two-bilayer dominated growth mode. The appearance of these two growth modes is discussed in terms of an initial surface defect-induced nucleation of bilayer-high Si islands and the formation of two-bilayers-high Si islands on top of the van der Waals like boron-covered surface, respectively. We suggest that these surface defects are of intrinsic nature, and their number only depends on the amount of boron at the surface. The oscillations become regular during further Si deposition with a bilayer period, indicating a diminishing influence of the layer/substrate interface on the growth processes.     

IR absorption enhancement for physisorbed methanol on Ag island films deposited on the oxidized and H-terminated Si(111) surfaces: effect of the metal surface morphology

Infrared absorption measurements using a multiple internal reflection geometry are reported for condensed methanol at 90 K on Ag island films deposited on the oxidized and hydrogen-terminated surfaces of Si(111). The attenuated total reflection (ATR) spectra obtained as a function of methanol exposure (up to 14 L) show that a 1-nm mass thickness of Ag island film on the oxidized Si(111) surface yields an absorption intensity 2–3 times larger than the intensity in the absence of Ag on the oxidized surface. Deposition of the same thickness of Ag on the hydrogen-terminated Si(111) surface results in approximately twice the enhancement. The different magnitudes of the enhancement are discussed based on SEM micrographs for Ag island films formed on the oxidized and H-terminated Si(111) surfaces. Received: 1 March 1999 / Accepted: 8 March 1999 / Published online: 5 May 1999     

Evolution of terahertz conductivity in ZnSe nanocrystal investigated with optical-pump terahertz-probe spectroscopy

The unique optical properties of nanoparticles are highly sensitive in respect to particle shapes, sizes, and localization on a sample. This demands for a fully controlled fabrication process. The use of femtosecond laser pulses to generate and transfer nanoparticles from a bulk target towards a collector substrate is a promising approach. This process allows a controlled fabrication of spherical nanoparticles with a very smooth surface. Several process parameters can be varied to achieve the desired nanoparticle characteristics. In this paper, the influence of two of these parameters, i.e. the applied pulse energy and the laser beam shape, on the generation of Si nanoparticles from a bulk Si target are studied in detail. By changing the laser intensity distribution on the target surface one can influence the dynamics of molten material inducing its flow to the edges or to the center of the focal spot. Due to this dynamics of molten material, a single femtosecond laser pulse with a Gaussian beam shape generates multiple spherical nanoparticles from a bulk Si target. The statistical properties of this process, with respect to number of generated nanoparticles and laser pulse energy are investigated. We demonstrate for the first time that a ring-shaped intensity distribution on the target surface results in the generation of a single silicon nanoparticle with a controllable size. Furthermore, the generated silicon nanoparticles presented in this paper show strong electric and magnetic dipole resonances in the visible and near-infrared spectral range. Theoretical simulations as well as optical scattering measurements of single silicon nanoparticles are discussed and compared.     

Origin of unusual work function change upon forming Tl nanoclusters on Si(111)-7x7 surface

We report an unusual behavior of the work function that remains unchanged while increasing the coverage (θ) of thallium (Tl) nanoclusters regularly arranged on a Si(111)-7×7 surface. Such a constant work function with increasing θ is understood in terms of a θ-dependent inter-cluster interaction that cancels the change induced by dipoles due to Tl nanoclusters. The spectral changes of Si 2p and Tl 5d_5/2 core-levels as a function of θ strongly indicates the presence of such an inter-clusters interaction, which is further supported by our theoretical calculation revealing a significant charge redistribution. PACS 73.20.-r; 73.30.+y; 68.47.Fg; 79.60.-i; 73.20.Hb     

Triangular Mott-Hubbard insulator phases of Sn/Si(111) and Sn/Ge(111) surfaces

The ground state of Sn/Si(111) and Sn/Ge(111) surface alpha phases is reexamined theoretically, based on ab initio calculations where correlations are approximately included through the orbital dependence of the Coulomb interaction (in the local density+Hubbard U approximation). The effect of correlations is to destabilize the vertical buckling in Sn/Ge(111) and to make the surface magnetic, with a metal-insulator transition for both systems. This signals the onset of a stable narrow gap Mott-Hubbard insulating state, in agreement with very recent experiments. Antiferromagnetic exchange is proposed to be responsible for the observed Gamma-point photoemission intensity, as well as for the partial metallization observed above 60 K in Sn/Si(111). Extrinsic metallization of Sn/Si(111) by, e.g., alkali doping, could lead to a novel 2D triangular superconducting state of this and similar surfaces.     

Measurement of ionization cross-sections and backscattering factors for use in quantitative AES

For the extraction of quantitative information from Auger electron spectra two parameters (apart from a number of instrumental parameters) are of utmost importance: the ionization cross-section and the backscattering factor. The determination of these two parameters is discussed. Ionization cross-section values for L- and M-shells have been determined based on existing values for K-shells from the literature. The backscattering factor for Si is calculated as a function of energy of the primary electrons and compared with absolute measurements of this factor using an adsorbed monolayer of sulfur or oxygen on a Si (100) surface.     

氧化锆纳米薄膜的激光诱导击穿光谱(LIBS)分析技术研究

为了发展一种实时、快速、非接触,能对金属氧化物纳米薄膜中元素成分进行分析的检测方法,搭建了一套基于LIBS技术的薄膜检测分析系统。该系统可同时实现样品平面的精确定位,样品烧蚀坑形貌实时观测,等离子体成像和光谱采集等功能。样品为在单晶硅基底上利用溶胶-凝胶法制备的约40 nm厚的氧化锆功能薄膜,实验中将其放置在一个位移精度为0.01 mm的三维平移台上。系统测试结果表明,在两束聚焦的连续激光辅助下,样品平面的定位精度达到了20 μm,LIBS单脉冲检测光谱信号的面积分强度的重复性的相对标准偏差(RSD)达到了1.6%。在室温和大气环境下,对等离子体空间分布、信号强度随激发能量、时间参数和LTSD(激光聚焦点到样品表面的距离)参数而变化的情况从光谱角度进行了实验研究,并优化了实验参数和探测参数。利用实验得到的光谱数据,用玻尔兹曼方计算了等离子体的电子温度,利用硅的原子线计算了电子密度,对定量检测所必须的局部热力学平衡(LTE)条件进行了评价。     

The role of the polarization in ATIR proximity sensing

The proximity sensing based on the attenuated total internal reflection (ATIR) is a technique that allows us to control the separation between two planes—the attenuating media surface and dielectric boundary, in the neighborhood of which the total internal reflection (TIR) takes place. This measurement supposes known optical constants of all media. An effect of lateral fringe shift and variations in intensity distribution of the interferometrical signal were observed during the measurements by an angular sheering polarization interferometer. The simple theory here proposed explains these effects. For that reason the relationship between the polarization effects, accompanying the ATIR and the output interferometrical signal had to be taken into account. To explain the variations of interference fringes intensity topography, the evolution of the Stokes parameters as a function of the gap width for Si attenuator is studied numerically.     

Effects of interplay between metal subwavelength slits on extraordinary optical transmission

In this paper we study the extraordinary optical transmission of one-dimensional multi-slits in an ideal metal film.The transmissivity is calculated as a function of various structural parameters.The transmissivity oscillates,with the period being just the light wavelength,as a function of the spacing between slits.As the number of slits increases,the transmissivity varies in one of three ways.It can increase,attenuate,or remain basically unchanged,depending on the spacing between slits.Each way is in an oscillatory manner.The slit interaction responsible for the oscillating transmission strength that depends on slit spacing is the subject of more detailed investigation.The interaction most intuitively manifests as a current distribution in the metal surface between slits.We find that this current is attenuated in an oscillating fashion from the slit corners to the center of the region between two adjacent slits,and we present a mathematical expression for its waveform.     

硅纳米颗粒在碳纳米管表面生长的分子动力学模拟

采用分子动力学模拟方法研究了硅纳米颗粒在碳纳米管上的生长，并分析了这种复合材料的基本结构.研究表明，由于硅原子和碳纳米管之间的相互作用以及碳纳米管的巨大的表面曲率，硅原子在碳纳米管表面不是形成覆盖碳纳米管的二维薄膜，而是生成具有三维结构的硅纳米颗粒.小纳米颗粒的结构和无基底条件下生成的颗粒结构基本一致.对于大纳米颗粒，不同于无基底条件下形成的球状纳米晶体硅结构，硅纳米颗粒沿管轴方向伸长，其结构为类似于硅晶体的无定形网络结构. 关键词： 纳米颗粒 碳纳米管 硅 分子动力学模拟     

Electronic structure of Si(111) surfaces

We report on new angle-resolved photoemission studies of Si(111) 2 × 1 and 7 × 7 surfaces. The emission from the 2 × 1 surface shows much structure. For normal emission the energy positions are insensitive to the photon energy in the range 19–27 eV. The emission has been interpreted as a probe of the surface density of states, SDOS, including both surface states, resonances and bulk-like states. The SDOS was also calculated as a function of parallel momentum k_∥ for a model of the Si(111) 2 × 1 surface obtained from energy minimization considerations. We identify emission from the dangling bond band, which has a positive dispersion of 0.6 eV, and also emission from surface resonances which have some character of the compressed and stretched back bonds. There are also other predicted surface resonances that correspond to experimental peaks which have not been identified in previous work. Except for the dangling bond band, the surface resonances are limited in k_∥ space, so that it is not possible to follow these resonance bands over all angles. Maximum intensity for the normal emission from the dangling bond is obtained at 23 eV, while the emission from the lowest s-like states monotonically increases towards 30 eV photon energy. When annealing the cleaved 2 × 1 surface to the 7 × 7 reconstructed surface, the spectra broaden significantly. The intensity of the dangling bond decreases and we see a very small metallic edge.     

Analysis of periodic Mo/Si multilayers: Influence of the Mo thickness

A set of Mo/Si periodic multilayers is studied by non-destructive analysis methods. The thickness of the Si layers is 5 nm while the thickness of the Mo layers changes from one multilayer to another, from 2 to 4 nm. This enables us to probe the effect of the transition between the amorphous and crystalline state of the Mo layers near the interfaces with Si on the optical performances of the multilayers. This transition results in the variation of the refractive index (density variation) of the Mo layers, as observed by X-ray reflectivity (XRR) at a wavelength of 0.154 nm. Combining X-ray emission spectroscopy (XES) and XRR, the parameters (composition, thickness and roughness) of the interfacial layers formed by the interaction between the Mo and Si layers are determined. However, these parameters do not evolve significantly as a function of the Mo thickness. It is observed by diffuse scattering at 1.33 nm that the lateral correlation length of the roughness strongly decreases when the Mo thickness goes from 2 to 3 nm. This is due to the development of Mo crystallites parallel to the multilayer surface.     

Laser removal of mold growth from paper

Femtosecond laser nano-processing by enhanced light scattered from nanospheres has received much attention. Enhanced scattered near field enables us to ablate nanoholes at nanometer scales below the diffraction limit. In addition, the interference between the scattered far field and the irradiated laser enables us to fabricate spatially controlled periodic surface structures. In this paper, we simulated the time evolution of scattered near field and far field during the free electron excitation in silicon (Si) by femtosecond laser irradiation. The optical property of Si changes from dielectric to metal-like Si due to the increase of the free electron number density excited by femtosecond laser pulse. It is elucidated that the scattered field of Si shifts from Mie scattering to plasmonic scattering during laser irradiation. We achieved the optimal free electron density and laser intensity for precisely controlled periodic surface structures fabrication. We explained the temporal behavior of the scattering near field and far field from the standpoint of dielectric function of the materials.     

Ab initio calculation of hyperfine and superhyperfine interactions for shallow donors in semiconductors

For the shallow group V donors in Si we show that the hyperfine interaction for the donor nucleus and the superhyperfine interactions for the first five shells of Si ligands can be quite accurately calculated using the local spin-density approximation of the density-functional theory. We treat the impurity problem in a Green's function approach. Since we have to truncate the long-ranged part of the defect potential, we do not obtain a localized gap state. Instead we identify the resonance above the conduction band with the paramagnetic defect state. We show that the hf and shf interactions thus obtained are at least as accurate as those obtained from one-electron theories with fitting parameters. Application of this first principles method to other shallow donors could be an essential help in defect identification.     

脉冲激光作用单晶硅的等离子体光谱分析

从激光与物质相互作用理论出发,对脉冲激光作用单晶硅的热特性进行分析。建立一套实验装置,所用激光光源的波长为1 064 nm,脉宽为10 ns,重复频率为1 Hz。得到单晶硅的等离子体谱及热辐射谱,在单晶硅的光电性质基础上对其热表面损伤进行理论分析。提取380~460 nm波段的单晶硅等离子体光谱,分析了谱图中SiⅠ390.52 nm,SiⅡ385.51 nm,SiⅡ413.12 nm三条谱线的相对强度与激光输出功率密度的对应关系。     

Interactions between charges and metallic surfaces

We report numerical results for the potential of interaction between two charged particles on a metallic surface. The density functional formalism approach is utilized. The interaction is cast in the form of an image interaction with a correction parameter λ, which depends on the positions of the particles. We find that for molecular distances, λ for the metal mediated interaction, between two charges is approximately given by the average of λ for the self interaction of there charges with the surface. However, the derivatives of the two charges λ, with respect to the particle positions (needed for the evaluation of forces and force constants), cannot be simply expressed as a function of single charge parameters.