Numerical analysis of the photoacoustic spectra of silicon samples with differently treated surfaces

This paper presents results of the photoacoustic (PA) spectral studies of a series of silicon samples with differently prepared surfaces. The PA amplitude and phase spectra of the samples indicated existence of the damaged surface layers. In the paper the two layer mathematical model of a sample with a damaged surface layer, that was used for numerical interpretation of the amplitude and phase PA spectra of the investigated samples, is presented and discussed. This model comprises the Urbach edge contribution to the optical absorption coefficient spectrum and it enabled observation of its influence on the PA amplitude spectra of silicon samples.     

Transmission and absorption based photoacoustic methods of determination of the optical absorption spectra of Si samples — Comparison

This paper is a comparison of two photoacoustic methods of determination of the optical absorption coefficient spectra of semiconductors illustrated with results obtained from Si samples. It presents experimental transmission and absorption photoacoustic spectra of Si samples as also the appropriate models leading to the determination of optical absorption coefficient spectra. The idea and the experimental set-up of the analyzed methods are presented in the paper too. From the fitting procedure of theoretical characteristics to experimental transmission and absorption photoacoustic spectra and after computations of the optical absorption coefficient spectra three components of the optical absorption coefficient spectra of Si samples were identified i.e. band to band transitions, Urbach tail and free carrier absorption. Their parameters are given and discussed in the paper. At the end the advantages and disadvantages of both methods are discussed. To the best of our knowledge, no such proof of the correctness of the PA method of determination of optical absorption spectra has been done.     

Infrared absorption spectra of amorphous silicon with gold

We determined the optical constants below the absorption edge of amorphous films of Si_1?xAu_x (x between 0.07 and 0.30) prepared by getter sputtering in argon. In the range 0.15 to 0.5 eV we found that the absorption increased with increasing Au content and could be described by Mott's formula for a.c. conductivity σ(ω) ≈ ω²[In (I₀/ω)]⁴ with I₀ ? 4.5 eV at x = 0.11 and 3.8 eV at x = 0.29; in the range 10–100 K it changed only slightly with temperature. The absorption is interpreted as due to direct transitions involving Au atoms.     

Quantitative analysis of photoacoustic IR spectra

Photoacoustic spectroscopy is a method for directly measuring the absorption properties of gases and condensed matter. We have developed a photoacoustic gas cell for a rapid-scan Fourier spectrometer. With this cell we can extend the spectral range down to 180 cm^–1; more than one octave lower than previously reported useful broadband measurements. The photoacoustic spectrum of a microscope cover glass was measured from 180 ... 200 cm^–1 and normalized with respect to the spectrum of carbon black as a reference material. Starting from the one-dimensional equation of heat conduction we derived an expression for the surface temperature of single and double-layer samples. We calculated the surface temperature of the glass lamella and divided it by the corresponding values for carbon black using the thermal conductivity of carbon black as a fitting parameter. We show that the one-dimensional model calculation reproduces the experimental spectrum over the whole spectral range.     

Infrared absorption spectra of 4H silicon carbide

We performed infrared absorption measurements on 4H-SiC samples with polarization E∥c and E⊥c at 8, 85 and 300 K. From the strong temperature dependence of the absorption lines, electronic transitions are separated from vibronic transitions. The electronic transition lines between 300 and 500 cm^-1 are assigned to the shallow nitrogen donor. It is found that the electronic transitions of the shallow nitrogen donor are polarization dependent. Zeeman spectroscopy was performed to study the influence of the magnetic field on the electronic transitions up to 15 T. The results show no linear Zeeman splitting and only a diamagnetic shift. This is consistent with the effective mass tensor of three different diagonal components in 4H-SiC. Received: 25 November 1999 / Accepted: 20 April 2000 / Published online: 2 August 2000     

Optical absorption spectra of adsorbates on metal surfaces

Optical absorption spectra of adsorbates on metal surfaces were calculated on the basis of the Newns-Anderson model supplemented with screening due to the surface plasmons. An inspection of the overall line shape calculated by the simplest second-order perturbation with respect to both the charge transfer and the adsorbate-surface-plasmon interactions prompted us to investigate the local structures of the excitations near the singly and doubly occupied adsorbate levels and near the Fermi level. Separately, analytic features of the absorption threshold were investigated in detail. With the aid of the image screening effect, the possibility occurred that the doubly occupied excited adsorbate state becomes the lowest final state. Furthermore, the exchange effect on the absorption threshold of the singly occupied adsorbate state was also studied.     

硅的间接跃迁双光子吸收系数谱

利用皮秒Nd:YAG脉冲激光器作为激发光源,测量出光子能量介于1.36 μm (0.912 eV)—1.80 μm (0.689 eV)之间的硅间接跃迁双光子吸收系数谱.尽管此波段范围内的激光光子能量小于硅间接带隙,但当激光辐照在硅基光电二极管受光面时,在二极管两电极端仍然探测到了显著的脉冲光伏信号.光伏信号峰值强度与入射光强呈二次幂函数关系,表明其是双光子吸收过程.采用pn结等效结电容充放电模型,将光伏响应信号峰值与入射光强相关联,从中提取出硅的间接跃迁双光子吸收系数,改变入射波长得到系数谱.研究表明:     

Ab initio absorption spectra and optical gaps in nanocrystalline silicon

The optical absorption spectra of Si(n)H(m) nanoclusters up to approximately 250 atoms are computed using a linear response theory within the time-dependent local density approximation (TDLDA). The TDLDA formalism allows the electronic screening and correlation effects, which determine exciton binding energies, to be naturally incorporated within an ab initio framework. We find the calculated excitation energies and optical absorption gaps to be in good agreement with experiment in the limit of both small and large clusters. The TDLDA absorption spectra exhibit substantial blueshifts with respect to the spectra obtained within the time-independent local density approximation.     

聚丁二炔Langmuir—Blodgett薄膜的光声谱研究：（Ⅰ）结构表征与光声？…

本在用Ｘ－射线小角衍射聚合与转载先后次序不同的ＰＤＡ ＬＢ膜进行结构表征的基础上,不同厚度的ＰＤＡ ＬＢ膜的光声新斩波频率效应说明,随着ＰＤＡ ＬＢ膜厚度增加,ＬＢ膜的热学性质逐渐明显。此外,还探讨了如何从ＰＤＡ ＬＢ膜的光声谱求得光声吸收系数问题,本的研究结果表明,光声谱技术是研究ＬＢ膜热学特性的有效手段。     

光声光谱检测装置中光声池的数值计算及优化

利用光声光谱技术进行痕量气体的检测具有独特的优势,光声池是系统装置中最为重要的核心部件,它决定着整机性能的优劣.以一圆柱形共振型光声池为研究对象,基于声学与吸收光谱学的基本理论,建立了光声池声场激发的数学模型;利用数值模拟方法对光声池空腔结构进行了声学模态仿真,获得了前8阶声学模态值以及声压可视化振型;在考虑热黏性声学损耗的作用下,对光声池进行了热-声耦合多物理场仿真计算;将仿真结果与解析计算和实验结果进行对比,明确了利用数值模拟方法来计算光声池有关指标的可靠性与可行性;针对光声池的优化问题,提出了一种将响应面代理模型与遗传算法相结合的优化算法,在将原光声池中的谐振腔两端形貌更改为喇叭口形的情况下,通过优化算法获得了以光声池品质因数Q及池常数C_(cell)为最大值寻优的Pareto最优解集;选取一组解进行考察,结果表明,代理模型预测值与数值模拟值指标最大误差仅为1.3%,优化后的新型光声池Q较之前增长了48.9%, C_(cell)增长了34.4%.研究方法可为光声光谱中光声池的优化设计提供参考借鉴.     

Photoacoustic investigations of the optical absorption spectra of porous silicon layers on a silicon backing

This paper presents a series of experimental photoacoustic spectra of porous silicon layers on crystalline silicon and their numerical analysis performed in the proposed two-layer model. The goal of the analysis was to calculate the optical absorption spectra of porous silicon from the photoacoustic spectra of porous silicon layers on a silicon background. The experimental character of the observed absorption band associated with the porous silicon was revealed. This is the first attempt at a theoretical interpretation of the photoacoustic spectra of porous silicon on a silicon backing.     

Origin of fine structure in si photoelectron spectra at silicon surfaces and interfaces

Using a first-principles approach, we investigate the origin of the fine structure in Si 2p photoelectron spectra at the Si(100)-(2 x 1) surface and at the Si(100)-SiO2 interface. Calculated and measured shifts show very good agreement for both systems. By using maximally localized Wannier functions, we clearly identify the shifts resulting from the electronegativity of second-neighbor atoms. The other shifts are then found to be proportional to the average bond-length variation around the Si atom. Hence, in combination with accurate modeling, photoelectron spectroscopy can provide a direct measure of the strain field at the atomic scale.     

Numerical analysis of ultrasonic transmission and absorption of oblique plane waves through the human skull.

Ultrasonic transmission and absorption of oblique plane waves through the human skull are analyzed numerically for frequencies ranging from 1/2 to 1 MHz. These frequencies are optimum for noninvasive ultrasound therapy of brain disorders where numerical predictions of skull transmission are used to set the phase and amplitude of source elements in the phased array focusing system. The idealized model of the skull is a three-layer solid with ivory outer and inner layers and a middle marrow layer. Each layer is modeled as a flat, homogeneous, isotropic, linear solid with effective complex wave speeds to account for focused energy losses due to material damping and scattering. The model is used to predict the amplitude and phase of the transmitted wave and volumetric absorption. Results are reported for three different skull thicknesses: 3 mm, 6 mm, and 9 mm. Thickness resonances are observed in the transmitted wave for 3 mm skulls at all frequencies and for the 6 mm skulls below 0.75 MHz. Otherwise, the transmission is dominated by the direct wave. Skull phase errors due to shear waves are shown to minimally degrade the power at the focus for angles of incidence up to 20 degrees from normal even for low material damping. The location of the peak volumetric absorption occurs either in the outer ivory or middle marrow layer and shown to vary due to wave interference.     

Determination of the low absorption coefficients of thick semi-conductor samples using rear side photoacoustic spectroscopy detection

Based on the Rosencwaig and Gersho theory, we have theoretically derived the photoacoustic signal in the gaz behind the unilluminated surface of thick samples and presented a simple analytical procedure for determining the low optical absorption spectra of these samples.The rear side photoacoustic spectroscopy (PAS) detection has been employed on thick samples of GaP (0,2-1mm) and optical absorption coefficients in the region between 15 to 200 cm^?1 were measured.Our results are discussed in comparison with those obtained by usual front side PAS detection and spectroscopic ellipsometry.     

Electronic structure and absorption spectra of silicon nanocrystals with a halogen (Br,Cl) coating

Ab initio calculations of the electronic structure and frequency dependence of the imaginary part of the dielectric function for 1–2 nm silicon nanocrystals with the surface fully passivated with Cl or Br halogen atoms have been performed. According to these calculations, passivation with halogens results in the strong localization of valence electrons near the surface. As a result, the width of the band gap of a nanocrystal is noticeably narrowed and its absorptance decreases as compared to the case of hydrogen passivation. These effects are more pronounced in bromine-passivated nanocrystals.     

Comparison of silicon LIII,II electron loss spectra and optical absorption

Threshold structure due to L_III,II excitation in the electron loss spectra of silicon at 100.2 ± 0.3 eV is in good agreement with optical absorption measurements of 100.1 ± 0.1 eV.