Effects of 9-cyanoanthracene and anthracene adsorption on the photoluminescence of porous silicon

The photoluminescence of porous silicon can be modified sensitively by surface adsorption of different kinds of molecules. A quite different effects of 9-cyanoanthracene and anthracene adsorption on the photoluminescence of porous silicon were observed. The adsorption of 9-cyanoanthracene induced the photoluminescence enhancement, while anthracene adsorption resulted in photoluminescent quenching. An explanation of the interaction of adsorbates with surface defect sites of porous silicon was suggested and discussed. (C) 1998 Elsevier Science S.A.

Micro-fabricated Al0.3Ga0.7As pyramids for potential SPM applications

A novel technique of manufacturing Al0.3Ga0.7As pyramids by liquid phase epitaxy (LPE) for scanning probe microscopy (SPM) sensors is reported Four meticulously designed conditions-partial oxidation, deficient solute, air quenching and germanium doping result in defect-free homogeneous nucleation and subsequent pyramid formation. Micrometer-sized frustums and pyramids are detected by scanning electron microscopy (SEM). The sharp end of the microtip has a radius of curvature smaller than 50 nm. It is believed that such accomplishments would contribute not only to crystal growth theory, but also to miniature fabrication technology.

Nonradiative recombination effect on photoluminescence decay dynamics in GaInNAs/GaAs quantum wells - art. no. 61180Z

The nonradiative recombination effect on the photoluminescence (PL) decay dynamics in GaInNAs/GaAs quantum wells is studied by photoluminescence and time-resolved photoluminescence under various excitation intensities and temperatures. It is found that the PL decay dynamics strongly depends on the excitation intensity. In particular, under the moderate excitation levels the PL decay curves exhibit unusual non-exponential behavior and show a convex shape. By introducing a new concept of the effective concentration of nonradiative recombination centers into a rate equation, the observed results are well simulated. In the cw PL measurement, a rapid PL quenching is observed even at very low temperature and is of the excitation power dependence. These results further demonstrate that the non-radiative recombination process plays a very important role on the optical properties of GaInNAs/GaAs quantum wells.

Intense room temperature near infrared emission from Al (3+) and Yb3+ ions

Intense near infrared emission was observed from Al3+ and Yb3+ ions co-implanted SiO2 film on silicon. It was found that the addition of Al3+ ions could remarkably improve the photoluminescence efficiency of Yb3+-implanted SiO2 film. No excitation power saturation was observed and trivial temperature quenching factor of 2 was achieved.

Temperature dependent photoluminescence of an In(Ga)As/GaAs quantum dot system with different areal density

We have systematically studied the temperature dependent photoluminescence of a self-assembled In(Ga)As/GaAs quantum dot (QD) system with different areal densities from similar to 10(9) to similar to 10(11) cm(-2). Different carrier channels are revealed experimentally and confirmed theoretically via a modified carrier equation model considering a new carrier transfer channel, i.e. continuum states ( CS). The wetting layer is demonstrated to be the carrier quenching channel for the low-density QDs but the carrier transfer channel for the high-density QDs. In particular, for the InGaAs/GaAs QDs with a medium density of similar to 10(10) cm(-2), the CS is verified to be an additional carrier transfer channel in the low temperature regime of 10-60 K, which is studied in detail via our models. The possible carrier channels that act on different temperature regimes are further discussed, and it is demonstrated that density is not a crucial factor in determining the carrier lateral coupling strength.

Self-Heating Effect on the Two-State Lasing Behaviors in 1.3-mu m InAs-GaAs Quantum-Dot Lasers

Experimental and theoretical study of the self-heating effect on the two-state lasing behaviors in 1.3-mu m self-assembled InAs-GaAs quantum dot (QD) lasers is presented. Lasing spectra under different injected currents, light-current (L-I) curves measured in continuous and pulsed regimes as well as a rate-equation model considering the current heating have been employed to analyze the ground-state (GS) and excited-state (ES) lasing processes. We show that the self-heating causes the quenching of the GS lasing and the ES lasing by the increased carrier escape rate and the reduced maximum modal gain of GS and ES.

Development of a self-thermal insulation miniature combustor

A novel miniature cylindrical combustor, whose chamber wall is made of porous material, has been designed and experimented for reducing heat loss and enhancing flame stability. The combustor has the function of reducing wall heat loss, extending residence time and avoiding radical chemical quenching with a self-thermal insulation concept in which heat loss reduction is obtained by the opposite flow directions between thermal energy transfer and mass flow. The methane/air mixture flames formed in the chamber are blue and tubular in shape. Between the flames and the porous wall, there is a thin unburned film that plays a significant role in reducing the flames' heat loss and keeping the flames stable. The porous wall temperature was 150-400 degrees C when the temperatures of the flames and exhaust gas were more than 1200 degrees C. When the equivalence ratio phi < 1.0, the methane conversion ratio was above 95%; the combustion efficiency was near 90%; and the overall sidewall heat loss was less than 15% in the 1.53 cm(3) chamber. Moreover, its combustion efficiency is stable in a wider combustion load (input power) range.

Enhanced photoluminescence of Nd2O3 nanoparticles modified with silane-coupling agent: Fluorescent resonance energy transfer analysis

A liquid laser medium with a lifetime of 492 mu s and a fluorescent quantum efficiency of 52.5% has been presented by stably dispersing dimethyl dichorosilane-modified Nd2O3 nanoparticles in dimethylsulfoxide. Its optical properties and mechanism were investigated and explained by fluorescence resonance energy transfer theory. The calculation result shows that the quenching of Nd-III F-4(3/2)-> I-4(11/2) transition via O-H vibrational excitation can be eventually neglected. The main reason is that the silane-coupling agent molecules remove the -OH groups on Nd2O3 nanoparticles and form a protective out layer. (c) 2007 American Institute of Physics.

Modeling and optimizing of high-concentration erbium-doped fiber amplifiers with consideration of ion-clusters

Starting from the modeling of isolated ions and ion-clusters, a closed form rate and power evolution equations for high-concentration erbium-doped fiber amplifiers are constructed. Based on the equations, the effects of the fraction of ion-clusters in total ions and the number of ions per cluster on the performance of high-concentration erbium-doped fiber amplifiers are analyzed numerically. The results show that the presence of the ion-clusters deteriorates amplifier performance, such as the signal power, signal gain, the threshold pump power for zero gain, saturated signal gain, and the maximum gain efficiency, etc. The optimum fiber length or other parameters should be modified with the ion-clusters being taken into account for the amplifiers to achieve a better performance. (c) 2007 Elsevier B.V. All rights reserved.

Preparation and luminescence properties of Eu2+ in SiO2 xerogels

Al-doped and B, Al co-doped SiO2 xerogels with Eu2+ ions were prepared only by sol-gel reaction in air without reducing heat-treatment or post-doping. The luminescence characteristics and mechanism of europium doping SiO2 xerogels were studied as a function of the concentration of Al, B, the europium concentration and the host composition. The emission spectra of the Al-doped and B, Al codoped samples all show an efficient emission broad band in the blue violet range. The blue emission of the Al-doped sample was centered at 437 nm, whereas the B, Al co-doped xerogel emission maximum shifted to 423 nm and the intensity became weaker. Concentration quenching effect occurred in both the Al-doped and B, Al co-doped samples, which probably is the result of the transfer of the excitation energy from Eu2+ ions to defects. The highest Eu2+ emission intensity was observed for samples with the Si(OC2H5)(4):C2H5OH:H2O molar ratio of 1:2:4. (c) 2006 Elsevier B.V. All rights reserved.

Formation and Properties of a Novel Heavy-Metal Chalcogenide Glass Doped with a High Dysprosium Concentration

A novel heavy-metal chalcogenide glass doped with a high dysprosium ion (Dy(3+)) concentration was prepared by the well-established melt-quenching technique from high-purity elements. The results show that when Cadmium (Cd) is introduced into chalcogenide glass, the concentration of Dy(3+) ions doped in GeGaCdS glasses is markedly increased, the thermodynamic performance improves, and the difference between T(g) and T(x) is >120 degrees C. The Vickers microhardness is also modified greatly, about 245 kgf/mm(2). The optical spectra indicate that all absorption and emission bands of Dy(3+) are clearly observed and red-shifted with increasing Dy(3+) concentration.

Ultra-dry oxygen atmosphere to protect tellurite glass fiber from surface crystallization

In this study we report on surface crystallization phenomena and propose a solution for the fabrication of long and robust tellurite glass fibers. The bulk tellurite glasses of interest were prepared by melting and quenching techniques. Tellurite glass preforms and fibers were fabricated by suction casting and rod-in-tube drawing methods, respectively. The surfaces of the tellurite bulk glass samples and of the drawn fibers prepared under different controlled atmospheres were examined by X-ray diffraction. When the tellurite glass fibers were drawn in ambient air containing water vapor, four primary kinds of small crystals were found to appear on the fiber surface, alpha-TeO(2), gamma-TeO(2), Zn(2)Te(3)O(8) and Na(2)Zn(3)(CO(3))(4)center dot 3H(2)O. A mechanism for this surface crystallization is proposed and a solution described, using an ultra-dry oxygen gas atmosphere to effectively prevent surface crystallization during fiber drawing. (C) 2010 Elsevier B.V. All rights reserved.

Multistage etching process for microscopically smooth tellurite glass surfaces in optical fibers

Bulk samples of tellurite glass with composition 75TeO(2)-20ZnO-5Na(2)O (TZN) were fabricated by melting and quenching techniques. In order to improve the surface quality of optical fiber preform made with this tellurite glass, the authors developed a multistage etching process. The relationship between successive etching treatments and roughness of the TZN glass surface was probed by using an atomic force microscope. The results demonstrate that this multistage etching method effectively improves this tellurite glass surface smoothness to a level comparable with that of a reference silica glass slide, and the corresponding chemical micromechanisms and fundamentals are discussed and confirmed by atomic force microscopy, potentially contributing to the development of multicomponent soft glass fibers and devices. (C) 2010 American Vacuum Society. [DOI: 10.1116/1.3437017]

关于半导体电极上极化成份的分解和电极反应动力学参数的确定

电极动力学在金属电极上已经发展得比较成熟，对半导体来说，由于电极反应的复杂性，还有不少问题比较模糊。其中之一是怎样根据表观的极化测量求得反映界面电荷转移的动力学参数。目前有关这方面的工作不多，还没有见到具体对极化成份进行分解以求解半导体电极反应的动力学系统的报导。本工作从一定的电极物理模型出发，在电化学测量的基础上结合电子计算技术，对这方面问题的研究提出了测量计算方法。本文提出的半导体电极的物理模型如图A所示。I，I_J, I_S, I_D, I_H均取阴极性电流方向为正，η则按电极电势的方向取阳极过电热为正，阴极过电势为负。模型中的J反映空间电荷层的Schottky结特性，其数学描述可概括（对n-型半导体）为：I_J = I_0[exp(-n_s/a)-1] I_0： 结的反向饱和电流。（a:k_T）/q或（RT）/F D反映界面电荷转移反应的特性，其数学描述适用Butler-Volmer方程：I_D = i~0[exp(-(1-β)n η_H)/a) - exp(βη-η_H/a)] i~0:交换电流。β：阳极传递系数 C_s，C_H分别表示空间电荷层和Helmholtz层的电容。R_B和R_l分别为半导体体内和溶液电阻。由于J，D二者特性不同，可以通过极化测量利用电子计算机加以鉴别。实验上针对电路主要是串联结构的特点，采用恒电流极化，利用恒电流恒电位仪实现一系列的电流阶跃（I_(K-1) I_K K = 1, 2, 3 ……）记录相应的电位随时间变化的响应曲线如图B所示。根据曲线各段的特点，利用电子计算机曲线拟合，分别求解有关参数。（1）在t = 0时，找出一系列不同I下的φ（I_K, t = 0）值，根据φ（I_K, t = 0) = φ(I = 0)-I_KR拟合求解φ（I = 0）， R。（2）找出一系列不同I下的稳态极化数据φ（I_K,t →∞），推导出电位随电流变化关系式，拟合求解I_0, j~0, β。（3）利用暂态过程的φ（I_K, t = 0) ～ t曲线，拟合求解每阶电位变化区间的C_s，C_H。由于在我们的电极模型中，D采取的是完整的Butler-Volmer表达式，没有作任何简化或近似，因而在数据处理和计算时，涉及隐式超越代数方程和隐式超越微分方程，无法通过一般解析法求解。为此我们把牛顿迭代法和Runge-Kutta法引入相应的曲线拟合计算程序中。这样做虽然计算上比较复杂困难，但方法的通用性更广泛。无论Helmholtz层处于线性极化，弱极化，或强极化区部同样适用。我们用BASIC语言编写了梯度法，线性化法联合使用的曲线拟合源程序及牛顿迭代法和Runge-Kutta法于程序。利用上述研究方法，对不同掺杂浓度的n-型GaAs电极在S~(2-)/S_x~(2-)体系中的电化学行为进行了研究。求解的电荷转移反应的动力学参数I_0, i~0, β分别在7.27 * 10~(-8) - 4.66 * 10~(-1) A/cm~2, 2.08 * 10~(-6)-4.62 * 10~(-6)A/cm~2, 0.70 - 0.78的范围之内。并于Pt电极连同一体系中的i~0，β进行了比较。i_（半导体）~0 < i_（导体）~0。但β在二种材料上差别不大。将半导体电极极化分解为空间电荷层极化和Helmholtz层极化两部分。测量了空间电荷层电学及Helmholtz层电容与电极电位的关系。从实际测量中证明，本文提出的电极模型比较恰当的反映了半导体电极的特性，本工作的测量和拟合计算方法对研究半导体电极行为是一种可行的方法。

THEORETICAL INVESTIGATION OF THE DYNAMIC PROCESS OF THE ILLUMINATION OF GAAS

The dynamic process of light illumination of GaAs is studied numerically in this paper to understand the photoquenching characteristics of the material. This peculiar behavior of GaAs is usally ascribed to the existence of EL2 states and their photodriven metastable states. To understand the conductivity quenching, we have introduced nonlinear terms describing the recombination of the nonequilibrium free electrons and holes into the calculation. Though some photoquenching such as photocapacitance, infrared absorption, and electron-paramagnetic-resonance quenching can be explained qualitatively by only considering the internal transfer between the EL2 state and its metastability, it is essential to take the recombination into consideration for a clear understanding of the photoquenching process. The numerical results and approximate analytical approach are presented in this paper for the first time to our knowledge. The calculation gives quite a reasonable explanation for n-type semiconducting GaAs to have infrared absorption quenching while lacking photoconductance quenching. Also, the calculation results have allowed us to interpret the enhanced photoconductance phenomenon following the conductance quenching in typical semi-insulating GaAs and have shown the expected thermal recovery temperature of about 120 K. The numerical results are in agreement with the reported experiments and have diminished some ambiguities in previous works.