Observation of quantum-confined Stark shifts in SiGe/Si type-I multiple quantum wells

Quantum-confined Stark shifts in SiGe/Si type-I multiple quantum wells are suggested by the bias dependence of the photocurrent spectra of p-i-n photodiodes. Both Stark redshift and blueshift have been observed for the same sample in the different ranges of electric fields applied to the quantum wells. The turnaround point corresponds to a certain electric field (named "critical" field). This phenomenon was generally predicted by Austin in 1985 [Phys. Rev. B 31, 5569 (1985)] and calculated in detail for SiGe quantum structure by Kim recently [Thin Solid Films 321, 215 (1998)]. The critical electric field obtained from the photocurrent spectra is in reasonable agreement with the theoretical prediction. (C) 2000 American Institute of Physics. [S0021-8979(00)03711-7].

Observation of field-driven blue shift in delta-doped Si n-i-p-i multiple quantum wells

We report the observation of the field-driven blue shift at near absorption edge in the photo-current response spectra of delta-doped Si n-i-p-i multiple quantum wells due to the widening of the effective energy gap. This phenomenon differs from the observed results in GaAs/AlGaAs and GeSi/Si superlattices, because the physical mechanisms of forming energy band in these superlattice samples are different. Our experimental results are interpreted satisfactorily by the theoretical calculation. (C) 1999 Elsevier Science Ltd. All rights reserved.

Modification of InAs quantum dot structure by the growth of the capping layer

InAs quantum dots inserted at the middle of a GaAs quantum well structure have been investigated by transmission electron microscopy and scanning transmission electron microscopy. We find that the growth condition of the overlayer on the InAs dots can lead to drastic changes in the structure of the dots. We attribute the changes to a combination of factors such as preferential growth of the overlayer above the wetting layers because of the strained surfaces and to the thermal instability of the InAs dots at elevated temperature. The result suggests that controlled sublimation, through suitable manipulation of the overlayer growth conditions, can be an effective tool to improve the structure of the self-organized quantum dots and can help tailor their physical properties to any specific requirements of the device applications. (C) 1998 American Institute of Physics.

an improved smart card based password authentication scheme with provable security

Password authentication has been adopted as one of the most commonly used solutions in network environment to protect resources from unauthorized access. Recently, Lee–Kim–Yoo [S.W. Lee, H.S. Kim, K.Y. Yoo, Improvement of Chien et al.'s remote user authentication scheme using smart cards, Computer Standards & Interfaces 27 (2) (2005) 181–183] and Lee-Chiu [N.Y. Lee, Y.C. Chiu, Improved remote authentication scheme with smart card, Computer Standards & Interfaces 27 (2) (2005) 177–180] respectively proposed a smart card based password authentication scheme. We show that these two schemes are both subject to forgery attacks provided that the information stored in the smart card is disclosed by the adversary. We also propose an improved scheme with formal security proof.

Production and characterization of biodiesel from tung oil

The feasibility of biodiesel production from tung oil was investigated. The esterification reaction of the free fatty acids of tung oil was performed using Amberlyst-15. Optimal molar ratio of methanol to oil was determined to be 7.5:1, and Amberlyst-15 was 20.8wt% of oil by response surface methodology. Under these reaction conditions, the acid value of tung oil was reduced to 0.72mg KOH/g. In the range of the molar equivalents of methanol to oil under 5, the esterification was strongly affected by the amount of methanol but not the catalyst. When the molar ratio of methanol to oil was 4.1:1 and Amberlyst-15 was 29.8wt% of the oil, the acid value decreased to 0.85mg KOH/g. After the transesterification reaction of pretreated tung oil, the purity of tung biodiesel was 90.2wt%. The high viscosity of crude tung oil decreased to 9.8mm(2)/s at 40 degrees C. Because of the presence of eleostearic acid, which is a main component of tung oil, the oxidation stability as determined by the Rancimat method was very low, 0.5h, but the cold filter plugging point, -11 degrees C, was good. The distillation process did not improve the fatty acid methyl ester content and the viscosity.

Production of biodiesel from soapstock using an ion-exchange resin catalyst

The feasibility of biodiesel production from soapstock containing high water content and fatty matters by a solid acid catalyst was investigated. Soapstock was converted to high-acid acid oil (HAAO) by the hydrolysis by KOH and the acidulation by sulfuric acid. The acid value of soapstock-HAAO increased to 199.1 mg KOH/g but a large amount of potassium sulfate was produced. To resolve the formation of potassium sulfate, acid oil was extracted from soapstock and was converted to HAAO by using sodium dodecyl benzene sulfonate (SDBS). The maximum acid value of acid oil-HAAO was 194.2 mg KOH/g when the mass ratio of acid oil, sulfuric acid, and water was 10:4:10 at 2% of SDBS. In the esterification of HAAO using Amberylst-15, fatty acid methyl ester (FAME) concentration was 91.7 and 81.3% for soapstock and acid oil, respectively. After the distillation, FAME concentration became 98.1% and 96.7% for soapstock and acid oil. The distillation process decreased the total glycerin and the acid value of FAME produced a little.

ENHANCEMENT EFFECT OF PLASMA-ENHANCED CHEMICAL-VAPOR-DEPOSITED SIN CAPPING LAYER ON DIELECTRIC CAP QUANTUM-WELL DISORDERING

Quantum well disordering of GaAs/AlGaAs multiple quantum well(MQW) has been accomplished with only plasma enhanced chemical vapor deposited (PECVD) SiN cap layer growth. The amount of blue shift increases with SiN growing time. This result has been explained by the vacancy indiffusion during PECVD SiN growth. Rapid thermal annealing (RTA) of the sample after SiN cap layer growth at 850 degrees C for 35 s caused a larger amount of blue shift than those obtained without RTA. By considering the model of Al diffusion from AlGaAs barrier into GaAs QWs together with the result from photoluminescence (PL) measurement, Al diffusion coefficients were calculated. The Al diffusion coefficient due to PECVD SiN was estimated at about 3 x10(-17) cm(2)/s. It was possible to extract the effect of RTA on the QW disordering, which showed that the amount of the blue shift and the Al diffusion coefficient due only to RTA increases with SiN cap layer thickness as reported by Chi et al.(10))

Linear and nonlinear optical properties of the PbSe quantum dots doped germane-silica glass optical fiber

We report development of a new fiber doped with PbSe quantum dots for nonlinear optical applications. PbSe quantum dots related absorption peaks were obtained at 1021, 1093 and 1351 nm. The resonant optical nonlinearity and attenuation at 1500 nm were measured to be 9.4 × 10−16 m2/W and 0.01 dB/m, respectively. The emission around 1540 nm was observed upon near resonant pumping at 1064 nm.

薄膜/基体界面的层裂行为研究

近年来，薄膜/基体结构在微/纳米工程中得到广泛的应用，研究薄膜/基体界面的力学性能得到广泛的关注。为了检验薄膜和基体间的黏结强度，早在20世纪50年代，人们就设计了一种简单的实验方法－－撕裂试验，该试验由于其简单易于操作至今仍在许多领域都有广泛的应用。早期对于撕裂试验的研究多局限于弹性撕裂，即将薄膜和基体都作为弹性材料来处理。后来，对于韧性薄膜，为了预测由于塑性耗散引起能量释放率(撕裂力)的增大，Kim及其合作者提出并采用了梁的弯曲模型对撕裂中的塑性耗散进行预测。Wei和Hutchinson在分析弹塑性薄膜的撕裂问题时，采用了一种不同于Kim的弯曲模型的方法，除已脱胶部分之外，薄膜的脱胶过程由平面应变弹塑性有限元方法模拟，在分析过程中采用黏聚力模型来刻画韧性薄膜沿基体界面被撕裂或脱胶过程，所得结果无论是在定性还是定量上均与Kim的梁弯曲模型结果有很大区别。\newline 鉴于几种关于韧性薄膜撕裂模型对结果预测的不一致，几位学者联名倡议提出开展关于薄膜撕裂实验刻画模型有效性检验的国际合作。在倡议中指出无论采用何种方法，都须采用黏聚力模型来刻画界面断裂过程。而此后有很多工作都采用黏聚力模型来刻画界面。\newline 如果考虑残余应力的影响，涂层或黏结于基体上的薄膜或多层材料中的单独片层等均不可避免地承受着残余应力的作用。薄膜中的残余应力的主要来源于沉积过程，外延，以及热膨胀失配等因素。承受应力状态的薄膜/基体体系往往在边缘处产生很强的应力集中，极易导致其在该处界面起始层裂发生。如何评价残余应力对薄膜/基体体系界面的影响也显得至关重要。\newline 本文将对撕裂试验以及热失配引起残余应力作用下的薄膜/基体体系的界面层裂行为展开研究。薄膜/基体体系界面采用混合性黏聚力模型来刻画。研究了对应不同薄膜材料参数和不同界面参数情况下的撕裂行为，给出了撕裂力和界面分离功以及界面分离强度等参数之间的关系，并对界面层裂模式进行了分析。同时对多角度撕裂的界面破坏形式进行了系统的研究，给出了稳态撕裂力及相角与撕裂角度之间的关系。\newline 对由于热失配引起残余应力作用下的薄膜/基体体系的界面层裂行为的研究。假设薄膜和基体材料分别为弹塑性材料和弹性材料。研究了对应不同薄膜材料参数和不同界面参数情况的界面层裂发生机制，给出了层裂发生时临界温度荷载与薄膜/基体几何参数、材料参数以及模型参数之间的关系。

Multi-strip MRPCs for FOPI

During the last years FOPI has developed a new ToF system as an upgrade of the existing detector based on Multi-strip Multi-gap Resistive Plate Chambers (MMRPCs). The intention is to increase the charged Kaon identification up to a laboratory momentum of 1 GeV/c and to enhance the azimuthal detector granularity. The new ToF barrel has an active area of 5 m(2) with 2400 individual strips (900 x 1.6 mm(2)) [A. Schuttauf, et al., Nucl. Phys. B 158 (2006) 52] which are read out on both sides by a custom designed electronics [M. Ciobanu, et al., IEEE Trans. Nucl. Sci. NS-54 (4) (2007) 1201; K. Koch, et al., IEEE Trans. Nucl. Sci. NS-52(3) (2005) 745]. To reach the envisaged goal a time resolution of 100 ps is needed, at a flight path of 1-1.3 m. Due to the rare production of the K- at SIS energies the efficiency of the MMRPCs has to be above 95%. We report on measurements with the detectors and electronics from the mass production line. For this purpose we used a proton beam at 2.0 and 1.25 GeV, at rates between 0.1 and 5 kHz/cm(2) to determine the timing, efficiency and rate capability of the MMRPCs