Influence of interdot electronic coupling on photoluminescence spectra of self-assembled InAs/GaAs quantum dots

The influence of interdot electronic coupling on photoluminescence (PL) spectra of self-assembled InAs/GaAs quantum dots (QDs) has been systematically investigated combining with the measurement of transmission electron microscopy. The experimentally observed fast red-shift of PL energy and an anomalous reduction of the linewidth with increasing temperature indicate that the QD ensemble can be regarded as a coupled system. The study of multilayer vertically coupled QD structures shows that a red-shift of PL peak energy and a reduction of PL linewidth are expected as the number of QD layers is increased. On the other hand, two layer QDs with different sizes have been grown according to the mechanism of a vertically correlated arrangement. However, only one PL peak related to the large QD ensemble has been observed due to the strong coupling in InAs pairs. A new possible mechanism to reduce the PL linewidth of QD ensemble is also discussed.

The enhancement of diffusion by strain of InAs quantum dots in a GaAs matrix

We investigate the annealing behavior of Photoluminescence (PL) from self-assembled InAs quantum dots (QDs) with different thicknesses GaAs cap layers. The diffusion introduced by annealing treatment results in a blue-shift of the QD PL peak, and a decrease in the integrated intensity. The strain present in QDs enhances the diffusion, and the QDs with the cap layers of different thicknesses will experience a strain of different strength. This can lend to a, better understanding of the larger blue-shift of the PL peak of the deeper buried QDs, and the different variance of the full width at half maximum of the luminescence from QDs with the cap layers of different thicknesses.

Annealing behavior of InAs/GaAs quantum dot structures

We investigate the annealing behavior of InAs layers with different thicknesses in a GaAs matrix. The diffusion enhancement by strain, which is well established in strained quantum wells, occurs in InAs/GaAs quantum dots (QDs). A shift of the QD luminescence peak toward higher energies results from this enhanced diffusion. In the case of structures where a significant portion of the strain is relaxed by dislocations, the interdiffusion becomes negligible, and there is a propensity to generate additional dislocations. This results in a decrease of the QD luminescence intensity, and the QD peak energy is weakly affected.

Pockels effect in GaN/Al-x,Gal(1-x)N superlattice with different quantum structures - art. no. 69841G

The linear electro-optic (Pockels) effect of wurtzite gallium nitride (GaN) films and six-period GaN/AlxGa1-xN superlattices with different quantum structures were demonstrated by a polarization-maintaining fiber-optical Mach-Zehnder interferometer system with an incident light wavelength of 1.55 mu m. The samples were prepared on (0001) sapphire substrate by low-temperature metalorganic chemical vapor deposition (MOCVD). The measured coefficients of the GaN/AlxGa1-xN superlattices are much larger than those of bulk material. Taking advantage of the strong field localization due to resonances, GaN/AlxGa1-xN SL can be proposed to engineer the nonlinear responses.

A Low Power Baseband Chain for CMMB Application

This paper presents experimental results of an analog baseband circuit for China Multimedia Mobile Broadcasting (CMMB) direct conversion receiver in 0.35um SiGe BiCMOS process. It is the first baseband of CMMB RFIC reported so far. A 8(th)-order chebyshev low pass filter (LPF) with calibration system is used in the analog baseband circuit, the filter provides 0.5 dB passband ripple and -35 dB attenuation at 6MHz with the cutoff frequency at 4MHz, the calibration of filter is reported to achieve the bandwidth accuracy of 3%. The baseband variable gain amplifier (VGA) achieves more than 40 dB gain tuning with temperature compensation. In addition, A DC offset cancellation circuit is also introduced to remove the offset from layout and self-mixing, and the remaining offset voltage and current consumption are only 6mV and 412uA respectively. Implemented in a 0.35um SiGe technology with 1.1 mm(2) die size, this tuner baseband achieves OIP3 of 25.5 dBm and dissipate 16.4 mA under 2.8-V supply.

Fast Locking and High Accurate Current Matching Phase-Locked Loop

In this paper, a charge-pump based phase-locked loop (CPLL) that can achieve fast locking and tiny deviation is proposed and analyzed. A lock-aid circuit is added to achieve fast locking of the CPLL. Besides, a novel differential charge pump which has good current matching characteristics and a PFD with delay cell has been used in this PLL. The proposed PILL circuit is designed based on the 0.35um 2P4M CMOS process with 3.3V/5V supply voltage. HSPICE simulation shows that the lock time of the proposed CPLL can be reduced by over 72% in comparison to the conventional PILL and its charge pump sink and source current mismatch is only 0.008%.

A Fast-Locking Phase-Locked Loop Using a Seven-State Phase Frequency Detector

A seven-state phase frequency detector (S.S PFD) is proposed for fast-locking charge pump based phase-locked loops (CPPLLs) in this paper. The locking time of the PLL can be significantly reduced by using the seven-state PFD to inject more current into the loop filter. In this stage, the bandwidth of the PLL is increased or decreased to track the phase difference of the reference signal and the feedback signal. The proposed architecture is realized in a standard 0.35 mu m 2P4M CMOS process with a 3.3V supply voltage. The locking time of the proposed PLL is 1.102 mu s compared with the 2.347 mu s of the PLL based on continuous-time PFD and the 3.298 mu s of the PLL based on the pass-transistor tri-state PFD. There are 53.05% and 66.59% reductions of the locking time. The simulation results and the comparison with other PLLs demonstrate that the proposed seven-state PFD is effective to reduce locking time.

Design and Verification of the Programming Circuit in an Application-Specific FPGA

In this paper we present a methodology and its implementation for the design and verification of programming circuit used in a family of application-specific FPGAs that share a common architecture. Each member of the family is different either in the types of functional blocks contained or in the number of blocks of each type. The parametrized design methodology is presented here to achieve this goal. Even though our focus is on the programming circuitry that provides the interface between the FPGA core circuit and the external programming hardware, the parametrized design method can be generalized to the design of entire chip for all members in the FPGA family. The method presented here covers the generation of the design RTL files and the support files for synthesis, place-and-route layout and simulations. The proposed method is proven to work smoothly within the complete chip design methodology. We will describe the implementation of this method to the design of the programming circuit in details including the design flow from the behavioral-level design to the final layout as well as the verification. Different package options and different programming modes are included in the description of the design. The circuit design implementation is based on SMIC 0.13-micron CMOS technology.

A Synthesis Tool for a Tile-Based Heterogeneous FPGA

A multi-mode logic cell architecture in a tile-based heterogeneous FPGA is proposed, and a logic synthesis tool, called Vsyn, based on this architecture is presented. The logic cell architecture design and its synthesis tool development are strongly influencing each other. Any feature or parameter from one needs to be fully exercised and verified on the other. In this paper, we presented experimental results based MCNC benchmarks to show that the integration of the synthesis tool and the FPGA architecture can achieve high performance in the targeted FPGA applications. In addition, Vsyn can also target embedded special-purpose macros for the heterogeneous FPGA.

Molecular-beam epitaxial growth of position controlled InAs islands on cleaved edge of InGaAs/GaAs superlattice

A promising approach for positioning of InAs islands on (110)GaAs is demonstrated. By combining self-assembly of quantum dots with solid source molecular beam epitaxy (MBE) on cleaved edge of InGaAs/GaAs superlattice (SL), linear alignment of InAs islands on the InGaAs strain layers have been fabricated The cleaved edge of InGaAs/GaAs SL acts as strain nanopattern for InAs selective growth. Indium atoms incident on the surface will preferentially migrate to InGaAs regions where favorable bonding sites are available. The strain nanopattern's effect is studied by the different indium fraction and thickness of InxGa1-xAs/GaAs SL. The ordering of the InAs islands is found to depend on the properties of the underlying InGaAs strain layers.

Anomalous shift of the beating nodes in illumination-controlled In1-xGaxAs/In1-yAlyAs two-dimensional electron gases with strong spin-orbit interaction

The beating patterns in the Shubnikov-de Haas oscillatory magnetoresistance originating from zero-field spin splitting of two-dimensional electron gases (2DEGs) in In0.52Al0.48As/InxGa1-xAs/In0.52Al0.48As quantum wells with silicon delta doped on the upper barrier layer have been investigated by means of magnetotransport measurements before and after illumination. Contrary to the expectation, after each illumination, the beating nodes induced by the zero-field spin-splitting effect shift to lower and lower magnetic field due to the decrease in the zero-field spin-splitting energy of the 2DEGs. The anomalous phenomenon of the shift of the beating nodes and the decrease in spin-orbit coupling constants after illumination cannot be explained by utilizing the previous linear Rashba model. It is suggested that the decrease in the zero-field spin-splitting energy and the spin-orbit coupling constant arise from the nonlinear Rashba spin splitting.

Optimization and comparison of single- and dual-pump fiber-optical parametric amplifiers with dispersion fluctuations

Solutions for fiber-optical parametric amplifiers (FOPAs) with dispersion fluctuations are derived using matrix operators. On the basis of the propagation matrix product and the hybrid genetic algorithm, we have optimized and compared single- and dual-pump FOPAs with zero-dispersion-wavelength variations. The simulations prove that the design of FOPAs involves multimodal function optimization problems. The numerical results show that dual-pump FOPAs are highly sensitive to dispersion fluctuations whereas dispersion variations have less impact on the gain of single-pump FOPAs. To increase signal gain and reduce ripple, dual-pump FOPAs, instead of single-pump FOPAs, have to be carefully optimized with a suitable multisegment fiber structure rather than a one-segment fiber structure. The different combinations of multisegment fibers can provide highly different gain properties. The increase in gain is at the cost of the ripple.

黄土高原沟壑区果园还耕对土壤水分的影响

在黄土高原沟壑区王东沟小流域,在塬面、梁地和坡地三种地貌类型上,分别选取了盛果期果园、老果园和退耕还田等3种土地利用与管理方式,测定土壤0~400 cm(塬面0~600 cm)水分含量分布,研究果园的利用管理方式变化对土壤剖面中水分含量变化的影响。结果表明,梁地和坡地上,盛果期果园、老果园土壤水分含量均接近作物凋萎湿度(10%);塬面上,盛果期果园、老果园土壤水分含量为15%。退果4年的耕地、坡地0~400 cm土层土壤储水量显著增加,梁地和塬面虽有增加但与老果园相比,剖面储水量并没有达到显著水平;退果4年耕地、坡地和梁地剖面中的60~140 cm和220~400 cm含水量提高最显著,塬面上220~600 cm土层中水分提高显著。

聚合物共混物的相容性及热力学研究

近二十年来，共混已经成为简便而有效地制备新的具有各种附加性能的高分子材料的一种重要方法。共混物间的相容性及相容的程度往往决定其最终性质，因此，高聚物共混物的相容性和相行为成为高聚物共混热力学研究的热点。我们选取了二乙酸纤维素(CDA)／聚乙烯基砒烙烷酮(PVP)、聚氧化乙烯(PEO)／聚醋酸乙烯酯(PVAc)和聚氧化乙烯(PEO)／聚甲基丙烯酸甲酯(PMMA)三个共混体系作为研究对象，用溶液法、量热法和红外光谱等实验手段研究了三个体系的相容性及相容程度，用现代热力学理论预测了它们的相容性及相行为。CDA/PVP共混体系 1．用DSC和DMA测定了不同组成的CDA/PVP共混物的玻璃化转变温度，所有组成的共混物均只有一个介于两个纯组分玻璃化转变温度之间的T_g，且不同组成共混物的玻璃化转变温度高于用Fox方程计算得到的玻璃化转变温度，说明该体系是相容性共混体系，且两组分间存在特殊相互作用。2．用奥氏粘度计和乌氏粘度计分别测定的不同组成共混物的绝对粘度和稀溶液特性粘数，都远远大于按相应比例的CDA和PVP计算的相关粘度和特性粘数的数学平均值，这应该归结为两组分间的特殊相互作用使共混溶液中线团的尺寸变大，导致粘度增大。3．共混物的红外光谱谱图显示PVP的加入使CDA的羟基吸收峰向低频率方向移动，同时，随CDA含量的增加，PVP的羰基吸收峰也向低频率方向移动，说明CDA与PVP分子间形成了氢键，使官能团的电子云密度降低，吸收峰频率降低。4．溶液量热法得到的不同组成共混物的混合热焓均为负值，直接证明该共混体系是热力学相容体系，而且随CDA含量的增加，混合热焓的绝对值也增大，表明体系的相容程度与共混组成有关。PEO/PVAc共混体系 1．用乌氏粘度计测定了不同组成不同分子量的PEO/PVAc共混物的稀溶液特性粘数，计算出表征分子间相互作用的参数，△b。发现随PVAc分子量的增加，体系的相容性降低，体系的相容程度还与共混组成及溶液浓度有关。2．用DSC方法首次得到不同分子量的PEO/PVAc共混体系的“云点”和混合热焓。随着PVAc分子量的增加，“云点”曲线向低温方向移动。不同组成共混物的混合热焓值都小于零，直接证明该体系是热力学相容体系。溶液量热法得到的混合热焓值也都为负，与DSC方法得到的结果一致。3．PEO与PVAc间只存在弱相互作用，所以，选择Hamada等人改进的Flory状态方程理论对PEO/PVAc共混体系的相容性和相行为进行预测。利用溶液量热法的混合热焓值，得到该体系的Flory相互作用参数与温度和共混组成的关系，结果表明该体系是相容性共混体系，而且应该存在LCST相行为，体系的相容程度随PEO含量的增加而降低。4．Sanchez-Lacombe(SL)格子流体理论也适用于PEO/PVAc共混体系，通过PVAc分子量与相互作用能的定量关系建立了该共混物的热力学参数的预测体系。将SL理论拟合出的spinodal曲线和binodal曲线组成的相图与DSC法得到的相图（用“云点”表示）对比，发现二者完全相符。随PVAc分子量的增加，binodal曲线向低温方向移动，临界点向富含PEO的方向移动。混合热焓的实验值也与理论值相符。Flory相互作用参数随温度和共混组成的变化与Hamada等人改进的Flory状态方程理论预测的结果一致。PEO/PMMA共混体系 1．用乌氏粘度计测定了不同组成、不同分子量的PEO/PMMA共混物的稀溶液特性粘数，计算出表征分子间相互作用的参数，Δb。发现PEO与分子量最小的PMMA共混的体系是相容性共混体系，与分子量最大的PMMA共混的是不相容性共混体系，说明随PMMA分子量的增加，体系的相容性降低。2．溶液量热法得到PEO/PMMA = 50/50（重量比）的共混物的混合热焓为负值，表明该体系是热力学相容体系。DSC法得到的不同组成共混物的混合热焓值也都为负，与溶液量热法结论一致。3．Hamada等人改进的Flory状态方程理论也适用于PEO/PMMA体系，因为PEO与PMMA之间不存在特殊相互作用。用溶液量热法得到的混合热焓值，计算出该体系的Flory相互作用参数与温度的关系，结果表明该体系是相容性共混体系，而且应该存在LCST相行为。