Photoluminescence study of InxGa1-xAs/InyAl1-yAs one-side-modulation-doped asymmetric step quantum wells

Fourier transform photoluminescence measurements were carried out to investigate the optical transitions in InxGa1-xAs/InyAl1-yAs one-side-modulation-doped asymmetric step quantum wells. Samples with electron density n(s) between 0.8 and 5.3 x 10(12) cm(-2) rue studied. Strong recombination involving one to three populated electron subbands with the first heavy-hole subband is observed. Fermi edge singularity (FES) clearly can be observed for some samples. The electron subband energies in the InGaAs/InAlAs step quantum wells were calculated by a self-consistent method, taking into account strain and nonparabolicity effects and the comparison with the experimental data shows a good agreement. Our results can help improve understanding for the application of InGaAs/InAlAs step quantum wells in microelectronic and optoelectronic devices. (C) 1998 Elsevier Science Ltd. All rights reserved.

Observation of the third subband population in modulation-doped InGaAs/InAlAs heterostructure

The population of the third (n = 3) two-dimensional electron subband of InGaAs/InAlAs modulation-doped structures has been observed by means of Fourier transform photoluminescence (PL). Three well resolved PL peaks centred at 0.737, 0.908, and 0.980eV are observed, which are attributed to the transitions from the lowest three electron subbands to the n = 1 heavy-hole subband. The subband separations clearly exhibiting the features of the stepped quantum well with triangle and square potentials are consistent with numerical calculation. Thanks to the presence of Fermi cutoff, the population ratio of these three subbands can be estimated. Temperature- and excitation-dependent luminescences are also analyzed.

Molecular beam epitaxy growth of Iny2Al1-y2As/In0.73Ga0.27As/Iny1Al1-y1As/InP P-HEMTs with enhancement conductivity using an intentional nonlattice-matched buffer layer

Pseudomorphic Iny2Al1-y2As/In0.73Ga0.27As/Iny1Al1-y1As (y1 greater than or equal to 0.52) modulation-doped heterostructures with an intentional nonlattice-matched buffer layer were successfully grown by molecular beam epitaxy on (100)InP substrates. Fourier transform photoluminescence and double crystal x-ray diffraction measurements show a superior crystalline quality in the high In content channel, when In mole fraction increases from y1=0.52 to 0.55 in the Iny1Al1-y1As buffer layer. In this case, an increasing of 16.3% and 23.5% for conductivity (mu xn(s)) and mobility, related to the strain compensation in the In0.73Ga0.27As channel, was achieved, respectively, comparing to the structure containing a well-lattice matched buffer layer. With increasing the mismatch further (y1=0.58), a morphology with cross-hatched pattern was observed due to the onset of a large amount of misfit dislocations, and the electronic characterization is not able to be improved continuously. Because we can realize high quality strained P-HEMTs in a relative wide range of equivalent beam flux (EBF) ratios, the stringent control over the constant EBF is not indispensable on this In-based material system. (C) 1997 American Vacuum Society.

The third subband population in modulation-doped InGaAs/InAlAs heterostructures

We have observed the population of the third (n=3) two-dimensional electron subband of InGaAs/ InAlAs modulation-doped structures with very dense sheet carrier density by means of Fourier transform photoluminescence (PL). Three well-resolved PL peaks centered at 0.737, 0.908, and 0.980 eV are observed, which are attributed to the recombination transitions from the lowest three electron subbands to the n=1 heavy-hole subband. The subband separations clearly exhibit the features of the stepped quantum well with triangle and square potential, consistent with numerical calculation. Thanks to the presence of the Fermi cutoff, the population ratio of these three subbands can be estimated. Temperature and excitation intensity dependence of the quantum well luminescence intensity is also analyzed. (C) 1997 American Institute of Physics.

AN EFFICIENT ITERATIVE DFT-BASED CHANNEL ESTIMATION FOR MIMO-OFDM SYSTEMS ON MULTIPATH CHANNELS

In this paper, an efficient iterative discrete Fourier transform (DFT) -based channel estimator with good performance for multiple-input and multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) systems such as IEEE 802.11n which retain some sub-carriers as null sub-carriers (or virtual carriers) is proposed. In order to eliminate the mean-square error (MSE) floor effect existed in conventional DFT-based channel estimators, we proposed a low-complexity method to detect the significant channel impulse response (CIR) taps, which neither need any statistical channel information nor a predetermined threshold value. Analysis and simulation results show that the proposed method has much better performance than conventional DFT-based channel estimators and without MSE floor effect.

Synthesis of GaN nanorods with vertebra-like morphology

GaN nanorods with vertebra-like morphology were synthesized by nitriding Ga2O3/ZnO films at 1000 degrees C for 20min. Ga2O3 thin films and ZnO middle layers were pre-deposited in turn on Si(111) substrates by r.f. magnetron sputtering system. In the flowing ammonia ambient, ZnO was reducted to Zn and Zu sublimated at 1000 degrees C. Ga2O3 was reducted to Ga2O and Ga2O reacted with NH3 to synthesize GaN nanorods in the help of the sublimation of Zn. The structure and morphology of the nanorods were studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM), The composition of GaN nanorods was studied by energy dispersive spectroscopy (EDS) and fourier transform infrared (FTIR) system.

Two-dimensional photonic crystals with a large photonic band gap designed by a rapid genetic algorithm

We used Plane Wave Expansion Method and a Rapid Genetic Algorithm to design two-dimensional photonic crystals with a large absolute band gap. A filling fraction controlling operator and Fourier transform data storage mechanism had been integrated into the genetic operators to get desired photonic crystals effectively and efficiently. Starting from randomly generated photonic crystals, the proposed RGA evolved toward the best objectives and yielded a square lattice photonic crystal with the band gap (defined as the gap to mid-gap ratio) as large as 13.25%. Furthermore, the evolutionary objective was modified and resulted in a satisfactory PC for better application to slab system.

The study of high temperature annealing of a-SiC : H films

A series of amorphous silicon carbide films were prepared by plasma enhanced chemical vapor deposition technique on (100) silicon wafers by using methane, silane, and hydrogen as reactive resources. A very thin (around 15 A) gold film was evaporated on the half area of the aSiC:H films to investigate the metal induced crystallization effect. Then the a-SiC:H films were annealed at 1100 degrees C for 1 hour in the nitrogen atmosphere. Fourier transform infrared spectroscopy (FTIR), X-Ray diffraction (XRD), and scanning electron microscopy (SEM) were employed to analyze the microstructure, composition and surface morphology of the films. The influences of the high temperature annealing on the microstructure of a-SiC:H film and the metal induced metallization were investigated.

Comparative study of the surface passivation on crystalline silicon by silicon thin films with different structures

Si thin films with different structures were deposited by plasma enhanced chemical vapor deposition (PECVD), and characterized via Raman spectroscopy and Fourier transform infrared (FTIR) spectroscopy. The passivation effect of such different Si thin films on crystalline Si surface was investigated by minority carrier lifetime measurement via a method, called microwave photoconductive decay (mu PCD), for the application in HIT (heterojunction with intrinsic thin-layer) solar cells. The results show that amorphous silicon (a-Si:H) has a better passivation effect due to its relative higher H content, compared with microcrystalline (mu c-Si) silicon and nanocrystalline silicon (nc-Si). Further, it was found that H atoms in the form of Si-H bonds are more preferred than those in the form of Si-H-2 bonds to passivate the crystalline Si surface. (C) 2009 Elsevier B.V. All rights reserved.

Electrical and Optical Phase Transition Properties of Nano Vanadium Dioxide Thin Films

Nano-vanadium dioxide thin films were prepared through thermal annealing vanadium oxide thin films deposited by dual ion beam sputtering. The nano-vanadium dioxide thin films changed its state from semiconductor phase to metal phase through heating by homemade system. Four point probe method and Fourier transform infrared spectrum technology were employed to measure and anaylze the electrical and optical semiconductor-to-metal phase transition properties of nano-vanadium dioxide thin films, respectively. The results show that there is an obvious discrepancy between the semiconductor-to-metal phase transition properties of electrical and optical phase transition. The nano-vanadium dioxide thin films' phase transiton temperature defined by electrical phase transiton property is 63 degrees C, higher than that defined by optical phase transiton property at 5 mu m, 60 degrees C; and the temperature width of electrical phase transition duration is also wider than that of optical phase transiton duration. The semiconductor-to-metal phase transiton temperature defined by optical properties increases with increasing wavelength in the region of infrared wave band, and the occuring temperature of phase transiton from semiconductor to metal also increases with wavelength increasing, but the duration temperature width of transition decreases with wavelength increasing. The phase transition properties of nano-vanadium dioxide thin film has obvious relationship with wavelength in infrared wave band. The phase transition properties can be tuned through wavelength in infrared wave band, and the semiconductor-to-metal phase transition properties of nano vanadiium dioxide thin films can be better characterized by electrical property.

Fast Discrimination of Commerical Corn Varieties Based on Near Infrared Spectra

The existing methods for the discrimination of varieties of commodity corn seed are unable to process batch data and speed up identification, and very time consuming and costly. The present paper developed a new approach to the fast discrimination of varieties of commodity corn by means of near infrared spectral data. Firstly, the experiment obtained spectral data of 37 varieties of commodity corn seed with the Fourier transform near infrared spectrometer in the wavenurnber range from 4 000 to 12 000 cm (1). Secondly, the original data were pretreated using statistics method of normalization in order to eliminate noise and improve the efficiency of models. Thirdly, a new way based on sample standard deviation was used to select the characteristic spectral regions, and it can search very different wavenumbers among all wavenumbers and reduce the amount of data in part. Fourthly, principal component analysis (PCA) was used to compress spectral data into several variables, and the cumulate reliabilities of the first ten components were more than 99.98%. Finally, according to the first ten components, recognition models were established based on BPR. For every 25 samples in each variety, 15 samples were randomly selected as the training set. The remaining 10 samples of the same variety were used as the first testing set, and all the 900 samples of the other varieties were used as the second testing set. Calculation results showed that the average correctness recognition rate of the 37 varieties of corn seed was 94.3%. Testing results indicate that the discrimination method had higher precision than the discrimination of various kinds of commodity corn seed. In short, it is feasible to discriminate various varieties of commodity corn seed based on near infrared spectroscopy and BPR.

平行六边形区域非均匀节点快速傅立叶变换

本文研究平行六边形区域上的非均匀节点离散傅立叶变换的快速算法及其实现.首先在晶格(Lattice)的框架下建立了平行六边形区域上的非均匀节点离散傅立叶变换(NDFTH).在此基础上设计了平行六边形区域上的非均匀节点快速傅立叶变换(NFFTH)算法.其核心思想是以局部性态良好的窗口函数为基底,以平行六边形区域上均匀节点快速傅立叶变换(FFTH)为时空域和频域转换工具,通过在时空域和频域上截取其展开级数的少量几项来快速近似计算,最终降低其计算复杂度.数值计算结果表明,本文算法是合理、稳定、高效的.

门限密码学中的离散傅里叶变换

基于离散傅里叶变换提出了一种通用的门限密码学中恢复分享秘密的算法。该算法是离散傅里叶变换在门限密码学中的首次应用。利用该算法，门限密码体制可以有效地达到鲁棒性和自适应安全性。同时，引入了设计有效的鲁棒自适应安全的门限密码体制的新的通用技术。

The fabrication of TiO2-supported zeolite with core/shell heterostructure for ethanol dehydration to ethylene

The TiO2-supported zeolite with core/shell heterostructure was fabricated by coating aluminosilicate zeolite (ASZ) on the TiO2 inoculating seed via in situ hydrothermal synthesis. The catalysts were characterized by transmission electron microscope (TEM), X-ray diffraction (XRD), nitrogen physisorption (BET), and Fourier transform infrared spectroscopy (FT-IR). The surface acidity of the catalysts was measured by pyridine-TPD method. The catalytic performance of the catalysts for ethanol dehydration to ethylene was also investigated. The results show that the TiO2-supported zeolite composite catalyst with core/shell heterostructure exhibits prominent conversion efficiency for ethanol dehydration to ethylene.