Evolution from point defects to arsenic clusters in low-temperature grown GaAs/AlGaAs multiple quantum wells

Optical transient current spectroscopy (OTCS), photoluminescence (PL) spectroscopy and excitonic electroabsorption spectroscopy have been used to investigate the evolution of defects in the low-temperature grown GaAs/AlGaAs multiple quantum well structures during the postgrowth rapid thermal annealing. The sample was grown at 350 degrees C by molecular beam epitaxy on miscut (3.4 degrees off (001) towards (111)A) (001) GaAs substrate. After growth, the sample was subjected to 30s rapid thermal annealing in the range of 500-800 degrees C. It is found that the integrated PL intensity first decreases with the annealing temperature, then gets a minimum at 600 degrees C and finally recovers at higher temperatures. OTCS measurement shows that besides As,, antisites and arsenic clusters, there are several relatively shallower deep levels with excitation energies less than 0.3 eV in the as-grown and 500 degrees C-annealed samples. Above 600 degrees C, OTCS signals from As,, antisites and shallower deep levels become weaker, indicating the decrease of these defects. It is argued that the excess arsenic atoms group together to form arsenic clusters during annealing. (C) 2000 Elsevier Science B.V. All rights reserved.

Exciton states and optical spectra in CdSe nanocrystallite quantum dots

By using the hole effective-mass Hamiltonian for semiconductors with the wurtzite structure, we have studied the exciton states and optical spectra in CdSe nanocrystallite quantum dots. The intrinsic asymmetry of the hexagonal lattice structure and the effect of spin-orbital coupling (SOC) on the hole states are investigated. It is found that the strong SOC limit is a good approximation for hole states. The selection rules and oscillator strengths for optical transitions between the conduction- and valence-band states are obtained. The Coulomb interaction of exciton states is also taken into account. In order to identify the exciton states, we use the approximation of eliminating the coupling of Gamma(6)(X, Y) with Gamma(1)(Z) states. The results are found to account for most of the important features of the experimental photoluminescence excitation spectra of Norris ct nl. However, if the interaction between Gamma(6)(X, Y) and Gamma(1)(Z) states is ignored, the optically passive P-x state cannot become the ground hole state for small CdSe quantum dots of radius less than 30 Angstrom. It is suggested that the intrinsic asymmetry of the hexagonal lattice structure and the coupling of Gamma(6)(X,Y) with Gamma(1)(Z) states are important for understanding the "dark exciton" effect.

Absorption spectra of nanocrystalline silicon embedded in SiO2 matrix

Nanocrystalline silicon (nc-Si) embedded SiO2 matrix has been formed by annealing the SiOx films fabricated by plasma-enhanced chemical vapor deposition (PECVD) technique. Absorption coefficient and photoluminescence of the films have been measured at room temperature. The experimental results show that there is an "aUrbach-like" b exponential absorption in the spectral range of 2.0-3.0 eV. The relationship of (alpha hv)(1/2) proportional to(hv - E-g) demonstrates that the luminescent nc-Si have an indirect band structure. The existence of Stokes shift between photoluminescence and absorption edge indicates that radiative combination can take place not only between electron states and hole states but also between shallow trap states of electrons and holes. (C) 2000 Elsevier Science B.V. All rights reserved.

Effect of arsenic precipitates on Fermi level in GaAs grown by molecular-beam epitaxy at low temperature

A simple model is presented to discuss the effect of As precipitates on the Fermi level in GaAs grown by molecular-beam epitaxy at low temperature (LT-GaAs). This model implements the compensation between point defects and the depletion of arsenic precipitates. The condition that the Fermi level is pinned by As precipitates is attained. The shifts of the Fermi level in LT-GaAs with annealing temperature are explained by our model. Additionally, the role of As precipitates in conventional semi-insulating GaAs is discussed. (C) 2000 American Institute of Physics. [S0021-8979(00)09905-9].

Effect of defects on optical phonon Raman spectra in SiC nanorods

Fabricated one-dimensional (1D) materials often have abundant structural defects. Experimental observation and numerical calculation indicate that the broken translation symmetry due to structural defects may play a more important role than the quantum confinement effect in the Raman features of optical phonons in polar semiconductor quantum wires such as SiC nanorods, (C) 1999 Elsevier Science Ltd. All rights reserved.

Photocurrent derivative spectra of ZnCdSe-ZnSe double multi-quantum wells

Photocurrent (PC) spectra of ZnCdSe-ZnSe double multi-quantum wells are measured at different temperature. Its corresponding photocurrent derivative (PCD) spectra are obtained by computing, and the PCD spectra have greatly enhanced the sensitivity of the relative weak PC signals. The polarization dependence of the PC spectra shows that the transitions observed in the PC spectra are heavy-hole related, and the transition energy coincide well with the results obtained by envelope function approximation including strain. The temperature dependence of the photocurrent curves indicates that the thermal activation is the dominant transport mechanism of the carriers in our samples. The concept of saturation temperature region is introduced to explain why the PC spectra have different temperature dependence in the samples with different structure parameters. It is found to be very useful in designing photovoltaic devices.

The intrinsic temperature effect of the Raman spectra of graphite

The Raman spectra of ion-implanted highly oriented pyrolytic graphite (HOPG) are reported, in which an additional mode at 1083 cm(-1) and three doublet structures in the positions of similar to 1350, similar to 2450, and similar to 2710 cm(-1) are revealed. Noticeable frequency shifts are observed for all the Raman bands between the spectra excited with different laser powers, which are interpreted as the pure temperature effect and a downshift in the C-C stretching frequency induced by the thermal expansion. Moreover, the pure temperature effect (d omega/dT)(V) without anharmonic contribution is achieved in pristine HOPG. The results suggest that the pure temperature effect without anharmonic contribution plays an important role in the frequency shifts with temperature. (C) 1999 American Institute of Physics. [S0003-6951(99)01313-3].

Interference effects in differential reflectance spectra of the GaAs epilayers grown on Si substrate

We report the observation of oscillating features in differential reflectance spectra from the GaAs epilayer grown on Si substrate in the energy range both below and above the fundamental band gap. It is demonstrated that the oscillating features are due to the difference in the interference between two neighboring areas of the sample. The interference arises from two light beams reflected from different interfaces of the sample. The calculated spectra in the nonabsorption region are in good agreement with measured data. It is shown that the interference effect can be used as a sensitive method to characterize the inhomogeneity of the semiconductor heterostructures. (C) 1998 American Institute of Physics. [S0021-8979(98)08723-4].

Linewidth of the infrared absorption spectra due to bound-to-continuum transition in GaAs/AlxGa1-xAs multiple quantum well structures

We have observed an extremely narrow absorption spectrum due to bound-to-continuum transition in GaAs/AlxGa1-xAs multiple quantum wells (MQWs). Its linewidth is only about one tenth of the values reported previously. Our calculation indicates that the broadening of the excited state in the continuum has little contribution to the absorption linewidth. We have grown a sample whose MQW region contains two kinds of wells with a minor thickness inhomogeneity. Its resultant absorption linewidth is six times as large as that of homogeneous well sample, which is in good agreement with our theoretical analysis. Thus we can suggest that the wider absorption spectra reported by many authors may be due to the well width inhomogeneity. (C) 1998 American Institute of Physics. [S0003-6951(98)03430-5]

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.

Preparation of I-2 clusters and their absorption spectra

Samples have been prepared at different temperatures by loading It molecules into the cages of zeolite 5A, and the measurements of the absorption spectra have been carried out for the prepared samples. It is shown that 12 molecular clusters are formed in the cages of zeolite 5A, and it is also found that molecular clusters which are bonded with intermolecular forces have an important feature, namely, the intermolecular distance in molecular clusters can be changed on different preparing conditions and the blue shift of absorption edges can not be as the criterion of forming molecular clusters.

Interfacial behaviour of a quantum well electrode/electrolyte: EER spectra of an SQW GaAs/AlxGa1-xAs electrode in HQ+BQ non-aqueous electrolyte

The quantum-confined Stark effect and the Franz-Keldysh oscillation of a single quantum well (SQW) GaAs/AlxGa1-xAs electrode were studied in non-aqueous hydroquinone + benzoquinone solution with electrolyte electroreflectance spectroscopy. By investigation of the relation of the quantum-confined Stark effect and the Franz-Keldysh oscillation with applied external bias, the interfacial behaviour of an SQW electrode was analysed. (C) 1997 Elsevier Science S.A.

Characterization of high-speed optoelectronics devices based optical and electrical spectra analyses - art. no. 68380Q

In this paper, we presents the characterization technique of high-speed optoelectronics devices based electrical and optical spectra, which is as important access to the devices performance as the prevalent vector network analyzer (VNA) sweeping method. The measurement of additional modulation of laser and frequency response of photodetector from electrical spectra, and the estimation of the modulation indexes and the chirp parameters of directly modulated lasers based on optical spectra analysis, are given as examples.

Time-resolved photoluminescence spectra of self-assembled InAs/GaAs quantum dots

Two types of InAs self-assembled Quantum dots (QDs) were prepared by Molecular beam epitaxy. Atomic force microscopy (AFM) measurements showed that, compared to QDs grown on GaAs substrate, QDs grown on InGaAs layer has a significantly enhanced density. The short spacing (several nanometer) among QDs stimulates strong coupling and leads to a large red-shift of the 1.3 mu m photoluminescence (PL) peak. We study systematically the dependence of PL lifetime on the QDs size, density and temperature (1). We found that, below 50 K, the PL lifetime is insensitive to temperature, which is interpreted from the localization effects. As T increases, the PL lifetime increases, which can be explained from the competition between the carrier redistribution and thermal emission at higher temperature. The increase of carriers in QDs migrated from barriers and wetting layer (WL), and the redistribution of carriers among QDs enhance the PL lifetime as T increases. The thermal emission and non-radiative recombination have effects to reduce the PL lifetime at higher T. As a result, the radiative recombination lifetime is determined by the wave function overlapping of electrons and holes in QDs, and QDs with different densities have different PL lifetime dependence on the QDs size. (c) 2005 Elsevier B.V. All rights reserved.

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.