Photoluminescence degradation in GaN induced by light enhanced surface oxidation

The exponential degradation of the photoluminescence (PL) intensity at the near-band-gap was observed in heavily doped or low-quality GaN with pristine surface under continuous helium-cadmium laser excitation. In doped GaN samples, the degradation speed increased with doping concentration. The oxidation of the surface with laser irradiation was confirmed by x-ray photoemission spectroscopy measurements. The oxidation process introduced many oxygen impurities and made an increase of the surface energy band bending implied by the shift of Ga 3d binding energy. The reason for PL degradation may lie in that these defect states act as nonradiative centers and/or the increase of the surface barrier height reduces the probability of radiative recombination.

Room-temperature spin-oriented photocurrent under near-infrared irradiation and comparison of optical means with Shubnikov de-Haas measurements in AlXGa1-XN/GaN heterostructures

The admixture of linear and circular photogalvanic effects and (CPGEs) in AlxGa1-xN/GaN heterostructures has been investigated quantitatively by near-infrared irradiation at room temperature. The spin-based photocurrent that the authors have observed solidly indicates the sizable spin-orbital interaction of the two-dimensional electron gas in the heterostructures. Further analysis shows consistency between studies by optical and magnetic (Shubnikov de-Haas) measurements on the spin-orbital coupling effects among different AlxGa1-xN/GaN heterostructures, indicating that the CPGE measurement is a good way to investigate the spin splitting and the spin polarization in semiconductors. (C) 2007 American Institute of Physics.

Optical microwave generation using two parallel DFB lasers integrated with Y-branch waveguide coupler

A new device of two parallel distributed feedback (DFB) lasers integrated monolithically with Y-branch waveguide coupler was fabricated by means of quantum well intermixing. Optical microwave signal was generated in the Y-branch waveguide coupler through frequency beating of the two laser modes coming from two DFB laser in parallel, which had a small difference in frequency. Continuous rapid tuning of optical microwave signal from 13 to 42 GHz were realized by adjusting independently the driving currents injected into the two DFB lasers.

Electron irradiation induced defects in high temperature annealed InP single crystal

Electron irradiation induced defects in InP material which has been formed by high temperature annealing undoped InP in different atmosphere have been studied in this paper. In addition to Fe acceptor, there is no obvious defect peak in the sample before irradiation, whereas five defect peaks with activation energies of 0.23 eV, 0.26 eV, 0.31 eV, 0.37 eV and 0.46 eV have been detected after irradiation. InP annealed in P ambient has more thermally induced defects, and the defects induced by electron irradiation have characteristics of complex defect. After irradiation, carrier concentration and mobility of the samples have suffered obvious changes. Under the same condition, electron irradiation induced defects have fast recovery behavior in the FeP2 ambient annealed InP. The nature of defects, as well as their recovery mechanism and influence on material property have been discussed from the results.

Frequency limitation in the calibration of microwave test fixtures

The problem of frequency limitation arising from the calibration of asymmetric and symmetric test fixtures has been investigated. For asymmetric test fixtures, a new algorithm based on the thru-short-match (TSM) method is outlined. It is found that the conventional TSM method does not have any inherent frequency limitation, but using the same procedure with an unknown match may lead to the said problem. This limitation can be avoided by using a different algorithm. The various calibration methods for symmetric test fixtures using known standards are also discussed and the origin of the frequency limitation is identified. Several ways in avoiding the problem are proposed. There is good agreement between the theories and experimental data.

The effects of pre-irradiation on the formation of Si1-xCx alloys

Carbon ions were implanted into crystal Si to a concentration of (0.6-1.5)at% at room temperature. Some samples were pre-irradiated with S-29(i)+ ions, while others were not pre-irradiated. Then the two kinds of samples were implanted with C-12(+) ions simultaneously, and Si1-xCx alloys were grown by solid phase epitaxy with high-temperature annealing. The effects of preirradiation on the formation of Si1-xCx alloys were studied. If the dose of implanted C ion was less than that for amorphizing Si crystals, the implanted C atoms would like to combine with defects produced during implantation, and then it was difficult for Si1-xCx alloys to form after annealine, at 950 degreesC. Pre-irradiation was advantageous for Si1-xCx alloy formation. With the increase of C ion dose, the damage produced by C ions increased. Pre-irradiation was unfavorable for Si1-xCx, alloy formation. If the implanted C concentration was higher than that for solid phase epitaxy solution, only part of the implanted C atoms form Si1-xCx alloys and the effects of pre-irradiation could be neglected. As the annealing temperature was increased to 1050 degreesC, Si1-xCx alloys in both pre-irradiated and unpreirradiated samples of low C concentration remained, whereas most part of Si1-xCx alloys in samples with high C concentration vanished.

Computation of resonant frequencies and quality factors of cavities by FDTD technique and Pade approximation

The finite-difference time domain (FDTD) technique and the Pade approximation with Baker's algorithm are used to calculate the mode frequencies and quality factors of cavities. Comparing with the fast Fourier transformation/Pade method, we find that the Fade approximation and the Baker's algorithm can obtain exact resonant frequencies and quality factors based on a much shorter time record of the FDTD output.

Experimental measurement of microwave-induced electron spin-flip time

The electron spin resonance (ESR) is optically detected by monitoring the microwave-induced changes in the circular polarization of the neutral exciton (X) and the negatively charged exciton (X-) emission in CdTe quantum wells with low density of excess electrons. We find that the circular polarization of the X and X- emission is a mapping of the spin polarization of excess electrons. By analyzing the ESR-induced decrease in the circular polarization degree of the X emission, we deduce the microwave-induced electron spin-flip time >0.1 mus, which is much longer than the recombination time of X and X-. This demonstrates that the optically detected ESR in type I quantum wells with low density of excess electrons does not obey the prerequisite for the conventional optically detected magnetic resonance. (C) 2001 American Institute of Physics.

The improvement characteristics of homoepitaxial GaAs grown by atomic hydrogen-assisted molecular beam epitaxy

The electrical activity of defects in GaAs grown on GaAs substrates doped with Si and Be by both conventional molecular beam epitaxy (MBE) and atomic hydrogen-assisted MBE (H-MBE) were characterized by deep level transient spectroscopy. The trap densities are significantly reduced in the homoepitaxial GaAs grown by H-MBE compared to that grown by MBE. The reduction of trap densities is attributed to in situ passivation of these defects by atomic H during the growth. The improvement characteristics of GaAs materials will be significance for fabrication of semiconductor devices.

Room-temperature microwave oscillation in AlAs/GaAs superlattices

Room-temperature microwave (MW) oscillations are observed in GaAs/AlAs (10 nm/2 nm) doped weakly coupled superlattices (SLs) in the first plateau of the I-V curve. Oscillations induced by sequential resonant tunneling are detected in a temperature range from 15 to 300 K by applying DC bias on the Si, diodes. The temperature dependence of current at small fixed bias voltage is also measured. Through analysis, it is found that the dominant transport mechanisms are sequential resonant tunneling and phonon-assisted tunneling when the temperature is below 300 K. The low bias voltage at which oscillations are realized is helpful to restrain thermionic emission through the X valley of AlAs barriers in the room-temperature transport. (C) 1999 Published by Elsevier Science B.V. All rights reserved.

Electron irradiation and thermal annealing effect on GaAs solar cells

This paper describes the effect of electron irradiation and thermal annealing on LPE AlGaAs/GaAs heterojunction solar cells with various p/n junction depths. The electron irradiation experiments were performed with energy of 3 MeV, fluences ranging from 1 x 10(14) to 5 x 10(15) e/cm(2). The results obtained demonstrate that the irradiation-induced degradation of performances of the cells is mainly in the short circuit current and could be mostly recovered by annealing at 260 degrees C for 30 min. Four electron traps, E-c - 0.24 eV, E-c - 0.41 eV, E-c - 0.51 eV, E-c - 0.59 eV, were found by DLTS analysis, only two shallow levels of which could be removed by the annealing. (C) 1998 Elsevier Science B.V. All rights reserved.

Neutron irradiation-infrared based measurement method for interstitial oxygen in heavily boron-doped silicon

A novel method, based on an infrared absorption and neutron irradiation technique, has been developed for the determination of interstitial oxygen in heavily boron-doped silicon. The new procedure utilizes fast neutron irradiated silicon wafer specimens. On fast neutron irradiation, the free carriers of high concentration in silicon can be trapped by the irradiated defects and the resistivity increased. The resulting calibration curve for the measurement of interstitial oxygen in boron-doped silicon has been established on the basis of the annealing behaviour of irradiated boron-doped CZ silicon.

Dependence of photoluminescence induced by carbon contamination on GeSi structure

We studied the dependence of photoluminescence induced by carbon contamination on the Ge/GeSi structure. It is found that a carbon and silicon defect complex may be formed in a special structure by opening the in situ high-energy electron diffraction test during growth. There is an important difference in the dependence of photoluminescence on the temperature between the defect complex in our samples and in bulk Si. where the impurity-active center is generated by high-energy electron (about several MeV) irradiation. The quenching temperature of the photoluminescence from the impurity-active center is higher in our Ge/GeSi structure than in bulk Si. The defect complex may serve as an impurity-active center for a possible application in making Si-based light-emitting diodes whose wavelength is around 1.3 mu m in the window of optical communication. (C) 1998 Elsevier Science B.V. All rights reserved.

Structural identification of a cubic phase in hexagonal GaN films grown on sapphire by gas-source molecular beam epitaxy

The structural characteristics of gallium nitride (GaN) films grown on sapphire(0001) substrates by gas source molecular beam epitaxy (GSMBE) have been investigated using high-resolution synchrotron irradiation X-ray diffraction and cathodoluminescence with a variable energy electron beam. Besides the well-known GaN hexagonal structure, a small portion of cubic phase GaN was observed. The X-ray measurements provide an essential means for the structural identification of the GaN layers. Arising from the variable penetration depth of the electron beam in the cathodoluminescence measurements, it was found that the fraction of the GaN cubic-phase typically increased as the probing depth was increased. The results suggest that the GaN cubic phase is mostly located near the interface between the substrate and GaN layer due to the initial nucleation.

Measurement and analysis of microwave frequency response of semiconductor optical amplifiers - art. no. 682406

The measurement and analysis of the microwave frequency response of semiconductor optical amplifiers (SOAs) are proposed in this paper. The response is measured using a vector network analyzer. Then with the direct-subtracting method, which is based on the definition of scattering parameters of optoelectronic devices, the responses of both the optical signal source and the photodetector are eliminated, and the response of only the SOA is extracted. Some characteristics of the responses can be observed: the responses are quasi-highpass; the gain increases with the bias current; and the response becomes more gradient while the bias current is increasing. The multisectional model of an SOA is then used to analyze the response theoretically. By deducing from the carrier rate equation of one section under the steady state and the small-signal state, the expression of the frequency response is obtained. Then by iterating the expression, the response of the whole SOA is simulated. The simulated results are in good agreement with the measured on the three main characteristics, which are also explained by the deduced results. This proves the validity of the theoretical analysis.