Baozhen Zheng

Temperature dependence of optical gain, quantum efficiency, and threshold current in GaAs/GaAlAs graded-index separate-confinement heterostructure single-quantum-well lasers

The temperature dependence of the optical gain in graded-index separate-confinement heterostructure single-quantum-well lasers with different quantum-well widths were investigated. The observed dependence of the kink temperature on cavity loss and quantum-well width and the differential quantum efficiency minimum at the kink temperature were analyzed in terms of the temperature variation of the gain spectra and peak gain curves. Dependences of the characteristic temperature T/sub 0/ on the quantum-well width, cavity loss, and temperature range are discussed in terms of the variation of the peak modal gain versus current relation with temperature and quantum-well width. >

Effect of growth interruption on the optical properties of InAs/GaAs quantum dots

The effect of growth interruption (GI) on the optical properties of InAs/GaAs quantum dots was investigated by cw and time-resolved photoluminescence (PL). It is found that this effect depends very much on the growth conditions, in particular, the growth rate. In the case of low growth rate, we have found that the GI may introduce either red-shift or blue-shift in PL with increase of the interruption time, depending on the InAs thickness. The observed red shift in our 1.7 monolayer (ML) sample is attributed to the evolution of the InAs islands during the growth interruption. While the blue-shift in the 3 ML sample is suggested to be mainly caused by the strain effect. In addition, nearly zero shift was observed for the sample with thickness around 2.5 ML.

Carrier loss resulting from Auger recombination in InGaAsP/InP double heterojunction laser diodes: Spectroscopy of 950 nm high energy emission

From 1.3 μm stripe InGaAsP/InP DH lasers, we observed a 950 nm emission band which originates from band-to-band recombination in InP confinement layers. This experimental result can be well explained by considering the overflow of injected carriers from InGaAsP into InP. The overflowed energetic carriers were created by Auger recombination in InGaAsP.

HOT CARRIER RELAXATION PROCESSES AND NONEQUILIBRIUM PHONON EFFECT IN MULTIPLE QUANTUM-WELL STRUCTURES

Abstract We have used a picosecond nonlinear optical correlation technique to investigate the relaxation processes of hot nonequilibrium carriers in multiple quantum well structures. The average energy loss time constant πavg of the hot electrons was measured. By solving the average energy loss rate equation of carriers and the hot phonon rate equation, taking both the phonon emission and reabsorption into account, we show that πavg is the sum of the electron-phonon interaction time constant and the nonequilibrium phonon decay time, assuming only a single mode of phonons are involved. It is found that the temperature will be higher and/or the energy loss rate of the electron system will be slower as the quantum well width decreases, or the excitation density increases, or the degree of the lattice mismatch between the well layer and the barrier layer increases in the case of strained quantum wells.

Experimental study on mechanical properties of clay soil under compression by ultrasonic test

This paper aims at investigating the ultrasonic and mechanical properties of clay soil under uniaxial compression by axial and lateral ultrasonic tests. Cylindrical soil specimens (50 mm diameter and 100 mm height) were produced to perform a series of uniaxial compressive strength test. The ultrasonic waves were transmitted and received by P-wave piezoelectric transducers (130 and 500 kHz for axial and lateral ultrasonic test, respectively), and the ultrasonic travel time and amplitude were recorded by an ultrasonic detector during the whole deformation process. From the test results, the ultrasonic pulse velocity (UPV) and transmission ratio (TR) increased with the increasing axial stress by axial ultrasonic test. The statistical correlations between UPV and TR and axial stress are presented and discussed. Also, the change in porosity was related to the increase in stress, relationship between porosity variation and UPV was deduced to analyse the stress-related compaction effect. In addition, an equation was proposed to link the relationship between UPV and width of cracks by lateral ultrasonic test, the width of cracks increased steadily with the increase in stress. Crack evolution can be reflected by UPV variation. Moreover, based on the relationship of width of cracks and UPV, continuous damage evolution equation and constitutive model were established, and compared with the testing data. These results confirm that stress-related compaction effect and damage cracking characteristics of clay soil are closely related to the ultrasonic parameters. In this regard, the ultrasonic tests can be suitably exploited for the mechanical study of soil.

Photoluminescence studies of GaAs/AlGaAs single quantum well intermixed by Ga+ ion implantation

Ga(+)ion implantation followed by rapid thermal annealing (RTA) was used to enhance the interdiffusion in GaAs/AlGaAs single Quantum Wells(SQWs). The extent of intermixing was found to be dependent on the well depth, number of implanted ions and annealing time. A very fast interdiffusion process occurs at the initial annealing stage. After that, the enhanced diffusion coefficient goes back to the umimplanted value. We propose a two-step model to explain the diffusion process as a function of the annealing time : a fast diffusion process and a saturated diffusion process. The interdiffusion coefficient of the fast diffusion was found to be of well depth dependence and estimated to be in the range of 5.4x10(-16) similar to 1.5x10(-15)cm(2)s(-1). Copyright (C) 1996 Published by Elsevier Science Ltd

Thermal population from hole mixing tunneling in asymmetric coupled double wells

The high energy tails in continuous wave photoluminescence spectra of asymmetric coupled double wells are studied under various excitation intensities and at different lattice temperatures. Hole mixing tunneling induced thermal population of hot carriers is firstly demonstrated and distinguished from photon heating. The corresponding thermalization process is shown to be dominated by acoustic phonon scattering.

Exciton dynamics in GaAs/GaAlAs, InGaAs/GaAs, and InGaAs/AlGaAs quantum wells

Photoluminescence (PL) and time-resolved PL measurements were used to study the exciton recombination processes in GaAs/AlGaAs, InGaAs/GaAs, and InGaAs/AlGaAs quantum wells (QWs). An increasing lifetime with decreasing well width has been observed in very narrow and high quality GaAs/AlGaAs samples, and attributed to the reduced overlap of the electron and hole wave functions and the increase of the exciton effective volume. In InGaAs/GaAs strained QWs the measured exciton lifetimes were found to be of In composition dependence: the more the indium composition, the shorter the lifetime. The mechanisms, including the random alloy disordering and the degeneration of quasi two-dimensional properties of excitons were inferred to explain the experimental results. The nonradiative recombination was stressed in our InGaAs/AlGaAs QWs. A combined analysis of cw PL and time-resolved PL measurements allows us to separate the radiative and nonradiative decay times in our sample. The observed dominant nonradiative recombination has been tentatively ascribed to the poor quality of AlGaAs.

The thermalization of photoexcited hot carriers in InxGa1−xAs/GaAs strained single quantum well structures

Abstract Both hot carrier photoluminescence and picosecond time-resolved luminescence demonstrate that photoinduced carrier relaxation processes in In x Ga 1− x As GaAs strained single quantum well structures are significantly slower than in the bulk and depend on the alloy composition of the InxGa1−xAs. These results are interpreted in terms of strain induced confinement of the LO-phonon as a possible source of the reduced electron-phonon interaction.

PHOTOLUMINESCENCE EXCITATION SPECTROSCOPY OF INXGA1-XAS/GAAS STRAINED-LAYER COUPLED DOUBLE QUANTUM-WELLS

Abstract Low-temperature photoluminescence excitation spectroscopy of the InxGa1−xAs/GaAs strained-layer coupled double quantum wells (CDQW) with the different barrier widths have been measured. The spectral behaviors show that the band configuration of the light hole is type II and the transitions associated with the light holes are still exciton-like. The calculated transition energies, taking into account both the strain and coupling of quantum wells, are in good agreement with the experimental data. By fitting the experimental results to our calculations, we obtaine that Qvh is about 0.2 for the samples with x=0.14. Comparing our result with other optical studies, we suggest that the band offset would be a function of the indium composition in InxGa1−xAs/GaAs heterojunction systems.