Zhongying Xu

Well width dependence of the exciton lifetime in narrow GaAs/GaAlAs quantum wells

We have studied the radiative excitonic lifetime as a function of the well width in GaAs/GaAlAs quantum wells. An increasing lifetime with decreasing well width has been observed in very narrow and high quality GaAs/GaAlAs quantum wells, and attributed to the reduced overlap of the electron and hole wave functions and the increase of the exciton effective volume. This is the first observation of its kind in the conventional GaAs/GaAlAs quantum wells.

Carrier capture into InAs/GaAs quantum dots detected by a simple degenerate pump–probe technique

We demonstrate that the carrier capture and relaxation processes in InAs/GaAs quantum dots can be detected by a simple degenerate pump-probe technique. We have observed a rising process in the transient reflectivity, following the initial fast relaxation in a GaAs matrix, and assigned this rising process to the carrier capture from the GaAs barriers to the InAs layers. The assignment was modeled using the Kramers-Kronig relations. The capture time was found to depend strongly on the InAs layer thickness as well as on the excitation density and photon energy

Wavelength tuning in GaAs/AlGaAs quantum wells by InAs submonolayer insertion

Wavelength tuning of exciton emissions has been achieved simply by inserting an InAs submonolayer at the centre of GaAs quantum wells during molecular beam epitaxy growth. Photoluminescence measurements show that the emission energy can be effectively tuned from the quantum-well-determined energy down to less than the band gap of GaAs, depending on the well width as well as the InAs layer thickness. Using the effective-mass approximation, the tuning effect can be well predicted theoretically. The results reported here may provide an alternative way to tune the wavelength in optoelectronic devices.

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.

GROWTH INTERRUPTION INDUCED INTERFACE MICROROUGHNESS IN SINGLE QUANTUM-WELLS

Growth interruption-induced microroughness is studied by photoluminescence (PL) of single quantum wells with different well widths and interruption times. Analysis of the peak splitting in the PL spectra shows that the adjacent peak splittings correspond to well width differences smaller than one monolayer. The number of split peaks increases with increasing well width, saturating when the well width exceeds 11 monolayers. This trend correlates well with the decrease in the lateral dimension of the exciton, which corresponds roughly to the minimum optically sampled area of the interface. For a given quantum well, a plot of the normalized integrated intensities of the split PL peaks versus the well width fluctuation is well described by a Gaussian distribution with an average fluctuation smaller than one monolayer. These results are consistent with the microroughness model.

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.

Hot carrier relaxation processes in GaAs-GaAlAs multiple quantum well structures

By using of the nonlinear luminescence correlation technique, a new time resolved optical spectroscopy technique has been developed and applied to investigating of hot carrier relaxation processes in GaAs-GaAlAs multiple quantum well structures. It has been found that the well width has a significant effect on the relaxation processes. For a sample with L z =40?, the time constant of the LO-phonon relaxation was found to be as long as 40ps. The physical me-chanism of this weakened electron-phonon interaction is also discussed.Abstract Based on the nonlinear luminescence correlation technique, a new time resolved spectroscopy technique has been developed and applied to the study of hot carrier relaxation processes in GaAs-GaAlAs multiple quantum well structures. We have found that the well width has a significant effect on the relaxation processes. For a sample with L Z =40 A, the time constant of the LO-phonon relaxation is found to be as long as 42 ps.