Showing papers by "Fanyao Qu published in 2001"

The pH dependence of the surface charge density in oxide-based semiconductor nanoparticles immersed in aqueous solution

Abstract: A new approach to calculate the surface charge density in semiconductor metal-oxide quantum dots (QDs) dispersed in alkaline aqueous colloids, at different pH values, is presented. Surface charge density up to about 0.3 C/m/sup 2/ was obtained for spherical nanoparticles in the diameter range of 8 to 14 nm, at pH 10 and 12. The calculated surface charge density, in very good agreement with the average value reported in the literature, is obtained as long as a proton transfer mechanism through the semiconductor-electrolyte interface takes place. The data reported here explain the stability found in ionic magnetic fluids at the usual lower/higher pH values and particle sizes.

Observation of the Fermi-edge singularity in n-doped single asymmetric quantum wells: the influence of residual acceptors

Abstract: The investigation of the evolution of the photoluminescence spectra, in single asymmetric quantum wells (SAQWs), from a typical emission spectrum to a Fermi-edge singularity, is carried out as a function of both the optical excitation intensity and the temperature. The three samples used here are n-doped, low carrier density (below 5×10 11 cm −2 ), GaAs/Al0.35Ga0.65As SAQWs grown by molecular beam epitaxy. The strong collective recombination of electrons with different k states up to the Fermi wave vector as well as the optical signature of the Fermi-edge singularity is observed in two samples containing residual acceptors inside the GaAs SAQW. In contrast, a third sample containing no experimental evidence of residual acceptors in the GaAs SAQW shows no optical signature of the Fermi-edge singularity.

The breakdown of the temperature dependence of the photoluminescence peak position in single asymmetric quantum wells

Abstract: Photoluminescence measurements performed in a wide range of temperature (10-100 K) and optical excitation intensity (0.03-90 W/cm2), and self-consistent numerical calculation is used to investigate n-doped GaAs/Al x Ga 1 − x As single asymmetric quantum wells. Under strong optical excitation intensity, a red-shift in the recombination energy with increasing temperature is observed. In the weak optical excitation condition, however, a breakdown of the recombination energy shift is found. The flat response was attributed to the combined effects of the temperature dependence of the bandgap energy, band bending, and bandgap renormalization.

New approach for tunable quantum well wire lasers

Abstract: A new approach for operation of tunable lasers based on quantum well wires is proposed. The laser operation uses the effect of an intense, long-wavelength laser field radiation applied to the semiconductor device. Different geometry's concerning the shape and size of the semiconductor quantum structure as well as the orientation of the applied laser field with respect to the quantum device was considered. We calculate the laser-dressed quantum well wire (QWW) potential energy, in the frame of the nonperturbative theory and finite difference method. Then, we show that when the intense, long-wavelength laser field radiation, is applied to the semiconductor-based quantum well wire device a significant optical Stark effect is observed for the bound state energy. Furthermore, under the action of the laser-dressed potential, a strong enhancement of the blue shift occurs for the electron-heavy hole recombination processes as the QWW lateral size is reduced. This effect may provide full control of the frequency operation of QWW-based lasers and thus would be of great help in tailoring the physical parameters of the semiconductor QWW device.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Anomalous shift of the recombination energy in single asymmetric quantum wells

Abstract: Self-consistent numerical calculation and photoluminescence (PL) measurements have been used to investigate the temperature dependence of the optical Stark effect in n-doped GaAs/AlGaAs single asymmetry quantum wells (SAQWs), grown by molecular beam epitaxy. In the low-temperature regime (5 to 40 K) a remarkable blue shift (9.8 meV) is observed in the PL peak energy, as the optical excitation intensity increases from 0.03 to 90 W/cm2. The blue shift is well explained by the reduction of the two-dimensional electron gas (2DEG) density, due to the charge-transfer mechanism. At about 80 K, however, an anomalous behavior of the PL peak energy was found, i.e. a red shift has been observed as the optical excitation intensity increases. This anomalous behavior has been explained by combining the effects of band gap renormalization, band bending, temperature dependence of the band gap, temperature dependence of the 2DEG density, and temperature dependence of the fundamental energy position.