Johnson Lee

Stark effect in the optical absorption in quantum wires

We have calculated the effect of a transverse electric field on the ground and the first few excited states of the electrons confined in a quantum wire and used these results to examine the effect of the electric field on the intersubband optical absorption in such quantum wires. The electric field removes the degeneracies between several of the excited states in the wire, which leads to peaks in the intersubband optical absorption of the wire. The application of the electric field leads to a Stark shift of the electron energies, which is quadratic in the field at low fields but becomes almost linear in the field at high fields. The electric field shifts the peaks in a manner which depends upon the polarization of the optical field with respect to the applied electric field. We have also investigated the range of wire radii and electric fields at which the infinite well model is valid in a material where the height of the confining potential barrier is finite. We have found that for small wire radii and s...

Temperature dependence of photoreflectance in InAs/GaAs quantum dots

Temperature dependent photoreflectance (PR) and photoluminescence experiments of the InAs/GaAs quantum dot (QD) structures were performed. At 20 K, effective band-gap transitions due to the InAs QDs, wetting layers, and GaAs buffer and cap layers were identified. Transition energies of the ground state and four excited states with nearly equal interlevel spacings (75–80 meV) were observed. The linewidth of the ground-state transition decreased as the temperature increased from 20 K to 100 K while the linewidth became broader at temperatures above 100 K. Energy features of the PR spectra originating from QDs and relating to the in-plane parabolic potentials were discussed.

Optical properties of self-assembled ZnTe quantum dots grown by molecular-beam epitaxy

The morphology and the size-dependent photoluminescence (PL) spectra of the type-II ZnTe quantum dots (QDs) grown in a ZnSe matrix were obtained. The coverage of ZnTe varied from 2.5 to 3.5 monolayers (MLs). The PL peak energy decreased as the dot size increased. Excitation power and temperature-dependent PL spectra are used to characterize the optical properties of the ZnTe quantum dots. For 2.5- and 3.0-ML samples, the PL peak energy decreased monotonically as the temperature increased. However, for the 3.5-ML sample, the PL peak energy was initially blueshifted and then redshifted as the temperature increased above 40K. Carrier thermalization and carrier transfer between QDs are used to explain the experimental data. A model of temperature-dependent linewidth broadening is employed to fit the high-temperature data. The activation energy, which was found by the simple PL intensity quenching model, of the 2.5, 3.0, and 3.5 MLs were determined to be 6.35, 9.40, and 18.87meV, respectively.

The impedance of the near ballistic electron transport device in n‐GaAs

The ac impedance of a two‐terminal near‐ballistic electron device is calculated by solving the equation of motion, Poisson’s equation, and the equation for current density. The influences of diffusion and collision drag are discussed. Our calculations show that this particular device has an equivalent circuit characterized by an antiresonance whose frequency shifts with increasing collision effect away from the plasma frequency towards lower values, and that diffusion and collision effects reduce substantially the magnitude and phase of the impedance. Even so, the impedance appears to be measurable.

Ballistic transport in a nonparabolic band structure

The orientation dependence of ballistic electron transport in GaAs at room temperature is investigated using the Kane band structure model, which is consistent with the pseudopotential model for values of wave vector k<1.5×109 m−1. It is shown that the variation of the effective mass with group velocity plays a very important role in the conservation of energy relation. The field distribution and the I‐V characteristic of a two‐terminal ballistic transport GaAs device are determined by solving Poisson’s equation for parabolic and nonparabolic band structures. For comparison, the I‐V characteristic of InSb is included also.

Quantum confinement of Te bound exciton in ZnSe/ZnSe1−xTex and ZnSe/Zn1−yCdySe quantum wells

Abstract The quantum confinements of an exciton bound to a single Te (X/Te) and to many Te (X/Te n ) were observed for the first time in ZnSe/Zn 1− y Cd y Se quantum wells with a thin ZnSe 1− x Te x (very low x 1− y Cd y Se quantum well. The binding energies of X/Te for the 5.0, 2.5, and 1.4xa0nm wells were 69, 81, and 83xa0meV, respectively. In the case of ZnSe/ZnSe 1− x Te x quantum well structures, the quantum confinement of a deep energy level, Te cluster bound exciton, was investigated. Current results imply a tiny wave-function size for the X/Te, X/Te n , and exciton bound to a Te cluster.

Analysis of heavily tailed size distributions of ZnTe/ZnSe quantum dot structures by using the bootstrap methodology

We have used the bootstrap methodology to analyze dot size distributions of ZnTe quantum dot (QD) structures. The photoluminescence (PL) spectrum indicates that the ZnTe QD structure belongs to a type-II band alignment. The broadness with small fluctuations in the PL represents the spatial inhomogeneity of the QD sizes. The Schrodinger equation together with the first-order perturbation correction was numerically solved to correlate the dot size and the photon energy. Using the bootstrap “loess” curve fitting method, the PL spectrum was determined to be a normal distribution with a high significance level of 46% tested by a null hypothesis H0. By examining the slope of the complementary cumulative distribution function, we found that the size distribution is heavy tailed.