Determination of electron effective mass from optical transition energy in InGaAs/InAlAs quantum wells

Nonparabolic effective mass of conduction subbands in InGaAs/InAlAs quantum wells (QWs), lattice-matched to InP, was quantitatively obtained by analyzing interband-optical transition spectra. Thickness of InGaAs well was 5.3, 9.4, and . Thickness of InAlAs barrier was about , and each QW was independent. Excellent agreement was obtained between experimental mass and theoretical mass predicted by Kane's three-level band theory on bulk InGaAs, in a wide energy range of from the bandedge. Method of experimental analysis on a relation between eigen energy and effective mass was described.     

A Criterion of Nonparabolicity by the Ricci Curvature

A complete manifold is said to be nonparabolic if it does admit a positive Green’s function. To ?nd a sharp geometric criterion for the parabolicity/nonparbolicity is an attractive question inside the function theory on Riemannian manifolds. This paper devotes to proving a criterion for nonparabolicity of a complete manifold weakened by the Ricci curvature. For this purpose, we shall apply the new Laplacian comparison theorem established by the ?rst author to show the existence of a non-constant bounded subharmonic function.     

Many-body effects on intersubband resonances in narrow InAs/AlSb quantum wells

Intersubband polarization couples to collective excitations of the interacting electron gas confined in a semiconductor quantum well (QW) structure. Such excitations include correlated pair excitations (repellons) and intersubband plasmons. The oscillator strength of intersubband resonances (ISBRs) strongly varies with QW parameters and electron density because of this coupling. Using the intersubband semiconductor Bloch equations for a two-conduction-subband model, we show that intersubband absorption spectra for narrow wells are dominated by the Fermi-edge singularity (via coupling to repellons) when the electron gas becomes degenerate and in the presence of large nonparabolicity. Thus the resonance peak position appears at the Fermi edge and the peak is greatly narrowed, enhanced, and red shifted as compared to the free particle result. Our results uncover a new perspective for ISBRs and indicate the necessity of proper many-body theoretical treatment in order for modeling and prediction of ISBR line shape.     

Carrier transport and bandgap shift in n-type degenerate ZnO thin films: The effect of band edge nonparabolicity

Contribution of band edge nonparabolicity to the charge carrier transport in degenerate n-type zinc oxide thin films has been investigated theoretically in order to understand the fundamental aspects of electron scattering in such thin films regardless of precise details of the preparation procedure. To conduct this, the theoretical evaluated results have been compared to the experimental values taken from literatures. The results indicate that the nonparabolicity (introducing through effective mass of charge carriers) has a strong effect on the total mobility of carriers in zinc oxide films so that a satisfactory agreement with experimental data is fulfilled. The dependence of nonparabolicity on bandgap shift is also discussed. Studying the optoelectronic properties of numerous moderately and heavily doped samples revealed that their optical bandgap has lower blueshift than the theoretical value obtained from the well-known Burstein–Moss effect. So, the observed bandgap shift was dependent on the carrier concentration and the total shift of bandgap was evaluated by combining the Burstein–Moss and bandgap narrowing effects. Two different cases were also examined; parabolic and nonparabolic (modified) Burstein–Moss effects. The results show that the modified Burstein–Moss effect leads to great agreement with experimental data.     

Saturation spectroscopy of electronic states in a magnetic field in InAs/AlxGa1−xSb single quantum wells

We have carried out saturation spectroscopy of cyclotron resonance in a semiconducting InAs/Al_0.5Ga_0.5Sb single quantum well using the UCSB free electron laser and have extracted an effective Landau level lifetime using an n-level rate equation model. The effective lifetime shows strong oscillations (>an order of magnitude) with frequency. Minima are shifted to higher frequencies than those given by the simple parabolic magnetophonon resonance condition due to large nonparabolicity in the InAs conduction band. We have also used this technique to investigate the origins of two lines: the X-line and cyclotron resonance in a “semimetallic” InAs/Al_0.1Ga_0.9Sb single quantum-well structure. Results show that the two lines are of different origin.     

An explicit FDM calculation of nonparabolicity effects in energy states of quantum wells

An explicit finite difference method (FDM) to solve the nonparabolic effective mass approximation of Schrodinger wave equation (SWE) for arbitrary quantum wells (QWs) is presented. The explicit nature of the presented method and its sparse matrices allow fast computation for energy states in QWs. The nonparabolicity effects are considered explicitly without iteration. This in turn results in faster and more stable calculations. The method is used to study the nonparabolicity effects in energy states and states overlapping in asymmetric AlGaAs/GaAs QWs.     

Quasibound states in semiconductor quantum well structures

We present a study on quasibound states in multiple quantum well structures using a finite element model (FEM). The FEM is implemented for solving the effective mass Schrödinger equation in arbitrary layered semiconductor nanostructures with an arbitrary applied potential. The model also includes nonparabolicity effects by using an energy dependent effective mass, where the resulting nonlinear eigenvalue problem was solved using an iterative approach. We focus on quasibound/continuum states above the barrier potential and show that such states can be determined using cyclic boundary conditions. This new method enables the determination of both bound and quasibound states simultaneously, making it more efficient than other methods where different boundary conditions have to be used in extracting the relevant states. Furthermore, the new method lifted the problem of quasibound state divergence commonly seen with many other methods of calculation. Hence enabling accurate determination of dipole matrix elements involving both bound and quasibound states. Such calculations are vital in the design of intersubband optoelectronic devices and reveal the interesting properties of quasibound states above the potential barriers.     

Spin susceptibility and effective mass of a 2D electron system in GaAs heterostructures towards the weak interacting regime

We determined the spin susceptibility χ and the effective mass m^* towards the high density limit. Using a tunable GaAs/AlGaAs heterostructure, we can vary the 2D electron density from to . From to our highest densities the mass values fall 10% below the band mass of GaAs. The enhancement of χ decreases monotonically from a factor of 3 to 0.88 with increasing density. It continues to follow a previously observed power law, which leads to an unphysical limit for n→∞. Band structure effects affecting mass and g-factor become appreciable for large n and, when taken into account, lead to the correct limiting behavior of χ. Numerical calculations are in qualitative agreement with our data but differ in detail.