A Theoretical Study of the Terahertz Dynamics of Magnetoexcitons in GaAs-(Ga,Al)As Quantum Wells

Abstract

Recent experimental measurements obtained via a highly sensitive optically detected resonance technique have shown that magnetoexcitons in GaAs-(Ga,Al)As semiconductor quantum wells (QWs) have discrete internal energy levels, with transition energies found in the far-infrared (terahertz) region. This work presents a theoretical study of some internal exciton transitions of light-and heavyhole confined magnetoexcitons in GaAs-(Ga,Al)As QWs under magnetic fields applied in the growth direction of the semiconductor heterostructure. We use a variational procedure in the effective-mass approximation, and a parabolic dispersion for electrons, and assume the spin-orbit splitting to be large enough so that the interaction between J ¼ 3/2 and J ¼ 1/2 hole states may be disregarded. The 1s ! 2s and 1s ! 2p þ theoretical transitions are in good agreement with optically detected resonance measurements for far-infrared intraexcitonic transition energies in GaAs-(Ga,Al)As QWs.Introduction The study of magneto-optical properties of type I and type II semiconductor superlattices by means of absorption, photoluminescence, magnetoreflectance, and other techniques [1-8] has attracted considerable interest. More specifically, recent optically detected resonance (ODR) experimental results have shown that excitons in GaAs-(Ga,Al)As quantum wells (QWs) have discrete internal energy levels, and transition energies are found in the far-infrared (FIR) region. In particular, Cerne et al. [5] investigated the terahertz (THz) dynamics of magnetoexcitons in GaAs-(Ga,Al)As undoped multiple quantum wells (MQWs) under magnetic fields applied perpendicular to the well interfaces, and observed resonant FIR absorption by the confined magnetoexcitons. The dominant resonance in GaAs-Ga 0.70 Al 0.30 As MQWs (with well and barrier widths of 100 and 150 A, respectively) was assigned to the 1s ! 2p þ intraexcitonic transition of a heavyhole exciton. The absorption feature was found to persist even when the FIR electric field was comparable to the electric field that binds the exciton. Analogous results were obtained by Nickel et al. [6,7] who performed a detailed ODR experimental study of internal transitions of confined magnetoexcitons in two GaAs-Ga 0.70 Al 0.30 As MQW structures (125 A well/150 A barrier, and 80 A well/150 A barrier), with several resonances assigned to 1s ! 2p þ , 3p þ , and 4p þ internal excitonic transitions.In the study reported here, we work within the effective-mass approximation and follow a variational procedure to investigate some properties of magnetoexcitons in GaAs-(Ga,Al)As QWs. In particular, we are interested in evaluating the internal magnetoexciton transitions in GaAs-Ga 1--x Al x As QWs to compare with the experimental data of Cerne et al. [5] and Nickel et al. [6,7].