Excited states of the light- and heavy-hole free excitons observed in photoreflectance

Abstract

Excitonic transitions are observed at 10 K in a 225-Å ${\mathrm{Al}}_{0.35}$ ${\mathrm{Ga}}_{0.65}$As-GaAs multiple-quantum-well structure in photoreflectance (PR) spectra obtained using a novel double monochromator spectrometer. With an instrumental resolution of 0.06 meV, complete resolution of excitonic line shapes in PR is achieved. The $1s$ and $2s$ states of the lowest-lying heavy- and light-hole free excitons ($1H$ and $1L$) are observed as well as the ground states of two forbidden excitons ($12H$ and $13H$). The PR data are analyzed using an effective-mass theory based on a modulation of the built-in electric field as the dominant modulation mechanism. The measured values of the difference in energy between the $1s$ and $2s$ states of both the heavy-hole free exciton and light-hole free excitons agree with their calculated values. No evidence is observed in PR for contributions of the two-dimensional band-to-band continuum (quantum confined Franz-Keldysh effect). The identification of excitonic features is further supported by comparison with corresponding photoluminescence excitation measurements made on the same sample.