Absolute transmission, reflection, and absorption studies in GaAs/AlAs quantum wells containing a photoexcited electron gas

Abstract

A study is presented of the absolute reflection, transmission, and absorption of several undoped mixed type-I--type-II GaAs/AlAs multiple quantum wells at T=2 K. These structures consist of alternating GaAs narrow and wide wells separated by AlAs barriers. They are designed so that a two-dimensional electron gas (2DEG) is photogenerated in the wide wells. The density of this gas is varied in the estimated range of 0\ensuremath{\le}${\mathit{n}}_{\mathit{e}}$\ensuremath{\le}3\ifmmode\times\else\texttimes\fi{}${10}^{11}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$ by varying the photoexcitation intensity in the range of 0\ensuremath{\le}${\mathit{I}}_{\mathit{L}}$\ensuremath{\le}100 mW/${\mathrm{cm}}^{2}$. In the presence of a 2DEG the (e1:hh1)1S and (e1:lh1)1S excitonic transitions broaden and weaken with increasing ${\mathit{n}}_{\mathit{e}}$, but their energy is virtually unchanged. The reflection and transmission spectra are analyzed by assuming a Lorentzian oscillator response function for the excitonic transitions and a broadened step function for the free-carrier (e1-hh1) and (e1-lh1) bands. Fitting the calculated optical spectra to the experimental ones (both line shape and absolute intensity) yields the dependence of the following parameters on ${\mathit{n}}_{\mathit{e}}$: exciton energies, exciton-photon interaction strength, exciton damping, and the onset energy of the free e-h interband transitions. It is found that the interaction strength of the (e1:hh1)1S and (e1:lh1)1S excitons with photons decreases and their damping increases with increasing ${\mathit{n}}_{\mathit{e}}$. \textcopyright{} 1996 The American Physical Society.