Luminescence studies of optically pumped quantum wells in GaAs-AlxGa1−xAsmultilayer structures

Abstract

Various properties derivable from the photoluminescence of a multilayer structure consisting of many thin GaAs layers (wells) separated by thin ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ layers (barriers) have been determined as a function of temperature from 7 K to greater than 150 K. These measurements, which were made utilizing both fixed-and tunable-frequency laser sources, include the following: the circular photoluminescence polarization $\ensuremath{\rho}$ generated by circularly polarized optical excitation, the magnetic field dependence of $\ensuremath{\rho}$ in the Voigt geometry (Hanle effect) to determine the electron lifetime $\ensuremath{\tau}$ and spin-relaxation time ${\ensuremath{\tau}}_{s}$, the excitation spectrum for the peak of the photoluminescence which shows thirteen exciton quantum transitions characteristic of multilayer samples, the photoluminescence spectra which exhibit five transitions in emission, the excitation intensity ${I}_{p}$ dependence of the integrated photoluminescence intensity, and $\ensuremath{\tau}$ and ${\ensuremath{\tau}}_{s}$ as a function of ${I}_{p}$. The temperature dependence of ${\ensuremath{\tau}}_{s}$ suggests that there is a center with a binding energy of about 5 meV that relaxes the electron spins at low temperatures. In the high-temperature region, $T\ensuremath{\gtrsim}100$ K, ${\ensuremath{\tau}}_{s}$ is close to that observed by others for lightly doped $p$-type bulk GaAs and is close to estimates of ${\ensuremath{\tau}}_{s}$ for the D'yakonov-Perel' spin-relaxation mechanism. The absence of optical alignment with linearly polarized pumping and the characteristics of the magnetic field dependence of $\ensuremath{\rho}$ both argue against significant hole polarization. The linear dependence of the integrated photoluminescence intensity on ${I}_{p}$ leads to a simple model for the radiative lifetime whose temperature dependence is found to be consistent with expectations. Also, these data are used to estimate the nonradiative lifetime ${\ensuremath{\tau}}_{n}=5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}$ sec which varies little with $T$ and the radiative lifetime ${\ensuremath{\tau}}_{r}(T)=3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$ sec at 7 K, which varies $\ensuremath{\sim}{T}^{\frac{3}{2}}$.