Photoluminescence due to positively charged excitons in undoped GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As quantum wells

We study the photoluminescence (PL) spectra of nominally undoped GaAs/${\mathrm{Al}}_{0.33}$ ${\mathrm{Ga}}_{0.67}$As quantum wells, as a function of well width, temperature, excitation energy, and intensity, and an applied magnetic field. A doublet is observed for temperatures below 10 K, whose components we demonstrate derive from the neutral (X) and positively charged (${\mathit{X}}^{+}$) excitons. The latter appears due to the binding of excitons to holes in the quantum well originating from the background concentration of acceptors in the ${\mathrm{Al}}_{0.33}$ ${\mathrm{Ga}}_{0.67}$As barriers. Our assignment of ${\mathit{X}}^{+}$ is motivated by the striking similarity of the PL spectra to those recorded on quantum wells with acceptors deliberately incorporated in the barriers. Consistent with our assignment, we see a strengthening of the ${\mathit{X}}^{+}$ transition when more holes are introduced into the well by photoexcitation. The dependence of the doublet splitting on well width is in close agreement with a Monte Carlo calculation of the second hole binding energy in ${\mathit{X}}^{+}$. The PL peak due to ${\mathit{X}}^{+}$ may have been falsely ascribed in previous work to a biexciton. © 1996 The American Physical Society.