Optical observation of impurity localized states at the edges of Landau subbands in doped quantum wells

We present experimental evidence for the existence of impurity localized electron and hole states in (in-well) p-type doped GaAs/${\mathrm{Al}}_{0.25}$${\mathrm{Ga}}_{0.75}$As multiple quantum wells (MQW’s) under applied magnetic fields (B<14 T) and at a temperature of T=2 K. We study the photoluminescence excitation (PLE) spectra with the monitored energy (${\mathit{E}}_{\mathit{m}}$) varied throughout the inhomogeneously broadened photoluminescence (PL) band. The main effect is a broadening of the Landau PLE transitions as ${\mathit{E}}_{\mathit{m}}$ decreases towards the low-energy side of the excitonic PL band. This effect is explained by an enhanced carrier relaxation rate between quantum states that are spatially localized in the vicinity of impurity centers. Also, the width of the observed transitions is nearly independent of the magnetic field. This confirms the long-range nature of the localizing extrinsic potential, which we suggest is due to compensated (charged) acceptors randomly distributed over the QW plane. We calculate the line shape dependence of the lowest Landau PLE band on ${\mathit{E}}_{\mathit{m}}$ and obtain a good agreement with the experimental spectra. In contrast, for an undoped MQW of similar well width, the width of the Landau PLE transitions is independent of ${\mathit{E}}_{\mathit{m}}$ and shows a √B dependence. This demonstrates the short-range potential carrier scattering nature of the Landau-level broadening. © 1996 The American Physical Society.