Radiative states in type-II GaSb/GaAs quantum wells

We have studied optical properties of staggered band line-up (type-II) heterostructures based on strained GaSb sheets in a GaAs matrix. The giant valence-band offset characteristic to this heterojunction leads to an effective localization of holes in ultrathin GaSb layers. An intense photoluminescence (PL) line caused by radiative recombination of localized holes with electrons located in the nearby GaAs regions is observed. The separation of nonequilibrium electrons and holes in real space results in a dipole layer and, thus, in the formation of quantum wells for electrons in the vicinity of the GaSb layer. The luminescence maximum shifts towards higher photon energies with rising excitation density reflecting the increase in the electron quantization energy. A bimolecular recombination mechanism is revealed in PL and in time-resolved PL studies. In the case of pseudomorphic monolayer-thick GaSb layers, the radiative exciton ground state does not exist. Accordingly, small absorption coefficients and a featureless behavior of the band-to-band calorimetric absoprtion spectrum are found in the vicinity of ${\mathit{k}}_{\mathit{x},}$y=0. Remarkable enhancement of the absorption coefficient with a characteristic onset is observed for heavy holes with ${\mathit{k}}_{\mathit{x},}$y>0. Radiative states in the continuum of heavy-hole subbands are revealed also in temperature-dependent PL studies. The experimentally measured onset energies point out the importance of GaSb heavy- and light-hole mixing effects. We demonstrate intense luminescence from staggered band line-up GaSb-GaAs heterostructures up to room temperature.