Type-I–type-II transition in ultra-short-period GaAs/AlAs superlattices

We report the direct observation of the transition from type I to type II in (GaAs${)}_{\mathrm{m}}$/(AlAs${)}_{\mathrm{n}}$ ultra-short-period superlattices with m=n by means of photoacoustic spectroscopy and high-excitation-intensity photoluminescence. The transition is found to occur when the constituent superlattice layers have a critical thickness of about 12 monolayers (m=n=12). Different temperature dependences of the type-I and type-II recombinations are observed, which are tentatively ascribed to the thermal transfer of the carriers between quantized subbands, in different critical points of the superlattice.