NONTHERMAL OCCUPATION OF GAMMA AND CHI-STATES IN GAAS/ALAS SUPERLATTICES

We have studied the photoluminescence (PL) emission from a GaAs/AlAs asymmetric-double-well superlattice with indirect band gap as a function of temperature and applied electric field. The PL spectrum consists of two peaks, with a separation of only 21 meV, originating from the recombination of wide-well heavy holes with either X electrons of the barrier or GAMMA electrons of the wide well, respectively. The PL intensity ratio of the two peaks shows a strong nonthermal population between GAMMA and X states near flatband, because in this sample the transfer from GAMMA to x after photoexcitation can be induced only by absorption of phonons with a large momentum. The small GAMMA-x separation and the long recombination time of X electrons results in the filling of the X states that are at the same energy as the GAMMA minimum, blocking GAMMA-x transfer by phonon emission or interface (roughness) scattering.