TUNNELING ESCAPE TIME OF ELECTRONS FROM THE QUASI-BOUND STARK LOCALIZED STATES IN ULTRATHIN BARRIER GAAS ALAS SUPERLATTICES/

Band-edge optical absorption spectra in two series of monoperiodic GaA s/AlAs superlattice diodes are investigated as a function of bias volt age by low-temperature photocurrent spectroscopy. In the superlattices the well width is fixed at 11 or 22 monolayers (ML) and only the ultr athin barrier thickness is varied between 2 and 6 ML. In the limit of high electric field conditions where the fundamental Stark ladder tran sition dominates, the linewidth of the heavy-hole excitonic peak is fo und to increase systematically with decreasing the barrier thickness. That is, the Linewidth for the superlattice with thinner barriers is b roader than that with the thick barrier superlattice, which is basical ly determined by the usual Inhomogeneous broadening. This enhancement of the spectral linewidth by more than 10 meV is due to a lifetime bro adening of the quasibound Stark localized states. This resonance energ y broadening due to the rapid tunneling escape of electrons is used to determine the tunneling time by the uncertainty principle. The tunnel ing escape time thus determined as a function of barrier thickness sho ws good agreement with values deduced from the confined electron level broadening for the biased double-barrier quantum-well structures base d on simple transfer matrix calculations.