Ultrafast Carrier Transfer in Quantum-Well- and Hetero-n-i-p-i-Structures

Carrier relaxation in 2D-semiconductor structures is of interest for applications in electrooptical devices and modulators. In many cases the relaxation process involves carrier transfer between neighboring potential wells which is studied in this paper via transient absorption changes. The mechanism for such a carrier transfer, thermionic emission, tunneling, or phonon assisted tunneling is of special interest. In a first part of this paper, carrier transfer from the quantum wells to the doping layers of type-II-hetero-n-i-p-i-structures is discussed. In the second part experimental data taken on modulation doped quantum well structures are presented. The carrier transfer from the first excited subband to potential minima in the barrier and vice versa is studied.