MICROSCOPIC BALANCE-EQUATIONS FOR THE REAL-SPACE TRANSFER IN MULTIPLE-QUANTUM WELLS

The parallel electronic transport in semiconductor multiple quantum we lls and the associated real-space transfer of electrons from high-mobi lity quantum wells into low-mobility barriers is treated on the basis of the microscopic Lei-Ting balance-equation theory. Our model system consists of a quasi-two-dimensional subband that is bound to the wells and a quasi-three-dimensional band for the extended states above the barriers. The real-space transfer between the two subbands is microsco pically described as intersubband scattering by phonons. In order to c apture the appropriate selection rules for these transitions, it is ne cessary to choose a set of orthogonal subband wavefunctions. A solutio n of the balance equations is presented for a GaAs-AlxGa1-xAs system i n which the real-space transfer causes negative differential resistanc e. Our approach shows how the transport properties of the system are i nterrelated with the different transfer scattering processes, and how system parameters influence the real-space transfer. Furthermore, it i s shown that the incorporation of screening requires a careful selecti on of the screening model, as both intersubband contributions and dyna mical effects are found to modify the results in the random-phase appr oximation.