EFFECT OF SPATIALLY VARYING POTENTIAL ON PHOTON-ASSISTED TUNNELING INQUANTUM DOTS

A formula for photon-assisted tunnelling (PAT) in semiconductor quantu m dots is derived for a case of low transmission rates through tunnell ing barriers, by using the generalized perturbation method developed b y Keldysh. We describe a new process giving rise to a de current even in the absence of bias voltage in which the dot is excited uniformly b y an ac signal including microwaves, and both the occupied and the uno ccupied quantized states participate in the electrical conduction. In addition, the de current flows through the left and the right barriers via different levels. An electron tunnels into the dot from the barri er in which the higher energy level pumped by absorbing a spatially un iform ac signal has a higher transfer rate and another one occupied in the lower level tunnels out through the barrier in which the lower en ergy level has a higher transfer rate. This is in marked contrast to a de current without bias voltage, which has been explained in terms of the process making use of a single quantized state in an asymmetrical ly excited quantum dot. In order to open a new channel, it is necessar y to prepare a particular level configuration, which is realized, for instance, by spatially varying the potential.