POLARON EFFECTS IN ASYMMETRIC SEMICONDUCTOR QUANTUM-WELL STRUCTURES

In this paper, polaron effects in asymmetric quantum-well structures ( QW's) are investigated by using second-order perturbation theory and t he modified Lee-Low-Pines (LLP) variational method. By applying the Gr een's-function method, explicit analytical expressions for the electro n extended-state wave functions and the density of states in a general step QW's are given. Within the framework of second-order perturbatio n theory, the ground-state polaron binding energy and effective mass i n step and asymmetric single QW's are studied as due to the interface optical phonons, confined bulklike LO and half-space LO phonons. The f ull energy spectrum is included in our calculations. The effects of th e finite electronic confinement potential and the subband nonparabolic ity are also considered. The relative importance of the different phon on modes is analyzed. By means of the modified LLP variational method, the binding energy of a polaron confined to asymmetric single QW's is also investigated. Our results show that in ordinary asymmetric QW's, the asymmetry of the QW's has a significant influence on the polaron effect, which has a close relationship to the interface phonon dispers ion. When the well width and one side barrier height of asymmetric sin gle QW's are fixed and identical with those of symmetric QW's, the pol aron binding energy in asymmetric QW's is always smaller than that in symmetric QW's. We have also found that it is necessary to include the continuum energy spectrum as intermediate states in the perturbation calculations in order to obtain the correct results; the subband nonpa rabolicity has a small influence on the polaron effect. Comparing our results obtained by using two different methods, good agreement is fou nd.