By using a modified Lee-Low-Pines variational method, we have investigated the groundstate binding energy of a polaron confined in asymmetric single and step quantum wells (QWs) due to interface phonons, confined bulk-like LO phonons, and half-space LO phonons. The relative importance of the different phonon modes is analysed in detail. Our results show that the asymmetry and the well width of the QWs have a significant influence on the polaron energy. The polaron binding energy has an intimate relation to the potential parameters of QWs. The subband nonparabolicity has a little influence to the polaron binding energy. Comparing with the results calculated with perturbation theory, a good agreement is found.
1996 Academic Press Limited
1. Introduction
Recently, some asymmetric polar semiconductor heterostructures, such as asymmetric single and step quantum wells (QWs), have led to widespread interest in some special device applications [1][2][3][4][5][6][7]. The optical-phonon modes, the electron-phonon interaction and scattering in asymmetric single and step QWs have been investigated in Refs [2-6], respectively. Some surprising results, such as the frequency-forbidden behavior of the interface phonons and the anomalous phenomenon of the electron-phonon interaction in asymmetric QWs, have been found. However, to date there has been little theoretical work on the polaron effects in asymmetric QWs which are of great practical importance at present. Quite recently, we have investigated the polaron energy and effective mass in asymmetric single QWs using the second-order perturbation theory [8]. The purpose of this paper is to further study the polaron binding energy in asymmetric single and step QWs using the modified Lee-Low-Pines (LLP) variational method [9], in which the confined bulk-like LO, the half-space LO and the interface optical phonon modes are all included in our calculations. The conduction band nonparabolicity and the finite barrier height effect are also considered. Comparing with our recent results calculated with perturbation theory [8], a good agreement is found.
This paper is organized as follows. The theory of the polaron binding energy is given in Section 2. In Section 3, numerical illustrations and discussion are shown. A summary is given in Section 4.