Quasisteady space-charge fields in photorefractive multiple quantum wells: Edge effects

An analytical solution has been obtained for the quasi-steady-state space-charge fields recorded in a photoconductive multiple quantum well (MQW) neglecting longitudinal mobility. The results apply to times longer than the transverse but shorter than the longitudinal relaxation time. The finite thickness of the structure has been explicitly taken into account by using a two-dimensional formulation, i.e., including parallel and perpendicular components for the field. The analysis refers to parallel geometry and has been applied to two different physical situations: (a) electron-hole generation from interband transitions and (b) electron generation from suitable donors. This latter case has been summarily discussed for comparison purposes. The longitudinal profiles for the trap densities as well as for the two components of the space-charge field have been determined. They show relevant edge effects that depend inversely on the ratio of dielectric constants in the MQW and buffer layers. The role of some relevant parameters such as grating period and applied field on these effects has been investigated. Finally, the results have been compared with the short-time solution and with previous works using a one-dimensional approach.