Extremely fast ambipolar diffusion in n-i-p-i doping superlattices investigated by an all-optical pump-and-probe technique

We report on investigations of the ambipolar diffusion process in n-i-p dio des and n-i-p-i doping superlattices performed by a new all-optical pump-an d-probe technique. This new technique allows not only the determination of the ambipolar diffusion coefficient but also the spatially resolved investi gation of the stationary distribution of the optically-induced excess carri ers, The n-i-p-i doping superlattice exhibited an extremely large ambipolar diffusion coefficient in the range of 10(4) cm(2) s(-1). The ambipolar dif fusion coefficient of the n-i-p and n-i-p-i structure was demonstrated not to be a constant but a function of the charge carrier density. This strongl y affects the spatial distribution of the excess carriers especially in the large signal regime (density of photo-generated carriers much larger than dark carrier concentration). The spatial distribution of the carriers in th e small as well as in the large signal case can be understood theoretically if the real dependence of the ambipolar diffusion coefficient on the carri er density is taken into account. (C) 1999 Academic Press.