A Simple Explanation and Conditions for Persistent Photoconductivity in n-Doped Al sub x Ga sub (1-x) AS.

New theoretical and experimental data will be presented to show that the persistent photoconductivity (PPC) in n-doped Al sub x Ga sub (1-x) As is related to the conduction band structure of Al sub x Ga sub (1-x) As and the position of the deep donor states attached to the L and X minima. The PPC is sensitive to both x and temperature, and is observed when at 300K the free carriers are shared between GAMMA, L and X valleys, and at the temperature of measurement (T) all the photoexcited carriers are transferred to the GAMMA valley, i.e.0 < f sub gamma (x, 300) < 1 and f sub gamma (x, T) ~ 1, where f sub gamma is the fraction of electrons in the GAMMA valley. Under this condition, Shubnikov-de Haas measurements at 4.2K indeed confirmed the presence of all photoexcited electrons in the Gamma valley. The Hall curves were linear against magnetic fields up to 12 Tesla which also indicated that after photoexcitation one type of carriers alone contributed the conduction process with no involvement of holes in the valence band or electrons in the L or X valleys. Significantly increased electron mobilities after photoexcitation also indicate the free carriers to be in the GAMMA valley alone.