Long-lifetime photoconductivity effect in n-type GaAlAs

A long-lifetime (τ∼hours, T≲60 °K) photoconductivity effect is observed in Te-doped Ga1−xAlxAs (0.25≲x≲0.7). Analysis of Hall-effect data showing a pronounced decrease in the electron mobility upon photoexcitation shows that a donor level is involved. Similar effects are observed in Se- and Sn-doped Ga1−xAlxAs (x=0.3). The magnitude of the effect which is typically of the order of the room-temperature electron concentration seems to correlate linearly with the concentration of Te, showing that a constant concentration background impurity is not responsible for this effect. A large lattice relaxation is indicated by the large difference between the thermal (0.12 eV) and optical (1.1 eV) ionization energy of the donor level. The potential barrier to electron capture by the donor level is estimated to be 180 meV (x=0.36) from time decay measurements of the photoexcited electron population at low temperatures. Extrapolation to room temperature gives a characteristic decay time of ∼0.5 nsec for the electron concentrations expected in injection lasers.