The observation of persistent photoconductivity in N-doped p-type ZnSe/GaAs heterojunctions

We report on the persistent photocurrent (PPC) in p-type nitrogen-doped ZnS e epilayers grown by molecular beam epitaxy on GaAs. Its time-evolution sca le ranges from several minutes to hours. The PPC is observed with a non-exp onential decay up to room temperature in some samples. A typical decay is c omposed of an initial transient, which is well described by a stretched exp onential, and a subsequent slower transient. The time constant of the first transient has a thermal activation energy of about 0.35 eV. Annealing samp les at 150 degrees C decreases both the magnitude of the initial transient of the PPC and the height of a deep-level transient spectroscopy (DLTS) sig nal from the interface states between ZnSe and GaAs, suggesting that there is a correlation between the persistent photocurrent and the interface stat es. Also the dependence of the PPC on the wavelength of the illumination su ggests that the initial transient originates from the hetero-interface. Fro m current-temperature measurements, we estimate the barrier at the heteroju nction to be 0.8 eV. This large value indicates that holes are trapped in a two-dimensional quantum well at the heterojunction to the GaAs substrate. We conclude that the PPC has two origins; namely the presence of metastable centres in the ZnSe layer, close to the hetero-interface, which are simila r to DX centres in GaAlAs, and tunnelling of trapped holes from the two-dim ensional quantum well through the barrier.