Observation of persistent photoconductivity in n-doped p-type ZnSe/GaAs heterojunctions

We report on the persistent photocurrent (PPC) in p-type nitrogen-doped ZnSe epilayers grown by molecular beam epitaxy on GaAs. Its time-evolution scale ranges from several minutes to hours. The PPC is observed with a non-exponential decay up to room temperature in some samples. A typical decay is composed of an initial transient, which is well described by a stretched exponential, and a subsequent slower transient. The time constant of the first transient has a thermal activation energy of about 0.35 eV. Annealing samples at 150°C decreases both the magnitude of the initial transient of the PPC and the height of a deep-level transient spectroscopy (DLTS) signal from the interface states between ZnSe and GaAs, suggesting that there is a correlation between the persistent photocurrent and the interface states. Also the dependence of the PPC on the wavelength of the illumination suggests that the initial transient originates from the hetero-interface. From current-temperature measurements, we estimate the barrier at the heterojunction 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 similar to DX centres in GaAlAs, and tunnelling of trapped holes from the two-dimensional quantum well through the barrier.