D. Seghier et Hp. Gislason, The observation of persistent photoconductivity in N-doped p-type ZnSe/GaAs heterojunctions, J PHYS D, 32(4), 1999, pp. 369-373
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.