Transient decay of photoinduced current in semiconductors and heterostructures

Abstract

Defects that exhibit some sort of lattice relaxation usually present an energy barrier for electron capture, and the possibility of developing the phenomenon known as persistent photoconductivity (PPC). In this effect, carriers induced in a metastable way remain in a conductive state forever, if the temperature is low enough to avoid the thermally excited retrapping of carriers by large lattice relaxation defects. Although this hypothesis is usually accepted to explain the origin of PPC, there are other principles such as the separation of excited carriers by random local-potential fluctuations. In this paper, we list many sorts of materials that exhibit the PPC effect and the attempts of several researchers to model the transient decay of PPC, seeking parameters in order to understand and model the electrical behavior of these materials. Besides, the PPC effect can be seen as the resulting electrical conduction obtained after removing the electromagnetic irradiation, and it is appealing for applications in a wide range of optoelectronic devices. A personal approach to modeling PPC decay is presented and applied to single crystalline semiconductors, nanocrystalline materials and heterostructures.

X-ray absorption spectroscopy and Eu3+-emission characteristics in GaAs/SnO2 heterostructure

“…In the case of InGaN LED excitation, the continuously decreasing permanent portion of conductivity means that only the intrabandgap defects are excited and the electrons are raised to a lower energy state, remaining in the heterostructure SnO 2 side. These electrons do not have enough thermal energy to overcome the interface potential barrier [25,26]. LED excitation (energy of 2.76 eV) is in good agreement with the PL data which show a broad band originated from a transition between Eu 3+ acceptors and oxygen vacancy donors [20].…”

“…The observed photo-induced current decay for the heterostructure GaAs/SnO 2 :2%Eu, with the described excitations, leads to temperature-dependent magnitude and is slower as the temperature is increased [25], which, although similar to Sb-doped SnO a surprising result, because the trapping by defects is a thermally activated process, and for higher temperature, the decay should be faster as observed for Er and Eu-doped SnO 2 [57,59]; besides, the shape of the decay curve shows dependence with light source, mainly at lower temperature [25,26]. Figure 7a is a diagram of the heterostructure sample structure along with the electric polarization scheme during the photo-induced conductivity decay measurement.…”

“…These excitations followed by the respective current decay have been explored for investigating the carrier trapping by defects, giving birth to a recent paper on a modeling where the heterostructure interface trapping and the Eu 3+ agglomerates play fundamental roles [25]. The decay of persistent photoconductivity in semiconductors and heterostructures has been reviewed recently [26] and is successfully applied for defects subject to some local lattice relaxation [55][56][57]. The relevance of its application is to electrically characterize these heterostructure samples in order to understand the electro-optical phenomena in this sort of semiconductor assembly and then contribute for optoelectronic devices operation.…”

“…The energy of the used laser is above the SnO 2 bandgap energy, and then, more excited carriers are expected, including electron-hole pair generation. Electrons may be excited to overcome the potential barrier at the heterostructure interface, becoming located at the GaAs side, which may return very slowly to the equilibrium state [25,26]. Then, at low temperature, the PPC is less evidenced for the excitation with the laser, since a significant part of the excited electrons does not take part in the conduction process, because they are trapped at the GaAs/SnO 2 interface.…”

“…It leads to regions where the Eu 3+ emission is more easily accomplished. Moreover, photoinduced current decay data for different temperatures, excited with below and above SnO 2 -bandgap light energies, reveal the possible confinement of electrons at the heterostructure interface region and that the persistent photoconductivity (PPC) phenomenon comes from intrabandgap states in the SnO 2 top layer [25,26], which presents typically local lattice relaxation.…”