Photovoltage and photo-induced charge trapping in porous silicon

Spectral and kinetic dependencies of photovoltaic effects in porous silicon-crystal substrate structures have been studied. Both as-prepared and aged in air samples were used. It is shown for the first time that besides the photovoltage component connected with a depleted region in the silicon substrate at the interface with the porous layer there is a photovoltage due to porous silicon itself. It is established that the electron states with the relaxation time about several minutes are located on the pores surface. The properties of these states changed with the thermal annealing of structures. The ''superslow'' hole traps with a relaxation time of about several hours were registered in the aged samples. These traps are located in the oxide on the silicon skeleton surface of the porous silicon: An energy diagram based on the data is suggested that explains the appearance of photovoltaic effects in the investigated structures.