Electronic transport in porous silicon of low porosity made on a p+ substrate

To analyze the carriers transport in metal–porous silicon (PS) junctions, we studied the current–voltage and impedance characteristics of a series of junctions, having a typical porosity of 30% and a layer thickness ranging from 1.5 to 30 μm. PS conductivity as a function of the annealing temperature showed two characteristic regions at 150 and 550°C where the conductivity abruptly increased by several orders of magnitude. These temperatures coincide with the temperature of dissociation of Si–H–B complexes and of hydrogen effusion. After short time rinsing of high temperature annealed PS in hydrofluoric acid its resistivity returned to the initial high value it had in the as-prepared state. These results imply that hydrogen plays a key role in determining the conduction properties of PS. We argue that the hydrogen present in PS in high concentration effectively passivates the boron doping atoms. As a result the space charge region that compensates surface charges significantly widens and becomes essentially larger than the silicon wires which leads to the high resistivity of PS. Current–voltage dependencies exhibit a region of space charge limited current, which allows for the determination of the energy distribution density of states in PS.