Transient and alternating current conductivity of nanocrystalline porous alumina thin films on silicon, with embedded silicon nanocrystals

Structural characterization and surface topography of porous alumina thin films on silicon with embedded silicon nanocrystals were performed using scanning and transmission electron microscopy. The nature of porous alumina thin films is nanocrystalline with a high density of uniformly distributed silicon nanocrystals. The pores were randomly distributed with an average size of 35 nm. ac impedance spectroscopy measurements were performed at room temperature, from 0.05 up to 3.0 V in the range of $1–105 Hz$ for both porous alumina thin films with and without embedded silicon nanocrystals. Transient current measurements were also performed from 0.5 up to 50.0 V in the time interval 1–100 s both in forward and reverse bias conditions. The electrical conduction is dominated by the porous alumina matrix and there is no evidence of participation of the contacts to the electrical properties of the thin films. ac conductivity results follow the dielectric universal response through the whole frequency range of investigation. The real part of the specific electrical conductivity σ^′ is voltage independent, in the samples studied, implying the presence of a conduction mechanism. The analysis of the experimental data reveals that the conductivity is governed by two different conduction mechanisms regardless of bias conditions, forward or reverse. In the low applied voltage region the conduction is due to thermally excited electrons, hopping from one state to another. This conduction mechanism is ohmic. For higher voltages the electrical conduction is space charge limited.