Size-controlled percolation pathways for electrical conduction in porous silicon

Abstract

1 . This raises the possibility of a silicon-based optoelectronic technology. The luminescence properties may be understood on the basis of the injection or creation of electrons and holes in the interconnected network of wires which recombine radiatively with high efficiency 1,2 . Elucidating the electron-transport mechanisms has been held back by several difficulties, particularly that of making stable, high-quality contacts to the porous material. Here we report experiments that probe the conduction process using photoemission stimulated by hard-ultraviolet/X-ray synchrotron radiation, obviating the need for good electrical contacts. We find that the conductivity of porous silicon films is temperature-dependent, and that the films become insulating at low temperatures. We suggest that these results may be understood in terms of a percolation process occurring through sites in the porous network in which conductivity is thermally activated, and we postulate that this activation may be the consequence of a Coulomb blockade effect 3,4 in the nanoscale channels of the film. This is consistent with our observation of optical 'unblocking' of conducting pathways. These results imply that the size distribution of the nanowires in the silicon backbone plays a key role in determining the conduction properties, and that porous-silicon light-emitting diodes may use only a small (and the least efficient) fraction of the material. Improvements in electroluminescence efficiency may be possible by taking into account the percolative nature of the conduction process.These studies probe photoelectron emission in the hard-ultraviolet to X-ray (X-UV) region, with photon energies between 90 eV and 120 eV. Such energies probe the Si L 23 edge transitions. This ensures that the primary excitation probes material characterized by Si-Si bonds: that is, the silicon backbone of the porous film. The sensitivity of the L 23 edge to local chemistry has been noted previously 5,6 . The X-ray attenuation length above the L edge for bulk silicon is ϳ500 Å , and the effects reported here were observed for films that are thicker than the attenuation depth, so that some fraction less than the whole thickness was directly excited by X-ray photons. We measured the total electron yield (TEY), which includes photoelectrons, primary Auger electrons, cascaded Auger electrons and secondary electrons. The TEY was measured by sensing the current flowing into the back of the Si wafer, which normally compensates exactly for the photoemitted electrons escaping from the surface. Such rapid charge compensation is always observed even for weakly conducting systems, because the small photoexcited current (typically 10 −10 A) is easily supplied. But we have found that porous silicon films do not behave in this way; instead the TEY steadily decreases as temperature is reduced, and may fall below background noise levels at low temperatures.TEY spectra obtained during a cooling cycle in which a conducting/insulating transition occurred are shown in Fig. 1. At the hi...

Reactions and luminescence in passivated Si nanocrystallites induced by vacuum ultraviolet and soft-x-ray photons

“…4͑c͔͒ indicates that this surface charging is relatively homogeneous. The presence of dynamic equilibrium charging is likely to arise from the slow neutralization of photoinduced surface charge through charge percolation pathways 26 ͑such transport is well known in granular systems͒. 27 In the slow sample the second Si2p component can be observed to evolve with photon irradiation ͓Fig.…”

Study of phase separation and photo‐ luminescent emission in silicon nanostructured PECVD systems

“…Photoluminescence in films constituted by silicon clusters embedded in SiO 2 or Si 3 N 4 dielectric matrix as well as in Si/SiO 2 multilayers has been attributed to confinement effects and interfacial phenomena [4][5][6][7]. In a previous work intense photoluminescence was observed in silicon-rich silicon oxynitride films, however the involved phenomena were not clear [8,9].…”

“…The photoemission phenomena in silicon-based films have attained much attention in the last decade since the observation of light emission in porous silicon raising the possibility of microelectronics and optoelectronics integration [1][2][3]. Photoluminescence in films constituted by silicon clusters embedded in SiO 2 or Si 3 N 4 dielectric matrix as well as in Si/SiO 2 multilayers has been attributed to confinement effects and interfacial phenomena [4][5][6][7].…”

Percolation phenomena in controlled drug release matrices studied by dielectric spectroscopy and the alternating ionic current method

“…For such systems the percolation threshold, signifying the conducting component volume fraction at which ion conducting channels spans the entire matrix, in many aspects determines the functionality of the system [1][2][3][4][5][6][7]. In spite of this, percolation studies in the literature analyzing binary mixtures of conducting and non-conducting materials hitherto have focused almost entirely on mixtures containing electron -and not ion -conductors [8][9][10][11][12]. One exception to this is within the pharmaceutical sciences where a few studies have been presented involving release of an ion conducting material or of a material that ionizes when in contact with a liquid in order to assess the percolation threshold of the mixture [7,13].…”