Conductivity Of Porous Silicon Research Articles

Lateral structuring of porous silicon: application to waveguides

The problem of lateral definition of waveguides and photonic structures has been tackled by means of direct laser scribing on porous silicon (PS). The low thermal conductivity of PS allows for local oxidation and even the complete ablation of the material with laser power of a few mW. The buried Laser Local Oxidized (LaLOx) method exhibits great versatility and efficiency in achieving lateral light confinement in waveguides.

Investigation of thermal transport in n-type porous silicon by photo-acoustic technique

Non-destructive thermal and optical characterization of materials can be successfully performed by the photo-acoustic technique. In this work, this technique has been applied to the measure of thermal conductivity in porous silicon by considering the photo-acoustic response at fixed frequency of samples having the same porosity but different thicknesses. Experimental data are interpreted in terms of a model which takes into account both scattering effects and the contribution to the photo-acoustic signal of the interstitial gas expansion. The measured thermal conductivity is found to be lower than the one reported for crystalline silicon by two orders of magnitude. A discussion of the photo-acoustic signal dependence on the morphology of the porous medium is also presented.

Conductivity Modulation of Porous Silicon by Formation Parameters

Conductivity of Porous Silicon (PS) can be modulated significantly by controlling its formation parameters like anodization current density and HF concentration in the electro-chemical cell and the doping concentration of the silicon wafer. It is established that not only the porosity but also the uniformity of the PS layer determines conductivity of the PS layer. A quantitative relationship between the effective conductivity and uniformity and porosity of the PS layer is presented and verified experimentally.

Effect of oxygen on the surface conductance of porous silicon: towards room temperature sensor applications

The surface conductance of an aluminum/p-type porous silicon device is reversibly changed on exposure to gaseous oxygen. The aluminum electrode gap has been found to cause a four-fold increase in initial rate of change of conductance on doubling the electrode separation on exposure to oxygen. Potential use of such devices in room temperature oxygen sensing is described.

Microwave Transient Photoconductivity Studies in Porous Semiconductors

AbstractThe dynamics of the photogenerated carriers in porous silicon and TiO2 anatase was studied at 35 GHz by measuring the change in time of the conductivity s and dielectric constanter. Localization of carriers leads to a positive change ofer, while quasifree carriers to a negative change. Size reduction in Si shortens the recombination time as long as the surface traps are not significant. Magnetic field investigations show opposite variation of conductivity in poroussilicon compared with TiO2.

Origin of emission from porous silicon: Temperature-dependence correlation with proton conductivity

The excitation, emission, and Fourier transform infrared (FTIR) spectra of freshly prepared and aged porous silicon (PS) are examined. The effects of aging and temperature changing on the emission and excitation spectra of fresh and aged PS samples is also studied. Aging in air is found to result in a redshift in the emission spectrum, but a blueshift in the excitation spectrum. Increasing the temperatures leads to a general decrease in the emission intensities of both samples: however, the green emission of fresh sample shows an unusually large and sudden increase in its intensity at the same temperature at which the proton conductivity in the silicon wafer increases. The temperature dependence of the O-H vibration intensity of silicon wafers shows a sudden decrease at that temperature. These results are consistent with a model in which the excitation leads to the formation of exciton in the confined nanostructure, and the formation of electron and hole that are trapped by surface traps, whose nature is dependent on the chemical composition of the surface (and thus its age). These conclusions are confirmed by the results of the FTIR spectra of the two samples, the temperature dependence of the emission, and its correlation with the proton conductivity in the fresh PS.

Electrical conduction in porous silicon: temperature dependence

The temperature dependent conduction of electrons in a porous silicon film is theoretically investigated by using the fact that the electrical conduction is closely related to the formation of a continuous network of the conducting Si nanocrytallites. For the first time, an analytical expression for the electrical conductivity of porous silicon has been obtained, which demonstrates that the overall temperature dependence of electrical conductivity cannot be described by a single Arrhenius relationship, and there is a change in the electrical transport mechanism at a critical temperature Tc. The critical temperature Tc and the activation energy are found to be dependent on the mean size of Si nanocrystals as well as their size distribution. The present results are supported by several recent experimental observations.

Probing Carrier Transport in Porous Silicon with Synchrotron Radiation

In this paper we demonstrate that photo-electron emission excited by X-UV synchrotron radiation can be used as a “contactless” probe of the gross conduction processes in porous silicon. Moreover we demonstrate that this approach reveals the underlying conduction geometry. We show that conduction in porous silicon is to some degree controlled by percolation phenomena and finally present data which support the notion that the fundamental blocking process may be Coulomb Blockade [P.A. Lee, Physica B 189, 1–5 (1993); D. Ali and H. Ahmed, Appl. Phys. Lett. 64, 2119–2120 (1994)].

Analyses of Ballistic Electron Transport in Nanocrystalline Porous Silicon

AbstractThe characteristics of ballistic electron transport in porous silicon (PS) are investigated in terms of the electron emission from PS diodes and the energy distribution of emitted electrons. The energy distributions show a behavior of ballistic electron emission that is quite different with the Maxwellian distribution. This is clearly observed at low temperatures below 150 K where the electrical conduction in PS is dominated by the tunneling mode. At 100 K, the peak position of distribution curve becomes more close to the energy corresponding to the energy gain expected from ballistic transport without any scattering losses. The simulated energy distribution suggests that the electrons having higher energies in a non-equilibrium state travel ballistically in PS via field-induced tunneling process. These results support that electrons can travel ballistically in nanocrystalline layer under a high electric field. The observed ballistic transport indicates the further technological potential of silicon nanocrystallites.

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.

AC Conductivity of Porous Silicon from Monte Carlo Simulations

The AC conductivity of a percolation model with local energetical disorder for porous Silicon in three dimensions, σ(ω), is studied by Monte Carlo simulations. The model includes both diffusion and recombination processes and σ(ω) is obtained by a Fourier transform of the mean-square displacement of the carriers, where hopping diffusion of a single type of carrier (either an electron or an exciton) and two types of carriers (an electron and a hole) are considered. It is found that at low temperatures, the behavior of σ(ω) depends sensitively on the type of carrier considered.

On the electrical conductivity in porous silicon under light and electron beams

The `photoconductive' properties of a porous silicon layer (∼10 μm thick) formed by anodization of p-type silicon wafer having aluminum ohmic contacts is studied. The porous silicon layer has a high dark resistivity, (>100 Ω m), a photoconductivity in visible light and for electron beams of 50–100 keV; a dependence of spectral response on bias voltage is also observed. These properties are interpreted as the result of an increase in the band gap in porous silicon correlated with the layer morphology and the formation of a heterojunction whose field separates generated pairs.

Photo-acoustic characterization of porous silicon samples

The photoacoustic technique is used largely for the thermal and optical non-destructive characterization of materials. In this paper we present a new and alternative method for the measure of thermal conductivity in porous silicon by considering the photoacoustic response at fixed frequency of samples with the same porosity but different thickness. The analysed samples are fabricated by etching n-type, 1 Ω cm, 550 μm thick crystalline silicon; they have a porosity ranging from 40-70% and thickness from 45-250 μm. The measured thermal conductivity is lower than the one reported for crystalline silicon by two orders of magnitude.

Two-dimensional tight-binding model of ac conductivity in porous silicon

A time-dependent tight-binding model has been developed to study the ac conductivity in porous silicon. Assuming that carriers are allowed to hop between isolated pairs of Si nanocrystals embedded in a SiO2 matrix, the tunneling times have been calculated according to different geometries. The geometrical structure of porous silicon has been modeled with simple percolationlike clusters. By using the tunneling times, the ac conductivity behavior in the high-frequency regime has been calculated in the pair approximation. The conductivity increases with the frequency according to a power law with an exponent lower than unity. It is found that there is a strong dependence of the ac conductivity on the thickness of the SiO2 interconnecting layer.

Size-controlled percolation pathways for electrical conduction in porous silicon

Silicon shows photo- and electroluminescence at visible wavelengths when chemically etched into a microporous network of ‘wires’ several nanometres thick1. 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 efficiency1,2. 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 effect3,4 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.

Conductivity in porous silicon

Admittance spectroscopy has been performed on Al dots deposited on porous Silicon grown on p + substrates. They exhibit d.c. current-voltage characteristics of a metal-insulator-semiconductor structure containing a large concentration of localized states in the insulator. The admittance is composed of a frequency (ω) independent conductance while the reactance varies as ω s with s = 1. This strongly suggests that conduction through porous Silicon layers is governed by hopping between surface states.

A comparative study of the Meyer–Neldel rule in porous silicon and hydrogenated amorphous silicon

We present two Meyer–Neldel Rules (MNRs) for the dc conductivity in porous silicon (PS), and compare them with the corresponding MNR in hydrogenated amorphous silicon (a-Si:H). One rule resembles the MNR observed in a-Si:H, while the other is very different. We critically analyze the new results in view of the existing theories of the MNR, and conclude that the second MNR in PS indicates that only the `statistical shift' model can account for the experimental data in the two materials.

Influence of short-term annealing on the conductivity of porous silicon and the transition resistivity of an aluminum-porous silicon contact

Measurements were made of the transition resistivities of an aluminum-porous silicon contact and the resistivity of the surface region of the porous material after short-term annealing in an inert medium at temperatures between 300 and 550 °C. It was shown that the parameters of the contacts are determined by the pore morphology, the annealing temperature, and the plasma-chemical etching process. For porous silicon formed on p-Si, annealing reduces the transition resistivities by several orders of magnitude. In this case, the resistivity of the surface region of porous silicon decreases sharply. The results are analyzed from the point of view of the passivating effect of hydrogen in the porous silicon.

One-dimensional localization in porous a-Si1−c Mnc

The temperature dependence of the conductance of porous silicon doped with manganese up to densities corresponding to the metallic side of the Anderson transition is investigated. It is found that in the temperature range below T=40–60 K the conductance decreases with T as G(T)∝T−1/3. This behavior corresponds to one-dimensional electron localization in silicon wires under conditions of inelastic electron-electron collisions with a small energy transfer.

Nanoscale nature and low thermal conductivity of porous silicon layers

Recently discovered phenomenon of low thermal conductivity of porous silicon (PS) layers is discussed in detail. A proposed theoretical model explains the considerable decrease of the thermal conductivity of nanoscale PS in comparison with meso-PS and bulk silicon. The thermal conductance dependence of Si/porous Si structures on the formation conditions of PS layers has been studied. By varying the values of anodisation current density and anodisation time it is possible to optimize the thermal conductance values of the Si/porous Si structures. In this way, an efficient thermal isolation can be obtained by forming thick PS layers.