Studies on Gold/Porous Silicon/Crystalline Silicon Junctions

Electrical transport in Gold/porous silicon/crystalline silicon junctions has been studied. The junctions are found to improve when the porous silicon is exposed to a hydrogen plasma before depositing the top metal. The hydrogen passivated junctions exhibited higher current levels and emitted light at lower voltages as compared to the unhydrogenated ones. Internal photoemission measurements were carried out to investigate the gold/porous silicon barrier. The barrier height determined from the Fowler plot is independent of the top material. The temperature dependence of the barrier height is similar to that of the crystalline silicon energy gap.     

Porous Silicon Studied with Time-Resolved Photoluminescence

Porous silicon has been studied with time-resolved photoluminescence, and growth as well as decay curves have been measured at several detection energies, with sample temperatures between 10 and 300 K. In the decay curves, three components are mainly observed, a small one which is very fast, with time scales of the order of nanoseconds or faster, the main component having time scales of the order of milliseconds, and a very small, very slow component, with time scales of the order of seconds. The main components can in most—but not all—cases be fitted well with stretched exponentials containing two fitting parameters. Of these, it comes out that the parameter accounting for disorder or the like depends only little upon detection energy and temperature, whereas the parameter accouting for the development in time decreases substantially for increasing temperature. The results are discussed.     

Laser Induced Degradation of Photoluminescence Intensity of Porous Silicon

The evolution, under vacuum, of the photoluminescence (PL) intensity of porous silicon (PS) has been studied as function of anodisation conditions, laser line and post-anodisation treatments. It was shown that the degradation of the PL intensity depends on the internal structure of PS. In particular, the degradation is important for PS layers formed essentially by crystallites having small size or where amorphous phase exists. The experimental results have been interpreted using a theoretical model, which takes into account the variation with time of the local concentration of the luminescent centers.     

Light Emission from Highly Reflective Porous Silicon Multilayer Structures

Porous silicon photoluminescence and electroluminescence can be controlled by periodically modulating the material porosity to form high quality multilayer stacks and microcavities. Important issues not yet fully addressed are (a) the precise role played by this microstructuring, given that the luminescence is distributed throughout the entire structure and that the low porosity layers are highly absorbing at short wavelengths, and (b) whether the quality of such microcavities could be sufficient to support lasing. Using both experimental and theoretical techniques, the emission and reflection properties of different porous silicon single and multilayer structures have been investigated in order to understand further and exploit the nature of light propagation within them.     

Conduction and Luminescent Properties of Wet Porous Silicon

Carrier transport and photoluminescence-quenching mechanisms in reverse-biased p-type porous silicon in contact with an aqueous electrolyte are investigated. Concerning transport mechanisms investigation, experiments are based on the study of the photo-induced current as a function of the porous layer thickness. The liquid-impregnated porous silicon skeleton is found under equipotential conditions. Transport of electrons (supplied by the substrate) in porous silicon is shown to be dominated by a diffusion process. Photoluminescence-quenching is investigated by using a reverse-biased p-type porous silicon illuminated at 365 and 809 nm simultaneoulsy. The first illumination generate photoluminescence and the second supplies carriers in the substrate. A progressive photoluminescence-quenching has been observed, under a constant applied voltage, by increasing progressively the electron concentration in the porous layer. This original experiment allows to reject the hypothesis of an electric-field-induced separation of carriers as the photoluminescence-quenching mechanism in wet porous silicon, while it strongly supports the mechanism based on Auger recombination.     

Correlation of Photoluminescence and Optical Absorption Spectra of Porous Silicon

Using a quantum confinement based-PL model, PS was modelled as a mixture of Quantum Dots (QDs) and Quantum Wires (QWs) having different concentrations and sizes. It was shown that in the optical absorption edge the PL peak energy and the Optical Absorption (OA) exhibit the same trend, depending on preparation conditions. The spectral behaviours of PL and OA are analysed and correlated throughout the shapes and the size distribution of the nanocrystallites forming PS. Using the quantum confinement formalism, the value of the effective band-gap energy determined from the lowest PL energy almost corresponds to that estimated from the optical absorption coefficient. These results suggest that the lowest radiative transition between the valence band and the conduction band corresponds to the largest luminescent wires, and that the radiative recombination process leading to the PL emission occurs in the c-Si crystallite core.     

Correlation of Photoluminescence and Bandgap Energies with Nanocrystal Sizes in Porous Silicon

We have measured the crystallite sizes, the bandgap energies, and the photoluminescence (PL) energies in porous silicon (PSi) samples having a wide range of porosities and kept in different ambient conditions. The dependence of the bandgap energy on the crystallite size agrees with theory. For PSi samples exposed to air and containing crystallites smaller than 5 nm, the PL intensity increases by several orders of magnitude and the PL peak energy shifts from the near infrared to the red, in agreement with the quantum confinement model for the PL. For crystallites smaller than 3 nm, there is a Stokes shift between the excitonic bandgap and PL energies, which increases to several hundreds of meV for sizes 2 nm, indicating that, in PSi exposed to air, the PL is not due to free excitons. Before exposure to air, very high porosity PSi samples emit at shorter wavelengths than after exposure to air, suggesting that the Stokes shift depends on the surface chemistry.     

Photoluminescence Decay and Hydrogen Desorption of n-Type Porous Silicon

The decay under illumination of the photoluminescence (PL) intensity ofn-type porous silicon (PS) samples prepared by electrochemical etching has been investigated. We have found that the PL evolution with illumination time presents two different stages: an initial very fast decay which lasts 300 s, followedby a second one, much slower, which extends for times longer than 10⁴ s. Thisevolution suggests that two different mechanisms could be responsible for the PLintensity decay. Samples subjected to different illumination times were studied byThermal Desorption Spectroscopy (TDS). The desorption rate of H₂ and SiH_x species was monitored during linear heating of the samples. A qualitative correlationbetween the decay of the PL intensity under illumination and the amount of H₂ and SiH_x species evolved from the illuminated samples has been observed. Experimentaldata suggest that H₂ could be desorbed from the sample during the illuminationtime through a photoinduced H₂ desorption process, inducing the decrease of the PLintensity.     

Optical Characterization of Free-Standing Porous Silicon Films

We report here the optical characterization of two free-standing porous silicon (PS) films of porosity 70% and 85%, grown on a p- and a n-type Si. We determine the optical band gap in these films from the excitation wavelength dependence of the photoluminescence band. As the excitation wavelength is changed from red (800 nm) to UV (355 nm), a blue shift of the photoluminescence (PL) band is observed. We attribute the observed blue shift of the PL band to the emission due to the distribution of bandgap in PS. Both samples are oxidized in air and we believe that the observed bandgap in these films arises from the inhomogeneous distribution of Si particle sizes. However, we find that intrinsic defects play a dominant role in the process of luminescence. Electron spin resonance measurement indicates the presence of defects leading to the saturation of the optical absorption spectra and a decrease in intensity of the PL band. The lineshape of the PL band is modeled using a weighted asymmetric Gaussian that selects a gap distribution function at each excitation wavelength. From Raman measurements in these two films, the quantum confinement effect in Si nanostructures is clearly observed in both films.     

Electroluminescence from Porous Silicon Studied Experimentally

Electroluminescence, photoluminescence, and current-voltage measurements have been carried out on a series of samples consisting of porous silicon on top of crystalline silicon. The electroluminescence spectral distributions are markedly shifted towards long wavelengths in comparison to those seen with photoluminescence. The results are discussed and explained in terms of the energy-band structure of porous silicon. The problem of injecting electrons as well as holes into the porous silicon layer is pointed out and discussed.     

多孔硅——聚合物PFTD复合膜的光致发光研究

文章采用直接浸泡法在多孔硅中嵌入了新型共轭有机聚合物聚[4-（N,N-二甲基胺丙氧基）苯-4,4’-二苯胺-9,9-二辛基芴-4,7-二噻吩-2-基-2,1,3-苯并噻二唑]（PFTD）,对比研究了多孔硅,PFTD/多孔硅,PFTD/硅以及PFTD在甲苯溶液中的光致发光特性。实验结果表明：PFTD/多孔硅复合膜光致发光强度要明显强于PFTD/硅,同时由于聚合物在固态时有效共轭程度增加导致PFTD/多孔硅复合膜的荧光光谱相对于甲苯溶液中的红光聚合物光谱有所红移。     

Diamond Coating of Porous Silicon

Diamond coatings on porous silicon (PS) samples have been obtained by the hot-filament chemical vapor deposition(CVD) technique. We focused our attention on the coating morphology, showing experimentally that high quality diamond coatings may be produced with the PS sample kept at 710°C. The deposited patterns consist of polycrystalline grains with a plane interface with the PS layer.At 790°C, the quality of the coating is improved but the PS layer becomes damaged, and at 650°C the coating consists of diamond-like carbon particles. Besides the temperature, other factors such as the porosity, roughness and chemical activity of the PS layer deserve attention. We observed that one of the limiting factors of the deposition process was the high nucleation time. Two nucleation mechanisms are involved in the growth process. The first nucleation mechanism occurs on the top of the sharp PS features, subsequently to the nucleation a superficial film, and then a second nucleation mechanism occurs over this surface, which allows the growth process to continue. We also observed the presence of ablue-shift in the luminescence spectra following the coating.     

Electronic Structure and PL Spectrum of the Diffusion-Limited Model for Porous Silicon

We present the results of theoretical calculations for electronic structures and photoluminescence (PL) spectrum of porous silicon whose morphology is generated through the diffusion limited aggregation process of pores in a two-dimensional honeycomb lattice. We have found that due to irregularity of the structure most of its eigenstates near band gap are localized while some of them are relatively delocalized. The localization of the eigenstates near band gap causes band-gap narrowing analogous to the quantum confinement effect. Solving the time-dependent equations for the occupation numbers of the eigenstates, we show that the present model reproduces the stretched exponential decay of PL intensity observed in the experiments.     

Electrical and Optical Properties of Conducting Polymer/Porous Silicon Structures

Electrical and optical properties of diode structures based on porous silicon (PS) and thin films of phenylene vinylene oligomer (PVO) have been studied. Steady-state photoluminescence spectroscopy show that the structure of the luminescence band depends on the PS morphology. We assign the observed effect to the morphology-dependent penetration of PVO material into the pores. Current-voltage characteristics of the PVO/PS diodes are studied and interpreted assuming Schottky emission and hopping transport of carriers aspossible mechanisms of d.c. electrical conduction.     

A Systematic Study of Synchrotron Light Induced Luminescence from Porous Silicon: Implications to Morphology and Chemical Effect Dependent Electronic Behaviour

Using synchrotron as a tunable excitation source, we have carried out a study on the photoluminescence systematics from a series of porous silicon samples prepared under different conditions, Luminescence spectra were recorded with excitation photon energies tuned to the Si L₃,2 absorption edge (100 eV). The luminescence yield was in turn used to monitor the Si L₃,2-edge absorption characteristics of porous silicon. A trend of luminescence wavelength and intensity as a function of preparation conditions emerges. Other related observations are also noted.     

Optical Absorption in Porous Silicon

The optical properties of porous silicon (p-Si) are calculated from the electronic band structure obtained by means of an sp₃s^* tight-binding Hamiltonian and a supercell model, in which the pores are columns detched in crystalline silicon (c-Si). The disorder in the pore sizes and the undulation of the silicon wires are considered by the existence of arandom perturbative potential, which produces non-vertical interband transitions, otherwise forbidden. A typical interval around each k-vector (optical window), where non-vertical transitions make an important contribution, depends on the value of the disorder and its order of magnitude is given by l^–1, where l is the localization length. The calculated absorption spectra are compared with experiments, showing good agreement.     

Cathodoluminescence from Implanted and Anodized Polycrystalline Silicon Films

Luminescence emission of LPCVD polycrystalline silicon films has been studied by cathodoluminescence (CL) in the scanning electron microscope. As-deposited films show visible luminescence with dominant blue band. The relative intensity of blue emission is enhanced by implantation and by slight anodization treatments. Our investigations are consistent with previous PL results and indicate that the origin of blue emission is related to quantum confinement effects. On the other hand, the effect of annealing in these samples is a reduction of the CL signal that could be related to the increase of the nanocrystals size.     

An Investigation of Porous Silicon by Means of Positron Annihilation

Positron lifetime spectroscopy has been used to investigate a porous silicon film subjected to heat treatments up to 1170°C. Annealings between 300 and 500°C resulted in a 17% mass increase of the film due to oxygen uptake following the effusion of hydrogen. The positron data also indicate that vacancy clusters are formed in the silicon oxide layer or the silicon oxide—silicon interface surrounding the nanocrystallites as oxygen replaces the effusing hydrogen. The vacancy cluster concentration, which may have a bearing on the photoluminescent properties, increased by a factor of three with heating to 500°C and then decreased to one-third the original value at higher temperatures. Above 900°C vacancy migration and clustering occurred, accompanied by visible deterioration of the film.     

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, (), is studied by Monte Carlo simulations. The model includes both diffusion and recombination processes and () 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 () depends sensitively on the type of carrier considered.     

Investigation of Morphology of Porous Silicon Formed on N+ Type Silicon

The properties of porous silicon (PS) are closely connected to its morphology, much investigation has been done in order to correlate the morphological characteristics of PS with the anodisation parameters. In this paper the results of morphological analysis of PS formed on N⁺type substrates of 1 0 0 and 1 1 1 orientation are presented. The dependences of the porosity, thickness of PS, density of pores and of the effective surface on the current density are obtained. Interpretation of these results in terms of diffusion layer and energy levels is given, with special attention given to the low current density case.