Light-emitting porous silicon

Although porous silicon has been known for more than 35 years, only in 1990 was it recognized that porous silicon shows an increased bandgap and efficient room-temperature photoluminescence in the visible. This paper will give an overview of porous silicon research, with special emphasis on the formation mechanism of microporous silicon in terms of a depletion of holes in the porous region due to quantum confinement and the understanding of the origin of the visible luminescence. The status of research on electroluminescent and other devices based on porous silicon will be discussed, as well as results for other luminescent forms of nanocrystalline silicon.     

The modification of the properties of n-type conductivity porous silicon by argon ion irradiation

The results of the research into the influence of argon ion irradiation at 3 keV on the composition and structure of porous silicon are presented. At a certain angle of incidence of the particles relative to the surface of the monocrystallites, an undulating λ∼60 nm nanorelief is formed, while the crystallite sizes and structure remain unchanged. The IR-spectroscopy data show that SiH groups are mainly localized in a thin 120 nm near-surface layer. During exposure of the samples to the air in the dark, monohydride groups are removed from the surface within a month. Dihydride groups, located in deeper layers, are oxidized considerably more slowly than the monohydride ones. The experimental data show that the 0.1 μm-thick surface layer serves as a diffusion barrier preventing oxygen from penetrating deep into the porous silicon.     

The dependence of hydrofluoric acid concentration during anodization on photoluminescence of porous silicon

The mixture of hydrofluoric (HF) acid and ethanol is used as an electrolyte during anodization of silicon. We investigated the effect of the ratio of HF acid to ethanol on photoluminescence. It is concluded that porous silicon anodized with the electrolyte containing 35 or 40% HF acid provides strong photoluminescence. The fact implies the existence of a chemical reaction including ethanol during anodization other than electrochemical reaction.     

MIS gas sensors based on porous silicon with Pd and WO3/Pd electrodes

Pd and WO₃/Pd gate metal-oxide-semiconductor (MIS) gas sensitive structures based on porous silicon layers are studied by the high frequency C(V) method. The chemical compositions of composite WO₃/Pd electrodes are characterized by secondary-ion mass spectrometry (SIMS). The atomic force microscopy (AFM) was used for morphologic studies of WO₃/Pd films. As shown in the experiments, WO₃/Pd structures are more sensitive and selective to the adsorption of hydrogen sulphide compared to Pd gate. The analyses of kinetic characteristics allow us to determine the response and characteristic times for these structures. The response time of MIS-structures with thin composite WO₃/Pd electrodes (the thickness of Pd is about 50 nm with WO₃ clusters on its surface) is slower compared to the structures with Pd electrodes. Slower sensor responses of WO₃-based gas sensors may be associated with different mechanism of gas sensitivity of given structures. The enhanced sensitivity and selectivity to H₂S action of WO₃/Pd MIS-structures can also be explained by the chemical reaction that occurs at the catalytic active surface of gate electrodes. The possible mechanisms of enhanced sensitivity and selectivity to H₂S adsorption of MIS gas sensors with WO₃/Pd composite gate electrodes compared to pure Pd have been analyzed.     

Blue luminescence from porous layers produced by metal-assisted chemical etching on low-doped silicon

Photoluminescent porous layers were formed on highly resistive p-type silicon by a metal-assisted chemical etching method using K₂Cr₂O₇ as an oxidizing agent. A thin layer of Ag is deposited on the (1 0 0) Si surface prior to immersion in a solution of HF and K₂Cr₂O₇. The morphology of the porous silicon (PS) layer formed by this method as a function of etching time was investigated by scanning electron microscopy (SEM). It shows that the surface is formed by macropores filled with microporous silicon. The porous layers were characterized by backscattering spectrometry (BS) as a function of etching time in random and channelling mode. Channelling spectra show that the porous layer remains crystalline after etching. On the other hand, random and channelling spectra show that the deposited silver diffuses into the pore. Luminescence from metal-assisted chemically etched layers was measured. It was found that the PL intensity increases with increasing etching time. This behaviour is attributed to increase of the density of the silicon nanostructure. Finally, the PL spectra show two peaks of emission at 450 and 600 nm.     

Visible luminescence in photo-electrochemically etched p-type porous silicon: Effect of illumination wavelength

The effect of low power density of ~ 5 μW/cm² monochromatic light of different wavelengths on the visible photoluminescence (PL) properties of photo-electrochemically formed p-type porous silicon (PS) has been investigated. Two-peak PL “red” and “green” is resolved in PS samples etched under blue-green wavelength illumination; 480, 533 and 580 nm. It is found that the weight of “green” PL has maxima for the sample illuminated with 533 nm wavelength. Whereas, PL spectra of PS prepared under the influence of red illumination or in dark does not exhibit “green” PL band, but shows considerable enhancement in the “red” PL peak intensity. Fourier transform infrared (FTIR) spectroscopic analysis reveals the relationship between the structures of chemical bonding in PS and the observed PL behavior. In particular, the PL efficiency is highly affected by the alteration of the relative content of hydride, oxide and hydroxyl species. Moreover, relative content of hydroxyl group with respect to oxide bonding is seen to have strong relationship to the blue PL. Although, the estimated energy gap value of PS samples shows a considerable enlargement with respect to that of bulk c-Si, the FTIR, low temperature PL and Raman measurements and analysis have inconsistency with quantum confinement of PS.     

Estimation of oxide related electron trap energy of porous silicon nanostructures

Estimation of electron trap energy (E_t), with respect to bulk Si valence band, of oxidized porous silicon (PS) nanostructures is reported. Photoluminescence (PL) spectra of oxidized PS prepared with different formation parameters have been investigated and the room temperature PL characteristics have been successfully explained on the basis of oxide related trap assisted transitions. PL peak energy for the oxidized samples with low porosity exhibited a blue shift with increasing formation current density (J). For the high porosity samples double peaks appeared in the PL spectra. One of these peaks remained constant at ∼730 nm while the other was blue shifted with increase in J. Evolution of PS nanostructure was correlated to the formation parameters using a simple growth mechanism. PS nanostructure was modelled as an array of regular hexagonal pores and the average value of E_t was estimated to be 1.67 eV.     

A technique for the fabrication of various multiparametric porous silicon samples on a single substrate

Porous silicon (PSi) samples generally have a uniform thickness and pore size according to specific anodization conditions, as the Si wafer is entirely immersed into hydrofluoric (HF) acid during the anodization process. In contrast, multiparametric (MP) PSi, as described in the present work, is fabricated by inserting a Si wafer gradually (or by stages) into a HF solution during the anodization process. Consequently, MP-PSi allows single layer fabrication with a pore-size and layer thickness gradient or various multilayers, on a single substrate. Therefore, MP-PSi can be readily used in sensor application areas to determine optimized detection conditions for various materials, such as gas, liquid, and bio-materials. MP-PSi layer with a lateral pore gradient distribution can also be used as size-exclusion matrix. In addition, the MP-PSi multilayer array is expected to open up application areas involving optical electronic nose systems.     

Gas sensor applications of porous Si layers

The porous silicone (PSi) is a relatively new and promising semiconductor material with special physical and chemical properties which somewhat differ from the properties of single crystal Si. Some of these properties are valuable in the field of gas sensor technology, but a lot of questions arise in connection with its application. Do we really need porous semiconductor material for proper gas sensing function? How can electrical properties of the PSi layer be measured if the electrical contacting is problematic? Is it possible to activate the PSi with catalytic noble metal layers or particles? What about the Fermi-level pinning in the PSi layer? The main target of this article is to seek answers to questions listed above and to give a short, but still comprehensive review of the application of the PSi layers on the field of the gas sensor technology, with special care on electrical output signal giving sensors.     

Crystallite-size-dependent characteristics of porous silicon

Porous silicon prepared with anodic currents of 5 to 30 mA/cm² are characterized for structural and electronic properties of surface using photoluminescence, grazing angle X-ray diffraction, photoconductivity, thermally stimulated exo electron emission and work function measurements. The observed results indicate that with increasing porosity the crystallite size decreases and the amount of silicon hydride and oxide-type species increases, exhibiting a tendency similar to that of hydrogenated amorphous silicon and hydrogenated microcrystalline silicon. Free-standing powder of porous silicon, characterized by bright photoluminescence at 730 nm, showed crystallites of nanometre dimensions under the transmission electron microscope.     

Some new results in porous silicon

Efforts have been made to see the effect of some standard microelectronic processing steps on porous silicon. Our diffusion experiments for making p-n junctions confirm that this material can withstand high temperatures of the order of 800°C to 1000°C. A new technique for photolithography has been suggested to obtain porous silicon in selected areas. Etch stop method to control the thickness of the porous layer and an organic protective layer for porous silicon have also been suggested. Models proposed by other workers to explain luminescence in porous silicon are not sufficient to explain many experimental observations. A hybrid model is suggested.     

Optical and structural analysis of porous silicon coated with GZO films using rf magnetron sputtering

In the production of porous silicon (PS) to optoelectronic application one of the most significant constrains is the surface defects passivation. In the present work we investigate, gallium-doped zinc oxide (GZO) thin films deposited by rf magnetron sputtering at room temperature on PS obtained with different etching times. The X-ray diffraction (XRD), Fourier transform infrared (FTIR) and atomic force microscopy (AFM) analysis have been carried out to understand the effect of GZO films coating on PS. Further, the XRD analysis suggests the formation of a good crystalline quality of the GZO films on PS. From AFM investigation we observe that the surface roughness increases after GZO film coating. The photoluminescence (PL) measurements on PS and GZO films deposited PS shows three emission peaks at around 1.9 eV (red-band), 2.78 eV (blue-band) and 3.2 eV (UV-band). PL enhancement in the blue and ultraviolet (UV) region has been achieved after GZO films deposition, which might be originated from a contribution of the near-band-edge recombination from GZO.     

Light emission from porous silicon

Room-temperature visible luminescence observed in porous silicon is one of the most significant discoveries of this decade as it opens up the possibility of silicon-based optoelectronics afresh. The exact mechanism of this different luminescence behaviour of porous silicon, compared to crystalline silicon, is not well established. In this paper results of a combination of infrared absorption, and photoluminescence emission and excitation spectroscopies will be described to show that the nanocrystallite nature of porous silicon and chemical environment at the surface are the important aspects of this novel luminescence behaviour.     

Magneto-transport in porous silicon

Magnetoresistance (MR) measurements have been performed in the temperature range 100–300 K on macroporous porous silicon (P.S.) samples. P.S. layers have been prepared by the anodic dissolution of Si in HF acid. The MR has been found to be positive in P.S. for the temperature range 100–300 K and for the entire range of magnetic field (0–5 kG). However the magnitude of the positive MR is found to be much less than expected on the basis of free electron conduction. Also the value of n in the relation Δρ/ρ_o∞Bⁿ for the temperature range 100–300 K is found to be < 2, implying that there is a contribution of some phenomenon other than the free electron conduction to MR. The measured data suggest that it is the contribution of localized state conduction near the Fermi level and in the localized states near the band edges.     

Raman scattering and photoluminescence study of porous silicon formed on n-type silicon

We report Raman scattering and photoluminescence studies on porous silicon film formed on n-type silicon. The Raman spectra over the sample surface exhibit considerable variation whereas the photoluminescence spectra are practically identical. Our results indicate that, well inside the film surface, it consists of spherical nanocrystals of typical diameter ≈ 100Å, while on the edge these nanocrystals are ? 300Å. We further observe that there is no correlation between the photoluminescence peak position and the nanocrystal diameter. This suggests that the origin of the photoluminescence is due to radiative recombination between defect states in the bulk as well as on the surface of the nanocrystal.     

Hydrogen in porous silicon — A review

This review is devoted to summarising the hydrogen-assisted properties and applications of porous silicon (PS). The role of hydrogen as an intermediate product in silicon porosification technology is accentuated. The regularities of hydrogen bonding in PS and its applications for hydrogen storage are listed. The models of hydrogen influence on luminescence and electrical properties of PS are analysed. The corresponding applications of PS for H₂ gas sensors and pH metres are illustrated. Hydrogen-assisted explosion and grafting of PS are discussed. Such a review can be useful for the tailoring of PS properties.     

CO2 and H2 detection with a CHx/porous silicon-based sensor

There has been great interest in the last years in gas sensors based on porous silicon (PS). Recently, a gas sensing device based on a hydrocarbon CH_x/porous silicon structure has been fabricated. The porous samples were coated with hydrocarbon groups deposited in a methane argon plasma. We have experimentally demonstrated that the structure can be used for detecting a low concentration of ethylene, ethane and propane gases [Gabouze N, Belhousse S, Cheraga H. Phy State Solidi (C), in press].In this paper, the CH_x/PS/Si structure has been used as a sensing material to detect CO₂ and H₂ gases. The sensitivity of the devices, response time and impedance response to different gas exposures (CO₂, H₂) have been investigated.The results show that current-voltage and impedance-voltage characteristics are modified by the gas reactivity on the PS/CH_x surface and the sensor shows a rapid and reversible response to low concentrations of the gases studied at room temperature.     

Determination of Vickers microhardness on porous silicon surfaces

The Vickers microhardness values of two different sets of porous silicon layers were determined at applied load of 98 mN. The sets consisted of Boron-doped substrates anodized at diverse current densities for two different amounts of hydrofluoric acid (HF) in the etching solution. We found that the microhardness of the samples with lower content of HF at the anodization process showed higher values, whereas the Vickers parameter diminishes consistently for higher current densities. A possible explanation of this behavior is proposed.     

Optical sensing of flammable substances using porous silicon microcavities

A porous silicon multilayer, constituted by a Fabry–Pèrot cavity between two distributed Bragg reflectors, is exposed to vapor of several organic species. Different resonant peak shifts in the reflectivity spectra, ascribed to capillary condensation of the vapor in the silicon pores, have been observed. Starting from experimental data, the layer liquid volume fractions condensed in the sensing stack have been numerically estimated. Values ranging between 0.27 (for ethanol) and 0.33 (for iso-propanol) have been found. Time-resolved measurements show that the solvent identification occurs in less then 10 s.     

制备温度对多孔硅光学特性的影响(英文)

本文不同的温度下制备多孔硅.通过荧光光谱、光吸收谱、X射线光电子谱研究了多孔硅的光和结构特性.研究结果表明存在着一个制备临界温度343 K,当制备温度从临界温度之下提高到临界温度之上时,多孔硅的荧光和光吸收从红移转向蓝移,同时硅2p电子结合能也从减小转向增大.