As-anodized Porous Silicon Research Articles

Morphology, chemical and electrical properties of CdO Nanoparticles on porous silicon

In this paper, CdO nanoparticles prepared by pulsed laser deposition techniqueonto a porous silicon (PS) surface prepared by electrochemical etching of p-type silicon wafer with resistivity (1.5-4Ω.cm) in hydrofluoric (HF) acid of 20% concentration. Current density (15 mA/cm2) and etching times (20min). The films were characterized by the measurement of AFM, FTIR spectroscopy and electrical properties.
 Atomic Force microscopy confirms the nanometric size.Chemical components during the electrochemical etching show on surface of PSchanges take place in the spectrum of CdO deposited PS when compared to as-anodized PS.
 The electrical properties of prepared PS; namely current density-voltage characteristics under dark, show that the pass current through the PS layer is more than that obtained from the CdO/PS/Si which is related to increasing junction resistivity that come from increasing in depletion width.

Quantification and Reduction of the Residual Chemical Reactivity of Passivated Biodegradable Porous Silicon for Drug Delivery Applications

The chemical reactivity of as-anodized porous silicon is shown to have an adverse effect on a model drug (Lansoprazole) loaded into the pores. The silicon hydride surfaces can cause unwanted reactions with actives during storage or use. Techniques such as thermal oxidation or surface derivitization can lower the reactivity somewhat, by replacing the reactive silicon-hydride species with a more benign oxide or functional group. However, by using a trio of analytical techniques (fluorometric dye assay, HPLC assay, and chemography) we show that residual hydride is still likely to be present and only after combining thermal oxidation with surface derivitization can the residual reactivity be reduced to those values typically observed with sol-gel (porous) silica. Potential sources of residual reactivity are discussed, with reference to data obtained by trace metal analysis, residual solvents, and pH measurements.

Raman scattering study on pristine and oxidized n-type porous silicon

We have investigated the effects of oxidation on Raman spectra of porous silicon (PS). Many Si–H x bonds were indicated in as-anodized PS. Compared to crystalline silicon, the background in the spectrum was stronger which indicates that the PS surface was rough due to the presence of pores. When oxidation was performed, the Raman spectrum revealed the presence of nanocrystalline silicon in freshly prepared PS whose diameter was around 5.22nm. The Raman analysis showed that the peak intensities sharply decrease after oxidation compared to the as-prepared because the bonds of Si–H x on the PS surface are greatly reduced after oxidation. The oxidation of PS by thermal oxidation is complete; therefore, thermal oxidation is very well suited for a passivation of PS films.

Effects of Alkali Metallization on the Luminescence Degradation of Porous Silicon

In this study, change in the intensity and stability of photoluminescence obtained by porous silicon were investigated with illumination time and metallization state. The porous silicon samples were metallized by immersing into solutions containing 3 mM LiNO3, KNO3 and NaNO3 metal salts using immersing plating method. The surface bond configurations of porous silicon were monitored by the Fourier transmission infrared spectroscopy and the results showed that the surfaces of the samples were oxidized after the metallization. The photoluminescence intensity increased after certain critical immersion times and photoluminescence spectrum shifted towards the high energy region after the metallization. Photoluminescence intensity of metallized porous silicon samples was more stable than as-anodized porous silicon samples. The experimental results suggested the possibility that oxygen and/or alkali metal (Li, K and Na) passivation of porous silicon surface could be a suitable way to obtain an efficient and stabilized photoluminescence.

The Effects of Chemical Etching of Porous Silicon on Raman Spectra

We have investigated the effects of chemical etching on Raman spectra of porous silicon. The as-anodized porous silicon consisted mainly of crystalline silicon, as indicated by the Raman spectra. The background in the spectrum was strong, indicating that the porous silicon surface was rough due to the presence of pores. When chemical etching was performed five times, the Raman spectrum revealed the presence of spherically shaped nanocrystalline silicon whose diameter was around 3.5 nm. Further chemical etching, however, extinguished the nanocrystallites, in addition to smoothing the surface morphology.

多孔質Si可視発光のその場フォトルミネセンス, ラマンおよび赤外分光法による研究

Visible light emission from porous silicon (PS) has been investigated in-situ with photoluminescence (PL), Raman, and transmission IR spectroscopies in an ultra-high vacuum. The PL intensity of as-anodized PS was significantly decreased by the first exposure to thermoelectrons accompanied by the intensity reduction of the IR absorption bands due to hydrogenated Si species (Si-Hx; x=1-3). Upon subsequent exposure to H atoms the lost PL intensity was almost recovered but never exceeded its original intensity. This PL recovery was accompanied by regeneration of the Si-Hx bonds. In contrast, an exceeding recovery was observed for the thermoelectron-treated PS after exposure to H2O or O3. Simultaneous IR measurements revealed that Si-OH or Si-O bonds were formed at the PS surface. These results demonstrate that the PL of the PS is closely related to the oxygen-included surface chemical bonds.

<I>In-situ</I> Photoluminescence, Raman, and IR Spectral Study of Porous Silicon during Exposure to Thermoelectrons/H atoms, /H<SUB>2</SUB>O and /O<SUB>3</SUB>

Photoluminescence (PL), Raman, and transmission IR spectral measurements of porous silicon (PS) have been carried out during exposure to thermoelectrons and also subsequent exposure to H atoms, H 2 O and O 3 . The PL band of as-anodized PS was significantly decreased by the first exposure to thermoelectrons accompanied by the intensity reduction of the IR bands due to hydrogenated Si species (Si-H x ; x = 1-3). Upon subsequent exposure to H atoms the PL band intensity was almost recovered but never exceeded its original intensity. This PL recovery was accompanied by re-generation of Si-H x bottds. In contrast, an overshooting recovery in the PL intensity took place when thermoelectron-treated PS was exposed to H 2 O or O 3 . The obtained IR spectra showed that Si-O and/or Si-OH bonds were formed at the PS surface. These results demonstrate that the PL of the PS is quite sensitive to the oxygen-included surface bonds.

Unusual Photoluminescence Decay of Porous Silicon Prepared by Rapid Thermal Oxidation and Quenching in Liquid Nitrogen

Rapid thermal oxidation and quenching in liquid nitrogen (RTOQN) has been examined on anodized porous silicon (PS). The as-anodized PS samples exhibit a photoluminescence peak at 750 nm that decays instantaneously upon discontinuance of 325-nm He–Cd laser irradiation. In contrast, PS samples after RTOQN show a luminescence peak at 560 nm that decays very slowly (>1 s). In this paper some detailed RTOQN conditions leading to such a slow-decay photoluminescence are defined.

Anisotropic polarization memory in thermally oxidized porous silicon

Visible photoluminescence (PL) from thermally oxidized porous silicon (PSi) has been investigated in terms of polarization memory (PM). The PSi samples were prepared by anodization of (100)p+-Si wafers in a HF/ethanol solution, followed by thermal oxidation at 700–1000 °C. These oxidized PSi samples show significantly anisotropic PM which depends largely on the polarization direction of the excitation light with respect to their crystallographic axes. In addition, the anisotropic PM from samples oxidized at 800 and 900 °C shows an anomalous emission-energy dependence. It is also observed that thermal oxidation at 1000 °C results in a significant decrease in the degree of PM, although it increases with increasing oxidation temperatures for ⩽900 °C. These experimental results suggest that the PL from oxidized PSi cannot be explained as a simple extension of the PL from as-anodized PSi and should be attributed to several different origins.

Porous Silicon Multilayer Mirrors and Microcavity Resonators for Optoelectronic Applications

AbstractPorous silicon multilayer structures are easily manufactured using a periodic current density square pulse during the electrochemical dissolution process. The difference in porosity profile, corresponding to a variation in current density, is attributed to a difference in refractive index. Manipulating the difference in refractive index, high quality optical filters can be made with a maximum reflectivity peak ˜ 100%. The next logical step to further exploit these optical mirrors is to incorporate them into an LED device. The benefit of adding a multilayer mirror below a luminescent film of porous silicon is to significantly reduce the amount of light loss to the silicon substrate and increase the light output. However, oxidation is required to stabilize the as-anodized porous silicon film. This disrupts the overall index profile of the multilayer stack, causing the peak reflectance to blue shift. This phenomenon must be quantified and accounted before device implementation. We present a detailed study on the effects of oxidation temperature, gas environment, and annealing time of porous silicon multilayer structures in a device configuration.

Diamond deposition on as-anodized porous silicon: some nucleation aspects

We have investigated diamond deposition on porous silicon (PS) substrates without any pretreatment for nucleation enhancement. We find that the widely held expectation that, the highly rough nature of the PS surface would necessarily lead to a significant enhancement in the diamond nucleation density, does not hold. For the diamond deposition conditions used in this study, the nucleation density on PS (10 6 cm −2) is similar to that on untreated c-Si substrate. This indicates an absence of extra high-energy nucleation sites in spite of the micro-columnar surface of PS. As a result, the incubation time was often much longer on PS than on heavily scratched c-Si for the same diamond deposition conditions. Under such conditions, the XRD patterns of deposit on PS showed silicon carbide-related peaks in addition to weak diamond peaks. The results are compared with the earlier observations regarding diamond deposition on PS pretreated with diamond abrasive or negative DC bias. The comparative study, which needs further explanation, indicates a synergistic relation between the negative DC bias pretreatment and the effect of porosity of the substrate on diamond nucleation on PS substrate.

Improvement of Current Injection of Porous Silicon

n-i-p-n hydrogenated amorphous silicon (a-Si:H) layers were deposited onto porous silicon (PS) formed on p-type crystalline silicon (c-Si) by anodization and used to improve the current injection of a PS-based light-emitting diode (LED). The electroluminescence (EL) spectrum for the obtained PS-based LED with n-i-p-n a-Si:H layers showed the similar tendency as that of a photoluminescence (PL) signal for an as-anodized PS. The current conduction mechanisms of this PS-based LED were also studied.

Photoluminescence peak energy evolution for porous silicon during photo-oxidation and gamma -ray oxidation

The evolution of photoluminescence (PL) from a series of as-anodized porous silicon (PS) samples with peaks in a wide energy range has been investigated systematically during oxidation enhanced by either laser illumination or gamma -ray irradiation. Under both oxidation conditions, PL spectra initially located in the infrared-red region undergo a blue shift, while those initially located in the orange-yellow region experience a red shift. Eventually, all the peaks are pinned at fixed energies which are in an energy range narrower than that for as-anodized PS samples. These experimental results can be explained by the quantum confinement-luminescence centres model, which argues that photoexcitation proceeds in nanoscale silicon and photoemission through the luminescence centres outside nanoscale silicon, and also by assuming that during oxidation the main luminescence centres change from one kind to another and/or the relative contributions to the PL intensity from various kinds of luminescence centres vary.

Influence of molecule adsorption on porous silicon photoluminescence

As-anodized porous silicon (PS) was investigated in situ by photoluminescence (PL) during the adsorption of H 2O, O 2, C 2H 5OH and C 2(CN) 4 molecules. The PL intensity was increased during H 2O adsorption (to 130%) and decreased by adsorption of O 2 (to 30%), C 2H 5OH (to 22%) and C 2(CN) 4 (to 5%). A strong blue shift was observed during the C 2H 5OH adsorption. The reversibility of the adsorption-induced PL changes was compared for the different atmospheres. Reversibility by the simple cycle high vacuum pumping (HV)-adsorption-HV could be reached only for C 2H 5OH. The PL changes induced by H 2O and C 2(CN) 4 adsorption were good (H 2O) or partially (C 2(CN) 4) reversible in combination with heat treatment at 420 K in HV. The O 2-adsorption-induced PL changes are not reversible by simple or modified cycles.

Relaxation mechanisms of electronic excitation in nanostructures of porous silicon

The temperature dependence of porous silicon (PS) photoluminescence and the dependence of the contact potential difference (CPD) in the dark after illumination on the photon energy were investigated for as-anodized PS and for PS oxidized in air for various times after anodization and HF treatment. The effect of the temperature-induced red shift (with decreasing temperature) decreases with increasing HF treatment time and is discussed under the assumption of different lifetimes for radiative and non-radiative recombination. Various photon energy thresholds could be observed by CPD, indicating the role of charge carrier injection in the PS properties.

Aging phenomena of light emitting porous silicon

Various aging phenomena observed for anodized porous silicon (PS) have been reviewed and discussed. Low-temperature oxidation of as-anodized PS, and photo-induced effects, thermal treatment effects and charged-particle irradiation effects on characteristic optical properties of PS films are included in the present article. The aging phenomena reported so far are discussed in relation to the structural instability of PS films and to the mechanism of visible light emissions from PS.

Luminescence Properties of Porous Silicon

ABSTRACTWe report the results of a systematic study of the influence of the anodization and etching conditions, and chemical and thermal treatments on the properties of light-emitting porous silicon (PSI). PSI layers with stable, high efficiency photoluminescence (PL) tunable from the near infrared to the yellow have been obtained using p-type and n-type substrates. As the temperature drops below 100 K, the as-anodized PSI layers display striking changes in the PL spectra, which are absent in PSI layers that have received a post-growth treatment.