Visible Photoluminescence Of Porous Silicon Research Articles

Quenching influence of cell culture medium on photoluminescence and morphological structure of porous silicon

In this work, the degradation of visible photoluminescence of porous silicon (PSi) under the influential actions of cell culture medium has been mainly studied in order to comprehend the quenching mechanisms necessitating the cell growth on spongy-like-silicon structures, which could form either micro- and/or nano-dimensional morphologies after stain-etching of the poly- or single-crystalline Si surfaces. Quenching effect of the neuron culture medium on visibly luminescent and non-luminescent porous silicon is found to be quite obvious so that this step of the culture process, especially, over nanostructured silicon is extremely essential for a variety of bionanotechnological applications.

Visible photoluminescence of porous silicon covered with an HfON dielectric layer

With HfON filling the holes in porous silicon (PS), films with improved photoluminescence (PL) at room temperature were prepared. A strong blue peak at 425 nm and a red peak at 690 nm were observed in PL spectra. It is believed that the quantum-limited effect (QLE) and the polycrystalline structure of HfON is responsible for the observed PL peaks. The stoichiometric proportion of N/O in the HfON layer has also a great influence on the intensity of blue light emission. Finally, the temperature quenching effect was observed to be greatly weakened for the incorporation of HfON.

Magnetic field effect on the visible photoluminescence of porous silicon

The excitation and emission spectra of red photoluminescence of Si wire nanostructures were studied in dependence on surface morphology and influence of magnetic field. The magnetic field of 0.5 T reduced the emission intensity by 10% at all emission spectral range. Comparison of the two photoluminescence models – the quantum confinement and the hot carrier ballistic ones – favours the second model based on the assumption of the excitation of interface oxide defect related luminescence by hot quazi-ballistic carriers created by illumination. The effective mobility of hot electrons is estimated as 6000 cm2/Vs, which greatly exceeds normal electron mobility (1900 cm2/Vs) in Si and thus confirms the presence of hot carrier ballistic motion. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Visible photoluminescence in porous silicon prepared in different conditions ‐ temperature dependence and decay

AbstractMeasurements of the visible photoluminescence and the decay of the photoluminescence, performed at different temperatures on porous silicon films, prepared by the anodization process under different conditions on (100) p‐type silicon wafers are reported. A separation of the broad visible band of the photoluminescence in two subbands with maxima situated at 1.54 and 1.72eV at room temperature related to the preparation conditions is obtained. This separation is still present at low temperatures but is very weak. The PL decay process is well described by a bimolecular recombination. The decay time α−1 is in the order of microseconds at room temperature and nanoseconds at liquid nitrogen temperature.

Polarization retention in the visible photoluminescence of porous silicon.

The polarization state of the visible photoluminescence of porous silicon excited with linearly polarized light has been examined. The photoluminescence was linearly polarized in the same direction as the excitation light, independent of the crystallographic orientation of the sample. The degree of linear polarization decreased significantly with increasing emission wavelength. However, no definite trend of its excitation-wavelength dependence was observed. An interesting correlation was found to exist between the photoluminescence efficiency and the degree of linear polarization, where polarized photoluminescence was more prominent in photoluminescence-inefficient samples. Furthermore, the polarization retention phenomenon was found to be independent of temperature. These experimental results can be explained by the radiative recombination of excitons localized within optically anisotropic and randomly distributed Si nanocrystallites.

Triplet excitons in porous silicon and siloxene

The microscopic nature of the excited state responsible for the strong visible photoluminescence in porous silicon and siloxene is determined by optically detected magnetic resonance (ODMR). The observation of dipole-forbidden Δm = ±2-transitions proves that this excited state is a triplet exciton. The allowed Δm = ±2-transitions has the characteristic shape of a Pake-doublett due to spin-spin interaction, with a linewidth of ≈500 G independent of the photon energy monitored. These results are qualitatively incompatible with geometric quantum confinement, and point to a molecular origin of the radiative center.

Interpretation of the temperature dependence of the strong visible photoluminescence of porous silicon

The temperature dependence of the strong visible photoluminescence (550–850 nm) is studied in differently prepared porous silicon samples. The variation in the photoluminescence intensity with temperature is a result of a decrease in the radiative decay time and an increase in the non-radiative recombination process with increasing temperatures. The strong visible photoluminescence is interpreted by a recombination of singlet and triplet excitons. The singlet-triplet splitting is comparable for all samples but depends on the detection wavelength and on sample preparation. We present similar data for the recombination process in siloxene which supports the model of a common origin of the strong visible photoluminescence in these very differently prepared samples.

Defect-based model for room-temperature visible photoluminescence in porous silicon.

Defect-based model for room-temperature visible photoluminescence in porous silicon.

Mechanisms of visible photoluminescence in porous silicon.

We propose a model for porous silicon, an irregular structure obtained by removing some silicon atoms randomly from a perfect silicon crystal. It is shown by using two-dimensional clusters with a tight-binding model that this model exhibits the energy-gap widening and the nonexponential decay of the photoluminescence (PL), which are in good agreement with the observed properties of porous silicon. In this model, the energy-gap widening is due to the localization of eigenstates caused by the randomness of the structure. The distributions in both size and position of the localized eigenstates, i.e., a statistical effect, yield nonexponential PL decay which is describable by the stretched exponential function. A dynamical effect due to electron and hole hopping between the eigenstates explains the observed strong energy dependence of the PL lifetimes.

Photo-assisted tuning of luminescence from porous silicon

A method is proposed to control the wavelength of visible photoluminescence (PL) from porous silicon (PS). This is based on the post-anodization illumination technique which employs a filtered light with various maximum cutoff photon energies. As the illumination time increases, the PL spectra shift toward the higher energy side. This blue shift tends to stop at the time when the PL peak energy becomes close to the cutoff photon energy. The emission wavelength can be controllably tuned in a wide spectral range by changing the cutoff filter. These results give us a strong indication that the visible PL of PS is closely related to interband excitation in Si nanocrystallites, not to some surface chemical compounds.

P b-defects and visible photoluminescence in porous silicon

We present the results of a correlated study of the neutral (1 1 1) P b point defect concentration and the intensity of the visible light emission of p + and n + porous silicon samples in as-prepared, ultrahigh vacuum (UHV) annealed (200–600°C) and 1000°C thermally oxidized samples of different porosities (65% and 80%). In all type of samples the (1 1 1) P b center is the dominant paramagnetic defect, whose concentration is increased by annealing or high temperature oxidation by up to two orders of magnitude. The visible photoluminescence (PL) intensity is strongly decreased after hydrogen desorption at T ⪖ 450°C. Our results on the vacuum annealed, as well as on the thermally oxidized samples, show a qualitative correlation between the (1 1 1) P b center concentration and the intensity of the photoluminescence band centered at ≈ 1.76eV only for defect concentrations higher than 5 × 10 11 cm −2.

Strong room-temperature photoluminescence of hydrogenated amorphous silicon oxide and its correlation to porous silicon

A set of hydrogenated amorphous silicon oxide (a-SiOx:H) films have been fabricated by plasma enhanced chemical vapor deposition. Some of the films exhibit strong room-temperature photoluminescence and others do so only after annealing at high temperature. The variation of photoluminescence after different annealing treatments for these films is found to be similar to that of porous silicon. Assisted by infrared spectra it is concluded that the photoluminescence from a-SiOx:H is originated from the small a-Si clusters or microcrystalline silicon embedded in the amorphous SiOx network. This argument may support a similar model proposed to explain the visible photoluminescence of porous silicon.

An oligosilane bridge model for the origin of the intense visible photoluminescence of porous silicon

Oligosilanes bridging microgaps between silicon microcrystallites were investigated as a possible model for the origin of the intense visible photoluminescence of porous silicon by means of the semiempirical molecular-orbital method. The incomplete structural relaxation of the oligosilane bridge after the photoexcitation was found to be a key factor to give the visible photoluminescence. The calculated structure-insensitive photoexcitation energy around 3.3 eV and the structure-sensitive light emission energy around 1.2–2.1 eV are consistent with the experimental evidence. The sufficient transition probabilities between the concerning electronic states support the high efficiency of photoluminescence. Durabiliy of the structure under the photoexcitation was also suggested. The model is valid even if the silicon microcrystallites are partly or thoroughly replaced by silicon oxides particles as is more realistic for the porous silicon exposed to the air.

Optical Properties of Porous Silicon and Small Silicon Clusters: Search for the Origin of Visible Photoluminescence of Porous Silicon

We have studied and compared the optical properties of both porous Si and the chemically synthesized small Si clusters. In planar and cubic Si clusters, broad photoluminescence (PL) spectra were observed at the visible region. The characteristics of broad visible PL and PL excitation spectra of porous Si were similar to those of small Si clusters.

Fast Photoluminescence from Porous Silicon

ABSTRACTTime-resolved studies of the visible photoluminescence in porous silicon with three different coverages of the internal surface are reported. We use aged, naturally oxidized porous Si (oxihydride), rapid thermal processed material (oxide) and samples stored in HF (pure hydride). A new, fast luminescence band in the blue-green spectral range and with response time less than 100 ns is observed at room temperature in each of the samples, although with different intensities. The observations prove that this is not an oxide-defect luminescence. We speculate on mechanisms for the origin of the fast luminescence in nanometer-size crystallites of Si.

Excitation Spectra of the Visible Photoluminescence of Anodized Porous Silicon

ABSTRACTPorous silicon samples photoluminous in orange, pink or greenish-yellow were formed by anodization. The shapes and wave length positions of their photoluminescence peaks were not influenced by the excitation wavelength. The photoluminescence intensity at any wavelength between 520nm and 800nm varied as a function of the excitation wavelength In accordance with the imaginary part of the spectral refractive index of bulk silicon. Thus the excitation process responsible for the visible photoluminescence of porous silicon seems to be bulk-silicon-like.