Visible Luminescence Of Porous Silicon Research Articles

Surface recombination model of visible luminescence in porous silicon

The most favourite candidate to explain visible photoluminescence (PL) of porous silicon (PS) is quantum confinement. It has been shown theoretically that decreasing size of silicon particles leads to the widening of the gap (physical confinement). According to calculations, red luminescence requires particle sizes that are too small to be directly observed in PS. On the other hand, chemical effects, other than etching, seem to affect the PL frequency. These effects indicate similarities to the PL of siloxene which is due to isolation of Si 6 rings by oxygen atoms (chemical confinement). Using self-consistent semi-empirical calculations, we have shown that the observed correlation between shift of the PL and Raman bands in PS can only be explained by chemical effects. Therefore, we suggest a surface recombination model of PL in PS, where the emission is determined primarily by physical confinement in features with nanometer dimensions on the surface of microcrystallites, while chemical substitutions tune the PL into the red. Empirical tight binding calculations on such particles show quantitative agreement with observed PL energies.

Recombination with larger than bandgap energy at centres on the surface of silicon microstructures

Previously it has been shown that the correlation between shifts in luminescence and Raman peak positions in various porous silicon samples cannot be explained by the size effect of the silicon microstructures, as postulated by the (physical) quantum confinement model. It is also unlikely that siloxene occurs in significant quantities in porous Si. In this contribution, a possibility is presented for radiative recombination of carriers on the surface of a silicon microstructure, to explain strong visible luminescence in porous silicon. Nanometre-size structures on the surfaces of silicon microcrystallites introduce localized resonances into the (bulk-like) bands of the crystallites, giving rise to larger-than-bandgap transitions. These surface structures act as electron- and hole-traps and, due to localization of the carriers, recombination is possible neglecting momentum selection rules. Results of semi-empirical calculations on model structures are presented.

Optical Properties of Silicon Nanostructures

AbstractStrong visible luminescence of porous silicon has triggered renewed general interest in the optical properties of silicon nanostructures. In addition to porous Si itself, Si‐based molecules, nanometer‐size Si particles, one‐dimensional Si chains, two‐dimensional Si layers, and many Si alloys are known to exhibit optical properties (absorption and luminescence) which strongly deviate from those of bulk crystalline Si. Here, we review and compare the main properties of the above‐mentioned systems and present a brief discussion of the experimental observations in terms of existing theories. Current prospects for Si‐based optoelectronic applications are also discussed.

Excitation spectroscopy of the visible photoluminescence of porous silicon

We present new results obtained from photoluminescence excitation spectroscopy experiments performed at 300 K on porous silicon layers with different porosities. The samples are made by electrochemical dissolution of (1 0 0) silicon wafers in a hydrofluoric solution. The data, which are essentially discussed in terms of quantum confinement in silicon wires, are actually in qualitative agreement with a quantum size effect. They show also that it would certainly be fruitful to investigate the carrier relaxation mechanims and the size distribution of the silicon wires or nanostructures responsible for the visible luminescence of porous silicon.

Origin of visible photoluminescence from porous silicon as studied by Raman spectroscopy

In this paper we discuss the different models proposed to explain the visible luminescence in porous silicon (PS). We review our recent photoluminescence and Raman studies on PS as a function of different preparation conditions and isochronal thermal annealing. Our results can be explained by a hybrid model which incorporates both nanostructures for quantum confinement and silicon complexes (such as SiH x and siloxene) and defects at Si/SiO2 interfaces as luminescent centres.

Role of hydrogen- and oxygen-terminated surfaces in the luminescence of porous silicon.

Room-temperature luminescence observed in porous silicon has been attributed either to direct radiative recombination in nanometer-size silicon in the porous silicon network, or to chemical complexes of silicon, hydrogen, and/or oxygen. In this paper, we attempt to correlate results of infrared and photoluminescence spectroscopy of freshly prepared and chemically treated porous silicon layers such that hydrogen or oxygen related species can be selectively enhanced on the surface of the porous silicon network. Our results indicate that it is difficult to establish a direct correlation between the visible luminescence in porous silicon and any particular chemical specie(s) such as silicon hydride(s), oxide, or siloxene.

Silicon Cluster Terminated by Hydrogen, Fluorine and Oxygen Atoms: A Correlation with Visible Luminescence of Porous Silicon

The effect of hydrogen, fluorine and oxygen surface termination atoms together with size dependence of the Si-cluster on the electronic structure are considered, using semiempirical molecular orbital calculations. The results show that the electronic structure of the Si-cluster is strongly affected by the presence of oxygen terminators which enhance the carrier localization. The transition probability between the electronic states support the high efficiency of visible luminescence. The hydrogen termination leads to a stable Si-cluster structure. Our calculations suggest that surface conditions of the Si-cluster and its size, are responsible for the visible luminescence properties of porous Si. The oxygen surface termination of the Si-ciuster is one of the requisite conditions to obtain luminescence in the red-orange region.

Size, Shape, and Crystallinity of Luminescent Structures in Oxidized Si Nanoclusters and H-Passivated Porous Si

ABSTRACTNear-edge and extended x-ray absorption fine structure measurements from a wide variety of H-passivated porous Si samples and oxidized Si nanocrystals, combined with electron microscopy, ir-absorption, α-recoil, and luminescence emission data, provide a consistent structural picture of the species responsible for the luminescence observed in these systems. For luminescent porous Si samples peaking in the visible region, i. e., ≤700nm, their mass-weighted-average structures are determined here to be particles–not wires, whose short-range character is crystalline – not amorphous, and whose dimensions – typically <15 Å – are significantly smaller than previously reported or proposed. These results depend only on sample luminescence behavior, not on sample preparation details, and thus have general implications in describing the mechanism responsible for visible luminescence in porous silicon. New results are also presented which demonstrate that the observed luminescence is unrelated to either the photo-oxidized Si species in porous Si or the interfacial suboxide species in the Si nanocrystals.

Electronic and structural properties of porous silicon

We review recent results concerning the structural properties and the origin of visible luminescence in porous silicon. The preparation of porous silicon is briefly described and current models for radiative recombination in porous Si are discussed. Special attention is given to a comparison between the properties of porous Si and those of siloxene and amorphous Si:O:H alloys, based on vibrational, optical, and spin resonance spectroscopy.

Effect of surface treatment on visible luminescence of porous silicon: Correlation with hydrogen and oxygen terminators

The effect of hydrogen and oxygen surface termination atoms on the visible luminescence of porous Si was investigated for underlying physical mechanism. Atmospheric thermal treatment up to 1000 °C was carried out to study the functional relationship between the surface coverage and photoluminescence (PL). The results show that oxygen incorporation induces surface modification that enhance the PL efficiency after the removal of all SiHx (x=1–3) species. The presence of oxygen atoms can also account for the observed PL redshift along with the usual blueshift. The molecular orbital calculations on the cluster modeling showed the significance of oxygen atoms in modifying the electronic structure of porous Si.

Electrical properties of luminescent porous silicon

The relation between the visible luminescence of porous silicon (PS) and its electrical properties has been investigated with an emphasis on carrier transport, photoconductivity, and the effects of an external electric field on the photoluminescence (PL). The PS layers are formed by anodization of p- or n-type Si wafers in an HF solution in the dark or under illumination. At first, the correlation of the electrical conduction mode with the PL efficiency is shown using a self-supporting PS film as a sample. Next, the photoconduction effects of PS for visible light are characterized for the experimental cells of the form (semitransparent thin Au film/PS/Si substrate/Al contact). Finally, the reversible electrical PL quenching is demonstrated. These results support the hypothesis that the visible luminescence of PS is based on surface-sensitive quantum confinement effects in Si nanocrystallites.

Temperature dependence of luminescence in porous silicon and related materials

We have investigated the temperature dependence of the visible luminescence of porous silicon and other Si-based luminescent materials (as-prepared and annealed siloxene, amorphous Si:O:H alloys) at temperatures between 10 and 359 K. In all samples, the decrease of the luminescence intensity, I( T), at temperatures > 100 K can be described by the relation I 0/ I( T) = 1 + exp( T/ T 0). At temperatures < 100 K, some of the strongly luminescent samples show a decrease of the luminescence intensity with decreasing temperature. We have found this decrease to be related to a “fine structure” in the luminescence spectra of yet unknown origin.

Excitons in silicon nanostructures

The electronic structure of σ-bonded silicon chains is calculated in the tight-binding approximation. The exciton spectrum is calculated by diagonalization of the full matrix of the Coulomb interaction taking into account the electron-hole exchange interaction. The energy, the size and the radiative lifetime of excitons are computed as a function of the size of the chain. The results show that chains of silicon atoms are probably not at the origin of the visible luminescence of porous silicon but could explain its fast band in the blue.

Slow decay dynamics of visible luminescence in porous silicon: Hopping of carriers confined on a shell region in nanometer-size Si crystallites.

We have studied the decay dynamics of visible photoluminescence (PL) from nanometer-sized Si crystallites fabricated by electrochemical etching of single-crystalline Si and laser breakdown of ${\mathrm{SiH}}_{4}$ gas. In the two types of Si crystallites, the slow-decay behavior of red PL in the time range ${10}^{\mathrm{\ensuremath{-}}6}$--${10}^{\mathrm{\ensuremath{-}}2}$ s is characterized by a stretched exponential function and the PL decay time scarcely depends on the size and the surface structure. The temperature dependence of the PL decay rate is identical with that of variable-range hopping of carriers in two-dimensional systems. It is concluded that the slow-decay PL is caused by the hopping-limited recombination in a quasi-two-dimensional interface region between the crystalline Si core and the surface layer.

Theory of optical properties of polysilanes: Comparison with porous silicon.

We have studied the electronic structure of \ensuremath{\sigma}-bonded silicon chains containing between 2 and 66 silicon atoms. Stable atomic configurations are obtained by minimization of the total energy. The exciton spectrum is calculated by diagonalization of the full matrix of the Coulomb interaction taking into account the electron-hole exchange interaction. We predict important atomic relaxations that give polaronic effects for free molecules and explain the observed Stokes shifts. We show that the luminescence properties of silicon chains strongly depend on the interactions between the molecules and the medium in which they are embedded. The energy, the size, and the radiative lifetime of excitons are computed and their dependence on the size of the molecules is analyzed. We deduce that chains of silicon atoms are probably not at the origin of the visible luminescence of porous silicon but could explain its fast band in the blue region.

Nanometer‐sized colloidal germanium particles: Wet‐chemical synthesis, laser‐induced crystallization and particle growth

Nanometer‐sized semiconductor particles are currently being intensively investigated in many fields of physics and chemistry—especially since the discovery of visible luminescence in porous silicon. This is one of the first papers to concentrate on the synthesis of IV – IV nanoparticles. A purely wet‐chemical room‐temperature synthesis is described that leads to stable nanometer colloidal germanium particles after laser illumination. The size of the crystallites can be controlled by adjusting the illumination conditions. The preparation technique detailed here is not restricted to germanium but can also be applied to other IV–IV semiconductor particles.

Electrical quenching of photoluminescence from porous silicon

We have studied the visible photoluminescence (PL) of porous silicon (PS) under the condition that a bias voltage is applied in the direction of the PS layer thickness. It is shown, for the first time, that the PL intensity is sharply decreased when increasing the voltage. This electrical PL quenching was completely reversible. This phenomenon is interpreted as to arise from field-enhanced tunneling of carriers between silicon crystallites. The result presented here strongly suggests that the visible luminescence of PS is not based on electronic transitions in some molecular substance, but on the radiative recombination in Si nanocrystallites.

Mechanism of the visible luminescence in porous silicon

Highly efficient photoluminescence (PL) in the visible light range has been found in porous Si (PS). The quantum confinement model (QCM) for the PL mechanism although explains why the PL energy of PS is in the visible light range, it meets great difficulties in explaining blue and especially red shifts of the PL peak position of PS with varying temperature, pressure, anodic-etching conditions and post-treatments. Considering the rational parts of the previous models and also the contradictions between these models and experimental results, a new model is proposed in this work which explains qualitatively a lot of important experimental results without severe difficulties. We emphasize that there is a lack of nonradiative recombination centers inside the nanoscale Si unit and there is a much greater probability of radiative recombination through luminescence centers outside nanoscale Si units than through band to band transitions inside the nanoscale Si units and these two factors explain why PS has a highly efficient PL. The suggested model may also be applicable to some other Si-based luminescent materials other than PS.