Polarization phenomena in the optical properties of porous silicon

We examine the polarization memory effect for porous Si excited by linearly polarized light. The various observations for the red-luminescing, slow band are discussed in the general framework of particle shape asymmetry. We show that because of the intrinsically nonlinear luminescence response, measurement parameters influence the polarization response. The preparation of porous Si with photoassisted etching is found to control the polarization retention parameter ρ. Using linearly polarized light during etching produces in-plane asymmetries. We find a substantial ρ-anisotropy linked to crystal symmetry planes and axes as a consequence of anisotropic etching. The effects are discussed with reference to current models of the light emission mechanism.     

Irreversible quenching of luminescence in porous silicon

The influence of applied voltage on photoluminescence (PL) in porous silicon was studied. A strong PL band around 680 nm was observed when excited by a 300 nm ultraviolet light with no voltage applied, but upon increasing the bias voltage, a strong and progressive decrease of the PL intensity was observed leading finally to a complete quenching of the emitted light at 1.80 V. The peak position of the emission appears to be stable. This effect is completely irreversible, and the spectra depend on the increased voltage to the sample and corresponding temperature increase. Nonradiative recombination resulting from the thermal oxidation was suggested to be responsible for the quenching.     

Solvent detection using porous silicon optical waveguides

Since the first report on the use of porous silicon as an optical waveguide medium in 1995, significant development has been made towards the understanding and applicability of such material. Here, the introduction of solvents (acetone, methanol, and propan-2-ol) into the pores is shown to dramatically reduce the loss of the waveguides, in a reversible manner. Both the magnitude and duration of this effect are sensitive to the solvent introduced. In some waveguides, for example, the measured loss (at 0.633 μm) falls by as much as 34 dB cm⁻¹ on the introduction of acetone. Theoretical estimates of the effect of solvents on the interfacial scattering loss confirm this as the origin of the observed reductions. These results, combined with the fact that a substantial portion of the guided-mode field interacts with the solvent, indicate an enhanced sensitivity for sensor applications may be achievable.     

Morphological changes in porous silicon nanostructures: non-conventional photoluminescence shifts and correlation with optical absorption

In this paper, we show that the photoluminescence (PL) shifts of p-type porous silicon (PS) are mainly attributed to some morphological changes related to anodisation conditions. We discuss how differences in the stirring and nature of the electrolytic solution can lead to morphological changes of the PS layers. It has been found that when PS is formed in pure aqueous HF solution, it can exhibit a non-conventional and reproducible “porosity – PL peak relationship”. By correlating the PL spectral behaviour and PS morphology throughout a quantum-confinement model, we explain both conventional and non-conventional PL shifts. Correlation of PL and optical absorption (OA) shows that the PL peak energy and the optical absorption edge of PS exhibit the same trend with size effect. The spectral behaviour of OA with regard to that of PL is well analysed within the quantum-confinement model throughout the sizes and shapes of the nanocrystallites forming PS. The value of the effective band gap energy determined from the calculated 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.     

All porous silicon microcavities: growth and physics

The spontaneous emission of a material can be controlled by placing it in a micron-sized optical cavity. In this paper we introduce the subject and we discuss the realization, the physics and perspective applications of all porous silicon microcavities. The emission properties of the cavities have been characterized as a function of the temperature, of the excitation power and of the response time. Coupled microcavities are demonstrated. Modeling of the structure have been performed on the basis of a transfer matrix approximation.     

Studies of diffusion coefficients in disordered porous matrices confined in a slit-pore

We present a series of molecular dynamics simulations to study the structure of porous matrices confined in a slit-pore. The matrices were prepared by two different methods. In the first method we used direct simulations of a fluid at a fixed density and the matrix was taken from the last configuration of its particles. In the second method we simulated a binary mixture where one of the components served as a template material and the final porous matrix configuration was obtained by removing template particles from the mixture. In both methods the matrices were confined by two parallel walls (slit-pore) modeled by continuous solid surfaces. The results show that the matrix structure and porosity were affected by the method of preparation of the porous matrices. Moreover, we found smaller void cavities in these matrices than in matrices prepared without walls. Finally, diffusion of a fluid inside the matrices was investigated and it was found that the diffusion coefficient did not decrease with the fluid density, and presented a maximum at certain values of the fluid density.     

Reverse biased porous silicon light-emitting diodes for optical intra-chip interconnects

We have reported recent progress in development of the integrated optoelectronic unit on a Si chip. The developed optoelectronic unit includes a porous Si light-emitting diode (LED) connected with a photodetector by an alumina waveguide. Main attention has been devoted to the enhancement of LED parameters. Quantum efficiency as high as 0.4% has been reached. The delay time of 1.2 ns and the rise time of 1.5 ns have been measured for the diodes. Further improvements are also discussed.     

Photoluminescence and electroluminescence from porous silicon

The properties and origins of the red, blue and infrared photoluminescence bands of porous silicon are reviewed and discussed in the light of the models that have been proposed to explain the experimental and theoretical results. The red band is due to quantum confinement possibly supplemented by surface states; the blue band is linked to the presence of silicon dioxide; the infrared band is correlated with dangling bonds and bandgap luminescence in large crystallites. The fabrication and characterization of light-emitting devices made of porous silicon are reported and discussed with respect to critical issues such as the device stability, efficiency, modulation speed, emission wavelength, and compatibility with microelectronic processing.     

Study of porous silicon optical waveguides impregnated with organic dyes

Planar waveguides were made using oxidised porous silicon layers. Then, they were impregnated with Congo Red or Disperse Red 1 dyes. Optical losses were investigated before and after impregnation. In our case, the losses of impregnated waveguides were always higher than those of non-impregnated ones. In order to achieve a better understanding of the origin of these losses, we not only studied the absorbance of solutions which would impregnate the porous layers but also the reflectance spectra of the obtained composite materials. According to the measurements, the increase in losses in the visible spectrum depends on the intrinsic absorption of the dye while in NIR, the increase would be due to an accumulation of dried dye on the surface of the waveguide which would give rise to the surface scattering losses.     

Annealing and amorphous silicon passivation of porous silicon with blue light emission

The photoluminescence (PL) of the annealed and amorphous silicon passivated porous silicon with blue emission has been investigated. The N-type and P-type porous silicon fabricated by electrochemical etching was annealed in the temperature range of 700-900 °C, and was coated with amorphous silicon formed in a plasma-enhanced chemical vapor deposition (PECVD) process. After annealing, the variation of PL intensity of N-type porous silicon was different from that of P-type porous silicon, depending on their structure. It was also found that during annealing at 900 °C, the coated amorphous silicon crystallized into polycrystalline silicon, which passivated the irradiative centers on the surface of porous silicon so as to increase the intensity of the blue emission.     

Optical properties of erbium-doped porous silicon waveguides

Planar and buried channel porous silicon waveguides (WG) were prepared from p⁺-type silicon substrate by a two-step anodization process. Erbium ions were incorporated into pores of the porous silicon layers by an electrochemical method using ErCl₃-saturated solution. Erbium concentration of around 10²⁰ at/cm³ was determined by energy-dispersive X-ray analysis performed on SEM cross-section. The luminescence properties of erbium ions in the IR range were determined and a luminescence time decay of 420 μs was measured. Optical losses were studied on these WG. The increased losses after doping were discussed.     

Voltage-tunable photo- and electroluminescence of porous silicon

Experimental results showing two electrically induced phenomena, namely the voltage-tunable electroluminescence (VTEL) and the voltage-induced quenching of porous silicon photoluminescence (QPL) are given. In both cases, a spectral shift as large as 300 nm can be recorded for an external bias variation of only 0.5 V. This spectral shift is characterised by a blue-shift of the whole EL line in the case of the VTEL whereas it results from a progressive and selective quenching starting by the low-energy part of the luminescence line in the case of the QPL experiments. The origin of this spectral shift is discussed in relation with an electrically induced selective carrier injection into the silicon nanocrystallites accompanied with an enhancement of the non-radiative recombination taking place by an Auger relaxation process. Finally, it is shown that a partial oxidation of the porous silicon layer leads to a complete loss of the selectivity of these two phenomena. This result is qualitatively discussed by considering the voltage drop distribution between the substrate and the silicon nanocrystallites. The voltage drops are modified by the growth of the oxide layer on the nanocrystallite surface leading to a modification of the energy barriers at the crystallite boundaries.     

Ellipsometric study of refractive index anisotropy in porous silicon

Porous Si layers of different thicknesses were prepared by anodising p⁺-type Si substrates with a resistivity of 0.01 Ω cm. The porosity of the samples ranged from 23% to 62%. The refractive index values for the ordinary and extraordinary rays were determined by multiple angle of incidence ellipsometry, from which an optical anisotropy parameter varying from 13% to 20% was obtained. The porous Si layers were modelled as uniaxially anisotropic films on an isotropic substrate, with an optical axis perpendicular to the sample surface. The morphological anisotropy which is typical for the p⁺-type porous Si with a predominating cylindrical geometry is responsible for these optical properties. All the porous Si layers studied were found to be optically negative.     

Stable ultraviolet photoluminescence emission in n-type porous silicon

This very paper is focusing on the investigation of porous silicon preparation with n-type silicon wafer by means of electrochemical anodization in the dark and, particularly, on its stable ultraviolet photoluminescence emission. A lateral electrical potential was applied, for this purpose, on silicon wafers, driving the electrons away and letting holes appear on the surface of the silicon wafer to enhance the electrochemical etching process. Characterizations have been made with scanning electronic microscope, fluorescence spectrophotometer and Fourier transform infrared spectroscope. An ultraviolet photoluminescence emission of 370 nm is found in the as-prepared n-type porous silicon, which seems to be well associated with the formation of oxygen-related species (twofold coordinated silicon defect) during the anodic oxidation. The result characterized by photo-bleaching performance indicates that the ultraviolet photoluminescence emission is so stable—only 7% reduction within 3600 s. Meanwhile the morphology of as-prepared n-type porous silicon is investigated.     

Photoluminescence studies of porous silicon microcavities

Narrow photoluminescence peaks with a full-width at half-maximum of 14–20 nm are obtained from porous silicon microcavities (PSM) fabricated by the electrochemical etching of a Si multilayer grown by molecular beam epitaxy. The microcavity structure contains an active porous silicon layer sandwiched between two distributed porous silicon Bragg reflectors; the latter were fabricated by etching a Si multilayer doped alternatively with high and low boron concentrations. The structural and optical properties of the PSMs are characterised by scanning electron microscopy and photoluminescence (PL). The wavelength of the narrow PL peaks could be tuned in the range of 700–810 nm by altering the optical constants.     

Steady-state and time-resolved spectroscopy of porous silicon

We present experimental data on steady-state properties, time-resolved properties and on polarization characteristics of porous silicon photoluminescence and models for the decay processes of the red-orange band. The manifold manifestation of inhomogeneous broadening of this band in emission, excitation, polarization, kinetics and degradation supports the model in which porous silicon is treated as a network of crystallites connected via an oxide interface. Spectral inhomogeneties of the red-orange band can be described in terms of varying shape and size of silicon clusters. The polarization of emission is explained by coexistence of dot-like and wire-like entities, i.e. spherical and non-spherical clusters. The relative weight of these species determines the polarization degree, whereas the kinetics are controlled by the transport of excitations among the clusters. The decay is modeled by a modified stretched exponential function with the local lifetime, the migration lifetime, and a scaling factor. The latter is determined by the dimensionality of the space available for migration which was found to be close to but less than unity. On the nanosecond range two distinct bands in the blue-green region are evaluated that need further studies for interpretation. Generally, arguments are proposed in favor of a quantum confinement origin of the red-orange band and a bridge between quantum-wire and quantum-dot models is provided.     

The room temperature oxidation of porous silicon

The room temperature oxidation of porous silicon was studied using isothermal methods. The oxidation was found to depend on the type of the porous silicon. The microcalorimetric signals from the oxidation of the p⁺- and n-type porous silicon in dry air were different. In humid air the signals from the oxidation could not be distinguished from the strong signal due to adsorption of water vapour, but when the samples were placed in water similar differences were observed. The reason for differences in reactions is discussed. The oxidation in different liquids was also studied. The signal from reactions in methanol and ethanol were found to be 100 times higher than in water. In FTIR studies the reaction gas produced by reactions between alcohols and the porous silicon, silane (SiH₄) was found in the gas. Traces of SiOCH₃ and SiOC₂H₅ groups were also found in FTIR spectra indicating Si---O---C_xH_y passivation of the surface.     

Surface analysis of thermally annealed porous silicon

Quasi-monocrystalline porous silicon (QMPS) has high potential for photovoltaic application for its enhanced optical absorption compared to bulk silicon in the visible range of solar spectrum. In this study, QMPS was formed from low porosity (∼20-30%) porous silicon (PS) produced by electrochemical anodization, and thermal annealing in the temperature range 1050-1100 °C under pure hydrogen ambient for a duration of 30 min. We analyzed the material surface by grazing incidence X-ray diffraction (GIXRD), field emission scanning electron microscopy (FESEM), atomic force microscopy (AFM) and dynamic secondary ion mass spectroscopy (SIMS) study. The crystallinity was confirmed by GIXRD while FESEM studies revealed that the surface layer is pore free with voids embedded inside the body. AFM studies indicated relatively smooth and uniform surface and the dynamic SIMS study showed the depth profiles of impurities present in the material.     

Local order in light emitting porous silicon studied by XEOL and TEY

The local structure of porous silicon has been studied exciting its optical luminescence by X-rays (XEOL). The photoluminescence yield and the total electron yield (TEY), recorded simultaneously as a function of the X-ray energy at the Si K edge, give rise to the extended X-ray absorption fine structures (EXAFS). Analysis of EXAFS data confirms that the optical luminescence of porous Si originates from the nanocrystalline cores and shows that XEOL–EXAFS and TEY–EXAFS are sensitive to different Si local environment. It can be assumed that XEOL–EXAFS is related only to the light emitting sites while TEY–EXAFS is sampling both luminescent and non-luminescent Si sites.     

Photoluminescence of porous silicon coated by SILD method with LaF3 nanolayers

The method of lanthanum fluoride passivating layer synthesis in the matrix of porous silicon by successive ionic layer deposition was elaborated and optimized. Luminescence and FTIR of obtained structures demonstrate the crucial role of the chemical composition of silicon nanocrystallite surface in the formation of radiative recombination channels and in the stability of porous silicon photoluminescence. The combination of high optical transparency of LaF₃ layers and low recombination losses in silicon covered with such layers allows to recommend the lanthanum fluoride film as an effective passivating coating for silicon optoelectronics devices.