Refractive Index Of Porous Silicon Research Articles

Modification of optical properties of oxidised porous silicon by pore filling

We report the experimental study of changes in optical properties of oxidised porous silicon induced by pore filling with salts from aqueous solutions. Our experiments demonstrate that pore filling is a simple and effective method for tuning the refractive index of porous silicon, which can be applied locally. This method can be used for creation of waveguides and other planar optical components. Our results correlate with previously reported works on the subject and provide information, which can be useful for the future investigation of porous materials.Our samples were thin porous layers on top of Si wafers. We used aqueous solutions of several phosphate salts with various concentrations for filling the pores. After the pore filling, the dry samples were examined using optical spectrometry, microscopy (optical and electron) and EDS methods.The paper provides description of the experimental procedures, data analysis and comparison with theoretical calculations.

Optical performance of hybrid porous silicon–porous alumina multilayers

In this work, we study the optical response of structures involving porous silicon and porous alumina in a multi-layered hybrid structure. We performed a rational design of the optimal sequence necessary to produce a high transmission and selective filter, with potential applications in chemical and biosensors. The combination of these porous materials can be used to exploit its distinguishing features, i.e., high transparency of alumina and high refractive index of porous silicon. We assembled hybrid microcavities with a central porous alumina layer between two porous silicon Bragg reflectors. In this way, we constructed a Fabry-Perot resonator with high reflectivity and low absorption that improves the quality of the filter compared to a microcavity built only with porous silicon or porous alumina. We explored a simpler design in which one of the Bragg reflectors is replaced by the aluminium that remains bound to the alumina after its fabrication. We theoretically explored the potential of the proposal and its limitations when considering the roughness of the layers. We found that the quality of a microcavity made entirely with porous silicon shows a limit in the visible range due to light absorption. This limitation is overcome in the hybrid scheme, with the roughness of the layers determining the ultimate quality. Q-factors of 220 are experimentally obtained for microcavities supported on aluminium, while Q-factors around 600 are reached for microcavities with double Bragg reflectors, centred at 560 nm. This represents a four-fold increase with respect to the optimal porous silicon microcavity at this wavelength.

Simple method for measuring the porosity, thickness and refractive index of porous silicon, based on the Fabry-Pérot interference spectrum

This article presents a technique to estimate, nondestructively, the thickness, porosity and refractive index of porous silicon samples. The technique is based on the Fabry-Perot interference spectrum analyzed by the Reflectometric Interference Fourier Transform Spectroscopy (RIFTS). The technique, also known as SLIM - Spectroscopic Liquid Infiltration Method (Segal 2007) takes into account the infiltration of a liquid with known refractive index inside the pores of the samples.

(Invited) Thermal Carbonization of Porous Silicon: The Current Status and Recent Applications

Porous silicon (PSi) is a versatile material for a wide range of applications. Its large surface area combined with easily adjustable pore morphology and surface chemistry offers an excellent platform for many different applications. For example, the refractive index of PSi is strictly depending on the porosity and can be controlled by the current density of electrochemical etching. This enables precise control of the refractive index and allows the fabrication of highly complex optical structures for many interesting applications. However, for decades, the biggest stumbling block for PSi applications has been the chemical instability of PSi. As-anodized PSi is hydrogen terminated and it slowly oxidizes even at room temperature. This instability of as-anodized PSi is unacceptable in most applications. On the other hand, intentional oxidation of PSi, in order to improve the stability, usually leads to too high resistivity for electrical applications and changes the optical properties of PSi too drastically. Moreover, the oxidized PSi is not chemically stable enough for certain type of applications, especially in aqueous conditions. It may react adversely with drugs and its functionalization needs to be done through silanization chemistry which shares the silicon oxide bonding related instability, e.g., for targeted drug delivery applications. Thermal carbonization by acetylene to stabilize PSi was introduced in 2000 [1]. It produces a non-stoichiometric silicon carbide layer on the PSi, which is very stable in harsh environments, like in NaOH, KOH, HF etc. solutions with extreme pH values. It also increases the conductivity of the PSi, which is beneficial for electrical sensor and energy storage applications and it has been found to be nontoxic both in in vitro and in vivo applications. Therefore, the thermally carbonized PSi (TCPSi) has been used in various different applications ranging from gas sensors and biosensors to targeted drug delivery, bioimaging and electrochemical supercapacitors since its introduction [2–9]. In the presentation, the current status and novel applications, in which thermally carbonized PSi has been used, will be discussed. In addition, some new information about the physical and chemical properties of TCPSi will be introduced. REFERENCES: [1] J. Salonen, V.-P. Lehto, M. Björkqvist, E. Laine, L. Niinistö, Studies of Thermally-Carbonized Porous Silicon Surfaces, Physica Status Solidi (a), 182 (2000) 123-126. [2] V. Torres-Costa, R.J. Martin-Palma, J.M. Martinez-Duart, J. Salonen, V.P. Lehto, Effective passivation of porous silicon optical devices by thermal carbonization, Journal of Applied Physics, 103 (2008) 4. [3] J. Tuura, M. Bjorkqvist, J. Salonen, V.P. Lehto, Electrically isolated thermally carbonized porous silicon layer for humidity sensing purposes, Sensors and Actuators B - Chemical, 131 (2008) 627-632. [4] T. Jalkanen, E. Mäkilä, Y.I. Suzuki, T. Urata, K. Fukami, K. Sakka, J. Salonen, Y.H. Ogata, Studies on Chemical Modification of Porous Silicon-Based Graded-Index Optical Microcavities for Improved Stability Under Alkaline Conditions, Advanced Functional Materials, 22 (2012) 3890-3898. [5] B. Sciacca, S.D. Alvarez, F. Geobaldo, M.J. Sailor, Bioconjugate functionalization of thermally carbonized porous silicon using a radical coupling reaction, Dalton Transactions, 39 (2010) 10847–10853. [6] P.J. Kinnari, M.L.K. Hyvönen, E.M. Mäkilä, M.H. Kaasalainen, A. Rivinoja, J.J. Salonen, J.T. Hirvonen, P.M. Laakkonen, H.A. Santos, Tumour homing peptide-functionalized porous silicon nanovectors for cancer therapy, Biomaterials. 34 (2013) 9134–9141. [7] L.M. Bimbo, M. Sarparanta, H.A. Santos, A.J. Airaksinen, E. Mäkilä, T. Laaksonen, L. Peltonen, V.-P. Lehto, J. Hirvonen, J. Salonen, Biocompatibility of thermally hydrocarbonized porous silicon nanoparticles and their biodistribution in rats, ACS Nano. 4 (2010) 3023–3032. [8] J. Salonen, Drug Delivery with Porous Silicon, in: L. Canham (Ed.) Handbook of Porous Silicon, Springer International Publishing, 2014, Ch. 91, pp. 909-919. [9] S. Chatterjee, R. Carter, L. Oakes, W.R. Erwin, R. Bardhan, C.L. Pint, Electrochemical and Corrosion Stability of Nanostructured Silicon by Graphene Coatings: Toward High Power Porous Silicon Supercapacitors, Journal of Physical Chemistry C. 118 (2014) 10893–10902.

Staggered Padé wavelength distribution for multi-Bragg photonic mirrors

High reflective mirrors are commonly used in solar concentration devices. It has been shown that photonic mirrors are suitable for this purpose. However, despite their high reflectivity, they absorb radiation and heat up showing degradation. To overcome this problem here we propose to enhance the reflectivity of the mirrors. We report an improved and easy method to design broadband high reflective mirrors and build them from porous silicon multilayers. Those are composed of a continuous arrangement of submirrors reflecting each one at a given wavelength. This method consists in staggering the wavelength distribution following a relation based on the Padé approximant. We simulate the reflectance spectra using the transfer matrix method taking into account the experimental complex refractive index of porous silicon. The comparison between the experimental and the reflectance spectra shows a good agreement. With this technique we have fabricated high quality dielectric mirrors that can be used for concentrated solar energy applications among others.

Electrochemical Fabrication of Porous Silicon Structures for Solar Cells

Porous Silicon (PS) has found its broad application for optoelectronics devices specially solar cells applications due to its efficient antireflection coating. This material has advantages such as broaden band gap, wide absorption spectrum and high optical transmission range from 700-1000nm. In this paper, the experimental study of electrochemically prepared porous silicon structures is presented. With an increase in etching time, the thickness and refractive index of porous silicon were found to be increased and decreased respectively. The XRD measurement has confirmed the crystallinity nature of porous silicon as that of bulk silicon. A broad peak of red emission has also confirmed by photoluminescence measurement and it has been attributed to SiHx groups which are confirmed by FTIR spectra. Pyramid like surface over the entire surface is confirmed by AFM measurement which is responsible for the reduction of reflection of photons and hence can enhance the conversion efficiency of solar cells.

Influence of current density on refractive index of p-type nanocrystalline porous silicon

Porous Silicon (PS) layers have been prepared from p-type silicon wafers of (100) orientation. SEM, XRD, FTIR and PL studies were done to characterize the surface morphological and optical properties of PS. The porosity of the PS samples was determined using the parameters obtained from SEM images by geometric method. The refractive index values of the PS samples as a function of porosity were determined by Effective Medium Approximation methods. The influence of current density on porosity and refractive index of PS, were discussed. SEM images indicated that the pores are surrounded by a thick columnar network of silicon walls. This porous silicon layer can be considered as a sponge like structure. The sizes of PS nanocrystallites were determined by XRD studies. FTIR spectra indicated that the porous layer contain SiHn complexes. PL study reveals that there is a prominent emission peak at 606 nm. No spectral shift was observed. These results suggest that this nanocrystalline porous silicon could be a potential candidate for optical as well as optoelectronic device applications.

A novel biosensor with enhanced sensitivity based on grating coupled double-layered porous silicon waveguide

The enhanced sensitivity of a guided mode biosensor is analyzed by employing double-layered porous silicon grating structures. The grating-coupled waveguide structure consists of two porous silicon grating layers with different refractive indices. Simulations are carried out by changing the refractive index, which is due to the binding of biological molecules on the porous silicon pore can increase the refractive index of porous silicon. The numerical results show that this novel guided mode biosensor with a double-layered grating can provide not only a very high sensitivity but also a better reflectivity characteristic.

Porous silicon as a promising material for photonics

Electrochemical etching – a usual technique in nanotechnology – creates porous silicon with novel and useful properties. The considerable and controllable changes in the electronic structure and refractive index of porous silicon make it a promising material for photonics in comparison with bulk silicon. In this paper, we review as well as report on some interesting and unique properties of porous silicon material. In studying porous silicon as a low-dimensional material, we focus on the effect of the surface passivation of silicon nanocrystals on photoluminescence characteristics of such zero-dimensional crystals. As an optical material, we demonstrate the fabrication method and optical properties of the planar waveguide as well as the active waveguide and optical interference filters operated in infrared wavelengths. In addition, we investigated the effect of energy transfer from silicon nanocrystals to erbium ions in the erbium-doped porous silicon waveguide and also elaborate on the origins of the difference between the reflectivity spectra from fabricated filters and that of the simulation program.

DETERMINATION OF THE COMPLEX REFRACTIVE INDEX OF POROUS SILICON LAYERS ON CRYSTALLINE SILICON SUBSTRATES

In this work, we show an algorithm to calculate the complex refractive index of porous silicon (PS) on its crystalline silicon (c-Si) substrate in UV-NIR range by means of the reflectance spectra only. The algorithm is based on the analytical relations established by Heavens to obtain both complex refractive index and thickness of an absorbing thin film on an absorbing substrate. Based on this model, some simplification is introduced at different wavelengths. We start with the NIR range (1000–2500 nm), where the c-Si substrate has a low extinction coefficient. Then, we continue with the near infrared to the optical range (300–1000 nm), where PS has a strong extinction coefficient and dispersion. The calculated n and k values are in agreement with those reported in the literature obtained from separated measurements of a free standing PS film. We consider that the algorithm can be applied to any thin film on a substrate with similar optical properties.

Reflection z-scan for measuring the nonlinear refractive index of porous silicon

An experimental investigation on the nonlinear refractive index of nanoporous silicon at wavelengths of 532 nm and 1064 nm is reported by the reflection z-scan (RZ-scan) method with picosecond pulses. The porous silicon (PS) does not need to be peeled from silicon substrate. The method uses a p-polarized beam with oblique incidence. The modification of the reflected beam intensity gives the information of the surface nonlinear refractive index. The index of porous silicon at 1064 nm is at the same order of magnitude as that obtained by the conventional transmission z-scan technique, and the measured absolute value of nonlinear refractive index n 2 at 532 nm is two orders of magnitude higher than that at 1064 nm.

Porous silicon as a low dimensional and optical material

The considerable and controllable changes in electronic structure and refractive index of porous silicon fabricated by electrochemical anodization make it become a promising material for photonics in comparison with bulk silicon. As the study of PS in terms of a low-dimensional material, we reviewed the effect of the surface passivation of silicon nanocrystals on photoluminescence spectra of such zero-dimensional crystals. In terms of an optical material, we show the fabrication method and optical properties of planar waveguide as well as active waveguide operated in the range of 1.54 μm wavelengths. We have also investigated the effect of energy transfer from silicon nanocrystals to erbium ions based on erbium-doped porous silicon waveguide.

Refractive index contrast in porous silicon multilayers

AbstractTwo of the most important properties of a porous silicon multilayer for photonic applications are flat interfaces and a relative large refractive index contrast between layers in the optical wavelength range. In this work, we studied the effect of the current density and HF electrolyte concentration on the refractive index of porous silicon. With the purpose of increasing the refractive index contrast in a multilayer, the refractive index of porous silicon produced at low current was studied in detail. The current density applied to produce the low porosity layers was limited in order to keep the electrolyte flow through the multilayer structure and to avoid deformation of layer interfaces. We found that an electrolyte composed of hydrofluoric acid, ethanol and glycerin in a ratio of 3:7:1 gives a refractive index contrast around 1.3/2.8 at 600 nm. Several multilayer structures with this refractive index contrast were fabricated, such as dielectric Bragg mirrors and microcavities. Reflectance spectra of the structures show the photonic quality of porous silicon multilayers produced under these electrochemical conditions. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Ellipsometry and ab initio approaches to the refractive index of porous silicon

Spectroscopic ellipsometry is used to determine the complex refractive index(n−ik), porosity, and thickness of porous silicon (PSi) films. These films are obtained by anodizingp-type crystalline silicon in a hydrofluoric acid bath. After etching, PSi samples are heated to750 °C in a controlled oxygen environment. A detailed analysis of the ellipsometry data is performed inorder to determine the complex refractive index of PSi thin film. This frequency dependence ofn and k is compared with the results of ab initio quantum mechanical calculations carried out bymeans of CASTEP codes within the density functional theory. The theoretical results showa diminution of the lattice constant as the oxygen content grows, in contrast to thehydrogen-saturated surface case.

Development of interference filters based on multilayer porous silicon structures

Porous silicon (PS) has a great potential in optical applications due to its tuneable refractive index. In particular, multilayer structures consisting of alternating PS layers with different refractive indices can be used as interference filters for applications in the field of optoelectronics and sensors. In the present work, the optical properties of PS single layers and multilayer structures were studied. Since the refractive index of PS varies depending on the air content of the porous matrix, the PS structures were modelled as an homogeneous mixture of silicon and air, according to the effective medium theories (EMTs). By adjusting the refractive index and thickness of each individual layer, we can obtain a stack of PS layers with the desired optical properties, resulting in interference filters of predetermined bandwidth.

Current-Induced Optical Effect in Porous Silicon Fabry-Perot Resonators

The refractive index of porous silicon (PS) was changed by electrical injected carriers in PS-based Fabry-Perot resonator (PS-FPR) diodes. According to measurements of time evolution, there are fast (millisecond order) and slow (several seconds order) components in the refractive index change. This phenomenon can be well explained by a theoretical estimation based on a model of carrier injection and subsequent accumulation. The maximum value of the relative refractive index change is about 0.1% at a diode current density of 40 mA/cm2. These results indicate further technological potential of PS for silicon-based opto-electronic devices.

Third-order nonlinear refraction in porous silicon: Theory versus experiment

Abstract A z-scan measurement of the nonlinear refractive index of porous silicon (PS) is performed at a photon energy well below the effective bandgap of the sample. The measure gives ReX (3) ≈−10−8 esu, that is four orders of magnitude larger than the bulk value. We also present a calculation of the enhancement of X (3) in PS due to the spatial auantum confinement of the exciton in the quasi-one-dimensional nanostructure. The calculated enhancement is in very good agreement with the experimental data. Finally, we performed a perturbative calculation of nonlinear absorption δα in a auantum wire, obtaining the material-independent dispersive scaling of the quantum wire nonlinear refractive index. The calculation suggests the presence of a ‘auantum-confinement-enhanced’ auadratic Stark effect in the quasi-one-dimensional structure that gives a Iow and almost constant value of Reχ (3) at photon energy around half-bandgap and large negative values in the range Egap/2 < hw < Egap . As we discuss, this predict...

Effective refractive index and composition of oxidized porous silicon films

The relationship between the refractive index of porous silicon, its porosity and oxidation degree has been calculated in the framework of the three-component Bruggeman effective medium model (silicon, SiO 2 and pores). It takes into account the fact that the gain of SiO 2 volume fraction, occurring in the course of oxidation, takes place with sacrifice in the silicon component, and the enhanced volume of the solid phase results in the decrease of porosity. Dependence of porosity on the effective refractive index for oxidized films with various initial porosity p in is limited by the curves for the two-component media: silicon and pores (Si+V) and porous oxide (SiO 2+V). Films with p in>56% remain porous even after the 100% oxidation of silicon carcass, and those with lower p in become poreless at a certain value of oxidation degree and form another two-component system: Si+SiO 2. It has been shown how the SiO 2 content in the oxidized porous silicon is obtained by using the calculated relations and graphs, when the experimental values of the refractive index and porosity (or the refractive index alone before and after oxidation) are known, and, vice versa, how the refractive index and the oxidation degree are determined from porosity variation resulting from oxidation. It has been found by ellipsometry in stain etched porous films that, as oxidation proceeds, the refractive index, the extinction coefficient and the optical thickness of the film decrease gradually, whereas the film thickness grows. Analysis of the film thickness dependence on the oxidation degree revealed that it obeys the law valid for a silicon film without pores.

Far-infrared dielectric constant of porous silicon layers measured by terahertz time-domain spectroscopy

We measure the refractive index and the absorption of porous silicon layers in the millimetric and submillimetric wavelength range using the terahertz time-domain spectroscopy technique. For the studied range of porosity (55%–76%), the refractive index of porous silicon is rather well described by mixture theories, in which the refractive index of bulk silicon enters as a main parameter.

Photosensitivity of porous silicon-silicon heterostructures

Data from experimental studies of the photoelectric properties of heterostructures of porous and single-crystal silicon are presented. Rectifying heterostructures with photosensitivities of up to 1 mA/W at 300 K in the spectral range 1.2–2.3 eV are obtained. Oscillations in the photocurrent due to interference of the light in the porous silicon layers are observed. The refractive index of porous silicon is estimated. The polarization dependence of the photosensitivity of the heterostructures is studied.