V. La Ferrara

Porous silicon in solar cells: a review and a description of its application as an AR coating

In this paper, we first review the potential applications of porous Si in solar cell structures. Then we describe the fabrication of this material by both electrochemical and chemical etching methods, providing some guidelines for a better comprehension of the influence of each process parameter. After that, the properties of porous Si in terms of morphology, structure, photoluminescence, and electroluminescence emissions are summarized together with their actual photovoltaic applications. The results of our study specifically address the creation of an antireflection (AR) coating for polycrystalline Si based solar cells. We have demonstrated the feasibility of a very efficient porous Si AR layer, prepared by a simple, cost effective, chemical etching method. The formation of a porous Si layer about 0.5 μm thick on the polycrystalline wafer results in an effective reflectance coefficient Reff lower than 5% in the wavelength region from 350 to 1150 nm. The drastic reduction of the optical losses is controllable by the process parameters and is almost independent of the starting substrate.

Gas detection with a porous silicon based sensor

Abstract Porous silicon (PS) layers with 60% porosity and 80 μm thick were prepared from n-type silicon wafer. We present the sensitivity of PS photoluminescence to 250 ppm of carbon monoxide. Besides the variation of conductivity of the device due to presence of organic vapors such as chloroform, methanol, ethanol and toluene have been carried out.

Gold-catalysed porous silicon for NOx sensing

Abstract Porous silicon (PS), obtained by electrochemical anodization of an n-type silicon wafer, was catalysed by sputtering gold onto the surface (4, 8, 15 and 40-nm nominal thickness). Investigation by Rutherford backscattering spectroscopy (RBS) and by electron microscopy showed that gold did not form a continuous layer, but rather formed clusters penetrating into the pores of PS by about 1 μm. A variation of the sample conductivity in the presence of a few parts per million of NO2 and NO was recorded at room temperature. We demonstrated that, as a result of Au catalysation, PS is suitable for sensing nitrogen oxides with negligible influence by interfering gases such as CO, CH4 or methanol. Indeed, we found that humidity appreciably affected the response.

Nonlinear optical refraction of free-standing porous silicon layers

Measurements of the nonlinear refractive index of free-standing porous silicon samples by means of the Z-scan technique are reported. A sensitive enhancement of the optical nonlinearity is found with respect to bulk silicon. The results are in agreement with a simple theoretical model which is also presented and discussed, that attributes the enhancement to quantum confinement of carriers. The negative sign of nonlinear refractive index suggests that optical Stark effect gives the dominating contribution to the nonlinearity. It is also found that the nonlinearity is mainly refractive, which is very promising in order to use porous silicon for nonlinear optical applications such as power limiting or optical switching.

Optical fiber tip templating using direct focused ion beam milling.

We report on a method for integrating sub-wavelength resonant structures on top of optical fiber tip. Our fabrication technique is based on direct milling of the glass on the fiber facet by means of focused ion beam. The patterned fiber tip acts as a structured template for successive depositions of any responsive or functional overlay. The proposed method is validated by depositing on the patterned fiber a high refractive index material layer, to obtain a ‘double-layer’ photonic crystal slab supporting guided resonances, appearing as peaks in the reflection spectrum. Morphological and optical characterizations are performed to investigate the effects of the fabrication process. Our results show how undesired effects, intrinsic to the fabrication procedure should be taken into account in order to guarantee a successful development of the device. Moreover, to demonstrate the flexibility of our approach and the possibility to engineering the resonances, a thin layer of gold is also deposited on the fiber tip, giving rise to a hybrid photonic-plasmonic structure with a complementary spectral response and different optical field distribution at the resonant wavelengths. Overall, this work represents a significant step forward the consolidation of Lab-on-Fiber Technology.

Sensitivity of Porous Silicon Photoluminescence to Low Concentrations of CH4 and CO

In this paper we report the sensitivity of porous silicon photoluminescence (PL) to diluted mixtures of methane and carbon monoxide in synthetic air. We also investigate the separate effect of synthetic air, purified nitrogen and relative humidity on both photoluminescence and conductance (G). Porous silicon samples have been prepared from n-type silicon substrates. We find that PL intensity and G decrease in synthetic air with respect to their values in N2. Presence of carbon monoxide reduces the PL intensity while methane provokes the opposite behaviour. The dependence of the PL spectra on methane and carbon monoxide concentrations has been investigated. The observed effects can be related to gas induced modifications in porous surface and suggest that porous silicon can be employed in gas sensor technology.

Light trapping efficiency of periodic and quasiperiodic back-reflectors for thin film solar cells: A comparative study

Recently, great efforts have been carried out to design optimized metallic nano-grating back-reflectors to improve the light absorption in thin film solar cells. In this work, we compare the performances of deterministic aperiodic backreflectors in the form of 1-D nanogratings based on the generalized Fibonacci deterministic aperiodic sequence with a standard periodic one. The case of study here analyzed relies on a realistic solar cell model, where light absorption is evaluated only in the intrinsic region of an amorphous silicon P-I-N junction. We found that the results of comparison are strongly influenced by the amorphous silicon extinction coefficient within the near-infrared wavelength range, where most photonic-plasmonic modes (responsible for the light absorption enhancement typically observed when structured metal nanogratings are employed) are excited. In particular, with device-grade hydrogenated amorphous silicon, we demonstrate that Fibonacci-like backreflectors are able to provide an absorpt...

Nanostructured porous silicon for gas sensor applications

Abstract The response of two different types of nanostructured gas sensor to oxygen has been investigated. The first (optical) is based on the photoluminescence quenching effect of a porous silicon sample, the second on the changes of the electrical conductance v. environment of a porous silicon free standing membrane on an insulating neutral substrate. The response of both the devices to oxygen have been measured and compared. The optical based gas sensor exhibits a quenching following the Stern-Volmer model. The corresponding reactivity rate constant is found to depend on a characteristic nanodimension of the wire. The electrically operated sensor is more sensitive to oxygen and shows an opposite behavior if exposed to a reducing environment.

Study of the structure of porous silicon via positron annihilation experiments

We performed two-dimensional angular correlation of the electron-positron annihilation radiation (2D-ACAR) and positron lifetime measurements on a porous Si sample. From the width of the narrow 2D-ACAR component, attributed to the positronium atom, we estimated the average size of the pores to be ~2.4 nm and did not find evidence of a preferential propagation of the pores. Moreover, by comparing the 2D-ACAR spectrum with that observed for a pure Si crystal, we isolated a further isotropic component attributable to crystal defects of unknown origin.

Stress Measurement Technique to Monitor Porous Silicon Processing

Macroscopic stress measurements are used to monitor Porous Silicon processing. Silicon wafer of 1Ω cm resistivity, n-type and 〈1 0 0〉 orientation were used as starting material. Porous Silicon layers with a porosity of 57% and a thickness of 85 μm, fabricated by electrochemical anodisation, were differently dried, then the evolution of the wafer deflection has been followed with storage time in air. Thermal treatments both in inert and oxidant atmosphere have been performed up to 1000°C. The stress behaviour vs. temperature allows to estimate the hydrogen desorption activation energy.