Cristian F. Ramirez-Gutierrez

Modeling the photoacoustic signal during the porous silicon formation

Within this work, the kinetics of the growing stage of porous silicon (PS) during the etching process was studied using the photoacoustic technique. A p-type Si with low resistivity was used as a substrate. An extension of the Rosencwaig and Gersho model is proposed in order to analyze the temporary changes that take place in the amplitude of the photoacoustic signal during the PS growth. The solution of the heat equation takes into account the modulated laser beam, the changes in the reflectance of the PS-backing heterostructure, the electrochemical reaction, and the Joule effect as thermal sources. The model includes the time-dependence of the sample thickness during the electrochemical etching of PS. The changes in the reflectance are identified as the laser reflections in the internal layers of the system. The reflectance is modeled by an additional sinusoidal-monochromatic light source and its modulated frequency is related to the velocity of the PS growth. The chemical reaction and the DC components...

Cooling rate effects on thermal, structural, and microstructural properties of bio-hydroxyapatite obtained from bovine bone

This article is focused on the study of cooling rate effects on the thermal, structural, and microstructural properties of hydroxyapatite (HAp) obtained from bovine bone. A three-step process was used to obtain BIO-HAp: hydrothermal, calcinations, and cooling. Calcined samples in a furnace and cooling in air (HAp-CAir), water (HAp-CW), and liquid nitrogen (HAp-CN2), as well as an air cooled sample inside the furnace (HAp-CFAir), were studied. According to this study, the low cooling rate that was achieved for air cooled samples inside the furnace produce single crystal BIO-HAp with better crystalline quality; other samples exhibited polycrystalline structures forming micron and submicron grains.

In situ photoacoustic characterization for porous silicon growing: Detection principles

There are a few methodologies for monitoring the in-situ formation of Porous Silicon (PS). One of the methodologies is photoacoustic. Previous works that reported the use of photoacoustic to study the PS formation do not provide the physical explanation of the origin of the signal. In this paper, a physical explanation of the origin of the photoacoustic signal during the PS etching is provided. The incident modulated radiation and changes in the reflectance are taken as thermal sources. In this paper, a useful methodology is proposed to determine the etching rate, porosity, and refractive index of a PS film by the determination of the sample thickness, using scanning electron microscopy images. This method was developed by carrying out two different experiments using the same anodization conditions. The first experiment consisted of growth of the samples with different etching times to prove the periodicity of the photoacoustic signal, while the second one considered the growth samples using three differe...

Optical interferometry and photoacoustics as in-situ techniques to characterize the porous silicon formation: a review

Abstract Porous Silicon (PSi) is a groundbreaking material because its physicochemical properties can be customized through its porosity. This means that monitoring and control of the growing parameters allows the fabrication of PSi-based systems with controlled properties. Interferometry and photoacoustics are non -invasive, non - contact, real-time (in-situ) techniques used to characterize the phenomena that takes place during the formation of PSi. This work presents the mathematical and experimental aspects related to the implementation of the techniques mentioned above, which are meant to characterize the PSi growth in fluoride-based electrolyte media. These methods can determine macroscopic parameters of PSi such as thickness, porosity profile trough effective medium approximation (EMA), refractive index, etching rate, and RMS roughness under 100 nm. The monitoring ability of these techniques is strongly dependent on the wavelength of radiation used. However, it is possible to monitor thickness from λ0/4 to ∼ 1/ α0, where α0 is the optical absorption coefficient at λ0. Also, these techniques can be implemented as feedback control on the etching processes for fabrication of PSi.

Porosity and roughness determination of porous silicon thin films by genetic algorithms

Abstract The problem of determining the porous silicon (PSi) optical constants, thickness, porosity, and surface quality using just reflectance data is board employing evolutionary algorithms. The reflectance measurements were carried out of PSi films over crystalline silicon (c-Si) substrate, and the fitting procedure was done by using a genetic algorithm (GA). The PSi is treated as a mixture of c-Si and air. Therefore, its effective optical constants can be correlated with the porosity through effective medium approximation (EMA). The results show that genetic fitting has a good match with the experimental measurements (near UV–vis reflectance) and the thickness obtained by scanning electron microscopy (SEM).

Development of modulated optical transmission system to determinate the cloud and freezing points in biofuels

This work is focused in the development of a modulated optical transmission system with temperature control to determine the thermal properties of biodiesels such as the cloud and freezing points. This system is able to determine these properties in real time without relying on the operator skills as indicated in the American Society for Testing Materials (ASTM) norms. Thanks to the modulation of the incident laser, the noise of the signal is reduced and two information channels are generated: amplitude and phase. Lasers with different wavelengths can be used in this system but the sample under study must have optical absorption at the wavelength of the laser.