Modeling the photoacoustic signal during the porous silicon formation

Abstract

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 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 of the heat sources are taken as an average value from the experimental data. The theoretical results are in agreement with the experimental data and hence provided a method to determine variables of the PS growth, such as the etching velocity and the thickness of the porous layer during the growing process.

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

“…In this kind of configuration, the incident radiation comes from the front, and the sensor ( e.g., piezoelectric transducer [22,23] or electret microphone [24,25]) is located at the back of the Si substrate. A p-type Si is usually used because the I-V curve does not change as a function of the lighting conditions.…”

“…By using the methodology developed for Rosencwaig and Gersho (RG) [25,52] the temperature distribution in the cell is…”

“…and the coefficients E j , U j , V j , and W j are obtained by substitution of these expressions [25] (equations 24, 25, and 26) in the equation 14 with the following boundary conditions:…”

“…The main techniques reported to study the in situ formation of PSi by electrochemical etching in hydrofluoric acid(HF) media are: in situ Fourier transform infrared spectrometry (FTIR) [10,11], optical interferometry [12][13][14][15][16][17] optical scattering [19,20], and photoacoustics [21][22][23][24][25].…”

“…It is useful to follow changes in optical and thermal properties in real time, the formation of multilayers, wetting process [21] , emptying and filling of porous media, and chemical reactions. Indeed, it is possible to perform an in situ monitoring of the PSi formation by using photoacoustic in HF aqueous media [21][22][23][24][25].…”