Electrochemical differential photoacoustic cell to study in situ the growing process of porous materials

Abstract

In order to study in situ the growing process of porous materials, a new electrochemical differential photoacoustic cell (DPC) was developed. This system allows to obtain the thermal signals coming from the growing process of the pores without the external noise component. The DPC is a good system to growth porous silicon and study their growing process with reproducibility. The porous silicon samples were obtained by using electrochemical etching of (100) n-type silicon wafers with different nominal resistivity values in the range of 1-25 Omega cm. The samples were formed in a solution of hydrofluoric acid and ethanol having a composition ratio of 1:1 in volume with etching voltage of 10 V and an etching time of 2 min using back illumination provided by a laser beam with a wavelength of 808 nm. The porous samples were characterized by means of Raman microscopy, x-ray diffraction, and scanning electron microscopy. The crystallite sizes of the samples were obtained through the analysis of the micro-Raman spectra using a phonon confinement model, and the analysis of the x-ray diffractograms.

In situ photoacoustic characterization for porous silicon growing: Detection principles

“…4, the change in the temperature of the sample is governed by the apparition of the porosity and changes in the thickness of the sample as shown in Eqs. (3) and (6). From a physics point of view, the reflectance during the etching process is periodic (see Figure 3 (a)).…”

“…The data reported by Gutierrez et al and Espinosa-Arbeláes et al showed an oscillatory character for the amplitude and phase signals of the photoacoustic signal. 6,7 This shows clearly that the photoacoustic signal depends on various parameters: substrate impurity concentration, current density, and the hydrofluoric acid (HF)/surfactant ratio.…”

“…Gutierrez et al 6 reported an in-situ technique based on the photoacoustic effect, to characterize the PS formation. However, the physical explanation of the origin and shape of the PA signals during the etching of Si were not described in detail.…”

Modeling the photoacoustic signal during the porous silicon formation

“…The coefficients W n and U n are obtained from the temperature and the flux continuity for each AC component of Eqs. (10)- (12). It is easy to show that σ's are related to the thermal diffusivity α as follows:…”

“…PA has been used to study kinetic processes, thermal and electronic properties of semiconductor materials. [10][11][12][13] Therefore, it is expected that the PA technique could be able to detect in situ the chemical reaction and the changes in the optical properties during the PS formation.…”