Optik 124 (2013) 5353–5356 Contents lists available at ScienceDirect Optik jou rn al homepage: www.elsevier.de/ijleo Study of porous silicon structure by Raman scattering R.S. Dariani ∗ , Z. Ahmadi Department of Physics, Alzahra University, Tehran, 1993893973, Iran a r t i c l e i n f o Article history: Received 22 October 2012 Accepted 20 March 2013 Keywords: Porous silicon Raman scattering Etching time Intensity Phonon confinement78.55.Mb, 52.38.Bv, 81.65.Cf, 42.60.Jf, 43.35.+d a b s t r a c t In this paper, the effect of etching time on light emitting porous silicon has been studied by using Raman scattering. Enhancement of Raman intensity by increasing the porosity is observed. Also there is a red shift, about 4 cm -1 , from the Raman peak of crystalline silicon to that of porous silicon. The phonon confinement model suggests the existence of spherical nanocrystalline silicon with diameter around 7 nm. But SEM images show that the samples have a sheetlike structure that confines phonons in one dimension. This should not cause any shift in their Raman spectra. It is suggested that the observed Raman peak shift is due to the spherical nanocrystals on the surface of these sheets. © 2013 Elsevier GmbH. All rights reserved. 1. Introduction Discovery of light emission of porous silicon (PS) at room tem- perature has been stimulated a lot of attempts to lead this material into optoelectronic industry. For this purpose, structural investiga- tion of PS is essential. The most common method for making porous silicon is electrochemical method which prepares PS in a low cost [1,2]. But the problem in this method is, a lot of parameters inter- fere and affect PS structure that make the PS structure somehow unpredictable and thus structural study of PS samples is always needed. Among variety of tools, Raman spectroscopy is a nondestructive tool that can give us a lot of information about the structure of solids [3]. For strongly absorbing materials such as many semiconductors Raman scattering is a perfect tool for studying their structural char- acters like stress, strain, and also disorders [4]. Raman peaks of well-defined phonons in single crystal semiconductors are very sharp. Finite-size effects in nano porous materials cause in peak broadening and a shift to lower frequencies in their Raman spectra [5]. There have been several reports and analysis on Raman spectra of PS [6]. The first-order Raman peak shift of c-Si is at 520.5 cm -1 [7]. It is reported to be symmetric and sharp with an FWHM of 3 cm -1 [7]. Also the Raman spectrum of amorphous silicon (a-Si) shows a broad peak near 480 cm -1 [7]. Because of massive sur- face area to volume ratio in PS structure, it is expected to see an interface of crystalline silicon with its oxides. It is proved that sil- icon oxides in PS form far more rapidly than in crystalline silicon ∗ Corresponding author. Tel.: +98 21 85692646; fax: +98 21 88047861. E-mail address: dariani@alzahra.ac.ir (R.S. Dariani). [8]. Thus an amorphous phase might be seen in Raman spectra of PS. It has been observed in PS that the peak shift relative to c- Si increases as nanocrystal size decreases [9]. Some studies have determined the local structure of PS is more like a sphere than a rod and polarization Raman measurements suggest that the struc- ture does not consist of a series of parallel columns [9,10]. The shift is the result of phonon confinement that happens by the phonons whose wave vectors extend out of the Brillouin zone. This happens for crystal sizes less than 100 ˚ A [11]. Using phonon confinement model, Richter et al. and Campbell et al. have theoretically studied the effect of crystal size on Raman spectra [4,5]. In this study, we report our results on PS Raman spectra with different etching times. Most researches have included analysis of peak shift and line width of spectra, but less considered on inten- sity changes. We tried to give our analysis by paying more attention on Raman spectra intensities. Speaking about Raman intensity of PS requires knowing about the porosity amounts, thus they also have been measured. Then, we confirmed our results with the help of SEM images from our samples. Finally, we speak about silicon nanocrystal sizes of our samples by calculations done by using phonon confinement model. 2. Experimental details Porous silicon films are prepared on p-type, (1 0 0) oriented sil- icon wafers with sheet resistance of 600 /, by electrochemical etching method in a HF (38–40%) = ethanol (1:1) etchant. Current density of 10 mA/cm 2 is kept constant during etching time. Raman spectra of samples are measured using NICOLET, Almega Dispersive Raman system consists of a laser source with 532 nm light beam wavelength. Fig. 1 shows the Raman spectra of PS samples that 0030-4026/$ – see front matter © 2013 Elsevier GmbH. All rights reserved. http://dx.doi.org/10.1016/j.ijleo.2013.03.129