Effects of boundary reflection on photoacoustic spectrum of porous silicon

Abstract

Photoacoustic (PA) amplitude and phase spectra are studied on porous silicon (PS) samples. For the sample with a thinner PS layer and a rough interface observed by field-emission scanning electron microscope (FE-SEM), PA amplitude decays rapidly at short wavelengths but stays at a higher level above 650 nm compared with a sample with a thicker PS layer and a smooth interface. In this latter longwavelength region, phase delay for the former sample is smaller. A model calculation for the two-layer model taking account of scattering of light in the porous media and interface reflection of light gives at least a qualitative explanation of these differences. 78.20.Hp; 65.90.+i Photoacoustic spectroscopy (PAS) detects the thermal signals generated in materials under light illumination and enables one to study the non-radiative recombination processes and evaluate both the optical and acoustic properties of the materials [1]. In contrast, photoluminescence (PL) spectroscopy characterizes the luminescent properties from the radiative processes. When these methods are applied to the scattering media, the analysis based on some models regarding the effect of scattering is required. In the case of PAS in the lightscattering media, Helander's model [2] can be employed in which the modified distribution of photons in the media is taken into account to estimate the heat generated by the scattered light. The light-scattering is expected to be important in the porous media. The complicated shape of pores can induce the complicated scattering of light in the media. PACS:The light emission of porous Si (PS) was discovered in 1990 [3]. The efficient visible-light emission in red was reported first, but the blue emission was reported to be related to the oxidized PS samples [4,5] and gave a promise for the Si-based optical devices. Although PL spectra were reported for PS [3,6], no careful analysis on the effect of scattering seems to have been considered and only a few photoacoustic (PA) measurements are reported in the PS samples [7,8]. The nano-structure that is expected to lead to the quantum confinement effect [9], believed to be essential for the light emission from PS, will also play a decisive role in the scattering effect.In this paper, both amplitude and phase PA signals are studied over a wavelength region 400-1600 nm on the two PS samples with different interface characteristics between the porous layer and the Si substrate. The difference in PA spectra is attributed to the combined effect of the scattering in the porous layer and the difference in roughness observed from the scanning electron microscope pictures of the interface between the PS layer and the Si substrate. The experimental PA data are qualitatively analyzed based on the calculated PA spectra in which the reflection effects of light at the interface and the subsequent scattering are included. Experimental proceduresThe PS films were made by the anodization of the 2-4 Ω cm p-Si wafers under galvanostatic conditions. The Si wafer was also used as one elec...

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

“…Photoacoustic techniques are nondestructive and are used to study thermal and optical properties of several materials, including porous media [33][34][35]. It is crucial to highlight that there are two methodologies to study materials using photoacoustic.…”