The porous silicon (PSi) layers were formed on p-type silicon (Si) wafer. The six samples were anodised electrically with 30？mA/cm2 fixed current density for different etching times. The structural, optical, and thermal properties of porous silicon on silicon substrates were investigated by photoluminescence (PL), photoacoustic spectroscopy (PAS), and UV-Vis-NIR spectrophotometer. The thickness and porosity of the layers were measured using the gravimetric method. The band gap of the samples was measured through the photoluminescence (PL) peak and absorption spectra, then they were compared. It shows that band gap value increases by raising the porosity. Photoacoustic spectroscopy (PAS) was carried out for measuring the thermal diffusivity (TD) of the samples. 1. Introduction When crystalline silicon (c-Si) wafers are electrochemically etched in hydrofluoric acid (HF) at specific current densities, pores are formed, which is known as a porous silicon (PSi) layer [1]. This is an interesting material due to its unique and unusual optical and electrical properties compared to bulk Si substrate. Structurally, PSi is very complicated [2]. Some published papers indicate that PSi layers consist of Si columns and pores or isolated nanocrystallites [3]. On the other hand, PSi may be considered as a system of interconnected quantum wells, the so-called quantum sponge [4]. Nevertheless, the properties of PSi, such as porosity, thickness, pore diameter and, microstructure of silicon, have been reported to depend on anodisation conditions, including the electrolyte, current density, wafer type and resistivity, etching time, and temperature [5]. The first report of room temperature visible photoluminescence (PL) from porous silicon (PSi) structures has attracted wide interest in the scientific community [6]. The mechanism of light emission in porous silicon is not fully understood. One popular hypothesis is that luminescence is due to quantum confinement of charge carriers in narrow crystalline silicon walls separating the pores [6]. The increase in the band gap of PSi is possible by reducing the size of the nanocrystallites [7]. Another hypothesis asserts the existence of luminescent surface species trapped on the inner pore walls as the source of light emission [8]. Another concludes that the origin of luminescence can be traced to the presence of surface-confined molecular emitters, such as siloxene (Si6O3H6) derivative, present in porous silicon [9]. It also has been indicated that the presence of oxygen shallow donors of binding energy in the 0.1？eV range shows
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