*2.2.1 Evolution of photoluminescence spectra of P-type porous silicon prepared at different etching time*

All of the PS samples showed a visible PL at a room temperature. **Figure 2** illustrates the PL spectra of samples of p-type elaborated at different etching times (60, 120, 180, and 240 s) as well as an etching current density of 15 mA/cm<sup>2</sup> .

**Figure 2** shows that the observed large band, that ranges from 550 to 780 nm, decreases in time with a peak of 668 nm, and a decreased full width at half maximum (FWHM) from 226 to 112 is obtained along with an increase of PL intensity from 0.07 to 21.06.

The width of this PL band is attributed to the wide size distribution of the silicon nanocrystallites which constitute the porous layer. The intense spectrum is characterized by a maximum at the energy of 1.86 eV and a width at mid-height of the order of 112 meV.

**Figure 2.** *Evolution of photoluminescence spectra of P-type porous silicon prepared at different etching times.*

In addition, we noticed a second less intense band compared to the first. This may be due to an oxide layer located in a band between 780 and 900 nm (**Table 1**).


#### **Table 1.**

*Optimized fitting parameters corresponding to the theoretical curves of porous silicon samples prepared at different etching time and current density of 15 mA/cm2 .*

**Figure 3** shows the variation of the PL intensity and the width at half height (FWHM) as a function of the anodization time.

#### **Figure 3.**

*The variation of the intensity, PL, and the width at half height (FWHM) as a function of the anodization time.*

From **Figure 3**, we notice that the width at half height of the PL spectra decreases as a function of the anodization time. As known the thickness of the porous layer increases as a function of the anodization time, then deduces that the width at half height decreases, and the intensity of PL increases as a function of the thickness of the porous layer. This is due to the decrease in the sizes of nanocrystallites. However, note that the intensity of PL increases as a function of the anodization time.
