Optical study of annealed cobalt–porous silicon nanocomposites

We report Raman and photoluminescence studies of cobalt–porous silicon nanocomposites (PS/Co). Cobalt was introduced in porous silicon (PS) by immersion method using CoCl2 aqueous solution. The presence of cobalt in PS matrix was identified by FTIR spectroscopy and EDX analyses. The Raman spectroscopy revealed the presence of Si bonded to cobalt oxide in PS/Co. We discuss also the Raman spectra of PS and PS/Co samples under different annealing temperatures ranging from room temperature (RT) to 600°C. The optical properties of PS and PS/Co were studied by photoluminescence (PL). The highest PL intensity was observed for an immersion time of 60min. For long duration, the deposited cobalt quantity acts as energy trap and promotes the non-radiative energy transfer; it is the autoextinction phenomenon. We have studied also the effect of the annealing temperature on the PL of both PS and PS/Co samples. For PS, the annealing process leads to a rapid oxidation of the Si nanocrystallites (nc-Si). In the case of PS/Co sample, two different mechanisms are proposed; one is the desorption of Si–Hx(x=2,3) with the formation of cobalt oxide for annealing temperature less than 450°C which causes the increasing of PL intensity and the stability of PL energy, the other mechanism is the transformation of the porous silicon to silica at high temperatures (≻450°C) which leads to the decreasing of the PL intensity and the blue shift of the PL curve.