Comparison of Porous Silicon Etched Gently and Under Illumination
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363 Mat. Res. Soc. Symp. Proc. Vol. 358 01995 Materials Research Society
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Figure 1: Room-temperature photoluminescence spectra from porous silicon samples QI and Q2 both prepared in the dark under "gentle" etching conditions. Etching times are 10 and 20 minutes respectively.
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Photoluminescence and Raman measurements from two of these "gently" prepared samples, Q1 and Q2, with etching times 10 and 20 minutes respectively were carried out. The spectra were obtained with a U-1000 Yvon-Jobin double monochromator and detected with a thermoelectrically cooled GaAs Hamamatsu PMT. The excitation source was the 4579 A line of an Ar ion laser. Both photoluminescence and Raman spectra were recorded during the same run and are shown in Figs. I & 2 respectively. As the photoluminescence peak increases in energy, the Raman peak shifts to lower energy [13]. Sample Q1 shows a photoluminescence peak at 1.72 eV and a Raman peak at 517 cm- 1 , while sample Q2, shows a photoluminescence peak at 1.91 eV and a Raman peak at 513 cm'T The above results correlate well with the quantum confinement model for the luminescence in porous silicon. The smaller is the size of the nanoparticle, the larger the Raman downshift from 521 cm- 1 for bulk silicon and the higher the photoluminescence peak energy [5]. Additional support to our conclusions is provided by thermal annealing studies. Samples from each of the two sets were annealed in a tube furnace in oxygen atmosphere at 450 TC and 650 TC for ten minutes. In agreement with previous reports [15], annealed samples from set-A show photoluminescence drastically reduced presumably due to the loss of surface passivation, and blue shifted. Although oxide may also provide passivation [16], hydrogen passivation is essential for efficient photoluminescence. Raman spectra from a setA sample are shown in Fig. 3. After annealing at 450 TC, the Raman line shifts to 514 cm-1, from 517 cm- 1 for the as prepared sample, indicating a further reduction in the particle size as a result of surface oxidation. For the 650 TC annealed samples, only the 521 cm" 1 peak from the silicon substrate remains, indicating that most of the nanoparticles have been consumed in the formation of SiO 2 .
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Figure 3: Room-temperature Raman spectra of a sample from set A: a. As anodised b. After 450 "Cannealing c. After 650 "C annealing. Note that Rayleigh scattering is quite high at c, as evidenced by the appearance of the Ar laser plasma lines, due to the degradation of the surface morphology.
The observations made with the first set of samples were then compared with a second set of samples. Set-B was anodised under illumination with a mercury light source. Since the strongest component of a mercury source is in the Ultra-Violet r
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