HRTEM images of porous Si layers obtained by depositing the exfoliated layers on carbon coated copper grids. (a) Bright field image of a low porosity (∼60%) sample formed with J = 10 mA/cm2, etching time = 5 min, and [HF] = 24%. A comparatively large crystallite along with a chunk detached from the substrate is also detected. (b) A non-spherical nc-Si obtained from a sample prepared with J = 80 mA/cm2, t = 15 min, and [HF] = 24%. (c) Randomly oriented nc-Si with the (111), (220), and (311) exposed planes with even smaller dimensions obtained from sample prepared with J = 100 mA/cm2, t = 15 min, and [HF] = 24%. (d) SAD pattern of the sample shown in (c) reveals the same planes.
(Color online) STM and AFM images of porous Si nanostructures. (a) and (b) are the STM images of as-grown, low (∼60%) and high (∼91%) porosity porous Si samples, respectively; (c) depicts the topographic image and the line profile of the marked particle, for the ncs obtained from the high porosity samples by exfoliation followed by deposition on cleaved mica; and (d) phase contrast AFM image of the sample shown in (c) depicting the formation of thick oxide shell, which manifest as humps in the line profile shown below the image.
(Color online) Room temperature PL spectra of high porosity samples prepared with J = 80 and 100 mA/cm2, t = 15 min, and [HF] = 24% showing the appearance of multiple peaks.
(Color online) Temperature dependent PL characteristics of porous Si samples having different porosities for temperatures varying from 10-300 K. (a) Sample having porosity 88%, prepared with J = 80 mA/cm2, t = 15 min, and [HF] = 24% and dried in clean atmosphere for 1 day. Two distinct peaks, one in the blue region (indicated as 1st peak) and one in the yellow/orange region (depicted as 2nd peak) of the visible spectrum, are clearly visible. A third peak seems to appear at ∼800 nm. (b) PL spectra of the same sample after 1 month ageing showing blue shift of the 1st and the 2nd peaks. The inset clearly shows the formation of a 3rd peak at ∼800 nm. (c) Sample prepared with J = 100 mA/cm2, t = 15 min, [HF] = 24%, and having porosity ∼91%. PL spectra were recorded after 1 day of preparation. Similar trends as the samples shown in (a) and (b) are clear. However, since this is a higher porosity sample, the 1st and the 2nd peaks are further blue shifted. The inset is the magnified 3rd peak appearing in the red-NIR regime. (d) Low porosity sample having porosity ∼60% showing a red shift of the PL peak beyond 200 K with increase in temperature and absence of features (like peak splitting, etc. as observed in the high porosity samples) over the entire temperature range.
(Color online) Variation of PL peak position and intensity with temperature for (a) the porous Si sample having porosity ∼88% prepared with J = 80 mA/cm2, t = 15 min, and [HF] = 24% and dried in clean atmosphere for 1 day; (b) for the same sample after ageing for 1 month; and (c) for the sample with porosity 91% prepared with J = 100 mA/cm2, t = 15 min, [HF] = 24% and aged for 1 day.
(Color online) Variation of the integrated PL intensity and FWHM with temperature of the blue peak for the sample with porosity 88% and aged for 1 day. The plot of ln(I int) vs. 1/T is shown in the inset.
(Color online) Schematic band diagram showing the different transition paths accounting for the different PL peaks in the temperature dependent spectra of porous Si.
(Color online) Normalized PL spectra (temperature dependent) showing only the high energy peak for the sample with porosity 88% and aged for 1 day.
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