Femtosecond Nonlinear Transmission Study of Free-Standing Porous Silicon Films
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Mat. Res. Soc. Symp. Proc. Vol. 405 ©1996 Materials Research Society
first stage, substrates were treated at electrical current densities from 7 to 100 mA cm-2 for 30 min to form the layers of required porosities. In the second stage, the current densities were sharply increased up to values 10 times larger than those in the first stage. This allowed the easy separation of thin porous films (thickness d = 5 - 20 gim) after the substrates were rinsed in ethanol and dried in an air stream. The PS films under investigation were fabricated as described above about two years ago and have been exposed to air until the present study. As distinct from light-emitting PS layers prepared on p-doped Si substrates, the films under investigation do not exhibit efficient PL. A week PL band recorded under UV excitation is located in the red region and has a maximum at 1.7 - 1.8 eV. Time-resolved nonlinear transmission spectra are measured with a femtosecond pump and probe set-up. The samples are excited at 3.1 eV by frequency-doubled 200-fs pulses from a regeneratively amplified mode-locked Ti-sapphire laser. Pump-induced transmission changes are probed by delayed pulses of a femtosecond continuum. A 0.15-m spectrometer and a cooled CCD array are used to measure the transmission in the absence (TO) and in the presence (T) of the pump, from which we derived the differential transmission spectra DTS = (T-T0 )/T0 =AT/T0 . 10 / EXPERIMENTAL RESULTS/ Fig. 1 shows bulk-equivalent absorption spectra of three samples prepared at current 2 densities 7 (#1), 20 (#2), and 100 mA cn- (#3). The sample porosities estimated from the measured values of the refractive index are 55% (#1), 60% (#2) and 64% (#3). The absorption edge shifts to high energies in going from sample #1 to sample #3. The analogous blueshift of the absorption edge with increased sample porosities was previously observed in PS (see, e.g., Ref. 8) and was explained in terms of size quantization within Si NC's of reduced dimensions.
The time-resolved DTS for sample #2
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measured at a pump fluence wp = 12 mJ cm-2 are Ploton Energy (eV) shown in Fig. 2 for delays At > 2.5 ps (for shorter delays the DTS are distorted by chirp of the FIG.1 Bulk-equivalent absorption in three broad-band probe pulse). The oscillatory PS films of different porosities. structure seen at the low energy side of the DTS results from an etalon interference, indicating the high optical quality of the samples. The DTS dynamics derived for fixed spectral energies and corrected for chirp are shown in Fig. 3. The differential transmission measured is negative in the whole spectral range, indicating photo-induced absorption. The DTS consist of one well resolved low-energy band (o1 around 1.23 eV and two closely-spaced high-energy bands at 1.96 eV (o2) and 2.11 eV (o3). Band co, has a risetime shorter than 500 fs and a fast initial decay with a relaxation constant of about 800 fs, followed by slower 20-ps relaxation (inset to Fig. 3). T
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