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559 Mat. Res. Soc. Symp. Proc. Vol. 452 01997 Materials Research Society

EXPERIMENTAL RESULTS AND DISCUSSION P-type Si wafers of 5 Qcm resistivity are used for preparation of the porous Si samples. The electrochemical etching is performed in a solution of 1: 1 by volume mixture of hydrofluoric acid (49% in H 20) and ethanol. The current density is 30 mA/cm2 . The samples are fabricated in the dark. Two experimental techniques are used. In the pump-probe experiments two linearly polarized beams are incident normally onto the same point of the PS sample surface. An intense CW beam from an Ar laser (515 nm, up to 500 W/cm 2) is used as a pump. The PL is excited by the relatively weak modulated probe beam from a HeCd laser (442 nm, below 5 W/cm 2). The lock-in detection technique allows us to separate the light emission from the porous layer caused by these two beams, so that only the PL, excited by the probe one, is detected. These experiments are done at room temperature. The optical fatigue experiments are performed at liquid helium temperature. The sample is exposed to the linearly polarized CW degrading beam (515 nm, 100 W/CM 2) for 20 min. After that the PL excited by the linearly polarized probe beam (442 nm, below 1 W/cm 2) is measured. In both cases the PL is collected by a conventional twolens condenser and focused onto the entrance slit of a monochromator. To determine the PL polarization a polarizer is placed between the lenses of condenser. A depolarizer is inserted near the entrance slit of the monochromator to avoid the polarization-dependent response of the detection system. The lifetime of the free carriers is the essential parameter governing the efficiency of the Auger process under CW optical excitation. In the system of isolated nanocrystals a long

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Energy (eV) Figure 1. Spectra of the PL from porous Si with and without additional illumination by the pump light (a); before and after the optical degradation (b).

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lifetime increases the probability to generate two electron-hole pairs per crystallite leading to the fast Auger recombination. The PL decay time is not a constant over the luminescence band of porous, Si and becomes longer toward the red edge of the PL band. Therefore, the low energy region of the PL should be more efficiently affected by the Auger recombination process. This is confirmed by the observation shown in Fig. l a, that the PL spectrum in the presence of the intense CW pump beam is shifted to the high energy range, the PL quenching is most pronounced at the red edge of the PL band. We would like to point out the similarity of this observation with the blue shift of the PL band of PS under intense CW optical excitation [8]. Fig. I b exhibits the PL spectrum of porous Si before and after a 20 min exposure of the sample to the intense CW illumination at liquid helium temperature. The PL intensity is quenched mainly in the high energy range 2 - 2.2 eV and does not recover for hours. The distinct diff