Carrier Dynamics in Porous Silicon: from the Femtosecond to the Second
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1. INTRODUCTION Four years after Canham's demonstration that porous silicon could photoluminesce efficiently in the red [1], our understanding of this material and our ability to manipulate its properties have improved considerably. For example, efficient luminescence is no longer restricted to the orangedeep red part of the spectrum: as shown in Figure 1, we have been able to prepare layers that emit with >0.1% efficiency from the blue/violet to past 1.5 gm [2]. We have shown that the luminescence of the infared-emitting samples measured at 77 K decays non-exponentially in the range of 50 nsec to >10 ptsec [2] and, in agreement with the measurements performed by many other groups, we have found that the red-orange luminescence decays as a stretched exponential with a characteristic time in the jisec range at room temperature and in the msec range at cryogenic temperatures. The dynamics of the blue luminescence, which are in the nanosecond regime, will be discussed below. Despite impressive progress on the experimental and theoretical fronts, the luminescence mechanisms have not been identified conclusively [3]. A promising approach to solve this difficult problem is to study the carrier dynamics following photoinjection using a short laser pulse. Such experiments are routinely performed in crystalline [4-6] and amorphous [7,8] semiconductors with a time resolution in the 100 femtosecond regime. From these time-resolved photoluminescence and photoinduced absorption measurements, one obtains information on mechanisms such as electron (hole) cooling and thermalization in the extended states, trapping in defect states, and recombination (either radiative or non-radiative, for example through Auger recombination). In this paper, we report on two types of measurements we have performed on light-emitting porous silicon (LEPSi), using time-resolved optical techniques. First, we have used picosecond time-resolved photoluminescence to characterize the fast blue luminescence in oxidized porous silicon. Second, we have used time-resolved pump-and-probe photoinduced absorption to study carrier dynamics in red-emitting porous silicon films with femtosecond time resolution. The results of these measurements are analyzed to identify the origin of the blue luminescence, which is related to the presence of silicon dioxide, and to demonstrate that porous silicon is an indirect 525
Mat. Res. Soc. Symp. Proc. Vol. 358 0 1995 Materials Research Society
bandgap semiconductor in which the trapping of carriers in surface states takes place on a subpicosecond time scale.
2. THE FAST BLUE LUMINESCENCE 2.1 Experimental techniques and sample preparation The time-resolved PL spectra were obtained using a gated optical multichannel analyzer after picosecond pulse excitation at 355 nm (3.5 eV). The maximum pulse fluence was 1 mJ/cm 2 , the repetition rate was 10 Hz and the overall time resolution was 15 nsec. The spectra were not corrected for the system's spectral response. The PL dynamics at selected wavelengths was measured using a single
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