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ABSTRACT A pulsed, high-power TEA CO2 laser with lines in the region from 9.2 to 10.6 pim has been used to irradiate luminescent porous Si samples. The IR laser pulses heat the sample on a time scale much shorter than the PL decay time which is at 300 K for the PL at 1.65 eV in the order of tenth of tis. One IR pulse serves to increase the temperature of the luminescing particles in -2 jis up to 100 0C. This increase of temperature leads to a efficient reduction of the photoluminescence (PL) intensity. However, the PL decay times are almost not affected by the heating pulse. Based on this measurement a picture of the recombination statistics that takes account of the granular nature of the material is developed.

INTRODUCTION Since the discovery of visible photoluminescence (PL) of porous silicon at room temperature 1990 by Canham [1] several models were proposed to explain this phenomenon. While the quantum-size nature of the radiative recombination processes is now generally accepted and is becoming predominant in the literature, a wide variety of models is applied to describe the time evolution of the PL under pulsed optical excitation [2,3,4,5,6]. At low temperatures, the temperature dependence of the PL intensity and the corresponding change in lifetimes has been explained by Calcott et al. [7] as a result of singlet-triplet splitting of the excitonic state. The variation of lifetimes and quantum yield versus temperature in the high temperature region has been reported by Vial et al. A model taking into account both radiative and nonradiative recombination has been proposed. We will discuss an appropriate description of the recombination statistics of the slow-red PL band at room temperature, taking into account the granular structure of the material. The general behavior of the PL lifetimes as a function of different measurement parameters is well documented. At room temperature the PL lifetimes depend strongly on the detection energy, ranging from several jts at 2.2 eV to tenth of jis at 1.6 eV. An increase of the sample temperature leads to faster luminescence lifetimes and to a decrease of the PL intensity. Several experiments were performed to clarify the physical processes responsible for the time evolution of the PL. The importance of Auger-recombination, leading to efficient damping of the PL was pointed out [8]. While this process with nonradiative lifetimes on the order of ns is important for the initial stage of the decay of the photoexcited carriers, the longtime component could only be addressed by conventional methods such as heating, oxidizing or changing the porosity of the samples. 553 Mat. Res. Soc. Symp. Proc. Vol. 452 © 1997 Materials Research Society

In this work we show that an IR laser source is able to heat the Si nanocrystals on a time scale much shorter than the PL decay time. Due to the strong absorption of the IR radiation by the Si-0 2 bonds in the overlayer on the Si nanocrystals, one IR pulse serves to heat the luminescing particles on a time scale of -2 p.s up to 100°C. This gi