Temperature-Dependent Photoluminescence of Silicon Nanocrystallites Prepared by Inert-Gas-Ambient Pulsed Laser Ablation
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ABSTRACT The inert-gas-ambient pulsed laser ablation technique is a promising method for preparing Si nanocrystallites. We measured the temperature dependence of photoluminescence (PL) spectra to investigate radiative and nonradiative recombination processes in the nanocrystallites prepared using this method. The Si nanocrystallites showed visible PL bands in the red (1.6 eV) and green (2.1 eV) spectral regions. The intensities of the red and green PL increased with decreasing temperature and then saturated below 80 K. This temperature dependence was compared with that of other photoluminescent Si materials. It was shown that the PL quantum efficiency of the Si nanocrystallites was larger than that of a-Si:H at high temperatures. One of the reasons for the difference in the temperature dependence between the Si nanocrystallite and a-Si:H is the change in the role of defects in the nonradiative recombination process. INTRODUCTION
In the past several years, extensive studies have been carried out on nanometersized silicon (Si) structures because visible photoluminescence (PL) spectra were observed at room temperature [1-3]. These nanometer-sized silicon structures are prepared using various methods. In the present study, we adopted the laser ablation technique combined with constant-pressure gas evaporation to prepare size-controlled spherical Si nanocrystallites [4]. Recent models of PL from nanometer-sized silicon crystals are basically classified as follows: (i) quantum confinement effects of Si nanocrystallites [1,5,61, (ii) silicon-based luminous compounds such as siloxene [7], polysilane [8], and defects in the SiO 2 layer [9], and (iii) localized states at the surface of nanocrystallites [10, 11]. However, the mechanism of the PL is still under debate [12, 13]. These previous reports were mainly focused on the origin of the PL band. A clarification of the nonradiative recombination processes is also important to produce efficient luminescent materials. The temperature dependence of PL provides important information on the radiative and nonradiative recombination processes. In bulk semiconductor crystals and in a-Si:H, the nonradiative recombination centers are deep defects [14, 15]. The electron and hole recombine nonradiatively via a multiphonon process through these defects. In many cases, including that of a-Si:H, the nonradiative recombination probability is expressed by thermal activation. We will discuss the differences between the nonradiative recombination processes of a-Si:H and Si 219
1998 Materials Research Society Mat. Res. Soc. Symp. Proc. Vol. 486 ©
nanocrystallites. EXPERIMENTAL Silicon nanocrystallites were prepared with a pulsed laser ablation method as follows [41. After the vacuum chamber was evacuated to 1.OxlO-6 Pa using a turbomolecular pump, helium (He) gas was introduced into the chamber and was kept at a constant pressure. An argon-fluoride (ArF) excimer laser (X: 193 nm, energy density: 1 J/(cm 2-pulse), pulse duration: 12 ns, repetition rate: 10 Hz) beam was focused onto a 3x1 mm
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