Identification of the luminescence center of porous silicon under low temperature thermal oxidation
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Identification of the luminescence center of porous silicon under low temperature thermal oxidation Kazuo Goda, Ryuji Tanaka1, Yukako Honda, Kazuhisa Inoue and Hideki Ohno2 Dept. of Physics, Meisei University, 2-1-1 Hodokubo, Hino City, Tokyo 191-8506, Japan 1 Koyo Electronics Industries Co., Ltd., Tokyo, Japan 2 Dept. of Physics, Tokyo National College of Technology, Tokyo, Japan ABSTRACT The relationship between chemical states and optical properties for porous silicon (PS) under initial oxidation by heating at low temperatures (150, 200, 250 οC) in air was investigated using an infrared (IR) spectroscopy and photoluminescence (PL) measurements. The measurements in the IR region show that the IR absorption peaks for the Si-H stretching band (2050-2150 cm-1) change with appearance of the Si-O-Si-H stretching band (2100-2300 cm-1) when the thermal oxidation time is increased. On the other hand, the peak in the PL spectrum shows a blue shift from 820 nm to 710 nm with the oxidation time. The observed blue shift of the PL spectrum is due to the decrease in the initial PL peak intensity at 820 nm and the increase at 710 nm. Moreover, the peak intensities in the PL spectra at 820 nm and 710 nm have clear relationship to the amounts of the Si-H bonds and Si-O-Si-H bonds, respectively, as a function of the oxidation time. These results indicate that luminescence center (LC) for as-prepared PS is ascribed to complexes including Si-H bonds. Also the LC for oxidized PS under the oxidation process at low temperatures is ascribed to complexes including Si-O-Si-H bonds covering the inner surface of PS. INTRODUCTION Since the discovery of efficient visible photoluminescence (PL) from porous silicon (PS), there have been caused many attempts to study PS not only from fundamental physical interests but also from expectation to develop optoelectronic devices[1-4]. Most of the studies have been directed to understanding the mechanism of the light emission. To explain the experimental results, two different models have been proposed for the mechanism of luminescence: (1) The quantum confinement effect model (QCM) that the strong visible emission is due to the modified band gap in Si nanocrystals [1,2,4-8]. (2) Si compounds model that the light emission is ascribed to compounds covering the surface of Si crystallites [9-11]. However, these models can explain PL properties only in the special cases [12]. The quantum confinement/luminescence center (QCLC) model, lately proposed by Qin et al., can explain many of the experimental results [12-15]. According to the QCLC model, electron-hole pairs are generated inside the Si nanocrystallites whose band gap is widened by quantum confinement F5.30.1
and then recombine through luminescence centers (LCs) outside the Si nanocrystallites. Although they assume that the luminescence centers are complexes of Si with O, H or F, the luminescence center in this model is still unclear. On the other hand, it is an important and useful problem to study the relation between properties of luminescence and
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