Estimation of Silicon Nanocrystalline Sizes from Photoluminescence Measurements of RF Co-Sputtered Si/SiO 2 Films.
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Estimation of Silicon Nanocrystalline Sizes from Photoluminescence Measurements of RF Co-Sputtered Si/SiO2 Films. A. Ramírez-Porras1, L.F. Fonseca2, and O. Resto2 1 Centro de Investigación en Ciencia e Ingeniería de Materiales and Escuela de Física, Universidad de Costa Rica, San Pedro 2060, Costa Rica. 2 Department of Physics, University of Puerto Rico, Río Piedras, PR 00931, U.S.A. ABSTRACT A stochastic distribution of nanocrystalline sizes model is applied to fit photoluminescence (PL) spectra of luminescent Si nanocrystals in a Si/SiO2 matrix synthesized by RF co-sputtering on the top of quartz substrates. With this method, the PL spectra from a diverse set of samples can be resolved mainly as the sum of two components: a contribution from a gaussian-like distribution of sizes of quantum dots (QD) and a similar component from a distribution of quantum wires (QW). These distributions of sizes and their associated PL energies agree well with the so-called Smart Quantum Confinement model (SQC). INTRODUCTION The interest in the study of nanometer size silicon crystals has improved since the discovery of PL in this material more than a decade ago [1]. The increase in quantum efficiency compared to the bulk is the key point to consider this material a candidate for optoelectronic applications [2]. Despite the large number of studies dealing with the modeling of the PL mechanism (see for example reviews in Refs. [3] and [4]), a controversy is still present and no conclusive model can explain all the facets of the problem. Nevertheless, it is generally accepted that the PL emission is a consequence of a widening in the band gap due to quantum confinement in the nanocrystals [5]. Several studies confirm the presence of nanocrystals (called “quantum dots”, QD) or aggregates of nanocrystals in long structures (“quantum wires”, QW) in luminescent specimens [6-12]. The evidence points at the existence of statistical distributions of nanocrystal sizes mainly because the process of production of the specimens is highly stochastic [13-17]. A model by John and Singh [13] proposes that the PL characteristics should be in accordance with such distribution of sizes, at least for nanocrystals non passivated on their surface with foreign species. This is partially true for air-exposed specimens (surface passivated with oxygen), where it turns out that samples with nanocrystal mean diameter of around 3 nm or less do not show further dependence between PL structure and diameter [18]. A model proposed by Wolkin et al [18] takes into account the transitions from localized-to-band states that originate from the trapping of an exciton in Si=O bonds in those nanocrystals, but assuming a sole size for them. As a result, a comprehensive model encompassing these two approaches has been proposed recently to explain the PL characteristics in porous silicon [19]. The aim of the present study is to apply the model to nanocrystalline Si/SiO2 films. THEORETICAL MODELING The theoretical model employed here has been detailed in Ref. 19 and wi
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