Optical Transitions in Light-Emitting Nanocrystalline Silicon Thin Films
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ABSTRACT Optical transitions in nanocrystalline Si (nc-Si) thin films with different mean crystal sizes ranging from < 2 nm to -3 nm have been studied by electroreflectance (ER) spectroscopy. At 293 K, ER signals are observed at 1.20-1.37 eV to be corresponding to fundamental gap in bulk crystalline Si. With a decrease in the mean crystal sizes of nc-Si, the transition energy of the fundamental gap is increased and the ER signal is intensified. The bandgap widening would be due to quantum confinement (QC) in nc-Si, and the increased signal indicates appearance of direct transition nature. The ER signals are also observed at 2.2 eV and at El (E0') direct gap of 3.1-3.4 eV, while photoluminescence (PL) peak energies are located at 1.65-1.75 eV and at 2.3 eV. With the reduced mean crystal size, the 1.7-eV PL peak energy is also increased, suggesting that QC is also responsible for the increased PL peak energy. INTRODUCTION Nanocrystalline Si (nc-Si) is one of the promising materials for optoelectronic devices [II. We have investigated thin-film light emitting diodes (TFLEDs) using plasma-deposited p-type ncSi as a light emitting active layer [2], because the thin-film devices have many advantages (e.g. low cost fabrication on a large-area substrate) which have been already realized on amorphous Si (aSi:H) devices [3]. In our previous reports, we have fabricated the nc-Si TFLED with a structure of SnO 2/p-type nc-Si/Al, and studied electroluminescence (EL) properties of the TFLEDs in conlunc-
tion with photoluminescence (PL) properties of the nc-Si thin films [2]. For further improvement of the luminescent efficiency from material aspects, characterization of the electronic energy structure of nc-Si should be required. In general, optical transitions measured by spectroscopic techniques provide a rich source of information on the electronic energy structure of semiconductors. Regarding porous Si which is the most intensively studied nc-Si, optical transitions have been mainly studied by optical transmittance or photoluminescence excitation (PLE) spectroscopy [4]. The obtained transmittance and PLE spectra, however, show featureless broadwide tails. Therefore it should be introduced an experimental approach with high resolution as well as high sensitivity for extraction of structures from the broadwide lineshape. Electroreflectance (ER) spectroscopy is an appropriate tool because the obtained lineshape is related to the derivative of the unperturbed dielectric function, thereby the structure-less background is largely reduced and sharp peaks should appear at the interband critical points. In addition, spectral structures with respect to confined systems (e.g. excitons, subband states in quantum wells) can be extracted by use of the ER spectroscopy [5]. In this article, we present experimental results of the electroreflectance (ER) measurements performed on the nc-Si thin films [6]. We discuss ER features observed at 1.2-1.4 eV, 2.2 eV, and 3.1-3.4 eV with a particular emphasis on the ER feature at 1.2-1.4 eV in compari
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