Electroreflectance Study of Light-Emitting Porous Silicon
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Electroreflectance Study of Light-Emitting Porous Silicon Toshihiko Toyama, Akihito Shimode and Hiroaki Okamoto Department of Physical Science, Graduate School of Engineering Science, Osaka University Toyonaka, Osaka 560-8531, Japan ABSTRACT Electroreflectance spectroscopy measurements have been performed on light-emitting porous Si (LEPSi). The transmission electron microscope measurements reveal that LEPSi includes Si nanocrystals with a mean crystal size of 1–2 nm. The ER features are observed at a transition energy of 3.4 eV in all of the samples, giving that LEPSi still keeps the threedimensional (3D) electronic structure. Changes in the transition energies are not found for LEPSi with the different mean crystal sizes. Furthermore, we directly observed interband transitions of quantized states due to quantum-confined electron-hole (e-h) pairs in LEPSi as two or three extra ER features being located between 1.2 and 3.1 eV which are never observed in bulk crystalline Si. Employing a simple effective mass approximation model, we have evaluated the reduced mass, the kinetic energies and the Coulomb attraction energies of the quantumconfined e-h pairs. We also found that the energy distance between the transition energies at the ground state and the photoluminescence (PL) peak energies basically corresponds to the Coulomb attraction energies. Finally, we propose a new luminescent model based on interband transitions involving the quantum-confined dense e-h plasma. INTRODUCTION Since the first discovery of strong photoluminescence (PL) at room temperature with emission energies being larger than the bandgap of crystalline Si (c-Si) [1], light-emitting porous Si (LEPSi) has been intensively studied as a physical phenomenon arising from quantum confinement (QC) effects found in low-dimensional semiconductors [2]. An alternative blueshift is found in the optical absorption edge [3], and this has been also considered as the bandgap widening due to the QC effects. However, it has been found that the blueshift in the optical absorption edge is much larger than that in the PL peak energy, when the mean crystal size is decreased to a few nanometers [4]. Therefore the luminescent models in which the optical absorption edge directly reflects the QC effects, while the PL occurs via luminescent centers, e.g., surface states on Si nanocrystals [5], have been proposed. So far, the luminescent models have been widely accepted [6]. However, there remain at least two crucial problems for the identification of the bandgap of LEPSi based on optical absorption or photoluminescent excitation spectra. First, the obtained spectra are very broad featureless, so that a specific definition is required for the bandgap determination such as Tauc gap used in amorphous semiconductors [7]. However, there has been no established definition with physical aspects. The alternative problem is spectroscopic signs assigned to the three-dimensional (3D) electronic structure in LEPSi. For example, spectral features associated with E1(E0') and E2 direct gap
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