Photoluminescence Study of Self-Limiting Oxidation in Nanocrystalline Silicon Quantum Dots
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Photoluminescence Study of Self-Limiting Oxidation in Nanocrystalline Silicon Quantum Dots
Kenta Arai, Junichi Omachi, Katsuhiko Nishiguchi, and Shunri Oda Research Center for Quantum Effect Electronics, Tokyo Institute of Technology 2-12-1, O-okayama, Meguro-ku, Tokyo 152-8552, JAPAN. ABSTRACT We have studied photoluminescence (PL) of surface oxidized nanocrystalline silicon quantum dots (QDs) for various oxidation periods and temperatures. With increasing oxidation period, the surface oxide grows and the Si QD core shrinks initially, then retardation of the oxidation process occurs which is ascribed to compressive stress at the interface between Si QD core and oxide. Upon oxidation, the PL spectrum peak shifts toward the shorter wavelength side followed by retardation of the blueshift or even manifestation of the redshift. The origin of PL is due to the localized excitons at the interface between Si QD core and oxide or amorphous SiOx (a-SiOx ) formed at the interface. The blueshift is associated with the increased quantum confinement or increased bandgap of a-SiOx . The redshift is due to the stress effect of the bandgap of Si QD core or a-SiOx . We have successfully confirmed the effect of compressive stress associated with the self-limiting oxidation by PL measurement. INTRODUCTION Recent interest in exploring the optical properties of Si nanostructures has intensified the search for a reliable technique in fabricating sub 5-nm Si structures [1]. Earlier work on the oxidation of nonplanar Si structures indicates that the oxidation rate decreases with decreasing structural dimensions because of the associated compressive stress normal to the Si/SiO2 interface [2–4]. In the case of spherical nanocrystalline Si quantum dots (QDs) fabricated by a very-high-frequency (VHF) plasma-enhanced chemical vapor deposition (PECVD), the saturation of the oxidation rate in the Si QDs has been observed by transmission electron microscopy (TEM) [5]. Although it is possible to measure the diameter of the Si QD cores by TEM, it is difficult to evaluate quantitatively the applied compressive stress to the Si QD core, which, on the other hand, can be characterized by optical measurements. By employing oxidation, the diameter of the Si QD cores can be reduced. On the other hand, the compressive stress, which is caused by the volume mismatch between Si and amorphous SiOx (a-SiOx ) (ΩSi = 20 ˚ A3 is the atomic volume of Si and ΩSiO2 = 45 ˚ A3 the molecular volume of SiO2 ), is applied to both the Si QD core and the surrounding a-SiOx layer. It is known that bulk crystalline Si has a pressure coefficient of about –1.4 meV/kbar [6] for the indirect bandgap at ∼1.1 eV and 5.2 meV/kbar [7] for the direct bandgap at ∼3.4 eV, while amorphous Si:H (a-Si:H) has a pressure coefficient of about –2 meV/kbar [8]. Although it is unclear, the pressure coefficient of a-SiOx might be a negative value by the analogy with a-Si:H. In this work, we show experimental observation of the effect of strain on the Si QDs system by photoluminescence (PL) spectroscopy.
A20.6.
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