Photoluminescence from Nanoscale Si in a-SiO x Matrix
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Photoluminescence from Nanoscale Si in a-SiOx Matrix I. Umezu1,2, K. Yoshida1,2 , M. Inada2, A. Sugimura1,2 Department of Applied Physics, Konan University, Kobe 658-8501, Japan 2 High Technology Research Center, Konan University, Kobe 658-8501, Japan 1
ABSTRACT We prepared a-SiOx (x < 2.0) films by co-sputtering Si and SiO2 targets and found that these films included nano-scaled a-Si regions in a-SiOx matrix. The structure of the films was evaluated by X-ray photoemission spectroscopy (XPS) and infrared absorption. The XPS spectra in the a-SiOx films showed two Si 2p peaks around 99.6 and 104 eV. This indicates that the a-SiOx films were composed of Si and a-SiO2 regions. The optical gap energy of this system rapidly increased when x exceeded 0.7. The rapid increase in the optical gap of this system indicates the formation of nanometer sized Si islands. The photoluminescence (PL) peak energy of this material decreased with increasing temperature above 60 K when x was less than 0.7. On the other hand, when x exceeds 0.7, the PL peak energy increases with temperature above 60 K. The quenching of PL intensity also takes place at 60 K. These results indicate a film structure change at x = 0.7, and recombination path and carrier distribution changes at 60 K. A correlation between film structure and PL properties are discussed. PL properties in this system are explained by superposition of the PL peak from nanoscale Si and a-SiO2 matrix.
INTRODUCTION Light emission from silicon based material is an important research area for optoelectronic applications. A great deal of work has been done on the visible light emission originating from Si nanostructures [1-4]. Amorphous silicon sub-oxide (a-SiOx) has possible applications as a light-emitting device since the PL energy can be controlled by changing the composition, x. The PL properties of nanocrystal silicon (nc-Si) surrounded by an SiOx layer is difficult to understand because the a-SiOx layer is also potoluminescent. Research on the PL properties of a-SiOx is important to understand the PL mechanism of Si nanostructure such as nc-Si. Although there is a long history of research on this material, the origin of the PL is not completely understood. The PL center in this material was considered to be due to defects or tail-to-tail states [5-9]. Recently, some groups have found a nanocrystal Si phase in a-SiOx and concluded that PL originates from the nanocrystal Si in a-SiOx matrix [10,11]. Adding to the difficulty in understanding the PL is the fact that the defect-related luminescence in the a-SiOx matrix is sometimes confused with luminescence from the nanocrystal. In this work, we prepared a-SiOx films by RF sputtering in order to study the correlation between the microstructure of the film and PL properties.
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EXPERIMENTAL The silicon sub-oxide (a-SiOx) film was prepared by co-sputtering Si and SiO2 targets in Ar gas. The input RF power was 40 W and the Ar gas pressure was 0.01 Torr. Film thickness was controlled to about 500 nm with the deposition time. The
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