Visible Light Emission and Absorption of Si Quantum Dots: Measurement and Analysis

• Main Authors: J. J. Chiu, 曲建仲
• Other Authors: T. P. Perng
• Format: Others
• Language: en_US
• Published: 1999
• Online Access: http://ndltd.ncl.edu.tw/handle/24602482639678865388

碩士 === 國立清華大學 === 材料科學工程學系 === 87 === Nanotechnology has become an increasingly hot and popular research subject recently. The physical and chemical properties such as optical, electronic, magnetic, mechanical, and chemical activity, etc., of nanocrystalline materials, such as nano-wires and nano-dots, are very different from those of conventional materials. Therefore, nanocrystalline materials can be used in chemical and biological sensors, photochemical devices, optical electronic devices and biomedical applications, etc. Among them, nanometer-sized silicon has attracted much attention in recent years because of its potential applications in microelectronics and the lower price. In this study, real zero dimensional Si quantum dots without any substrate were fabricated. Any possible substrate effects in the measurement of optical and electronic properties are obviated. Si quantum dots with particle sizes varying from 20 nm to 1 nm were prepared, and the size distribution was well controlled. Some other nanostructures, such as nano-nets, were also formed by thermal evaporation. According to the XRD patterns, the average grain size of Si quantum dots prepared by thermal evaporation was about 10 nm. From the TEM observation, the particles were spherical and the size varied from 1 to 20 nm. The atomic image of the Si quantum dots was also observed by HRTEM. Some areas with ordered lattice points revealed that there were some single crystalline regions in a Si quantum dot. From FTIR spectrum and ESCA analysis, Si nanoparticles with smaller size absorbed more oxygen, hydrogen, and nitrogen. Si quantum dots without the substrate effect showed visible light emission from 5000 A to 9000 A, and the maximum PL intensity occurred at about 8200 A. The PL intensity increased as the particle size reduced, implying that the surface area of Si nanoparticles dominated the light emission process. Si quantum dots that absorbed oxygen on the surface were deoxidized by H2 when heated by a He-Cd laser, resulting in disappearance of photoluminescence. This result illustrated that the oxide layer on the surface of Si quantum dots played an important role in the luminescence. Based on the surface state mechanism and the above results, a new model "oxide induced surface state mechanism" was proposed to explain the experimental results In addition, from the UV-visible spectrum, the fundamental gap of Si quantum dots had a blue shift from 2.70 eV to 3.54 eV. It revealed that quantum size effect dominated the optoelectric properties of silicon.