| Summary: | 碩士 === 國立臺灣大學 === 材料科學與工程學研究所 === 98 === Hydrogenated amorphous silicon has long been a subject of great interest mainly due to its important role in the fabrication of large area electronic devices such as photovoltaic cells and liquid-crystal displays. Nevertheless, many of its important properties, such as the structure of defects and the local bonding environments of hydrogen, are still not fully understood yet. Furthermore, at present there is no satisfactory theoretical treatment has been established regarding the influence of the Si-H bonding on the electronic properties of amorphous silicon though it has been well known that its band gap increases with the hydrogen content.
In this study, we employed first principles molecular dynamic simulations to prepare various realistic structural models of amorphous silicon via different procedures with hydrogen content ranging from zero to 20%. The evolution of the structure, electrical, and the optical properties of amorphous silicon with the hydrogen content is mainly focused. The structure properties of amorphous silicon in short-ranged order are characterized using radial distribution function, structure factor, dihedral angle and bond angle distributions, respectively. In addition, the evolution of the structure properties in medium-ranged order is characterized using the ring distribution analysis. Moreover, the evolution of the electrical and optical properties of amorphous silicon with the hydrogen content is investigated by calculating the electronic density of states and the dielectric function.
We found that increasing the hydrogen content do help relax the internal strain in the amorphous bond network but does not really have significant influence on the structure properties of amorphous silicon in short-ranged order. For the medium-ranged order characteristics, our results showed that the total number of rings in amorphous silicon bond network can be reduced by the addition of hydrogen atoms. Nevertheless, the ring size distribution is almost unchanged. Furthermore, our simulations showed that hydrogen atoms are not homogeneously distributed in amorphous silicon, and the addition of hydrogen atoms may help induce the formation of large voids in the amorphous bond network. Most interestingly, our results showed that, in addition to the number of Si-H bonding, the volume of voids/the density of the system also play an important role in determining the electronic and optical properties of amorphous silicon.
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