以第一原理計算與分子動態模擬探討氫化過程對非晶矽的結構與電性之影響

Abstract

近年來氫化非晶矽在太陽能電池與液晶顯示器等大面積電子裝置的廣泛應用使其成為極具競爭力的半導體材料並引起研究學者們的高度關注。然而，目前有許多複雜且具有挑戰性的問題，例如非晶矽結構中的缺陷與矽氫鍵結的種類等，仍待更進一步的探討與釐清。此外，文獻中關於氫化非晶矽的能隙值隨氫含量的增加而上升的現象尚未有令人滿意的理論解釋。在本研究中我們運用第一原理計算與分子動態模擬來建構合理的純非晶矽以及不同氫含量的氫化非晶矽結構模型，並探討系統中氫含量的增加對非晶矽的結構性質、電子態密度與能帶結構、以及光學性質所造成之影響。根據我們所建構的非晶矽結構模型，我們利用各種結構分析工具，諸如徑向分佈、結構因子、鍵角分佈、二面角分佈、配位數分佈以及環結構分析等，來探討在不同氫含量下非晶矽模型的中、短程距離結構特徵，並計算其振動態密度來驗證我們所產生氫化非晶矽結構模型的合理性。另外，經由電子態密度和介電函數的計算，我們探討了氫化非晶矽結構中氫含量的變化對其電子與光學性質的影響，並進一步分析其與結構特徵間的關聯性。我們研究的結果顯示，在非晶矽中加入氫原子會導致結構中產生較大的孔隙而降低了系統的密度，且大部份的氫原子在結構中會聚集在孔隙的周圍而呈現不均勻的分布。此外，氫原子的加入確實有鬆弛非晶矽內部的應力而達到改善其結構性質的效果。在電子與光學性質方面，我們的結果顯示矽氫鍵結的數量並非影響非晶矽光、電子性質的唯一因素，而結構中孔隙的體積或是系統的密度亦扮演著極為重要的角色。

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.