為發展太陽能電池所進行之矽奈米粒子電子結構的研究

Abstract

利用量子侷限效應(quantum confinement effect)的特性，將矽做成奈米尺度的大小，將會大大的改變它原有的電子結構和光學特性，而在近年來矽奈米晶體已經被廣泛的討論，且也應用在半導體、太陽能電池、生醫等等各領域。 而在我們的研究當中我們將針對在真空中孤立的矽奈米晶體去計算其電子結構和吸收光譜，並且我們也將表面分成兩種情況去討論:表面用氫將表面鈍化與表面用氫氧根去鈍化，並且去計算其電子結構和光學能譜。且在我們計算中我們將會在標準的密度泛函計算(generalized gradient approximation or local density approximation)中加入高階的修正，如考慮基態電子密度的Heyd-Scuseria-Ernzerhof混合泛函(hybrid functional)計算和使用激發態近似的與時有關的密度泛函理論(time dependent density functional theory)計算等，這些高階修正會幫我們得到更正確的電子結構和光學性質。 而我們將會計算矽奈米晶體在不同尺寸下的能隙值，且我們用與時有關的密度泛函理論中算出矽奈米晶體的吸收光譜，而在矽奈米晶體表面用氫氧根處理的狀況下將也可以看到當奈米晶體的尺度達到很小的時候表面效應的影響可能會大於量子侷限效應。在我們的計算中，我們結合了Heyd-Scuseria-Ernzerhof混合泛函和緊束縛法，利用這兩種方法可以就可以去計算尺寸由小到大的矽奈米晶體能隙值。

Electronic and optical properties can vary significantly from bulk to nanostructures because of quantum confinement effects. During the past 20 years, silicon nanostructures have been extensively investigating in semiconductors, solar-cells, and biophysical systems. We calculate the electronic structures and optical absorption spectra in isolated nanocrystals. In this thesis, the silicon nanocrystals with two different surfaces are studied: the hydrogen- terminated and hydroxiyl-terminated. To obtain accurate electronic and optical properties, we improve standard density functional theory methods (local density approximation or generalized gradient approximation) with the ground-state method Heyd-Scuseria-Ernzerh of hybrid functional and the excited-state approach time dependent density functional theory. For large sizes of silicon nanocrystals, we use the semi-empirical tight-binding method. We calculate the energy gaps of different sizes of silicon nanocrystals. We further applied the time dependent density functional theory method to obtain absorption spectra. In small sizes of hydroxiyl-terminated silicon nanocrystals, the surface effect can dominant over the size effect. We combine the calculations of Heyd-Scuseria-Ernzerh and tight-binding to obtain the energy gaps of silicon nanocrystals over the full range of their sizes.