等溫電容暫態頻譜儀在無摻雜非晶矽薄膜內缺陷能態的電特性量測上的應用

Abstract

在本論文中，我們提出一個新的測量元件結構"金屬/無摻雜非晶矽薄膜/ 低阻值單晶矽基材"來取代原有的蕭特基(Schottky)或 P/N接面結構，而 能將等溫電容暫態頻譜儀(ICTS)應用到高阻值的無摻雜非晶矽薄膜內缺陷 能態電特性參數的量測。利用此新的元件結構作ICTS量測時，我們讓空乏 區完全的蓋過非晶矽薄膜並深入到低阻值單晶矽基材中，如此，能反映非 晶矽薄膜內缺陷能態的電荷變化的暫態空乏區電容便能被量測而不會受到 非晶矽薄膜本身的大電阻所影響。利用此一技術，我們不僅成功的量得無 摻雜非晶矽薄膜內缺陷的能態分佈，並且更深入的測得其缺陷能態的電子 捕獲截面積隨不同能階的變化。進一步的，我們觀察了缺陷能態分佈及電 子捕獲截面積隨不同能階的變化分佈，隨不同的外加偏壓應力而改變的行 為。隨著外加偏壓應力時間的增長，缺陷能態的量不斷增加但電子捕獲截 面積卻沒有改變，這清楚的指出在外加偏壓應力下產生的缺陷與對應到相 同能階的原有缺陷是屬於同一種類。除此，不同能階的缺陷能態在不同的 外加偏壓應力下的增加量差異很大，又告訴我們非晶矽薄膜內的斷鍵種類 的確有很多種，隨周圍原子排列不同而有不同。這將有助於深入了解非晶 矽材料不穩定性的發生機構。最後，我們討論不同成長壓力下經電漿輔助 氣相沈積方法成長的無摻雜非晶矽薄膜的特性。實驗發現：成長壓力不僅 會影響非晶矽薄膜內缺陷的量，也會改變缺陷的能階位置以及其電子捕獲 截面積隨不同能階的變化分佈。成長壓力越大則成長的薄膜內缺陷越多， 因此其材料的不穩性也較差。 In this thesis, a novel structure, metal/undoped a-Si:H/n-type c-Si substrate, is proposed for determining the density-of- state distribution g(E) of gap states of undoped a-Si:H via isothermal capacitance transient spectroscopy (ICTS) method. In addition to the g(E), the electron-capture cross section Sn(E) of gap states in undoped a-Si:H can also be obtained via the structure. Then, carrier induced changes in the g(E) and Sn(E) of gap states of undoped a-Si:H are studied using the technique. It is found that the density of g(E) increases but the Sn(E) does not change sig- nificantly after the electron injection. These indicate that the defects created during the electron injection are of the same type as those of the as- deposited films for the corresponding energy level. In addition, the contour of g(E) reconstructs with respect to different energy levels as the stress bias voltage increases, indicating that the gap states at different energy levels are corresponding to distinct types of defects. Finally, the effects of the silane gas pressure (Ps) on the properties of gap states of undoped a-Si:H are investigated. Experimental results show that, in addition to modifying their total density, the Ps also affect the energy level and the Sn(E) of gap states.