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標題: | 以第一原理計算探討二氧化矽基材內的缺陷與摻雜以及高介電常數二氧化鉿絕緣層對二硫化鉬電子性質與其載子遷移率變化之影響 First-Principles Study of the Doping Effect on MoS2 from the Impurities and Intrinsic Point Defects in the underlying SiO2 Substrate and the Origin of the Enhanced Carrier Mobility by the Top High-k HfO2 Layers |
作者: | An Ho 何安 |
指導教授: | 郭錦龍(Chin-Lung Kuo) |
關鍵字: | 二硫化鉬,第一原理計算,二氧化矽,二氧化鉿,蕭特基能障, MoS2,first-principles calculation,SiO2,HfO2,Schottky barrier, |
出版年 : | 2015 |
學位: | 碩士 |
摘要: | 由於具有獨特且優異之物理性質,二硫化鉬在各種電子元件上的應用是極具發展潛力的二維奈米材料,而金氧半場效電晶體(MOSFET) 是其在眾多可能的應用中被認為最為可行的方向之一。這主要是因為二硫化鉬具有良好的載子遷移率與電流開關比,所以有機會取代矽作為新一代電晶體中的導電通道。然而,目前對於單層二硫化鉬的基本物理性質仍有許多不清楚和未解決的地方,特別是其與基材之間的相互作用如何導致二硫化鉬電子性質改變的物理機制仍不清楚。本論文的研究目標即是運用第一原理密度泛函計算來探討二氧化矽基材內部的缺陷與摻雜以及外加高介電常數二氧化鉿絕緣層對二硫化鉬電子性質與其載子遷移率變化產生之影響與其相關物理機制分析。
在第一部分研究中,我們主要探討了當二硫化鉬置於二氧化矽基材時其n型載子之來源以及硫空缺對其電子性質可能產生的影響。我們首先建構了數個含有固有點缺陷以及硼、鈉等常見雜質的二氧化矽基材結構模型,並依此個別分析其對單層二硫化鉬產生的影響。我們計算的結果顯示,二氧化矽基材中固有的點缺陷、與硼、鈉等雜質皆可能會對二硫化鉬產生n型摻雜的效果,而其中硼摻雜所引起的n型載子來源主要為硼雜質本身以及與其伴隨產生的二氧化矽本質結構缺陷。這些摻雜物以及伴隨而生的本質缺陷可能會在二氧化矽能隙中產生電子佔據或未佔據之缺陷能態,其中部分可能成為二硫化鉬之電子施體,因而造成二氧化矽基材中的缺陷電子移轉至二硫化鉬的現象。此外,二硫化鉬之硫空缺會在導帶下方產生兩個未佔據的缺陷能態,而此缺陷能態將有利於二氧化矽基材中缺陷電子的轉移,因而導致二硫化鉬中n型載子濃度的上升。在另一方面,計算的結果也顯示鈉原子對於二硫化鉬可以產生n型摻雜效果。然而,鈉元素在二氧化矽玻璃材料中主要是以鈉離子形態存在,不會對二硫化鉬產生任何摻雜的效果。因此,我們認為二氧化矽中的鈉雜質不會是二硫化鉬主要的n型載子來源。 接著我們也探討了含氮、磷、砷等摻雜之非晶二氧化矽基材對二硫化鉬電子性質之影響,我們希望瞭解是否能透過這些摻雜物來增加二硫化鉬中電洞載子的濃度,進而引導實驗達成p型場效電晶體之應用。計算結果顯示,氮和砷摻雜於二氧化矽基材中所產生的雜質缺陷以及其伴隨產生的二氧化矽本質結構缺陷對於二硫化鉬是有效的電子受體,可使二硫化鉬的價帶電子轉移至二氧化矽基材中而產生p型摻雜的效果。此外,由於原子半徑大小與電負度等基本性質的差異,磷和砷雖為同族的元素,它們在二氧化矽結構中會傾向形成不同的鍵結形式,因而使得磷在二氧化矽結構中無法有效對二硫化鉬產生p型摻雜的效果。 在第二部份的研究中,我們主要針對高介電常數二氧化鉿絕緣層對二硫化鉬中載子遷移率增強效應之物理原因進行探討。研究的動機主要在於實驗上觀察到當二硫化鉬導電通道上方披覆一層高介電常數二氧化鉿絕緣材料作為閘極氧化層時,二硫化鉬中的載子遷移率可獲得大幅的提升,但是目前實驗上對此現象並無法提供合理的物理解釋。我們計算的結果發現,無缺陷的二氧化鉿對於二硫化鉬之電子性質並無顯著影響;然而,當二氧化鉿中含有氧空缺時,其會對於二硫化鉬會產生顯著n型摻雜的效果。我們的結果也顯示此n型摻雜的效果可有效降低二硫化鉬和源極金屬間的蕭特基能障,因而使其接觸電阻下降,最終使得二硫化鉬電晶體之載子遷移率提升。此外,我們同時探討n型摻雜對於二硫化鉬介電性質之影響。計算的結果顯示,n型摻雜僅能些微提高二硫化鉬垂直平面方向之介電常數,對於介電屏蔽效應的增強效果十分有限。因此,我們認為二氧化鉿層對於二硫化鉬介電性質的影響並非是使其載子遷移率增加的主要原因,而主要的物理因素是其對二硫化鉬產生的n型摻雜能夠有效降低二硫化鉬與源極金屬間的蕭特基能障所致。 MoS2 is a promising candidate for the new nano-electronic devices primarily due to its outstanding physical and electronic properties. In practical applications, monolayer MoS2 has been successfully integrated into a MOS transistor showing a mobility of >200 cm2/V·s with an on/off current ratio >108. However, the MoS2-based transistors still have some disadvantages that require further improvements to uplift their performance. Therefore, it would be of great interest to develop detailed atomistic understanding of this material system in many fundamental aspects, particularly for the interactions between MoS2 and the insulating dielectric substrates. In the first part of the thesis, we employed first-principles calculations to explore the origins of the n-type doping effect on MoS2 from the underlying SiO2 substrate. We first constructed various structure models of a-SiO2 containing boron, sodium and the relevant Si point defects in the substrates and then investigate the electronic structure changes of the MoS2/SiO2 hybrid system. Our calculated results show that B atoms can form various stable bonding configurations such as •BO-SiO3, •SiO2-BO2, SiO3-BO2 and B2O3 with the associated Si point defects like E’ and S centers in a-SiO2. Furthermore, those dopant configurations and the associated Si point defects can induce the formation of electronic defect states in the band gap of SiO2, some of which can be effective electron donors inducing electron transfer from a-SiO2 to monolayer MoS2. We also found that this n-type doping effect can be much enhanced by the appearance of S vacancies in MoS2 mainly attributed to the induced unoccupied defect states at ~0.7 eV below its conduction band minimum. On the other hand, Na atom in a-SiO2 was found to be an effective electron donor on MoS2 as well. Nevertheless, since Na generally appears as Na+ ions in a-SiO2 glasses, it is not expected to be the major source contributing to the n-type conducting behavior in MoS2 monolayer. We next investigated the effects of the N-, P-, and As-doped SiO2 substrates on the electronic property changes of MoS2 monolayer. Our calculations unambiguously show that some of the bonding configurations of N and As atoms with the associated Si point defects in a-SiO2 can become effective electron acceptors for monolayer MoS2, providing one possible route to fabricate the MoS2-based p-MOSFET. Nevertheless, the P-doped SiO2 is not possible to induce any p-type doping on MoS2 monolayer though As and P are both the Group VA elements in the periodic table. In the second part of the thesis, we intend to reveal the physical origin of the enhanced carrier mobility in MoS2 by the deposition of the top high-k HfO2 layers. We first investigated the influence of HfO2 layer on the electronic property of monolayer MoS2, and then probed for the effect of the induced electronic doping on the dielectric property changes of MoS2 as well as the contact resistance between the S/D metal and MoS2 layer. Our results show that perfect HfO2 layer has no significant effect on the electronic property change of MoS2, but as O vacancy is present in the high-k layer, it was found to induce significant amount of n-type doping on monolayer MoS2. Our calculations further show that the induced n-type doping from HfO2 can effectively reduce the Schottky barrier height between the S/D metal and monolayer MoS2, thereby largely enhancing the electron mobility in the MoS2-based MOS transistor. In addition, our calculations show that the dielectric screening along the z axis (?zz) of MoS2 is nearly unchanged upon n-type doping. Therefore, the reduction of coulomb scattering arising from the charge traps in SiO2 cannot be used to account for the enhanced electron mobility in the MoS2 channel by the deposited HfO2 layer. |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/52972 |
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顯示於系所單位: | 材料科學與工程學系 |
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