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標題: | 第一原理研究鍺元件高介電氧化層介面與新穎通道彈道傳輸研究 First-Principle Study of High-k/ Ge Interface and Ballistic Transport of Novel Channels |
作者: | Hung-Chih Chang 張弘志 |
指導教授: | 劉致為(Chee Wee Liu) |
關鍵字: | 第一原理,鍺,二氧化鍺,二氧化鋯,二硫化錳,應變,烏采,彈道傳輸, First-Principle Theory,Germanium,GeO2,ZrO2,MoS2,Strain,Wurtzite, Ballistic Transport, |
出版年 : | 2014 |
學位: | 博士 |
摘要: | 由於互補式金氧半元件的尺寸微縮逐漸到達物理極限,為了讓摩爾定律繼續延續下去,使用新穎材料已是無法避免的趨勢。鍺被認為極有可能取代矽成為下一世代金氧半元件的通道材料。 在使用含有無缺陷鍺次級氧化物的氧化鍺與鍺所建立的介面模型中,並沒有發現任何介面帶隙能態,意味著單純的鍺次級氧化物並不是造成元件特性降低的來源。因此,在本論文中,我們採取第一原理密度泛函的方法來建立具有懸浮鍵的鍺/二氧化鍺介面模型,進一步探討介面上處在不同氧化態的鍺原子,也就是鍺次級氧化物中的鍺原子之上的懸浮鍵所造成的帶隙能態能量位置的差異,還有不同氧化態對能量位置的影響,藉以提出之能帶中不同位置之帶隙能態的可能來源,同時我們也解釋了在二氧化鍺中的氧化層固定電荷來源。此外,由於預期高介電常數金屬氧化物在未來極有可能被運用在鍺元件上,我們探索二氧化鋯的氧化層性質。二氧化鋯上使用四氟化碳氣體作後閘極式電漿處理,發現二氧化鋯仍可以四方晶相形成。在等效氧化層厚度為0.4奈米的金氧半電容元件上,電性遲滯現象可以改善,原因可歸於鋯-氟鍵結的形成,減少了二氧化鋯中的氧空缺缺陷數量。
論文第二部分是使用鍺以外的新材料,並研究在低維度狀況下的彈道傳輸效能。過渡金属硫族化合物體材料是間接帶隙半導體, 而單層材料是直接帶隙半導體。我們研究單層二硫化錳的能帶結構及施加應變後的變化,並導入彈道傳輸理論,計算其在應變下的增強效果。而另外一種新材料則是使用六方晶系的烏采結構矽或鍺。雖然屬於在幾十年就被發現的結構,但是該系的電子結構並未廣泛的研究。在最後的章節,利用第一原理計算出合理的能隙與能帶結構後,同樣討論應變後的變化及彈道傳輸的效能。 Since the traditional Si complementary metal-oxide-semiconductor (CMOS) scaling is approaching to its physical limits, introduction of performance boosters by alternative materials has become necessary to continue the scaling trend. Ge has been regarded as one of the promising candidates for emergent devices. Within Ge/GeO2 interface, a defect-free suboxide (GeOx) transition region did not reveal any gap states within the Ge band gap, suggesting that the suboxide itself should not be invoked as the cause of any electrical degradation. In this work, calculations based on the density functional theory predict the formation of gap states due to the presence of the dangling bonds of Ge0+, Ge 1+, and Ge 2+ atoms at the Ge/GeO2 interface. The oxygen-vacancy-related defect in GeO2 indicates the +2 charged state can be positive fixed-charge in the GeO2 and the theoretical result is examined by capacitance-voltage measurement of Al/GeO2/Ge MOS. Moreover, to achieve low equivalent oxide thickness, utilizing remote NH3/H2 plasma on GeO2/Ge and followed by ALD ZrO2 forms the nearly-free interfacial layer and ~0.4 nm EOT. The fluorine incorporation by CF4 plasma is demonstrated to effectively passivate these defects both experimentally and theoretically. In the second part of the thesis, the band structures of two-dimensional monolayer molybdenum disulfide with different strains are calculated. The symmetry of hexagonal geometry is broken down with uniaxial strain and the valleys become anisotropic. Both tensile strain and compressive strain reduces bandgap. Isotropic strain reduces bandgap more than the uniaxial strain. The compressive strain causes the shift of conduction band minimum from K point to Q point in reciprocal lattice and degrade the current density. The uniaxial tensile strain along the zigzag direction reduces the conductive effective mass and enhances the ballistic current significantly. Wurtzite (WZ) Ge can move the lowest conduction valley into Γ point to form the direct bandgap with no degeneracy as compared to 4-fold degenerated diamond Ge, and even breaks the symmetry of Γ point to yield a small effective mass on the hexagonal plane, and a large quantization mass along the c-axis. Due to the high injection velocity, the WZ Ge channel has a 1.6X ballistic current of the best Ge (111 plane)/[110 channel direction] double gate FETs with 8.5nm channel thickness. |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/18821 |
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顯示於系所單位: | 電子工程學研究所 |
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