有机气相沉积过程中分子位相转变和双层石墨烯带隙打开的理论和模拟研究
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摘要
近年来，有机半导体薄膜（OSTFs）受到了人们的广泛关注。与传统的无机半导体薄膜相比，OSTFs具有沉积成本低、柔性、可大面积加工等优点。OSTFs作为有源层在电子器件中有潜在的应用，如有机场效应晶体管（OFETs）、有机光伏电池、有机发光二极管。在生产高端产品方面，虽然OFETs还无法与硅金属氧化物半导体场效应晶体管（MOSFETs）相比，但是它们可以应用于低成本的柔性记忆卡、智能价格标签、显示器的像素驱动等。有机气相沉积（OVD）是制备OFETs有源层的一种有效方法。OFETs的性能在很大程度上依赖于电介质附近初始几层分子层的形态以及分子位相，而它们反过来由OSTFs的初始生长阶段控制。目前，人们主要集中在对OSTFs的生长机制、形态以及生长动力学进行研究，而对于初始生长阶段的研究已经远远落后，这主要是因为直接的实验观测具有空间和时间尺度上的局限性。所以对OVD过程中OSTFs的初始生长阶段的研究是非常迫切的。
另一方面，在高端电子电路中，现代数字逻辑基于硅互补金属氧化物半导体（CMOS）技术，其中逻辑栅极由硅MOSFETs构成。几十年来，MOSFETs的小型化已经是改善数字逻辑的关键。然而，由于硅的物理极限和短沟道效应，MOSFETs的小型化正在接近极限。为了持续这一领域的发展，有必要寻找硅的替代物来作为场效应晶体管（FETs）的沟道材料。石墨烯是由碳原子紧密堆垛成蜂房晶格的二维片层。如果一个FET具有薄的势垒层和薄的栅极控制区域，那么就可以把短沟道效应降低到很短的栅极长度。因此，能在原子层厚度制备沟道成为石墨烯应用于FETs的最吸引人的特征。此外，石墨烯具有极高的载流子迁移率（μ）。这些优异的性质使石墨烯成为高速FETs的潜在沟道材料。然而，由于石墨烯没有带隙，所以纯石墨烯FETs的开/关电流比（Ion/Ioff）非常低，从而导致高的静态能耗。因此，在石墨烯中打开一个大的带隙同时不降低它的μ值是其应用于FETs的前提。
基于以上考虑，我们通过热力学建模和分子动力学模拟，研究了OVD过程中OSTFs的初始生长阶段。此外通过第一原理密度泛函理论（DFT）计算，我们系统研究了来自基底和有机分子的双掺杂对双层石墨烯（BLG）带隙打开的影响，并且发展了一种满足技术要求的双掺杂BLG。主要研究成果分为以下三部分：
一、建立了一个统一的热力学模型来描述OVD过程中盘状和棒状有机分子的初始生长阶段。在高的基底温度和低的沉积速率条件下，初始生长的平行簇可以在一个临界分子数Nc时转变成一个垂直簇。Nc由分子的表面能γf（对于盘状分子，γf指垂直于π-π交互作用方向的分子面的表面能；对于棒状分子，γf指平行于分子长轴的分子面的表面能）与基底的表面能γsub之间的竞争决定。通过对酞菁分子的OVD过程进行热力学分析和分子动力学模拟，我们进一步证实了平行到垂直的位相转变机制。当N(5/7)γsub时，初始生长的平行簇倾倒形成一个垂直簇。结果表明，可以通过表面能来控制位相转变从而形成稳定的垂直位相，以用于OFETs。
二、通过第一原理DFT计算定量确定BLG在N,N-二甲基对苯二胺/BLG（DMPD/BLG）系统中呈现p掺杂，而在四氰乙烯/BLG（TCNE/BLG）系统中呈现n掺杂。来自表面具有O2–的非晶SiO2的相反p掺杂使DMPD/BLG系统的带隙从106增加到253meV。相似地，来自具有碳缓冲层的4H-SiC(0001)的相反n掺杂使TCNE/BLG系统的带隙从98增加到211meV。增加的带隙可以提高FETs的Ion/Ioff值。同时，由于弱的界面交互作用，BLG的μ值可以在很大程度上保留下来。此工作为基于BLG的FETs的进一步发展提供了理论基础。
三、通过来自双（五甲基环戊二烯）钴（DMC）的n掺杂和来自功能化非晶SiO2栅极电介质的p掺杂，我们利用第一原理DFT计算预测了BLG的带隙打开。在BLG上存在MDC单层和非晶SiO2栅极电介质表面存在最大浓度的O2时，这个双掺杂的BLG呈现出390394meV的带隙和5952meV的狄拉克能级偏移。前者非常接近400meV的技术要求，后者恰好位于栅极电压可达到的300meV的范围内。高的μ值在很大程度上保留下来，同时Ion/Ioff满足104107的技术要求。在此方法中没有加外电场，从而避免了准备双栅极结构的复杂制备过程和添加一个额外栅极所引起的μ和Ion/Ioff值的大幅下降。这些优异的性质使这个双掺杂BLG有可能用作现代数字逻辑中单栅FETs的沟道材料。
In recent years, organic semiconductor thin films (OSTFs) have attracted considerableattention. Comparing with traditional inorganic semiconductor thin films, OSTFs possess theadvantages of low-cost deposition, flexibility, and large area processing. As active layers,OSTFs have potential applications in electronic devices, such as organic field-effecttransistors (OFETs), organic photovoltaic cells, and organic light-emitting diodes. AlthoughOFETs are not expected to compete with silicon metal-oxide-semiconductor field-effecttransistors (MOSFETs) in the production of high-end products, they can be used in lost-costflexible memory cards, smart price tags, pixel drivers for displays, and so on. Organic vapordeposition (OVD) is an effective way to fabricate active layers for OFETs. The performanceof OFETs depends strongly on the morphology and molecular orientation of the first fewmolecular layers near gate dielectrics, which are in turn governed by the initial growth stageof OSTFs. Until now, many efforts have concentrated on growth mechanism, morphology,and growth dynamics of OSTFs. However, the studies on the initial growth stage havelagged far behind due to the limitations of spatial and temporal scales in direct experimentalobservation. Therefore, the investigations on the initial growth stage of OSTFs during OVDprocess become apparently urgent.
On the other hand, in high-end electronic circuits, modern digital logic is based onsilicon complementary metal-oxide-semiconductor (CMOS) technology, where the logicgates consist of silicon metal-oxide-semiconductor field effect transistors (MOSFETs). Fordecades, making MOSFETs smaller has been the key to improve the digital logic. However,MOSFETs scaling is approaching the limits due to the physical limitation of silicon as wellas the short-channel effects. To continue the development in this field, a suitable substitute ofsilicon is necessary as the channel material of field effect transistors (FETs). Graphene is atwo-dimensional sheet of carbon atoms tightly packed into a honeycomb lattice. If a FET hasa thin barrier and a thin gate-controlled region, it will be robust against short-channel effectsdown to very short gate lengths. Therefore, the possibility of making channels at atomiclayer thick is considered as the most attractive feature of graphene for use in FETs. Moreover,graphene has an extremely high carrier mobility (μ). These impressive properties make graphene a very promising channel material for high-speed FETs. However, due to theabsence of a bandgap, pristine graphene-based FETs have a very low on/off current ratio(Ion/Ioff), leading to high static power dissipation. Therefore, opening a sizable bandgap ofgraphene without degrading the μ value is the prerequisite for use in FETs.
Based on above considerations, by using thermodynamic modeling and moleculardynamics simulations, we investigate the initial growth stage of OSTFs during OVD process.Furthermore, by using first-principles density functional theory (DFT) calculations, wesystematically study the effect of dual-doping from substrates and organic molecules on thebandgap opening of bilayer graphene (BLG), and develop a dual-doped BLG that satisfiesthe technical requirements. The main results are divided into three parts as following:
Firstly, a unified thermodynamic model is established to characterize the initial growthstage of disk-like and rod-like organic molecules during OVD process. Under high substratetemperature and low deposition rate, the initially grown parallel cluster can transfer to thenormal one at a critical molecular number Nc, which is determined by the competitionbetween surface energy of molecule γf(surface energy of the molecular surface normal toπ-π interaction direction for disk-like molecule or parallel to the molecular axis for rod-likemolecule) and that of substrate γsub. By thermodynamic analyses and molecular dynamicssimulations for the OVD process of phthalocyanine, we further confirm this parallel-normaltransformation mechanism. When N(5/7)γsub, the grown parallelcluster tilts down to form a normal one. The results show that the orientation transformationcan be controlled by surface energy to get a stable normal orientation for use in OFETs.
Secondly, our first-principles DFT calculations have quantitatively identified that BLGis n-doped in N,N-dimethyl paraphenylenediamine/BLG (DMPD/BLG) system while it isp-doped in tetracyanoethylene/BLG (TCNE/BLG) system. The opposite p-doping fromamorphous SiO2substrate with O2–on its surface increases the bandgap of DMPD/BLGsystem from106to253meV. Similarly, the bandgap of TCNE/BLG system is enhancedfrom98to211meV by the opposite n-doping from4H-SiC(0001) with a C buffer layer. Theincreased bandgap can improve Ion/Ioffvalue of FETs. Meanwhile, velue of BLG is largelymaintained due to the weak interaction at the interface. Thus, this work provides thescientific basis for further development in BLG-based FETs.
Thirdly, through n-doping from decamethylcobaltocene (DMC) and p-doping fromfunctionalized amorphous SiO2gate dielectric, the bandgap opening in BLG is predicted by first-principles DFT calculations. With DMC monolayer on BLG and maximal O2on amorphous SiO2, the dual-doped BLG presents a bandgap of390394meV and a Dirac levelshift of5952meV. The former is close to the technical requirement of400meV, whilethe latter lies in the gate voltage accessible range of300meV. The high value largelyremains with Ion/Ioffsatisfying the technical requirement of104107. No external electricfield is employed in this method, avoiding a complex fabrication step for preparing adual-gate structure and a substantial reduction in and Ion/Ioffvalues induced by adding anextra gate. These impressive properties promise this dual-doped BLG as the channel of asingle-gate FET used in modern digital logic.
另一方面，在高端电子电路中，现代数字逻辑基于硅互补金属氧化物半导体（CMOS）技术，其中逻辑栅极由硅MOSFETs构成。几十年来，MOSFETs的小型化已经是改善数字逻辑的关键。然而，由于硅的物理极限和短沟道效应，MOSFETs的小型化正在接近极限。为了持续这一领域的发展，有必要寻找硅的替代物来作为场效应晶体管（FETs）的沟道材料。石墨烯是由碳原子紧密堆垛成蜂房晶格的二维片层。如果一个FET具有薄的势垒层和薄的栅极控制区域，那么就可以把短沟道效应降低到很短的栅极长度。因此，能在原子层厚度制备沟道成为石墨烯应用于FETs的最吸引人的特征。此外，石墨烯具有极高的载流子迁移率（μ）。这些优异的性质使石墨烯成为高速FETs的潜在沟道材料。然而，由于石墨烯没有带隙，所以纯石墨烯FETs的开/关电流比（Ion/Ioff）非常低，从而导致高的静态能耗。因此，在石墨烯中打开一个大的带隙同时不降低它的μ值是其应用于FETs的前提。
基于以上考虑，我们通过热力学建模和分子动力学模拟，研究了OVD过程中OSTFs的初始生长阶段。此外通过第一原理密度泛函理论（DFT）计算，我们系统研究了来自基底和有机分子的双掺杂对双层石墨烯（BLG）带隙打开的影响，并且发展了一种满足技术要求的双掺杂BLG。主要研究成果分为以下三部分：
一、建立了一个统一的热力学模型来描述OVD过程中盘状和棒状有机分子的初始生长阶段。在高的基底温度和低的沉积速率条件下，初始生长的平行簇可以在一个临界分子数Nc时转变成一个垂直簇。Nc由分子的表面能γf（对于盘状分子，γf指垂直于π-π交互作用方向的分子面的表面能；对于棒状分子，γf指平行于分子长轴的分子面的表面能）与基底的表面能γsub之间的竞争决定。通过对酞菁分子的OVD过程进行热力学分析和分子动力学模拟，我们进一步证实了平行到垂直的位相转变机制。当N
二、通过第一原理DFT计算定量确定BLG在N,N-二甲基对苯二胺/BLG（DMPD/BLG）系统中呈现p掺杂，而在四氰乙烯/BLG（TCNE/BLG）系统中呈现n掺杂。来自表面具有O2–的非晶SiO2的相反p掺杂使DMPD/BLG系统的带隙从106增加到253meV。相似地，来自具有碳缓冲层的4H-SiC(0001)的相反n掺杂使TCNE/BLG系统的带隙从98增加到211meV。增加的带隙可以提高FETs的Ion/Ioff值。同时，由于弱的界面交互作用，BLG的μ值可以在很大程度上保留下来。此工作为基于BLG的FETs的进一步发展提供了理论基础。
三、通过来自双（五甲基环戊二烯）钴（DMC）的n掺杂和来自功能化非晶SiO2栅极电介质的p掺杂，我们利用第一原理DFT计算预测了BLG的带隙打开。在BLG上存在MDC单层和非晶SiO2栅极电介质表面存在最大浓度的O2时，这个双掺杂的BLG呈现出390394meV的带隙和5952meV的狄拉克能级偏移。前者非常接近400meV的技术要求，后者恰好位于栅极电压可达到的300meV的范围内。高的μ值在很大程度上保留下来，同时Ion/Ioff满足104107的技术要求。在此方法中没有加外电场，从而避免了准备双栅极结构的复杂制备过程和添加一个额外栅极所引起的μ和Ion/Ioff值的大幅下降。这些优异的性质使这个双掺杂BLG有可能用作现代数字逻辑中单栅FETs的沟道材料。
In recent years, organic semiconductor thin films (OSTFs) have attracted considerableattention. Comparing with traditional inorganic semiconductor thin films, OSTFs possess theadvantages of low-cost deposition, flexibility, and large area processing. As active layers,OSTFs have potential applications in electronic devices, such as organic field-effecttransistors (OFETs), organic photovoltaic cells, and organic light-emitting diodes. AlthoughOFETs are not expected to compete with silicon metal-oxide-semiconductor field-effecttransistors (MOSFETs) in the production of high-end products, they can be used in lost-costflexible memory cards, smart price tags, pixel drivers for displays, and so on. Organic vapordeposition (OVD) is an effective way to fabricate active layers for OFETs. The performanceof OFETs depends strongly on the morphology and molecular orientation of the first fewmolecular layers near gate dielectrics, which are in turn governed by the initial growth stageof OSTFs. Until now, many efforts have concentrated on growth mechanism, morphology,and growth dynamics of OSTFs. However, the studies on the initial growth stage havelagged far behind due to the limitations of spatial and temporal scales in direct experimentalobservation. Therefore, the investigations on the initial growth stage of OSTFs during OVDprocess become apparently urgent.
On the other hand, in high-end electronic circuits, modern digital logic is based onsilicon complementary metal-oxide-semiconductor (CMOS) technology, where the logicgates consist of silicon metal-oxide-semiconductor field effect transistors (MOSFETs). Fordecades, making MOSFETs smaller has been the key to improve the digital logic. However,MOSFETs scaling is approaching the limits due to the physical limitation of silicon as wellas the short-channel effects. To continue the development in this field, a suitable substitute ofsilicon is necessary as the channel material of field effect transistors (FETs). Graphene is atwo-dimensional sheet of carbon atoms tightly packed into a honeycomb lattice. If a FET hasa thin barrier and a thin gate-controlled region, it will be robust against short-channel effectsdown to very short gate lengths. Therefore, the possibility of making channels at atomiclayer thick is considered as the most attractive feature of graphene for use in FETs. Moreover,graphene has an extremely high carrier mobility (μ). These impressive properties make graphene a very promising channel material for high-speed FETs. However, due to theabsence of a bandgap, pristine graphene-based FETs have a very low on/off current ratio(Ion/Ioff), leading to high static power dissipation. Therefore, opening a sizable bandgap ofgraphene without degrading the μ value is the prerequisite for use in FETs.
Based on above considerations, by using thermodynamic modeling and moleculardynamics simulations, we investigate the initial growth stage of OSTFs during OVD process.Furthermore, by using first-principles density functional theory (DFT) calculations, wesystematically study the effect of dual-doping from substrates and organic molecules on thebandgap opening of bilayer graphene (BLG), and develop a dual-doped BLG that satisfiesthe technical requirements. The main results are divided into three parts as following:
Firstly, a unified thermodynamic model is established to characterize the initial growthstage of disk-like and rod-like organic molecules during OVD process. Under high substratetemperature and low deposition rate, the initially grown parallel cluster can transfer to thenormal one at a critical molecular number Nc, which is determined by the competitionbetween surface energy of molecule γf(surface energy of the molecular surface normal toπ-π interaction direction for disk-like molecule or parallel to the molecular axis for rod-likemolecule) and that of substrate γsub. By thermodynamic analyses and molecular dynamicssimulations for the OVD process of phthalocyanine, we further confirm this parallel-normaltransformation mechanism. When N
Secondly, our first-principles DFT calculations have quantitatively identified that BLGis n-doped in N,N-dimethyl paraphenylenediamine/BLG (DMPD/BLG) system while it isp-doped in tetracyanoethylene/BLG (TCNE/BLG) system. The opposite p-doping fromamorphous SiO2substrate with O2–on its surface increases the bandgap of DMPD/BLGsystem from106to253meV. Similarly, the bandgap of TCNE/BLG system is enhancedfrom98to211meV by the opposite n-doping from4H-SiC(0001) with a C buffer layer. Theincreased bandgap can improve Ion/Ioffvalue of FETs. Meanwhile, velue of BLG is largelymaintained due to the weak interaction at the interface. Thus, this work provides thescientific basis for further development in BLG-based FETs.
Thirdly, through n-doping from decamethylcobaltocene (DMC) and p-doping fromfunctionalized amorphous SiO2gate dielectric, the bandgap opening in BLG is predicted by first-principles DFT calculations. With DMC monolayer on BLG and maximal O2on amorphous SiO2, the dual-doped BLG presents a bandgap of390394meV and a Dirac levelshift of5952meV. The former is close to the technical requirement of400meV, whilethe latter lies in the gate voltage accessible range of300meV. The high value largelyremains with Ion/Ioffsatisfying the technical requirement of104107. No external electricfield is employed in this method, avoiding a complex fabrication step for preparing adual-gate structure and a substantial reduction in and Ion/Ioffvalues induced by adding anextra gate. These impressive properties promise this dual-doped BLG as the channel of asingle-gate FET used in modern digital logic.
引文
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