金属-多层二维MoS2异质结的理论研究

THEORETICAL STUDY OF METAL-MULTILAYERED TWO-DIMENTIONAL MoS2 HETEROJUNCTIONS

王倩

11649012

博士

物理学

石兴强

2020-06-11

2020-07-15

哈尔滨工业大学

深圳

近十多年来，二维材料凭借非比寻常的物理特性和在超薄电子和光电子器件中的潜在应用引起了人们极大的兴趣，并引发了对其基本性质的深入研究。然而当二维半导体材料应用于电子器件时，在二维半导体和金属电极的界面会出现界面电阻，如何降低界面接触电阻是目前二维半导体材料应用面临的重要挑战。肖特基势垒是界面接触电阻最重要的成因，它的形成非常复杂，与界面附近多种物理化学因素相关。二维半导体的表面没有悬挂键，与多数金属电极接触通过物理吸附，从而在肖特基势垒的基础上产生了一个附加的隧穿势垒。最近，在二维多层半导体-三维金属异质结中发现了层数依赖特性并实现了肖特基-莫特极限，这一发现为欧姆接触的实现带来了新的希望。然而，目前层数依赖机制和肖特基-莫特极限实现的条件还不为人们所知 ，是目前迫切需要解决的科学问题。二维半导体的另一个重要应用是II型能带排列（type-II band alignment）的半导体异质结。II型能带排列的半导体异质结在载流子光子转换中起着重要作用，是光催化和光电器件的核心。然而半导体异质结中的动量失配降低了光子激发电子空穴对的跃迁效率，制约了它的发展，所以实现动量空间匹配的II型能带排列对于光电器件应用非常重要。本论文的研究主要集中在降低金属和半导体界面的接触势垒和解决半导体异质结中的动量失配问题，详细信息如下：基于密度泛函理论计算，首先证明了面内应变可以作为减小二维半导体-金属界面电阻的一个可行手段。 以二维半导体单层二硫化钼（MoS2）-金属异质结为例，随着拉伸应变的增大，异质结的n型肖特基势垒和隧穿势垒均显著降低，特别是肖特基势垒可以在达到一定程度的应变（~ 4%）时减小到零。由反键轨道贡献的MoS2导带底随拉伸应变的增加远离真空能级，导致了n型肖特基势垒随着拉伸应变的增加而减小。因此，其他具有反键导带底的半导体型二维过渡金属硫化物的肖特基势垒也可以通过拉伸应变降低到零。应变为实现二维半导体-金属接触电阻最小化提供了一种通用方法。对于二维多层半导体-三维金属异质结，发现在二维半导体层数较少的情况下（< 5层），费米能级钉扎系数对于半导体层数的变化很敏感。以多层MoS2-金属异质结为例，金属和MoS2界面会产生强烈的钉扎而MoS2范德华层间则会出现去钉扎现象。在此基础上，将金属-单层半导体费米能级钉扎理论扩展到多层半导体中， p型肖特基势垒在多层二维半导体-金属异质结中相对容易实现。多层二维半导体-金属异质结中半导体的层数依赖理论促进了一个新的研究领域的产生，即二维半导体的层数可以作为一个新的自由度来调控异质结的性能。基于多层二维半导体-金属异质结的研究，如果将二维范德华双层半导体作为吸附在金属表面的半导体同质结，则可以解决II型能带排列半导体异质结中动量失配的难题。由于二维范德华层状半导体层间固有的取向一致性，动量空间匹配的能带结构比异质结更容易实现。另外在通过金属吸附诱导的简并掺杂的半导体同质结中，层间费米能级的相对刚性平移实现了层间能带结构的“相互平行”，可以获得布里渊区内大范围的动量空间匹配。通过使用不同功函数的金属衬底，可以在大能量范围内调节半导体层间能带的相对偏移，使半导体同质结的能带排列方式由II型转变为III型，或转变为反向的II型。此外，以二维金属为衬底可获得n型和p型的欧姆接触。这种基于金属表面的范德华半导体同质结来实现II型能带排列的方法值得更多实验上的尝试和理论应用上的研究。基于以上对于二维多层半导体-金属异质结的研究，发现肖特基势垒和半导体范德华层间的能带偏移与金属-半导体界面和半导体-半导体界面的电荷分布息息相关，所以给出一个全面的界面电荷密度分布以及能带排列的机理显得至关重要。对于双层MoS2-金属体系，多种相互作用机制参与了金属-MoS2界面和MoS2-MoS2界面处的电荷重新分布，不同体系的主导机制取决于金属电极的维度及其功函数的大小。对于MoS2-三维金属界面，物理吸附引起的金属表面电子回推效应和金属诱导的间隙态对界面电荷的分布起主导作用。对于二维金属衬底，类共价准键特征出现在MoS2与中等功函数金属的界面处，而电荷转移机制在MoS2与功函数很大或很小的金属界面起主导作用。在双层MoS2-二维金属体系中，半导体-半导体界面继承了金属-半导体界面的电荷重新分布行为；而在双层MoS2-三维金属体系中，半导体-半导体界面的电荷分布与金属-半导体界面的电荷分布方向相反，产生了去钉扎效应。二维多层半导体-金属异质结中两类界面的详细研究填补了理论上的空白，并为以后的研究提供了普适的理解和新概念。

The potential of two-dimensional (2D) semiconductors in ultrathin electronics and optoelectronics has stimulated intensive research of their fundamental properties in the last more than ten years. For 2D semiconductor connecting to external circuit, a bottleneck problem to realize high-performance 2D devices is the unexpectedly high resistance at the metal-2D semiconductor interface. Recently, layer-number depend performance and Schottky-Mott limit are reported in metal–semiconductor junctions (MSJ) with multilayered 2D semiconductors (MmSJ), which brings light to the realization of Ohmic contacts. But till present, people have little knowledge about the layer-number dependence. Another important research direction of 2D semiconductor is semiconductor heterojunctions with type-II band alignment, which play an important role in carrier-photon conversion. However, the momentum-mismatch is an intractable problem which reduces the efficiency of photon excited electron-hole pairs with a radiative transition.Based on state-of-the-art density functional theory calculations, firstly the contact resistances between monolayer MoS2 and metal surfaces can be reduced by in-plane strain. The Schottky barrier (SB) and tunneling barrier (TB), both of which contribute to the interface resistance, are lowered significantly by tensile strain. Especially, the SB can reduce to zero with increasing tensile strain. The mechanism of SB height reduction under tensile strain is attributed to the increase of the electron affinity energy of MoS2 since the monolayer MoS2 conduction band minimum (CBM) is in anti-bonding. From this mechanism, the SB height in other semiconducting transitional metal dichalcogenides (TMDCs) with anti-bonding CBM could also be reduced to zero by tensile strain. Secondly, taking MmSJ with MoS2 as a typical example, the Fermi-level pinning (FLP) factor depends sensitively on the layer-number of MoS2, especially for few-layer (less than 5 layers) MoS2. Strong pinning arises right at the metal-1st-layer-MoS2 interface while depinning occurs between MoS2 layers. The depinning effect between MoS2 layers makes FLP decrease as a function of layer-number, and makes p-type SB contact favored in MmSJ than that in metal-2D monolayer MoS2 junctions, especially for junctions with large work-function metals. Thirdly, to solve the intractable problem of momentum-mismatch in 2D semiconductor heterojunctions, 2D multilayered van der Waals (vdW) semiconductor homojunctions (mSHs) supported on 2D metals could offer a universal approach to obtain type II band alignment with the advantage of wide range momentum-space-match in the Brillouin zone by band-nesting effect. There are two advantages in mSHs than 2D semiconductor heterojunctions: 1) momentum-matched band alignments are easy to achieve due to the inherent lattice-orientation-match between vdW layers of the same material in homojunction, and moreover, a wide-range momentum-space-match can be obtained by metal-induced Fermi-level rigid shift to achieve ‘parallel’ band dispersions (due to the 1st-layer of mSH is degenerately doped); and 2) largely tunable band offsets make band alignment change from Type II to Type III to inversed Type II due to charge redistribution at the interface. Moreover, the mSH is better to be supported on 2D metal rather than three-dimensional (3D) bulk metal, with the advantage of free of metal-induced-gap-states and easily obtained n- and p-type Schottky-barrier-free contacts. Fourthly, a detailed mechanism study about the nature of bonding and electron redistribution at the metal-semiconductor (M-S) and semiconductor-semiconductor (S-S) interfaces of MmSJ is revealed. Multiple mechanisms simultaneously contribute to the electron redistribution at M-S and S-S interfaces, and, the dominant mechanism depends on both the metal electrode’s dimension (2D vs 3D) and their work function. At 3D M-S interfaces, the pushback effect and metal-induced gap states play a dominant role. At 2D M-S interfaces, the covalent-like quasi-bonding feature appears for 2D metal with medium work function, while charge transfer plays the main role for 2D metals with extremely large or small work functions. The S-S interface inherits the electron-redistribution behavior at the M-S interface in MmSJ with 2D metal, while a depinning effect appears across the S-S interface in MmSJ with 3D metal.Our discoveries about strain shed a new and general light toward minimizing the contact resistance of semiconducting TMDCs-metal based contacts which can also prove applicable to other 2D semiconductors with an anti-bonding CBM (or banding VBM for p-type contacts). The most important innovation of this thesis is that the importance of the layer-number of 2D semiconductors in MSJ is highlighted, this is unnoticed before. Our work of layer-number engineering in MmSJ promotes an emerging field that layer-number can be used as a new degree of freedom for manipulating the pinning factor and SB in MmSJ and sheds light on the recent controversial experimental observations. Besides, vdW mSHs supported on 2D metals, with unexpected excellent properties than heterojunctions, stimulates experimental and theoretical studies for their various applications. Therefore, the detailed mechanism study about interfaces in MmSJ provides general insights and new concepts to better understand and use of MmSJ.

密度泛函理论计算 金属-多层半导体异质结 费米能级钉扎 肖特基势垒 II型能带排列

Density functional theory calculations metal-multilayered semiconductor heterojunction Fermi-level pinning Schottky barrier type II band alignment

中文

联合培养

王倩. 金属-多层二维MoS2异质结的理论研究[D]. 深圳. 哈尔滨工业大学,2020.