文献阅读:06

标题:Strong Interlayer Coupling in van der Waals Heterostructures Built from Single-Layer Chalcogenides
作者:Hui Fang, Corsin Battaglia, Carlo Carraro, Slavomir Nemsak, Burak Ozdole, Jeong Seuk Kang, Hans A. Bechtel, Sujay B. Desai, Florian Kronast, Ahmet A. Unalh, Giuseppina Conti, Catherine Conlon, Gunnar K. Palsson, Michael C. Martin, Andrew M. Minor, Charles S. Fadley, Eli Yablonovitch, Roya Maboudian and Ali Javey
期刊:PNAS
日期:2014

简介:这是一篇关于半导体异质结的文献,标题为“单层硫化物构建的范德华异质结中的强层间耦合作用”。大致翻译了一下,翻译仅供参考,请以原文为准。如翻译有不妥之处,欢迎一起讨论。

摘要

Semiconductor heterostructures are the fundamental platform for many important device applications such as lasers, light-emitting diodes, solar cells, and high-electron-mobility transistors. Analogous to traditional heterostructures, layered transition metal dichalcogenide heterostructures can be designed and built by assembling individual single layers into functional multilayer structures, but in principle with atomically sharp interfaces, no interdiffusion of atoms, digitally controlled layered components, and no lattice parameter constraints. Nonetheless, the optoelectronic behavior of this new type of van der Waals (vdW) semiconductor heterostructure is unknown at the single-layer limit. Specifically, it is experimentally unknown whether the optical transitions will be spatially direct or indirect in such heterobilayers. Here, we investigate artificial semiconductor heterostructures built from single-layer WSe2 and MoS2. We observe a large Stokes-like shift of $\sim 100 \rm{meV}$ between the photoluminescence peak and the lowest absorption peak that is consistent with a type II band alignment having spatially direct absorption but spatially indirect emission. Notably, the photoluminescence intensity of this spatially indirect transition is strong, suggesting strong interlayer coupling of charge carriers. This coupling at the hetero-interface can be readily tuned by inserting dielectric layers into the vdW gap, consisting of hexagonal BN. Consequently, the generic nature of this interlayer coupling provides a new degree of freedom in band engineering and is expected to yield a new family of semiconductor heterostructures having tunable optoelectronic properties with customized composite layers.

半导体异质结是激光、发光二极管、太阳能电池和高电子迁移率晶体管等重要器件应用的基础平台。与传统的异质结类似,我们可以通过将独立的单层过渡金属二硫化物组装成功能性多层结构来设计和构建层状的异质结,但这种异质结原则上具有原子级别的清晰界面、没有原子间的相互扩散、数字控制的层状组分和无晶格参数约束条件。然而,这种新型的 van der Waals(vdW)半导体异质结在单层极限下的光电特性仍然是未知的。具体地说,在这种异质双层结构中,光跃迁在空间上是直接的还是间接的,在实验上是未知的。在这里,我们研究了由单层 WSe2 和 MoS2 构成的半导体异质结。我们观察到在光致发光峰和最低吸收峰之间有一个很大的类斯托克斯(Stokes-like)位移 $\sim 100 \rm{meV}$,这与具有空间直接吸收但空间间接发射的 II 型能带匹配一致。值得注意的是,这种空间间接跃迁的光致发光强度很强,这表明电荷载流子的层间耦合很强。这种异质界面上的耦合可以非常容易地通过将六角 BN 介电层插入 vdW 间隙中来调节。因此,这种层间耦合的一般性质在能带工程中提供了一个新的自由度,并有望产生具有可调控光电特性以及可定制复合层的一类半导体异质结。

意义

A new class of heterostructures consisting of layered transition metal dichalcogenide components can be designed and built by van der Waals (vdW) stacking of individual monolayers into functional multilayer structures. Nonetheless, the optoelectronic properties of this new type of vdW heterostructure are unknown. Here, we investigate artificial semiconductor heterostructures built from single-layer WSe2 and MoS2. We observe spatially direct absorption but spatially indirect emission in this heterostructure, with strong interlayer coupling of charge carriers. The coupling at the hetero-interface can be readily tuned by inserting hexagonal BN dielectric layers into the vdW gap. The generic nature of this interlayer coupling is expected to yield a new family of semiconductor heterostructures having tunable optoelectronic properties through customized composite layers.

我们可以使用范德华(van der Waals,vdW)相互作用将独立的单层堆叠成具有功能性的多层结构,并由此设计和构建一种由层状过渡金属二硫化物组成的新型异质结。然而,这种新型 vdW 异质结的光电特性尚不清楚。在此,作者研究了由单层 WSe2 和 MoS2 构建的半导体异质结。在这种异质结中,作者观测到了空间上的直接吸收和空间上的间接发射,并伴随有很强的电荷载流子的层间耦合作用。通过在 vdW 间隙插入六角 BN 介电层,我们可以非常容易地调节异质界面的耦合作用。这种层间耦合的一般性质有望产生具有可调控光电特性以及可定制复合层的一类半导体异质结。

前言

Two-dimensional layered transition metal dichalcogenide (TMDC) semiconductors such as MoS2 and WSe2 have established themselves as strong contenders for next-generation electronics and optoelectronics[1–6] and are promising building blocks for novel semiconductor heterostructures.[7–11] Conventional heterostructures are mainly based on group IV, III-V, or II-VI semiconductors with covalent bonding between atoms at the hetero-interface. Owing to atomic interdiffusion during growth, the resulting atomic-scale interface roughness and composition variation at the hetero-interface inevitably smear the density of states profile and consequently compromise the performance of these heterostructures, especially as the film thicknesses are reduced toward a single atomic layer. In addition, the choice of material components for conventional heterostructures is strongly dictated by lattice mismatch.

二维层状过渡金属二硫化物(transition metal dichalcogenide,TMDC)半导体,如 MoS2 和 WSe2 已经成为下一代电子和光电器件的有力竞争者[1–6],并且是作为新型半导体异质结的具有潜力的基石[7–11]。传统的异质结主要是基于 IV、III-V 或者 II-VI族半导体构建的,而这些半导体中的原子和异质界面是靠共价键连接起来的。由于生长过程中原子的相互扩散,由此产生的原子尺度界面粗糙度和异质界面成分的变化不可避免地影响了态密度分布,从而影响了这些异质结构的性能,尤其是当薄片的厚度减小到单原子尺度时。另外,晶格失配对选择组成传统异质结的材料也有影响。

In TMDCs, however, individual layers are held together by van der Waals (vdW) forces, without surface dangling bonds.[12] Semiconductor heterostructures built up from monolayer TMDCs would in principle offer atomically regulated interfaces and thereby sharp band edges. Theoretical studies have predicted different electronic structures and optical properties from TMDC heterobilayers;[13–17] however, to date there have been no experimental results. Whereas previous experimental efforts have focused on graphene-based layered heterostructures,[8–11, 18–26] we present an experimental study on the electronic interlayer interaction in a heterostructure built from two singlelayer TMDC semiconductors, namely, MoS2 and WSe2. The hetero-bilayers are characterized by transmission electron microscopy, X-ray photoelectron microscopy, electron transport studies, and optical spectroscopy to elucidate the band alignments, optoelectronic properties, and the degree of the electronic layer coupling in this novel material system.

然而,在 TMDC 中,各单层之间通过范德华(van der Waals,vdW)力连接在一起,表面没有悬挂键[12]。由单层 TMDC 构成的半导体异质结理论上具有原子级别可控的界面,从而具有清晰的带边。理论研究预测了不同 TMDC 异质双层的电子结构和光学性质[13–17] ;然而,到目前为止都还没有实验的结果。鉴于以往的实验工作都聚焦在基于石墨烯的层状异质结上[8–11, 18–26],作者在此展示了一项关于由双层 TMDC 半导体,即 MoS2 和 WSe2,构建的异质结中电子层间相互作用的实验研究。作者使用透射电子显微镜、X 射线光电子显微镜、电子输运研究和光谱研究来表征这个异质双层结构,并以此来阐述这个新型材料体系的能带匹配关系、光电特性以及电子层间耦合的程度。

图表

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<b>Figure 1</b>. WSe<sub>2</sub>/MoS<sub>2</sub> hetero-bilayer illustration, optical image, and TEM images. (A) Atomistic illustrations of the heterostructure of single-layer (SL) WSe<sub>2</sub> on SL MoS<sub>2</sub> with their respective lattice constants and a misalignment angle $\phi$. (B) Optical microscope image of a WSe<sub>2</sub>/MoS<sub>2</sub> hetero-bilayer on a Si/SiO<sub>2</sub> substrate ($260\ \rm{nm}$ SiO<sub>2</sub>). (C) HRTEM images of a boundary region of SL MoS<sub>2</sub> and the hetero-bilayer, showing the resulting Moire pattern. (D) The electron diffraction pattern of the hetero-bilayer shown in B, with the pattern of MoS<sub>2</sub> and WSe<sub>2</sub> indexed in green and blue colors, respectively.
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<b>图 1</b>. WSe<sub>2</sub>/MoS<sub>2</sub> 异质双层的结构、光学图像和 TEM 图像。(A)在单层(single layer,SL)MoS<sub>2</sub>上叠加 SL WSe<sub>2</sub> 的异质结原子结构示意图,以及它们各自的晶格常数和偏心角度 $\phi$。(B)在 Si/SiO<sub>2</sub> 衬底上($260\ \rm{nm}$ SiO<sub>2</sub>)的 WSe<sub>2</sub>/MoS<sub>2</sub> 异质双层的光学显微镜图像。(C)SL MoS<sub>2</sub> 和异质双层的 HRTEM 图像,显示了由此产生的莫尔(Moire)条纹。(D) 异质双层的电子衍射图案如图B所示,绿色和蓝色的部分分别表示 MoS<sub>2</sub> 和 WSe<sub>2</sub> 的图案。
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<b>Figure 2</b>. XPS core level shift analyses of WSe<sub>2</sub>/MoS<sub>2</sub> heterostructures. (A) Sketch of the spatially resolved PEEM experiment. (B) Comparison of W 4f core level doublet from WSe<sub>2</sub> and WSe<sub>2</sub>/MoS<sub>2</sub> indicating a $220\ \rm{meV}$ shift to lower binding energy, corresponding to a negative net charge on the WSe<sub>2</sub> top layer. (C) Comparison of Mo 3d core level doublet and S 2s singlet from MoS<sub>2</sub> and WSe<sub>2</sub>/MoS<sub>2</sub> indicating a shift of $190\ \rm{meV}$ to higher binding energy, corresponding to a positive net charge on MoS<sub>2</sub>. The single peak at $224.4 \sim 224.6\ \rm{eV}$ is identified as S 2s, which shows the same shift as Mo 3d, as expected.
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<b>图 2</b>. WSe<sub>2</sub>/MoS<sub>2</sub> 异质结的 XPS 核级位移分析。(A)空间分辨 PEEM 实验的示意图。(B)比较 WSe<sub>2</sub> 和 WSe<sub>2</sub>/MoS<sub>2</sub> 中 W 4f 轨道的核能级双峰,表明在 $220\ \rm{meV}$ 处移向较低的结合能,对应于 WSe<sub>2</sub> 顶层的负净电荷。(C)比较 MoS<sub>2</sub> 和 WSe<sub>2</sub>/MoS<sub>2</sub> 中 Mo 3d 轨道的核能级双峰和 S 2s 单峰,表明在 $190\ \rm{meV}$ 处移向更高的结合能,对应于 MoS<sub>2</sub> 上的正净电荷。$224.4 \sim 224.6\ \rm{eV}$ 处的单峰可以确定为 S 2s,这显示了与 Mo 3d 相同的偏移,正如预期的那样。
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<b>Figure 3</b>. Photoluminescence and absorption from WSe<sub>2</sub>/MoS<sub>2</sub> hetero-bilayers. (A) PL spectra of single-layer WSe<sub>2</sub>, MoS<sub>2</sub>, and the corresponding hetero-bilayer. (B) Normalized PL (solid lines) and absorbance (dashed lines) spectra of single-layer WSe<sub>2</sub>, MoS<sub>2</sub>, and the corresponding hetero-bilayer, where the spectra are normalized to the height of the strongest PL/absorbance peak. (C) Band diagram of WSe<sub>2</sub>/MoS<sub>2</sub> hetero-bilayer under photo excitation, depicting (1) absorption and exciton generation in WSe<sub>2</sub> and MoS<sub>2</sub> single layers, (2) relaxation of excitons at the MoS<sub>2</sub>/WSe<sub>2</sub> interface driven by the band offset, and (3) radiative recombination of spatially indirect excitons. (D) An atomistic illustration of the heterostructure of single-layer WSe<sub>2</sub>/single-layer MoS<sub>2</sub> with few-layer $h$-BN spacer in the vdW gap. (E) Normalized PL spectra from single-layer WSe<sub>2</sub>/single-layer MoS<sub>2</sub> heterostructure with $n$ layers of $h$-BN ($n = 0, 1, 3$).
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<b>图 3</b>. WSe<sub>2</sub>/MoS<sub>2</sub> 异质双层的光致发光和吸收。(A)单层 WSe<sub>2</sub>、MoS<sub>2</sub> 和相应的异质双层的 PL 频谱。(B)单层 WSe<sub>2</sub>、MoS<sub>2</sub> 和相应的异质双层的归一化 PL(实线)和吸光率(虚线)频谱,其中频谱被归一化至最强的 PL/吸光率峰的高度。(C)光激发下 WSe<sub>2</sub>/MoS<sub>2</sub> 异质双层的能带图,描述了(1)WSe<sub>2</sub> 和 MoS<sub>2</sub> 单层的吸收和激子产生,(2)受能带偏移驱动的 MoS<sub>2</sub>/WSe<sub>2</sub> 界面的激子弛豫,以及(3)空间间接激子的辐射复合。(D)单层 WSe<sub>2</sub>/单层 MoS<sub>2</sub> 异质结的原子结构示意图,其中在 vdW 间隙中有几层 $h$-BN 间隔层。(E)单层 WSe<sub>2</sub>/单层 MoS<sub>2</sub> 异质结的归一化 PL 频谱,其中在层间有 $n$ 层 $h$-BN($n = 0, 1, 3$)。
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<b>Figure 4</b>. Electrical transport across the WSe<sub>2</sub>/MoS<sub>2</sub> hetero-interface. (A) Optical microscope image of a device encompassing single-layer WSe<sub>2</sub>, WSe<sub>2</sub>/MoS<sub>2</sub> hetero-bilayer, and single-layer MoS<sub>2</sub> on a Si/SiO<sub>2</sub> substrate. Electrodes are numbered 1–7 from bottom to top. (Right) A color-coded PL peak energy map. (Scale bar, $2\ \rm{μm}$.) (B) A qualitative band diagram of the single-layer WSe<sub>2</sub>/hetero-bilayer/single-layer MoS<sub>2</sub> device, corresponding to the device between electrodes 2 and 3. (C) $I-V$ characteristic when measuring between electrodes 2 and 3, with 2 grounded and 3 biased. A back-gate voltage of $50\ \rm{V}$ was applied to reduce the contact resistance to MoS<sub>2</sub> and patterned NO<sub>2</sub> doping was used near the WSe<sub>2</sub> contact for reducing the contact resistance.
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<b>图 4</b>. 跨越 WSe<sub>2</sub>/MoS<sub>2</sub> 异质界面的电子输运。(A)器件的光学显微镜图像,包括在 Si/SiO<sub>2</sub> 衬底上的单层 WSe<sub>2</sub>、WSe<sub>2</sub>/MoS<sub>2</sub> 异质双层和单层 MoS<sub>2</sub>。电极从下到上编号为 1-7。(右)彩色编码的 PL 峰能量图。(比例尺,$2\ \rm{μm}$)(B)单层 WSe<sub>2</sub>/异质双层/单层 MoS<sub>2</sub> 器件的定性能带图,对应于电极 2 和 3 之间的器件。(C)在电极 2 和 3 之间测量得到的 $I-V$ 特性,其中 2 接地,3 偏置。作者采用 $50\ \rm{V}$ 的背栅电压来降低 MoS<sub>2</sub> 的接触电阻,并在 WSe<sub>2</sub> 接触附近采用图案化的 NO<sub>2</sub> 掺杂来降低接触电阻。
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结论

In summary, we have fabricated and characterized an artificial vdW heterostructure by stacking monolayer TMDC building blocks and achieved electronic coupling between the two 2D semiconductor constituents. Strong PL with a large Stokes-like shift was observed from the WSe2/MoS2 hetero-bilayer, consistent with spatially indirect luminescence from a type II heterostructure. We anticipate that our result will trigger subsequent studies focused on the bottom-up creation of new heterostructures by varying chemical composition, interlayer spacing, and angular alignment. In addition, the focus will be on the fabrication of vdW semiconductor heterostructure devices with tuned optoelectronic properties from customized single-layer components. Particularly, electroluminescene efficiency of vdW heterostructures needs to be explored experimentally to examine their viability for use as nanoscale light-emitting/lasing devices.

综上所述,作者通过堆叠单层 TMDC 来构建和表征一种人为制造的 vdW 异质结,并且实现了两层 2D 半导体之间的电子耦合。作者从 WSe2/MoS2 异质双层中观测到了较强的光致发光(photoluminescence,PL)和较大的类斯托克斯位移,这与 II 型异质结中的空间间接发光一致。作者预期其结果将引发后续关于通过改变化学组分、层间距以及角度对准等方式来自下而上地创建新异质结的研究。此外,研究的重点将放在通过定制单层的组分来制备具有光电特性可调控的 vdW 半导体异质结器件。尤其是 vdW 异质结的电致发光效率亟需实验来验证其作为纳米发光/激光器件的可行性。

文章作者: 喵函数
文章链接: https://eigenmiao.site/2020/03/13/article-06/
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