[论文解读] High-quality BN/WSe2/BN heterostructure and its quantum oscillations

Shuigang Xu, Yu Han+13 more|arXiv (Cornell University)|Mar 29, 2015

2D Materials and Applications参考文献 29被引用 5

一句话总结

本研究通过选择性h-BN刻蚀，实现了封装的少层WSe2的高质量BN/WSe2/BN范德华异质结构，获得了超高的场效应迁移率（最高达6236 cm²/V·s）和低接触电阻（0.2–0.5 kΩ·μm），使本征量子振荡（包括Shubnikov–de Haas振荡和金属-绝缘体转变）得以观测，揭示了WSe2的真实输运特性。

ABSTRACT

Tungsten diselenide (WSe2), a typical semiconducting transition metal dichalcogenides (TMDs), have recently emerged as a promising two-dimensional (2D) material for potential applications in electronic and optoelectronic devices. Extraordinary physical phenomena such as valley Hall effect, valley Zeeman effect, thickness-dependent band structures have been discovered recently in TMDs mainly through optical characterizations. The intrinsic transport characteristics of TMDs, however, have not been well studied because of extrinsic factors such as the electrode contact problems and charged traps in TMD devices. These problems are also reflected by the low carrier mobility in WSe2 (the highest mobility so far reported is about 600 cm2/V s). Here, we demonstrate a new kind of van der Waals heterostructure which consists of a few-layer WSe2 encapsulated between insulating hexagonal boron nitride (h-BN) sheets. High-quality Ohmic contacts at low temperatures are achieved between the metal electrodes and WSe2 based on a selective etching process for h-BN, and the electrode contact resistance is in the range of 0.2-0.5 k{\Omega}{\mu}m which is sufficiently low for detecting the intrinsic transport properties of few-layer WSe2 and its quantum oscillations. Because of clean interfaces between h-BN and WSe2, BN/WSe2/BN heterostructured devices show ultrahigh field-effect mobility (up to 6236 cm2/V s) at low temperatures. The high quality of the BN/WSe2/BN devices is further verified by the high electron mobility extracted from Hall-effect measurements (over 4200 cm2/V s), metal-insulator transitions at low carrier density (in the order of 1011 cm-2) and interesting features of Shubnikov-der Haas (SdH) oscillations.

研究动机与目标

• 为克服电极接触电阻和电荷陷阱等外部限制因素，这些因素会掩盖WSe2器件中的本征输运行为。
• 开发高质量的范德华异质结构，实现h-BN与WSe2之间洁净的界面，以保持本征电子特性。
• 通过h-BN的选择性刻蚀工艺，实现与少层WSe2的低电阻欧姆接触。
• 通过最小化散射和无序，使本征量子振荡（如Shubnikov–de Haas振荡）的观测成为可能。
• 在受控的封装二维异质结构中测量并验证超高空穴迁移率和金属-绝缘体转变。

提出的方法

• 利用机械剥离和干法转移技术制备BN/WSe2/BN异质结构，以确保界面洁净且原子级平整。
• 采用选择性h-BN刻蚀工艺，选择性地去除WSe2沟道周围的h-BN层，从而实现低电阻欧姆接触。
• 采用双栅场效应晶体管结构，调节载流子密度，并在低温下测量输运特性。
• 采用低温输运测量（最低至4.2 K），以解析量子振荡并从霍尔测量中提取迁移率。
• 对Shubnikov–de Haas振荡进行定量分析，以提取载流子有效质量和费米面特性。
• 在低载流子密度（~10¹¹ cm⁻²）下表征金属-绝缘体转变，以探究二维系统中的多体效应。

实验结果

研究问题

• RQ1 具有低接触电阻的BN/WSe2/BN异质结构能否揭示少层WSe2的本征输运特性？
• RQ2 在封装WSe2中可实现的最大场效应迁移率是多少？与先前报道的数值相比如何？
• RQ3 洁净的界面和低无序是否能实现WSe2中如Shubnikov–de Haas振荡等量子振荡的观测？
• RQ4 在封装的少层WSe2中，金属-绝缘体转变在何种载流子密度下发生？这反映了何种多体效应？
• RQ5 选择性h-BN刻蚀如何降低接触电阻并改善二维异质结构中电接触的质量？

主要发现

• BN/WSe2/BN异质结构在低温下实现了高达6236 cm²/V·s的场效应迁移率，显著超过先前报道的数值（约600 cm²/V·s）。
• 霍尔效应测量证实电子迁移率超过4200 cm²/V·s，表明载流子输运质量高且无序度低。
• 清晰观测到Shubnikov–de Haas振荡，证实存在明确的费米面，从而可提取有效质量与载流子密度。
• 在载流子密度约为10¹¹ cm⁻²时检测到金属-绝缘体转变，表明局域化或关联效应开始显现。
• 通过选择性h-BN刻蚀，接触电阻降低至0.2–0.5 kΩ·μm，实现了对本征输运现象的高保真测量。
• h-BN与WSe2之间的洁净界面抑制了电荷杂质和散射，从而增强了量子相干性并实现了长平均自由程。