[论文解读] Ultrahigh-Gain Phototransistors Based on Graphene-MoS2 Heterostructures

Wenjing Zhang, Chih‐Piao Chuu+8 more|arXiv (Cornell University)|Feb 5, 2013

2D Materials and Applications参考文献 52被引用 5

一句话总结

该论文展示了一种石墨烯-MoS₂异质结光电晶体管，其光电响应度超过10⁷ A/W，实现了超高光电增益。该成果通过化学气相沉积法（CVD）生长并转移的大面积、连续单层MoS₂与石墨烯结合实现。由于能带排列有利，MoS₂中的光激发电子-空穴对被高效分离并注入石墨烯，从而在保持超薄、平面结构的同时实现高增益，且可通过栅压实现响应可调。

ABSTRACT

Due to its high carrier mobility, broadband absorption, and fast response time, graphene is attractive for optoelectronics and photodetection applications. However, the extraction of photoelectrons in conventional metal-graphene junction devices is limited by their small junction area, where the typical photoresponsivity is lower than 0.01 AW-1. On the other hand, the atomically thin layer of molybdenum disulfide (MoS2) is a two-dimensional (2d) nanomaterial with a direct and finite band gap, offering the possibility of acting as a 2d light absorber. The optoelectronic properties of the heterostructure of these two films is therefore of great interest. The growth of large-area graphene using chemical vapour deposition (CVD) has become mature nowadays. However, the growth of large-area MoS2 monolayer is still challenging. In this work, we show that a large-area and continuous MoS2 monolayer is achievable using a CVD method. Both graphene and MoS2 layers are transferable onto desired substrates, making possible immediate and large-scale optoelectronic applications. We demonstrate that a phototransistor based on the graphene/MoS2 heterostructure is able to provide a high photoresponsivity greater than 107 A/W while maintaining its ultrathin and planar structure. Our experiments show that the electron-hole pairs are produced in the MoS2 layer after light absorption and subsequently separated across the layers. Contradictory to the expectation based on the conventional built-in electric field model for metal-semiconductor contacts, photoelectrons are injected into the graphene layer rather than trapped in MoS2 due to the alignment of the graphene Fermi level with the conduction band of MoS2. The band alignment is sensitive to the presence of a perpendicular electric field arising from, for example, Coulomb impurities or an applied gate voltage, resulting in a tuneable photoresponsivity.

研究动机与目标

• 开发一种用于可扩展光电器件应用的高增益、超薄光电晶体管，基于二维材料。
• 通过异质结工程克服传统金属-石墨烯结器件光电响应度低（<0.01 A/W）的问题。
• 展示通过化学气相沉积（CVD）实现大面积、连续单层MoS₂生长，以实现实际集成。
• 通过可控的能带排列，实现从MoS₂到石墨烯的高效光生电子注入。
• 通过外部栅压或库仑杂质实现光电响应度的可调性。

提出的方法

• 采用化学气相沉积（CVD）方法在大面上合成连续单层MoS₂，并转移至目标基底上。
• 通过CVD生长石墨烯并转移形成与MoS₂的异质结，保持其结构与电子特性完整性。
• 采用顶栅结构制造光电晶体管器件，以施加垂直电场。
• 在光学照射下测量光电响应度，以评估增益与响应速度。
• 利用费米能级对齐与栅压调制分析能带排列与载流子动力学。
• 研究库仑杂质与电场效应对能带弯曲的影响，以解释电子注入行为。

实验结果

研究问题

• RQ1 能否可靠地合成并转移大面积、连续的单层MoS₂以实现器件集成？
• RQ2 为何光生电子会注入石墨烯而非被束缚在MoS₂中，这与传统金属-半导体结模型相反？
• RQ3 何种机制使得二维异质结光电晶体管的光电响应度超过10⁷ A/W？
• RQ4 施加的栅压或局域电场如何调制石墨烯-MoS₂体系中的光电响应？
• RQ5 通过外部控制能带排列，光电响应度的可调范围有多大？

主要发现

• 在石墨烯-MoS₂异质结光电晶体管中实现了超过10⁷ A/W的光电响应度，相比传统金属-石墨烯器件增益提升超过10⁹倍。
• 通过CVD成功生长并转移了大面积、连续的单层MoS₂，缺陷极少，适用于可扩展器件制造。
• 由于有利的能带排列，MoS₂中的光生电子被高效注入石墨烯，其石墨烯费米能级与MoS₂导带对齐。
• 通过外部栅压或库仑杂质产生的局域电场可调制异质结界面处的能带弯曲，从而实现光电响应度的调制。
• 器件保持超薄、平面结构，响应速度快，适用于柔性与透明光电器件的集成。
• 该机制违背了传统内建电场模型，尽管缺乏强肖特基势垒，电子仍能有效注入。