Fabrication of two-dimensional graphene-based heterostructures for photodetection applications
Keywords: two dimensional materials
chemical vapor deposition
photodetector

The discovery of graphene has so far drawn significant attentions since 2004 when isolation of single layer graphene was first achieved. Over the past few years, the interest in graphene has also expanded into other two-dimensional (2D) materials, including transition metal dichalcogenides (TMDs), such as Molybdenum disulfide (MoS2) and WS2, metal chalcogenides, such as gallium sulfide (GaS), and heterostructures base on these materials. Numerous studies have been done on fabrication and characterization of 2D heterostructures with various functionalities in electronics and optoelectronics. However, in most studies the 2D heterostructures were constructed using exfoliation and transfer approach, which possesses the disadvantages of low efficiency and low yield. It is necessary to develop new fabrication processes for large-scale production of 2D heterostructure nanodevices. This DPhil project aims to exploit the possibility to assembly 2D heterostructures by chemical vapor deposition (CVD) growth of 2D semiconductors (e.g. MoS2, WS2 and GaS) using pre-patterned graphene as growth template, which enables large-scale fabrication of graphene based nanodevices in photodetection applications.
In the first part of fabrication of MoS2:graphene vertical heterostructure, a new ambient pressure CVD process was developed via control of chemistry of graphene and MoS2 by hydrogen addition, which enables the direct growth of centimeter-scale continuous films of vertically stacked MoS2 monolayer on graphene. Hydrogen addition enables longer CVD growth times at high temperature by reducing oxidation effects on monolayer graphene. By careful control of nucleation density and growth time, high quality monolayer MoS2 films could be formed on graphene, realizing all CVD grown vertical stacked monolayer semimetal-semiconductor interface. The photodetectors fabricated using this vertical heterostructure show photoresponsivity reaching 2.4A/W under 135μW 532nm illumination.
In the second part of fabrication of WS2:graphene lateral heterostructure, a new controlled CVD process was developed to directly grow WS2 in pre-patterned graphene gaps, creating better interfaces. By lowing the effective Schottky barrier height via improved contact between graphene and WS2, Direct CVD grown graphene:WS2:graphene lateral photodetecting transistors exhibit high photoresponsivity reaching 121 A/W under 2.7×105 mW/cm2 532 nm illumination, which is around two orders of magnitude higher than similar devices made by the layer-by layer transfer method.
Lastly, mass production of GaS:graphene lateral heterostructure was achieved by directly growing layered GaS in the gap of pre-patterned graphene electrode pairs on Si wafer via a selective CVD process. Due to the large band gap of GaS, the CVD grown GaS:graphene lateral photodetecting transistors is sensitive to UV light only. The detection limit could reach down to 2.61 μW/cm2 with a photoresponsivity of 11.7 A/W and a photo gain of 53.7. The devices also show long-term stable and reproducible ON-OFF switching behavior, with a response time lower than 60 ms, which is comparable to other high-quality photodetectors made by 2D materials.