Enhanced Photoresponse in MoTe2 Photodetectors with Asymmetric Graphene Contacts

Xia Wei,Kaiyou Wang,Wenkai Zhu,Ce Hu,Faguang Yan,Amalia Patanè,Quanshan Lv

doi:10.1002/adom.201900190

Abstract

AbstractAtomically thin 2D materials are promising candidates for miniaturized high‐performance optoelectronic devices. This study reports on multilayer MoTe2 photodetectors contacted with asymmetric electrodes based on n‐ and p‐type graphene layers. The asymmetry in the graphene contacts creates a large (Ebi ∼ 100 kV cm−1) built‐in electric field across the short (l = 15 nm) MoTe2 channel, causing a high and broad (λ = 400–1400 nm) photoresponse even without any externally applied voltage. Spatially resolved photovoltage maps reveal an enhanced photoresponse and larger built‐in electric field in regions of the MoTe2 layer between the two graphene contacts. Furthermore, a fast (∼10 µs) photoresponse is achieved in both the photovoltaic and photoconductive operation modes of the junction. The findings can be extended to other 2D materials and offer prospects for the implementation of asymmetric graphene contacts in future low‐power optoelectronic applications.

Highlights

Two dimensional (2D) van der Waals crystals have received great attention due to their excellent properties and versatility for a wide range of potential applications in optoelectronics.[1,2] In particular, transition metal dichalogenides (TMDs) with their finite and tunable bandgap energy and strong light absorption offer opportunities for a variety of optoelectronic devices.[2,3,4] Amongst the TMDs, MoTe2 is an attractive semiconductor
Unlike other TMDs, such as MoS2 and WS2, photodetectors based on MoTe2 can have a broadband photoresponse that extends from the visible (VIS) to the near infrared (NIR) spectral range.[8,9,10]
We report on photodetectors based on MoTe2 with vertical asymmetric graphene contacts

Summary

Introduction

Two dimensional (2D) van der Waals crystals have received great attention due to their excellent properties and versatility for a wide range of potential applications in optoelectronics.[1,2] In particular, transition metal dichalogenides (TMDs) with their finite and tunable bandgap energy (from Eg = 1.1 to 2.1 eV) and strong light absorption offer opportunities for a variety of optoelectronic devices.[2,3,4] Amongst the TMDs, MoTe2 is an attractive semiconductor. In the monolayer form, it has a direct bandgap, Eg = 1.10 eV at room temperature, larger than that of bulk MoTe2, which has an indirect bandgap (Eg = 0.85 eV).[5,6,7] Thus, unlike other TMDs, such as MoS2 and WS2, photodetectors based on MoTe2 can have a broadband photoresponse that extends from the visible (VIS) to the near infrared (NIR) spectral range.[8,9,10] In particular, in MoTe2-based field effect transistors (FETs), the photoresponsivity (R) can be enhanced by a photogating effect and achieve values of up to R = 24 mA W-1 under illumination with NIR light.[9] Although Si has a similar bandgap to that of MoTe2, its absorption coefficient in the NIR spectral range is smaller than that of MoTe2: for Si, the absorption coefficient is 8.17 cm-1 at = 1064 nm, which is smaller than that for MoTe2 (4.9×104cm-1).[11] In contrast to traditional bulk semiconductors such as Si, Ge or III-V compounds, 2D vdW crystals have surfaces that are free of dangling bonds.[2] This unique feature arises from their atomic structure: the atoms are arranged into layers that are held together by strong covalent inplane bonds; in contrast, in the out-of plane direction, the atomic layers interact with weak vdW interactions. This offers opportunities to combine them with other materials without the limitations of lattice mismatch that apply to covalent crystals.[2,12] For example, MoTe2 has been used in different multilayer structures: in MoTe2/MoS2 heterojunctions, the on/off photocurrent ratio can reach values of about 780;[13] also, the photoconductive gain in MoTe2/graphene heterostructures can be as large as 4.69×108.[9] More generally, asymmetric contact barriers between two electrodes and a 2D vdW crystal can be exploited to construct high performance photodetectors:[14,15,16,17,18,19,20,21] Au and In Schottky contacts to a 2D material can be used to realize selfpowered photodetectors with high photoresponsivity (R = 110 mA W-1).[14] Also, graphene can form a clean interface with 2D materials and its near perfect optical transparency makes it suitable for use as the top electrode of vertical heterostructure photodetectors.[22,23,24,25] Au/MoTe2/graphene vertical heterostructures have good photoresponsivity and photoreponse time of about 96 ms.[15] However, the photoresponse of 2D vdW heterostructure devices in the current literature remain still slow due to relatively long optically active channels and/or charge traps at the metal/2D material interface.[14,15,26,27,28,29] Thus, both the length of the channel and the quality of the contacts should be carefully chosen to optimize the photoresponse. In this study, we report on photodetectors based on MoTe2 with vertical asymmetric graphene contacts. We use p-type graphene grown by chemical vapour deposition (CVD) as the top contact and n-type exfoliated graphene as the bottom contact. This asymmetry in the graphene contacts is adopted to break the mirror symmetry of the internal electric field profile, thus creating a large built-in electric field Ebi. This feature combined with the short length of the MoTe2 optically active channel enables an efficient and fast photoresponse. The heterostructure exhibits a high and broad spectral photoresponse from the VIS to the NIR range of the electromagnetic spectrum (λ = 400 – 1400 nm) without any external applied voltage: the photoresponsivity is R = 12.38 mA W-1 at λ = 1064 nm and R = 27.64 mA W-1 at λ = 550 nm. Through scanning photovoltaic mapping, an enhanced light absorption is clearly observed in the overlapping region of the graphene and MoTe2 layers. Furthermore, because of the short (l = 15 nm) MoTe2 channel between the top and bottom graphene contacts in the vertical heterostructure, the response time of the device can be as short as ~ 6.15 μs, which is 1-3 orders of magnitude faster than that reported before for MoTe2-based photodetectors.[1,9,15,30,31]

Results and discussion

Conclusions

Experimental sections