MOCVD Grown Type-II TMDC Heterostructures for Photodetecting Devices

2D transition metal dichalcogenides (TMDCs) are highly promising materials for ultrathin, flexible and large-area photodetectors due to their tiny thickness in the nm range and their high absorption coefficient. However, the high exciton binding energy in these materials leads to fast recombination processes. This limits efficient charge carrier separation as required for photodetecting devices. To overcome this limitation type II heterostructures based on 2D materials have been developed in the past with a strong focus on micrometer-scale prototypes either prepared by mechanical exfoliation and stacking or via chemical vapor deposition (CVD). Here, we present our progress on large-area MoS₂/WS₂ monolayer heterostructures grown on sapphire by MOCVD that enables us to realize type-II-heterostructures by a scalable process without poorly defined transfer steps and contaminations [1].

Clear signatures of both, MoS₂ and WS₂, in Raman as well as in photoluminescence (PL) spectroscopy, confirm that no alloying takes place during growth. Low temperature PL spectroscopy reveals an emission feature at λ = 865 nm. This is attributed to the interlayer exciton, in agreement with reports on similar CVD heterostructures [2]. Kelvin probe force microscopy measurements under illumination confirm an efficient separation of optically generated charge carriers: Electrons accumulate in the MoS₂, while holes are transferred to the WS₂.

The efficient charge carrier separation in our direct grown type II heterostructure enables the fabrication of high performance photodetectors. In a first concept, photoconductors are realized by defining lateral metal contacts on top of the heterobilayer in an interdigital layout. The type II heterostructure device yields photocurrents in the mA range [3] and a responsivity of up to 16 A/W. The responsivity is increased by about 4 orders of magnitude with respect to the MoS₂ and WS₂ monolayer references devices as a consequence of the efficient charge carrier separation at the heterointerface. In a second concept the MoS₂/WS₂ heterostructure is integrated into a vertical, mm²-sized p-n-photodiode architecture. A pronounced photocurrent is generated even at zero bias. An ultra-high photoswitching ratio of 8.4*10⁵ is obtained under illumination with a photon flux density of φ_ph = 10¹⁷ cm^-2 s^-1 at an excitation wavelength of λ_exc = 620 nm. Photon flux densities down to φ_ph = 9*10¹¹ cm^-2 s^-1 can be detected with an external quantum efficiency of EQE = 4.5 %. Transient photocurrent measurements exhibit a fast photoresponse with a cut-off frequency of fc = 80 MHz [4].

[1] A. Grundmann et al., MRS Advances 31, 1625-1633 (2020)
[2] Gong et al., Nat. Mat. 13, 1135 (2014)
[3] U. Hutten et al., 2D Materials 8, 045015 (2021)
[4] Y. Beckmann et al., ACS Photonics 11, 2228 (2024)