Experimental and theoretical studies of native deep-level defects in transition metal dichalcogenides

Abstract

Transition metal dichalcogenides (TMDs), especially in two-dimensional (2D) form, exhibit many properties desirable for device applications. However, device performance can be hindered by the presence of defects. Here, we combine state of the art experimental and computational approaches to determine formation energies and charge transition levels of defects in bulk and 2D MX2 (M = Mo or W; X = S, Se, or Te). We perform deep level transient spectroscopy (DLTS) measurements of bulk TMDs. Simultaneously, we calculate formation energies and defect levels of all native point defects, which enable identification of levels observed in DLTS and extend our calculations to vacancies in 2D TMDs, for which DLTS is challenging. We find that reduction of dimensionality of TMDs to 2D has a significant impact on defect properties. This finding may explain differences in optical properties of 2D TMDs synthesized with different methods and lays foundation for future developments of more efficient TMD-based devices.

Role of chalcogen vacancies and hydrogen in the optical and electrical properties of bulk transition-metal dichalcogenides

“…In the −1 charge state, one of the states is half occupied. The −2 charge state is unstable within the band gap for all the studied TMDs, in agreement with previous studies [58,69,70].…”

“…The formation energy of the chalcogen vacancy as a function of Fermi level is shown in figure 3 for all the studied bulk TMDs. The chalcogen vacancy in bulk TMDs is a deep acceptor with the (0/−1) transition level deep inside the band gap in agreement with previous studies [58,69,70]. The relaxation around the vacancy is very minimal.…”

Long lived photogenerated charge carriers in few-layer transition metal dichalcogenides obtained from liquid phase exfoliation

“…We envisage that the recently developed covalent interconnection of nanosheets, 42,43 which enables efficient inter-nanosheet charge transport, will ultimately enable the fabrication of macroscopic photovoltaic cells using tiled films of 2d semiconductor as the active layer. While photocurrents have already been demonstrated, 60 photovoltaic performance will require a more in-depth study of the ink processing 61 and charge stabilisation 55,62,63 to find the optimal balance between long lifetimes and high mobilities of photogenerated carriers.…”

“…While photocurrents have already been demonstrated, 60 photovoltaic performance will require a more in-depth study of the ink processing 61 and charge stabilisation 55,62,63 to nd the optimal balance between long lifetimes and high mobilities of photogenerated carriers.…”

Chalcogen Vacancy Engineering of Two-Dimensional Transition Metal Dichalcogenides for Electronic Applications

“…For example, crystal defects in MoS 2 include mono-S vacancies (V S ) and di-S vacancies (V S2 ), vacancy complexes containing Mo and three nearby S atoms (V MoS3 ) and Mo and six nearby S atoms (V MoS6 ), as well as antisites wherein Mo replaces two S atoms (Mo S2 ) and those wherein two S atoms replace one Mo (S 2Mo ). ,, The formation energies of these point defects at their theoretically most stable charge states are calculated and plotted against the Fermi level (Figure g). The V S have been shown to contain the lowest formation energy among all the studied point defects. − Similarly, other TMDCs, such as MoSe 2 , MoTe 2 , WS 2 , WSe 2 , and WTe 2 , had the lowest formation energies for V X (X: S or Se), − suggesting that excess chalcogen vacancies (CVs) (generally, S vacancies (SVs) and Se vacancies (SeVs)) are unavoidably and easily generated during LPE and EE processing.…”