Defects as a factor limiting carrier mobility in WSe2: A spectroscopic investigation

Abstract

The electrical performance of two dimensional transitional metal dichalcogenides (TMDs) is strongly influenced by the amount of structural defects inside. In this work, we provide an optical spectroscopic characterization approach to correlate the amount of structural defects and the electrical performance of WSe 2 devices. Low temperature photoluminescence (PL) spectra of electron beam lithography (EBL) processed WSe 2 presents a clear defect-induced PL emission due to excitons bound to defects, which would strongly degrade the electrical performance. By adopting an e-beam-free transfer-electrode technique, we are able to prepare backgated WSe 2 device with limited amount of defects. A maximum hole-mobility of about 200 cm 2 /Vs was achieved due to reduced scattering sources, which is the highest reported value among its type.This work would not only provide a versatile and nondestructive method to monitor the defects in TMDs, but also a new route to approach the room temperature phonon-limited mobility in high performance TMDs devices.

Experimental evidence of exciton capture by mid-gap defects in CVD grown monolayer MoSe2

“…1d). Similar PL peaks (X B peaks) have also been observed for mid-gap defects in ion beam-irradiated exfoliated TMDs 15,41,42 and as grown CVD MoS 2 , 43 which are associated with chalcogen atom vacancies (or impurity atoms occupying the vacancies) induced by the ion beam or during the growth process due to high volatility of chalcogenides, with energy level 0.1-0.3 eV above the valence band maximum or below the conduction band minimum. The defect peak shown in our PL data not only indicates a rich source of mid-gap defects in the CVD grown MoSe 2 sample, but also confirms the ability of the mid-gap defect to rapidly capture the photoexcited excitons and assist their radiative recombination process.…”

Spectroscopic investigation of defects in two-dimensional materials

“…where τ is the excitonic lifetime exceeding 100ps, τ 0 is effective scattering time, and E A is activation energy [48]. By fitting the data in Figure 9C with the above equation, we get E A of 43 meV and the ratio τ/τ 0 of 259 for bound exciton in monolayer WSe 2 .…”

“…In Figure 9F, in order to quantify defects, the intensity ratio of I Xb /I X0 is obtained, where X 0 peak was used for normalization, similar to defect characterization in graphene by Raman intensity ratio of D and G peaks (I D /I G ) [14]. It can be seen that such ratio shows very good linear dependence with the irradiation electron dosage in the range of <60 × 10 6 μm −2 , which suggests that the intensity of X b peak can be used as a standard approach to characterize and monitor the defects in WSe 2 sample [48]. The electrical properties of WSe 2 can be greatly influenced by the presence of defects, as shown in Figure 9G.…”

“…Figure 9A shows low-temperature PL spectra (80 K) of single-layer WSe 2 with and without electron-beam irradiation during the electron beam lithography (EBL) process [48]. As can be seen, a strong X b peak presents in the PL spectrum of electron-beam irradiated region, which suggests that PL spectroscopy can be used to monitor the structural defects in TMDs introduced by electron-beam irradiation.…”

“…For TMDs, some new Raman peaks also appear after the introduction of defects, and their intensities are related to the density of defects [47], but the sensitivity is not very high. At same time, in low-temperature PL measurement, a defect-related PL peak of TMDs could be observed due to the emission of excitons bound to the defect sites, which could be effectively used to monitor the numbers of defects in TMDs [48].…”