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

Abstract

In two dimensional (2D) transition metal dichalcogenides, defect-related processes can significantly affect carrier dynamics and transport properties. Using femtosecond degenerate pump-probe spectroscopy, exciton capture, and release by mid-gap defects have been observed in chemical vapor deposition (CVD) grown monolayer MoSe 2 . The observed defect state filling shows a clear saturation at high exciton densities, from which the defect density is estimated to be around 0.5 × 10 12 /cm 2 . The exciton capture time extracted from experimental data is around~1 ps, while the average fast and slow release times are 52 and 700 ps, respectively. The process of defect trapping excitons is found to exist uniquely in CVD grown samples, regardless of substrate and sample thickness. X-ray photoelectron spectroscopy measurements on CVD and exfoliated samples suggest that the oxygenassociated impurities could be responsible for the exciton trapping. Our results bring new insights to understand the role of defects in capturing and releasing excitons in 2D materials, and demonstrate an approach to estimate the defect density nondestructively, both of which will facilitate the design and application of optoelectronics devices based on CVD grown 2D transition metal dichalcogenides.

Competition between Free Carriers and Excitons Mediated by Defects Observed in Layered WSe2 Crystal with Time‐Resolved Terahertz Spectroscopy

“…To further understand the dynamics of free carriers and excitons, the peak values and as a function of pump fluence are studied in Figure b. The peak value shows a trend of saturation with the pump fluence, which suggests nonlinear absorption by the free carriers and more excitons would be formed at high pump fluence . Based on previous theoretical model, the response of nonlinear free carrier absorption could be attributed to the strong electron–photon interaction and electron–impurity interaction .…”

Accelerated carrier recombination by grain boundary/edge defects in MBE grown transition metal dichalcogenides

“…With resonant detection, the reflection change is dominated by the absorption coefficient change due to the phasespace filling effect. 8,9 For both samples (exfoliated MoSe 2 on the Si substrate and the MoSe 2 thin film grown on the sapphire substrate by MBE), the reflection change is proportional to the absorption coefficient change, according to the calculation with the transfer matrix method (supplementary material). 8,9 Thus, the response of negative reflection change ∆R means that the absorption coefficient decreases after the pump excitation, which is consistent with the Pauli blocking/repulsion induced by the phase-space filling effect.…”

“…6 While oxygen impurities occupying chalcogen vacancy sites can eliminate the mid-gap states, 7 oxygen impurities taking up molybdenum vacancy sites keep those mid-gap states and play the role as effective carrier trappers. 8,9 Comparing with exfoliated and CVD synthesized samples, studies on the photoexcited carrier dynamics in MBE-grown TMDs are rare. Our previous work has shown that one of the main a Author to whom correspondence should be addressed: yaguo.wang@austin.utexas.edu 2166-532X/2018/6(5)/056103/7 6, 056103-1 © Author(s) 2018…”

Carrier Trapping by Oxygen Impurities in Molybdenum Diselenide

“…the transition from the localized defect states which possess broad momenta to high energy states in the conduction band, becomes available, leading to an increase in the absorption. In our previous study 16 , oxygen impurities have been proposed to be responsible for the carrier-trapping events, because the defect trapping effect could only be observed in CVD grown films but not in exfoliated flakes, and the oxygen content is only found in CVD as-grown films. Here, the fact that the defect-capturing-carrier signature can again be observed after the generation of oxygen impurities in the exfoliated sample by plasma irradiation, serves as strong evidence that oxygen impurities in MoSe2 samples are indeed the effective carrier-trapping defects.…”