Exploring Carrier Dynamics and Localization in Transition Metal Dichalcogenides for Integration into Semiconductor Devices

Abstract

Low carrier mobility, oftentimes affected by the formation of localized states, is the major bottleneck of utilizing the unique quantum transport properties of transition metal dichalcogenides (TMDCs) in the context of electronic applications. Moreover, during the degradation period with aging carrier dynamics of TMDCs are still ambiguous, though time-dependent photo-physical properties can be a deterministic factor to their implementation in practical semiconductor devices as the defect-induced valley-specific transitions can drastically alter its properties. In this study, we propose an effective method for quantifying the localization energy by analyzing the temperature-dependent spectral variation of photoluminescence (PL) in monolayer (ML) WSe_2, both in its pristine form and when encapsulated with hexagonal boron nitride (h-BN). Considering the protecting capability of h-BN against contamination and degradation, while not affecting the electronic structure as an insulating dielectric, the localization energy was comparatively extracted out of PL spectra in pristine and encapsulated ML WSe_2. In pristine ML WSe_2, two distinctive energy traces were resolved with an energy difference of about 17 meV, which was associated with the localized state revealed below 200 K. A clear signature of the carrier localization was also apparent in the integrated PL intensity trend with temperature, as the intensity from pristine ML clearly deviates from the dark-exciton-like behavior of ML WSe_2 violating the spin selection rule of the lowest exciton state. In clear contrast, the temperature dependency of the h-BN encapsulated ML WSe_2 in PL spectra matches well with the typical Varshni formula of free excitonic peaks and the integrated intensity trace of thermally populated spin subbands. More interestingly, we use this scheme of localization energy quantification to disclose a long-term (4-year) ambient aging effect on the mechanically exfoliated monolayer WSe_2. While freshly exfoliated air-exposed WSe_2 exhibits carrier localization, which becomes more pronounced with time, h-BN encapsulated WSe_2 conforms well to the typical Varshni formula for up to two years of aging; however, after four years of aging, it also becomes susceptible to defects. Our findings endorse that the h-BN encapsulation could suppress the carrier localization channels by avoiding surface oxidation due to air exposure for a certain period of time and the prospect of h-BN encapsulation for WSe_2 ML as pragmatic components of the electronics industry. Such understanding and managing the long-term dynamic interaction of TMDCs with their environment could be useful in achieving their practical device applications.