Enhancement of valley polarization in CVD grown monolayer MoS2 films

Abstract

The development of future valley based electronics or valleytronics requires a high degree of valley polarization (VP) in large area monolayer (1L)-MoS2. Though it is possible to synthesize 1L-MoS2 films with large area coverage, VP property of as-grown films is found to be very poor. Here, we investigate the role of physisorbed air molecules and strain on the luminescence and the VP characteristics of large area monolayer MoS2 grown on various substrates by a microcavity based chemical vapor deposition (CVD) technique. The study shows that the removal of adsorbates from sulfur vacancy ( VS) sites not only suppresses the broad sub-bandgap luminescence feature that typically dominates low temperature photoluminescence (PL) spectra of these films but also significantly enhances VP. Post-growth transfer of the 1L-MoS2 film from sapphire to a SiO2/Si substrate by a polystyrene assisted process is found to be highly effective in improving the polarization characteristic (∼80%) of K-valleys through relaxation of the biaxial tensile strain and the removal of physisorbed air molecules from the VS sites. The process is also found to provide long lasting protection for MoS2 films from air. The finding, thus, creates much needed opportunity to use CVD grown large area 1L-MoS2 for realization of valleytronics of the future.

Recombination dynamics and manybody effect of excitons in large-area monolayer MoS2 capped with (111) NiO epitaxial layer

“…On the other hand, large area coverage and continuity of the 1L-MoS 2 films, which are essential to develop integrated electronics on this material platform, can be achieved by CVD technique [24,25]. However, CVD grown layers often suffer from imperfections, such as sulphur vacancies (V S ) [26][27][28][29][30], grain boundaries [31] and the residual strain [32,33], which deteriorate the electrical and optical performance of the material. Presence of sulphur vacancies makes these films more sensitive to the environmental conditions [34,35].…”

“…V S defects attached with air molecules are also responsible for the appearance of a broad luminescence band at low temperatures in CVD grown 1L-MoS 2 [30,32]. It has been found that upon vacuum annealing, the conductivity of the films improves by several orders of magnitude [29] and at the same time, defect luminescence is substantially suppressed [30,32]. Further, the valley polarization property of the films enhances quite significantly [30,32].…”

“…It has been found that upon vacuum annealing, the conductivity of the films improves by several orders of magnitude [29] and at the same time, defect luminescence is substantially suppressed [30,32]. Further, the valley polarization property of the films enhances quite significantly [30,32]. These findings have generated an optimism that controlled annealing followed by capping with a suitable material that can protect the film from further exposure to atmosphere might be the way to make CVD grown 1L-MoS 2 films ready for device applications.…”

Vapor–Liquid–Solid-Mediated Layer-by-Layer Growth of Stepped-Wedge-Shaped WS2 Microribbons Using the Chemical Vapor Deposition Technique

“…Transition-metal dichalcogenides (TMDCs), which are two-dimensional (2D)-layered semiconductors, have great potential for electronic and optoelectronic device applications. − The class of materials exhibits several fascinating properties like large exciton binding energy; existence of many body bound states, e.g., trions and biexcitons; and thickness dependence of the electronic properties . Strong spin-valley coupling in monolayers also leads to the valley polarization effect, , which can be exploited to inject spin-polarized carriers and excitons for future spintronics and valleytronics. Due to their layered nature, lower dielectric constant, higher electron effective mass, and larger band gap than that of silicon, TMDCs are superior in suppressing the short channel effects that facilitate greater integration of devices in a chip.…”

In situ doping effect in monolayer MoS2 via laser irradiation

“…Atomically thin transition metal dichalcogenides (TMDs), expected to be excellent candidate materials for photodetectors, electroluminescent devices, transistors and spintronics, have attracted extensive interests owing to its strong photoluminescence, remarkable mechanical strength and flexibility, atomic-scale thickness, and intriguing coupled spin-valley physics [1][2][3][4][5][6][7][8][9][10][11]. One distinctive feature of monolayer TMDs is the existence of a direct bandgap.…”