High Optical Quality of MoS2

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January 1, 2020 – MoS2 monolayers grown by chemical vapor deposition can match exfoliated ones. Researchers from Toulouse, Jena, Ulm and NIMS Tsukuba counted the vacancies in the grown layers atom by atom with the Cc/Cs-corrected SALVE microscope at 60 kV and found a defect density of the same order as in exfoliated monolayers. Encapsulated in hBN, the grown layers emit light with lines as narrow as those of the best exfoliated samples.

A semiconductor only three atoms thick can be grown over large areas on a cheap silicon wafer, but for years such grown layers were thought too imperfect for careful optical work. Monolayers of transition metal dichalcogenides (TMDs) such as MoS2 and WSe2 are semiconductors with a direct band gap in the visible to near-infrared[1],[2]. Their strong interaction with light and their transport properties make them candidates for a range of future devices[3]–[6]. To show their intrinsic properties, they usually have to be encapsulated in hexagonal boron nitride (hBN), which shields them from their surroundings[7]–[11]. Many applications also need layers hundreds of micrometres across, many flakes on one substrate or even continuous films[12]–[14].

Such large areas come from chemical vapor deposition (CVD), while monolayers peeled from bulk crystals, the usual choice for fundamental studies, are only tens of micrometres wide. A team from the Université de Toulouse, FSU Jena, Ulm University and the National Institute for Materials Science (NIMS) in Tsukuba has now shown that conventional CVD can deliver MoS2 monolayers of high quality. With the Cc/Cs-corrected SALVE microscope at 60 kV, the Ulm group counted the missing atoms in the grown layers directly, one by one, and found a defect density of the same order as in exfoliated monolayers.

Clean light from a grown layer
The team removed the grown MoS2 monolayers from their growth substrate, encapsulated them in hBN and measured their photoluminescence (PL) and reflectivity at T = 4 K. The result was unexpected: the optical transitions were as narrow as in exfoliated samples, about 5 meV wide, compared with about 50 meV for the layers still on their silicon oxide[8],[15]. The emission came from free excitons, not from defects. The layers even showed excited exciton states that appear only in high-quality material[16],[17], and polarized laser light created large valley coherence and valley polarization[18]. Spin and valley physics, so far studied almost only in small exfoliated flakes, can thus be explored in CVD-grown samples of large area.

Which defects, and how many
Whether a grown layer can match an exfoliated one depends first of all on its crystal. Which type of defect is present is not always clear[19]. In naturally occurring MoS2 and in CVD-grown films, sulfur vacancies often dominate, whereas metal vacancies and antisites prevail in synthetic crystals grown by chemical vapor transport (CVT)[2]. The crystals of this study were grown in a modified CVD process in which a Knudsen-type effusion cell delivers a controlled flow of sulfur[12]. For comparison, the researchers also prepared monolayers by exfoliation from bulk MoS2 with adhesive tape and identified them under an optical microscope by their contrast on silicon oxide[20].

HRTEM image of a MoS2 monolayer with diffraction pattern and a magnified area with coloured atoms and circled vacancies
Fig. 1: CVD-grown MoS2 at 60 kV.

Counting vacancies with SALVE
The SALVE microscope images both kinds of sample at atomic resolution over a wide field of view (Fig. 1a). The diffraction pattern shows that the whole imaged area is a single, highly ordered crystal (Fig. 1b). In the enlarged view, the molybdenum atoms and the pairs of stacked sulfur atoms can be told apart, and missing sulfur atoms, mostly double sulfur vacancies as typically found in MoS2 monolayers[12],[21]–[23], can be recognized directly (Fig. 1c).

The team counted these vacancies in clean areas of the images and divided their number by the area. In the CVD-grown monolayers, the density was 7.9 × 1013 vacancies per cm2; the values of three areas ranged from 6.9 to 9 × 1013 cm−2. In the exfoliated monolayers, four areas gave between 3.4 and 4.1 × 1013 cm−2. How the vacancies were made visible and how the uncertainty was estimated is explained in the box below (Figs. 2 and 3).

▸ SALVE: counting vacancies at 60 kV

As good as exfoliated
The defect density of the CVD-grown monolayers is thus of the same order of magnitude as that of exfoliated monolayers[2],[25],[26], and it agrees well with values reported for MoS2 from naturally occurring bulk[27],[28]. A low density of sulfur vacancies means high structural quality, and that is what made the high optical quality in emission and absorption expected in the first place.

What it opens up
Counting defects atom by atom in low-voltage HRTEM proves to be a direct way to measure the structural quality of new TMD materials right after growth. The study shows that large-area CVD-grown MoS2, once lifted from its growth substrate and encapsulated in hBN, can be used where so far only exfoliated flakes would do, and opens new possibilities for (opto-)electronics and spintronics.

Resource:
Shree, S., George, A., Lehnert, T., Neumann, C., Benelajla, M., Robert, C., Marie, X., Watanabe, K., Taniguchi, T., Kaiser, U., Urbaszek, B., Turchanin, A. (2020).
High optical quality of MoS2 monolayers grown by chemical vapor deposition.
2D Mater. 7(1), 015011.
https://doi.org/10.1088/2053-1583/ab4f1f

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