Narrow excitonic lines and large-scale homogeneity of transition metal dichalcogenides grown by MBE on hBN

Monolayer transition metal dichalcogenides (TMDs) exhibit exceptional optical properties such as high oscillator strength and narrow excitonic resonances. However, above effects have been so far explored only for structures produced by techniques involving mechanical exfoliation and encapsulation in hBN inevitably inducing considerable large-scale inhomogeneity. On the other hand, techniques which are essentially free from this disadvantage, such as molecular beam epitaxy (MBE), have to date yielded only structures characterized by considerable spectral broadening, which hinders most of interesting optical effects.
We report [1] for the first time on the MBE-grown TMD exhibiting narrow and fully resolved spectral lines of neutral and charged exciton. Moreover, our MBE-grown TMD exhibits unprecedented high spatial homogeneity of optical properties, with variation of the exciton energy as small as 0.16 meV over a distance of tens of micrometers. Our recipe for MBE growth is presented for MoSe2 and includes extremely slow growth rate and the use of atomically flat hexagonal boron nitride (hBN) substrate. Importantly, good optical properties are achieved for as-grown sample, without any post growth mechanical treatment.
[1] W. Pacuski et al., Nano Letters 20, 3058 (2020).