PCSI2023 Session PCSI-MoM1: 2D and VdW Materials

Wafer-scale synthesis of semiconducting transition metal dichalcogenide (TMDs) monolayers is of significant interest for device applications to circumvent size limitations associated with the use of exfoliated flakes. Promising results have been demonstrated for epitaxial films deposited by vapor phase techniques such as CVD and MOCVD. However, the three-fold symmetry of TMDs such as MoS₂ and WSe₂, results in two energetically equivalent domain alignments, often referred to as 0°and 60° domains, when grown on flat high symmetry substrates such as c-plane sapphire. The oppositely oriented domains give rise to inversion domain boundaries (IDBs) upon coalescence which exhibit a metallic character and are generally undesirable. In this study, we demonstrate the epitaxial growth of unidirectional TMD monolayers on 2” diameter c-plane sapphire substrates with a significantly reduced density of inversion domains. Steps on the sapphire surface are shown to play a key role in TMD nucleation and impart a preferred orientation to the domains depending on the step edge structure and chemistry.

Metalorganic chemical vapor deposition (MOCVD) was used for the epitaxial growth of WSe₂ monolayers on c-plane sapphire, miscut ~0.2^o toward the m-axis, in a cold-wall horizontal quartz-tube reactor. A three-step nucleation-ripening-lateral growth process, carried out at temperatures ranging from 850^oC to 1000^oC, was used to achieve epitaxial films using W(CO)₆ and H₂Se as precursors in a H₂ carrier gas. Density functional theory calculations demonstrate that the extent of Se passivation of the sapphire surface and the presence of oxygen remnants near the step edge are critical factors in determining the location of TMD nucleation on the step edge and the subsequent domain orientation relative to the underlying sapphire. By changing the reactor pressure, which modifies the sapphire surface via changes in the chemical potential of H and Se, the site of WSe₂ nucleation can be modified which switches the preferred orientation of the domains. Fully coalesced TMD monolayers are obtained with a reduced density of inversion domain boundaries (<15% areal coverage).The results demonstrate the important role of step structure and chemistry in nucleation and epitaxial growth of TMD monolayers.

Monolayer transition-metal dichalcogenides (TMDs) are two-dimensional materials with exceptional optical properties such as high oscillator strength, valley-related excitonic physics, efficient photoluminescence, and several narrow excitonic resonances. Long time the above effects have been explored for structures produced by techniques involving mechanical exfoliation or post-growth transfer, and more recently, also 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 yielded only structures characterized by considerable spectral broadening, which hinders most of the interesting optical effects. In this talk, I will present the MBE-grown TMD (MoSe₂) exhibiting narrow and fully resolved spectral lines of neutral and charged exciton [1]. Moreover, our monolayers exhibit 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. Importantly, good optical properties are achieved for as-grown samples, without any post-growth exfoliation and encapsulation in hBN. Our best recipe for MBE growth includes an extremely slow growth rate, the annealing at very high temperatures, and the use of atomically flat hBN substrate in the form of flakes exfoliated from bulk. Moreover, comparable results we also obtained using an hBN buffer that we grow by MOCVD on 2” Al₂O₃ wafers [2]. Our optical characterization includes Raman scattering, second-harmonic generation, and low-temperature photoluminescencere. Thanks to sharp and intense exciton lines, we were able to perform also magneto-spectroscopy and time resolved spectroscopy [3], which reveal subtle differences between 2D epilayers and mechanically exfoliated materials. We compare structural and optical properties of MoSe₂ grown on hBN to properties of various TMDs (MoTe₂ [4,5], NiTe₂ [6], WSe₂, VSe₂) grown on various substrates (2D, 3D, polycrystalline). This reveals particularly high diffusion parameters of transition metals on hBN [5], the role of distribution of orientation of TMD grain domains, the tendency to merge grains, form bilayers and 1D or 3D structures.

[1] W. Pacuski et al., Nano Letters 20, 3058 (2020).

[2] K. Ludwiczak et al., ACS Appl. Mater. Interfaces 13, 47904 (2021).

[3] K. Oreszczuk et al., in preparation (2022) .

[4] Z. Ogorzałek et al., Nanoscale 12, 16535 (2020).

[5] B. Seredyński et al., J. Cryst. Growth 596, 126806 (2022).

[6] B. Seredyński et al., Cryst. Growth Des. 21, 5773 (2021).