Substrate Lattice-Guided MoS2 Crystal Growth: Implications for van der Waals Epitaxy

Two-dimensional (2D) monolayer molybdenum disulfide (MoS2) semiconductors are an emerging material with interesting device applications. MoS2 crystals grown on a substrate often have random orientations due to weak van der Waals (vdW) interaction with the substrate. This leads to multiple grain boundaries when random orientated crystals coalesce. Understanding the conditions and mechanism to grow 2D crystals with an aligned orientation is crucial for high-quality single-crystal growth. Here, we study the introduction of oxidation etching in chemical vapor deposition to grow aligned MoS2 crystals and elucidate the mechanism of the guided growth by a sapphire lattice through vdW interaction. Under proper oxygen flow conditions, single crystals are found to grow in two preferential orientations with triangle crystal edges aligned to the [11 (2) over bar0] or [1 (1) over bar 00] direction of the sapphire substrate. These two orientations correspond to a superlattice of (3x3) MoS2 on (2x2) sapphire and (5x5) MoS2 on (3x3) sapphire and occur in Mo oxide- and sulfur-rich growth environments, respectively. This aligned orientation growth is realized by a carefully balanced etching and growth competition, which acts as a selection mechanism to grow energetically stable structures while etching less stable structures away. The commeasure of MoS2 crystals with the sapphire lattice in the superlattice increases the bonding of MoS2 to the sapphire lattice, thereby becoming the preferred stable structure for nucleation orientations. This study demonstrates the important role of etching-growth competition in the substrate lattice-guided 2D material growth and paves the way for the future development of vdW single-crystal epitaxy.