Low-temperature plasma-enhanced atomic layer deposition of 2-D MoS₂: large area, thickness control and tuneable morphology

Abstract

Low-temperature controllable synthesis of monolayer-to-multilayer thick MoS2 with tuneable morphology is demonstrated by using plasma enhanced atomic layer deposition (PEALD). The characteristic self-limiting ALD growth with a growth-per-cycle of 0.1 nm per cycle and digital thickness control down to a monolayer are observed with excellent wafer scale uniformity. The as-deposited films are found to be polycrystalline in nature showing the signature Raman and photoluminescence signals for the mono-to-few layered regime. Furthermore, a transformation in film morphology from in-plane to out-of-plane orientation of the 2-dimensional layers as a function of growth temperature is observed. An extensive study based on high-resolution transmission electron microscopy is presented to unravel the nucleation mechanism of MoS2 on SiO2/Si substrates at 450 °C. In addition, a model elucidating the film morphology transformation (at 450 °C) is hypothesized. Finally, the out-of-plane oriented films are demonstrated to outperform the in-plane oriented films in the hydrogen evolution reaction for water splitting applications.

Large area few-layer TMD film growths and their applications

“…A related technique, plasma enhanced ALD (PEALD) has also been employed for the growth of MoS 2 . In this process, a plasma-exposure step was introduced between the precursor pulse cycles; with the plasma serving to enhance out-of-plane growth [66]. As with PECVD, this produced multilayer films on the scale of 4 inches at temperatures low enough for flexible substrates, but small domain size was still an issue [66].…”

“…In this process, a plasma-exposure step was introduced between the precursor pulse cycles; with the plasma serving to enhance out-of-plane growth [66]. As with PECVD, this produced multilayer films on the scale of 4 inches at temperatures low enough for flexible substrates, but small domain size was still an issue [66]. Plasma enhanced CVD and ALD technology is promising for its ability to synthesize large area homogenous TMDs at low temperatures, but the technology needs further development in areas such as layer number control and surface defect management in order to reach its full potential.…”

Polarized Raman spectroscopy to elucidate the texture of synthesized MoS2

“…4,[9][10][11][12][13] However, in order to adopt these materials in the field of nanoelectronics, the development of scalable synthesis methods for 2D TMDs such as MoS 2 and WS 2 are essential. [14][15][16][17] This is closely intertwined with the development of reliable characterization techniques for synthesized TMDs, in which Raman spectroscopy is of particular interest. Raman spectroscopy has already emerged as the go-to technique for characterization of exfoliated single-crystalline TMDs yielding insight into e.g.…”

“…[18][19][20][21] In contrast to the large single-crystalline flakes, the synthesized material is typically nanocrystalline in nature with grain size ranging from at best a few hundred nanometres down to tens of nanometres. 15,16,22,23 Larger grain sizes have so far only be obtained with greatly reduced growth rates or with specially optimized workflows not compatible with largearea synthesis. [24][25][26] Furthermore, synthesized TMDs exhibit a wide range of textures depending on the synthesis conditions as reported for e.g.…”

“…atomic-layer deposition (ALD) and chemical vapour deposition (CVD). 12,15,17,[27][28][29][30][31] Synthetic MoS 2 films can be amorphous or crystalline with a texture of the crystalline material ranging from in-plane oriented grains (i.e. horizontally aligned layers) to complex 3D structures with out-of-plane oriented (OoPO) grains (i.e.…”