Atomic layer deposition of single and few layered transition metaldichalcogenides for future logic applications.
The group-VI transition metal dichalcogenides (MX2), such as tungsten disulfide (WS2), emerge as two-dimensional (2D) semiconductors that can complement the workhorse silicon as channel material in ultra-scaled nanoelectronic devices. Manufacturable approaches that develop highly crystalline MX2 layers, tailor the layer number down to the atomic level, and remain compatible with temperature sensitive structures, are essential to unlock the desired material functionality. However, fundamental understanding is lacking on how to design chemical deposition processes for 2D MX2, such as chemical vapor deposition (CVD) and atomic layer deposition (ALD). Therefore, in this PhD manuscript, we portray the growth and nucleation mechanism of WS2 ALD, plasma-enhanced ALD (PEALD) and CVD processes at low deposition temperatures (< 450 °C), based on analysis of the composition, structure and morphology.
The model system under consideration combines the tungsten hexafluoride (WF6) and dihydrogen sulfide (H2S) precursors, and explores the growth of WS2 on two starting surfaces with a contrasting reactivity towards the precursors, i.e., alumina and silica. The WS2 structure and crystal grain size depend on the precursor adsorption rate on the starting surface and the mobility of the adsorbed precursor species. The inherently less reactive SiO2 starting surface enables slow precursor adsorption, as such providing time for diffusion of adsorbed precursors species and lateral growth of crystals. However, the growth rate for ALD and CVD remains low which limits practical applications. A higher growth rate is obtained by considering a reducing agent, a H2 plasma, in a ternary PEALD process. The H2 plasma fulfills a dual role: it enables WF6 adsorption by creating reactive surface sites; and it reduces the adsorbed W6+ surface species. The WS2 PEALD process proceeds by a diffusion-mediated nucleation mechanism on SiO2: WS2 develops predominantly by lateral growth from the WS2 crystal edges, which are more reactive than the basal plane. Based on these insights in the growth and nucleation mechanism, we tune the composition, basal plane orientation and dimensions of the WS2 crystals during PEALD, either at random locations or from seeds at predetermined locations. The latter, denoted as “controlled seeding”, exploits the inherent chemical selectivity of ALD and the anisotropic growth component to grow WS2 crystals at predetermined positions on large area surfaces. This concept holds the promise to keep grain boundaries in WS2 outside the functional device area. While other WS2 ALD processes reported in literature yield nanocrystalline layers with restricted crystal grain size, a window opens for the WS2 PEALD that enables crystal grain size tuning even at low deposition temperatures (< 450 °C) based on the insight in the growth mechanism.