Nucleation mechanism during WS2 plasma enhanced atomic layer deposition on amorphous Al2O3 and sapphire substrates

Abstract

The structure, crystallinity and properties of as-deposited two-dimensional (2D) transition metal dichalcogenides are determined by nucleation mechanisms in the deposition process. 2D materials grown by atomic layer deposition (ALD) in absence of a template, are polycrystalline or amorphous. Little is known about their nucleation mechanisms.Therefore, we investigate the nucleation behavior of WS2 during plasma enhanced ALD from WF6, H2 plasma and H2S at 300 °C on amorphous ALD Al2O3 starting surface and on monocrystalline, bulk sapphire. Preferential interaction of the precursors with the Al2O3 starting surface promotes fast closure of the WS2 layer. The WS2 layers are fully continuous at WS2 content corresponding to only 1.2 WS2 monolayers. On amorphous Al2O3, (0002) textured and polycrystalline WS2 layers form with grain size of 5 nm to 20 nm due to high nucleation density (~10 14 nuclei/cm 2 ). The WS2 growth mode changes from 2D (layer-by-layer) growth on the initial Al2O3 surface to threedimensional (Volmer-Weber) growth after WS2 layer closure. Further growth proceeds from both WS2 basal planes in register with the underlying WS2 grain, and from or over grain boundaries of the underlying WS2 layer with different in-plane orientation. In contrast, on monocrystalline sapphire, WS2 crystal grains can locally align along a preferred in-plane orientation. Epitaxial seeding occurs locally albeit a large portion of crystals remain randomly oriented, presumably due to the low deposition temperature.The WS2 sheet resistance is 168 MΩµm suggesting that charge transport in the WS2 layers is limited by grain boundaries.3

Crystalline tungsten sulfide thin films by atomic layer deposition and mild annealing

“…So far, the best crystallinity seems to have been obtained using plasmaenhanced ALD, but the use of plasma adds process complexity and may limit the conformality of the films. Detailed studies on issues critical to the growth of 2D films, including nucleation, grain growth, film closure, and effect of substrate have been performed with the WF6+H2 plasma+H2S process, [43][44][45][46] whereas corresponding understanding is lacking for the other reported processes. Thus, further studies on our process as well as the other processes are needed in the future to assess their full potential in the growth of 2D WS2.…”

“…42 On the contrary, only a few processes have been demonstrated for WS2. WF6 does not enable film growth with H2S unless a H2 plasma pulse [43][44][45][46] is added before the H2S pulse or either a ZnS substrate or a ZnEt2 pulse [47][48][49] is applied. W(CO)6 can be used to deposit amorphous or nanocrystalline WS2 films with either H2S 50,51 or H2S plasma 52 reactants.…”

“…The use of WCl5 with H2S has also been claimed, but very limited information is available about the precursor as well as the process and its ALD characteristics. 53 Furthermore, in many cases the ALD WS2 films have been tens to hundreds of nm in thicknessindeed, in addition to the WCl5+H2S process, ultrathin (<10 nm) films have only been deposited by the W(CO)6+H2S plasma 52 and WF6+H2 plasma+H2S [43][44][45][46] PEALD processes operated at 350 and 250-450 °C, respectively.…”

Controlling Atomic Layer Deposition of 2D Semiconductor SnS2 by the Choice of Substrate

“…However, ALD also has certain challenges in the deposition of 2D materials due to the typically low reactivity of ALD precursors on the basal surfaces of 2D materials. [44][45][46] The growth rates may be low, [44,47,48] rough flake-like films are often obtained, [48][49][50][51][52] and the low growth temperature and high nucleation density lead to small grain size which limits the electrical performance of the films. [38,41,44] The substrate plays a crucial role in surface-reaction controlled techniques such as ALD and in 2D film growth in general.…”

“…[44][45][46] The growth rates may be low, [44,47,48] rough flake-like films are often obtained, [48][49][50][51][52] and the low growth temperature and high nucleation density lead to small grain size which limits the electrical performance of the films. [38,41,44] The substrate plays a crucial role in surface-reaction controlled techniques such as ALD and in 2D film growth in general. In addition to the common SiO 2 /Si, a range of substrates have been tested for the ALD of 2D materials, such as metal oxides, [44,47,48] gold, [49] carbon fiber paper, [53] polyimide, [41] metal foams, [54] and sapphire.…”