Metal‐sulfide assisted rapid crystallization of highly (001)‐textured tungsten disulphide (WS₂) films on metallic back contacts

Abstract

Highly (001)-textured, photoactive WS 2 films, which could be deposited before only on insulating substrates, have been prepared on polycrystalline metal layers, which can be used as back contacts for electronic WS 2 devices, for instance for thin film solar cells. The WS 2 films were prepared by the amorphous-solid-liquid-crystalline-solid (aSLcS) crystallization process from a Ni-S eutectic. However, normal polycrystalline metallic films can not be used as back contact layers, caused by the diffusion of the thin metal promoter (Ni) into the back contact layer, before a nickel-sulfur eutectic can be formed. Preventing the diffusion of the metal promoter into the back contact allows using the well-known metal-promoter assisted crystallization to grow photoactive films on different back contacts, like TiN:O. Additionally, WS 2 films on tungsten layers could be grown by Ni-induced sulfidation of W films. Based on these experiments the model of the rapid nickel sulfide induced crystallization was modified: Ni assists the growth of WS 2 crystallites already at temperatures between 500 and 600 8C leading to WS 2 films with differently oriented crystallites. At temperatures above the Ni-S eutectic temperature (637 8C) liquid NiS x droplets induce, as reported earlier, a rapid recrystallization, which leads to WS 2 films with strong (001) orientation.

Preparation of highly (001)-oriented photoactive tungsten diselenide (WSe2 ) films by an amorphous solid-liquid-crystalline solid (aSLcS) rapid-crystallization process

“…. The higher value of the interplanar spacing at lower temperatures is connected with a high density of stacking faults and dislocations in the films, which was found recently also for the very similar compound tungsten disulfide . The formation of large (001)‐oriented WSe 2 crystallites can be explained by the amorphous solid–liquid‐crystalline solid model (aSLcS), developed recently in our group .…”

Highly (001)-textured p-type WSe2 Thin Films as Efficient Large-Area Photocathodes for Solar Hydrogen Evolution

“…This method allows the preparation of highly (001)-textured thin films over a large area for a prospective large-scale solar energy conversion. We have shown that the (metal M) promoter MSe x enhances the crystallization of amorphous WSe 2+x films considerably and improves the (001) texture and photoactivity of the WSe 2 films 21–25 .…”

Fabrication of WS2/GaN p-n Junction by Wafer-Scale WS2 Thin Film Transfer

“…1. It is well-known that highly type II textured thin films can be grown on molten Ni promoter on sapphire substrates at high temperatures by rheotaxy which is attributed to the minimization of the film-substrate interface energy by forming molten NiS x droplets2122. In our experiments, the application of an ultra-thin Ni as a texture promoter is found to dramatically improve the crystal quality and carrier mobility of the WS 2 thin film.…”

“…S6 and Table S1. It was reported that the residual NiS x phases distributed at the grain boundaries do not strongly impact on the electrical properties of the WS 2 film21. Figure 3c shows the intensity of the HXRD pattern for type C film which is much lower than either type A or type B films.…”

“…Compared to type A film, carrier mobility of type B film was remarkably increased from 3.25 cm 2 /Vs to 63.3 cm 2 /Vs with the bulk carrier (hole) concentration of 7.12 × 10 16  cm −3 . The improvement in the crystallinity of the type B film is attributed to the use of Ni texture promoter which facilitates the formation of liquid NiS x phase, with a melting point of 637 °C2122, that exists at the grain boundaries during the sulfurization process. It is well known that rheotaxy is a growth technique based on a molten substrate.…”