Molybdenum dichalcogenide films (MoX₂, X=S, Se) were fabricated on 6-inch wafers by a breakthrough technique utilizing cracked small X-molecules. Uniformly deposited Mo-metal films were sulfurized or selenized by reactive chalcogen (X_a, a= 1- 2) small molecules, and MoX₂ film was approximately 3 times thicker than the Mo metal precursor. The behavior of reaction chamber pressure according to the cracking temperature indicated that one large molecule could crack into 4 – 5 small particles in experimental condition used in this work. The formation of protrusions was also effectively inhibited to obtain MoX₂ films of a device quality. First, I will explain how effectively large chalcogen molecules (normally X₈) were cracked, and the cracking effect on the film quality. By controlling the cracking temperature separately, we could lower the growth temperature down to 400 ^oC or lower. The effect of cracking and X-source evaporation temperatures on the film quality will be also presented. The film quality was dependent on the deposition technique of Mo metal films. The films were characterized using x-ray photoelectron spectroscopy (XPS), Raman scattering, atomic force microscopy (AFM), and transmission electron microscopy (TEM). Cross sectional high resolution TEM image, energy-dispersive x-ray spectroscopy, and angle-resolved XPS analysis of 6 nm-thick MoS₂ sulfurized at 570 ^oC showed that the film has well-aligned layer-by-layer crystalline structure and the composition and chemical states were uniform through depth.

Normally, layer-by-layer grown MoX₂ films on very flat surfaces are studied for device applications as reported in earlier works. But, practical devices require the growth of semiconductor films on rough and/or patterned surfaces. Then, we studied the formation of a few nm-thick and a few tens nm-thick MoX₂ films on rough surfaces, and found that the MoX₂ films could be successfully grown on a very rough substrate (root mean square roughness = 38 nm) using the method introduced in this work.

When MoX₂ films were grown by chalcogenizing Mo, the crystalline orientation of MoX₂ films depended on the thickness of Mo films. When the Mo film is thinner than 5 nm, we could obtain layer-by-layer structured, i.e., MoX₂ films. We had reported the successful application of the horizontally grown MoX₂ film to the field effect transistors in the previous work [1]. Contrarily, when the Mo film was 5 nm or thicker, the formed MoX₂ films had vertically oriented crystalline structure. It is because the stress due to 3 times volume expansion should be released by changing the crystalline orientation from planar layer-by-layered to vertically oriented structures. In this work, we utilized the vertically oriented MoX₂ films as an interlayer between the semiconductor layer and the electrode. By adopting the MoX₂ interlayer in amorphous silicon (a-Si:H) thin film photovoltaic devices having the inverted structure, we could obtain greatly improved cell performances. In the substrate type photovoltaic cells, the shunt resistance increased from 10 kΩ (no MoSe₂) to 18 kΩ (with MoSe₂) and the open circuit voltage increased from 0.717 V to 0.785 V. The a-Si:H photovoltaic cell having 26 % visible light transmittance on a glass substrate showed the conversion efficiency of 7.7 % under blue light irradiation of 7 mW/cm².

In conclusion, the horizontally and vertically oriented MoX₂ films of high crystallinity were formed by chalcogenizing Mo metal films using reactive cracked small X-molecules, and the use of MoX₂ film as an interlayer greatly improved the performance of a-Si:H thin film photovoltaic devices. The growth method introduced in this work would be one of the most promising methods to obtain uniform and high quality metal dichalcogenide films on a large substrate. The most important advantage of this method is that it can be easily applied to other metal chalcogenide materials.

[1] Jung, K. H.; Yun, S. J.; Choi, Y.; Cho, J. H.; Lim, J. W.; Chai, H.-J.; Cho, D.-H.; Chung, Y.-D.; Kim, G. Nanoscale 2018, 10, 15213-15221.