Formation of Horizontally and Vertically Oriented MoS2 and MoSe2 Films Using Cracked Small S- and Se-Molecules and Its Applications for Transparent a-Si:H Thin Film Photovoltaic Devices
Authors: Yun Sun Jin , Jung Kwang Hoon , Kim So Hyun 
Published: February 2020

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 (Xa, 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.