First principle study of formation mechanism of molybdenum-doped amorphous silica in MoO<sub>3</sub>/Si interface
Authors: Chen Dong-Yun , Gao Ming , Li Yong-Hua , Xu Fei , Zhao Lei , Ma Zhong-Quan ,   SHU-SOEN's R&D  Lab, Department of Physics, College of Sciences, Shanghai University, Shanghai 200444, China
Published: December 2020

An amorphous mixing layer (3.5–4.0 nm in thickness) containing silicon (Si), oxygen (O), molybdenum (Mo) atoms, named <i>α</i>-SiO<sub><i>x</i></sub>(Mo), is usually formed by evaporating molybdenum trioxide (MoO<sub>3</sub>) powder on an n-type Si substrate. In order to investigate the process of adsorption, diffusion and nucleation of MoO<sub>3</sub> in the evaporation process and ascertain the formation mechanism of <i>α</i>-SiO<sub><i>x</i></sub>(Mo) on a atomic scale, the first principle calculation is used and all the results are obtained by using the Vienna <i>ab initio</i> simulation package. The possible adsorption model of MoO<sub>3</sub> on the Si (100) and the defect formation energy for substitutional defects and vacancy defects in <i>α</i>-SiO<sub>2</sub> and <i>α</i>-MoO<sub>3</sub> are calculated by the density functional theory. The results show that an amorphous layer is formed between MoO<sub>3</sub> film and Si (100) substrate according to <i>ab initio</i> molecular dynamics at 1500 K, which are in good agreement with experimental observations. The O and Mo atoms diffuse into Si substrate and form the bonds of Si—O or Si—O—Mo, and finally, form an <i>α</i>-SiO<sub><i>x</i></sub>(Mo) layer. The adsorption site of MoO<sub>3</sub> on the reconstructed Si (100) surface, where the two oxygen atoms of MoO<sub>3</sub> bond with two silicon atoms of Si (100) surface, is the most stable and the adsorption energy is －5.36 eV, accompanied by the electrons transport from Si to O. After the adsorption of MoO<sub>3</sub> on the Si substrate, the structure of MoO<sub>3</sub> is changed. Two Mo—O bond lengths of MoO<sub>3</sub> are 1.95 Å and 1.94 Å, respectively, elongated by 0.22 Å and 0.21 Å compared with the those before the adsorption of MoO<sub>3</sub> on Si substrate, while the last bond length of MoO<sub>3</sub> is little changed. The defect formation energy value of neutral oxygen vacancy in <i>α</i>-SiO<sub>2</sub> is 5.11 eV and the defect formation energy values of neutral oxygen vacancy in <i>α</i>-MoO<sub>3</sub> are 0.96 eV, 1.96 eV and 3.19 eV, respectively. So it is easier to form oxygen vacancy in MoO<sub>3</sub> than in SiO<sub>2</sub>, which implies that the oxygen atoms will migrate from MoO<sub>3</sub> to SiO<sub>2</sub> and forms a 3.5–4.0-nm-thick <i>α</i>-SiO<sub><i>x</i></sub>(Mo) layer. As for the substitutional defects in MoO<sub>3</sub> and SiO<sub>2</sub>, Mo substitutional defects are most likely to form in SiO<sub>2</sub> in a large range of Mo chemical potential. So based on our obtained results, the forming process of the amorphous mixing layer may be as follows: the O atoms from MoO<sub>3</sub> bond with Si atoms first and form the SiO<sub><i>x</i></sub>. Then, part of Mo atoms are likely to replace Si atoms in SiO<sub><i>x</i></sub>. Finally, the ultra-thin buffer layer containing Si, O, Mo atoms is formed at the interface of MoO<sub>3</sub>/Si. This work simulates the reaction of MoO<sub>3</sub>/Si interface and makes clear the interfacial geometry. It is good for us to further understand the process of adsorption and diffusion of atoms during evaporating, and it also provides a theoretical explanation for the experimental phenomenon and conduces to obtaining better interface passivation and high conversion efficiency of solar cell.