MoO_3/Si界面区钼掺杂非晶氧化硅层形成的第一性原理研究

采用基于密度泛函理论的第一性原理计算方法,通过模拟MoO_3/Si界面反应,研究了MoO_x薄膜沉积中原子、分子的吸附、扩散和成核过程,从原子尺度阐明了缓冲层钼掺杂非晶氧化硅(a-SiO_x(Mo))物质的形成和机理.结果表明,在1500 K温度下, MoO_3/Si界面区由Mo, O, Si三种原子混合,可形成新的稳定的物相.热蒸发沉积初始时, MoO_3中的两个O原子和Si成键更加稳定,同时伴随着电子从Si到O的转移,钝化了硅表面的悬挂键. MoO_3中氧空位的形成能小于SiO_2中氧空位的形成能,使得O原子容易从MoO_3中迁移至Si衬底一侧,从而形成氧化硅层;替位缺陷中, Si替位MoO_3中的Mo的形成能远远大于Mo替位SiO_2中的Si的形成能,使得Mo容易掺杂进入氧化硅中.因此,在晶硅(100)面上沉积MoO_3薄膜时, MoO_3中的O原子先与Si成键,形成氧化硅层,随后部分Mo原子替位氧化硅中的Si原子,最终形成含有钼掺杂的非晶氧化硅层.

First principle study of formation mechanism of molybdenum-doped amorphous silica in MoO3/Si interface

An amorphous mixing layer (3.5-4.0 nm in thickness) containing silicon (Si), oxygen (O), molybdenum (Mo) atoms, named α-SiO_x(Mo), is usually formed by evaporating molybdenum trioxide (MoO₃) powder on an n-type Si substrate. In order to investigate the process of adsorption, diffusion and nucleation of MoO₃ in the evaporation process and ascertain the formation mechanism of α-SiO_x(Mo) on a atomic scale, the first principle calculation is used and all the results are obtained by using the Vienna ab initio simulation package. The possible adsorption model of MoO₃ on the Si (100) and the defect formation energy for substitutional defects and vacancy defects in α-SiO₂ and α-MoO₃ are calculated by the density functional theory. The results show that an amorphous layer is formed between MoO₃ film and Si (100) substrate according to ab initio 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 α-SiO_x(Mo) layer. The adsorption site of MoO₃ on the reconstructed Si (100) surface, where the two oxygen atoms of MoO₃ 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 MoO₃ on the Si substrate, the structure of MoO₃ is changed. Two Mo-O bond lengths of MoO₃ 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 MoO₃ on Si substrate, while the last bond length of MoO₃ is little changed. The defect formation energy value of neutral oxygen vacancy in α-SiO₂ is 5.11 eV and the defect formation energy values of neutral oxygen vacancy in α-MoO₃ are 0.96 eV, 1.96 eV and 3.19 eV, respectively. So it is easier to form oxygen vacancy in MoO₃ than in SiO₂, which implies that the oxygen atoms will migrate from MoO₃ to SiO₂ and forms a 3.5-4.0-nm-thick α-SiO_x(Mo) layer. As for the substitutional defects in MoO₃ and SiO₂, Mo substitutional defects are most likely to form in SiO₂ 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 MoO₃ bond with Si atoms first and form the SiO_x. Then, part of Mo atoms are likely to replace Si atoms in SiO_x. Finally, the ultra-thin buffer layer containing Si, O, Mo atoms is formed at the interface of MoO₃/Si. This work simulates the reaction of MoO₃/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.