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
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摘要
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.
An amorphous mixing layer(3.5 –4.0 nm in thickness) containing silicon(Si), oxygen(O), molybdenum(Mo) atoms, named a-SiO_x(Mo), is usually formed by evaporating molybdenum trioxide(MoO_3) powder on an n-type Si substrate. In order to investigate the process of adsorption, diffusion and nucleation of MoO_3 in the evaporation process and ascertain the formation mechanism of a-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 MoO_3 on the Si(100) and the defect formation energy for substitutional defects and vacancy defects in a-SiO_2 and a-MoO_3 are calculated by the density functional theory. The results show that an amorphous layer is formed between MoO_3 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 a-SiO_x(Mo) layer. The adsorption site of MoO_3 on the reconstructed Si(100) surface, where the two oxygen atoms of MoO_3 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_3 on the Si substrate, the structure of MoO_3 is changed. Two Mo—O bond lengths of MoO_3 are 1.95 ? and 1.94 ?, respectively, elongated by 0.22 ? and 0.21? compared with the those before the adsorption of MoO_3 on Si substrate, while the last bond length of MoO_3 is little changed. The defect formation energy value of neutral oxygen vacancy in a-SiO_2 is 5.11 eV and the defect formation energy values of neutral oxygen vacancy in a-MoO_3 are 0.96 eV, 1.96 eV and 3.19 eV,respectively. So it is easier to form oxygen vacancy in MoO_3 than in SiO_2, which implies that the oxygen atoms will migrate from MoO_3 to SiO_2 and forms a 3.5 –4.0-nm-thick a-SiO_x(Mo) layer. As for the substitutional defects in MoO_3 and SiO_2, Mo substitutional defects are most likely to form in SiO_2 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_3 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 MoO_3/Si. This work simulates the reaction of MoO_3/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.
An amorphous mixing layer(3.5 –4.0 nm in thickness) containing silicon(Si), oxygen(O), molybdenum(Mo) atoms, named a-SiO_x(Mo), is usually formed by evaporating molybdenum trioxide(MoO_3) powder on an n-type Si substrate. In order to investigate the process of adsorption, diffusion and nucleation of MoO_3 in the evaporation process and ascertain the formation mechanism of a-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 MoO_3 on the Si(100) and the defect formation energy for substitutional defects and vacancy defects in a-SiO_2 and a-MoO_3 are calculated by the density functional theory. The results show that an amorphous layer is formed between MoO_3 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 a-SiO_x(Mo) layer. The adsorption site of MoO_3 on the reconstructed Si(100) surface, where the two oxygen atoms of MoO_3 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_3 on the Si substrate, the structure of MoO_3 is changed. Two Mo—O bond lengths of MoO_3 are 1.95 ? and 1.94 ?, respectively, elongated by 0.22 ? and 0.21? compared with the those before the adsorption of MoO_3 on Si substrate, while the last bond length of MoO_3 is little changed. The defect formation energy value of neutral oxygen vacancy in a-SiO_2 is 5.11 eV and the defect formation energy values of neutral oxygen vacancy in a-MoO_3 are 0.96 eV, 1.96 eV and 3.19 eV,respectively. So it is easier to form oxygen vacancy in MoO_3 than in SiO_2, which implies that the oxygen atoms will migrate from MoO_3 to SiO_2 and forms a 3.5 –4.0-nm-thick a-SiO_x(Mo) layer. As for the substitutional defects in MoO_3 and SiO_2, Mo substitutional defects are most likely to form in SiO_2 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_3 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 MoO_3/Si. This work simulates the reaction of MoO_3/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.
引文
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[16]Anez R,Sierraalta A,Díaz L,Bastardo A,Coll D 2015 Appl.Surf.Sci.335 160
[17]Lu S Q,Wang C,Jin Y X,Bu Q Q,Yang Y 2012 J.Synthetic Crystals 41 1037
[18]Pavlova T V,Zhidomirov G M,Eltsov K N 2018 J.Phys.Chem.C 122 1741
[19]Wan Y Z,Gao M,Li Y,Du H W,Li Y H,Guo H B,Ma Z Q2017 Appl.Phys.Lett.110 213902
[20]Tao P C,Huang Y,Zhou X H,Chen X S,Lu W 2017 Acta Phys.Sin.66 118201(in Chinese)[陶鹏程,黄燕,周孝好,陈效双,陆卫2017物理学报66 118201]
[21]Coquet R,Willock D J 2005 Phys.Chem.Chem.Phys.7 3819
[22]Scopel W L,Silva A J R D,Orellana W,Fazzio A 2004 Appl.Phys.Lett.84 1492
[23]Liu H F,Yang R B,Yang W F,Jin Y J,Lee C J J 2018Appl.Surf.Sci.439 583
[2]Bullock J,Cuevas A,Allen T,Battaglia C 2014 Appl.Phys.Lett.105 232109
[3]Battaglia C,Yin X T,Zheng M,Sharp I D,Chen T,McDonnell S,Azcatl A,Carraro C,Ma B W,Maboudian R,Wallace R M,Javey A 2014 Nano Lett.14 967
[4]Battaglia C,Nicolás S M D,Wolf S D,Yin X T,Zhang M,Ballif C,Javey A 2014 Appl.Phys.Lett.104 113902
[5]Geissbühler J,Werner J,Nicolas S M D,Barraud L,HesslerWyser A,Despeisse M,Nicolay S,Tomasi A,Niesen B,Wolf S D,Ballif C 2015 Appl.Phys.Lett.107 081601
[6]Gerling L G,Voz C,Alcubilla R,Puigdollers J 2016 J.Mater.Res.32 260
[7]Gao M,Chen D Y,Han B C,Song W L,Zhou M,Song X M,Xu F,Zhao L,Li Y H,Ma Z Q 2018 ACS Appl.Mater.Interfaces 10 27454
[8]Chen D Y,Gao M,Wan Y Z,Li Y H,Guo H B,Ma Z Q2019 Appl.Surf.Sci.473 20
[9]Kresse K,Furthmüller J 1996 Phys.Rev.B 54 11169
[10]Perdew J P,Burke K,Ernzerhof M 1996 Phys.Rev.Lett.773865
[11]Bl?chl P E 1994 Phys.Rev.B 50 17953
[12]Lambert D S,Murphy S T,Lennon A,Burr P A 2017 RSCAdv.7 53810
[13]NoséS 1984 J.Chem.Phys.81 511
[14]Fialko E F,Kikhtenko A V,Goncharov V B,Zamaraev K I1997 J.Phys.Chem.A 101 8607
[15]Oliveira J A,Almeida W B D,Duarte H A 2003 Chem.Phys.Lett.372 650
[16]Anez R,Sierraalta A,Díaz L,Bastardo A,Coll D 2015 Appl.Surf.Sci.335 160
[17]Lu S Q,Wang C,Jin Y X,Bu Q Q,Yang Y 2012 J.Synthetic Crystals 41 1037
[18]Pavlova T V,Zhidomirov G M,Eltsov K N 2018 J.Phys.Chem.C 122 1741
[19]Wan Y Z,Gao M,Li Y,Du H W,Li Y H,Guo H B,Ma Z Q2017 Appl.Phys.Lett.110 213902
[20]Tao P C,Huang Y,Zhou X H,Chen X S,Lu W 2017 Acta Phys.Sin.66 118201(in Chinese)[陶鹏程,黄燕,周孝好,陈效双,陆卫2017物理学报66 118201]
[21]Coquet R,Willock D J 2005 Phys.Chem.Chem.Phys.7 3819
[22]Scopel W L,Silva A J R D,Orellana W,Fazzio A 2004 Appl.Phys.Lett.84 1492
[23]Liu H F,Yang R B,Yang W F,Jin Y J,Lee C J J 2018Appl.Surf.Sci.439 583