The Stability Of B-doped Silicon Quantum Dots By First Principles Calculation

Doping silicon quantum dots have different optical and electrical properties from bulk silicon materials and have attracted widespread attention in the scientific community.Through doping,the optical,electrical and magnetic properties of silicon quantum dots can be adjusted to make it have more potential application prospects,such as solar cells,photovoltaics,photovoltaics and biomedical imaging.Actually,Boron(B)is the most widely used p-type dopant.Although many experiments and theoretical work have shown that the doping behavior of B atoms in Si quantum dots is closely related to the size,site,shape,and surface chemistry,even the difference in surface passivation also affects the distribution of B atoms in Si quantum dots.However,in the study of the stability of B-doped Si quantum dots,the site and size effect of impurity formation energy,and the qualitative or quantitative contribution of shape role on impurity formation energy are still unknown.In addition,the effect of halogenation on B doping Si quantum dots has not been systematically studied.In this paper,the first principles calculation based on density function theory(DFT)is used to investigate the impurity formation energy of B-doped Si quantum dots,and the three-dimensional analysis of the size,site,and shape of B doping is formed.Moreover,the effect of surface halogenation on the formation energy and doping site of B-doped Si quantum dots is also studied,which provides theoretical guidance for the research of Si quantum dots.The main contents are summarized as following:(1)The site discrimination and size effects on the formation energy of the B atoms in B-doped Si quantum dots with truncated octahedral shape were studied in this work by introducing a new parameter,the adjusted coordination number(ACN),which includes second-nearest neighbor atoms.It was found that the B-doping sites in the Si quantum dots can be well discriminated by ACN,and the change trends of the formation energy with either size or site become clear if one uses ACN as a describer.When the site is given,the formation energy has a decreasing trend when size increases.However,when considering the sites with the same coordination number(CN)in a given Si quantum dot,their values of the formation energy increase with increasing ACN.Moreover,it was observed that the sites with ACN=3.375 are the usually preferred B doping sites in these Si quantum dots when they exist.(2)The shape effect of B doping formation energy E_f in Si quantum dots was investigated by first principles calculation.Si quantum dots with octahedron(OT),tetrahedron-centered(TC)and tetrahedron(TH)shapes were constructed respectively.B doping site can be simply divided into vertex(′v′),edge(′e′)and facet(′f′)on the surfaces of Si quantum dots.Under a given size and site,there is a sequence of E_f(TH)<E_f(TC)<E_f(OT).This is closely related with their shape,since their shape factor?has the similar sequence of?(TH)<?(TC)<?(OT).However,for any shape of TH,TC,and OT,the shape factor?is not a constant but shows the increases trend when size increases,which is opposite to the effect of the size on formation energy.Even though this,shape effect on formation energy still cannot be ignored,and the shape of Si quantum dots with smaller?values are in favor of B doping in them.(3)We explored the effects of surface halogen passivants including fluorine(F),chlorine(Cl)and bromine(Br)on B-doped Si quantum dots.The formation energy E_f of B doping at different sites(′v′,′e′and′f′site on the surface)of Si quantum dots with different surface halogen passivation numbers was obtained by using first-principle calculation.Compared with pure Si quantum dots,the value of E_f is greatly lowered as long as halogen passivants exit.Considering B doping site on surface,it was found that in the partially halide-Si quantum dots,B prefers to stay at the sites whose first nearest neighbors have no halogen atoms.Moreover,these sites can be well discriminated by considering the number of second-nearest halide atoms k.When B is doped at?v?site,?e?and?f?sites,E_f decreases when the number of k increases.Obviously,the second-nearest neighbors clearly play an important role in E_f value.Based on these results,except for fully halogenated Si quantum dots,the most preferred sites of B doing are found to be the same without considering halogen types if the passivation ratio is constant.In a word,through considering the number of halgon elements at the first and second nearest neighbors of B doping site,the change rule of E_f can be well clarified to a large extent.