AlN和TiO_2掺杂体系的电子结构和光学性质的理论研究
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摘要
AlN是一种很有发展前途的新型光电子材料,在AlN中掺杂Si元素,可以得到较理想的n型AlN薄膜材料,提高系统的电导率,改善其导电性能和光学性质。本征AlN是n型半导体,存在较多本征施主缺陷(如氮空位V_N),很难经过掺杂实现p型转变,因而无法制得氮化铝p-n结结构,这在一定程度上限制了氮化铝基光电器件的开发应用,因此AlN的p型掺杂就成了热点问题。
锐钛矿相半导体TiO_2在光催化方面有广泛应用,其带隙为3.2eV,光催化所需最大波长为387nm,位于紫外光区,太阳光的利用率很低。因此,为提高太阳光的催化效率,就需要把TiO_2的光学吸收边从紫外区移至可见光区,掺杂是实现这一目的的有效方法。
本文利用第一性原理方法研究了Si和C掺杂AlN系统、氧空位与Sc掺杂锐钛矿TiO_2系统的电子结构和光学性质,并与可能的实验结果进行了比较。我们的计算工具是基于密度泛函理论的CASTEP软件。
(1)研究了纯AlN和Si掺杂AlN系统的电子结构和光学性质。结果表明,纯AlN是直接带隙半导体,带隙为6.2eV,其价带主要来源于N2p态的贡献,少部分来源于Al3s、3p态的贡献;导带主要来源于Al3p态的贡献。Si掺杂后,杂质能级位于导带底附近,与Al3p能级形成复合导带底,费米能级插在导带中间,使系统发生莫特相变,由半导体变为金属。纯AlN的介电函数虚部在8.83eV、11.46eV和13.24eV附近有吸收峰,光吸收主要分布在6～15eV的能量范围内,在9eV附近的吸收最强,峰值达到3.01×10~5cm~(-1);折射率n_0约为1.5,其峰值主要分布在6.7～8.7eV的能量范围内,当能量大于7.76eV时,折射率随能量的增加逐渐减小;消光系数和光电导率实部的峰值位置与吸收系数的峰值位置对应,能量损耗的主峰在14.1eV附近。Si掺杂后,在6～15eV能区内对系统的光学性质影响很小,而在1.5～3.5eV的低能(可见光)区有新的吸收峰出现,量级达10~5cm~(-1),能量损耗降低。
(2)对于AlN系统,研究了C替代Al位(C_(Al))、N位(C_N)以及C_(Al)与C_N共掺(C_(Al)-C_N)三种情况的杂质形成能和电子结构。结果表明,掺杂后系统的禁带宽度都变小;C_(Al)的杂质形成能最小,最容易形成:C_(Al)是n型掺杂,施主能级位于导带底下约1.27eV处。C_N的杂质形成能最大,最难形成;但C_N可使系统实现p型转变,受主能级位于价带顶约0.21eV处。C_(Al)和C_N共存时,C_(Al)对C_N有补偿作用,这对系统的p型转变不利。
(3)对于锐钛矿TiO_2系统,研究了Sc掺杂、含氧空位以及氧空位与Sc掺杂共存时系统的电子结构和光学性质。结果表明,纯TiO_2是间接带隙半导体,光吸收集中在紫外区;Sc掺杂对系统的主要贡献在价带区,掺杂后系统在可见光区有明显的吸收;氧空位可以使系统发生莫特相变,由半导体变为金属,掺杂后系统在可见光区也有较强的吸收;氧空位与Sc掺杂共存时系统在可见光区的吸收相干加强,光吸收的峰值可达9.1×10~4cm~(-1),从而可以明显地改善系统在可见光区的光催化活性。
AlN is a new kind and promising photoelectric material.Better n-type AlN thin films can be achieved by Si doping.Thus,the conductivity of the system can be enhanced,and the conductive properties and optical properties can be improved.The as-grown AlN is an n-type semiconductor with many donor defects such as N vacancy(V_N),so it is very difficult to make AlN a p-type semiconductor by doping,accordingly the useful p-n joint is hard to be made.Thus,the developments and applications of the AlN-based optoelectronic devices are greatly limited.So, the p-type doping of AlN has become a focus task.
Anatase TiO_2 plays an important role in photocatalysis,its band gap is 3.2eV.The maximum of the photocatalysis wavelength is 387nm,located in the ultraviolet(UV) light region, so the sunlight utilization efficiency is very low.It is necessary to extend the absorption edge from UV region to visible region for making efficient uses of the solar energy.Doping is an effective way for the purpose.
Using the first-principles approach,we have studied the electronic structure and optical properties of the AlN system doped with Si and C,and the TiO_2 system doped with Sc,oxygen vacancy(V_o),and Sc-V_o coexisting.Comparisons of our results with the possible experiment results have been given.Our calculations are implemented by the density functional theory based CASTEP package.
(1)The electronic structure and optical properties of pure AlN and Si-doped AlN systems have been investigated.The obtained results show that pure AlN is a direct band gap semiconductor with gap of 6.2eV.The valence band is primarily derived from N2p states, partially derived from Al3s and 3p states.The conduction band is primarily derived from Al3p states.Si impurity energy levels are located near the bottom of the conduction band of the host AlN,which and the Al3p levels together make the complex conduction band bottom.Fermi level lies within the conduction band,a Mott phase transition takes place,and the system transforms from semiconductor into metal.There are three peaks in the imaginary part of the electronic dielectric function of pure AlN,which are located at about 8.83,11.46,and 13.24eV. respectively.Optical absorption distributes mainly in the 6～15eV energy region.The main absorption peak with the height as big as 3.01×10~5 cm~(-1) is in the vicinity of 9eV.The refractive index n_0 is about 1.5.The refractive index peak distributes mainly in the energy region of 6.7 8.7eV.When the energy is higher than 7.76eV,the refractive index gradually decreases with the increase in energy.The peak positions of the extinction and the real part of optical conductivity is homologous to the peak position of the absorption.The main peak of the energy loss is at about 14.1eV.In the energy region of 6～15eV,Si doping has little effects on the optical properties of the system;while in the low energy region of 1.5～3.5eV,there exists a new absorption peak with the height as big as 10~5 cm~(-1).The energy loss decreases.
(2) Three kinds of defects,the substitution of Al by C,N by C,and two kinds of substitution coexisting in AlN,have been studied.The impurity formation energy and electronic structure of the systems have been studied.The obtained results show that the band gaps of the doped systems become smaller.When Al is substituted by C,the impurity formation energy is the least, i.e.it is easy to be formed.The substitution of Al by C is n-type,and the donor levels are formed below the bottom of the conduction band about 1.27eV.When N is substituted by C,the impurity formation energy is the biggest,i.e.it is hard to be formed,but the p-type transformation of the system can be achieved,the acceptor levels are formed nearby the top of the valence band about 0.21eV.When the two kinds of substitution coexist,the acceptor levels are compensated for all cases,which is unfavorable for the p-type transformation of the system.
(3) The electronic structure and optical properties of the anatase TiO_2 systems doped with So,V_o,and Sc-V_o coexisting,have been studied,respectively.The obtained results show that pure TiO_2 is an indirect band gap semiconductor.Optical absorption mainly distributes in the UV region.The contribution by the doped Sc mainly lies in the valence bands,and the light absorption in the visible distinct is obvious.A Mott phase transformation takes place in the presence of oxygen vacancies,and the system transforms into metal from semiconductor,the light absorption in the visible distinct is also obvious.Especially,the visible light absorptions of the two cases enhance coherently,thus the photocatalysis of the system can be improved greatly.
锐钛矿相半导体TiO_2在光催化方面有广泛应用,其带隙为3.2eV,光催化所需最大波长为387nm,位于紫外光区,太阳光的利用率很低。因此,为提高太阳光的催化效率,就需要把TiO_2的光学吸收边从紫外区移至可见光区,掺杂是实现这一目的的有效方法。
本文利用第一性原理方法研究了Si和C掺杂AlN系统、氧空位与Sc掺杂锐钛矿TiO_2系统的电子结构和光学性质,并与可能的实验结果进行了比较。我们的计算工具是基于密度泛函理论的CASTEP软件。
(1)研究了纯AlN和Si掺杂AlN系统的电子结构和光学性质。结果表明,纯AlN是直接带隙半导体,带隙为6.2eV,其价带主要来源于N2p态的贡献,少部分来源于Al3s、3p态的贡献;导带主要来源于Al3p态的贡献。Si掺杂后,杂质能级位于导带底附近,与Al3p能级形成复合导带底,费米能级插在导带中间,使系统发生莫特相变,由半导体变为金属。纯AlN的介电函数虚部在8.83eV、11.46eV和13.24eV附近有吸收峰,光吸收主要分布在6～15eV的能量范围内,在9eV附近的吸收最强,峰值达到3.01×10~5cm~(-1);折射率n_0约为1.5,其峰值主要分布在6.7～8.7eV的能量范围内,当能量大于7.76eV时,折射率随能量的增加逐渐减小;消光系数和光电导率实部的峰值位置与吸收系数的峰值位置对应,能量损耗的主峰在14.1eV附近。Si掺杂后,在6～15eV能区内对系统的光学性质影响很小,而在1.5～3.5eV的低能(可见光)区有新的吸收峰出现,量级达10~5cm~(-1),能量损耗降低。
(2)对于AlN系统,研究了C替代Al位(C_(Al))、N位(C_N)以及C_(Al)与C_N共掺(C_(Al)-C_N)三种情况的杂质形成能和电子结构。结果表明,掺杂后系统的禁带宽度都变小;C_(Al)的杂质形成能最小,最容易形成:C_(Al)是n型掺杂,施主能级位于导带底下约1.27eV处。C_N的杂质形成能最大,最难形成;但C_N可使系统实现p型转变,受主能级位于价带顶约0.21eV处。C_(Al)和C_N共存时,C_(Al)对C_N有补偿作用,这对系统的p型转变不利。
(3)对于锐钛矿TiO_2系统,研究了Sc掺杂、含氧空位以及氧空位与Sc掺杂共存时系统的电子结构和光学性质。结果表明,纯TiO_2是间接带隙半导体,光吸收集中在紫外区;Sc掺杂对系统的主要贡献在价带区,掺杂后系统在可见光区有明显的吸收;氧空位可以使系统发生莫特相变,由半导体变为金属,掺杂后系统在可见光区也有较强的吸收;氧空位与Sc掺杂共存时系统在可见光区的吸收相干加强,光吸收的峰值可达9.1×10~4cm~(-1),从而可以明显地改善系统在可见光区的光催化活性。
AlN is a new kind and promising photoelectric material.Better n-type AlN thin films can be achieved by Si doping.Thus,the conductivity of the system can be enhanced,and the conductive properties and optical properties can be improved.The as-grown AlN is an n-type semiconductor with many donor defects such as N vacancy(V_N),so it is very difficult to make AlN a p-type semiconductor by doping,accordingly the useful p-n joint is hard to be made.Thus,the developments and applications of the AlN-based optoelectronic devices are greatly limited.So, the p-type doping of AlN has become a focus task.
Anatase TiO_2 plays an important role in photocatalysis,its band gap is 3.2eV.The maximum of the photocatalysis wavelength is 387nm,located in the ultraviolet(UV) light region, so the sunlight utilization efficiency is very low.It is necessary to extend the absorption edge from UV region to visible region for making efficient uses of the solar energy.Doping is an effective way for the purpose.
Using the first-principles approach,we have studied the electronic structure and optical properties of the AlN system doped with Si and C,and the TiO_2 system doped with Sc,oxygen vacancy(V_o),and Sc-V_o coexisting.Comparisons of our results with the possible experiment results have been given.Our calculations are implemented by the density functional theory based CASTEP package.
(1)The electronic structure and optical properties of pure AlN and Si-doped AlN systems have been investigated.The obtained results show that pure AlN is a direct band gap semiconductor with gap of 6.2eV.The valence band is primarily derived from N2p states, partially derived from Al3s and 3p states.The conduction band is primarily derived from Al3p states.Si impurity energy levels are located near the bottom of the conduction band of the host AlN,which and the Al3p levels together make the complex conduction band bottom.Fermi level lies within the conduction band,a Mott phase transition takes place,and the system transforms from semiconductor into metal.There are three peaks in the imaginary part of the electronic dielectric function of pure AlN,which are located at about 8.83,11.46,and 13.24eV. respectively.Optical absorption distributes mainly in the 6～15eV energy region.The main absorption peak with the height as big as 3.01×10~5 cm~(-1) is in the vicinity of 9eV.The refractive index n_0 is about 1.5.The refractive index peak distributes mainly in the energy region of 6.7 8.7eV.When the energy is higher than 7.76eV,the refractive index gradually decreases with the increase in energy.The peak positions of the extinction and the real part of optical conductivity is homologous to the peak position of the absorption.The main peak of the energy loss is at about 14.1eV.In the energy region of 6～15eV,Si doping has little effects on the optical properties of the system;while in the low energy region of 1.5～3.5eV,there exists a new absorption peak with the height as big as 10~5 cm~(-1).The energy loss decreases.
(2) Three kinds of defects,the substitution of Al by C,N by C,and two kinds of substitution coexisting in AlN,have been studied.The impurity formation energy and electronic structure of the systems have been studied.The obtained results show that the band gaps of the doped systems become smaller.When Al is substituted by C,the impurity formation energy is the least, i.e.it is easy to be formed.The substitution of Al by C is n-type,and the donor levels are formed below the bottom of the conduction band about 1.27eV.When N is substituted by C,the impurity formation energy is the biggest,i.e.it is hard to be formed,but the p-type transformation of the system can be achieved,the acceptor levels are formed nearby the top of the valence band about 0.21eV.When the two kinds of substitution coexist,the acceptor levels are compensated for all cases,which is unfavorable for the p-type transformation of the system.
(3) The electronic structure and optical properties of the anatase TiO_2 systems doped with So,V_o,and Sc-V_o coexisting,have been studied,respectively.The obtained results show that pure TiO_2 is an indirect band gap semiconductor.Optical absorption mainly distributes in the UV region.The contribution by the doped Sc mainly lies in the valence bands,and the light absorption in the visible distinct is obvious.A Mott phase transformation takes place in the presence of oxygen vacancies,and the system transforms into metal from semiconductor,the light absorption in the visible distinct is also obvious.Especially,the visible light absorptions of the two cases enhance coherently,thus the photocatalysis of the system can be improved greatly.
引文
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