Aln And Tio <sub> 2 </ Sub>-doped Theoretical Study Of Electronic Structure And Optical Properties Of The System,

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 TiO₂ 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 TiO₂ 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 3.01×10⁵ cm^-1 is in the vicinity of 9eV.The refractive index n₀ 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 10⁵ 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 TiO₂ systems doped with So,V_o,and Sc-V_o coexisting,have been studied,respectively.The obtained results show that pure TiO₂ 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.