Study On Mn Doped Mg₂Si Thin Films

Silicide Mg₂ Si is a kind of environment-friendly semiconductor materials because of its non-toxic,no pollution,as well as the abundance of their raw materials in natural resources.The materials have important application in the field of optoelectronic devices,electronics,and energy devices.In this thesis,Mn doped Mg₂ Si and its structure and photoelectric properties were studied theoretically and experimentally.Firstly,the effects of Mn doping on electronic structure and photoelectric properties of Mg₂ Si were researched based on the Density Functional First Principles Pseudo Potential Plane Wave method with Mn content of 0,0.125,0.25 and 0.5,respectively.The energy band structure,density of states and optical parameters were calculated.The results show that the Mg₂ Si is an indirect semiconductor material with the band gap value of 0.22 eV.Near the Fermi surface,the total density of states is zero.The conduction band of Mg₂ Si is mainly composed of Mg 3s,3p and Si 3p;and the valence band is mainly composed of Si 3p,Mg 3s and 3p.Its static dielectric constant is 21.459,the refractive index is 4.632,the extinction coefficient is 2.857,the light absorption is in the range of 0²5.106 eV,the main photoelectric conductance peaks appear in the range of 2.299².856 eV.After Mn doping,the band gap become zero because of the electronic structure of Mn 3d54s2 and the non zero density of states near the Fermi level.With the increase of the concentration of Mn,the static dielectric constant increases,the imaginary part of the complex dielectric function appears red shift and the peak value decreases,absorption peak shift to blue firstly and then to red,but the value of peak does not change significantly,the range of reflection spectrum first increases and then decreases,the optical conductivity shows a blue shift and the peak increases firstly then decreases,the refractive index and extinction coefficient appears red shifted,and the refractive index increases,the extinction coefficient decreased slightly.Secondly,Mg/Mn films were deposited on Si substrates by thermal evaporation,and Mg₂-xMnxSi?x=0,0.02,0.04,0.06,0.08?thin films were prepared by annealing at low vacuum?10-1 Pa-10-2 Pa?atmosphere.The structure and morphology of the film was characterized by XRD,SEM and AFM.The results show that Mn doping of increases the film density but does not change the crystal structure of Mg₂ Si film.With the increase of Mn doping amount,the Mn is present in the Mg₂ Si film in the form of substitution,and the surface roughness of the Mg₂-xMnxSi film increased and the average particle size decreased.Finally,the photoelectric properties of Mg₂-xMnxSi?x=0,0.02,0.04,0.06,0.08?films were measured by Holzer effect measurement and spectrophotometer.The results show that when Mn content is low,the doping of Mn does not change the conductivity type of Mg₂ Si itself.With the increase of Mn amount,the conductivity type of Mg₂-xMnxSi films is changed from N to P,and the strength of ionic bond and covalent bond decrease,resulting in the binding force of atom on electron is weakened,lead to increase the carrier concentration and Holzer mobility and to reduce the resistance of film of Mg₂-xMnxSi.The absorbance of Mg1.96Mn0.04 Si reached 17% at a wavelength of 2000 nm,the reflectivity reached 90% at a wavelength of 1500 nm,and transmittance reached 4% at a wavelength of 1250 nm.