First Principles Study Of Electronic Structure And Optical Properties Of GaN Doping

As a typical representative of the third generation semiconductor materials,gallium nitride(GaN)has attracted more and more attention.Wurtzite GaN is a wide band gap semiconductor material.It has high thermal conductivity,high breakdown voltage,high melting point,high thermal conductivity and good chemical stability,Which can be widely used in high power,high temperature,high electron mobility,and other optoelectronic devices,such as microelectronic devices,light emitting diodes(LED)and photodetectors.Doping and adsorption are two ways to modulate the properties of materials.The lattice constants of materials will be changed through the substitutional doping or surface adsorption,and the energy band structure and photoelectric properties of materials can also be changed.Based on the first principles calculations,the Material Studio software is adopted to simulate the photoelectric properties of GaN(0001)surface modified by intrinsic point defects and adatoms and the magnetic properties of GaN monolayer structure doped with transition metal atoms.The main contents and achievements of this dissertation are outlined as follows:1.By comparing the electronic properties of GaN bulk structure,GaN(0001)surface and GaN monolayer,the direct band gap of the GaN bulk structure and indirect band gap of GaN(0001)surface and G-GaN monolayer(ML)are observed.GaN(0001)surface corresponds to the largest band gap,while GaN ML corresponds to the smallest band gap.The density of states of the three structures have little change,but the energy range corresponding to the lower valence band,the upper valence band,and the conduction band changed.2.The effects of different defects on the photoelectric properties of GaN(0001)surface were studied by analyzing the band structure,partial density of states(PDOS),dielectric function,absorption coefficient,reflectivity and the refractive index.The results show that the band gaps are significantly reduced and even disappear.In some cases of point defects,the Fermi level locates in the conduction band or valence band is observed.From the DOS we can see that the location and the energy range of the valence band,upper valence band changes,so does to the electronic states that form them.The maximum value of each optical parameter decreases obviously,the energy range of the optical parameter moves towards to the direction of low energy.It is seen that the visible light and infrared region are covered by optical absorption curve,which implies the possibility of applying to the photoelectric field such as infrared detection.3.By introducing different concentration point defects to the surface of GaN(0001),we can see that the band gaps decrease further.Different point defects cause different changes of the energy range of the upper valence band,lower valence band,and the conduction band.In terms of optical properties,the energy range covered by optical curves is still concentrated on the low energy regions.Each optical parameter changes to some extent depending on the type of defect,but the trend of variation is different.4.The photoelectric properties of the GaN(0001)surfaces adsorbed different active metals(Na,K,Al,Ni,Ru,Au)were studied by analyzing the band structure,PDOS,work function,dielectric function,absorption coefficient,reflectivity and refractive index.The results show that the band gaps decrease significantly,energy band shifts from the indirect band gap to the direct band gap.In some cases of absorption,the band gap disappearance.From the DOS we can see that the electronic states that make up of the valence band,lower valence band,and the conduction band change,the electronic states of the metal atoms are introduced into different energy regions.There is orbital hybridization between adatoms and GaN(0001)surfaces,a certain number of electrons are transferred between them.Except for Au adsorption,the work function of GaN(0001)surface decreased significantly after adsorption by other atoms.The maximum value of each optical parameter decreases obviously,the energy range covered by the optical curve narrows and shifts toward the low energy direction.The visible light and infrared region are covered by optical curve in the low energy,which implies the possibility of applying to the photoelectric field such as infrared detection.5.By adsorbing different concentration metal atoms on the surface of GaN(0001),we can see that the band gaps disappear except for Ru adsorption,even the band gaps decrease between the impurity energy level and the bottom of conduction band or the top of valence band.From the DOS we can see that the orbital hybridization near the Fermi level and charge transfer change.In terms of optical properties,the energy ranges covered by optical characteristic curves increased.Each optical parameter changes to some extent depending on the type of adatoms,but the trend of variation is different.6.Some 3d and 4d series TM atoms(Sc,Ti,Cr,Ni,Y,Zr,Mo,and Pd)are selected to tune the magnetic properties of graphene like GaN ML by analyzing band structure,electron transfer and spin polarized density of states.The results show that the slope of band changes,which implies a certain number of charges transfer takes place between GaN ML and TM atoms.In the case of Sc,Y,Ti,and Zr doping,the band gaps value change.The symmetric between the spin up electrons and spin down electrons in the energy band is observed,which implies zero magnetic moment.In the case of other TM atoms doping,the band gaps get small or even disappeare.In the spin up and spin down energy band,the asymmetry of electrons occupying degenerate and nondegenerate orbitals leads to nonzero magnetic moments.For Cr or Rh doped GaN ML,the system exhibits semi-metallic property,corresponding to the 100% spin polarization of the carrier,which is the best candidate for spintronic devices.7.By introducing 2 times TM atoms into the GaN ML,it is found that the magnetic moment of Fe,Co,Rh,and Pd doped GaN ML is two times that of its single atom doping.The magnetic moment of GaN ML doped with double Cr,Ni,Mo and Ru atoms is less than two times that of its single atom doping.The relative position between the spin up or spin down electrons and Fermi level changes after the increase of doping concentration due to the movement of the electronic state,resulting in the change of the properties of some doped systems.In the case of the co-doping of two different TM atoms,the total magnetic moment is not equal to the sum of magnetic moments due to the influence of hybridization from each other.