| Silicon Carbide(SiC)is a very promising wide band gap semiconductor material,which has excellent physical and chemical properties.Whether as structural materials or functional materials,the surface structures of SiC as well as the associate properties is extremely crucial to the multiple-applications in many fields.For the hydrogen-passivated 4H-SiC(0001)surface,particularly,the atomistic investigation is becoming very imperative due to its unique advantages in the fabrication of new generation electronic devices and the preparation of two dimensional nano-films.Based on the first-principles calculation,the influences of seven typical intrinsic point defects,including ISi,IC,VSi,VC,CSi,SiC,CSiSiC,on the lattice structure of hydrogen-passivated 4H-SiC(0001)surface have been systematically studied at the atomic scale.The adsorption strengths of Ag and Mo atoms,the two representative metals usually applied in the surface modification of ion-implantation,on these defective sites are compared with their effects to the electronic band structures of hydrogen-passivated 4H-SiC(0001)surface explored.The detailed researches are summarized as follows:Firstly,the structures and formation energies of the seven defects aformentioned on the hydrogen-passivation 4H-SiC(0001)surface are calculated.A correlation between the formation energy of the defect and the resultant lattice deformation has been therefore established.The general order of the formation energies of the defects on the hydrogen-passivated surface(HP-surface),clean surface(CL-surface)and bulk phase of 4H-SiC is concluded with their thermodynamic concentrations estimated.Moreover,the effects of the cell size and the chemical potentials to the formation energies of these defects are also discussed.The structures of the interstitial defects in the HP-surface were not stable.Except the carbon interstitials and carbon vacancies,the formation energies of the other defects showed general increase with the penetration from the HP-surface to the deeper layers.Meanwhile,except the antisite defects,the other defects,in general,had lower formation energies(Ef0s)than those of the bulk defects but higher ones than those in the CL-surface.Secondly,based on the calculation of the binding energies,the adsorption strengths of the intrinsic defects on the hydrogen-passivated(0001)surface to two metal atoms(Ag,Mo)are compared.The bonding situation and charge transfer between the metal atoms and the surface defects are careful characterized and analyzed by charge density differences(CDDs),which could explain the reason for the different adsorption strengths.Meanwhile,the microscopic physics under the surface metallization and wettability by the surface modification of ion-implantation are illustrated.Compared with the HP-surface with free-defect,the defects with dangling bonds created by vacancies or interstitials are able to increase the adsorption capacities of the HP-surface to the Ag(Mo)evidently with the highest increase ratio reaching ~ 4150%(238.1%).Thirdly,by calculating the electronic band structures and the total densities of states(DOS),we investigate the influences of the surface defects and their adsorbed metal atoms to the electronic properties of the hydrogen-passivated 4H-SiC(0001)surface.The simulated results show that hydrogen-passivated surface can eliminate additional energy levels of the clean surface,which can lead to the upshifting of conduction band minimum(CBM)and valence band maximum(VBM).The presence of surface defects as well as the absorbed metal atoms,however,can create additional energy levels in the forbidden band,most of which are deep energy levels.The metal Ag(or Mo)can increase the magnetic moment of the system and the more additional energy levels can be introduced by Mo adsorptions. |
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