Work Function Of Metal Gate, And Magnetic Properties And Spin Modulation Of Fe-based Materials: First-principles Study

Since the sixties from the 20th century, integrated circuit, micro electronic technique has been developed rapidly with Moore's law. The size of MOSFET devices are shrinked continuously in certain proportion.The continuous downscaling of the traditional SiO2 gate is now near its limit.So it is necessary to use high-k dielectrics to substitute the traditional SiO2 gate dielectric. The polysilicon gate must be replaced by metal gate because the polysilicon gate encounters many problems when integrated with high-k dielectric, such as depletion effect, high gate resistance,feimi level pinning, and boron penetration.So metal gate have attracted a lot of research interests.In this dissertation, we focus on the effects of surface adsorption, spin polarization,alloying on the work function of metal gates using first-principles methods based on density-functional theory. Results of our research reveal that adsorption and spin-polarization have a significant effect on the work function of metal gates; For the same coverage of adatom, the values of work functions obtained for different coverage structures are basely same, which indicates that the distribution of adatom on the surface layer has little effect on the work function.The magnetic moment and workfunctions of Fe(100) has also been investigated. Each layer of magnetic moment is symmetric. There is much difference between paramagnetic work functions and ferromagnetic work functions,which show spin polarization have a significant effect on the work function.The work functions of FeCr alloys has also been investigated.It also be found that alloying and spin polarization have a significant effect on the work function. This dissertation aims at providing some unavailable results from experiment by means of a first principles calculation. The main contents are as follows:First of all, the research background were introduced, including scaling issues of traditional SiO2 gate dielectric and the difficulty of replacement of polysilcon gate by metal gate, magnetic materials. the details to the theoretical basis of first-principles calculation and the software of VASP involved in this thsis were also presented.Secondly, effects of oxygen adsorption on work functions of Mo(110) surface and substrate are investigated using first-principles methods. Results of our calculations reveal that oxygen adsorption on Mo(110) surface has a significant effect on the work function of Mo(110). the effects of the second as well as farther oxygen adsorption are negligible. For surface adsorption, the work functions increase with increasing coverage content. For the same coverage of oxygen atoms, the values of work functions obtained for different coverage structures are basely same, which indicates that the distribution of oxygen atoms on the surface layer has little effect on the work function.All results suggests surface adsorption as a promising way of tuning the work function of metal gate.Thirdly, magnetism and work function on Fe(100) are investigated using first-principles methods. Calculated results reveal that each layer of magnetic moment is symmetric.It is obvious that the magnetic of surface is enhanced.Orbital magnetic moment of d were found to have most important contribution to the average magnetic moment of Fe(100). There is much difference between paramagnetic work functions and ferromagnetic work functions,which show spin polarization have a significant effect on the work function. Results suggests spin polarization as a promising way of tuning the work function of metal gate.Finally, the magnetic and work functions of Cr doped on Fe(100),(110),(111) surface are investigated using first-principles methods. Calculated results reveal that surface doping and different surface doping density has a significant effect on the work functions.Different surface orientation and spin polarization also be found to have a significant effect on the work functions.