First-principles study of defect properties in ultrathin MOS-based devices and diluted magnetic semiconductors

Abstract

Based on first-principles calculations, we study some critical issues of (1) Si-based microelectronics, (2) diluted magnetic semiconductors(DMSs), and (3) graphene-based nanoelectronics. First, we focus on the reliability issues of the gate oxides such as ultra-thin SiO$_{\\\\\\\\rm 2}$ and HfO$_{\\\\\\\\rm 2}$ in Si metal-oxide-semiconductor (MOS) devices. In Mn-doped GaN, which is one of the most studied DMS materials, we investigate the magnetic interactions between Mn ions and the influence of intrinsic defects on the electronic and magnetic properties. Finally, the effect of atomic-range disorders on the electronic structure of graphene is discussed. We study the electron tunneling through ultra-thin SiO$_{\\\\\\\\rm 2}$ layers with different oxide thicknesses and crystal phases by using first-principles nonequilibrium Green\\\\`s function calculations. The Si-induced gap states, which result from an exponential decay of the wave functions of Si electrodes into the oxide region, are responsible for the tunneling currents. In the presence of oxygen vacancies across the oxide region, leakage currents are greatly enhanced due to O-vacancy-related defect states even at low gate voltages. We investigate the hydrogenation effect on the defect properties of oxygen vacancies (V$_{\\\\\\\\rm O}$) in HfO$_{\\\\\\\\rm 2}$, which is a good candidate for alterative dielectrics to SiO$_{\\\\\\\\rm 2}$ in MOS devices. A defect complex of V$_{\\\\\\\\rm O}$ and H behaves as a shallow donor for a wide range of Fermi levels, with a positive charge state, and this complex is energetically stable against its dissociation into V$_{\\\\\\\\rm O}$ and H. We suggest that the V$_{\\\\\\\\rm O}$--H complex is responsible for the formation of positive fixed charges, which neutralize negative fixed charges during the post-annealing process of SiO$_{\\\\\\\\rm x}$/HfO$_{\\\\\\\\rm 2}$ stack. The magnetic properties of Mn-doped GaN are investigated through first-principles pseudop...