표면위에 자기조립적으로 형성된 저차원나노구조물에서의 전자구조에 관한 연구

Abstract

Low-dimensional nano structures fabricated on surfaces have been widely investigated for both fundamental and technological aspects. In these substrate sup- ported low-dimensional nanostructure, the electronic and structural properties are highly related with the parent substrate and shows exotic interplay between adsorbate- adsorbate and adsorbate-surface interaction, which result in structural phase transition, metal-insulator transition, Peierls instability, or Rashba spin splitting. In this project, I study the electronic and structural properties of the self-assembled quasi- one-dimensional (1D) metal atomic wire on semiconductor surfaces with the two simplest cases, that is, indium nanowires on (111) surface of silicon [In/Si(111)] and bismuth atomic chain on (110) surface of galium arsenide [Bi/GaAs(110)]. First, I investigate the In/Si(111) nanowire array as the prototype 1D atomic chain on semiconductor surfaces. The controversial issue of (4×1)/(8×2) structural phase transition accompanying metal-insulator transition, whether it is orig- inated from Peierls instability or simple energy lowering due to a periodic lattice distortion, has been addressed. In the present study, using the van der Waals (vdW) energy corrected hybrid density-functional theory (DFT) calculation, I predicts that the low-temperature 8×2 hexagonal structure is energetically favored over the room- temperature 4×1 structure. I show that the correction of self-interaction error and inclusion of vdW interactions play crucial roles in describing the covalent bonding, band-gap opening, and energetics of hexagon structures. The results manifest that the formation of hexagons in the low temperature 8×2 phase is driven by a energy gain from lattice distortion enhanced by indium-indium bonding and vdW interaction between indium chains. Next, I study the effect of external perturbation, such as in-plane strain and electric field, in the tunability of the structural stability of In/Si(111) nanowire. I demonstrate that the incorporation of compressive strain or electron doping into In/Si(111) nanowire stabilizes electronically phase-separated ground state where the 8×2 in- sulating and 4×1 phases coexist. The mechanism of external perturbation induced phase transition is discussed either quantitatively or qualitatively. It is also found that tuning the external purturbation and the mechanical stress along the chain, which is controlled via the electric field and the lattice contraction or expansion, respectively, induces reversible electronic and structural phase transition. The present results not only extend the realm of electronic phase separation from strongly correlated d-electron materials to weakly interacting sp-electron systems, but also have important implications in understanding the underlying driving forces involved in various phase transitions of the reduced dimensionality. Finally, I move on to 1D atomic Rashba system, Bi/GaAs(110), where the Bi zigzag chains are formed on a heterogeneous GaAs(110) surfaces. I predict a giant Rashba-type spin splitting of surface states. This giant spin splitting is attributed to the interplay between the asymmetric surface charge distribution due to the strong spin-orbit coupling effect that result in hybridization of px, py, and pz orbitals of the Bi and the out-of-plane and in-plane electric field generated by two geometrically and electronically inequivalent Bi atoms bonding to Ga and As atoms. Especially, the spin texture exhibits anisotropic 1D feature including the presence of in-plane and out-of- plane spin components and a combined effect of Rashba and Dresselhaus spin-orbit interactions. I thus argue that the structural and bulk inversion asymmetries of the Bi/GaAs(110) surface system cause a giant spin splitting with complex spin texture.