Practical application of the method of Kona-Shema. Pseudopotentials method

INTRODUCTION

Apart from the application aspect, much interest in the structure and stability of the polar (001) surface has also been caused by the rich variety of surface reconstructions which can be observed depending on the surface preparation. The GaAs(001) surface shows a с(4×4) structure for As-rich conditions but changes its periodicity to (2×4)/с(2×8) and finally (4×2)/с(8×2) as the surface gets more cation rich.

Vienna Ab-initio Simulation
Package (VASP)
http://cms.mpi.univie.ac.at

G. Kresse, J. Hafner et al.

full-potential method

pseudopotential approach

PSEUDOPOTENTIAL APPROACH (VASP)

The values of lattice parameters for different approximation of exchange-correlation effects

E(k) of bulk GaAs

Total and local DOS of GaAs

β2

β2

ζ-

a)

b)

c)

d)

Ga-terminated

As-terminated

S4

S2

S3

S1

* S.M. Lee, S.H. Lee, M. Scheffler. Phys.Rev., B69, 125317 (2004).

Локальные ПЭС для Cs на ζ-GaAs(001)-(4×2)

More preferable site

Adsorption of cesium is due to the energy gain for different adsorbates sites and their number on the structural unit cell

Chlorine adsorption- local DOS for
Cl - ζ-GaAs(001)-(4×2)

Cl on Ga-terminated
ζ-GaAs(001)-(4×2)

Ga-
termination

As
termination

The distribution of valence charge difference along binding with Cs nearest atoms: As (left) and Ga (right) for two interface.

The valence charge difference shows the charge redistribution in the surface layers with dipole formation

Electrons accumulation region along Cs-Ga bonding in Т’2 site expresses more pronounced in comparison with T3-site.

Cs adsorption on ζ-GaAs(001)-(4×2) (coverage increase)

S2-S5

S1-S5

S1-S4

S2-S4

The increase of Cs coverage up to 0.25 ML on ζ-surface leads to increase of charge transfer from Cs to substrate. Apart the factors mentioned for one atoms adsorption the role of lateral Cs-Cs interaction can influence adsorption energies.

The energy of ionization (I) for β2-GaAs(001) is 4.86 eV for Ga-terminated surface and 5.38 eV (5.43, 5.61 –other calculation, 5.5 eV –experiment) for As-terminated surface.
I=~4.9 eV for ζ-GaAs(001)-(4×2)

The values of work function (eV) for S4-S1, S4-S2, S5-S1 and S5-S2 geometries in comparison with Cs adsorption in S4 and S5 sites and their changes in respect to clean surface

Sites dependent changes in the work function is shown in our calculations

S5

S5-S2

S4-S1

The decrease of work function at higher Cs concentration is due to filling of the conduction band edge at the addition of the second atom

Total, local surface DOS of ζ-GaAs(001)-(4×2) with adsorbed oxygen and cesium

ζ-GaAs(001)-(4×2)

O –S1

O –S1
Cs -S5

Cs -S5

The change of E(k) upon adsorption

S1-S5 geometry does not influence significantly the work function. The adsorption of Cs and O in dimer bridge S1 position decreases the value of the work function by ~0.3 eV. So the formation of dipole oriented perpendicular to surface at thin coverage leads to better quantum effect.

U =-1.5 V (filled) U =1.8 V (empty)

Experiment by Vedeev A.A. and Eltsov K.H. JETP Letters, 2005. – V.82. – P.46 –51.

calculation

S.H. Lee et al. Phys. Rev. Lett. 85, 3890 (2000)