Ab initio studies of Cs on GaAs (100) and (110) surfaces

GaAs with an atomic monolayer of Cs is one of the best known photoemissive materials. The results of density functional theory calculations of Cs adsorption on the GaAs(100)-$\left(4\times 2\right)$ gallium-terminated reconstructed surface and the GaAs(110) surface are presented in this work. Coverage of up to 4 Cs ${\mathrm{atoms}/\mathrm{nm}}^2$ on GaAs surfaces has been studied to predict the work-function reduction and adsorption energies accurately. The high mobility of Cs atoms on the (110) surface allows formation of ordered structures, whereas the low mobility of Cs of the (100) surface causes amorphous growth.

Figure 1

(a) Top and (b) side view of the relaxed Ga-terminated GaAs(100)-$\left(4\times 2\right)$ surface slab. (c) Top and (d) side view of the relaxed GaAs(110) surface. Both cells have a vacuum spacing of 25 Å, which sufficiently reduces interactions between the slabs. The Ga atoms are shown in red and the As atoms are shown in blue. For the (100) surface the $x,y$, and $z$ axes indicate the [011], $\mathrm{[}0\overline{1}1$], and [100] directions, respectively. For the (110) surface the $x,y$, and $z$ axes indicate the $\mathrm{[}00\overline{1}$], $\mathrm{[}\overline{1}10$], and [110] directions, respectively.

Figure 2

The energy barrier for the diffusion of Cs, (a) along the trench ([011] direction) on GaAs(100), (b) across the dimer $(\mathrm{[}01\overline{1}$] direction) on GaAs(100), (c) between two adjacent $X$ sites along the $\mathrm{[}00\overline{1}$] direction on GaAs(110), and (d) between two adjacent $X$ sites along the perpendicular $\mathrm{[}1\overline{1}0$] direction on GaAs(110). Ga atoms are shown in red and As atoms in blue. The initial position of the diffusing Cs atom is shown as a yellow sphere, and subsequent images along the minimum-energy path of the Cs atom are shown as circles, with the darkest circle denoting the final position.

Figure 3

Top view of the Cs position distributions on the (100) surface for Cs surface densities of (a) 0.78, (b) 2.35, and (c) 3.91 $\mathrm{atoms}/{\mathrm{nm}}^2$, respectively. Top view of Cs position distributions on the (110) surface for Cs densities of (d) 0.74, (b) 2.21, and 3.69 $\mathrm{atoms}/{\mathrm{nm}}^2$, respectively. The Cs position distributions are overlaid on the surface atoms of the GaAs slabs. The blue circles represent the As atoms and the red circles the Ga atoms. The intensity of the position distribution is indicated by the color bar. The position distributions on the (100) and (110) surfaces are normalized to the maximum of the distribution on that surface over all the Cs densities.

Figure 4

(a) Work-function reduction $\left(\Delta \langle W\rangle \right)$ as a function of Cs surface density for the GaAs(100) and the GaAs(110) surfaces. The experimental values [13, 15] agree well with those calculated using DFT. Chemical potential of adsorbed Cs as a function of Cs surface density for (b) the GaAs(100) and (c) the GaAs(110) surfaces. The experimental values [16, 17] are also presented.