Formation of the Cs/GaAs(001) interface: Work function, cesium sticking coefficient, and surface optical anisotropy

Abstract

We have studied at room temperature (RT) and at near liquid-nitrogen temperature [low temperature (LT)], the adsorption of cesium on clean gallium-rich GaAs(001) prepared by sulfide passivation, and annealing in ultrahigh vacuum. We monitor the optical transitions of gallium surface dimers, using reflectance anisotropy spectroscopy (RAS), and the changes of work function and cesium sticking coefficient. The clear correlations between these three quantities allow us to distinguish three adsorption phases: (i) At LT and up to 0.7 ML coverage, cesium adsorption induces a decrease of the RA signal, which perfectly corresponds with the work function decrease. The RA decrease is, therefore, caused by the surface dipole due to charge transfer from cesium to the solid. (ii) At RT, for a coverage smaller than 0.15 ML, both the RA signal and the work function change are different from their LT behavior, so that a distinct adsorption process occurs. This indicates that cesium diffuses to specific surface sites at which the bonding is different. (iii) At a coverage larger than 0.7 ML, and independently on adsorption temperature, no charge transfer to the solid occurs, since no change of RA spectrum and work function is observed. The cesium sticking coefficient is reduced by a factor of 2. Comparison with results obtained on As-decapped GaAs(001) shows the similarity of the formation of the interface for the two surfaces. However, the results are clearer for the surface prepared by sulfide passivation, which seems to be of higher quality.

Electron work function decrease in SIMS analysis induced by neutral cesium deposition

“…For most samples, no significant differences between the Ga + /Cs • and the Cs + /Cs • bombardment can be seen for the work function variations with Table 2 Maximal electron work function decrease in eV for Si, GaAs, InP, Al and Ni samples [11,12,30,36,40,41] respect to the Cs surface concentration. A fast decrease of Φ at low C Cs is followed by minimal values of Φ and for some samples with a small increase of Φ at the highest C Cs .…”

“…This increase of the analysis sensitivity can cover several orders of magnitude [9]. The deposition or incorporation of alkali metals (and in particular cesium) has been shown to decrease the electron work function of the sample [10][11][12][13][14] which induces an increase of the negative secondary ion sensitivity [15,16]. Hence, the negative secondary ion yields depend strongly on the stationary Cs surface concentration [9,15,[17][18][19].…”

A TRIDYN study: Comparison of experimental SIMS useful yields with simulated Cs concentration evolution

“…Whereas Cs adsorbs easily on other surfaces, the sticking coefficient of Cs on itself is low limiting Cs adsorption on most substrates to one monolayer [12,39,40]. This implies that for high Cs deposition rates during the experiments only a part of the incident Cs • atoms sticks on the surface and contributes to the sensitivity increase in SIMS analysis, while in the simulations all incident Cs • atoms stick on the surface producing a Cs surface concentration larger than the experimental concentration.…”

“…The concentration averaged over a depth of 1.5Å is three times larger than the concentration averaged over 49.5Å. As the Cs atoms deposited on the sample surface contribute most to the decrease of the sample work function [10][11][12][13][14] and the Cs surface concentration is highest for an averaging depth equal to 1.5Å, the concentrations obtained at 1.5Å are used in order to compare the experimental results to the electron-tunneling model [27].…”

“…This increase of the analysis sensitivity can cover several orders of magnitude [9]. It has been shown that the deposition or incorporation of alkali metals (in particular cesium) decreases the electron work function of the sample [10][11][12][13][14] which induces an increase of the negative secondary ion sensitivity [15,16]. So the negative secondary ion yields strongly depend on the stationary Cs surface concentration [9,15,[17][18][19].…”