Variations of work function and surface conductivity on clean cleaved zinc oxide surfaces by annealing and by hydrogen adsorption

Abstract

Crystals are cleaved in UHV normal and parallel to the c axis exposing polar and prism faces. Work function (Kelvin) and surface conductivity are measured, also after annealing at elevated temperatures in UHV and after adsorption of atomic hydrogen. The work function of all three faces decreases irreversibly by annealing in UHV, but only on the Zn face a measurable surface conductivity appears. Also hydrogen adsorption diminuishes the work function. Simultaneously an increasing surface conductivity can be observed on all three types of surfaces. Corresponding structures in the annealing curves demonstrate a correlation between work function and surface conductivity. By means of space charge calculations band bending values are derived and there-by the electron affinity, which is not accessible to direct measurement. It is concluded that the two clean cleaved polar faces exhibit a depletion layer, which remains on the O face also after annealing in UHV. On the prism face nearly flat band situation is found. The decrease of work function with increasing hydrogen coverage, i.e. surface electron density, can be interpreted by means of space charge calculations using the measured surface conductivity. For the two polar faces the band bending alone causes the decrease, whereas for the prism face an additional increase of electron affinity has to be assumed. The variation of electron affinity by annealing or by hydrogen adsorption might be due to a change of atomic distances in the uppermost layer, although no superstructure has been found in LEED studies.