Interaction of oxygen with Si(111)7 × 7 and Si(100)2 × 1 surfaces modified with small amounts of Cs

Abstract

The effect of small amounts of Cs (∼ 0.2 of the saturation Cs coverage at 300 K) on the oxidation of Si(111)7 × 7 and Si(100)1 × 2 at reaction temperatures of 300–700 K has been studied by means of Auger and electron energy loss (EEL) spectroscopies, thermal desorption and work function (WF) measurements. It has been found that at these low Cs coverages only the first stage of the oxidation process — the initial oxygen uptake — is affected. The result is: (i) an increase and temperature independence of the initial sticking coefficient, S 0; (ii) coverage independence of S upto certain (reaction temperature determined) oxygen coverages. EEL and WF data have shown that at 300 K oxygen occupies preferentially Cs promoted sites. The liberation of oxygen from this Cs influenced state in order to form usual Si-O bonding configurations takes place at temperatures exceeding 400 K as evidenced by the rise of sharp SiO related ∼ 5 and 7 eV loss peaks and reduction of the work function. Thermal desorption data showed that the oxidation of Si leads to an appearance of a higher temperature desorption state of Cs. Because of this oxygen induced stabilization of the Cs layer the removal of Cs requires annealing the mixed layer at 950 K. At oxygen coverages less than 1 ML this annealing procedure led also to complete SiO desorption.