TDS and LEED studies of H2O adsorption on GaAs(110)

The UHV cleaved (110) face has been exposed to water in the range from 10 L to 2 × 10⁴ L. The main TDS peak in H₂O desorption appears at 350 K, independent of coverage. The low desorption energy of 0.7 eV (16 kcal/mol) is reasonable for oxygen atoms bound via the lone pair orbital to As as was earlier derived from UPS measurements. A broad spur between 450 and 600 K may be related to O-Ga bonds. The sticking probability shows values below 10^-4; only near 4.8 × 10³ L (6 × 10¹⁵ cm^-2 s^-1 H₂O molecules for 300 s) corresponding to a coverage of about 0.4 monolayes a steep maximum appears. At about one monolayer saturation is observed. Exposures to more than 10⁴ L of water quench the intensity of the (10) LEED spot considerably stronger than the intensity of the (11) spot. A comparison of the I(E) curves with existing model calculations suggests that the observed behaviour of the LEED spots is caused by a change in surface structure towards the unrelaxed configuration. The higher sticking coefficient observed near 0.4 monolayers may be connected with this rearrangement of surface atoms.