Water adsorption on a GaP(110) surface: a UHV study

The interaction of molecular water with the GaP(110) surface has been studied under ultrahigh vacuum (UHV) conditions with a combination of X-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and temperature programmed desorption (TPD) methods. An adlayer of water at 1 ML coverage (referenced to the number of undercoordinated Ga surface atoms) desorbs from the GaP(110) surface over the temperature range of 250 - 370 K in TPD experiments. This water monolayer exhibited a c(2 × 2) surface structure, observed with both LEED and STM, consisting of alternating OH and H2O surface-bound species, as determined by XPS data. However, at water coverages below 1 ML the surface concentrations of OH species were higher than that of molecular H2O, and in the 0 - 0.75 ML surface coverage range the (OH : H2O) ratio was constant at 2 : 1. We also observed another stable surface adlayer at 4/3 ML water coverage that exhibited quasi-ordering with a c(3 × 2) LEED pattern. TPD spectra showed that this extra 1/3 ML desorbed near 210 K before the onset of the desorption of the 1-ML coverage.