Elastic scattering of low-energy electrons from oxygen-covered Si surfaces

We present results for the elastic scattering of low-energy (4-eV) electrons from Si(100)(2x1) and Si(111)(7x7) surfaces during O2 adsorption experiments. We observe striking variations in the elastic peak height as a function of surface condition. The peak height for the clean surfaces, which contain metallic surface states, or for surfaces covered with physisorbed O2 (at 20 K), is more than an order of magnitude larger than that of surfaces that have reacted with oxygen to form Si-O-Si or are slightly contaminated with carbon. These observations are qualitatively different from previously reported results for metals