Hydrogen interactions with Si(111) and Si(100) surfaces studied by ultraviolet photoemission spectroscopy

H interactions with thermally cleaned Si(111) and Si(100) surfaces at 300-650 K have been studied by ultraviolet photoemission spectroscopy. Experimental results are consistent with the existence of weakly adsorbed H atoms at 300 K, which revealed surface resonance states around 7 eV below Fermi level. The weakly adsorbed H atoms desorb around 500 K from the Si(100) surface to form the monohydride phase, while they are irreversibly transformed into the trihydride phase by annealing at 650 K on the Si(111) surface. This indicates the different natures of Si-H chemical reactions on the two surfaces.