SCANNING TUNNELING MICROSCOPY STUDY OF THE ADSORPTION AND RECOMBINATIVE DESORPTION OF HYDROGEN FROM THE SI(100)-2X1 SURFACE

A scanning tunneling microscopy and spectroscopy study of the adsorption and recombinative desorption of hydrogen from the Si(100)-2 X 1 surface is presented. Initially at room temperature, hydrogen atoms singly occupy the Si dimer units resulting in the formation of isolated Si-H bonds and unpaired dangling bonds. At higher temperatures (approximately 630 K) these hydrogen atoms tend to pair up on the dimer units. Thus there is a thermodynamic preference for the hydrogen atoms to occupy the dimers in pairs. This pairing phenomenon is due to a pi interaction between the dangling bonds on the Si(100)-2 X 1 surface which favors pairing the unpaired dangling bonds on the dimer units. This pairing of dangling bonds in turn forces the hydrogen atoms to pair. Following desorption from the saturated surface, pairs of dangling bonds are found localized on the Si dimer units, suggesting that desorption involves the direct recombination of hydrogen atoms from the dimers units of the monohydride surface. The implications of this desorption mechanism are discussed and compared to earlier models.