Kinetics of H2 (D2) desorption from a Ge(100)-2x1:H (D) surface studied using scanning tunneling microscopy and temperature programmed desorption

The kinetics of H-2 (D-2) desorption from a Ge(100)-2x1:H (D) surface was studied using scanning tunneling microscopy (STM) and temperature programmed desorption (TPD). Inspection of STM images of surfaces at the saturation coverage of H (D) (theta(H(D))similar or equal to1.0 ML) revealed a 2x1 monohydride (monodeuteride) phase in which most H (D) atoms were paired on Ge-dimers. By counting the sites of H-2 (D-2) desorption in STM images taken after desorption of H-2 (D-2) at temperatures in the range T-s=500-550 K, the desorption of H-2 (D-2) was found to follow first order kinetics with an activation energy of E-d=1.65+/-0.1 eV (1.65+/-0.1 eV) and a pre-exponential factor of nu(d)=(2.7+/-0.5)x10(13) s(-1) [(1.2+/-0.5)x10(13) s(-1)]. These values of E-d and nu(d) were used to simulate TPD spectra for the desorption of H-2 (D-2) from a Ge(100)-2x1:H (D) surface. The simulated spectra were in good agreement with the experimental TPD spectra. In contrast to the surfaces with saturated H coverage, which are characterized by pairs of H atoms on Ge-dimers, at the low H coverage of theta(H)similar or equal to0.05 ML unpaired H atoms as well as paired H atoms were observed on the Ge-dimers on the surface, causing the desorption process to follow second order kinetics. At T(s)similar to300 K, the singly occupied dimers (SODs) appear to be favored over doubly occupied dimers (DODs). However, upon increasing the temperature (T-s) from 300 to 500 K, most SODs were rapidly converted into the thermodynamically favored DODs by the migration of H atoms. On the other hand, it is observed that even above T(s)similar to500 K, the onset temperature for H-2 desorption from DODs, a non-negligible number of SODs remain on the surface due to the large entropic barrier to pairing. These results suggest that H adsorption in the low coverage is strongly influenced by the energetics of the pairing of H atoms. (C) 2003 American Institute of Physics.