AVS 46th International Symposium
    Surface Science Division Tuesday Sessions
       Session SS2+EM-TuA

Paper SS2+EM-TuA7
Dissociative Adsorption and Recombinative Desorption of H2 on Si(100)-2x1

Tuesday, October 26, 1999, 4:00 pm, Room 6C

Session: Semiconductor Surface Chemistry
Presenter: F.M. Zimmermann, Rutgers University
Authors: F.M. Zimmermann, Rutgers University
X. Pan, Rutgers University
Correspondent: Click to Email
The interaction of hydrogen with Si(100) is of considerable technological importance since hydrogen desorption is the rate-limiting step in low-temperature chemical vapor deposition of Si. Furthermore, this system is intriguing from a fundamental point of view, due to a multitude of unusual and remarkable experimental observations, such as desorption kinetics intermediate between first order and second order, and the so-called 'barrier puzzle': A large barrier to adsorption is seen in adsorption experiments, whereas in desorption dynamics experiments the desorbing molecules paradoxically show no signs of having traversed such a barrier. Using surface second harmonic generation, we have measured the adsorption and desorption kinetics as a function of surface temperature, H coverage, and exposure pressure. These measurements reveal more highly unusual kinetic behavior. Contrary to what is expected from Langmuirian site-blocking considerations, the sticking probability increases markedly with coverage, indicating a self-catalyzed adsorption mechanism. In addition, the sticking probability exhibits a pronounced dependence on exposure pressure, ruling out most 'simple' adsorption mechanisms. The observations are interpreted in terms of a self-catalyzed, barrier-less, inter-dimer adsorption mechanism (observed with the STM by Biedermann, Knoesel, Hu, and Heinz, submitted to Phys. Rev.). Our model quantitatively explains the measured adsorption and desorption kinetics (temperature, coverage, and pressure dependence), and provides a natural explanation of the vexing barrier puzzle. Fitted model parameters are in very good agreement with independently obtained experimental and theoretical values.