Accession Number:

ADA273504

Title:

Adsorption, Desorption, and Decomposition of HCl and HBr on Ge(100): Competitive Pairing and Near-First-Order Desorption Kinetics

Descriptive Note:

Technical rept.

Corporate Author:

RENSSELAER POLYTECHNIC INST TROY NY DEPT OF CHEMISTRY

Report Date:

1993-11-01

Pagination or Media Count:

38.0

Abstract:

We have investigated the surface chemistry of coadsorbed hydrogen and halogen atoms on Ge100, produced by dissociative chemisorption of HCL and HBr, by temperature-programmed desorption. The initial sticking probability, Ssub 0 , for HCL decreases from 0.6 at a substrate temperature of 270 K to 0.05 at 400 K, indicative of a precursor state to adsorption. For HBr Ssub 0 is constant at 0.7 over the same temperature range. A fraction f of adsorbed hydrogen atoms desorb associatively as H2 near 570 K, while the remaining 1-f H atoms recombine with adsorbed halogen atoms and desorb as the hydrogen halide HX near 580 - 590 K. The activation energies for desorption of H2, HCL, and HBr are all approximately 40 kcalmol. For both HCL and HBr f is 0.7 at low initial coverage and decreases slightly to 0.6 at saturation. The fraction f of adsorbed halogen atoms left on the surface following the competitive desorption of H2 and HX desorb as the dihalides GeC12 and GeBr2 near 675 K and 710 K, respectively. Desorption of H2, HCL, and HBr occurs with near-first-order kinetics, similar to the behavior of hydrogen adsorbed alone, which we attribute to preferential pairing induced by the pi bond on unoccupied Ge dimers. We introduce and solve a generalized doubly-occupied dimer model incorporating competitive pairing of HH, HX, and XX on Ge dimers to explain the near-first order kinetics. The model quantitatively accounts for both the desorption kinetics and the relative yields of H2 and HX with pairing energies of approx. kcalmol. Germanium, Desorption kinetics, Chemical vapor deposition, Atomic layer epitaxy.

Subject Categories:

  • Inorganic Chemistry
  • Physical Chemistry
  • Thermodynamics

Distribution Statement:

APPROVED FOR PUBLIC RELEASE