Hydrogen diffusion on silicon surfaces

Abstract

Diffusion of atomic hydrogen on silicon serves as a model system for the investigation of thermally activated diffusion processes of covalently bound adsorbates on semiconductor surfaces. Over the past two decades, a detailed understanding of the hopping mechanisms for H/Si(0 0 1) and H/Si(1 1 1) has been obtained using a variety of experimental and theoretical methods. Hydrogen diffusion on silicon is in general characterized by energy barriers that are substantially larger than for adsorbate diffusion on metal surfaces, by the occurrence of different pathways on one surface, as well as by a strong participation of the underlying lattice in the hopping process.

In the case of the flat Si(0 0 1) surface, three diffusion pathways were identified: site exchange within one Si dimer, hopping along dimer rows, and hopping across dimer rows, with barriers of 1.4, 1.7 and 2.4 eV, respectively. These barriers correlate with the distances of the involved adsorption sites of 2.4, 3.8 and 5.2 Å. While hydrogen diffusion on Si(0 0 1) is strongly anisotropic at surface temperatures below 700 K, the measurement of high hopping rates by means of a combination of pulsed laser heating and scanning tunneling microscopy reveals similar jump frequencies around 10⁸ s⁻¹ at 1400 K. Diffusion across steps is found to occur with similar speed as diffusion along dimer rows.

Hydrogen diffusion on Si(1 1 1) 7 × 7 involves 4.4-Å-long jumps between restatom and adatom sites, accompanied by strong distortions of the adatom backbonds. Crossing the unit-cell boundaries via a 6.7-Å-long migration pathway between two adatoms is the rate limiting process for diffusion on macroscopic length scales, which has an activation energy of 1.5 eV.