Carbide Contamination of Silicon Surfaces

Abstract

The techniques of grazing angle reflection high‐energy electron diffraction (RHEED), mass spectroscopy, and electron microscopy have been used to study carbide contamination of silicon samples heated in ultra‐high vacuum. The RHEED data showed that the surfaces of silicon crystals heated between 800°–1000°C were covered with β‐SiC crystallites well oriented with respect to the substrate. Heating between 1000°–1100°C formed a nonoriented β‐SiC phase, and further heating above 1100°C removed the surface carbide. CO and CO₂ evolution was observed when samples were heated above 800°C. The carbide reappears with heating to 800°C if the carbide free surface is exposed to atmosphere or left in the vacuum environment for a sufficient time. Electron micrographs of platinum‐shadowed carbon replicas from samples contaminated with β‐SiC indicated that the carbide existed within protuberances at the surface. Data from samples heated to 950° and 1050°C showed particles ∼400 Å in size, of cylindical shape and tapering towards the top; the surface density of the protuberances was observed to vary from 2×10⁷ to 6×10⁹/cm². In all cases of carbide formation, the carbide results from the chemical reaction of a carbon‐containing adsorbate with the silicon surface. Although a gaseous origin for an adsorbate has been demonstrated, additional adsorption probably also results from chemical cleaning of the sample. Surface steps on the silicon are correlated with the presence of β‐SiC protuberances (or other artifacts) and appear to have been pinned by these particles.