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.