Growth of high quality electronic grade silicon carbide using methane as carbon source
In short: Methane is hardly ever used for growing silicon carbide, but it is interesting to study to shed light on the differences in gas phase reactions using different hydrocarbons and the differences in surface reactions of the specie that reach the surface. We observe that methane plays a considerable role for the growth of silicon carbide and it has a much higher probability to stick to the surface than previously believed. We also show that high quality layers may be grown using methane if the carbon to silicon ratio is carefully tuned.
It is generally considered that the most simple hydrocarbon molecule, methane (CH4), is not a suitable carbon source for growth of silicon carbide (SiC) epitaxial layers by Chemical Vapor Deposition (CVD). This since methane based processes renders layers with rough surface and high density of surface defects. Such layers are naturally useless for electronic devices. The inferior surface roughness obtained in from methane is explained by a lower thermal decomposition of methane as compared to larger hydrocarbons with carbon-carbon bonds e. g. propane (C3H8), in combination with a lower surface reactivity. The lower reactivity for methane can be attributed to the highly symmetrical tetrahedral arrangement of the molecule.
Methane is the only hydrocarbon that reasonably easy can be obtained in an isotopically enriched form with the 12C isotope. When using enriched precursors in the SiC CVD process, isotopically enriched 28Si12C can be grown which augments the material properties; for instance, the thermal conductivity is expected to increase markedly compared to natural SiC. Since methane is the only enriched hydrocarbon available at present, the ability to use methane in SiC CVD becomes vital in order to realize isotopically enriched material.
We now demonstrate that SiC layers with high-quality surfaces in fact can be grown using methane. A key factor in obtaining high-quality is tuning the carbon to silicon ratio of the process gas mixture to a region where the growth is limited neither by carbon nor by silicon supplies. The required growth chemistry is shown to be confined to a narrower range of parameters for CVD using methane compared to CVD using conventional ethylene. This is suggested to be due to differences in growth chemistry caused by the different gas phase chemistries of methane compared to hydrocarbons with carbon–carbon bonds. Based on the observed differences in growth behavior, we suggest that the methane molecule plays an active role in the growth and has a substantial sticking probability which is contrary to what is commonly believed today.
These findings are presented in Journal of Crystal Growth 390, 24 (2014)
- Knut and Alice Wallenberg Foundation (KAW).
- Swedish Foundation for Strategic Research (SSF).
- Swedish Research Council (VR).
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Last updated: 02/11/14