High dose implantation in 6H-SiC


High dose implantation in 6H-SiC

Heera, V.; Skorupa, W.; Stoemenos, J.; Pécz, B.

There are two major reasons to study the effects of high dose implantation in SiC. First, the production of low-resistivity, p-type layers requires high acceptor concentrations which are far above the equilibrium solubility. It is essential to know the critical acceptor concentration for thermally stable, homogeneously doped SiC. On the other hand, there is a lack of information about the precipitation of new phases in SiC, their properties and influence on the SiC matrix. Such information is needed for the assessment whether insulating or conductive layers with device quality can be formed in SiC by ion beam synthesis. Two examples of high dose implantation in 6H-SiC are presented.

In the first example high doses of 350 keV Al+ ions were implanted in 6H-SiC single crystals at temperatures between 400oC and 800oC in order to preserve the crystallinity of the SiC matrix. The phase formation was studied by TEM, SIMS and AES. A critical Al concentration of about 10 at% was found below that the 6H-SiC structure remains stable during implantation at 500°C. The Al atoms occupy preferentially Si sites in the SiC lattice. At higher Al concentrations the SiC matrix is decomposed and precipitates of Si and Al4C3 are formed. It was found that the Al4C3 precipitates have a perfect epitaxial orientation to the SiC matrix. The phase transformation is accompanied by atomic redistribution and strong volume swelling. The observed atomic depth profiles can be accounted for by a a simple chemical reaction model. Implantation at 800°C leads to the formation of Si and Al precipitates accompanied by a dramatic volume swelling which results in surface cracking and material outflow. Post-implantation annealing above 1500°C severely deteriorate the SiC matrix by local melting of Al and Si precipitates.

In the second example the evolution of the binary system C -> SiC was studied as function of implantation temperature. 1 1018 cm-2, 60 keV carbon ions were implanted into single crystalline 6H silicon carbide (SiC) at elevated temperatures. The formation of carbon phases in the crystalline SiC lattice was investigated by cross sectional transmission electron microscopy. An amorphous, carbon rich phase was produced at 300oC. Precipitates of graphite were obtained at 600oC, whereas at 900oC small diamond grains were produced. These grains are in perfect epitaxial relation with the surrounding SiC lattice. The mechanism of the diamond formation by IBS in SiC is not quite clear. It can be assumed that the tetrahedrally coordinated SiC lattice, which is preserved during the high temperature implantation, acts as a template for the growth of diamond. In addition, local nonequilibrium conditions in the ion cascades (thermal spikes, shock waves) or temporary stress on interstitially incorporated carbon atoms by the surrounding SiC lattice could contribute to the diamond nucleation.

Keywords: high dose implantation; silicon carbide; 6H-SiC; ion beam synthesis

  • Lecture (Conference)
    ECSCRM2000, Kloster Banz, Germany, Sep. 3-7, 2000 Mater.Sci. Forum Vols. 353-356 (2001) 579-582

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