Mev Ion Beam Synthesis of Well-Defined Buried 3C-Sic Layers in Silicon
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EXPERIMENTAL Two kinds of implantations have been performed: 0.8 MeV C4 implantations into Si(l 11) at a constant target temperature of 500 K, and 1.1 MeV C4 implantations into Si(l00) at 620 K. Doses of up to 1.2 x 10"8 C/cm2 were achieved in areas of 1 cm 2 using maximum current densities of 877 Mat. Res. Soc. Symp. Proc. Vol. 396 01996 Materials Research Society
4 pA/cm2 . Some of the samples were annealed for several hours in the flowing Ar ambient of a quartz tube furnace at 1250 'C without using any surface capping layers. This temperature was chosen since it has been shown earlier [3] in keV-IBS experiments that this temperature is necessary to induce carbon redistribution into a well-defined buried SiC layer with sharp interfaces to the Si top layer and the Si bulk. Samples were analysed by means of Guinier X-ray diffraction (XRD), Rutherford backscattering spectroscopy (RBS) and channeling with 3.5 and 4 MeV He ions, as well as cross-sectional transmission electron microscopy (XTEM) and electron diffraction (TED). RESULTS According to TRIM [9] simulations, the doses required to obtain the stoichiometry of SiC at the peak of the ion distribution are 1.0 and 1.1 x 1018 C/cm 2 for 0.8 and 1.1 MeV implantations, respectively, where the concentration maxima are located 1.36 and 1.63 pm beneath the surface. 0.8 MeV C+ => Si( 11) at 500 K. as-implanted state Carbon implantations at 500 K lead to the formation of a buried amorphous layer above a threshold dose between 1.8 and 3.6 x l0ol C/cm 2, as can be deduced from RBS/channeling measurements and depth profiles of the optical reflectivity change [10]. Up to the highest dose considered (1.2 x 1018 C/cm 2 ) the amorphous layer thickness increases, but a crystalline Si top layer of approximately 0.9 pm thickness remains. Apart from a narrow, highly dislocated near-interface zone, this Si top layer appears to be free of damage in XTEM investigations (Fig. 1). RBS/channeling studies reveal that the carbon concentration maximum is located within the amorphous layer. No indications of the formation of crystalline SiC precipitates have been obtained either in XRD studies or in XTEM and TED investigations of high-dose samples in the as-implanted state. 0.8 MeV C+ => Si(1 11) at 500 K. annealed state Annealing of high-dose (1.1 x 1018 C/cm 2) samples for 1 h at 1250 °C leads to the crystallization of SiC, as shown by the XRD spectrum in Fig. 2. This is in agreement to results in [11] reporting a crystallization temperature of 850 'C for amorphous Si containing high concentrations of carbon. The sharp Si peaks in Fig. 2 signalize that in addition to SiC, polycrystalline Si has been formed during annealing. The RBS spectra in Fig. 3 demonstrate that even after 11 h annealing at 1250 'C the carbon depth distribution is nearly unchanged. Only a minor change in profile shapes is observed at the upper flank of the carbon depth profile (marked by an arrow) in a depth zone where the silicon crystal had not been amorphized. This is different to our observations in keV-lBS experi
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