X-Ray Diffraction Analysis of the Strain of SiGeC/(100)Si Alloys
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ABSTRACT Samples of Sil-xyGexCy were analyzed with a triple axis high resolution x-ray diffractometer to produce reciprocal space maps. Films with compositions of approximately Si0.77 Ge0 .20C0 .01, with thicknesses ranging from 120 nm to 750 nm were found to be pseudomorphic with a tetragonal distortion, 6T, near 1%. The tetragonal distortion in pseudomorphic samples with compositions near Si 0 .47 Ge 0 .50 C 0 .03 with thicknesses ranging from 61 nm to 115 nm was found to be near 2%. The strain increased linearly with Ge concentration even though the Ge:C ratio remained nearly constant. The strain in samples with similar compositions was not a function of thickness. These strain measurements correlated well with results from ion channeling analysis. INTRODUCTION Alloys of Sil.x-yGexCy grown epitaxially on (100)Si have been investigated for their potential as bandgap engineered materials. Since the lattice of Ge is 4.2% larger than that of Si, compressive strains can develop which create stability problems. This limits the thickness that a pseudomorphic film can be grown. Carbon can be introduced substitutionally into the lattice at temperatures below 800 'C without the formation of the thermodynamically favored P3-SiC phase[I-3]. Since the diamond phase of C is 54% smaller than Si, this should compensate for the compressive strain in the film. If 100% of the C is incorporated into substitutional sites and Vegard's law is obeyed, complete compensation will result with a Ge:C ratio of 8.2:1. The strain characteristics of SiGeC has been studied previously using double-crystal x-ray diffractometry. In studies by Strane et al.[4], preamorphized Si1 _,Gey was implanted with C and regrown through solid phase epitaxy. Multiple implants were used to obtain uniform C profiles of 0.7 and 1.4 at. %. It was found that the perpendicular strain in the regrown layers was less than the pre-implant SilxGey. Thus the C compensated the strain. Similar successes in reducing strain in SiGeC grown through molecular beam epitaxy were reported by Powell et al.[5]. In this study the strain in CVD grown Sil.xyGexCy layers on (100) Si was characterized as functions of Ge content, and Ge:C ratio. High resolution x-ray diffraction was used to generate reciprocal space maps. The separation between the layer and substrate peak, Aco, can be used to quantify both the parallel and perpendicular lattice constant. These lattice constants were converted to a tetragonal distortion strain parameter and compared to ion channeling results performed subsequently on the same samples in experiments detailed by Sego et al.[6]. EXPERIMENTAL The samples analyzed in this experiment were layers of Si1 .~xyGexCygrown on Si(100) substrates at atmospheric pressure with an Epsilon One model E-2 single wafer automated CVD reactor. 461 Mat. Res. Soc. Symp. Proc. Vol. 399 ©1 99 6 Materials Research Society
The precursors used were SiH 2 CI2 as the Si source, GeH 4 as the Ge source and source.
C 2H 4
as the C
The film areal densities were quantified with Rutherford
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