We show the effect of C incorporation sites in SiGeC intermediate layer to SiGe layer relaxation. First of all, atomically flat (rms roughness less than 0.3 nm), fully strained Si1-xGex/Si1-x-yGexCy/Si1-xGex heterostructures with the same Ge content (x = 0.2) and different C contents were grown on Si substrates by using ultrahigh vacuum chemical vapor deposition (UHV-CVD). The thickness of intermediate SiGeC layer was 10 nm, and those of both upper and lower SiGe layers were 85 nm, which is extremely thin compared to conventional graded buffer layers (~ µm range). Following the growth of heterostructures, layers were subjected to an anneal at 1000 °C for 30 sec using rapid thermal process (RTP) to relax the layers while suppressing surface atom diffusion.
As a result, smooth surfaces with rms roughness less than 1 nm was obtained in all SiGe layers including control-SiGe layer without C containing layer. However, uniform and high-degree relaxation appeared only in the layers with the SiGeC intermediate layer. The degree of relaxation achieved by SiGeC intermediate layer was 65% and 60% for with and without interstitial C, respectively. Their relaxation was factor of two larger than that of the control-SiGe layer subjected to RTP, despite the fact that the control-SiGe layer without the SiGeC intermediate layer was more strained for relaxation. The small difference in relaxation and roughness between samples with and without interstitial C indicates that the substitutionality of C in SiGeC layer is not the main reason for the relaxation enhancement.
However, AFM images show the difference in density and step height of crosshatch patterns, implying different distribution of misfit dislocation pile-up. Cross sectional TEM results reveals that SiGeC intermediate layer relaxes SiGe layer by the formation of dislocation via modified Frank-Read (MFR) mechanism. Therefore, the different distribution of the misfit dislocation pile-up is caused by the difference of distribution of pinning sites for MFR mechanism and optimal SiGe relaxation can be achieved by adjusting the substitutionality of C in SiGeC intermediate layer. |