Assessment of the growth/etch back technique for the production of Ge strain-relaxed buffers on Si
Résumé
Thick Ge layers grown on Si(0 0 1) are handy for the production of GeOI wafers, as templates for the epitaxy of III-V and GeSn-based heterostructures and so on. Perfecting their crystalline quality would enable to fabricate suspended Ge micro-bridges with extremely high levels of tensile strain (for mid IR lasers). In this study, we have used a low temperature (400 degrees C)/high temperature (750 degrees C) approach to deposit with GeH4 various thickness Ge layers in the 0.5 mu m - 5 mu m range. They were submitted afterwards to short duration thermal cycling under H-2 (in between 750 degrees Cand 875-890 degrees C) to lower the Threading Dislocation Density (TDD). Some of the thickest layers were partly etched at 750 degrees C with gaseous HCl to recover wafer bows compatible with device processing later on. X-ray Diffraction (XRD) showed that the layers were slightly tensile-strained, with a 104.5-105.5% degree of strain relaxation irrespective of the thickness. The surface was cross-hatched, with a roughness slightly decreasing with the thickness, from 2.0 down to 0.8 nm. The TDD (from Omega scans in XRD) decreased from 8 x 10(7) cm(-2) down to 10(7) cm(-2) as the Ge layer thickness increased from 0.5 up to 5 mu m. The lack of improvement when growing 5 mu m thick layers then etching a fraction of them with HCl over same thickness layers grown in a single run was at variance with Thin Solid Films 520, 3216 (2012). Low temperature HCl defect decoration confirmed those findings, with (i) a TDD decreasing from slightly more 10(7) cm(-2) down to 5 x 106 cm(-2) as the Ge layer thickness increased from 1.3 up to 5 mu m and (ii) no TDD hysteresis between growth and growth then HCl etch-back.