SiGe Epilayer Stress Relaxation:Quantitative Relationships Between Evolution of Surface Morphology and Misfit Dislocation Arrays

摘要

Stress relaxation mechanisms have been investigated during growth of SiGe/Si heterostructures in a regime where misfit dislocations and surface morphological evolution strongly interact.Stress evolution was measured in real time using in situ wafer curvature measurements during molecular beam epitaxy of Ge_(0.3)Si_(0.7) on Si (001 ).This real-time data has been combined with detailed analysis of the evolving surface morphology and misfit dislocation density using atomic force microscopy and transmission electron microscopy.Several distinct stages of microstructural evolution were observed during epitaxial growth of Ge_(0.3)Si_(0.7) alloys at intermediate temperatures (550 deg C) and relatively high deposition rate (0.9 Ats).Initial planar,coherent growth was followed by pyramidal pit formation in the metastable wetting layer for 15nm thick films.Subsequent cooperative nucleation of island ridges occurs along <100> directions surrounding the pits.Dislocations are next introduced into the film abruptly at 5Onm film thickness and relieve 40% of the film stress by 1 OOnm of growth.Cross-hatch features aligned along the <110> directions accompany the introduction of misfit dislocations.As the dislocation density increases in the film,the coherent island/pit structures begin to disappear and the film morphology is dominated by the evolving cross-hatch.Implicatjons for competitive and/or correlated relaxation of strain by misfit dislocations and surface morphology based upon this data will be discussed.This work demonstrates that combining real-time stress measurements with microscopy enables mechanistic and quantitative understanding of coupled relaxation modes in strained layer heteroepitaxy.