Control of tensile strain and interdiffusion in Ge/Si(001) epilayers grown by molecular-beam epitaxy

Tensile-strained and n-doped Ge has emerged as a potential candidate for the realization ofoptoelectronic devices that are compatible with the mainstream silicon technology. TensilestrainedGe/Si epilayers can be obtained by using the difference of thermal expansion coefficientsbetween Ge and Si. We have combined various surface, structural, and compositionalcharacterizations to investigate the growth mode and the strain state in Ge/Si epilayers grown bymolecular-beam epitaxy. The Ge growth was carried out using a two-step approach: a low temperaturegrowth to produce relaxed and smooth buffer layers, which is followed by a hightemperaturegrowth to get high quality Ge layers. The existence of a substrate temperature windowfrom 260 to 300 C is evidenced, which allows to completely suppress the Ge/Si Stranski-Krastanov growth. As a consequence of the high temperature growth, a tensile strain lying in therange of 0.22%–0.24% is obtained. Concerning the effect of thermal annealing, it is shown thatcyclic annealing may allow increasing the tensile strain up to 0.30%. Finally, we propose anapproach to use carbon adsorption to suppress Si/Ge interdiffusion, which represents one of themain obstacles to overcome in order to realize pure Ge-based optoelectronic devices