We have investigated the surface morphology of relaxed, compositionally graded GexSi1−x structures, to illustrate the influence of defect-related strain fields on film growth. Quantitative topographic measurements via scanning force microscopy show that the roughness associated with the cross-hatch patterns, due to underlying misfit dislocations beneath the surface, increases as the final Ge concentration or the grading rate increases. We further show that strain fields arising from the termination of threading dislocations at the surface result in shallow depressions.
Modulation-doped Si/GexSi1−x/Ge/GexSi1−x structures were fabricated in which a thin Ge layer was employed as the conduction channel for the two-dimensional hole gas. The strained heterostructure was fabricated on top of a low threading dislocation density, totally relaxed, GexSi1−x buffer layer with a linearly graded Ge concentration profile. The best mobility of the two-dimensional hole gas is 55 000 cm2/V s at 4.2 K with a concentration-dependent hole effective mass of ≤0.10m0. The effect of the Ge/GeSi interface roughness on the 2DHG mobility was studied.
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