Reduction of threading dislocation density beyond the saturation limit by optimized reverse grading

The threading dislocation density (TDD) in plastically relaxed Ge/Si(001) heteroepitaxial films is commonly observed to decrease progressively with their thickness due to mutual annihilation. However, there exists a saturation limit, known as the geometrical limit, beyond which a further decrease of the TDD in the Ge film is hindered. Here, we show that such a limit can be overcome in SiGe/Ge/Si heterostructures thanks to the beneficial role of the second interface. Indeed, we show that Si0.06Ge0.94/Ge/Si(001) films display a TDD remarkably lower than the saturation limit of Ge/Si(001). Such a result is interpreted with the help of dislocation dynamics simulations. The reduction of TDD is attributed to the enhanced mobility acquired by preexisting threading dislocations after bending at the new interface to release the strain in the upper layer. Importantly, we demonstrate that the low TDD achieved in Si0.06Ge0.94/Ge/Si layers is preserved also when a second, relaxed Ge layer is subsequently deposited. This makes the present reverse-grading technique of direct interest also for achieving a low TDD in pure-Ge films.