Strain Relaxation of He⁺ Implanted, Pseudomorphic Si_1−xGe_x Layers on Si(100)

Abstract

Strain relaxed Si_1−xGe_x buffer layers are of great importance as virtual substrates for Si_1−xGe_x/Si quantum well structures and devices. We apply He⁺ ion implantation and subsequent annealing on pseudomorphic, MBE-grown Si_1−xGe_x/Si(100) heterostructures with an implantation depth of about 100 nm below the Si_1−xGe_x/Si interface. A narrow defect band is generated inducing the formation of strain relieving misfit dislocations during subsequent thermal annealing. Efficient strain relaxation was demonstrated for Si_1−xGe_x layers with Ge fractions up to 30 at. %. The variation of the implantation dose and the annealing conditions changes the dislocation configuration and the He bubble structure. At a dose of 2×10¹⁶ cm⁻² a high degree of relaxation is accompanied by a low density of threading dislocations of about 10⁷ cm⁻² for a Ge content of 30%. An additional increase of the Ge content can be achieved by annealing in oxygen. The oxidation of Si_1−xGe_x leads to the formation of SiO₂ while the Ge atoms are rejected from the oxide leading to a pile-up of Ge below the oxidation front. The heterostructures were analyzed using X-ray diffraction, Rutherford backscattering/channeling spectrometry and transmission electron microscopy.