Strain relaxation of SiGe/Si heterostructures by helium ion implantation and subsequent annealing: Helium precipitates acting as dislocation sources

Abstract

The strain relaxation process of pseudomorphic SiGe/Si(100) heterostructures has been investigated by ex-situ and in-situ transmission electron microscopy, Rutherford backscattering spectroscopy and ion channelling. SiGe layers with Ge contents between 19 and 27 at% were implanted with doses of 0.7 − 1.5×10¹⁵ He/cm⁻² and annealed at temperatures from 200°C up to 950°C. Helium precipitates in overpressurised, platelet shaped volume defects underneath the heterostructure interface during low annealing temperatures of 400°C. The precipitates decay into arrangements of a larger central precipitate surrounded by a ring system of smaller bubbles. At higher temperatures a transformation from platelet-like to a spherical bubble is observed and for temperatures above 800°C coalescence of entire bubbles is found in in-situ TEM heating experiments. Particular emphasis is placed on the onset of the strain relaxation process which occurs after thermal treatment around 600°C. The processes involved are verified by in-situ experiments. The nucleation of dislocation loops at the helium filled precipitates, their enlargement and the formation of misfit dislocation segments elongating via the movement of threading dislocation segments are observed. Thus, helium precipitates are unambiguously identified as dislocation sources and, therefore, promote the strain relaxation of epitaxial SiGe layers.