Origin of residual strain in heteroepitaxial films

Heterogeneous integration of diverse materials structures is critical to the scaling of electronic and photonic integrated circuits. For a model system of Ge-on-Si, we experimentally examine the roles of lattice misfit and thermal expansion misfit in determining the residual strain in as-grown and annealed heteroepitaxial films. We present data for Ge-on-Si growth from 400 to 730 degrees C followed by heat treatment from 500-900 degrees C. We show that strain fluctuations of 5.02% enable misfit dislocation formation, and we propose a comprehensive model for the conversion of compressive misfit strain to tensile elastic strain. The model is expressed in terms of three regimes: (1) misfit control for the low temperature growth regime at 400 degrees C; (2) point defect control via annealing in the point defect recovery regime at 500-650 degrees C; and (3) thermal expansion control for growth or anneal at T > 650 degrees C in the dislocation recovery regime. Growth from 400 to 730 degrees C exhibits near complete misfit strain relief by misfit dislocations leaving a consistent residual compressive strain of 0.09%. Growth at 400 degrees C followed by post growth heat treatment at 600 degrees C results in vertical threading dislocation density reduction via a point defect-mediated climb mechanism that gives minimal strain relief. Anneal above 650 degrees C promotes strain relief by dislocation glide. Temperature excursions at T > 730 degrees C followed by cooling to room temperature yield plastic strain in the Ge film that cannot be further relieved by thermal expansion misfit accommodation. Growth at 400-730 degrees C retains a residual compressive strain that represents the nucleation threshold for misfit dislocations.

Automated summary

Heterogeneous integration of diverse materials structures is critical to the scaling of electronic and photonic integrated circuits. The roles of lattice misfit and thermal expansion misfit in determining the residual strain in as-grown and annealed heteroepitaxial films are experimentally examined using Ge-on-Si as a model system. The study reveals the formation of misfit dislocations and proposes a comprehensive model for the conversion of compressive misfit strain to tensile elastic strain. Different temperature regimes exhibit different mechanisms for strain relief.