Strain relaxation under compressive or tensile stress

The mechanisms of strain relaxation in strained heterostructures is still the matter of controversial debate. As a matter of fact experimentally determined critical thicknesses are usually much larger than the values calculated using equilibrium theoretical approaches. This fact shows that mechanisms of dislocation nucleation and/or multiplication play a central role. Moreover for the same material system a different behavior is expected for tensile or compressive stress. In fact in the framework of the continuum elasticity theory dissociation of perfect dislocations into partials leads to a critical thickness for layers under tension lower than for layers under compression. An ideal material system for the comparison of tensile and compressive strain is InXGa1-As-X/InP where the initial misfit can be varied over large intervals from tensile to compressive by varying the In composition below or above x=0.53. In this paper we report a systematic study of strain relaxation in MOVPE grown InGaAs/InP epilayers of different composition and thickness. Samples were characterized by RES-Channeling, X-ray diffraction, AFM, TEM and CL. The main results may be summarized as follows. Layers under compression relax following the same dependence on layer thickness determined for MBE grown InGaAs/GaAs showing no composition or growth technique effect. For tensile stress the strain relaxation is asymmetric showing faster relaxation along [110] than along [1-10]. This asymmetry increases by increasing the tensile misfit. The critical thickness for tensile strain relaxation is larger than for compressive strain relaxation even for the fast relaxing [110] direction. Cracks are found only along [110], but their density is not sufficient to explain the amount of strain relaxation. Moreover carefull inspection of morphology features and of Misfit Dislocations (MD) suggests that cracks are formed after the growth has been completed, probably assisted by thermal strain.