Strain relief in compositionally graded Si1-xGex formed by high dose Ion implantation

Abstract

Strain relief mechanisms have been investigated in alloys of Si and Ge formed by high dose ion implantation followed by solid phase epitaxy. Both compressive and tensile strain states were studied by implanting: (i) 200 keV Ge into (001) Si to form Si-rich alloys and (ii) 150 keV29Si into (001) Ge to form Ge-rich alloys. We report that Si-rich compressively strained alloys above a critical implant dose relax via the introduction of a/6(112) partials (bounding stacking faults) when regrown by solid phase epitaxy below ≈600° C. Alloys formed by implanting Si into (001) Ge and regrown at 450° C also undergo strain relaxation above a critical dose but, in this case, relaxation proceeds via the introduction of planar defects + partials, Lomers, and 60° dislocations. The high dose ion implantation technique produces alloys in which the concentration, and hence lattice mismatch, is a Gaussian function of depth from the surface. In this work we present a modification of the conventional critical thickness calculation which includes: (i) balancing forces acting on defects in a strain gradient to define an interface above which strain relief occurs and (ii) integration of the strain energy in the portion of the compositionally graded film above this interface. Critical dose implant plots based on these calculations are presented. The model provides a good fit to cross-sectional and TEM observations of regrown alloys with implant doses above and below the predicted critical dose. A TEM characterization of the strain relieving defects for both compressive and tensile films is presented.