Strain Relief by Tilting of Epitaxial GaAs Films on Si
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STRAIN RELIEF BY TILTING OF EPITAXIAL GaAs FILMS ON Si L.J. SCHOWALTER*, E.L. HALL*, N. LEWIS*, *Physics Dept. and Ctr. Integrated Electronics, NY 12181 **GE Research & Development Ctr., Schenectady, -Physics Dept., State University of New York at
AND SHIN HASHIMOTORensselaer Polytechnic Institute, Troy, NY 12301 Albany, Albany, NY 12222
ABSTRACT In this paper, a general formula is calculated relating the misalignment of a heteroepitaxial overlayer with respect to the substrate to the defects at the interface. High-resolution TEM studies are presented which show that the density of dislocations along the interface of GaAs grown on vicinal Si(100), with Burgers vector perpendicular to the interface, is consistent with the amount of tilt observed in ion-channeling studies. Arguments are also presented which demonstrate that the tilt observed is an effective form of strain relief but that this mechanism can only operate when the heteroepitaxial growth is nucleated by island growth. One significant conclusion is that the lower bound observed on the threading dislocation density in GaAs on Si is due to this mechanism of strain relief by tilting. INTRODUCTION There is substantial interest in the commercial development of GaAs on Si. A continuing problem, however, is the relatively high number of dislocations (about 107 cm- 2) which thread through the epitaxial GaAs layer. We argue here that this dislocation density may be an intrinsic property of the strain-relief mechanism during the growth of the GaAs layer (which has a 4% larger lattice spacing than the underlying Si substrate). When a heteroepitaxial layer, such as GaAs on Si, initiates growth by forming islands on the substate, there exists a new mechanism for strain relief of lattice mismatch during growth. By introducing dislocations with an edge-like component of their Burgers vectors perpendicular to the interface, epitaxial islands can change their tilt with respect to the substrate so long as there exist side walls which are free to move. In the case of a vicinal Si(001) substrate in which the (001) axis is deliberately tilted away from the surface normal, the GaAs islands can relieve strain by tilting back toward the surface normal through this mechanism. The amount of tilt which occurs will depend on the aspect ratios of the islands when they coalesce. This implies that there will be some amount of statistical fluctuation in the amount of tilt across the film which will result in the formation of low-angle grain boundaries (LAGB). These LAGB will then act as sources of "threading dislocations." The strain-relief mechanism presented here should be equally effective in relieving strain in any lattice mismatched system in which the epitaxial layer is nucleated by island growth. In this paper, we first relate the tilt in a coherent epitaxial layer to the defects at the overlayer/substrate interface. In the second part of the paper, a high-resolution transmission electron microscopy (HRTEM) study is presented which relates the observed dislocation densities at
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