Threading Dislocation Densities in Mismatched Heteroepitaxial (001) Semiconductors
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THREADING DISLOCATION DENSITIES IN MISMATCHED HETEROEPITAXIAL (001) SEMICONDUCTORS
J. E. AYERS*, S. K. GHANDHI, AND L. J. SCHOWALTER Rensselaer Polytechnic Institute, Troy, NY 12180 *presently at University of Connecticut, Storrs, CT 06269
ABSTRACT In this paper we propose a theory which accounts for the thickness dependence of threading dislocation densities in mismatched heteroepitaxial (001) semiconductors. This theory predicts that, for thick, planar, highly-mismatched heteroepitaxial layers with equilibrium strain, the threading dislocation density should be proportional to f/h, where f is the lattice mismatch and h is the film thickness. These predictions are in good agreement with experimental results in the GaAs on Si(001) system. One very important consideration in the growth of mismatched heteroepitaxial layers is the density of threading dislocations. Dislocations act as non-radiative recombination centers in compound semiconductors (1,2], and degrade the performance of minority carrier devices (3,4). Dislocations and strain both cause rapid degradation in injection lasers (5, 6,7] . It is therefore important to clarify the thickness-dislocation density relationship, and to understand the origin of the threading dislocations, for the application of mismatched heteroepitaxial semiconductors to devices. Most thick, mismatched (f > 0.2%) heteroepitaxial layers exhibit broad x-ray rocking curves due to the large threading dislocation densities. In most heteroepitaxial systems, it is found that the dislocation density reduces with the reciprocal of the film thickness. Systems with the D - 1/h behavior include GaAs/Si (see figure one) (8), ZnSe/GaAs [9], InP/GaAs [10) and InAs/InP. We therefore expect that this is a general phenomenon. In this paper we propose an explanation for the thickness dependence of the threading dislocation densities in mismatched heteroepitaxial materials, based on a half-loop mechanism for the annihilation of misfit dislocations. In developing this model, it is assumed that a relatively high density of threading dislocations is introduced during lattice relaxation, by some means. Neighboring threading dislocations will often have opposing Burgers vectors; they will therefore attract one another and coalesce. Upon coalescence, a half-loop such as the one shown in figure two will be formed. The strain and line-tensional forces acting on this half-loop will determine whether it glides back to
Mat. Res. Soc. Symp. Proc. Vol. 209. 01991 Materials Research Society
662
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Figure 1. Threading Dislocation Density versus Reciprocal of Layer Thickness for Annealed GaAs/Si (001) (8). The postgrowth annealing was performed in an AsH3 /H 2 ambient at 850 0 C for 30 minutes, so as to achieve saturation of the dislocation density.
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