Asymmetric Dislocation Densities in Forward-Graded ZnSySe1−y/GaAs (001) Heterostructures

Research paper by J. F. Ocampo, B. Bertoli, P. B. Rago, E. N. Suarez, D. Shah, F. C. Jain, J. E. Ayers

Indexed on: 24 Feb '10Published on: 24 Feb '10Published in: Journal of Electronic Materials

Abstract

We report an experimental and modeling study of ZnSySe1−y/GaAs (001) structures, all of which comprised a uniform top layer of ZnS0.014Se0.986 grown on a compositionally graded buffer layer or directly on the GaAs substrate. High-resolution x-ray diffraction was used to estimate dislocation densities on type A slip systems, with misfit dislocation (MD) line segments oriented along the $$[1\bar{1}0]$$ direction, and type B slip systems, with MD line segments oriented along a [110] direction. A control sample having no graded buffer exhibits equal dislocation densities on the two types of slip systems (DA ≈ DB ≈ 1.5 × 108 cm−2), but a forward-graded (FG) structure (grading coefficient of 27 cm−1) exhibits 20% more dislocations on the type B slip systems (DA ≈ 1.6 × 108 cm−2 and DB ≈ 1.9 × 108 cm−2) and a steep forward-graded structure (grading coefficient of 54 cm−1) exhibits 50% more type B dislocations (DA ≈ 2 × 108 cm−2 and DB ≈ 3 × 108 cm−2). The insertion of an overshoot interface reduced the dislocation densities in the uniform top layer by promoting annihilation and coalescence reactions, but type B dislocations were removed more effectively. Based on equilibrium calculations the overshoot graded layer in the steep graded overshoot structure is expected to exhibit large compressive and tensile strains, with a reversal in the sign of the strain near its middle, which may promote annihilation and coalescence reactions between threading dislocations.