Asymmetric Dislocation Densities in Forward-Graded ZnS y Se1−y /GaAs (001) Heterostructures

Abstract

We report an experimental and modeling study of ZnS y Se 1Ày /GaAs (001) structures, all of which comprised a uniform top layer of ZnS 0.014 Se 0.986 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 ½1 " 10 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 (D A % D B % 1.5 9 10 8 cm À2 ), but a forward-graded (FG) structure (grading coefficient of 27 cm À1 ) exhibits 20% more dislocations on the type B slip systems (D A % 1.6 9 10 8 cm À2 and D B % 1.9 9 10 8 cm À2 ) and a steep forwardgraded structure (grading coefficient of 54 cm À1 ) exhibits 50% more type B dislocations (D A % 2 9 10 8 cm À2 and D B % 3 9 10 8 cm À2 ). 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.

Low-Temperature and Metamorphic Buffer Layers

“…Buffer layers with linearly-graded composition, and therefore lattice constant, have been extensively investigated in a number of material systems, including In x Ga 1Àx As/GaAs [25,26,51,[96][97][98][99][100][101][102][103][104], In x Al 1Àx As/GaAs [34,75,103,[105][106][107][108][109][110], In x Al y Ga 1ÀxÀy As/GaAs [18,19,23,35,80,95,111], Si 1Àx Ge x /Si [112][113][114][115][116], In x Ga 1Àx P/GaAs [117][118][119], In x Ga 1Àx P/ GaP [120], ZnS y Se 1Ày /GaAs [102,121], and In x Ga 1Àx Sb/GaSb [122,123]. A possible advantage of continuous grading is that layer-by-layer growth may be maintained without the intrusion of island growth associated with large, abrupt changes in composition [119].…”

Comparison of Nonlinear Grading Approaches for ZnSSe/GaAs (001) Heterostructrures

“…The realization of semiconductor devices on a lattice-mismatched substrate is typically achieved by metamorphic (partly-relaxed) growth (1)(2)(3)(4)(5)(6), which is accompanied by elastic strains and threading dislocation defects (7). Compositionally-graded buffer layers are commonly employed to accommodate the lattice mismatch (8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25)(26), with the goal of reducing the density of threading dislocations and, in some cases, with the added goal of controlling the residual strain. Use of such buffer layers increases the complexity and cost of the device processing, so it is desirable to maximize the effectiveness of the buffer layer with the minimum requirement on thickness.…”

“…Typically metamorphic buffer layers have utilized linear grading of composition (8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19). This distributes the misfit dislocations through a considerable thickness of the buffer instead of concentrating them at the substrate interface, and this may limit pinning interactions with substrate defects (28).…”