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X-ray analysis of metamorphic InxGa1-xAs/InyGa1-yAs superlattices on GaAs (001) substrates
TIME:2017-5-31 15:32:00

The authors present a detailed x-ray dynamical diffraction analysis of metamorphic InxGa1-xAs/InyGa1-yAs superlattices grown epitaxially on GaAs (001) substrates. The x-ray rocking curve analysis was conducted for a number of hkl reflection profiles, including 004, 115, 026, and 117, assuming Cu k1 radiation, by using the mosaic crystal model for dynamical diffraction. The authors show that the threading dislocation density in the superlattice can be estimated from nondestructive x-ray rocking curve measurements, by observing the superlattice peak widths.
I. INTRODUCTION

Semiconductor device heterostructures involving superlattices are of great interest for a number of applications, in which they may enhance device performance directly or act as buffer layers and dislocation filters.1C10 Within a device structure, a strained-layer superlattice may be used to control the threading dislocation density in one of two ways. If the superlattice is placed below a graded layer, it could modify the misfit dislocation length, while placing the superlattice on the top of a metamorphic buffer could promote annihilation and coalescence reactions between threading dislocations. In either application, the superlattice is metamorphic, or partly lattice relaxed, and it is of interest to be able to determine the threading dislocation density within the superlattice by means of nondestructive characterization.
Superlattices may be characterized by high-resolution x-ray dynamical diffraction,11,12 allowing for profiling of the composition, strain, and dislocation density. In this work, we report an x-ray dynamical diffraction analysis from InxGa1-xAs/InyGa1-yAs superlattices grown epitaxially on GaAs (001) substrates with and without dislocations, using the mosaic crystal model. The x-ray rocking curve analysis was conducted for a number of hkl reflection profile including 004, 115, 026, and 117, assuming Cu k1 radiation. We show that the threading dislocation density in the superlattice may be estimated from nondestructive x-ray rocking curve measurements, using the rocking curve widths for superlattice diffraction peaks. Several x-ray diffraction techniques have been reported extensively in the literature to characterize semiconductor structures. Either kinematical13C17 or dynamical18C23 simulations allow for strain and composition profiling, yet these are based on perfect crystals and generally do not apply in the metamorphic case. A successful extension of dynamical simulations involving regular dislocation geometry was reported by Krivoglaz and Ryaboshapka24 and Levine and Thomson,25 but this work does not apply directly to practical crystals with random configurations of dislocations.
Another extension of dynamical simulations reported in the literature used an average mosaic block size, L, and a mean disorientation angle, , and was developed by Li et al.26 The Li et al. model is based on the calculation of the dynamical diffraction profiles, after which the incoherent intensity associated with dislocations is added. However, this model fails to predict the reduction of peak intensities from heteroepitaxial layers that exhibit high dislocation densities. The reciprocal space maps based on dynamical diffraction theory developed by Fewster27 and Fewster et al.28 also take into account the effects of dislocations, but only consider uniform dislocation density and are not applicable to depth profiling.
Recently, the method of dynamical simulations has been extended to metamorphic structures by using the phase-invariant11 and mosaic crystal12 models. While both models are fundamentally based on the Takagi-Taupin equation,18C20 they differ slightly in their details. The mosaic crystal model used here is more generally applicable and has low computation times, allowing for depth profiling of strain, composition, and dislocations by comparison to experimentally measured x-ray rocking curves. In a previous investigation, we used the mosaic crystal model to study the x-ray diffraction from linearly graded and step-graded InxGa1-xAs buffer layers on GaAs substrates.12 For step-graded layers we showed that the widths of the primary diffraction peaks could be used to estimate the threading dislocation densities. Until now, no work has considered the dynamical diffraction of superlattices containing dislocations using the mosaic crystal model. The impetus of the present paper was to determine if the widths of the superlattice peaks could be used to estimate the threading dislocation density in much the same way as primary peaks could be used for step-graded structures.


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