X-ray diffraction analysis of step-graded InxGa1−xAs buffer layers grown by molecular beam epitaxy

doi:10.1016/j.jcrysgro.2010.11.173

Journal of Crystal Growth

Volume 323, Issue 1, 15 May 2011, Pages 17-20

https://doi.org/10.1016/j.jcrysgro.2010.11.173 Get rights and content

Abstract

High resolution X-ray diffraction reciprocal space mapping (RSM) is used to study the crystal quality of step-graded In_xGa_1−xAs buffer layers grown on GaAs (0 0 1) substrates by molecular beam epitaxy (MBE) using two growth methods. The lateral correlation length of the buffer layers are described by the FWHM of diffraction peaks along the lateral directions extracted from RSM, which are good indicators of the layer's crystalline quality. The quality improvement of In_xGa_1−xAs buffer layers grown at low temperature with in-situ annealing could be determined by XRD-RSM, which is also consistent with TEM results.

Introduction

Relaxed In_xGa_1−xAs buffer layers on GaAs substrates are useful for many devices because they can provide a good virtual substrate with a desired lattice constant by simply varying the In content. Therefore the strain induced in subsequent active layers can be engineered. One recently proposed application is to provide a virtual substrate for growth of bi-axially tensile-strained Ge [1], [2]. Fischetti and Laux [3] predicted that about 1.7% tensile strain can modify the Ge band structure such that it becomes a direct bandgap material. The key criteria for the In_xGa_1−xAs buffer layers are low threading dislocation density (TDD), efficient strain relief, and a smooth surface. Transmission electron microscopy (TEM) is widely used to study the crystal quality of In_xGa_1−xAs buffer layers. However, it requires extensive sample preparation and considerable experience to take a good TEM image. It has been reported that high resolution X-ray diffraction (XRD) is an accurate tool for measuring the strain relaxation through the position of diffraction peaks in reciprocal space mapping (RSM). XRD-RSM is also a good method to investigate the crystallographic quality of epitaxial layers through diffraction peak analysis, such as layer tilting [4], [5], [6].

In this paper, we report the XRD-RSM results of step-graded In_xGa_1−xAs buffer layers grown by solid source MBE using two different methods. The full width at half maximum (FWHM) of buffer layer peaks in RSM along the lateral direction and the mosaic direction were extracted to characterize the crystallographic quality of the In_xGa_1−xAs layers. Here we show that the larger value of FWHM along the lateral direction indicates smaller value of the lateral correlation length, which corresponds to a higher dislocation density. Cross-section TEM (XTEM) images confirm the results. XRD-RSM FWHM analysis could be used in other lattice-mismatched material systems.

Section snippets

Experimental procedures

Step-graded In_xGa_1−xAs buffer structures consisting of two In_xGa_1−xAs layers were grown on GaAs (1 0 0) substrates using a modified Varian Gen II MBE system. The oxide layers on the GaAs wafers were removed by thermal cleaning at 680 °C (all the temperature readings refer to thermo-couple readings) in the MBE chamber. After a 100 nm GaAs buffer layer was grown, a clear (2×4) RHEED pattern was confirmed. Then step-graded In_xGa_1−xAs layers were grown by two different methods. In the first method both

Results and discussion

Fig. 1, Fig. 2 show the (2 2 4) RSM of sample A (grown by the first method) and sample B (grown by the second method). The vertical and horizontal axes represent the reciprocal lattice of the [0 0 1] (q_y) and [1 1 0] (q_x) directions. In each figure, the GaAs substrate (the top peak) and the two In_xGa_1−xAs buffer layers are well resolved. The In content and the relaxation of each In_xGa_1−xAs buffer layer are calculated from the buffer layer peak position relative to the substrate peak, corrected by the

Conclusion

In conclusion, XRD-RSM has been used to study the crystalline quality of step-graded In_xGa_1−xAs buffer layers grown by MBE on GaAs substrates using two different methods. The results are consistent between XRD and TEM analysis. The crystalline quality of the In_xGa_1−xAs layers greatly improves by using low temperature growth and in-situ annealing. The FWHM of the diffraction peaks along the lateral directions extracted from RSM are good indicators of layer crystal quality. This technique is

Acknowledgements

This study was supported by Air Force Office of Scientific Research (contract number FA9550-05-1-0532-P00) and was partially supported through the Connectivity Center of the Focus Center Research Programs (FCRP).

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