Macro- and microstrains in MOCVD-grown GaN
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Internet Journal Nitride Semiconductor Research
Macro- and microstrains in MOCVD-grown GaN A. Usikov1, V.V. Ratnikov1, R. Kyutt1, W. V. Lundin1, B. Pushnyi1, N. M. Shmidt1 and M.P. Scheglov1 1Ioffe
Physical-Technical Institute,
(Received Monday, June 22, 1998; accepted Wednesday, October 21, 1998)
Undoped and Si-doped GaN films were grown by low pressure MOCVD on (0001) sapphire substrates. The angular distribution of the X-ray diffraction corresponding to the (0002), (0004), (1010), (2020), and (1124) reflections has been measured by means of double- and triple -crystal diffractometry with Mo Kα1 and Cu Kα1 radiation under conditions of symmetrical and asymmetrical Bragg- and Laue-geometry. In our experiments a non-coplanar geometry was also applied. On the basis of the performed studies, five independent components of the tensor of microdistortion were evaluated and the average grain-size in two directions was determined. The type, position, and density of dislocations were established as well. The role of dislocations in strain relaxation and their influence on the optical and electrical properties are discussed.
1
Introduction
In recent years remarkable progress in the fabrication of bright UV and blue-green light emitting diodes and laser diodes has been achieved for III-N epilayers grown mainly on (0001) sapphire substrates [1]. The peculiarities specific to the growth of GaN films on lattice-mismatched sapphire substrates cause remarkable macroand micro deformations in the epilayers for different thermal expansion coefficients of the film and the substrate. X-ray diffraction is used to study the material structural properties as well as macro- and micro-strains in GaN and related compounds. Defects in epitaxial layers are usually inspected using measurement of the angular full width at half maximum (FWHM) of the Xray diffraction rocking curves. However, it is insufficient to establish a type of defect and to clarify the mechanism of stress relaxation and its influence on the optical and electrical properties of the epilayers. In this paper both undoped and Si-doped GaN epilayers have been studied comprehensively by means of double- and triple-crystal diffractometry. 2
Experiment
The GaN single epilayers with specular surfaces, both undoped and doped with Si, were grown in a conventional MOCVD system at a low pressure, 200 mbar. Ammonia and trimethylgallium were applied as component precursors. Purified hydrogen was used as a carrier gas. Silane was used for doping. The substrates were
(0001) optical-grade polished sapphire. The growth procedure included GaN nucleation layer deposition at a low temperature of 500 °C followed by epilayer growth at a higher temperature of 1040 °C The typical layer thickness was 2-4 µm. The two-dimensional growth mode appeared to be dominant on the specular surfaces. Hexagonal growth features were lacking, whereas growth steps of ~0.3 nm height were well defined by AFM. The angular distribution of the X-ray diffraction corresponding to the (0002), (0004), (1010), (2020
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