Characterization of InGaN and InAlN Epilayers by Microdiffraction X-Ray Reciprocal Space Mapping
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Characterization of InGaN and InAlN Epilayers by Microdiffraction X-Ray Reciprocal Space Mapping V. Kachkanov1, I.P. Dolbnya1, K.P. O’Donnell2, K. Lorenz3, S. Pereira4, R.W. Martin2, P.R. Edwards2, I.M. Watson5 1
Diamond Light Source Ltd, Didcot, United Kingdom Department of Physics, SUPA, University of Strathclyde, Glasgow, Scotland, United Kingdom 3 Instituto Tecnologico e Nuclear, Sacavem, Portugal. 4 CICECO, Departamento de Fisica and I3N, Universidade de Aveiro, Aveiro, Portugal 5 Institute of Photonics, SUPA, University of Strathclyde, Glasgow, Scotland, United Kingdom 2
ABSTRACT We report a study of InGaN and InAlN epilayers grown on GaN/Sapphire substrates by microfocused three-dimensional X-ray Reciprocal Space Mapping (RSM). The analysis of the full volume of reciprocal space, while probing samples on the microscale with a focused X-ray beam, allows us to gain uniquely valuable information about the microstructure of III-N alloy epilayers. It is found that “seed” InGaN mosaic nanocrystallites are twisted with respect to the ensemble average and strain free. This indicates that the growth of InGaN epilayers follows the Volmer-Weber mechanism with nucleation of “seeds” on strain fields generated by the a-type dislocations which are responsible for the twist of underlying GaN mosaic blocks. In the case of InAlN epilayer formation of composition gradient was observed at the beginning of the epitaxial growth. INTRODUCTION The III-nitride semiconductors (GaN, InN, AlN and their alloys) have become a subject of intense research in the past fifteen years due to the unique physical properties of these materials, which include a wide direct bandgap, remarkable mechanical strength and high melting temperatures, as well as their huge commercial utility. The technological breakthrough in the epitaxial growth of InGaN structures [1] has revolutionized the optoelectronic industry, leading recently to robust and compact all-solid state light sources covering the broad spectral range from infra-red to ultraviolet. However, further advances in nitride technology, for example nitride laser diodes emitting in the green, red and ultra-violet spectral regions, require a deeper understanding of the microstructure and nanostructure of InxGa1-xN and InxAl1-xN alloy films. X-ray Reciprocal Space Mapping (RSM) is a powerful tool to explore the structure of materials. However, RSMs are usually measured in two dimensions (2D) ignoring the third dimension of diffraction space volume. The idea of full three-dimensional (3D) diffraction space mapping to obtain information on the 3D structure of materials was first introduced by Fewster et al [2]. In this paper we report the study of InGaN and InAlN epilayers by 3D RSM while probing the material with a microfocused X-ray beam.
EXPERIMENT The samples studied were two nominally undoped wurtzite InxGa1-xN epilayers, labelled A and B, and one InxAl1-xN epilayer, grown by Metalorganic Chemical Vapor Deposition (MOCVD) on GaN/Al2O3 (0001) substrates. The nominal thickness of the InGaN and I
