Statistical Analysis of Local Composition and Luminescence in InGaN Grown by Molecular Beam Epitaxy
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PO Box 330440, 28334 Bremen, Germany ** Institute of Materials Physics and Structural Research, University of Bremen,
PO Box 330440, 28334 Bremen, Germany Institute of Experimental Physics, University of Magdeburg, PO Box 4120, 39016 Magdeburg, Germany Cite this article as: MRS Internet J. Nitride Semicond. Res. 4S1, G3.33(1999) Abstract InGaN layers grown by molecular beam epitaxy are investigated in terms of their compositional homogeneity using transmission electron microscopy and cathodoluminescence spectroscopy performed with high spatial resolution. Strong fluctuations of the indium content were found in bulklike layers, which could be partially reduced by modulating the indium flux during growth, i. e. by nominally growing a short period GaN/InGaN superlattice. For indium compositions above x - 0.1 this approach fails. Strained InGaN in quantum wells exhibits lateral fluctuations on an atomic scale and on a scale of several hundred nanometers. The results are discussed in view of the origin of inhomogeneous indium incorporation. Introduction Although the ternary alloy InGaN is established as the active material in optoelectronic devices operating in the short wavelength region, its epitaxial growth raises significant problems making it a subject of numerous investigations. The large lattice mismatch between InN and GaN is the reason for a miscibility gap in the phase diagram of InGaN, which could result in a demixing into phases of different composition during growth [1,2]. Moreover, the binding energy is significantly lower for InN compared to GaN, such that the thermal decomposition of InGaN causing the formation of indium droplets on the growth surface is a possible source of lateral variations of the composition [3]. Furthermore, compositional fluctuations arise from lattice relaxation of strained InGaN films on GaN due to V-shaped defects [4] and the composition pulling effect [5]. Direct evidence of compositional fluctuations in InGaN bulk-like layers and quantum well structures was given by spatially resolved measurements of optical and structural properties utilizing cathodoluminescence (CL) spectroscopy [6] and transmission electron microscopy (TEM) [7,8J. In this paper the attempt is started to distinguish between possible reasons for these fluctuations is with the object to draw conclusions for the optimization of expitaxial growth. Therefore, InGaN/GaN heterostructures were grown under different conditions and investigated by CL and TEM. Experimental Epitaxial growth was performed in a standard molecular beam epitaxy (MBE) system equipped with Knudsen cells for gallium and indium and a radio frequency plasma source (EPI Unibulb) operated with pure nitrogen gas. C-plane sapphire substrates were first nitridated using the nitrogen G 3.33
Mat. Res. Soc. Symp. Proc. Vol. 537 ©1999 Materials Research Society
0 plasma followed by the growth of a GaN layer of variable thickness. Its growth temperature was 650 C for small layer thicknesses of 30 nm, 750'C for thicker layers and 830'C for cladding lay
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