Growth of quaternary AlInGaN/GaN Heterostructures by Plasma Induced Molecular Beam Epitaxy with high In Concentration
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Growth of quaternary AlInGaN/GaN Heterostructures by Plasma Induced Molecular Beam Epitaxy with high In Concentration A. P. Limaa, C. R. Miskysa, L. Görgens a, O. Ambachera, A. Wenzelb, B. Rauschenbachc, and M. Stutzmanna a
Walter Schottky Institut, Technische Universität München, Garching, Germany Institut für Physik, Universität Augsburg, Augsburg, Germany c Institut für Experimentalphysik II, Universität Leipzig and Institut für Oberflächenmodifizierung Leipzig, Leipzig, Germany b
ABSTRACT
Growth of AlInGaN/GaN heterostructures on sapphire substrates was achieved by plasma induced molecular beam epitaxy. Different alloy compositions were obtained by varying the growth temperature with constant Al, In, Ga and N fluxes. The In content in the alloy, measured by Rutherford Backscattering Spectroscopy, increased from 0.4% to 14.5% when the substrate temperature was decreased from 775 to 665 °C. X-Ray reciprocal space maps of asymmetric AlInGaN (2.05) reflexes were used to measure the lattice constants and to verify the lattice match between the quaternary alloy and the GaN buffer layers. INTRODUCTION
Theoretically, the growth control of the quaternaries AlInGaN would allow an independent variation of the lattice constant and the band gap. The possibility to obtain AlInGaN/GaN lattice matched structures is also interesting in order to study and explore the strong piezoelectric effect and the spontaneous polarization present in III-V nitride structures.[1-3] Nevertheless, the epitaxial growth of such layers is a very difficult task, because Al containing layers normally require much higher growth temperatures than In compounds. Indium has a relatively high vapor pressure, therefore the growth temperature must be lowered in order to increase the indium incorporation and to reduce the thermal dissociation of In-N bonds. AlInGaN films grown by MOCVD were already obtained by few groups. [2, 4-8] These quaternary layers were deposited on GaN buffer layers grown on sapphire [4-7] or 6H-SiC [2,6] substrates at fixed growth temperatures ranging from 750 °C to 1000 °C. The alloy composition variation was obtained by changing the metal flux. The aim of this article is to describe the growth conditions to obtain AlInGaN on a GaN buffer layer by Molecular Beam Epitaxy. EXPERIMENTAL
Our approach to grow the series of quaternary samples consisted keeping the flux of Ga, In and Al as well as the nitrogen plasma conditions constant and varying the growth temperature in order to achieve different alloy compositions. The fluxes of the Al, In and Ga for the quaternary growth were chosen such that the total flux corresponds to the flux of Ga for a GaN optimized Ga-rich growth (φmetal = 1015cm-2s-1). The heterostructure consist of a thin high temperature AlN nucleation layer (≈ 5 nm), to ensure Ga-face polarity growth on cAl2O3 substrates,[1] followed by 500 nm GaN and 130 to 230 nm of AlxInyGa1-x-yN. The growth were carried out using a Tectra MBE chamber equipped with conventional effusion cells for Ga, In, Al, Si, Mg and an Oxford Ap
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