Cathodoluminescence of MBE-grown cubic AlGaN/GaN multi-quantum wells on GaAs (001) substrates
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Cathodoluminescence of MBE-grown cubic AlGaN/GaN multi-quantum wells on GaAs (001) substrates D. J. As, S. Potthast, U. Köhler, A. Khartchenko and K. Lischka, University of Paderborn, Faculty of Science, Department of Physics, Warburger Strasse 100, D-33095 Paderborn, Germany, [email protected]
ABSTRACT Cubic phase GaN/AlxGa1-xN Multi Quantum Well structures were grown by rf-plasma assisted molecular beam epitaxy (MBE) on GaAs (001) substrates. X-ray measurements showed a high phase purity of the epilayers and revealed an Aluminum incorporation between 9 % and 49 %, respectively. The QW luminescence was tuned between 3.25 eV and 3.4 eV by means of the variation of QW barrier Aluminum content and QW width. Strong Cathodoluminescence (CL) from the GaN QWs and the underlying cubic AlxGa1-xN bulk material was observed at room temperature. The spatial localization of the QW emission was unambiguously determined by depth-resolved CL measurements. Combined with a model of energy-dependent penetration, diffusion, and recombination, these variations indicate a value of about 20 nm for the minority carrier diffusion length within the AlxGa1-xN confinement layer. The assignment of AlxGa1-xN bulk and GaN luminescence was further supported by employing a simple effective-mass quantum mechanical model.
INTRODUCTION In optoelectronic and microelectronic applications of group-III nitrides, heterostructures incorporating AlxGa1-xN and GaN epilayers are important ingredients in, e.g. lasers, LEDs and high electron mobility transistors (HEMTs). However, the commonly used hexagonal AlxGa1-xN/GaN heterostructures show an inherently strong spontaneous polarisation oriented along the hexagonal c-axis as well as strain induced piezoelectric polarization. Such polarization induced electric fields have a detrimental effect on the electrical and optical characteristics of GaN-based quantum well (QW) devices with wurtzite lattice configuration and increases the threshold current and redshifts the emission wavelength in laser diodes [1-3].The absence of built-in field in these structures will improve the performance and will significantly simplify the design of such devices. The metastable cubic phase of group III nitrides offers the possibility to avoid these problems by growing the samples on (001) oriented substrates. In this contribution we report on the optical properties of cubic AlxGa1-xN/GaN multi quantum well (MQW) structures. The samples were grown by radio-frequency plasma-assisted molecular beam epitaxy (MBE) on GaAs (001) substrates and depth resolved cathodoluminescence (CL) was used for optical characterization. The spatial localization of the QW emission as well as the minority diffusion length were unambiguously determined by depthresolved CL measurements.
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EXPERIMENTAL Multi Quantum Well (MQW) structures of cubic phase AlxGa1-xN barriers and GaN wells were grown on GaAs (001) substrates in a Riber 32 MBE system using a RF plasma-assisted Nitrogen source. Solid source Knudsen cells were used in order to s
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