Optical Properties of MBE Grown Cubic AlGaN Epilayers and AlGaN/GaN Quantum Well Structures
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Optical Properties of MBE Grown Cubic AlGaN Epilayers and AlGaN/GaN Quantum Well Structures D.J. As*, T. Frey*, M. Bartels*, A. Khartchenko*, D. Schikora*, K. Lischka*, R. Goldhahn**, S. Shokhovets**, * Universität Paderborn, FB-6 Physik, Warburger Strasse 100 D-33095 Paderborn, Germany, [email protected] ** TU Ilmenau, Institut für Physik, PF 100565, D-98684 Ilmenau, Germany ABSTRACT Cubic AlyGa1-yN/GaN heterostructures on GaAs(001) substrates were grown by radio-frequency plasma-assisted molecular beam epitaxy. High resolution X-ray diffraction, spectroscopic ellipsometry and cathodoluminescence were used to characterize the structural and optical properties of the alloy epilayers. X-ray diffraction reciprocal space maps demonstrate the good crystal quality of the cubic AlyGa1-yN films. Both SE as well as room temperature CL of the AlyGa1-yN epilayer show a linear increase of the band gap with increasing Al-content. A pseudomorphically strained cubic 10 x (2.4 nm GaN/ 4.8 nm Al0.12Ga0.88N) multi-quantum well (MQW) structure has been realized. Cathodoluminescence clearly demonstrates strong radiative recombination due to quantized states in the GaN well layer at a photon energy of 3.323 eV.
INTRODUCTION The first electroluminescence of cubic-phase GaN light emitting diodes (LEDs) grown on GaAs (001) substrates by molecular beam epitaxy (MBE) and metal organic chemical vapor deposition (MOCVD) has been reported recently by As et al. [1] and Yang et al. [2], respectively. For advanced optoelectronic and electronic devices, like light emitting diodes (LEDs), or laser diodes (LDs) Al containing cladding layers or barriers are necessary. The commonly used hexagonal AlyGa1-yN/GaN heterostructures show an inherently strong spontaneous polarization [3] oriented along the hexagonal c-axis as well as strain induced piezoelectric polarization. Such polarization induced electric fields in strained quantum wells can cause the spatial separation of electrons and holes resulting in a severe reduction of optical recombination efficiency. Using the metastable cubic modifications of AlyGa1-yN and GaN such piezoelectric effects can be avoided if the samples are grown in the (001) direction [4]. In this contribution we report on the optical properties of cubic AlyGa1-yN/GaN heterostructures and of a pseudomorphically strained cubic 10 x (2.4 nm GaN/ 4.8 nm Al0.12Ga0.88N) multi-quantum well (MQW) structure. The samples were grown by radiofrequency plasma-assisted molecular beam epitaxy (MBE) on GaAs (001) substrates and high resolution x-ray diffraction (HRXRD), spectroscopic ellipsometry (SE) and cathodoluminescence (CL) were used for characterization.
EXPERIMENTAL The cubic AlyGa1-yN/GaN films (0.07 < y < 0.20) and the Al0.12Ga0.88N/GaN MQW structure were grown on GaAs (001) substrates by rf-plasma assisted MBE. On top of a 350 nm thick G5.9.1
c-GaN layer 70 nm to 400 nm thick c-AlyGa1-yN films were deposited at a growth temperature of T = 835°C. The MQW structure (10 x (2.4 nm GaN/4.8 nm Al0.12Ga0.88N) was grown at T=835°C o
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