Interface and Luminescence Properties of Pulsed Laser Deposited Mg x Zn 1-x O/ZnO Quantum Wells with Strong Confinement
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0957-K07-23
Interface and Luminescence Properties of Pulsed Laser Deposited MgxZn1-xO/ZnO Quantum Wells with Strong Confinement Susanne Heitsch, Gregor Zimmermann, Alexander Müller, Jörg Lenzner, Holger Hochmuth, Gabriele Benndorf, Michael Lorenz, and Marius Grundmann Institut für Experimentelle Physik II, Universität Leipzig, Linnéstr. 5, Leipzig, 04103, Germany
ABSTRACT MgxZn1−xO/ZnO/MgxZn1−xO quantum wells (QWs) (0.12 ≤ x ≤ 0.15) have been grown on aplane sapphire substrates by pulsed laser deposition. The nominal ZnO well layer thickness lies between 1.2 nm and 6 nm. Atomic force microscopy (AFM) investigations at ZnO/MgxZn1−xO heterostructures show the film-like structure of the ZnO layers. Their root mean square surface roughness of ~ 0.5 nm gives information about the interface roughness in the QWs. AFM results from the MgxZn1−xO barrier layers show the same surface structure and roughness. We confirmed the lateral homogeneity of the Mg distribution in the MgxZn1−xO barrier layers by scanning cathodoluminescence measurements. The QWs show a bright and laterally homogeneous luminescence, suggesting good crystalline quality of the ZnO wells. The measured QW photoluminescence energies agree well with calculated values and display the presence of the quantum-confined Stark effect. As a result of quantum confinement a high-energy shift of the ZnO excitonic photoluminescence of 222 meV is observed in the thinnest QW. INTRODUCTION MgxZn1-xO/ZnO quantum wells (QWs) have attracted many scientists’ interest in the last years, because they represent structures which can emit UV light of definite wavelength. Furthermore, they are advantageous over GaN based structures, because ZnO possesses not only strong radiation hardness, but also a large exciton binding energy of 60 meV. This leads to the possibility to observe and use excitonic effects at room temperature. In MgxZn1-xO/ZnO QWs a high-energy shift of the excitonic emission from ZnO and an increase of the exciton binding energy can be observed due to quantum confinement [1]. However, due to built-in electric fields in the structure, the quantum-confined Stark effect (QCSE) counteracts these effects in QWs with sufficiently large well widths [2, 3]. The production of MgxZn1-xO/ZnO QWs holds two major challenges which have to be coped with in order to achieve structures which are suitable for the use in optoelectronic devices. These are the preparation of alloy barrier layers with laterally homogeneous Mg content and the formation of smooth interfaces within the structure, in order to possess a laterally homogeneous QW depth throughout the structure. Despite these requirements, the interface roughness and particularly the lateral Mg distribution in MgxZn1-xO/ZnO QWs have only rarely been studied [26]. Although there are numerous reports about the optical and structural properties of MgxZn1-xO/ZnO QWs grown by molecular beam epitaxy [1], corresponding structures grown by pulsed laser deposition (PLD) have hardly ever been reported [7, 8]. Krishnamoorthy et al. [7]
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