Cathodoluminescence Study of Hydrothermal Zn 1-x Mg x O Alloy Crystals
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Cathodoluminescence Study of Hydrothermal Zn1-xMgxO Alloy Crystals Julio Mass1,2, Manuel Avella1, Juan Jiménez1, Michael Callahan3, E. Grant3, K. Rakes3, David Bliss3, and Buguo Wang4 1 Física de la Materia Condensada, Universidad de Valladolid, E.T.S.I.I, Paseo del Cauce s/n, Valladolid, 47011, Spain 2 Dpto. Matemáticas y Física, UniNorte, UniNorte, Km5, Barranquilla, Colombia 3 Sensors Directorate, Air Force Research Laboratory, Hanscom AFB, MA, MA 01731 4 Solid State Scientific Corporation, Hollis, NH, NH 03049 ABSTRACT Hydrothermal ZnMgO crystals were studied by cathodoluminescence. The high energy shift of the excitonic luminescence demonstrates the Mg incorporation in the ZnO lattice at a few percent, estimated at ≈3%. The spectral parameters of the luminescence emission show a marked dependence of the incorporation of Mg and defects on the growth facet. This growth sector selectivity shows similar trends to those observed in hydrothermal ZnO crystals. The depth distribution of Mg was studied varying the acceleration voltage of the excitation e-beam, showing a slight gradient of Mg towards the surface. INTRODUCTION ZnO has received a renewed interest in view of its application for UV optoelectronics (1). A crucial step for the development of ZnO based optoelectronic devices is band gap engineering, which can be achieved by alloying ZnO with other metallic oxides. The two candidates to engineer the band gap of ZnO are MgO to enlarge the bandgap (2), and CdO to shrink it (3). MgZnO is the candidate of choice to achieve the confinement barriers in ZnO quantum wells (QW). The band gap of MgZnO alloys increases linearly with the Mg atomic fraction up to an atomic fraction of 0.36 (4). Most of the work carried out concerns thin films grown by different methods (5,6). Little has been done on the hydrothermal (HTT) growth of ZnMgO alloyed crystals (7). One of the reasons is the low solid solubility of MgO in ZnO, because HTT is an equilibrium process the incorporation of Mg is limited to a few percent (4%), unlike the metastable phase of up to 36% Mg that can be grown epitaxially. Hydrothermal growth has been demonstrated as a technique useful for mass production of ZnO crystals for the production of substrates suitable for ZnO and GaN based optoelectronics. The potential interest of large ZnMgO crystals lies in the possibility to obtain alloys with uniform composition, fully lattice matched to GaN, (8) and their role as transparent substrates for both ZnO and GaN optoelectronics. We present herein a cathodoluminescence (CL) study of HTT ZnMgO crystals. In this work we study the incorporation of Mg, paying special emphasis to the influence of faceting. Also, the presence of deep levels responsible for visible luminescence is analyzed; the results are compared to those obtained for HTT pure ZnO crystals. SAMPLES AND EXPERIMENTAL SET-UP The HTT samples were grown at the AFRL laboratory in Hanscom. The hydrothermal growth of ZnO can be achieved at low temperature between 150 and 400ºC. However, the growth of
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