Electronic Structures and Nature of Host Excitation in Gallates
- PDF / 109,287 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 116 Downloads / 237 Views
Electronic Structures and Nature of Host Excitation in Gallates P. C. Schmidt+, J. Sticht+, M. Stephan+, V. Eyert++ and K. C. Mishra* + Institut für Physikalische Chemie,Technische Universität Darmstadt, Darmstadt, Germany ++ Institut für Physik, Universität Augsburg, Augsburg, Germany * Research and Development, OSRAM SYLVANIA, Beverly, MA Dedicated to Dr. Franz Kummer on the Occasion of his 60th Birthday ABSTRACT It is an interesting exercise in materials science to explore simple rules relating the electronic properties of ternary systems to those of their binary constituents. In the present work, we have investigated the electronic structures of the large band gap gallates MGa2O4 (M=Mg, Ca, Ba and Zn) and the corresponding binary oxides MO and Ga2O3. Using first-principles band structure methods, we find that the metal atoms in MO control the width of the O 2p-like valence band and the size of the optical band gap. Covalent metal-oxygen bonding is much more pronounced in Ga2O3 and leads to characteristic structure in the valence band density of states. These basic features are retained in the ternary compounds where the covalent admixture to the chemical bond is largest between Ga and O, and the transitions across the band gap involve the Ga2O3 sublattice. INTRODUCTION There has been resurgence of interest in gallates, particularly zinc gallate, for application in thin film electro luminescent devices (TFEL), vacuum fluorescent displays (VFD), field emission display (FED) utilizing low-voltage cathode luminescence [1]. Gallates in M2O-Ga2O3 and MO-Ga2O3 systems activated by divalent manganese was first investigated by Hoffman and Brown [2]. Magnesium gallate activated by manganese was used for a while as a green phosphor in specialty lamps. Recently zinc gallate has gained much attention as a blue emitter and manganese activated zinc gallate as a green emitter for VFDs and TFEL devices [1]. Understanding the electronic properties of the ternary oxides like MgGa2O4, ZnGa2O3 and CaGa2O4 from those of their binary building blocks CaO, MgO, ZnO and Ga2O3 is of importance from the perspective of materials design for specific application. This issue has already been the subject of our previous studies on a class of ternary oxides containing anionic groups with strong intra-atomic covalent bonding [3-5]. There we showed that the band gap transitions in the ternary compounds do not necessarily involve the highest occupied molecular orbitals (HOMO) and the lowest unoccupied molecular orbitals (LUMO) of the anionic groups. In the present work, we report results from electronic structure calculations of the ternary gallates MGa2O4 (M=Mg, Ca, Ba and Zn) and their binary building blocks, MO and Ga2O3. In particular, we investigate how the HOMO-LUMO transitions change on going from the binary compounds MO and Ga2O3 to the ternary compounds. A systematic understanding of the correlations among the binary oxides and higher order oxides is helpful for predicting physical and chemical properties of more complex systems, and for e
Data Loading...
