Chemical and electrical properties of interfaces between magnesium and aluminum and tris-(8-hydroxy quinoline) aluminum

The chemistry, electronic structure, and electron injection characteristics at interfaces formed between tris(8-hydroxy quinoline) aluminum (Alq₃) and magnesium (Mg) and aluminum (Al) are studied via x-ray photoemission spectroscopy, ultraviolet photoemission spectroscopy, and current-voltage (I-V) measurements. Both metal-on-Alq₃ and Alq₃-on-metal interfaces are investigated. All interfaces are fabricated and tested in ultrahigh vacuum in order to eliminate extrinsic effects related to interface contamination. The propensity for Mg and Al to form covalent metal-carbon bonds leads to broad and heavily reacted interfaces when the metal is deposited on the organic film. For this deposition sequence, we propose the formation of an organometallic structure where a single metal atom attaches to the pyridyl side of the quinolate ligand of the molecule and coordinates with an oxygen atom of another ligand or of a neighboring molecule. The other deposition sequence leads to significantly more abrupt interfaces where the chemical reaction is limited to the first molecular layer in contact with the metal surface. Both types of interface exhibit chemistry-induced electronic gap states, the position of which depends on the chemical structure of the interface. The interface width, chemical structure, and electronic states appear to play no significant role in electron injection in metal/Alq₃/metal sandwich structures, the I-V characteristics for top and bottom injection being identical over several decades of current.