Growth and structure of internal Cu/Al2O3 and Cu/Ti/Al2O3 interfaces

Thin Cu films and Cu/Ti bilayers were grown on (0001)α-Al2O3 by molecular beam epitaxy (MBE) at substrate temperatures ranging from 373 K to 473 K. Results on growth behaviour, low-energy interface orientation relationships and the nature of bonding at the interfaces are reported. Transmission electron microscopy (TEM) and in-situ reflection high-energy electron diffraction (RHEED) studies show that Cu/Ti bilayers nucleate and grow epitaxially on the Al2O3 substrates with an (111)Cu〈110〉Cu∥(0001)Ti〈21̄1̄0〉Ti∥(0001)Al2O3 〈101̄0〉Al2O3 orientation relationship. In contrast, Cu films on (0001)α-Al2O3 grow in two stages. Initially, the Cu nuclei have random orientation but they realign with increasing film thickness into an heteroepitaxial (111)Cu〈110〉Cu∥(0001)Al2O3〈101̄0〉Al2O3 orientation relationship. In high-resolution transmission electron microscopy (HRTEM) no reaction phases are observed at the atomically sharp Cu/Al2O3 and Ti/Al2O3 interfaces. Electron energy-loss spectroscopy (EELS) of the interfaces reveals bonding between copper or titanium and the oxygen sublattice of Al2O3, The interfacial widths of copper and titanium atoms involved in the bonding are estimated using the characteristic absorption edge data. In the case of Ti/Al2O3 the interfacial width is estimated to be 0.5 nm±0.1 nm. The lower affinity of Cu to oxygen results in a smaller interfacial width of 0.34 nm±0.06 nm. These values correspond to the apparent fraction of the metal layers that participate in the bonding with the oxygen sublattice of (0001)α-Al2O3.