Epitaxial integration of polar oxides with wide band gap polar semiconductors presents the possibility of tunable 2D charge carriers at polar interfaces and integration of non-linear dielectric properties. To achieve such coupling, defect densities must be low, and microstructures must have minimal roughness. However, conventional growth techniques fail to achieve this in materials with such highly dissimilar structure and symmetry. For example, the terminal polar 3D rocksalt surface is determined by the tendency for the highenergy (111) polar orientation to form (100)-oriented low-energy facets. Therefore, realizing smooth polar oxide films on GaN requires the ability to overcome this (100)-faceting tendency. In this presentation we will discuss a new surfactant approach to MBE and PLD growth of rocksalt oxides (MgO, CaO) on GaN, where water vapor is utilized during growth to hydroxylate the rocksalt (111) surfaces, changing the equilibrium habit from cubic to octahedral, eliminating the (100)-faceting tendency. We demonstrate unambiguously using RHEED that surfactant incorporation enables a 2D growth mode of (111) CaO and MgO thin films with a step-and-terrace morphology. For both MgO and CaO, temperature dependent ab initio thermodynamic surface energy calculations predict the experimentally observed temperature and pressure window in which 2D growth occurs. Additionally, solid solutions between MgO and CaO by both PLD and MBE that offer perfect lattice match to (0002) GaN are presented. In all cases, epitaxy stabilizes the system against phase separation and RHEED shows layer-by-layer growth. TEM analysis of defects of the lattice-matched interface will be presented. Current efforts focus on samples designed to probe the possibility of conductive oxide-GaN interfaces. Collectively, these results demonstrate that one can rationally engineer surface chemistry during growth and create a local equilibrium promoting a specific crystallographic habit and growth mode otherwise unavailable. Demonstrating this using two materials and two growth techniques suggests the generic nature of this methodology. The utility of this method is illustrated by electrical property measurements that reveal drastically reduced leakage current densities (approximately 1000X lower) for surfactant-assisted films as compared to those grown using conventional means. Finally, current work is now extending surfactant work to include the BaTiO3/GaN interface. Previously, we have demonstrated growth of high quality, epitaxial BT on GaN and recent PFM results suggest coupling between the polarizations across the BT/GaN interface.
E.A. Paisley,1 M. Biegalski,2 J.M. Lebeau,1 B. Gaddy,1 S. Mita,1 R. Collazo,1 Z. Sitar,1 and D. Irving1