A focused international research collaboration linking North Carolina State University, the Technical University Berlin (TUB) and the Leibniz-Institut für Kristallzüchtung (IKZ) in Berlin, Germany, explores the fundamental science of epitaxial interfaces that define a discontinuity in electrical polarization in nitride-nitride and nitride-oxide systems. The aim is to understand the structural, chemical, and electronic features that regulate the interfacial electronic properties. The effort is based on a hypothesis that vertical and lateral heteropolar interfaces that approach 'semiconductor-grade' quality can be prepared, and that their study will lead to exciting opportunities for fundamental advances in interface physics and optic/electronic functionality. Each member of the international team brings a unique and leading edge technical capability that collectively enables this research. The Sitar group (NC State) brings a metalorganic chemical vapor deposition technology that can control polarity in thin films of nitride semiconductors both laterally and vertically; the Maria group (NC State) developed a surfactant-assisted molecular beam epitaxy technique that can for the first time prepare cubic oxides on GaN surfaces with a 2-D growth mode and dramatically reduced defect concentrations; the Hoffmann group (TUB) brings in optical characterization of polar materials and heterostructures; and the Albrecht group (IKZ) brings aberration-corrected transmission electron microscopy characterization with sub-Å spatial resolution and sub-eV energy resolution. The research refines synthesis techniques that enable hybrid thin film structures that seamlessly merge oxides and nitrides with heretofore unachievable material quality. By combining synthesis and characterization the investigators are able to identify and understand the structural and chemical features that enable, interfere, or enhance heteropolar interface coupling.
Achieving an understanding of heteropolar interface-property relationships will propel research in hybrid systems that lead to new physics, new functionality, an expanded set of low dimension optoelectronic devices, and a pathway to smart structures that utilize in a new way the non-linear dielectric response of ferroelectrics. Through this collaboration, students and faculty from NCSU, TUB, and IKZ have access to mentoring expertise and world-class laboratory facilities in technical areas that are absent from the home institutions. U.S. and German students participate in a 6-month graduate sabbatical in program year three to ensure substantive international experience. During their first two years, U.S. students are encouraged take two semesters of German to acquire basic communication skills. The work of the German participants is supported by the German Research Foundation (DFG).
