Technical: This project aims for greater understanding of fundamental materials science issues in epitaxial growth and conduction mechanisms of thin-film heterostructures of conducting and insulating perovskite oxides that exhibit an anisotropic percolative conductivity transition tunable by film thickness, voltage and composition. The transition is thought to be similar to a metal-insulator transition but is additionally length dependent, involving electron localization triggered by a voltage bias. The approach includes transport measurements as a function of temperature and field (electrical and magnetic) under various conditions (film thickness, cell size, composition, electrode types) and material science synthesis and processing issues in materials combinations (electrodes, matrix:conductor, substrate) and to compare behaviors of materials combinations. Also, quantum mechanical modeling of electron transport along discrete elements consisting of contacts and conducting islands will be performed taking into account self energy, state occupancy (Fermi-Dirac) statistics and electrostatics (solving equivalent circuits made of essentially capacitors), and effects of charge trapping.
The project addresses basic research issues in a topical area of electronic/photonic materials science with high technological relevance. It is considered a high risk/high potential pay-off project. The interdisciplinary nature of the research and the combined experiment and modeling approach provide additional opportunities for students to broaden their educational experience.
Percolation is common phenomenon in nature, but how it manifests in the nano world is not known. The goal of this project is to explore percolation at the length scale of several to several tens of nanometers using conductor/insulator mixtures in the form of thin films. It especially focuses on atomic mixtures, of two conductor and insulator compounds from the same structure family, so that after mixing the composite still maintains the same structure. It is even possible to obtain an entire film that is a single crystal. The expectation is that these films will behave as a new conductor or a new insulator with properties not known in ordinary conductors or insulators. Such films may find applications in electronic devices. In this project, we have succeeded in making such mixture films and discovered that the size matters: whether the film is conducting or insulating depends on the thickness. Moreover, such films have properties not known before: a conducting film can be uniformly converted to an insulating film when a critical voltage is applied, and the reverse transformation occurs under an opposite voltage. This opens the possibility of using such films as electronic switches and memories. The project also trained three PhD, one already graduated and working in the US industry, to become experts in the design, fabrication and characterization of thin films and electronic devices. The project further provided research opportunities for one master student and several undergraduates. Finally, this research opens a new avenue for further exploration into electronically percolating nano materials which promise to be a fertile ground for inventions.
