A pathway to creating low-resistance Ohmic contacts at nanoscale has been reported in Advanced Materials this week; a development which is of critical importance to the on-going advancement of oxide electronics.
The collaboration of researchers from the London Centre for Nanotechnology (LCN), the Pacific Northwest National Laboratory (PNNL) and the University of California, Davis, has worked to determine the physical drivers required to facilitate formation of an “Ohmic contact”, which is highly desirable type of an electrical junction, between a metal and an oxide semiconductor.
When a metal and a semiconductor are joined, there are two possible types of contact that can result. An “Ohmic contact”, in which electrical current can pass in either direction, and a “Schottky barrier”, in which case the current has preferential direction.
The type of contact depends on the combination of metal and semiconductor used but the vast majority of metals form Schottky barriers when deposited on oxide surfaces. Not only do Ohmic contacts rarely occur but also little is known at an atomistic level about what leads to a good Ohmic contact on a wide-gap oxide.
The study, which reports both experimental and theoretical results, suggests that an overlayer of Chromium metal deposited on the (001) surface of Niobium-doped strontium titanate (SrTi03), an oxide semiconductor of considerable interest in science and technology, forms a low-resistance Ohmic contact. It is revealed that in-diffusion of metal atoms into the first few atomic planes of the oxide is of critical importance to both anchoring the overlayer of Chromium for good adhesion and metalizing the oxide surface for very low contact resistance. These results provide a new strategy for generating Ohmic contacts in other metal/oxide interfaces as well as optimizing their characteristics.
Dr Peter Sushko, one of the authors of the paper, commented “We think this effect of in-diffused metal atoms is generic and can manifest itself in many other interfacial phenomena”.
It is believed that the principle of near surface doping by incoming metal atoms will have a great impact in the overall field of oxide electronics and provides a new degree of freedom in materials design.
Journal link: Chambers, S. A., Gu, M., Sushko, P. V., Yang, H., Wang, C. and Browning, N. D. (2013), Ultralow Contact Resistance at an Epitaxial Metal/Oxide Heterojunction Through Interstitial Site Doping. Adv. Mater.. doi: 10.1002/adma.201301030
Figure: In-diffused Cr atoms stabilise and dope with electrons the interface between a thin Cr film and SrTiO3 substrate.