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Ultralow contact resistance at an epitaxial metal/oxide heterojunction through interstitial site doping


06 May 2013 

Scott A. Chambers, Meng Gu, Peter V. Sushko, Hao Yang, Chongmin Wang, Nigel D. Browning, Advanced Materials (published online).

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.

fig_13_AdvMat_CrSTO_TEM

TEM image of Cr/SrTiO3.

The collaboration of researchers from the University Colloge London, 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.

fig_13_AdvMat_CrSTO_DFT

Electrons near the Fermi level in Cr/SrTiO3.

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 (SrTiO3), 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.

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.