Cleaner junctions for higher efficiency

A crucial step in making electronic components is to join metals and semiconductors together, with an efficient junction between them. An international team including Xiangfeng Duan at the University of California, USA, and Imran Shakir at King Saud University, Saudi Arabia, have developed some of the smoothest-ever metal-semiconductor junctions, which show exceptional performance for use in transistors and photodiodes¹.

A metal semiconductor junction presents an energy barrier to electron flow. In theory, the height of this barrier, called the Schottky-barrier, can be predicted based on how electron energy levels align across the junction. However, most junctions don’t come near this ideal situation in reality, due to chemical disorder at the surfaces and an effect called Fermi-level pinning, whereby electrons are trapped at a nearly fixed position within the band gap. 

This results in greater resistance to current flow, consuming extra energy.

Duan, Shakir and co-workers realised that commonly-used methods of building electronic components such as high-energy deposition, lithography and evaporation of metals were likely to contribute to disorder formation at junctions, because they introduce strain, impurities and defects. Instead, the team prepared atomically flat metal films that they simply laid down onto a flat semiconducting layer of molybdenum disulfide. 

The two flat surfaces are laminated perfectly, held in place only by weak intermolecular van der Waal’s forces, and the metal could even be peeled off without damaging either surface (see image).

The researchers showed that their junctions perform very close to the Schottky-Mott limit, a theory first predicted in the 1930s. By adding metal films to the semiconductors in various combinations, they have also built several types of transistor and photodiode structures that show promising energetic properties. 

In their paper, published in Nature, the researchers say "our study not only experimentally validates the fundamental limit of ideal metal–semiconductor junctions, but also defines a highly efficient and damage-free strategy for metal integration that could be used in high-performance electronics and optoelectronics."

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