In a development that could lead to more advanced computer chips and light-emitting diodes, researchers from the University of California Los Angeles have created a new method for building semiconductor devices that eliminates efficiency-sapping atomic-scale defects.
Standard methods used to create semiconductor devices produce tiny defects that can trap electrons passing in between the semiconductor and its neighbouring metal electrodes, which makes these devices less effective than they might be in theory.
The electrodes in semiconductor-based devices allow electrons to go to and from the semiconductor, carrying computing data or energy with them.
Typically, metal electrodes in semiconductor devices are created using a method known as physical vapor deposition.
In this technique, metallic materials are vaporized into atoms or atomic groupings that then settle onto the semiconductor, which can be silicon or a similar material.
The metal atoms adhere to the semiconductor through robust chemical bonds, ultimately developing a thin film of electrodes on top of the semiconductor.
One problem with that method is the metal atoms are generally different sizes or shapes from the atoms in the semiconductor materials that they’re connecting to.
Consequently, the layers cannot form ideal one-to-one atomic contacts, which is why little gaps or defects appear. Those gaps can snare electrons passing across them, and the electrons need more energy to get through those places.
Study author Yu Huang, professor of materials science and engineering at UCLA, compared the problem of trying to combine different atoms to trying to fit Lego blocks with similar plastic blocks from a competitor.
You can force the two different blocks together, but the fit will not be perfect. With semiconductors, those imperfect chemical bonds lead to gaps where the two layers join, and those gaps could extend as defects beyond the interface and into the materials.”
Yu Huang, Study Author
According to a new report published in the journal Nature, the UCLA team was able to develop a technique that prevents defects from developing by placing a thin sheet of metal atop the semiconductor layer through a basic lamination sequence.
Rather than using chemical bonds to keep the two parts together, the new process uses weak electrostatic connections know as van der Waals forces, which are triggered when atoms are very close to one another.
Van der Waals forces are weaker than chemical bonds, but they’re sufficiently strong to keep the materials together in a semiconductor device due to how thin they are; 10 nanometres thick or less.
Even though they are different in their geometry, the two layers join without defects and stay in place due to the van der Waals forces.”
Yu Huang, Study Author
In addition to being an effective solution to a nagging problem, the new research finally confirms a scientific theory that was first put forward in the 1930s.
The Schottky-Mott rule proposed the lowest amount of energy electrons must have to move between a metal and a semiconductor under suitable conditions.
Making use of the theory, engineers ought to be able to pick a metal that permits electron travel across the connection between metal and semiconductor with the lowest required quantity of energy.
But due to those small defects that have always appeared during manufacturing, semiconductor devices have always required electrons with more energy than the theoretical minimum.
The UCLA team is the first to validate the Schottky-Mott rule in trials with various combinations of metals and semiconductors. Since the electrons didn’t have to overcome the usual defects, they could travel with the minimum quantity of energy expected by the Schottky-Mott rule.
Our study for the first time validates these fundamental limits of metal–semiconductor interfaces. Broadly, this can be applied to the fabrication of any delicate material with interfaces that were previously plagued by defects.”
Xiangfeng Duan, Study Author