UCLA scientists and engineers have developed a new process for assembling semiconductor devices. The advance could lead to much more energy-eﬃcient transistors for electronics and computer chips, diodes for solar cells, light-emitting diodes, and other semiconductor-based devices.
Their method joins a semiconductor layer and a metal electrode layer without the atomic-level defects that typically occur when other processes are used to build semiconductor-based devices. Even though those defects are minuscule, they can trap electrons traveling between the semiconductor and the adjacent metal electrodes, which makes the devices less eﬃcient than they could be. The electrodes in semiconductor-based devices are what enable electrons to travel to and from the semiconductor; the electrons can carry computing information or energy to power a device.
Generally, metal electrodes in semiconductor devices are built using a process called physical vapor deposition. In this process, metallic materials are vaporized into atoms or atomic clusters that then condense onto the semiconductor, which can be silicon or another similar material. The metal atoms stick to the semiconductor through strong chemical bonds, eventually forming a thin ﬁlm of electrodes atop the semiconductor. One issue with that process is that the metal atoms are usually different sizes or shapes from the atoms in the semiconductor materials that they’re bonding to. As a result, the layers cannot form perfect one-to-one atomic connections, which is why small gaps or defects occur. Those defects trap electrons traveling across them, so the electrons need extra energy to get through those spots.
The UCLA method prevents the defects from forming, by joining a thin sheet of metal atop the semiconductor layer through a simple lamination process. Instead of using chemical bonds to hold the two components together, the new procedure uses van der Waals forces — weak electrostatic connections that are activated when atoms are very close to each other — to keep the molecules “attached” to each other. Van der Waals forces are weaker than chemical bonds, but they’re strong enough to hold the materials together because of how thin they are — each layer is around 10 nanometers thick or less.
The research is the ﬁrst work to validate a scientiﬁc theory that originated in the 1930s. The Schottky-Mott rule proposed the minimum amount of energy electrons need to travel between metal and a semiconductor under ideal conditions. Using the theory, engineers should be able to select the metal that allows electrons to move across the junction between metal and semiconductor with the smallest amount of energy. But because of those tiny defects that have always occurred during manufacturing, semiconductor devices have always needed electrons with more energy than the theoretical minimum.
The UCLA team is the ﬁrst to verify the theory in experiments with different combinations of metals and semiconductors. Because the electrons didn’t have to overcome the usual defects, they were able to travel with the minimum amount of energy predicted by the Schottky-Mott rule.
Besides electrode contacts on semiconductors, it could be used to assemble ultra-energy-eﬃcient nanoscale electronic components, or optoelectronic devices such as solar cells.