"Contact resistance often limits the performance of nanoelectronic devices," explains research team leader Ali Javey of the University of California at Berkeley. "This is particularly true for ultrathin materials, such as semiconducting monolayers. We have now succeeded in electron doping few-layered tungsten selenide (WSe2) and molybdenum sulphide (MoS2) using surface charge transfer and show that we can obtain high electron densities in these materials."

Dichalcogenides are easily processed semiconducting films that might be used to make circuitry for low-power electronics, low cost or flexible displays, high-performance smart cards, sensors and even flexible electronics that can be coated onto a wide variety of surfaces. They have the chemical formula MX2, where M is a transition metal (such as Mo or W) and X is S, Se and Te. The materials go from being indirect bandgap semiconductors in the bulk to direct bandgap semiconductors when scaled down to monolayers – a property that could be ideal for making a variety of optoelectronics device applications such as light-emitting diodes and solar cells.

p and n polarities

Javey and colleagues have already managed to make high mobility n-type field-effect transistors (FETs) from the dichalcogenides they studied using their technique. This new research complements their previous work on making high mobility p-type FETs from WSe2, which means that this material can now be used to transport both holes and electrons. "This is the first 2D semiconductor (excluding graphene, which is a semimetal) to have been configured for both polarities, p and n," Javey told nanotechweb.org. "This is an important result since it will allow us to make high-performance CMOS devices based on a single material system."

The California team doped their dichalcogenides by evaporating potassium atoms on top of the 2D layers in a sealed vacuum chamber. Potassium strongly donates electrons to dichalcogenides – an effect that can be exploited to controllably dope the materials. The density of electrons in the sheets can also be controlled by the amount of potassium atoms covering the surface, explained Javey.

"We achieved high electron sheet densities of 1.0 x 1013 cm–2 and 2.5 x 1012cm–2 for MoS2 and WSe2 respectively," he added. "These values result in degenerate doping of both materials, something that is crucial for forming ohmic metal contacts in transistors and other devices made from the semiconductors."

No crystal damage

The mechanism behind surface charge transfer doping is completely different from that behind conventional semiconductor doping techniques, where dopant atoms are substituted for atoms in the semiconductor lattice. In contrast to this type of doping, no crystal damage occurs during surface charge transfer.

The n-doping scheme has allowed the researchers to explore how electrons travel in dichalcogenide FETs and discover how well these devices can actually perform. And while they used potassium atoms as a dopant, there is nothing to stop them from trying different molecules and atoms – especially those that remain stable over time.

Indeed, the team says that it is now going to look at making CMOS devices from WSe2 using different doping species and schemes.

The present work is published in Nano Letters.