Material Controls Excitons at Room Temperature

A material that can control excitons — bound pairs of electrons and electron holes — at room temperature could enable optoelectronic devices for commercial applications.

Excitons are created when a laser is shone onto a semiconductor device. They can transport energy without transporting net electric charge. Inside the device, excitons interact with each other and their surroundings, and then convert back into light that can be detected by sensitive CCD cameras.

Previously, researchers from the University of California, San Diego, and the University of Manchester had been working on structures based on gallium arsenide (GaAs), a material commonly used in the semiconductor industry. Unfortunately, the team said the devices they developed had a fundamental limitation: They required cryogenic temperatures (below 100 K) to operate, which ruled out commercial applications.

Now the team has reported a material change that should bring the excitonic devices up to room temperature.

"Our previous structures were built from thin layers of GaAs deposited on top of a substrate with a particular layer thickness and sequence to ensure the specific properties we wanted," said UCSD graduate student Erica Calman.

Erica Calman and Chelsey Dorow align optics required to collect measurements from a molybdenum disulfide sample. Courtesy of Calman.

To make the new devices the physicists turned to new structures built from a specially designed set of ultrathin layers of materials: molybdenum disulfide (MoS₂) and hexagonal boron nitride (hBN), each a single atom thick.

The structures were produced via the famous "Scotch tape" or mechanical exfoliation method developed by the group of Andre Geim, a physicist awarded a Nobel Prize in physics in 2010 for his groundbreaking work on the 2D material graphene.

"Our specially designed structures help keep excitons bound more tightly together so that they can survive at room temperature — where GaAs excitons are torn apart," Calman said.

Excitons can form a special quantum state known as a Bose-Einstein condensate, which occurs within superfluids and enables currents of particles without losses. The team discovered a similar exciton phenomenon at cold temperatures with GaAs materials.

"The results of our work suggest that we may be able to make new structures work all the way up to room temperature," Calman said. "We set out to prove that we could control the emission of neutral and charged excitations by voltage, temperature, and laser power ... and demonstrated just that."

The research was published in Applied Physics Letters (doi: 10/10.1063/1.49432).