By Josh Perry, Editor
[email protected]

Researchers at Monash University (Australia) and the U.S. Department of Energy (DOE) Lawrence Berkeley National Laboratory (Berkeley, Calif.) demonstrated, for the first time, room temperature electrical switching in ultrathin sodium bismuthide (Na₃Bi), a topological Dirac semimetal, which could be a breakthrough for next-generation transistors.

 James Collins, a researcher at Monash University in Australia, works on an experiment at Beamline 10.0.1, part of Berkeley Lab’s Advanced Light Source. (Marilyn Chung/Berkeley Lab)

According to a report from Berkeley Lab, topological materials are considered a potential advancement in transistor technology because they reduce energy loss and power consumption at room temperature – unlike superconductors that require ultracold temperatures to operate.

The material was grown using X-rays at the lab’s Advanced Light Source (ALS) and was easily switched from a conducting state to an insulating state with a low voltage. Monash University researchers developed the means for growing single layers of the material in a honeycomb pattern.

“In the latest study, researchers grew the material samples, measuring several millimeters on a side, on a silicon wafer under ultrahigh vacuum at the ALS Beamline 10.0.1 using a process known as molecular beam epitaxy,” the article explained. “The beamline allows researchers to grow samples and then conduct experiments under the same vacuum conditions in order to prevent contamination.”

The material became conductive when an electrical field was applied to it and a slightly higher electrical field caused the material to switch to insulating. Using electricity to create these changes is important because previous research relied on chemical doping or mechanical strain.

The research was recently published in Nature. The abstract read:

“The electric-field-induced quantum phase transition from topological to conventional insulator has been proposed as the basis of a topological field effect transistor. In this scheme, ‘on’ is the ballistic flow of charge and spin along dissipationless edges of a two-dimensional quantum spin Hall insulator, and ‘off’ is produced by applying an electric field that converts the exotic insulator to a conventional insulator with no conductive channels.

“Such a topological transistor is promising for low-energy logic circuits, which would necessitate electric-field-switched materials with conventional and topological bandgaps much greater than the thermal energy at room temperature, substantially greater than proposed so far. Topological Dirac semimetals are promising systems in which to look for topological field-effect switching, as they lie at the boundary between conventional and topological phases.

“Here we use scanning tunnelling microscopy and spectroscopy and angle-resolved photoelectron spectroscopy to show that mono- and bilayer films of the topological Dirac semimetal Na₃Bi are two-dimensional topological insulators with bulk bandgaps greater than 300 millielectronvolts owing to quantum confinement in the absence of electric field.

“On application of electric field by doping with potassium or by close approach of the scanning tunnelling microscope tip, the Stark effect completely closes the bandgap and re-opens it as a conventional gap of 90 millielectronvolts. The large bandgaps in both the conventional and quantum spin Hall phases, much greater than the thermal energy at room temperature (25 millielectronvolts), suggest that ultrathin Na₃Bi is suitable for room-temperature topological transistor operation.”