Chips Hit a New Low with ‘Valleytronics’

By George Leopold

October 30, 2019

A potential interim step between conventional semiconductors and quantum devices has emerged, promising improved information processing schemes that outperform current electronic charge- and spin-based chip architectures.

The emerging quantum process dubbed “valleytronics” focuses on low energy “valleys” or extremes in the electronic band structure of semiconductors. Those valleys of electrons can be used to encode, process and store information, according to researchers at the University of California at Riverside.

The researchers focused on a two-dimensional chip substrate based on tungsten diselenide  (WSe2) with two distinct electronic valleys. Among the semiconductor material’s properties is a hexagonal crystalline structure.

“Excited” electronics tend to relax and accumulate in those valleys, the university researchers note. The resulting valley indices can be used to encode information as either zero or one. Meanwhile, excitons and trions also can occupy valleys in the WSe2 substrate.

An exciton is a quantum bound state of an electron and an electron hole. A trion is a quantum bound state of three charged particles. The semiconductor substrate hosts both “bright” and “dark” excitons and trions. “Dark excitons and trions in monolayer WSe2 have much longer lifetime and better valley stability than the common bright excitons and trions,” said aid Chun Hung “Joshua” Lui, an assistant professor in the Department of Physics and Astronomy at UC-Riverside.

“The dark excitons and trions, therefore, serve as excellent candidates for valleytronic applications,” added Lui, who led the research

The next step was developing a method for reading the valley indices of dark excitons and trions. The researchers came up with a method of that distinguished the dark valley indices. “We observed a new decay process of dark excitons and trions in monolayer WSe2, which allows us to identify their valley indices,” Lui said. “A dark exciton or trion can decay into a pair of photon and phonon with a distinctive valley signature.”

A photon is a quantum representation of an electromagnetic wave while a phonon represents a quantum of an atomic vibration in the substrate material. Depending on the direction of dark exciton decay—either right or left hand in this example—the “handedness of the emitted photon is a clear signature of the valley indices of the dark excitons and trions,” Liu explained.

The resulting ability to read the dark-state valleys could lead to eventual applications of valleytronic technology as conventional charge and spin electronics run out of stream, the researchers predicted.

Their research results were published this month in the journal Physical Review Research.

Earlier research used the same substrate to accelerate control of electrons at the quantum level for an application called “lightware valleytronics.” That approach could lead to new logic devices, investigators said

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