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Universal law for light absorption in 2D semiconductors

By accurately probing 2D indium arsenide (InAs), researchers have discovered the magnitude of step-wise absorptance is independent of membrane thickness and electron band structure

From solar cells to optoelectronic sensors to lasers and imaging devices, many of today’s semiconductor technologies hinge upon the absorption of light.

Absorption is especially critical for nano-sized structures at the interface between two energy barriers, known as quantum wells, in which the movement of charge carriers is confined to two-dimensions.

Now, a team of researchers with the U.S. Department of Energy (DOE)’s Lawrence Berkeley National Laboratory say they have, for the first time, demonstrated a simple law of light absorption for 2D semiconductors.

Working with ultrathin membranes of InAs, the researchers discovered a quantum unit of photon absorption, which they have dubbed “AQ,” which should be general to all 2D semiconductors, including III-Vs.

This discovery not only provides new insight into the optical properties of 2D semiconductors and quantum wells, it should also open doors to exotic new optoelectronic, photonic and solar technologies.

“We used free-standing indium arsenide membranes down to three nanometres in thickness as a model material system to accurately probe the absorption properties of 2D semiconductors as a function of membrane thickness and electron band structure,” says Ali Javey, a faculty scientist in Berkeley Lab’s Materials Sciences Division and a professor of electrical engineering and computer science at the University of California (UC) Berkeley.

(From left) Eli Yablonovitch, Ali Javey and Hui Fang discovered a simple law of light absorption for 2D semiconductors that should open doors to exotic new optoelectronic and photonic technologies (Credit: Roy Kaltschmidt)

“We discovered that the magnitude of step-wise absorptance in these materials is independent of thickness and band structure details.”

Javey is one the authors of a paper describing this research in the Proceedings of the National Academy of Sciences (PNAS).

Previous work has shown that graphene, a two-dimensional sheet of carbon, has a universal value of light absorption. Javey, Yablonovitch and their colleagues have now found that a similar generalised law applies to all 2D semiconductors. This discovery was made possible by a unique process that Javey and his research group developed in which thin films of indium arsenide (InAs) are transferred onto an optically transparent substrate, in this case calcium fluoride.

InAs has an electron mobility and velocity that make it an outstanding candidate for future high-speed, low-power optoelectronic devices

“This provided us with ultrathin membranes of indium arsenide, only a few unit cells in thickness, that absorb light on a substrate that absorbed no light,” Javey says. “We were then able to investigate the optical absorption properties of membranes that ranged in thickness from three to 19 nm as a function of band structure and thickness.”

Using Fourier transform infrared spectroscopy (FTIR) the team measured the magnitude of light absorptance (the ratio of absorbed to incident radiation) in the transition from one electronic band to the next at room temperature. They observed a discrete stepwise increase at each transition from InAs membranes with an AQ value of approximately 1.7 percent per step.

In this FTIR microspectroscopy study, light absorption spectra are obtained from measured transmission and reflection spectra in which the incident light angle is perpendicular to the membrane

“This absorption law appears to be universal for all 2D semiconductor systems,” says Yablonovitch. “Our results add to the basic understanding of electron-photon interactions under strong quantum confinement and provide a unique insight toward the use of 2D semiconductors for novel photonic and optoelectronic applications.”

This research is described in detail in the paper, “Quantum of optical absorption in two-dimensional semiconductors,” by Hui Fang et al in Proceedings of the National Academy of Sciences, 110, 29, 11688–11691. The full text may be accessed via the following link www.pnas.org/cgi/doi/10.1073/pnas.1309563110

This research was supported by DOE’s Office of Science and the National Science Foundation.

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