2-D semiconductors demonstrate universal light absorption law

A simple law of light absorption observed in 2-D semiconductors could make exotic new optoelectronic and photonic technologies a reality.

Many of today’s semiconductor technologies hinge upon the absorption of light. It’s especially critical for nanosized structures at the interface between two energy barriers called quantum wells, in which the movement of charge carriers is confined to two dimensions.

Previous work has shown that graphene has a universal value of light absorption. Now, researchers from Lawrence Berkeley National Laboratory (Berkeley Lab) have found that a similar generalized law applies to all 2-D semiconductors. This discovery was the outcome of a process they developed in which ultrathin membranes of indium arsenide were transferred onto an optically transparent substrate – in this case, calcium fluoride.

“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,” said 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, Berkeley. “We were then able to investigate the optical absorption properties of membranes that ranged in thickness from 3 to 19 nm as a function of band structure and thickness.”

In this Fourier transform IR microspectroscopy study, light absorption spectra are obtained from measured transmission and reflection spectra in which the incident light angle is perpendicular to the membrane. (InAs = indium arsenide; CaF₂ = calcium fluoride). Photo ccourtesy of Hui Fang, courtesy of Berkeley Lab

As they worked with the indium arsenide membranes, the investigators discovered a quantum unit of photon absorption, which they call AQ; it should be general to all 2-D semiconductors, including the compound semiconductors of the III-V family, favored for solar films and optoelectronic devices.

“This absorption law appears to be universal for all 2-D semiconductor systems,” said electrical engineer Eli Yablonovitch, who holds joint appointments with Berkeley Lab and UC Berkeley. “Our results add to the basic understanding of electron-photon interactions under strong quantum confinement and provide a unique insight toward the use of 2-D semiconductors for novel photonic and optoelectronic applications.”

Via Fourier transform IR spectroscopy carried out at Beamline 1.4.3 at Berkeley Lab’s Advanced Light Source, the researchers measured the light absorptance in indium arsenide’s transition from one electronic band to the next at room temperature. They observed a discrete stepwise increase at each transition from the membranes with an AQ value of about 1.7 percent per step.

“We used free-standing indium arsenide membranes, down to 3 nm in thickness, as a model material system to accurately probe the absorption properties of 2-D semiconductors as a function of membrane thickness and electron band structure,” Javey said. “We discovered that the magnitude of step-wise absorptance in these materials is independent of thickness and band structure details.”

The study, supported by the US Department of Energy’s Office of Science and the National Science Foundation, appears in PNAS (doi: 10.1073/pnas.1309563110).