A trick of light: Researchers turn silicon into direct bandgap semiconductor

The development, subject of a paper recently published as the cover story in the journal ACS Nano, hinges on the UC Irvine researchers' conversion of pure silicon from an indirect to a direct bandgap semiconductor through the way it interacts with light.

The UC Irvine team, in collaboration with scientists from Russia's Kazan Federal University and Tel Aviv University, explored an innovative approach by conditioning the light rather than changing the material itself. They confined photons on sub-3-nanometer asperities near the bulk semiconductor, granting light a novel property -- expanded momentum -- that opens new interaction pathways between light and matter. By "decorating" the silicon surface, the researchers said, they achieved a boost in light absorption by orders of magnitude, along with a significant increase in device performance.

"In direct bandgap semiconductor materials, electrons transition from the valence band to the conduction band. This process requires only a change in energy; it's an efficient transfer," noted lead author Dmitry Fishman, UC Irvine adjunct professor of chemistry. "In indirect bandgap materials, like silicon, an additional component -- a phonon -- is needed to provide the electron the momentum necessary for the transition to occur. Since the likelihood of a photon, phonon and electron interacting at the same place and time is low, silicon's optical properties are inherently weak."

He said that as an indirect bandgap semiconductor, silicon's poor optical properties limit the development of solar energy conversion, and optoelectronics in general, which is a drawback considering that silicon is the second-most abundant element in Earth's crust and the foundation on which the world's computer and electronics industries were built.

"Photons carry energy but almost no momentum, but if we change this narrative explained in textbooks and somehow give photons momentum, we can excite electrons without needing additional particles," said co-author Eric Potma, UC Irvine professor of chemistry. "This reduces the interaction to just two particles, a photon and an electron, similar to what occurs in direct bandgap semiconductors, and increases light absorption by a factor of 10,000, completely transforming light-matter interaction without changing the chemistry of the material itself."

Co-author Ara Apkarian, UC Irvine Distinguished Professor emeritus of chemistry, said: "This phenomenon fundamentally changes how light interacts with matter. Traditionally, textbooks teach us about so-called vertical optical transitions, where a material absorbs light with the photon changing only the electron's energy state. However, momentum-enhanced photons can change both the energy and momentum states of electrons, unlocking new transition pathways we hadn't considered before. Figuratively speaking, we can 'tilt the textbook,' as these photons enable diagonal transitions. This dramatically impacts a material's ability to absorb or emit light."

According to the researchers, the development creates an opportunity to exploit recent advances in semiconductor fabrication techniques at the sub-1.5-nanometer scale, which has the potential to affect photo-sensing and light-energy conversion technologies.

"With the escalating effects of climate change, it's more urgent than ever to shift from fossil fuels to renewable energy. Solar energy is key in this transition, yet the commercial solar cells we rely on are falling short," Potma said. "Silicon's poor ability to absorb light means that these cells require thick layers -- almost 200 micrometers of pure crystalline material -- to effectively capture sunlight. This not only drives up production costs but also limits efficiency due to increased charge carrier recombination. The thin-film solar cells that are one step closer to reality due to our research are widely seen as the solution to these challenges."

Other co-authors on this study included Jovany Merham and Aleksey Noskov of UC Irvine; Kazan Federal University researchers Elina Battalova and Sergey Kharintsev; and Tel Aviv University investigators Liat Katrivas and Alexander Kotlyar. The project received financial support from the Chan Zuckerberg Initiative.