How black silicon, a prized material used in solar cells, gets its dark, rough side

Researchers at the U.S. Department of Energy’s Princeton Plasma Physics Laboratory (PPPL) have developed a new theoretical model explaining one way to make black silicon, an important material used in solar cells, light sensors, antibacterial surfaces and many other applications.

Black silicon is produced when the surface of ordinary silicon is etched to produce tiny nanoscale pits on the surface. These pits change the color of the silicon from gray to black and, importantly, capture more light, an essential feature of efficient solar cells.

While there are many ways to produce black silicon, including some using the fourth charged state of matter known as plasma, the new model focuses on a process that uses only fluorine gas. Yuri Barsukov, a postdoctoral research associate at PPPL, said the choice to focus on fluoride was intentional: the PPPL team wanted to fill a gap in publicly available research. While some papers have been published on the role of charged particles called ions in the production of black silicon, little has been published on the role of neutral substances, such as fluorine gas.

“We now know – with great precision – the mechanisms that cause these pits to form when fluorine gas is used,” said Barsukov, one of the authors of a new paper on this work, which appeared in the journal Journal of Vacuum Science and Technology A.

“This type of information, publicly published and freely accessible, benefits all of us, whether we are delving deeper into the basic knowledge behind these processes or seeking to improve manufacturing processes,” Barsukov added.

Model reveals bond breakage based on orientation of atoms on surface

The new etching model explains precisely how fluorine gas breaks certain bonds in silicon more often than others, depending on the orientation of the bond on the surface. Since silicon is a crystalline material, the atoms bond together in a rigid pattern. These bonds can be characterized based on how they are oriented in the pattern, with each orientation type, or plane, identified by a number in parentheses, such as (100), (110), or (111).

“If you etch silicon using fluorine gas, the etch proceeds along the (100) and (110) crystal planes but does not etch (111), resulting in a rough surface after etching,” he said. explained Barsukov. As the gas attacks the silicon unevenly, pits form on the surface of the silicon. The rougher the surface, the more light it can absorb, making rough black silicon ideal for solar cells. Smooth silicon, on the other hand, makes an ideal surface for creating the atomic-scale patterns needed for computer chips.

“If you want to etch silicon while leaving a smooth surface, you need to use a reagent other than fluorine. It needs to be a reagent that evenly etches all crystal planes,” Barsukov said.

PPPL extends its expertise to quantum chemistry

The research is also notable because it represents an initial success in one of PPPL’s ​​newest areas of research.

“The lab is diversifying,” said Igor Kaganovich, senior research physicist and co-author of the paper. “This is a first for PPPL to carry out this type of quantum chemistry work.”

Quantum chemistry is a branch of science that studies the structure and reactivity of molecules using quantum mechanics, the laws of physics governing very small and very light objects, such as electrons and nuclei.

Other researchers who contributed to the paper include Joseph Vella, associate research physicist; Sierra Jubin, graduate student at Princeton University; and former research assistant at PPPL Omesh Dhar Dwivedi.