Alberi Validates New Theory, Sheds Light on Semiconductors
A new theoretical framework on defect formation in semiconductors-materials used in a multitude of NREL-developed technologies such as solar cells, light-emitting diodes (LEDs), and power electronics-indicates that shining light on them during growth can lead to improved crystalline quality and, ultimately, better device performance.
NREL Materials Physics Scientist Kirstin Alberi and University of Utah Materials Science and Engineering Associate Professor Mike Scarpulla made the discovery when they found that light can suppress native defect formation during semiconductor growth. When Alberi and Scarpulla began discussing the concept of how light can affect semiconductor growth processes, they realized that free carriers-or moving particles that carry an electric charge-generated by light in the crystal can be used to control the electron potential, or Fermi level. By turning the Fermi level into a tunable parameter, researchers can achieve greater control over the probability of defect formation in a variety of devices.
"One thing that limits a semiconductor's performance is the potential for native defects in a crystal during synthesis, such as an atom being in a place it shouldn't be," explained Alberi, comparing the atomic arrangement in a semiconductor crystal to a stack of Lego building blocks. Just as the pegs and holes of Legos are laid out in such a way that pieces easily fit together, so are the atoms in the crystal. But when an atom appears in a crystal in a location it shouldn't be, it creates a defect.
"Some of these defects have fixed charges that act as recombination centers and scatter free carriers, limiting the semiconductor's transport and optical properties and lowering device efficiencies," said Alberi. Reducing defect concentrations is therefore important for advancing the performance of many semiconductor-based devices.
The researchers' findings were recently published in a Scientific Reports paper, which details the model they developed to evaluate the role of the light-induced change in the Fermi level on defect formation energy. Their theory can be applied to all semiconductors, yet the most significant changes can be found in compound semiconductors where there are more native defect types. Such materials include GaAs (gallium arsenide), CdTe (cadmium telluride), and GaN (gallium nitride), which are used for cell phones, solar panels, and LED lightbulbs.
Reducing native defects by shining light on semiconductors may also improve the ability to reach higher doping levels. As Alberi pointed out, native defects often compensate for charged carriers that are intentionally added by doping, or introducing a small amount of another element to the material to alter its conductive properties. High levels of compensation can limit a device's performance, Alberi said, because it may not have the right concentration of charged carriers to achieve a particular conductive behavior. As Alberi and Scarpulla's theory suggests, greater doping control can be achieved by suppressing defects with light, which may allow higher efficiencies in solar panels, greater lifespan for LED lights, and faster performance in electronics such as cell phones.
"When you think about the basic way solar cells work, the photons that get absorbed by the semiconductor are converted into electrons and holes, which are then extracted by the electric circuit. We found that those carriers can also have all sorts of effects on the growth of semiconductors," said Alberi, adding that light-stimulated growth techniques could eventually be applied to overcome existing challenges in manufacturing optoelectronic devices. Alberi and her team, Dan Beaton and Kwangwook Park, are studying this effect experimentally.
"We have plenty more questions," said Alberi. "But our goal is still to understand how light affects a growing crystal and apply our model to other materials to see where this approach could work best."
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