Terahertz spectroscopy helps improve performance of solar cells

https://optics.org/news/11/1/89

Researchers at NIST make the most sensitive measurements to date of silicon conductivity.

A project at the US National Institute of Standards and Technology (NIST) has made the most sensitive measurements so far of how quickly electric charge moves in silicon, a gauge of its performance as a semiconductor.

In particular, NIST has found a way to measure the charge-carrier mobility at extremely low levels of electric charge, in a way that does not require physical contact with the silicon sample. The new results may suggest ways to improve semiconductor materials and their applications, including solar cells and high-speed cellular networks, and were reported in Optics Express.

The findings build on previous NIST research into the use of time-resolved terahertz spectroscopy (TRTS) to assess other semiconductor materials, and represent the first comparison between the spectroscopic technique and more conventional contact-based methods to measure charge-carrier behavior.

The technique involves using a pulse of visible, ultraviolet or infrared light to first excite a resonant transition in the semiconductor and create a controlled density of charge carriers, effectively "photo-doping" the material.

Coupling a probe pulse of THz illumination into the photoexcited region then allows the motion of those charge carriers to be measured, as the probe irradiation is integrated over all the conduction pathways inside the interaction volume of the material, and THz light can penetrate even opaque materials such as silicon semiconductor samples. How much of that light is absorbed by the sample then depends on how many charge carriers are freely moving, allowing the charge mobility to be calculated.

In addition, the low-intensity pulsed light used in this study simulates the kind of low-intensity light a solar cell would receive from the Sun, so a greater understanding of charge-carrier behavior could lead to real-world benefits for power generation.

"The light we use in this experiment is similar to the intensity of light that a solar cell might absorb on a sunny spring day," said Tim Magnanelli of NIST. "So the work could potentially find applications in improving solar-cell efficiency."

New discoveries about silicon

A key breakthrough was the use of two-photon excitation for the photo-doping process. Earlier work using single-photon excitation had been limited by the small penetration depth achieved into the sample, usually around 10 to 100 nanometers. That meant that surface variations were an inevitable complication, negating the advantage of a contactless measurement technique.

But a two-photon technique can penetrate deeper, and more effectively complement the ability of THz light to completely penetrate a sample. The project successfully used the TRTS method to study a number of un-doped, n-, and p-doped Si wafer samples about half a millimeter thick.

"Combining two-photon excitation with a terahertz probe serves as a more accurate method to extract absolute and integrated carrier mobility, by minimizing of the impact of surface defects and providing an explicit depth of photoexcitation," commented the team in its published paper.

Having successfully lowered the threshold for measuring free holes and electrons, the NIST researchers discovered that carrier mobility eventually plateaus once the density of careers drops to a certain level. Although this effect had been noted before, the new project was able to determine that this plateau occurs at a lower carrier density than previously thought.

"An unexpected result like this shows us things we didn’t know about silicon before," said NIST's Ted Heilweil. "And though this is fundamental science, learning more about how silicon works could help device makers use it more effectively, for example if some semiconductors can be made to work better at lower doping levels than currently used."

To test whether these findings only held for silicon, the project tested gallium arsenide (GaAs) as well, and found that there too the carrier mobility continues to increase with lower charge carrier density, to a point about 100 times lower than the conventionally accepted limit.

"This observation for both Si and GaAs suggests that the effect is not uniquely due to silicon's indirect bandgap structure, and may be the product of differing selection rules or effective masses for populated carriers," commented the team.

Future work may now involve applying different photodoping techniques to samples, or experimenting with thicker samples. "Using the two-photon method on thicker samples may produce even lower carrier densities, that we can then probe with the THz pulses," said Heilweil.

NIST uses terahertz waves to make ultrasensitive silicon measurements

To create electric charges in silicon, researchers shine pulsed laser light onto a sample. One-photon tests using visible light only penetrate a tiny way into a silicon sample--on the order of micrometers or smaller. But the new two-photon tests using near-infrared light penetrate much, much deeper into silicon--on the order of millimeters or longer. The one-photon tests create a lot of electric charge (shown here as pluses and minuses) in a relatively small volume. By contrast, the two-photon test creates far fewer electric charges in a much larger volume.

Sean Kelley/NIST

National Institute of Standards and Technology (NIST) scientists have made the most sensitive measurements to date of silicon’s conductivity in order to improve future solar cell and semiconductor applications.

Gail Overton

https://www.laserfocusworld.com/test-measurement/article/14168789/nist-uses-terahertz-waves-to-make-ultrasensitive-silicon-measurements

Silicon, the best-known semiconductor, is ubiquitous in electronic devices including cellphones, laptops and the electronics in cars. Now, researchers at the National Institute of Standards and Technology (NIST) have made the most sensitive measurements to date of how quickly electric charge moves in silicon, a gauge of its performance as a semiconductor. Using a novel method, they have discovered how silicon performs under circumstances beyond anything scientists could test before--specifically, at ultralow levels of electric charge. The new results may suggest ways to further improve semiconductor materials and their applications, including solar cells and next-generation high-speed cellular networks. The NIST scientists detail their terahertz spectroscopy technique in Optics Express.

Unlike previous techniques, the new method does not require physical contact with the silicon sample and allows researchers to easily test relatively thick specimens, which enable the most accurate measurements of semiconductor properties.

The NIST researchers had previously done a proof-of-principle test of this method using other semiconductors. But this latest study is the first time researchers have pitted the new light-based technique against the conventional contact-based method for silicon.

It’s too soon to say exactly how this work might be used someday by industry. But the new findings could be a foundation for future work focused on making better semiconducting materials for a variety of applications, including potentially improving efficiency in solar cells, single-photon light detectors, LEDs, and more. For example, the NIST team’s ultrafast measurements are well-suited to tests of high-speed nanoscale electronics such as those used in fifth-generation (5G) wireless technology, the newest digital cellular networks. In addition, the low-intensity pulsed light used in this study simulates the kind of low-intensity light a solar cell would receive from the Sun.

When researchers want to determine how well a material will perform as a semiconductor, they assess its conductivity. One way to gauge conductivity is by measuring its “charge carrier mobility,” the term for how quickly electric charges move around within a material. Negative charge carriers are electrons; positive carriers are referred to as “holes” and are places where an electron is missing.

The conventional technique for testing charge carrier mobility is called the Hall method. This involves soldering contacts onto the sample and passing electricity through those contacts in a magnetic field. But this contact-based method has drawbacks: The results can be skewed by surface impurities or defects, or even problems with the contacts themselves.

To get around these challenges, NIST researchers have been experimenting with a method that uses terahertz (THz) radiation.

NIST’s THz measurement method is a rapid, noncontact way to measure conductivity that relies on two kinds of light. First, ultrashort pulses of visible light create freely moving electrons and holes within a sample--a process called “photodoping” the silicon. Then, THz pulses, with wavelengths much longer than the human eye can see, in the far infrared to microwave range, shine on the sample.

Unlike visible light, THz light can penetrate even opaque materials such as silicon semiconductor samples. How much of that light penetrates or is absorbed by the sample depends on how many charge carriers are freely moving. The more freely moving charge carriers, the higher the material’s conductivity.

For the full story, see the source link below.

SOURCE: NIST; https://www.nist.gov/news-events/news/2020/02/how-low-can-you-go-lower-ever