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A better way to find defects


Tohoku University team's omnidirectional photoluminescence teaches old spectroscopy new tricks

Tohoku University researchers have improved a method for probing semiconducting crystals with light to detect defects and impurities. The details of their 'omnidirectional photoluminescence (ODPL) spectroscopy' set-up were published in the journal Applied Physics Express, and could help improve the fabrication of materials for electric cars and solar cells.

"Our technique can test materials at very low temperatures and can find even small amounts of defects and impurities," says Tohoku University materials scientist Kazunobu Kojima.

Kojima and his colleagues demonstrated their approach using GaN crystals. GaN can develop defects and impurities during its fabrication, which can affect performance. Currently available methods for testing these crystals are expensive or too invasive.

The ODPL spectroscopy, on the other hand, is a non-invasive technique that can test the crystals, but only at room temperature. Being able to change the crystal's temperature is important to properly test its properties.

Kojima and his colleagues found a way to set up an ODPL instrument so that the crystal can be cooled. The process involves placing a crystal on an aluminium plate connected to a cooling device. This is placed under an 'integrating sphere,' which collects light coming from many directions. External light is shone through the sphere onto the crystal, exciting it.

The crystal emits light back into the sphere in order to return to its initial unexcited state. The two lights, from the external source and the crystal, are integrated within the sphere and measured by a detector. The result reveals the crystal's 'internal quantum efficiency,' which is reduced if it contains defects and impurities, and can be measured even at very low temperatures.

The team's modification - placing the crystal outside the sphere and connecting it to something that cools it - means the temperature change crucially happens only within the crystal and not within the sphere. The scientists were able to measure the internal quantum efficiency of samples using this technique at temperatures ranging from -261degC to about 27degC.

"We next plan to use our method for testing other materials, such as perovskites for use in highly efficient solar cells and boron nitride as an atomically thin 2D material," says Kojima.

'Temperature dependence of internal quantum efficiency of radiation for the near-band-edge emission of GaN crystals quantified by omnidirectional photoluminescence spectroscopy' by Kazunobu Kojima et al; Applied Physics Express, Volume 13, Number 10

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