Teaching an old spectroscope new tricks

Date:
October 6, 2020
Source:
Tohoku University
Summary:
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 could help improve the fabrication of materials for electric
cars and solar cells.



FULL STORY ========================================================================== 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 gallium
nitride crystals. Gallium nitride is a semiconducting crystal that
has been used in energy-saving light-emitting diodes (LEDs) since the
2000s. It has interesting optic and electronic properties, making it
attractive for many applications, including power-switching devices in
electric vehicles. But it 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 gallium
nitride crystal on an aluminum 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
gallium nitride 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 two-dimensional material," says Kojima.


========================================================================== Story Source: Materials provided by Tohoku_University. Note: Content
may be edited for style and length.


========================================================================== Journal Reference:
1. Kazunobu Kojima, Kenichiro Ikemura, Shigefusa
F. Chichibu. Temperature
dependence of internal quantum efficiency of radiation for
the near-band- edge emission of GaN crystals quantified by
omnidirectional photoluminescence spectroscopy. Applied Physics
Express, 2020; 13 (10): 105504 DOI: 10.35848/1882-0786/abb788 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/10/201006114249.htm

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