Charge Carrier Diffusion in Blue Light Emitting Diodes
Collin Epstein and Tim Gfroerer, Davidson College, Davidson, NC
Yong Zhang, University of North Carolina at Charlotte, Charlotte, NC
Abstract
Blue light emitting diodes (LEDs) remain subject to widespread research because they exhibit detrimental behavior that LEDs of other colors do not. While most LEDs become more efficient at converting electrical
power to light as the drive current increases, blue LEDs only increase in efficiency up to a certain current threshold, at which point they become progressively less efficient. The decrease in efficiency is called droop.
Previous research in the Gfroerer lab has shown that when a laser excites a blue LED device, the device emits light from regions that are not directly excited. We investigate this phenomenon by comparing
operational conditions under optical and electrical excitation.
Motivation: Blue LED Droop
PL Efficiency of Excited Region
Efficiency vs. Generation Rate
Low Excitation
High Excitation
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One common characteristic of most LEDs is that the devices operate
more efficiently with higher current input. However, blue LEDs only
increase in efficiency up to a certain threshold, and then become less
efficient as current increases. This phenomenon is known as droop.
Data and graph courtesy of Grace Watt ‘15.
Experiment Diagram
The Photo-Luminescent (PL) efficiency of the excited region of the device is a measure of
the proportion of electron-hole pairs generated by incident laser light that recombine to
emit light within the excited region rather than emitting heat or diffusing into other
regions of the device. In this regime, PL efficiency increases with increasing generation
rate (simulating increasing current) and also increases with decreasing temperature.
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Defect
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Electron
Hole
The radiative efficiency is influenced by the travel distance between
electron-hole pair generation and recombination. At low generation rates,
electrons are more likely to encounter a defect before a hole, allowing for
defect-related trapping and heat-generating recombination. At high
generation rates, electron-hole pairs are more likely to find each other and
produce light before reaching a defect. Adapted from an image courtesy of
Mac Read ‘10.
EL Efficiency of Device
Efficiency vs. Temperature
Low Temperature
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High Temperature
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Observation: Global Emission
The Electro-Luminescent (EL) efficiency of the device is a measure of the proportion of
the electron-hole pairs generated by electrical excitation that recombine to emit light
rather than heat. In this regime, EL efficiency increases with increasing generation rate
(increased current). EL efficiency also increases with decreasing temperature.
Electron
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Thermal Diffusion
Low Temperature
High Temperature
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When excited by a ring-shaped laser, the device emits light from
regions that are not directly excited as well as regions that are. The
emission from unexcited regions appears to decrease with increasing
excitation power and increase with increasing temperature.
Hole
Radiative electron-hole pair recombination produces photons. Since
photons have very little momentum, the momenta of the electron and the
hole must be equal and opposite in order to conserve momentum during
photon emission. At low temperatures, momentum vectors are shorter and
have a smaller distribution, so it is easier to satisfy this condition. At high
temperatures, the wide distribution of momenta decreases the probability
of suitable collisions. Adapted from an image courtesy of Mac Read ‘10.
ELPE of Device
The Electro-Luminescence due to Photo-Excitation (ELPE) is a measure of the proportion
of electron-hole pairs generated in the laser excitation region that recombine via the EL
(current-driven) mechanism throughout the device. The result is luminescence in
regions of the device that are not directly excited by laser light. ELPE increases with
increasing temperature and decreases with increasing generation rate.
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The test device is a 1mm2 InGaN blue LED. For open circuit
measurements, the current switch is opened. For short-circuit
measurements, the SourceMeter is set to source zero volts and
measure current. Image courtesy of Dr. Gfroerer.
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Electron
Excitation Area
Average thermal velocity increases with temperature. Therefore, as
temperature increases, electrons and holes can travel further before
recombining. This mechanism increases the likelihood of recombination in
unexcited regions of the device. Adapted from an image courtesy of Mac
Read ‘10.
Conclusions
• In the excitation regime of this investigation, radiative efficiency increases with increasing generation rate and decreasing temperature.
• The magnitude of the ELPE effect is much greater than anticipated (global lateral movement of charge carriers is often neglected in LED research).
• Since ELPE decreases with increasing excitation density, reduced lateral mobility may limit efficiency and contribute to droop at high current.
Acknowledgements
We would like to thank Ben Stroup ’16, Grace Watt
‘15 and Mac Read ‘10 for their prior work.
We also thank Yong Zhang (UNC-Charlotte) for
providing the test device for this investigation.