Green Light Emitting Diodes under Photon Modulation
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1040-Q03-08
Green Light Emitting Diodes under Photon Modulation Yufeng Li1,2, Jayantha Senawiratne1,2, Yong Xia1,2, Mingwei Zhu1,2, Wei Zhao1,2, Theeradetch Detchprohm1,2, and Christian M Wetzel1,2 1 Future Chips Constellation, Rensselaer Polytechnic Institute, Troy, NY, 12180 2 Department of Physics, Applied Physics and Astronomy, Rensselaer Polytechnic Institute, Troy, NY, 12180 ABSTRACT With an external laser excitation, the electroluminescence (EL) of GaInN/GaN green light emitting diodes (LEDs) grown on sapphire by metal organic vapor phase epitaxy has been investigated. The EL was found significantly enhanced under this bias and the difference can not merely be attributed to additional photoluminescence (PL). Under 325 nm photon bias, the EL enhancement starts at its highest value and decreases along with an increase of the LED current. Under 408 nm and 488 nm bias, it increases first to a value smaller than that of the 325 nm bias and then decreases with a much lower rate along the current increase. The EL enhancement is attributed to the more efficient carrier injection into quantum wells (QWs) resulting from the screening of the QW polarization by photon bias. Therefore, an enhanced balance of majority and minority carriers was obtained resulting in a better radiative recombination rate. Meanwhile, the current-voltage characteristics show a negative current first and then a voltage reduction as forward voltage increases. The reverse photocurrent indicates carrier loss due to the solar cell effect in our LED device while at high current region the carrier loss is attributed to another effect controlled by the external electrical field. At the balance point of those two effects, the EL enhancement is the highest. These findings clarify the transition from highly efficient radiative recombination at low current density to the region of efficiency droop at high current densities. INTRODUCTION Although great improvements have been achieved in GaInN/GaN LEDs, the “green efficiency gap” still remains as a big scientific challenge. Many works have been done about improving the efficiency as well as eliminating the “droop effect” under high current density. Piezoelectric polarization of the QWs is believed to be one of the main reasons of inefficient radiative recombination.1 Recently, this problem has partly been solved by growing QWs along non-polar or semi-polar crystal axes.2 However, the role that carrier dynamics plays in determining the EL efficiency in the polarized structure still remains unclear. In our experiment, by applying different photon bias, we modulate the QW to different extent. From significant EL enhancements in combination with other results, we find evidence that polarization indeed is one of the reasons of low radiative recombination efficiency. We find that a balance of the majority and minority carriers in QWs is the physical origin of the efficiency maximum at low current densities.
EXPERIMENT We use two green LEDs in our experiment that share many characteristics with a large set of s
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