Loss of Quantum Efficiency in Green Light Emitting Diode Dies at Low Temperature
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Loss of Quantum Efficiency in Green Light Emitting Diode Dies at Low Temperature Yufeng Li1,2, Wei Zhao1,2, Yong Xia1,2, Mingwei Zhu1,2, Jayantha Senawiratne1,2, Theeradetch Detchprohm1,2, E. Fred Schubert1,3, and Christian M Wetzel1,2 1 Future Chips Constellation, Rensselaer Polytechnic Institute, 110 Eighth Street, Troy, NY, 12180 2 Department of Physics, Applied Physics, and Astronomy, Rensselaer Polytechnic Institute, 110 Eighth Street, Troy, NY, 12180 3 Department of Electrical, Computer and Systems Engineering, Rensselaer Polytechnic Institute, 110 Eighth Street, Troy, NY, 12180 The electroluminescence, photoluminescence and cathodoluminescence of GaInN/GaN multiple quantum well light emitting diode dies are analyzed at variable low temperature. Three dies of nominally identical structure but strongly different RT performance from 510 nm to 525 nm have been studied. The electroluminescence peak energy exhibits a blue shift from RT to 158 K followed by a red shift for lower temperature. In the same low-temperature range, a secondary emission peak appears near 390 nm (3.18 eV) that resembles a donor-acceptor pair transition from GaN. Depth profiling spectroscopy of this transition at 77 K reveals its location either in the unintentionally doped quantum barriers or within the n-GaN layer, rather than the commonly believed Mg doped p-type GaN layers. The external quantum efficiency of each die increases as the temperature is lowered. A maximum is reached for all near 158 K while for lower temperature as low as 7.7 K, the efficiency continuously drops. The pronounced efficiency maximum is tentatively assigned to a combination of temperature dependent mobility and shift of the actual pn-junction location. Keywords: GaInN/GaN, green LED, external quantum efficiency, low temperature, donor-acceptor transition
Introduction High-brightness, reliable and efficient pure green light emitting diodes (LEDs) are of high economic value. However, when the emission wavelength of a multiple quantum well (MQW) LED device moves into the green region (beyond 500 nm) the quantum efficiency drops sharply. The radiative recombination processes in group-III nitride heterostructures remain the subject of continued investigation in order to increase performance of high brightness LEDs. The external quantum efficiency (EQE) shows a strong dependence on the injection current and temperature. For typical operation current, EQE is only a fraction of its maximum value at low current. A detailed spectroscopic analysis is therefore warranted to identify the limiting mechanism and to enhance performance in next generation solid-state lighting. Here we spectrally analyze the electroluminescence (EL) and EQE as a function of variable low temperature in three dies of nominally identical structure but very different RT output power near 520 nm.
Experimental Pseudomorphic GaInN/GaN MQW structures of nominally identical structure have been grown by metal-organic vapor-phase epitaxy on c-plane sapphire substrates using the technique of lo
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