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Piprek J.,NUSOD Institute LLC | Simon Li Z.M.,Crosslight Software Inc.
Applied Physics Letters | Year: 2013

III-nitride light-emitting diodes (LEDs) suffer from efficiency droop, which is partially attributed to electron leakage into the p-doped layers. Only very few direct measurements of such leakage are published. We here analyze leakage measurements on AlGaN LEDs with an emission wavelength near 260 nm. The electron leakage disappears after insertion of a thin undoped electron blocking layer (EBL). In good agreement with these measurements, we show that the electron blocking effect is extremely sensitive not only to the EBL material composition but also to the conduction band offset and to the net polarization, which are both not exactly known. © 2013 American Institute of Physics.

Piprek J.,NUSOD Institute LLC
Proceedings of SPIE - The International Society for Optical Engineering | Year: 2013

High-power broad-area laser diodes often suffer from a widening of the lateral far-field with rising current injection. This effect is also referred to as thermal blooming, since self-heating is considered the main cause. The non-uniform temperature profile inside the waveguide leads to a lateral refractive index profile that enhances the index guiding of laser modes (thermal lens). This paper presents a self-consistent electro-thermal-optical simulation and analysis of the thermal blooming effect, including the non-uniform heat power distribution inside the laser as well as the non-uniform carrier and gain distributions inside the quantum wells. The calculated results are in good agreement with measurements. The simulations demonstrate that thermal blooming is not only caused by the rising order of lateral modes but also by the far field widening of each individual mode with increasing current. © 2013 Copyright SPIE.

Piprek J.,NUSOD Institute LLC
Physica Status Solidi (A) Applications and Materials Science | Year: 2010

Nitride-based light-emitting diodes (LEDs) suffer from a reduction (droop) of the internal quantum efficiency with increasing injection current. This droop phenomenon is currently the subject of intense research worldwide, as it delays general lighting applications of GaN-based LEDs. Several explanations of the efficiency droop have been proposed in recent years, but none is widely accepted. This feature article provides a snapshot of the present state of droop research, reviews currently discussed droop mechanisms, contextualizes them, and proposes a simple yet unified model for the LED efficiency droop. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

This Letter investigates the efficiency enhancement achieved by tunnel junction insertion into the InGaN/GaN multi-quantum well (MQW) active region of blue light emitting diodes (LEDs). The peak quantum efficiency of such LED exceeds 100%, but the maximum wall-plug efficiency (WPE) hardly changes. However, due to the increased bias, the WPE peaks at much higher input power, i.e., the WPE droop is significantly delayed, and the output power is strongly enhanced. The main physical reason for this improvement lies in the non-uniform vertical carrier distribution typically observed within InGaN MQWs. © 2014 AIP Publishing LLC.

Piprek J.,NUSOD Institute LLC
Applied Physics Letters | Year: 2015

GaN-based light-emitting diodes (LEDs) exhibit a strong efficiency droop with higher current injection, which has been mainly attributed to Auger recombination and electron leakage, respectively. Thus far, the few reports on direct measurements of these two processes do not confirm their dominating influence on the droop unambiguously. Advanced numerical simulations of experimental characteristics are shown to validate one or the other explanation by variation of uncertain material parameters. We finally demonstrate how the comparative simulation of temperature effects enables a clear distinction between both models. Contrary to common assumptions, the consistently measured efficiency reduction of blue LEDs with higher ambient temperature eliminates electron leakage as primary cause of the efficiency droop in these devices. © 2015 AIP Publishing LLC.

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