NUSOD Institute LLC

Newark, DE, United States

NUSOD Institute LLC

Newark, DE, United States

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

We analyze efficiency droop reductions in InGaN/GaN light-emitting diodes caused by a chirped AlGaN/GaN multi-quantum barrier (MQB). Such electron barriers are expected to create an additional forbidden energy range above the natural conduction band edge, which reduces the electron leakage current. Advanced numerical device simulations reveal that energy band bending practically eliminates this MQB effect. Instead, we find that the measured efficiency improvement has its origin in enhanced hole injection, which can be more easily accomplished using a single thin AlGaN layer. © 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
Proceedings of SPIE - The International Society for Optical Engineering | Year: 2012

The development and application of nitride-based light-emitting diodes (LEDs) is hindered by the low hole conductivity of Mg-doped layers. As an alternative, polarization-induced hole doping of graded p-AlGaN layers was recently demonstrated. Using previously manufactured 440nm LEDs as device examples, this paper evaluates the effect of polarization doping by advanced numerical device simulation, both for Ga-face and N-face growth. Recently published material parameters are employed in the simulation, including new data for the Auger coefficients. The simulations reveal that Auger recombination is the main carrier loss mechanism in these devices, electron leakage seems to exert a much smaller influence on the internal quantum efficiency. The importance of internal physical mechanism is studied in detail, including the Poole-Frenkel field ionization of Mg acceptors, which is commonly held responsible for polarization doping effects. Surprisingly, we find that the field ionization inside the graded p-AlGaN layers is not stronger than in conventional electron blocking layers. © 2012 Copyright Society of Photo-Optical Instrumentation Engineers (SPIE).


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.


Piprek J.,NUSOD Institute LLC
Applied Physics Letters | Year: 2014

This Letter investigates the output power enhancement achieved by tunnel junction insertion into the InGaN multi-quantum well (MQW) active region of a 410nm vertical-cavity surface-emitting laser which enables the repeated use of carriers for light generation (carrier recycling). While the number of quantum wells remains unchanged, the tunnel junction eliminates absorption caused by the non-uniform MQW carrier distribution. The thermal resistance drops and the excess bias lead to a surprisingly small rise in self-heating. © 2014 AIP Publishing LLC.


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
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.


Piprek J.,NUSOD Institute LLC
IEEE Photonics Technology Letters | Year: 2013

High-power broad-area laser diodes often suffer from a widening of the lateral far-field with increasing current (slow-axis far-field blooming). This effect mainly originates in self-heating that generates a lateral thermal lens in the active region. Pedestal heat sinking was recently shown to mitigate the far-field blooming. In this letter, utilizing self-consistent electro-thermal optical simulations, we analyze such a far-field improvement and link it to the formation of an inverse thermal lens. At high injection current, the inverse thermal lens is shown to weaken high-order lateral modes and to even narrow the slow-axis far-field. © 2013 IEEE.


Piprek J.,NUSOD Institute LLC
Optical and Quantum Electronics | Year: 2012

The development and application of nitride-based light-emitting diodes (LEDs) is handicapped by the low hole conductivity of Mg-doped layers. Mg-doping becomes increasingly difficult with higher Al-content of the p-AlGaN layers as required for ultraviolet (UV) light emission. Polarization-induced hole doping of graded AlGaN was recently demonstrated as an alternative doping method. Using advanced numerical device simulation, this paper investigates the impact of polarization-doping on the internal device physics ofUVLEDs and compares the conventional Ga-face growth to the novel N-face growth direction. Various LED design options are explored to maximize the internal quantum efficiency. © Springer Science+Business Media, LLC. 2011.


Piprek J.,NUSOD Institute LLC
Physica Status Solidi - Rapid Research Letters | Year: 2014

InGaN/GaN light-emitting diodes (LEDs) are known to exhibit a strongly non-uniform vertical carrier distribution within the multi-quantum well (MQW) active region. We propose to eliminate "dark" quantum wells by insertion of multiple tunnel junctions into the MQW which allow for the repeated use of electrons and holes for photon generation. In good agreement with available measurements, we demonstrate by self-consistent numerical simulation that such tunnel junction LED design promises quantum efficiencies as high as 250% as well as a strongly enhanced output power at high input power, compared to conventional LED concepts. Energy band diagram and photon emission rate for the proposed LED with four sets of two quantum wells separated by three tunnel junctions. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

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