Effects of direct lateral Current Injection on the Performance, Overall Efficiency and Emission Distribution in GaN LED
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Effects of direct lateral Current Injection on the Performance, Overall Efficiency and Emission Distribution in GaN LED structures: a 2D Computational Study Pyry Kivisaari1 , Jani Oksanen, and Jukka Tulkki 1 Department
of Biomedical Engineering and Computational Science, Aalto University, P.O. Box 12200, 00076, Finland. pyry.kivisaari@tkk.fi
ABSTRACT We study and demonstrate the potential benefits of using a transverse junction structure in GaN LEDs by simulating and comparing the structure with conventional vertical injection structures. The direct current injection component enabled by the transverse structure significantly reduces the height of the polarization-induced potential barriers and facilitates the electron flow into the active material, improving the overall efficiency. In addition, the transverse junction structure enables a more even radiative recombination distribution from different quantum wells. We estimate the attainable optical output efficiency and also discuss the influence of the active layer design on the quantum efficiency. Based on the obtained results, shifting from the conventional 1-dimensional LED structures to genuinely 2-dimensional structures may allow new possibilities to optimize LED performance. INTRODUCTION Solid-state lighting based on light-emitting diodes (LEDs) is expected to revolutionize the lighting industry within the next few years with their electricity-to-light conversion efficiency potentially unparalled by any other existing lighting technology. Currently, however, the efficiency of IIINitride LEDs is limited by several factors, many of which still remain a subject of intense research. One of the most studied effects related to LEDs made of InGaN quantum wells is a strong efficiency droop at high current densities, which so far limits the use of LEDs in high-power solutions. Several explanations have been suggested for the drooping (See e.g. Ref. [1] and references therein) but the origin of the phenomenon is still unclear. Other challenges related to III-Nitrides include poor acceptor activation and high dislocation density [2]. In addition to the widely studied quantized losses determining the external quantum efficiency, the wall-plug efficiency of an LED is also affected by other losses related to current transport in the
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Figure 1: Thin-film flip-chip (TFFC) and transverse-vertical junction (TVJ) LED structures. The shaded area is present only in the TVJ structure and it enables direct current injection to the QWs from the side in addition to the vertical injection. The total width of the structures is 100 μm and the width of the n-type contact is 5 μm. The GaN barriers between the QWs are 10 nm thick, and the thickness of the uppermost GaN barrier is 20 nm. Note that the material layer thicknesses are not scaled correctly in the figures. LED. The latter losses are manifested as unnecessarily large forward voltages [3, 4] and originate from resistive losses and other related energy losses. The recent overwhelming attention received by the droop has left these secondary
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