Recombination dynamics in planar and three-dimensional InGaN/GaN light emitting diode structures
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Tilman Schimpke, Adrian Avramescu, and Martin Strassburg OSRAM Opto Semiconductors GmbH, 93055 Regensburg, Germany (Received 20 February 2017; accepted 10 May 2017)
The spectrally and temporally resolved luminescence of three-dimensional (3D) InGaN/GaN microrods and planar light emitting diode (LED) structures is studied for different energy densities of fs-laser excitation pulses and for different sample temperatures. We find an energy density threshold above which irreversible modifications of the structures take place, which leads to a decrease of the luminescence intensity and a change in the intensity ratio of the GaN to the InGaN luminescence. Due to the quantum confined Stark effect, a biexponential decay characteristic is found in the planar structure, while the 3D microrods with nonpolar InGaN quantum wells on their sidewalls show a monoexponential decay of the InGaN luminescence. For both structures, the decay of the luminescence becomes faster with increasing energy density per pulse. However, the luminescence of the planar LED decays faster with increasing temperature, while the opposite trend is found for the 3D sample.
I. INTRODUCTION
Currently, the market for light emitting diodes (LEDs) is constantly growing and the state-of-the-art devices reach photon extraction efficiencies of about 80% for LEDs with active InGaN layers.1 Nevertheless, it is desirable to further increase the efficiency and reduce the production costs. For that, the reduction of droop, which is the decrease of efficiency with higher current densities, would be very desirable. The three-dimensional (3D) core–shell microrod structures represent one approach to reach this aim.2–5 The defect density of these structures is low due to their small footprint on the substrate. Beyond this, the small footprint improves the crystal quality irrespective of the substrate material. Additionally, the active area of microrod LED structures can exceed their substrate area because the InGaN quantum wells (QWs) are wrapped around the microrods. Since aspect ratios of 10–40 can be reached, this results in a drastic increase of the light emitting area, and hence to a drastic reduction of the current density at the same total currents, which also reduces the droop. Standard planar LEDs are currently grown in the c-axis GaN direction leading to polar InGaN-QWs. Therefore, their emission properties are influenced by the quantum confined Stark Contributing Editor: Winston V. Schoenfeld a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2017.212
effect (QCSE) which results in band bending and the separation of electron and hole wave functions.6 For 3D structures, the growth of QWs on the nonpolar m-plane sidewalls of GaN cores reduces the impact of the QCSE due to the lack of spontaneous and piezoelectric polarization in this direction. Therefore, the better overlap of the wave functions leads to faster recombination dynamics7–9 and at the same time increases the absorption.10 For the optical characterization of LED structures
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