Carrier localization and recombination in InGaN quantum wells on a sub-micrometer length scale
Final Report Abstract
The project addresses the physical processes related to light emission in light emitting diodes (LEDs) which are currently replacing conventional light sources in a wide range of applications. The goal of the project was to develop a deeper understanding of the internal quantum efficiency in InGaN quantum wells, which constitute the light emitting region of high-brightness LEDs in the violet to green spectral region. Strong spatial fluctuations of the intensity, wavelength, and linewidth of the electroluminescence or photoluminescence on a micrometer and sub-micrometer length scale are characteristic for these InGaN quantum wells. Within the project we measure the local internal quantum efficiency (IQE) on a sub-micrometer length scale with a confocal microscope which was developed for this particular purpose. This local IQE is related to the LED efficiency which is usually measured on LED device level. Our main focus was to study the influence of dislocations and other defects on light emission. A comparison between AFM and photoluminescence imaging showed that V-shaped pits which act as markers of threading dislocations under certain growth conditions correlate with dark meandering structures. It was thus proven that in samples with a threading dislocation density of the order of 109 cm-2 threading dislocations and dark areas of the meandering structure are strongly linked. However, there are samples, especially those emitting at a wave length of 515 nm and longer, that show a meandering structure of the photoluminescence intensity not unlike the one created by threading dislocations even at threading dislocations of 5×107 cm-2 and below. In this case additional mechanisms, like indium segregation, must be responsible for the observed flucutations. With the help of temperature dependent spatially resolved measurements we were able to prove that these meandering features are not the cause for the phenomenon called S-shape that is observed in InGaN/GaN quantum wells. Instead, the resolution of our setup provides an upper boundary for the length scale of the features responsible for S-shape. During these measurements we were also able to see first indications that this type of meandering structure is created by a spatial variation of the point defect density. Strong evidence for a variation of non-radiative recombination is provided by excitation density dependent measurements. The change of the local I with rising excitation density can be described with the help of three different recombination processes, one particle non-radiative Shockley-Read-Hall recombination, two particle radiative recombination, and three particle non-radiative Auger process. Our spatially resolved excitation density dependent measurements revealed constant radiative recombination and Auger processes throughout the whole sample while the one particle non-radiative recombination fluctuates. This fluctuation is correlated with the meandering structure. Emission fluctuations of quantum well structures are not limited to the sub-micron length scale. Fluctuations can also be observed on a length scale of several tens of microns. With the help of simulations of the quantum well and external bias dependent measurements we were able to link such fluctuations to variations of the energy gap which is either attributed to indium or quantum well width fluctuations. The model describing the local quantum efficiency in InGaN quantum wells, which was developed in the current project, helps to further improve the efficiency of LEDs, in particular in the blue to green spectral region. During the project it also became obvious that we need to develop a further understanding of carrier transport with in the complex energy landscape of InGaN quantum wells. The methods developed within the project should also be utilized for the study of semi- and non-polar InGaN quantum wells.
Publications
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„Correlation of surface morphology and photoluminescence fluctuation in green light emitting InGaN/GaN quantum wells,” Physica Status Solidi (C), 6(S2), S747 (2009)
J. Danhof, C. Vierheilig, U.T. Schwarz, T. Meyer, M. Peter, B. Hahn, M. Maier, and J. Wagner
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„Temperaturedependent photoluminescece measurements on a sub-micrometer length scale on green light emitting InGaN/GaN quantum wells“, Phys. Status Solidi B 248 (5), 1270 (2011)
J. Danhof, C. Vierheilig, U. T. Schwarz, T. Meyer, M. Peter, and B. Hahn
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„Time-of-flight measurements of charge carrier diffusion in In_{x}Ga_{1−x}N/GaN quantum wells,” Physical Review B, 84(3), 035324 (2011)
J. Danhof, U.T. Schwarz, A. Kaneta, and Y. Kawakami
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„Local internal quantum efficiency of a green light emitting InGaN/GaN quantum well“, Phys. Status Solidi B 249, 600 (2012)
J. Danhof, U. T. Schwarz, T. Meyer, C. Vierheilig, and M. Peter
