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Charge carrier dynamics in InGaN quantum wells: Stimulated emission depletion and lateral charge carrier motion

: Solowan, H.-M.
: Schwarz, U.T.; Wöllenstein, J.

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Freiburg/Brsg., 2015, IV, 126 pp.
Freiburg/Brsg., Univ., Diss., 2015
URN: urn:nbn:de:bsz:25-freidok-105079
Dissertation, Electronic Publication
Fraunhofer IAF ()

Over the last decade Indium Gallium Nitride (InGaN) based LEDs have become one of the dominating light sources in cases where high efficiency and compact form factors are required. Despite having already reached a very high efficiency level, there is still room for further improvement. Charge carrier motion and recombination dynamics are essential processes when it comes to understanding and improving solid state based lighting technology on a fundamental level. Two experimental techniques based on confocal microscopy are used in the present work to study charge carrier dynamics and motion in InGaN based quantum well LED structures. It focuses on the investigation of stimulated emission depletion (STED) and lateral charge carrier motion, i.e. motion of charge carriers within the quantum well plane. In the first part the properties of the STED effect in blue InGaN quantum wells are studied using different continuous wave and pulsed laser sources. The samples are excited at 405 nm and depletion of the excited charge carriers is induced by stimulated emission using a second laser source with a wavelength in the long wavelength part of the fluorescence spectrum. It is found that only a partial fluorescence suppression of about 10% can be achieved in the investigated samples. This is not sufficient to use the STED effect for spatial resolution enhancement like it has already been successfully demonstrated for fluorescence dyes in studies of other authors. Two main processes are identified to be responsible for the partial fluorescence suppression. The first is the availability of only a limited number of localized states in the band structure being susceptible to depletion by stimulated emission. Their number is too small to allow for a sufficient depletion rate. The second effect is strong two photon absorption induced by the depletion laser. The enhanced non-linear absorption leads to an increase in fluorescence intensity and thus counteracts the depletion effect over a wide range of depletion laser powers. Extending the rate equation model normally used to describe STED by two photon absorption shows, that there might still be a chance to achieve higher fluorescence suppression outside of the experimentally accessible parameter space. A confocal time of flight method is used in the second part to measure for the first time the temperature and charge carrier density dependent in-plane diffusion constant of a turquoise InGaN multi quantum well LED structure. The spectral range is extended towards blue by studying a second shorter wavelength sample of similar structure. The room temperature diffusion constant for the turquoise sample with a central emission wavelength of 470 nm is about 0.25 cm² s(-1). The second sample is emitting at 430 nm and has a diffusion constant of 0.19 cm² s(-1) at 250 K. The diffusion constant decreases continuously for both samples with decreasing temperature. At 10K and below it is no longer possible to resolve any lateral charge carrier motion and the charge carriers are localized. When looking at the charge carrier dependence of lateral charge carrier motion, a decreasing diffusivity with decreasing charge carrier density is found at 120K and 240 K. An important contribution to the overall lateral motion dynamics is found to be caused by the intrinsic polarization fields along the c-direction of the InGaN crystal, which is perpendicular to the quantum well plane. It influences the lateral mot-ion of charge carriers in several ways, whereas the effect is similar in all cases. An increase in charge carrier density reduces the influence of the polarization fields and leads to an enhanced in-plane charge carrier motion.