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Lumineszenzdynamik in InGaN/GaN-basierten Mikrosäulen- und Fin-LED-Strukturen
von Angelina JarosNowadays gallium-nitride-based (GaN) light-emitting diodes (LEDs) with indium gallium nitride (InGaN) quantum wells are an integral part of the lighting market. One approach to increase the efficiency of LEDs and to reduce the production costs are three-dimensional (3D) core-shell structures. The quantum wells are grown as a shell around the n-doped GaN cores resulting in a large surface area in relation to the substrate and a mostly non-polar crystal orientation. So far, the focus of attention lay on core-shell microrod structures, but so-called fin structures (walls) are of increasing interest.
In this thesis, the luminescence properties and in particular the charge carrier dynamics of three-dimensional microrod and fin structures are investigated and conclusions are drawn about the underlying physical processes on the basis of time-integrated and time-resolved photoluminescence spectroscopy. Complementary information is obtained by means of cathodoluminescence spectroscopy. The material quality and concentration of free charge carriers are examined with Raman spectroscopy and scanning capacitance microscopy.
For the investigation of the charge carrier dynamics, fs laser pulses with a low repetition rate are used, which have pulse powers in the range of several tens of megawatts. This leads to high fluences close to the damage threshold of GaN. For the utilized titanium- sapphire laser system a damage threshold for GaN layers of 0,01 J/cm2 is determined for a pulse duration of 100 fs, a repetition rate of 1 kHz and a photon energy of 3,81 eV. Excitation densities close to the damage threshold are used for the measurements on the LED structures.
The results of the 3D structures are compared with the properties of a conventional planar structure. The greatest differences in the decay dynamics can be traced back to the influence of the quantum-confined Stark effect. This effect determines the recombination dynamics in planar samples grown in the polar c-direction but is negligible in the core- shell structures whose quantum wells exhibit a non-polar growth direction. This leads to faster decay times of the InGaN luminescence in the 3D structures. Furthermore, charge carriers are more strongly localized in the quantum wells of the microrod and fin structures as compared to the planar LEDs and thermally activated non-radiative processes play a minor role. An additional influence on the charge carrier dynamics of the quantum wells can be assigned to the surrounding layers. In the case of non-resonant excitation of the samples, thicker GaN capping layers lead to longer decay dynamics due to diffusion processes. The luminescence properties of the structures are additionally influenced by defects both in the shell layers and in the n-doped GaN cores and also by inhomogeneities in the thickness and indium concentration of the InGaN layers.
In this thesis, the luminescence properties and in particular the charge carrier dynamics of three-dimensional microrod and fin structures are investigated and conclusions are drawn about the underlying physical processes on the basis of time-integrated and time-resolved photoluminescence spectroscopy. Complementary information is obtained by means of cathodoluminescence spectroscopy. The material quality and concentration of free charge carriers are examined with Raman spectroscopy and scanning capacitance microscopy.
For the investigation of the charge carrier dynamics, fs laser pulses with a low repetition rate are used, which have pulse powers in the range of several tens of megawatts. This leads to high fluences close to the damage threshold of GaN. For the utilized titanium- sapphire laser system a damage threshold for GaN layers of 0,01 J/cm2 is determined for a pulse duration of 100 fs, a repetition rate of 1 kHz and a photon energy of 3,81 eV. Excitation densities close to the damage threshold are used for the measurements on the LED structures.
The results of the 3D structures are compared with the properties of a conventional planar structure. The greatest differences in the decay dynamics can be traced back to the influence of the quantum-confined Stark effect. This effect determines the recombination dynamics in planar samples grown in the polar c-direction but is negligible in the core- shell structures whose quantum wells exhibit a non-polar growth direction. This leads to faster decay times of the InGaN luminescence in the 3D structures. Furthermore, charge carriers are more strongly localized in the quantum wells of the microrod and fin structures as compared to the planar LEDs and thermally activated non-radiative processes play a minor role. An additional influence on the charge carrier dynamics of the quantum wells can be assigned to the surrounding layers. In the case of non-resonant excitation of the samples, thicker GaN capping layers lead to longer decay dynamics due to diffusion processes. The luminescence properties of the structures are additionally influenced by defects both in the shell layers and in the n-doped GaN cores and also by inhomogeneities in the thickness and indium concentration of the InGaN layers.