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Register a new userA Photonic Atom Probe Analysis of the Effect of Extended and Point Defects on the Luminescence of InGaN/GaN Quantum Dots
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We report a correlative microscopy study of a sample containing three stacks of InGaN/GaN quantum dots (QDs) grown at different substrate temperature, each stack consisting of 3 layers of QDs. Decreasing the substrate temperature along the growth axis leads to the proliferation of structural defects. However, the luminescence intensity increases towards the surface, in spite of the higher density of threading dislocations, revealing that the QD layers closer to the substrate behave as traps for non-radiative point defects. During atom probe tomography experiments combined with in-situ micro-photoluminescence, it was possible to isolate the optical emission of a single QD located in the topmost QD stack, closer to the sample surface. The single QD emission line displayed a spectral shift during the experiment confirming the relaxation of elastic strain due to material evaporation during atom probe tomography.
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We derive an energy-dependent decay-time distribution function from the multi-exponential decay of the ensemble photoluminescence (PL) of InGaN/GaN quantum dots (QDs), which agrees well with recently published single-QD time-resolved PL measurements. Using eight-band k.p modelling, we show that the built-in piezo- and pyroelectric fields within the QDs cause a sensitive dependence of the radiative lifetimes on the exact QD geometry and composition. Moreover, the radiative lifetimes also depend heavily on the composition of the direct surrounding of the QDs. A broad lifetime distribution occurs even for moderate variations of the QD structure. Thus, for unscreened fields a multi-exponential decay of the ensemble PL is generally expected in this material system.
We demonstrate a series of InGaN/GaN double quantum well nanostructure elements. We grow a layer of 2 {mu}m undoped GaN template on top of a (0001)-direction sapphire substrate. A 100 nm SiO2 thin film is deposited on top as a masking pattern layer. This layer is then covered with a 300 nm aluminum layer as the anodic aluminum oxide (AAO) hole pattern layer. After oxalic acid etching, we transfer the hole pattern from the AAO layer to the SiO2 layer by reactive ion etching. Lastly, we utilize metal-organic chemical vapor deposition to grow GaN nanorods approximately 1.5 {mu}m in size. We then grow two layers of InGaN/GaN double quantum wells on the semi-polar face of the GaN nanorod substrate under different temperatures. We then study the characteristics of the InGaN/GaN quantum wells formed on the semi-polar faces of GaN nanorods. We report the following findings from our study: first, using SiO2 with repeating hole pattern, we are able to grow high-quality GaN nanorods with diameters of approximately 80-120 nm; second, photoluminescence (PL) measurements enable us to identify Fabry-Perot effect from InGaN/GaN quantum wells on the semi-polar face. We calculate the quantum wells cavity thickness with obtained PL measurements. Lastly, high resolution TEM images allow us to study the lattice structure characteristics of InGaN/GaN quantum wells on GaN nanorod and identify the existence of threading dislocations in the lattice structure that affects the GaN nanorods growth mechanism.
The optical emission of InGaN quantum dots embedded in GaN nanowires is dynamically controlled by a surface acoustic wave (SAW). The emission energy of both the exciton and biexciton lines is modulated over a 1.5 meV range at ~330 MHz. A small but systematic difference in the exciton and biexciton spectral modulation reveals a linear change of the biexciton binding energy with the SAW amplitude. The present results are relevant for the dynamic control of individual single photon emitters based on nitride semiconductors.
High resolution coherent nonlinear optical spectroscopy of an ensemble of red-emitting InGaN quantum dots in GaN nanowires is reported. The data show a pronounced atom-like interaction between resonant laser fields and quantum dot excitons at low temperature that is difficult to observe in the linear absorption spectrum due to inhomogeneous broadening from indium fluctuation effects. We find that the nonlinear signal persists strongly at room temperature. The robust atom-like room temperature response indicates the possibility that this material could serve as the platform for proposed excitonic based applications without the need of cryogenics.
We present an eight-band k.p model for the calculation of the electronic structure of wurtzite semiconductor quantum dots (QDs) and its application to indium gallium nitride (InGaN) QDs formed by composition fluctuations in InGaN layers. The eight-band k.p model accounts for strain effects, piezoelectric and pyroelectricity, spin-orbit and crystal field splitting. Exciton binding energies are calculated using the self-consistent Hartree method. Using this model, we studied the electronic properties of InGaN QDs and their dependence on structural properties, i.e., their chemical composition, height, and lateral diameter. We found a dominant influence of the built-in piezoelectric and pyroelectric fields, causing a spatial separation of the bound electron and hole states and a redshift of the exciton transition energies. The single-particle energies as well as the exciton energies depend heavily on the composition and geometry of the QDs.
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