InGaN based blue and near-ultraviolet light emitting diodes and laser diodes have been successfully commercialized for many applications such as general lighting, display backlighting and high density optical storage devices. Despite having a comparably high defect density, these devices are known for their efficient operation, which is attributed to localization in potential fluctuations preventing carriers from reaching the centers of nonradiative recombination. Nitride research is currently headed towards improving deep ultraviolet AlGaN and green InGaN emitters with higher Al and In molar fractions. The efficiency of these devices trails behind the blue counterparts as the carrier localization does not seem to aid in supressing nonradiative losses. In addition, the operation of ternary nitride heterostructure based devices is further complicated by the presence of large built-in electric fields. Although the problem can be ameliorated by growing structures in nonpolar or semipolar directions, the step from research to production still awaits. In this thesis, carrier dynamics and localization effects have been studied in three different nitride ternary compounds: AlGaN epitaxial layers and quantum wells with high Al content, nonpolar m-plane InGaN/GaN quantum wells and lattice matched AlInN/GaN heterostructures. The experimental methods of this work mainly consist of spectroscopy techniques such as time-resolved photoluminescence and differential transmission pump-probe measurements as well as spatial photoluminescence mapping by means of scanning near-field microscopy. The comparison of luminescence and differential transmission measurements has allowed estimating the localization depth in AlGaN quantum wells. Additionally, it has been demonstrated that the polarization degree of luminescence from m-InGaN quantum wells decreases as carriers diffuse to localization centers.What is more, dual-scale localization potential has been evidenced by near-field measurements in both AlGaN and m-InGaN. Larger scale potential fluctuation have been observed directly and the depth of nanoscopic localization has been estimated theoretically from the recorded linewidth of the near-field spectra. Lastly, efficient carrier transport has been observed through AlInN layer despite large alloy inhomogeneities evidenced by broad luminescence spectra and the huge Stokes shift. Inhomogeneous luminescence from the underlying GaN layer has been linked to the fluctuations of the built-in electric field at the AlInN/GaN interface. / <p>QC 20121101</p>

InGaN based light emitting devices operating in the blue and near UV spectral regions are commercialized and used in many applications. InGaN heterostructures experience compositional inhomogeneity and thus potential fluctuations, such that regions of higher indium composition are formed and correspond to lower potentials. The indium rich regions form localization centers that save carriers from non-radiative recombination at dislocations, thus despite the large defect density, their quantum efficiency are surprisingly large. However, the conventional c-plane InGaN QWs suffer from high internal piezoelectric and spontaneous fields. These fields are detrimental for the performance of such structures as they lead to the quantum confined stark effect causing red-shift of the emission as well as reducing the electrons and holes wavefunctions overlap, thereby reducing the radiative recombination rate. However, growth of InGaN QWs on semipolar and nonpolar planes greatly reduced the polarization fields. Semipolar and nonpolar QWs experience an outstanding property which is polarized luminescence, opening a new frontier for applications for InGaN emitting devices. While nonpolar QWs have larger degree of polarized emission than semipolar QWs, semipolar QWs can emit in longer wavelengths due to their higher indium uptake. In this thesis, semipolar 20¯21 and nonpolar m-plane InGaN/GaN QWs were investigated. Photoluminescence, spectral and polarization dynamics were all studied in order to form a whole picture of the carrier dynamics in the QWs. Time resolved photoluminescence measurements were conducted for following carriers distribution between extended and localized states. Both the semipolar and nonpolar samples showed efficient luminescence through short radiative recombination times, as well as carrier localization in lower potential sites after thermal activation of excitons. Carrier localization was found to be benign as it didn’t degrade the performance of the samples or decrease the polarization ratio of their emission. However, the structures showed modest potential variations with the absence of deep localization centers or quantum dots. High polarization ratios were measured for both samples, which is well-known for nonpolar QWs. The high polarization ratio for the semipolar sample is of great importance, thus semipolar 20¯21 QWs should be considered for longer wavelength emitters with highly polarized spontaneous emission.

Diese Arbeit befasst sich mit den optischen Eigenschaften von dünnen GaN-Schichten gewachsen in verschiedenen Orientierungen. Hierbei werden die optischen Eigenschaften von verspannten M- und A-plane sowie unverspannten C-plane GaN-Schichten untersucht und die Ergebnisse im Rahmen von Bandstrukturberechnungen diskutiert. Im Rahmen dieser Arbeit werden die Bandstrukturverschiebungen theoretisch mittels eines k.p-Näherungsansatzes untersucht. Diese Bandverschiebungen beeinflussen sowohl die Übergangsenergien als auch die Oszillatorstärken. Man findet, dass die C-plane Schicht im Falle einer isotropen Verspannung in der Filmebene keine Anisotropie der optischen Polarisation zeigt. In beiden Fällen zeigen die drei Übergänge von den drei oberen Valenzbändern in das untere Leitungsband andere Polarisationseigenschaften als die entsprechenden Übergänge in C-plane GaN-Schichten. Es wird beobachtet, dass für einen bestimmten Wertebereich der Verspannung in der Filmebene diese Übergänge nahezu vollständig x-,z- bzw. y-artig polarisiert sind. Die verwendeten Schichten wurden auch mittels Transmissionspektroskopie untersucht. Im Falle der M-plane GaN-Schichten können zwei fundamentale Übergänge identifiziert werden, wobei der elektrische Feldvektor E des einfallenden Lichtes einmal parallel (z-Polarisation) und einmal senkrecht (x-Polarisation) auf der c-Achse steht. Die M-plane GaN-Schicht besitzt unterschiedliche Dielektrizitätskonstanten für z-Polarisation und x-Polarisation, welche zu zusätzlichem Dichroismus und Doppelbrechung führen. Als Resultat findet eine Filterung der Polarisation für einfallendes, linear polarisiertes Licht statt. Die elektrische Feldkomponente mit x-Polarisation wird stärker absorbiert als die Komponente mit z-Polarisation. Diese Polarisationsfilterung äußert sich für schmalbandiges Licht in Form einer Drehung der Polarisationsebene in Richtung der c-Achse, wobei ein maximaler Rotationswinkel von 40 Grad gefunden wurde. / In this work, we focus on the optical response of GaN thin films grown along various orientations. The optical properties of strained M- and A- and unstrained C-plane GaN thin films are investigated, and the results are explained with help of band-structure calculations. We calculate the strain-induced band-structure modification using the k.p perturbation approach. The valence-band (VB) states are modified affecting both the transition energies as well as the oscillator strengths. We observe that C-plane GaN does not show any in-plane polarization anisotropy, when an isotropic in-plane strain is applied. For the case of M- and A-plane GaN, one expects to see an in-plane polarization anisotropy even for the unstrained case. Additionally, the in-plane strain significantly changes the band structure and the symmetry of the VB states. The three transitions, involving electrons in the conduction band (CB) and holes in the top three VBs, will exhibit a very different polarization characteristic than the ones for C-plane GaN. These transitions are predominantly x, z, and y polarized, respectively, for a certain range of in-plane strain values, present in our samples. For M-plane GaN thin films, two fundamental transitions can be identified, which occur when the electric field vector E is perpendicular (x-polarization) and parallel c (z-polarization). These transitions give rise to a transmittance spectrum separated by 50 meV at room temperature with respect to each other. This result in a polarization filtering of an incident linearly polarized light beam after transmission, because the electric field component with x-polarization is more strongly absorbed than with z-polarization. This filtering manifests as a rotation of the polarization vector toward the c axis and can be as large as 40 degrees for an initial angle of 60 degrees, for our samples.