Advanced optical signatures of single, wurtzite GaN quantum dots : from fundamental exciton coupling mechanisms towards tunable photon statistics and hybrid-quasiparticles

University of Technology Sydney. Faculty of Science.The present work treats the fundamental optical signatures of individual, hexagonal GaN quantum dots embedded in AlN. The conducted experiments established the basis for numerous, novel observations, which are not only of interest for this particular quantum dot system, but also of general value for the entire quantum dot community. The presented analysis of the interaction in between quantum dot excitons and charged defects, as well as phonons, culminates in the first-time demonstration of quantum-optical device concepts for the ultraviolet (UV) spectral range. Here, a highlight is constituted by an optically pumped two-photon source based on the biexciton cascade, which maintains its highly promising photon statistics up to a temperature of 50K. Further in-detail studies of this biexciton cascade even lead to the description of so-called hybrid-quasiparticles in this work, with prominent consequences for a wide range of exciton-based quantum light sources. The first part of this thesis is dedicated to the preparation of multiexcitonic states. Based on the conjunction of excitation power dependent and time-resolved micro-Photoluminescence, an entire zoo of multiexcitonic complexes is identified for the first time. Here, the determination of relaxation times presents an observation with direct consequences for applications. Furthermore, it is demonstrated that the initial carrier capture process is predominantly realized by Auger-processes that dominate any multi-phonon contributions. However, in terms of intra-quantum dot carrier relaxation, it is exactly these multi-phonon processes that present the limiting factor, a phenomenon known as the “phonon-bottleneck” effect. As the emission of these excitons in hexagonal GaN is affected by “spectral diffusion”, a strong emission line widths broadening occurs, which still limits future applications but also any more fundamental analysis. A line width statistic is obtained by analyzing hundreds of individual GaN quantum dots, allowing an indirect determination of the average, defect-induced electric field, whose fluctuations originate the line widths broadening. A continuative statistical analysis is given for the coupling between excitons and longitudinal-optical (LO) phonons. As a result, the corresponding Huang-Rhys factors and LO-phonon energies are extracted for an elevated number of quantum dots. Finally, a microscopic parameter, known as the exciton-LO-phonon interaction volume was approximated for the first time, based on the presented detailed statistical analysis. Due to this extended, optical analysis of individual GaN quantum dots it was possible to characterize the optical traces of the biexciton cascade over a wide spectral range. Here, for a certain transitional range, a unique balance between one- and two-photon processes is observed, which arises from the biexciton decay and can be tuned means of temperature and excitation density. Especially the two-photon emission is a promising candidate for future applications as its temperature stability is demonstrated up to 50K. Interestingly, the particular case of biexcitonic complexes also forms the basis for the description of an entire new class of hybrid-quasiparticles with so far unknown spin configurations. An extended analysis of the optical properties of these hybrid-quasiparticles presents highly unconventional decay characteristics, demonstrating the outmost importance of the dark-excitons in hexagonal GaN quantum dots based on the present thesis

Effects of Strain on the valence band structure and exciton-polariton energies in ZnO

ABSTRACT The uniaxial stress dependence of the band structure and the exciton-polariton transitions in wurtzite ZnO is thoroughly studied using modern first-principles calculations based on the HSE+G0W0 approach, k p modeling using the deformation potential framework, and polarized photoluminescence measurements. The ordering of the valence bands (A(G7), B(G9), C(G7)) is found to be robust even for high uniaxial and biaxial strains. Theoretical results for the uniaxial pressure coefficients and splitting rates of the A, B, and C valence bands and their optical transitions are obtained including the effects of the spin-orbit interaction. The excitonic deformation potentials are derived and the stress rates for hydrostatic pressure are determined based on the results for uniaxial and biaxial stress. In addition, the theory for the stress dependence of the exchange interaction and longitudinal-transversal splitting of the exciton-polaritons is developed using the basic exciton functions of the quasi-cubic approximation and taking the interaction between all exciton states into account. It is shown that the consideration of these effects is crucial for an accurate description of the stress dependence of the optical spectra in ZnO. The theoretical results are compared to polarized photoluminescence measurements of different ZnO substrates as function of uniaxial pressure and experimental values reported in the literature demonstrating an excellent agreement with the computed pressure coefficient

Structural and optical investigation of non-polar (1-100) GaN grown by the ammonothermal method

We studied the structural and optical properties of state-of-the-art non-polar bulk GaN grown by the ammonothermal method. The investigated samples have an extremely low dislocation density (DD) of less than 5 × 104cm-2, which results in very narrow high-resolution x-ray rocking curves. The a and c lattice parameters of these stress-free GaN samples were precisely determined by using an x-ray diffraction technique based on the modified Bond method. The obtained values are compared to the lattice parameters of free-standing GaN from different methods and sources. The observed differences are discussed in terms of free-electron concentrations, point defects, and DD. Micro Raman spectroscopy revealed a very narrow phonon linewidth and negligible built-in strain in accordance with the high-resolution x-ray diffraction data. The optical transitions were investigated by cathodoluminescence measurements. The analysis of the experimental data clearly demonstrates the excellent crystalline perfection of ammonothermal GaN material and its potential for fabrication of non-polar substrates for homoepitaxial growth of GaN based device structures. © 2013 AIP Publishing LLC

Phonon pressure coefficients and deformation potentials of wurtzite AlN determined by uniaxial pressure-dependent Raman measurements

© 2014 American Physical Society. We studied bulk crystals of wurtzite AlN by means of uniaxial pressure-dependent Raman measurements. As a result, we derive the phonon pressure coefficients and deformation potentials for all zone center optical phonon modes. For the A1 and E1 modes, we further experimentally determined the uniaxial pressure dependence of their longitudinal optical-transverse optical (LO-TO) splittings. Our experimental approach delivers new insight into the large variance among previously reported phonon deformation potentials, which are predominantly based on heteroepitaxial growth of AlN and the ball-on-ring technique. Additionally, the measured phonon pressure coefficients are compared to their theoretical counterparts obtained by density functional theory implemented in the siesta package. Generally, we observe a good agreement between the calculated and measured phonon pressure coefficients but some particular Raman modes exhibit significant discrepancies similar to the case of wurtzite GaN and ZnO, clearly motivating the presented uniaxial pressure-dependent Raman measurements on bulk AlN crystals

Growth and structure of In0.5Ga0.5Sb quantum dots on GaP(001)

Stranski-Krastanov (SK) growth of In0.5Ga0.5Sb quantum dots (QDs) on GaP(001) by metalorganic vapor phase epitaxy is demonstrated. A thin GaAs interlayer prior to QD deposition enables QD nucleation. The impact of a short Sb-flush before supplying InGaSb is investigated. QD growth gets partially suppressed for GaAs interlayer thicknesses below 6 monolayers. QD densities vary from 5 × 109 to 2 × 1011 cm−2 depending on material deposition and Sb-flush time. When In0.5Ga0.5Sb growth is carried out without Sb-flush, the QD density is generally decreased, and up to 60% larger QDs are obtained

Visible spectrum quantum light sources based on InxGa1–xN/GaN Quantum Dots

We present a method for designing quantum light sources, emitting in the visible band, using wurtzite InxGa1−xN quantum dots (QDs) in a GaN matrix. This system is significantly more versatile than previously proposed arsenide- and phosphide-based QDs, having a tuning range exceeding 1 eV. The quantum mechanical configuration interaction method, capturing the fermionic nature of electrons and associated quantum effects explicitly, is used to find shapes and compositions of dots to maximize the excitonic dipole matrix element and optimize the biexciton binding energy. These results provide QD morphologies tailored for either bright single-photon emission or entangledphoton- pair emission at any given wavelength in the visible spectrum