Polariton effects in the dielectric function of ZnO excitons obtained by ellipsometry

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Appl. Phys. Lett. 96, 031904 (2010) and may be found at https://doi.org/10.1063/1.3284656.The complex dielectric tensor of ZnO in the regime of the excitonic transitions is determined with ellipsometry and analyzed concerning the quantization of the electromagnetic field in terms of coupled polariton-eigenmodes. Negative sections in the real part indicate the significant formation of polaritons for the dipole-allowed excitons of the three upper valence-bands Γ7,Γ9,Γ7. The transverse-longitudinal splittings which separate the upper polariton branch from the lower branch, corresponding to the k-vector of the used light, are deduced precisely for each subband. Mainly for E∥c, additional absorption peaks are observed at the longitudinal B-exciton and closely above. One is considered to be a mixed-mode and the other is seen as a consequence of interference effects in an exciton free surface layer which is also visible in reflectance anisotropy spectroscopy.EC/FP7/218570/EU/MULTIFUNCTIONAL NANOMATERIALS CHARACTERISATION EXPLOITING ELLIPSOMETRY and POLARIMETRY/NANOCHAR

Compensation effects in GaN:Mg probed by Raman spectroscopy and photoluminescence measurements

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in J. Appl. Phys. 113, 103504 (2013) and may be found at https://doi.org/10.1063/1.4794094.Compensation effects in metal organic chemical vapour deposition grown GaN doped with magnesium are investigated with Raman spectroscopy and photoluminescence measurements. Examining the strain sensitive E2(high) mode, an increasing compressive strain is revealed for samples with Mg-concentrations lower than 7 × 1018 cm−3. For higher Mg-concentrations, this strain is monotonically reduced. This relaxation is accompanied by a sudden decrease in crystal quality. Luminescence measurements reveal a well defined near band edge luminescence with free, donor bound, and acceptor bound excitons as well as a characteristic donor acceptor pair (DAP) luminescence. Following recent results, three acceptor bound excitons and donor acceptor pairs are identified. Along with the change of the strain, a strong modification in the luminescence of the dominating acceptor bound exciton and DAP luminescence is observed. The results from Raman spectroscopy and luminescence measurements are interpreted as fingerprints of compensation effects in GaN:Mg leading to the conclusion that compensation due to defect incorporation triggered by Mg-doping already affects the crystal properties at doping levels of around 7 × 1018 cm−3. Thereby, the generation of nitrogen vacancies is introduced as the driving force for the change of the strain state and the near band edge luminescence.DFG, 43659573, SFB 787: Halbleiter - Nanophotonik: Materialien, Modelle, Bauelement

Electronic excitations stabilized by a degenerate electron gas in semiconductors

Excitons in semiconductors and insulators consist of fermionic subsystems, electrons and holes, whose attractive interaction facilitates bound quasiparticles with quasi-bosonic character. In the presence of a degenerate electron gas, such excitons dissociate due to free carrier screening. Despite their absence, we found pronounced emission traces in the below-band-edge region of bulk, germanium-doped GaN up to a temperature of 100 K, mimicking sharp spectral features at high free electron concentrations (3.4E19–8.9E19 cm−3). Our interpretation of the data suggests that a degenerate, three-dimensional electron gas stabilizes a novel class of quasiparticles, which we name collexons. These many-particle complexes are formed by exchange of electrons with the Fermi gas. The potential observation of collexons and their stabilization with rising doping concentration is enabled by high crystal quality due to the almost ideal substitution of host atoms with dopants.DFG, 43659573, SFB 787: Semiconductor Nanophotonics: Materials, Models, Device

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

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Journal of Applied Physics 113, 203513 (2013) and may be found at https://doi.org/10.1063/1.4807581.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 × 104 cm−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

Lithium related deep and shallow acceptors in Li-doped ZnO nanocrystals

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Journal of Applied Physics 107, 024311 (2010) and may be found at https://doi.org/10.1063/1.3275889.We study the existence of Li-related shallow and deep acceptor levels in Li-doped ZnO nanocrystals using electron paramagnetic resonance (EPR) and photoluminescence (PL) spectroscopy. ZnO nanocrystals with adjustable Li concentrations between 0% and 12% have been prepared using organometallic precursors and show a significant lowering of the Fermi energy upon doping. The deep Li acceptor with an acceptor energy of 800 meV could be identified in both EPR and PL measurements and is responsible for the yellow luminescence at 2.2 eV. Additionally, a shallow acceptor state at 150 meV above the valence band maximum is made responsible for the observed donor-acceptor pair and free electron-acceptor transitions at 3.235 and 3.301 eV, possibly stemming from the formation of Li-related defect complexes acting as acceptors.DFG, 43659573, SFB 787: Halbleiter - Nanophotonik: Materialien, Modelle, Bauelement

Decay dynamics of excitonic polarons in InAs/GaAs quantum dots

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Journal of Applied Physics 110, 074303 (2011) and may be found at https://doi.org/10.1063/1.3639310.We present time-resolved studies of the exciton-phonon interaction in self-assembled InAs/GaAs quantum dots. Different scattering and luminescence processes were investigated by time-resolved spectroscopy exciting resonantly into the quantum dot’s electronic structure. By studying the characteristic decay times of the ground state and of several phonon-assisted recombinations we were able to distinguish a resonant Raman process from a phonon-assisted photoluminescence process which are always simultaneously present and can interfere with each other. While lifetimes under 30 ps were observed for the coherent Raman process, the incoherent phonon-assisted recombination exhibited typical lifetimes of around 1 ns independently of the excitation energy. We conclude that under resonant excitation the dominant radiative recombination process in this system always involves an electronic state of the ground state of the quantum dot’s electronic structure. Combining temperature-dependent and time-resolved measurements we show that a weak phonon-bottleneck is present in the low temperature regime (< 130 K), while it disappears for higher temperatures.DFG, 43659573, SFB 787: Halbleiter - Nanophotonik: Materialien, Modelle, BauelementeDFG, 53182490, EXC 314: Unifying Concepts in Catalysi

Diabetes is associated with impaired myocardial performance in patients without significant coronary artery disease

<p>Abstract</p> <p>Background</p> <p>Patients with diabetes mellitus (DM) have high risk of heart failure. Whether some of the risk is directly linked to metabolic derangements in the myocardium or whether the risk is primarily caused by coronary artery disease (CAD) and hypertension is incompletely understood. Echocardiographic tissue Doppler imaging was therefore performed in DM patients without significant CAD to examine whether DM per se influenced cardiac function.</p> <p>Methods</p> <p>Patients with a left ventricular (LV) ejection fraction (EF) > 35% and without significant CAD, prior myocardial infarction, cardiac pacemaker, atrial fibrillation, or significant valve disease were identified from a tertiary invasive center register. DM patients were matched with controls on age, gender and presence of hypertension.</p> <p>Results</p> <p>In total 31 patients with diabetes and 31 controls were included. Mean age was 58 ± 12 years, mean LVEF was 51 ± 7%, and 48% were women. No significant differences were found in LVEF, left atrial end systolic volume, or left ventricular dimensions. The global longitudinal strain was significantly reduced in patients with DM (15.9 ± 2.9 vs. 17.7 ± 2.9, p = 0.03), as were peak longitudinal systolic (S') and early diastolic (E') velocities (5.7 ± 1.1 vs. 6.4 ± 1.1 cm/s, p = 0.02 and 6.1 ± 1.7 vs. 7.7 ± 2.0 cm/s, p = 0.002). In multivariable regression analyses, DM remained significantly associated with impairments of S' and E', respectively.</p> <p>Conclusion</p> <p>In patients without significant CAD, DM is associated with an impaired systolic longitudinal LV function and global diastolic dysfunction. These abnormalities are likely to be markers of adverse prognosis.</p

Decoding the Fucose Migration Product during Mass-Spectrometric analysis of Blood Group Epitopes

Fucose is a signaling carbohydrate that is attached at the end of glycan processing. It is involved in a range of processes, such as the selectin-dependent leukocyte adhesion or pathogen-receptor interactions. Mass-spectrometric techniques, which are commonly used to determine the structure of glycans, frequently show fucose-containing chimeric fragments that obfuscate the analysis. The rearrangement leading to these fragments—often referred to as fucose migration—has been known for more than 25 years, but the chemical identity of the rearrangement product remains unclear. In this work, we combine ion-mobility spectrometry, radical-directed dissociation mass spectrometry, cryogenic IR spectroscopy of ions, and density-functional theory calculations to deduce the product of the rearrangement in the model trisaccharides Lewis x and blood group H2. The structural search yields the fucose moiety attached to the galactose with an α(1→6) glycosidic bond as the most likely product

Nitrogen and vacancy clusters in ZnO

Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.Understanding the interaction of group V impurities with intrinsic defects in ZnO is important for developing p-type material. We have studied N-doped ZnO thin films and N-doped bulk ZnO crystals, with positron annihilation spectroscopy, in contrast to earlier studies that have concentrated on N-implanted ZnO crystals. We show that the introduction of N impurities into ZnO, irrespective of whether it is done during the growth of thin films or bulk crystals or through implantation and subsequent thermal treatments, leads to the formation of stable vacancy clusters and negative ion-type defects. Interestingly, the stability of these vacancy clusters is found almost exclusively for N introduction, whereas single Zn vacancy defects or easily removable vacancy clusters are more typically found for ZnO doped with other impurities.DFG, SFB 787, Halbleiter - Nanophotonik: Materialien, Modelle, Bauelement