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- ItemMode competition in broad-ridge-waveguide lasers(Bristol : IOP Publ., 2020) Koester, J.-P.; Putz, A.; Wenzel, H.; Wünsche, H.-J.; Radziunas, M.; Stephan, H.; Wilkens, M.; Zeghuzi, A.; Knigge, A.The lateral brightness achievable with high-power GaAs-based laser diodes having long and broad waveguides is commonly regarded to be limited by the onset of higher-order lateral modes. For the study of the lateral-mode competition two complementary simulation tools are applied, representing different classes of approximations. The first tool bases on a completely incoherent superposition of mode intensities and disregards longitudinal effects like spatial hole burning, whereas the second tool relies on a simplified carrier transport and current flow. Both tools yield agreeing power-current characteristics that fit the data measured for 5-23 µm wide ridges. Also, a similarly good qualitative conformance of the near and far fields is found. However, the threshold of individual modes, the partition of power between them at a given current, and details of the near and far fields show differences. These differences are the consequence of a high sensitivity of the mode competition to details of the models and of the device structure. Nevertheless, it can be concluded concordantly that the brightness rises with increasing ridge width irrespective of the onset of more and more lateral modes. The lateral brightness W mm-1at 10 MW cm-2 power density on the front facet of the investigated laser with widest ridge (23 µm) is comparable with best values known from much wider broad-area lasers. In addition, we show that one of the simulation tools is able to predict beam steering and coherent beam coupling without introducing any phenomenological coupling coefficient or asymmetries. © 2020 The Author(s). Published by IOP Publishing Ltd.
- ItemTerahertz quantum-cascade lasers for high-resolution absorption spectroscopy of atoms and ions in plasmas(Bristol : IOP Publ., 2023) Lü, X.; Röben, B.; Biermann, K.; Wubs, J.R.; Macherius, U.; Weltmann, K.-D.; van Helden, J.H.; Schrottke, L.; Grahn, H.T.We report on terahertz (THz) quantum-cascade lasers (QCLs) based on GaAs/AlAs heterostructures, which exhibit single-mode emission at 3.360, 3.921, and 4.745 THz. These frequencies are in close correspondence to fine-structure transitions of Al atoms, N+ ions, and O atoms, respectively. Due to the low electrical pump power of these THz QCLs, they can be operated in a mechanical cryocooler in continuous-wave mode, while a sufficient intrinsic tuning range of more than 5 GHz is maintained. The single-mode operation and the intrinsic tuning range of these THz QCLs allow for the application of these lasers as radiation sources for high-resolution absorption spectroscopy to determine the absolute densities of Al atoms, N+ ions, and O atoms in plasmas.
- ItemEffects of post metallization annealing on Al2O3 atomic layer deposition on n-GaN(Bristol : IOP Publ., 2022) Tadmor, Liad; Brusaterra, Enrico; Treidel, Eldad Bahat; Brunner, Frank; Bickel, Nicole; Vandenbroucke, Sofie S. T.; Detavernier, Christophe; Würfl, Joachim; Hilt, OliverThe chemical, physical and electrical properties and the robustness of post metallization annealed Al2O3 atomic layers deposited on n-type GaN are investigated in this work. Planar metal insulator capacitors are used to demonstrate a gate-first with following ohmic contacts formation at elevated temperature up to 600 °C process flow. X-ray photoelectron spectroscopy indicates that no new bonds in the Al2O3 layer are formed due to exposure to the elevated annealing temperature. X-ray diffraction measurements show no crystallization of the oxide layer. Atomic force microscopy shows signs of degradation of the sample annealed at 600 °C. Electrical measurements indicate that the elevated annealing temperature results in an increase of the oxide depletion and the deep depletion capacitances simultaneously, that results in a reduction of the flat band voltage to zero, which is explained by fixed oxide charges curing. A forward bias step stress capacitance measurement shows that the total number of induced trapped charges are not strongly affected by the elevated annealing temperatures. Interface trap density of states analysis shows the lowest trapping concentration for the capacitor annealed at 500 °C. Above this temperature, the interface trap density of states increases. When all results are taken into consideration, we have found that the process thermal budget allows for an overlap between the gate oxide post metallization annealing and the ohmic contact formation at 500 °C.
- ItemTerahertz quantum-cascade lasers for high-resolution absorption spectroscopy of atoms and ions in plasmas(Bristol : IOP Publ., 2023) Lü, X.; Röben, B.; Biermann, K.; Wubs, J.R.; Macherius, U.; Weltmann, K.-D.; van Helden, J.H.; Schrottke, L.; Grahn, H.T.We report on terahertz (THz) quantum-cascade lasers (QCLs) based on GaAs/AlAs heterostructures, which exhibit single-mode emission at 3.360, 3.921, and 4.745 THz. These frequencies are in close correspondence to fine-structure transitions of Al atoms, N+ ions, and O atoms, respectively. Due to the low electrical pump power of these THz QCLs, they can be operated in a mechanical cryocooler in continuous-wave mode, while a sufficient intrinsic tuning range of more than 5 GHz is maintained. The single-mode operation and the intrinsic tuning range of these THz QCLs allow for the application of these lasers as radiation sources for high-resolution absorption spectroscopy to determine the absolute densities of Al atoms, N+ ions, and O atoms in plasmas.
- ItemTerahertz quantum-cascade lasers for high-resolution absorption spectroscopy of atoms and ions in plasmas(Bristol : IOP Publ., 2023) Lü, X.; Röben, B.; Biermann, K.; Wubs, J.R.; Macherius, U.; Weltmann, K.-D.; van Helden, J.H.; Schrottke, L.; Grahn, H.T.We report on terahertz (THz) quantum-cascade lasers (QCLs) based on GaAs/AlAs heterostructures, which exhibit single-mode emission at 3.360, 3.921, and 4.745 THz. These frequencies are in close correspondence to fine-structure transitions of Al atoms, N+ ions, and O atoms, respectively. Due to the low electrical pump power of these THz QCLs, they can be operated in a mechanical cryocooler in continuous-wave mode, while a sufficient intrinsic tuning range of more than 5 GHz is maintained. The single-mode operation and the intrinsic tuning range of these THz QCLs allow for the application of these lasers as radiation sources for high-resolution absorption spectroscopy to determine the absolute densities of Al atoms, N+ ions, and O atoms in plasmas.
- ItemMWCNT induced negative real permittivity in a copolyester of Bisphenol-A with terephthalic and isophthalic acids(Bristol : IOP Publ., 2020) Özdemir, Zeynep Güven; Daşdan, Dolunay Şakar; Kavak, Pelin; Pionteck, Jürgen; Pötschke, Petra; Voit, Brigitte; SüngüMısırlıoğlu, BanuIn the present study, the negative real permittivity behavior of a copolyester of bisphenol-A with terephthalic acid and isophthalic acid (PAr) containing 1.5 to 7.5 wt% multi-walled carbon nanotubes (MWCNTs) have been investigated in detail. The structural and morphological analysis of the melt-mixed composites was performed by Fourier transform infrared spectroscopy using attenuated total reflection (FTIR-ATR), atomic force microscopy (AFM), X-ray diffraction (XRD), and light microscopy. The influences of the MWCNT filler on the AC impedance, complex permittivity, and AC conductivity of the PAr polymer matrix were investigated at different operating temperatures varied between 296 K and 373 K. The transition from a negative to positive real permittivity was observed at different crossover frequencies depending on the MWCNT content of the composites whereas pure PAr showed positive values at all frequencies. The negative real permittivity characteristic of the composites was discussed in the context of Drude model. © 2020 The Author(s). Published by IOP Publishing Ltd.
- ItemElastic properties of single crystal Bi12SiO20 as a function of pressure and temperature and acoustic attenuation effects in Bi12 MO20 (M = Si, Ge and Ti)(Bristol : IOP Publ., 2020) Haussühl, Eiken; Reichmann, Hans Josef; Schreuer, Jürgen; Friedrich, Alexandra; Hirschle, Christian; Bayarjargal, Lkhamsuren; Winkler, Björn; Alencar, Igor; Wiehl, Leonore; Ganschow, SteffenA comprehensive study of sillenite Bi12SiO20 single-crystal properties, including elastic stiffness and piezoelectric coefficients, dielectric permittivity, thermal expansion and molar heat capacity, is presented. Brillouin-interferometry measurements (up to 27 GPa), which were performed at high pressures for the first time, and ab initio calculations based on density functional theory (up to 50 GPa) show the stability of the sillenite structure in the investigated pressure range, in agreement with previous studies. Elastic stiffness coefficients c 11 and c 12 are found to increase continuously with pressure while c 44 increases slightly for lower pressures and remains nearly constant above 15 GPa. Heat-capacity measurements were performed with a quasi-adiabatic calorimeter employing the relaxation method between 2 K and 395 K. No phase transition could be observed in this temperature interval. Standard molar entropy, enthalpy change and Debye temperature are extracted from the data. The results are found to be roughly half of the previous values reported in the literature. The discrepancy is attributed to the overestimation of the Debye temperature which was extracted from high-temperature data. Additionally, Debye temperatures obtained from mean sound velocities derived by Voigt-Reuss averaging are in agreement with our heat-capacity results. Finally, a complete set of electromechanical coefficients was deduced from the application of resonant ultrasound spectroscopy between 103 K and 733 K. No discontinuities in the temperature dependence of the coefficients are observed. High-temperature (up to 1100 K) resonant ultrasound spectra recorded for Bi12 MO20 crystals revealed strong and reversible acoustic dissipation effects at 870 K, 960 K and 550 K for M = Si, Ge and Ti, respectively. Resonances with small contributions from the elastic shear stiffness c 44 and the piezoelectric stress coefficient e 123 are almost unaffected by this dissipation. © 2020 The Author(s). Published by IOP Publishing Ltd.