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- ItemRoadmap on quantum nanotechnologies(Bristol : IOP Publ., 2021) Laucht, Arne; Hohls, Frank; Ubbelohde, Niels; Fernando Gonzalez-Zalba, M.; Reilly, David J.; Stobbe, Søren; Schröder, Tim; Scarlino, Pasquale; Koski, Jonne V.; Dzurak, Andrew; Yang, Chih-Hwan; Yoneda, Jun; Kuemmeth, Ferdinand; Bluhm, Hendrik; Pla, Jarryd; Hill, Charles; Salfi, Joe; Oiwa, Akira; Muhonen, Juha T.; Verhagen, Ewold; LaHaye, M D; Kim, Hyun Ho; Tsen, Adam W; Culcer, Dimitrie; Geresdi, Attila; Mol, Jan A.; Mohan, Varun; Jain, Prashant K.; Baugh, JonathanQuantum phenomena are typically observable at length and time scales smaller than those of our everyday experience, often involving individual particles or excitations. The past few decades have seen a revolution in the ability to structure matter at the nanoscale, and experiments at the single particle level have become commonplace. This has opened wide new avenues for exploring and harnessing quantum mechanical effects in condensed matter. These quantum phenomena, in turn, have the potential to revolutionize the way we communicate, compute and probe the nanoscale world. Here, we review developments in key areas of quantum research in light of the nanotechnologies that enable them, with a view to what the future holds. Materials and devices with nanoscale features are used for quantum metrology and sensing, as building blocks for quantum computing, and as sources and detectors for quantum communication. They enable explorations of quantum behaviour and unconventional states in nano- and opto-mechanical systems, low-dimensional systems, molecular devices, nano-plasmonics, quantum electrodynamics, scanning tunnelling microscopy, and more. This rapidly expanding intersection of nanotechnology and quantum science/technology is mutually beneficial to both fields, laying claim to some of the most exciting scientific leaps of the last decade, with more on the horizon.
- ItemImpact of the capture time on the series resistance of quantum-well diode lasers(Bristol : IOP Publ., 2020) Boni, A.; Wünsche, H.J.; Wenzel, H.; Crump, P.Electrons and holes injected into a semiconductor heterostructure containing quantum wells are captured with a finite time. We show theoretically that this very fact can cause a considerable excess contribution to the series resistivity and this is one of the main limiting factors to higher efficiency for GaAs based high-power lasers. The theory combines a standard microscopic-based model for the capture-escape processes in the quantum well with a drift-diffusion description of current flow outside the quantum well. Simulations of five GaAs-based devices differing in their Al-content reveal the root-cause of the unexpected and until now unexplained increase of the series resistance with decreasing heat sink temperature measured recently. The finite capture time results in resistances in excess of the bulk layer resistances (decreasing with increasing temperature) from 1 mΩ up to 30 mΩ in good agreement with the experiment. © 2020 The Author(s). Published by IOP Publishing Ltd.
- 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.
- ItemHigh-mobility 4 μm MOVPE-grown (100) β-Ga2O3 film by parasitic particles suppression(Bristol : IOP Publ., 2023) Chou, Ta-Shun; Seyidov, Palvan; Bin Anooz, Saud; Grüneberg, Raimund; Rehm, Jana; Tran, Thi Thuy Vi; Fiedler, Andreas; Tetzner, Kornelius; Galazka, Zbigniew; Albrecht, Martin; Popp, AndreasIn this work, we comprehensively investigate the development of unwanted parasitic particles in the MOVPE chamber while growing μm level films. The density of the parasitic particles is found to be pronounced at film thicknesses starting from >1.5 to 2 μm. These particles seem to induce structural defects such as twin lamellae, thereby harming the electrical properties of the grown film. The origin of the parasitic particle is attributed to the parasitic reactions within the chamber triggered by the promoted gas-phase reactions during the growth process, which can be largely reduced by increasing the total gas flow and decreasing the showerhead distance to the susceptor. A film thickness of up to 4 μm has been achieved after minimizing the density of parasitic particles. Thereby, RT Hall measurements reveal carrier mobilities of 160 cm2V−1s−1 at carrier concentrations of 5.7 × 1016cm−3
- ItemStructural and luminescence imaging and characterisation of semiconductors in the scanning electron microscope(Bristol : IOP Publ., 2020) Trager-Cowan, C.; Alasmari, A.; Avis, W.; Bruckbauer, J.; Edwards, P.R.; Ferenczi, G.; Hourahine, B.; Kotzai, A.; Kraeusel, S.; Kusch, G.; Martin, R.W.; McDermott, R.; Naresh-Kumar, G.; Nouf-Allehiani, M.; Pascal, E.; Thomson, D.; Vespucci, S.; Smith, M.D.; Parbrook, P.J.; Enslin, J.; Mehnke, F.; Kuhn, C.; Wernicke, T.; Kneissl, M.; Hagedorn, S.; Knauer, A.; Walde, S.; Weyers, M.; Coulon, P.-M.; Shields, P.A.; Bai, J.; Gong, Y.; Jiu, L.; Zhang, Y.; Smith, R.M.; Wang, T.; Winkelmann, A.The scanning electron microscopy techniques of electron backscatter diffraction (EBSD), electron channelling contrast imaging (ECCI) and cathodoluminescence (CL) hyperspectral imaging provide complementary information on the structural and luminescence properties of materials rapidly and non-destructively, with a spatial resolution of tens of nanometres. EBSD provides crystal orientation, crystal phase and strain analysis, whilst ECCI is used to determine the planar distribution of extended defects over a large area of a given sample. CL reveals the influence of crystal structure, composition and strain on intrinsic luminescence and/or reveals defect-related luminescence. Dark features are also observed in CL images where carrier recombination at defects is non-radiative. The combination of these techniques is a powerful approach to clarifying the role of crystallography and extended defects on a material's light emission properties. Here we describe the EBSD, ECCI and CL techniques and illustrate their use for investigating the structural and light emitting properties of UV-emitting nitride semiconductor structures. We discuss our investigations of the type, density and distribution of defects in GaN, AlN and AlGaN thin films and also discuss the determination of the polarity of GaN nanowires. © 2020 The Author(s). Published by IOP Publishing Ltd.
- ItemTheoretical investigation of a miniature microwave driven plasma jet(Bristol : IOP Publ., 2020) Klute, Michael; Porteanu, Horia-Eugen; Stefanovic, Ilija; Heinrich, Wolfgang; Awakowicz, Peter; Brinkmann, Ralf PeterRadio frequency driven plasma jets are compact plasma sources which are used in many advanced fields such as surface engineering or biomedicine. The MMWICP (miniature micro wave ICP) is a particular variant of that device class. Unlike other plasma jets which employ capacitive coupling, the MMWICP uses the induction principle. The jet is integrated into a miniature cavity structure which realizes an LC-resonator with a high quality factor. When excited at its resonance frequency, the resonator develops a high internal current which—transferred to the plasma via induction—provides an efficient source of RF power. This work presents a theoretical model of the MMWICP. The possible operation points of the device are analyzed. Two different regimes can be identified, the capacitive E-mode with a plasma density of ne ≈ 5 × 1017 m−3, and the inductive H-mode with densities of ne ⩾ 1019 m−3. The E to H transition shows a pronounced hysteresis behavior.
- ItemLow-index quantum-barrier single-pass tapered semiconductor optical amplifiers for efficient coherent beam combining(Bristol : IOP Publ., 2020) Albrodt, P.; Niemeyer, M.; Elattar, M.; Hamperl, J.; Blume, G.; Ginolas, A.; Fricke, J.; Maaßdorf, A.; Georges, P.; Lucas-Leclin, G.; Paschke, K.; Crump, P.The requirements for coherent combination of high power GaAs-based single-pass tapered amplifiers are studied. Changes to the epitaxial layer structure are shown to bring higher beam quality and hence improved combining efficiency for one fixed device geometry. Specifically, structures with large vertical near field and low wave-guiding from the active region show 10% higher beam quality and coherent combining efficiency than reference devices. As a result, coherent combining efficiency is shown to be limited by beam quality, being directly proportional to the power content in the central lobe across a wide range of devices with different construction. In contrast, changes to the in-plane structure did not improve beam quality or combining efficiency. Although poor beam quality does correlate with increased optical intensities near the input aperture, locating monolithically-integrated absorption regions in these areas did not lead to any performance improvement. However, large area devices with subsequently improved cooling do achieve higher output powers. Phase noise can limit coherent combining, but this is shown to be small and independent of device design. Overall, tapered amplifiers are well suited for high power coherent combining applications. © 2020 The Author(s). Published by IOP Publishing Ltd.
- ItemHigh-brightness broad-area diode lasers with enhanced self-aligned lateral structure(Bristol : IOP Publ., 2020) Elatta, M.; Brox, O.; Della Casa, P.; Maaßdorf, A.; Martin, D.; Wenzel, H.; Knigge, A.; Crump, P.Broad-area diode lasers with increased brightness and efficiency are presented, which are fabricated using an enhanced self-aligned lateral structure by means of a two-step epitaxial growth process with an intermediate etching step. In this structure, current-blocking layers in the device edges ensure current confinement under the central stripe, which can limit the detrimental effects of current spreading and lateral carrier accumulation on beam quality. It also minimizes losses at stripe edges, thus lowering the lasing threshold and increasing conversion efficiency, while maintaining high polarization purity. In the first realization of this structure, the current block is integrated within an extreme-triple-asymmetric epitaxial design with a thin p-doped side, meaning that the distance between the current block and the active zone can be minimized without added process complexity. Using this configuration, enhanced self-aligned structure devices with 90 µm stripe width and 4 mm resonator length show up to 20% lower threshold current, 21% narrower beam waist, and slightly higher (1.03 ) peak efficiency in comparison to reference devices with the same dimensions, while slope, divergence angle and polarization purity remain almost unchanged. These results correspond to an increase in brightness by up to 25%, and measurement results of devices with varying stripe widths follow the same trend. © 2020 The Author(s). Published by IOP Publishing Ltd.
- 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.
- ItemModelling of a miniature microwave driven nitrogen plasma jet and comparison to measurements(Bristol : IOP Publ., 2021) Klute, Michael; Kemaneci, Efe; Porteanu, Horia-Eugen; Stefanović, Ilija; Heinrich, Wolfgang; Awakowicz, Peter; Brinkmann, Ralf PeterThe MMWICP (miniature microwave ICP) is a new plasma source using the induction principle. Recently Klute et al presented a mathematical model for the electromagnetic fields and power balance of the new device. In this work the electromagnetic model is coupled with a global chemistry model for nitrogen, based on the chemical reaction set of Thorsteinsson and Gudmundsson and customized for the geometry of the MMWICP. The combined model delivers a quantitative description for a non-thermal plasma at a pressure of p = 1000 Pa and a gas temperature of Tg = 650–1600 K. Comparison with published experimental data shows a good agreement for the volume averaged plasma parameters at high power, for the spatial distribution of the discharge and for the microwave measurements. Furthermore, the balance of capacitive and inductive coupling in the absorbed power is analyzed. This leads to the interpretation of the discharge regime at an electron density of ne ≈ 6.4 × 1018 m−3 as E/H-hybridmode with an capacitive and inductive component.