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- ItemContinuous tuning of two-section, single-mode terahertz quantum-cascade lasers by fiber-coupled, near-infrared illumination(New York : American Institute of Physics, 2017) Hempel, Martin; Röben, Benjamin; Niehle, Michael; Schrottke, Lutz; Trampert, Achim; Grahn, Holger T.The dynamical tuning due to rear facet illumination of single-mode, terahertz (THz) quantum-cascade lasers (QCLs) which employ distributed feedback gratings are compared to the tuning of single-mode QCLs based on two-section cavities. The THz QCLs under investigation emit in the range of 3 to 4.7 THz. The tuning is achieved by illuminating the rear facet of the QCL with a fiber-coupled light source emitting at 777 nm. Tuning ranges of 5.0 and 11.9 GHz under continuous-wave and pulsed operation, respectively, are demonstrated for a single-mode, two-section cavity QCL emitting at about 3.1 THz, which exhibits a side-mode suppression ratio better than -25 dB.
- ItemSelf-Assembly of Well-Separated AlN Nanowires Directly on Sputtered Metallic TiN Films(Weinheim : Wiley-VCH, 2020) Azadmand, Mani; Auzelle, Thomas; Lähnemann, Jonas; Gao, Guanhui; Nicolai, Lars; Ramsteiner, Manfred; Trampert, Achim; Sanguinetti, Stefano; Brandt, Oliver; Geelhaar, LutzHerein, the self-assembled formation of AlN nanowires (NWs) by molecular beam epitaxy on sputtered TiN films on sapphire is demonstrated. This choice of substrate allows growth at an exceptionally high temperature of 1180 °C. In contrast to previous reports, the NWs are well separated and do not suffer from pronounced coalescence. This achievement is explained by sufficient Al adatom diffusion on the substrate and the NW sidewalls. The high crystalline quality of the NWs is evidenced by the observation of near-band-edge emission in the cathodoluminescence spectrum. The key factor for the low NW coalescence is the TiN film, which spectroscopic ellipsometry and Raman spectroscopy indicate to be stoichiometric. Its metallic nature will be beneficial for optoelectronic devices using these NWs as the basis for (Al,Ga)N/AlN heterostructures emitting in the deep ultraviolet spectral range.
- ItemThe Interaction of Extended Defects as the Origin of Step Bunching in Epitaxial III–V Layers on Vicinal Si(001) Substrates(Weinheim : Wiley-VCH, 2019) Niehle, Michael; Rodriguez, Jean-Baptiste; Cerutti, Laurent; Tournié, Eric; Trampert, AchimSeveral nanometer high steps are observed by (scanning) transmission electron microscopy at the surface and interfaces in heteroepitaxially grown III–Sb layers on vicinal Si(001) substrates. Their relations with antiphase boundaries (APBs) and threading dislocations (TDs) are elaborated. An asymmetric number density of TDs on symmetry-equivalent {111} lattice planes is revealed and explained according to the substrate miscut and the lattice misfit in the heteroepitaxial material system. Finally, a step bunching mechanism is proposed based on the interplay of APBs, TDs, and the vicinal surface of the miscut substrate.
- ItemEpitaxial synthesis of unintentionally doped p-type SnO (001) via suboxide molecular beam epitaxy(Melville, NY : AIP, 2023) Egbo, Kingsley; Luna, Esperanza; Lähnemann, Jonas; Hoffmann, Georg; Trampert, Achim; Grümbel, Jona; Kluth, Elias; Feneberg, Martin; Goldhahn, Rüdiger; Bierwagen, OliverBy employing a mixed SnO2 + Sn source, we demonstrate suboxide molecular beam epitaxy (S-MBE) growth of phase-pure single-crystalline metastable SnO (001) thin films on Y-stabilized ZrO2 (001) substrates at a growth rate of ∼1.0 nm/min without the need for additional oxygen. These films grow epitaxially across a wide substrate temperature range from 150 to 450 °C. Hence, we present an alternative pathway to overcome the limitations of high Sn or SnO2 cell temperatures and narrow growth windows encountered in previous MBE growth of metastable SnO. In situ laser reflectometry and line-of-sight quadrupole mass spectrometry were used to investigate the rate of SnO desorption as a function of substrate temperature. While SnO ad-molecule desorption at TS = 450 °C was growth-rate limiting, the SnO films did not desorb at this temperature after growth in vacuum. The SnO (001) thin films are transparent and unintentionally p-type doped, with hole concentrations and mobilities in the range of 0.9-6.0 × 1018 cm-3 and 2.0-5.5 cm2 V-1 s-1, respectively. These p-type SnO films obtained at low substrate temperatures are promising for back-end-of-line (BEOL) compatible applications and for integration with n-type oxides in pn heterojunctions and field-effect transistors.
- ItemSynthesis of quasi-free-standing bilayer graphene nanoribbons on SiC surfaces(London : Nature Publishing Group, 2015) Oliveira, Myriano H., Jr.; Lopes, Joao Marcelo J.; Schumann, Timo; Galves, Lauren A.; Ramsteiner, Manfred; Berlin, Katja; Trampert, Achim; Riechert, HenningScaling graphene down to nanoribbons is a promising route for the implementation of this material into devices. Quantum confinement of charge carriers in such nanostructures, combined with the electric field-induced break of symmetry in AB-stacked bilayer graphene, leads to a band gap wider than that obtained solely by this symmetry breaking. Consequently, the possibility of fabricating AB-stacked bilayer graphene nanoribbons with high precision is very attractive for the purposes of applied and basic science. Here we show a method, which includes a straightforward air annealing, for the preparation of quasi-free-standing AB-bilayer nanoribbons with different widths on SiC(0001). Furthermore, the experiments reveal that the degree of disorder at the edges increases with the width, indicating that the narrower nanoribbons are more ordered in their edge termination. In general, the reported approach is a viable route towards the large-scale fabrication of bilayer graphene nanostructures with tailored dimensions and properties for specific applications.
- ItemCritical aspects of substrate nanopatterning for the ordered growth of GaN nanocolumns(London : BioMed Central, 2011) Barbagini, Francesca; Bengoechea-Encabo, Ana; Albert, Steven; Martinez, Javier; Sanchez García, Miguel Angel; Trampert, Achim; Calleja, EnriquePrecise and reproducible surface nanopatterning is the key for a successful ordered growth of GaN nanocolumns. In this work, we point out the main technological issues related to the patterning process, mainly surface roughness and cleaning, and mask adhesion to the substrate. We found that each of these factors, process-related, has a dramatic impact on the subsequent selective growth of the columns inside the patterned holes. We compare the performance of e-beam lithography, colloidal lithography, and focused ion beam in the fabrication of hole-patterned masks for ordered columnar growth. These results are applicable to the ordered growth of nanocolumns of different materials.
- ItemDelayed crystallization of ultrathin Gd2O3 layers on Si(111) observed by in situ X-ray diffraction(London : BioMed Central, 2012) Hanke, Michael; Kaganer, Vladimir M.; Bierwagen, Oliver; Niehle, Michael; Trampert, AchimWe studied the early stages of Gd2O3 epitaxy on Si(111) in real time by synchrotron-based, high-resolution X-ray diffraction and by reflection high-energy electron diffraction. A comparison between model calculations and the measured X-ray scattering, and the change of reflection high-energy electron diffraction patterns both indicate that the growth begins without forming a three-dimensional crystalline film. The cubic bixbyite structure of Gd2O3 appears only after a few monolayers of deposition.
- ItemElectron Tomography of Pencil-Shaped GaN/(In,Ga)N Core-Shell Nanowires(New York, NY [u.a.] : Springer, 2019) Nicolai, Lars; Gačević, Žarko; Calleja, Enrique; Trampert, AchimThe three-dimensional structure of GaN/(In,Ga)N core-shell nanowires with multi-faceted pencil-shaped apex is analyzed by electron tomography using high-angle annular dark-field mode in a scanning transmission electron microscope. Selective area growth on GaN-on-sapphire templates using a patterned mask is performed by molecular beam epitaxy to obtain ordered arrays of uniform nanowires. Our results of the tomographic reconstruction allow the detailed determination of the complex morphology of the inner (In,Ga)N multi-faceted shell structure and its deviation from the perfect hexagonal symmetry. The tomogram unambiguously identifies a dot-in-a-wire configuration at the nanowire apex including the exact shape and size, as well as the spatial distribution of its chemical composition. © 2019, The Author(s).
- ItemToward edges-rich MoS2 layers via chemical liquid exfoliation triggering distinctive magnetism(Milton Park : Taylor & Francis, 2016) Gao, Guanhui; Chen, Chi; Xie, Xiaobin; Su, Yantao; Kang, Shendong; Zhu, Guichi; Gao, Duyang; Eckert, Jürgen; Trampert, Achim; Cai, LintaoThe magnetic function of layered molybdenum disulfide (MoS2) has been investigated via simulation, but few reliable experimental results have been explored. Herein, we developed edges-rich structural MoS2 nanosheets via liquid phase exfoliation approach, triggering exceptional ferromagnetism. The magnetic measurements revealed the clear ferromagnetic property of layered MoS2, compared to the pristine MoS2 in bulk exhibiting diamagnetism. The existence of ferromagnetism mostly was attributed to the presence of grain boundaries with abundant irregular edges confirmed by the transmission electron microscopy, magnetic force microscopy and X-ray photoelectron spectroscopy, which experimentally provided reliable evidences on irregular edges-rich states engineering ferromagnetism to clarify theoretical calculation.
- ItemEngineering grain boundaries at the 2D limit for the hydrogen evolution reaction([London] : Nature Publishing Group UK, 2020) He, Yongmin; Tang, Pengyi; Hu, Zhili; He, Qiyuan; Zhu, Chao; Wang, Luqing; Zeng, Qingsheng; Golani, Prafful; Gao, Guanhui; Fu, Wei; Huang, Zhiqi; Gao, Caitian; Xia, Juan; Wang, Xingli; Wang, Xuewen; Zhu, Chao; Ramasse, Quentin M.; Zhang, Ao; An, Boxing; Zhang, Yongzhe; Martí-Sánchez, Sara; Morante, Joan Ramon; Wang, Liang; Tay, Beng Kang; Yakobson, Boris I.; Trampert, Achim; Zhang, Hua; Wu, Minghong; Wang, Qi Jie; Arbiol, Jordi; Liu, ZhengAtom-thin transition metal dichalcogenides (TMDs) have emerged as fascinating materials and key structures for electrocatalysis. So far, their edges, dopant heteroatoms and defects have been intensively explored as active sites for the hydrogen evolution reaction (HER) to split water. However, grain boundaries (GBs), a key type of defects in TMDs, have been overlooked due to their low density and large structural variations. Here, we demonstrate the synthesis of wafer-size atom-thin TMD films with an ultra-high-density of GBs, up to ~1012 cm−2. We propose a climb and drive 0D/2D interaction to explain the underlying growth mechanism. The electrocatalytic activity of the nanograin film is comprehensively examined by micro-electrochemical measurements, showing an excellent hydrogen-evolution performance (onset potential: −25 mV and Tafel slope: 54 mV dec−1), thus indicating an intrinsically high activation of the TMD GBs.