Dr. Benedikt Haas
Profil
Forschungsthemen3
Pumpen, Messen und Modellieren atomarer Vibrationen im Elektronenmikroskop
Quelle ↗Förderer: DFG Sachbeihilfe Internationale Kooperation Zeitraum: 06/2024 - 06/2027 Projektleitung: Dr. Benedikt Haas
Pumpen, Messen und Modellieren atomarer Vibrationen im Elektronenmikroskop
Quelle ↗Förderer: DFG Sachbeihilfe Internationale Kooperation Zeitraum: 06/2024 - 06/2027 Projektleitung: Dr. Benedikt Haas
SFB 1772/1: Struktur und kollektive Anregungen in mol2Dmat-Heterostrukturen durch schnelle Elektronen (TP A05)
Quelle ↗Förderer: DFG Sonderforschungsbereich Zeitraum: 10/2025 - 06/2029 Projektleitung: Prof. Christoph T. Koch, PhD, Dr. Benedikt Haas
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Stand: 26.4.2026, 19:48:44 (Top-K=20, Min-Cosine=0.4)
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Publikationen25
Top 25 nach Zitationen — Quelle: OpenAlex (BAAI/bge-m3 embedded für Matching).
ACS Nano · 156 Zitationen · DOI
Controlled substitutional doping of two-dimensional transition-metal dichalcogenides (TMDs) is of fundamental importance for their applications in electronics and optoelectronics. However, achieving <i>p</i>-type conductivity in MoS<sub>2</sub> and WS<sub>2</sub> is challenging because of their natural tendency to form <i>n</i>-type vacancy defects. Here, we report versatile growth of <i>p</i>-type monolayer WS<sub>2</sub> by liquid-phase mixing of a host tungsten source and niobium dopant. We show that crystallites of WS<sub>2</sub> with different concentrations of substitutionally doped Nb up to 10<sup>14</sup> cm<sup>-2</sup> can be grown by reacting solution-deposited precursor film with sulfur vapor at 850 °C, reflecting the good miscibility of the precursors in the liquid phase. Atomic-resolution characterization with aberration-corrected scanning transmission electron microscopy reveals that the Nb concentration along the outer edge region of the flakes increases consistently with the molar concentration of Nb in the precursor solution. We further demonstrate that ambipolar field-effect transistors can be fabricated based on Nb-doped monolayer WS<sub>2</sub>.
Journal of Applied Physics · 103 Zitationen · DOI
This work presents the first atomic scale evidence for ferroelectric polarization inversion on the unit cell level in a wurtzite-type material based on epitaxial Al0.75Sc0.25N thin films. The electric field induced formation of Al-polar inversion domains in the originally N-polar film is unambiguously determined by atomic resolution imaging using aberration-corrected scanning transmission electron microscopy (STEM). Anisotropic etching supports STEM results confirming a complete and homogenous polarization inversion at the film surface for the switched regions and the virtual absence of previous inversion domains in as-deposited regions. Local evidence of residual N-polar domains at the bottom electrode interface is observed and can be explained by both stress gradients and electric field deflection. The epitaxial relationship of the sapphire/AlN/Mo/AlScN multilayer stack is discussed in detail. Selected-area electron diffraction experiments and XRD pole figures reveal a Pitsch–Schrader type orientation relation between the Mo electrode and the AlScN film.
Advanced Functional Materials · 76 Zitationen · DOI
Abstract The recent emergence of wurtzite‐type nitride ferroelectrics such as Al 1‐ x Sc x N has paved the way for the introduction of all‐epitaxial, all‐wurtzite‐type ferroelectric III‐N semiconductor heterostructures. This paper presents the first in‐depth structural and electrical characterization of such an epitaxial heterostructure by investigating sputter deposited Al 1‐ x Sc x N solid solutions with x between 0.19 and 0.28 grown over doped n‐GaN. The results of detailed structural investigations on the strain state and the initial unit‐cell polarity with the peculiarities observed in the ferroelectric response are correlated. Among these, a Sc‐content dependent splitting of the ferroelectric displacement current into separate peaks, which can be correlated with the presence of multiple strain states in the Al 1‐ x Sc x N films is discussed. Unlike in previously reported studies on ferroelectric Al 1‐ x Sc x N, all films thicker than 30 nm grown on the metal (M)‐polar GaN template feature an initial multidomain state. The results support that regions with opposed polarities in as‐grown films do not result as a direct consequence of the in‐plane strain distribution, but are rather mediated by the competition between M‐polar epitaxial growth on an M‐polar template and a deposition process that favors nitrogen (N)‐polar growth.
Advanced Materials · 76 Zitationen · DOI
Atomically thin transition metal dichalcogenide crystals (TMDCs) have extraordinary optical properties that make them attractive for future optoelectronic applications. Integration of TMDCs into practical all-dielectric heterostructures hinges on the ability to passivate and protect them against necessary fabrication steps on large scales. Despite its limited scalability, encapsulation of TMDCs in hexagonal boron nitride (hBN) currently has no viable alternative for achieving high performance of the final device. Here, it is shown that the novel, ultrathin Ga<sub>2</sub> O<sub>3</sub> glass is an ideal centimeter-scale coating material that enhances optical performance of the monolayers and protects them against further material deposition. In particular, Ga<sub>2</sub> O<sub>3</sub> capping of monolayer WS<sub>2</sub> outperforms commercial-grade hBN in both scalability and optical performance at room temperature. These properties make Ga<sub>2</sub> O<sub>3</sub> highly suitable for large-scale passivation and protection of monolayer TMDCs in functional heterostructures.
Ultramicroscopy · 68 Zitationen · DOI
The influence of AlN buffer over the polarity and the nucleation of self-organized GaN nanowires
2015Journal of Applied Physics · 61 Zitationen · DOI
We experimentally investigate the influence of AlN buffer growth on the nucleation and the polarity of a self-organized assembly of GaN nanowires (NWs) grown on Si. Two complementary growth mechanisms for AlN buffer deposited on Si are demonstrated. Both emphasize the aggregation of Si on the AlN surface and the growth of large cubic crystallites, namely, AlN pedestals. Further growths of GaN NWs assembly reveal that the GaN 2D layer found at the bottom of the NW assembly is the result of the coalescence of Ga-polar pyramids, whereas AlN pedestals are observed as preferential but not exclusive NW nucleation sites. NWs are N-polar or exhibit inversion domains with a Ga-polar core/N-polar shell structure. This suggests that N-polarity is a necessary condition to trigger NW self-organized nucleation due to a different facets energy hierarchy between the Ga- and the N-polar sides.
ACS Nano · 55 Zitationen · DOI
High dielectric constant (high-<i>k</i>) ultrathin films are required as insulating gate materials. The well-known high<i>-k</i> dielectrics, including HfO<sub>2</sub>, ZrO<sub>2</sub>, and SrTiO<sub>3</sub>, feature three-dimensional lattice structures and are thus not easily obtained in the form of distinct ultrathin sheets. Therefore, their deposition as ultrathin layers still imposes challenges for electronic industries. Consequently, new high-<i>k</i> nanomaterials with <i>k</i> in the range of 40 to 100 and a band gap exceeding 4 eV are highly sought after. Antimony oxide nanosheets appear as a potential candidate that could fulfill these characteristics. Here, we report on the stoichiometric cubic polymorph of 2D antimony oxide (Sb<sub>2</sub>O<sub>3</sub>) as an ideal high<i>-k</i> dielectric sheet that can be synthesized <i>via</i> a low-temperature, substrate-independent, and silicon-industry-compatible liquid metal synthesis technique. A bismuth-antimony alloy was produced during the growth process. Preferential oxidation caused the surface of the melt to be dominated by α-Sb<sub>2</sub>O<sub>3</sub>. This ultrathin α-Sb<sub>2</sub>O<sub>3</sub> was then deposited onto desired surfaces <i>via</i> a liquid metal print transfer. A tunable sheet thickness between ∼1.5 and ∼3 nm was achieved, while the lateral dimensions were within the millimeter range. The obtained α-Sb<sub>2</sub>O<sub>3</sub> exhibited high crystallinity and a wide band gap of ∼4.4 eV. The relative permittivity assessment revealed a maximum <i>k</i> of 84, while a breakdown electric field of ∼10 MV/cm was observed. The isolated 2D α-Sb<sub>2</sub>O<sub>3</sub> nanosheets were utilized in top-gated field-effect transistors that featured low leakage currents, highlighting that the obtained material is a promising gate oxide for conventional and van der Waals heterostructure-based electronics.
Applied Physics Letters · 55 Zitationen · DOI
Using correlated experiments on single nanowires (NWs) by microphotoluminescence (μ-PL) and high resolution scanning transmission electron microscopy, we attribute the 3.45 eV luminescence of GaN NWs grown by plasma assisted molecular beam epitaxy (PA-MBE) to the presence of prismatic inversion domain boundaries (pIDBs). This attribution is further strengthened by a recent publication demonstrating the observation of pIDBs in PA-MBE grown GaN NWs. A statistical study of the presence of 3.45 eV lines in NWs PL spectra allows to estimate the ratio of single NWs nucleating with a pIDB to be 50% in the sample under scrutiny.
Langmuir · 54 Zitationen · DOI
Ultrasmall water-soluble silver nanoclusters are synthesized, and their properties are investigated. The silver nanoclusters have high colloidal stability and show fluorescence in the red. This demonstrates that like gold nanoclusters also silver nanoclusters can be fluorescent.
Microelectronic Engineering · 49 Zitationen · DOI
Applied Physics Letters · 35 Zitationen · DOI
High quality III/V-layers grown on Si enable a variety of optoelectronic devices. The performance of such devices is limited by anti-phase domains forming at monoatomic steps on the Si-surface. To date the atomic structure of anti-phase boundaries, which affects the charge distribution at polar interfaces, is unknown. Here, we use CS-corrected scanning transmission electron microscopy to reveal the atomic structure of the anti-phase boundaries in III/V-semiconductors, choosing GaP as a model system. We observe boundaries on (110) lattice planes which are atomically abrupt and also facetted ones, which introduces locally charged regions influencing device performance.
Nonpolar<i>m</i>-plane GaN/AlGaN heterostructures with intersubband transitions in the 5–10 THz band
2015Nanotechnology · 31 Zitationen · DOI
Graphene monolayers can be used for atomically thin three-dimensional shell-shaped superscatterer designs. Due to the excitation of the first-order resonance of transverse magnetic (TM) graphene plasmons, the scattering cross section of the bare subwavelength dielectric particle is enhanced significantly by five orders of magnitude. The superscattering phenomenon can be intuitively understood and interpreted with Bohr model. Besides, based on the analysis of Bohr model, it is shown that contrary to the TM case, superscattering is hard to occur by exciting the resonance of transverse electric (TE) graphene plasmons due to their poor field confinements.
Physical Review Letters · 30 Zitationen · DOI
We report the observation of spin-to-charge current conversion in strained mercury telluride at room temperature, using spin pumping experiments. We show that a HgCdTe barrier can be used to protect the HgTe from direct contact with the ferromagnet, leading to very high conversion rates, with inverse Edelstein lengths up to 2.0±0.5 nm. The influence of the HgTe layer thickness on the conversion efficiency is found to differ strongly from what is expected in spin Hall effect systems. These measurements, associated with the temperature dependence of the resistivity, suggest that these high conversion rates are due to the spin momentum locking property of HgTe surface states.
Nanotechnology · 29 Zitationen · DOI
The structural and optical properties of axial GaN/InGaN/GaN nanowire heterostructures with high InN molar fractions grown by molecular beam epitaxy have been studied at the nanoscale by a combination of electron microscopy, extended x-ray absorption fine structure and nano-cathodoluminescence techniques. InN molar fractions up to 50% have been successfully incorporated without extended defects, as evidence of nanowire potentialities for practical device realisation in such a composition range. Taking advantage of the N-polarity of the self-nucleated GaN NWs grown by molecular beam epitaxy on Si(111), the N-polar InGaN stability temperature diagram has been experimentally determined and found to extend to a higher temperature than its metal-polar counterpart. Furthermore, annealing of GaN-capped InGaN NWs up to 800 °C has been found to result in a 20 times increase of photoluminescence intensity, which is assigned to point defect curing.
Nano Letters · 27 Zitationen · DOI
Phonon scattering at grain boundaries (GBs) is significant in controlling the nanoscale device thermal conductivity. However, GBs could also act as waveguides for selected modes. To measure localized GB phonon modes, milli-electron volt (meV) energy resolution is needed with subnanometer spatial resolution. Using monochromated electron energy loss spectroscopy (EELS) in the scanning transmission electron microscope (STEM) we have mapped the 60 meV optic mode across GBs in silicon at atomic resolution and compared it to calculated phonon densities of states (DOS). The intensity is strongly reduced at GBs characterized by the presence of 5- and 7-fold rings where bond angles differ from the bulk. The excellent agreement between theory and experiment strongly supports the existence of localized phonon modes and thus of GBs acting as waveguides.
ACS Nano · 20 Zitationen · DOI
Indium nitride (InN) has been of significant interest for creating and studying two-dimensional electron gases (2DEG). Herein we demonstrate the formation of 2DEGs in ultrathin doped and undoped 2D InN nanosheets featuring high carrier mobilities at room temperature. The synthesis is carried out via a two-step liquid metal-based printing method followed by a microwave plasma-enhanced nitridation reaction. Ultrathin InN nanosheets with a thickness of ∼2 ± 0.2 nm were isolated over large areas with lateral dimensions exceeding centimeter scale. Room temperature Hall effect measurements reveal carrier mobilities of ∼216 and ∼148 cm<sup>2</sup> V<sup>-1</sup> s<sup>-1</sup> for undoped and doped InN, respectively. Further analysis suggests the presence of defined quantized states in these ultrathin nitride nanosheets that can be attributed to a 2D electron gas forming due to strong out-of-plane confinement. Overall, the combination of electronic and plasmonic features in undoped and doped ultrathin 2D InN holds promise for creating advanced optoelectronic devices and functional 2D heterostructures.
The Journal of Physical Chemistry C · 20 Zitationen · DOI
In recent years, inorganic cesium–lead–halide perovskites, CsPbX3 (X = I, Br, Cl), have attracted interest for optoelectronic applications such as highly efficient thin-film light-emitting diodes or wide-gap absorber materials for photovoltaics. However, phase segregation and secondary phases in as-deposited thin films are still considered to be limiting factors for devices based on CsPbX3. Here, we report a correlative electron microscopy and spectroscopy approach for the identification of secondary phases and their distributions in Cs–Pb–Br thin films, deposited by solution-based and coevaporation methods on various substrates. We identified phases by their compositional, structural, and optoelectronic properties, using X-ray diffraction, spectroscopy, and a variety of microscopy techniques. We found that the Cs–Pb–Br films contain ternary Cs4PbBr6 and CsPb2Br5 phases in addition to CsPbBr3, a finding consistent with calculations of formation enthalpies by means of the density functional theory showing that these values are very similar for the three ternary phases. We find that these phases can exhibit different spatial distributions inside the film and discuss the influence of the deposition method and synthesis parameters on the resulting phase composition of the Cs–Pb–Br layers.
Advanced Materials · 20 Zitationen · DOI
Optical and electrical properties of 2D transition metal dichalcogenides (TMDCs) grown by chemical vapor deposition (CVD) are strongly determined by their microstructure. Consequently, the visualization of spatial structural variations is of paramount importance for future applications. This study demonstrates how grain boundaries, crystal orientation, and strain fields can unambiguously be identified with combined lateral force microscopy and transverse shear microscopy (TSM) for CVD-grown tungsten disulfide (WS<sub>2</sub> ) monolayers, on length scales that are relevant for optoelectronic applications. Further, angle-dependent TSM measurements enable the fourth-order elastic constants of monolayer WS<sub>2</sub> to be acquired experimentally. The results facilitate high-throughput and nondestructive microstructure visualization of monolayer TMDCs and insights into their elastic properties, thus providing an accessible tool to support the development of advanced optoelectronic devices based on such 2D semiconductors.
Photoluminescent thermometry based on europium-activated calcium sulphide (hyperthermia application)
1982Physics in Medicine and Biology · 20 Zitationen · DOI
A photoluminescent thermometer, based on the transient emission response of a europium-activated calcium sulphide phosphor, is described in detail. This optical thermometry system has special advantages for temperature measurements in microwave and RF fields and potential application in electromagnetically induced clinical hyperthermia. A laboratory system has been constructed which utilises a fibreoptic probe with external diameter 0.8 mm. A system temperature resolution of 0.1 degree C has been achieved in the range 37-47 degrees C.
Nanoscale Research Letters · 19 Zitationen · DOI
The enhancement of the performance of advanced nitride-based optoelectronic devices requires the fine tuning of their composition, which has to be determined with a high accuracy and at the nanometer scale. For that purpose, we have evaluated and compared energy dispersive X-ray spectroscopy (EDX) in a scanning transmission electron microscope (STEM) and atom probe tomography (APT) in terms of composition analysis of AlGaN/GaN multilayers. Both techniques give comparable results with a composition accuracy better than 0.6 % even for layers as thin as 3 nm. In case of EDX, we show the relevance of correcting the X-ray absorption by simultaneous determination of the mass thickness and chemical composition at each point of the analysis. Limitations of both techniques are discussed when applied to specimens with different geometries or compositions.
Nanotechnology · 18 Zitationen · DOI
It is demonstrated that growing InGaN nanowires in metal-rich conditions on top of GaN nanowires results in a widening of the InGaN section. It is shown that the widening is eased by stacking faults (SFs) formation, revealing facets favorable to In incorporation. It is furthermore put in evidence that partial dislocations terminating SFs efficiently contribute to elastic strain relaxation. Indium accumulation on top of the InGaN section is found to result in an axial growth rate decrease, which has been assigned to increased N-N recombination and subsequent effective nitrogen flux decrease, eventually leading to the formation of InGaN nano-umbrellas/nanoplatelets.
Journal of Power Sources · 18 Zitationen · DOI
Nonpolar m-plane GaN/AlGaN heterostructures with intersubband transitions in the 5 to 10 THz band
2015PubMed · 16 Zitationen · DOI
This paper assesses intersubband (ISB) transitions in the 1-10 THz frequency range in nonpolar m-plane GaN/AlGaN multi-quantum-wells deposited on free-standing semi-insulating GaN substrates. The quantum wells (QWs) were designed to contain two confined electronic levels, decoupled from the neighboring wells. Structural analysis reveals flat and regular QWs in the two perpendicular in-plane directions, with high-angle annular dark-field scanning transmission electron microscopy images showing inhomogeneities of the Al composition in the barriers along the growth axis. We do not observe extended structural defects (stacking faults or dislocations) introduced by the epitaxial process. Low-temperature ISB absorption from 1.5 to 9 THz (6.3-37.4 meV) is demonstrated, covering most of the 7-10 THz band forbidden to GaAs-based technologies.
Journal of Applied Physics · 16 Zitationen · DOI
The crystalline structure and orientation of perfluoropentacene (C22F14, PFP) fibers formed upon thin-film deposition onto SiO2 substrates have been studied by means of transmission electron microscopy (TEM), atomic force microscopy (AFM), and x-ray diffraction. The synopsis of TEM micrographs and diffraction patterns enhances the understanding of local crystal orientation on small length scales. The relationship of the PFP fiber morphology with the crystalline arrangement of PFP molecules within single fibers was established using this technique. Radiation damage, which is a critical problem for TEM investigations of organic materials, is described and the sample morphology after TEM investigations is correlated with AFM measurements of samples previously examined by TEM.
Nano Letters · 15 Zitationen · DOI
The integration of metallic contacts with two-dimensional (2D) semiconductors is routinely required for the fabrication of nanoscale devices. However, nanometer-scale variations in the 2D/metal interface can drastically alter the local optoelectronic properties. Here, we map local excitonic changes of the 2D semiconductor MoS<sub>2</sub> in contact with Au. We utilize a suspended and epitaxially grown 2D/metal platform that allows correlated electron energy-loss spectroscopy (EELS) and angle resolved photoelectron spectroscopy (nanoARPES) mapping. Spatial localization of MoS<sub>2</sub> excitons uncovers an additional EELS peak related to the MoS<sub>2</sub>/Au interface. NanoARPES measurements indicate that Au-S hybridization decreases substantially with distance from the 2D/metal interface, suggesting that the observed EELS peak arises due to dielectric screening of the excitonic Coulomb interaction. Our results suggest that increasing the van der Waals distance could optimize excitonic spectra of mixed-dimensional 2D/3D interfaces and highlight opportunities for Coulomb engineering of exciton energies by the local dielectric environment or moiré engineering.
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Bestätigte Forscher↔Partner-Paare aus HU-FIS — Gold-Standard-Positive für das Matching.
Pumpen, Messen und Modellieren atomarer Vibrationen im Elektronenmikroskop
other
SFB 1772/1: Struktur und kollektive Anregungen in mol2Dmat-Heterostrukturen durch schnelle Elektronen (TP A05)
university
Pumpen, Messen und Modellieren atomarer Vibrationen im Elektronenmikroskop
university
SFB 1772/1: Struktur und kollektive Anregungen in mol2Dmat-Heterostrukturen durch schnelle Elektronen (TP A05)
university
Stammdaten
Identität, Organisation und Kontakt aus HU-FIS.
- Name
- Dr. Benedikt Haas
- Titel
- Dr.
- Fakultät
- Zentralinstitut Center for the Science of Materials Berlin
- Telefon
- +49 30 2093-7937
- HU-FIS-Profil
- Quelle ↗
- Zuletzt gescrapt
- 26.4.2026, 01:05:35