arXiv subjects

H. Riedl

Publications and source records attributed to H. Riedl.

8 recordsLinked to original sources

arXiv2026

Ultra-high-vacuum cluster tool for epitaxial synthesis and optical spectroscopy of reactive 2D materials

The large-area synthesis of high-crystalline-quality two-dimensional (2D) materials is at the core of novel material integration for semiconductor technology. This effort relies on developing fabrication and characterization techniques that can uncover the material's intrinsic properties by preserving its pristine conditions. In this article, we present an all ultra-high-vacuum cluster for the growth using molecular beam epitaxy of 2D semiconductors that are unstable under ambient conditions and optical spectroscopy using low temperature (20 K) photoluminescence and Raman scattering. The optical chamber of the setup provides micrometer scale spatial resolution and the ability to scan the entire wafer. The performance of its setup regarding spatial resolution, temperature control over a temperature range of 20-300 K using a closed-cycle cryostat and long-term preservation are demonstrated using as-grown post-transition metal monochalcogenides. Furthermore, we introduce a deconvolution-based algorithm to recover spatial information under vibration using a system-specific point-spread function. This enables in situ analysis of the structural and optoelectronic properties of as-grown materials in their pristine form, providing rich and reproducible feedback for both fundamental studies and the optimization of scalable 2D material growth toward integration in advanced devices.

cond-mat.mtrl-sci↗

arXiv2026

Electrically tunable orbital coupling and quantum light emission from O-band quantum dot molecules

We present the observation of electrically tunable quantum coupling of orbital states in individual InAs/InGaAs quantum dot molecules emitting in the telecom O-band (~1300 nm). By tuning the static electric field along the growth axis of the QD-molecule, we observe pronounced anticrossings between excitonic transitions and determine the dependence of the interdot electron tunnel coupling on the interdot separation. As the electric field applied along the growth axis of the QD-molecules increases, positively charged exciton complexes sequentially emerge in the time-integrated emission spectra due to electron escape from the system while holes remain trapped. Moreover, for strong pumping, biexciton emission from the O-band molecules is identified. We demonstrate single-photon emission from the InAs/InGaAs QD-molecule emitting around 1300 nm with a g(2)(0) = 0.017(2) and explore the impact of tuning orbital coupling on the second-order correlation function.

cond-mat.mes-hall↗

arXiv2025

Unveiling the growth mode diagram of GaSe on sapphire

The growth of two-dimensional epitaxial materials on industrially relevant substrates is critical for enabling their scalable synthesis and integration into next-generation technologies. Here we present a comprehensive study of the molecular beam epitaxial growth of gallium selenide on 2-inch c-plane sapphire substrates. Using in-situ reflection high-energy electron diffraction (RHEED), in-situ Raman spectroscopy, optical and scanning electron microscopies, we construct a diagram of the gallium selenide growth modes as a function of substrate temperature (530-650 °C) and Se/Ga flux ratio (5-110). The growth mode diagram reveals distinct regimes, including the growth of layered post-transition metal monochalcogenide GaSe with an unstrained in-plane lattice constant of 0.371$\pm$0.001 nm and a partial epitaxial alignment on sapphire. This work demonstrates a RHEED-based pathway for synthesizing gallium selenide of specific phase and morphology, and the construction of a phase diagram for high vapor pressure III-VI compounds that can be applied to a wide range of other metal chalcogenide materials.

cond-mat.mtrl-sci↗

arXiv2025

Three-fold Superstructured Superlattice HfN/HfAlN Thin Films for Enhanced Toughness

To simultaneously achieve high hardness and high toughness in protective coatings remains a fundamental challenge. Here, we harness the superlattice architecture to combine Koehler hardening while the coherent interfaces reduce the crack driving force and improve toughness, enabling coatings that are both hard and damage tolerant. We design and fabricate epitaxial HfN$_{1.33}$/Hf$_{0.76}$Al$_{0.24}$N$_{1.15}$ superlattices, deposited on MgO(001) substrates using low-energy, high-flux ion-assisted reactive magnetron sputtering. These superlattices with bilayer periods ranging from 6 to 20 nm, exhibit a unique three-fold superstructure, confirmed by X-ray diffraction and reciprocal space mapping (RSM). Each constituent forms distinct 3D checkerboard superstructures, with a period of 7.5 Å for HfN and 12.5 A for HfAlN. RSMs further reveal low mosaicity, high crystalline quality, and in-plane compressive strains, indicating well preserved coherence across interfaces. Mechanical testing shows that the superlattices maintain the high hardness of HfAlN (\~36 GPa) independent of bilayer period, while surpassing the softer HfN (~27 GPa), consistent with interface-driven Koehler strengthening. Micropillar compression shows brittle fracture on the {110}<110> system, yet with distributed cracking and faster mechanical recovery compared to monolithic films, suggesting improved toughness. Cube-corner indentation further corroborate this behavior, with pile-up and suppressed fracture events. These results demonstrate that epitaxial HfN/HfAlN superlattices uniquely combine high hardness with improved toughness, enabled by their three-fold superstructured architecture. Leveraging the intrinsic high-temperature stability of HfN-based materials, this design offers a robust pathway toward next-generation protective coatings capable of maintaining performance under extreme conditions.

cond-mat.mtrl-sci↗

arXiv2025

Growth Mechanisms and Mechanical Response of 3D Superstructured Cubic and Hexagonal Hf$_{1-x}$Al$_x$N Thin Films

Transition metal aluminum nitrides are a technologically important class of multifunctional ceramics, however, the HfAlN system remains largely unexplored. We investigate phase stability, nanostructure design, and mechanical behavior of Hf$_{1-x}$Al$_x$N$_y$ thin films deposited on MgO(001) substrates using ion-assisted reactive magnetron sputtering. Compared to growth temperature and ion assistance, backscattered Ar neutrals are shown to have a dominant influence on the film structure. The Al-rich (x > 0.41) films form a nanocrystalline morphology consisting of Hf- and Al-rich nanodomains in a wurtzite-hexagonal(h) 0001 fiber-texture exhibiting about 22 GPa hardness, considerably higher than that of a binary AlN. For low Al contents, x < 0.30, surface-driven spinodal decomposition by energetic Ar neutrals during deposition in combination with quenching of sub-surface diffusion results in an unusual - and unique for nitrides - three-dimensional checkerboard superstructure of AlN- and HfN-rich nanodomains in the single-crystal rocksalt-cubic (c) phase. Lattice-resolved scanning transmission electron microscopy complemented with x-ray and electron diffraction reveals that the superstructure periodicity extends along <100> directions and the size increases linearly from 9 to 13 A with rising Al content. Consequently, the nanoindentation hardness increases sharply from 26 GPa for HfN$_y$, to \~38 GPa for c-Hf$_{1-x}$Al$_x$N$_y$, due to dislocation pinning at the superstructure strain fields. Micropillar compression of c-Hf$_{0.93}$Al$_{0.07}$N$_{1.15}$ shows a considerably higher yield stress compared to HfN$_y$ and controlled brittle fracture occurs via {110}<011> slip systems, attributed to superstructure inhibited dislocation motion. In contrast, nanocrystalline h-Hf$_{0.59}$Al$_{0.41}$N$_{1.23}$ exhibits a high yield stress and limited plasticity before strain burst failure.

cond-mat.mtrl-sci↗

arXiv2015

Non-linear two-photon resonance fluorescence on a single artificial atom

We report two-photon resonance fluorescence of an individual semiconductor artificial atom. By non-linearly driving a single quantum dot via a two-photon transition, we probe the linewidth of the two-photon processes and show that, similar to their single-photon counterparts, they are close to being Fourier limited at low temperatures. The evolution of the population of excitonic states with the Rabi frequency exhibits a clear s-shaped behavior, indicative of the non-linear response via the two-photon excitation process. We model the non-linear response using a 4-level atomic system representing the manifold of excitonic and biexcitonic states of the quantum dot and show that quantitative agreement is obtained only by including the interaction with LA-phonons in the solid state environment. Finally, we demonstrate the formation of dressed states emerging from a two-photon interaction between the artificial atom and the excitation field. The non-linear optical dressing induces a mixing of all four excitonic states that facilitates the tuning of the polarization selection rules of the artificial atom.

cond-mat.mes-hall↗

arXiv2015

Controlled tunneling induced dephasing of Rabi rotations for ultra-high fidelity hole spin initialization

We report the sub-picosecond initialization of a single heavy hole spin in a self-assembled quantum dot with >98.5 % fidelity and without external magnetic field. Using an optically adressable charge and spin storage device we tailor the relative electron and hole tunneling escape timescales from the dot and simultaneously achieve high-fidelity initialization, long hole storage times and high efficiency readout via a photocurrent signal. We measure electric field-dependent Rabi oscillations of the neutral and charged exciton transitions in the ultrafast tunneling regime and demonstrate that tunneling induced dephasing (TID) of excitonic Rabi rotations is the major source for the intensity damping of Rabi oscillations in the low Rabi frequency, low temperature regime. Our results are in very good quantitative agreement with quantum-optical simulations revealing that TID can be used to precisely measure tunneling escape times and extract changes in the Coulomb binding energies for different charge configurations of the quantum dot. Finally, we demonstrate that for sub-picosecond electron tunneling escape TID of a coherently driven exciton transition facilitates ultrafast hole spin initialization with near-unity fidelity.

cond-mat.mes-hall↗

arXiv2014

Origin of High Temperature Oxidation Resistance of Ti-Al-Ta-N Coatings

Alloying Ti-Al-N coatings with Ta has proven to enhance their hardness, thermal stability, and oxidation resistance. However, especially for arc-evaporated Ti-Al-Ta-N coatings only limited information on the detailed influence of the elements on various properties is available. Therefore, we have developed arc-evaporated Ti1-x-yAlxTayN coatings with various Al (x = 0.50 - 0.65) and Ta (y = 0.00 - 0.15) contents. While the thermal stability of our coatings during annealing in inert He atmosphere increases with increasing Ta content, best results are obtained for specific Ta-Al ratios during oxidation. Single phase cubic Ti0.32Al0.60Ta0.08N yields a mass-gain of only ~5 % after 5 h at 950 °C in synthetic air, whereas Ti0.35Al0.65N is completely oxidized after 15 min. This is in part based on the suppressed anatase and direct rutile TiO2 formation at a defined Ta-Al content. Consequently, the anatase-to-rutile transformation, generally observed for Ti1-xAlxN, is absent. This reduces the generation of pores and cracks within the oxide scale and especially at the nitride-oxide interface, leading to the formation of a protective rutile and corundum based oxide scale. This is also reflected in the pronounced decrease in activation energy for the protective scale formation from 232 kJ/mol for Ti0.35Al0.65N down to 14.5 kJ/mol for Ti0.32Al0.60Ta0.08N. Based on our results we can conclude that especially phase transformations within the oxide scale need to be suppressed, as the connected volume changes lead to the formation of cracks and pores.

cond-mat.mtrl-sci↗