arXiv subjects

Lutz Kirste

Publications and source records attributed to Lutz Kirste.

9 recordsLinked to original sources

arXiv2026

Observation of narrow-band $γ$ radiation from a boron-doped diamond superlattice with an 855 MeV electron beam

We report the first observation of narrow band 1.3 MeV $γ$ radiation from a crystalline diamond micro-undulator. A diamond superlattice was grown with a periodical varying boron doping profile. Four sinusoidally deformed (110) periods resulted with a period length of 5.0 $μ$m and an amplitude of 0.098 nm. A channeling experiment was performed with the 855 MeV electron beam of the Mainz Microtron MAMI accelerator facility. A clear peak was detected with a large sodium iodide scintillation detector close to the expected photon energy of 1.28 MeV. Key characteristics of the peak, including photon energy, width and intensity, were reproduced fairly well by Monte-Carlo simulation calculations. Based on the latter, optimized boron doping profiles were designed for a hypothetical 3 GeV electron beam, enabling preparation of a highly directional $γ$-ray beam with a photon energy of 14.5 MeV. The predicted spectral bandwidth is 13\%, however, with a high energy continuum tail. The on-target photon flux at a beam current of 100 $μ$A would be about $10^{12}$/s.

physics.ins-det↗

arXiv2026

Delta-Doped Diamond via in-situ Plasma-Distance Control

We present an approach for the CVD growth of diamond, where the sample is placed in a defined distance from the reactor baseplate, to which the plasma couples. We observe two previously unknown growth regimes. In the first case, the sample is positioned within three to five millimeters of the plasma, leading to a decreased growth rate, compared to a position inside the plasma and, additionally, to an increased nitrogen incorporation, allowing the fabrication of delta-doped layers with a thickness below 30 nm. In another regime, where the sample is more than 10 mm away from the plasma, no growth is observed. Instead, we assume a deposition of nitrogen-rich species on the diamond surface, which is incorporated during the growth of the following layer. All fabricated layers show NV emission, where the intensity correlates with the nitrogen incorporation. The growth techniques could allow the fabrication of highly doped thin films for quantum sensing applications, as well as layers with low NV concentration, for quantum computing. The new approaches are applicable not only for nitrogen incorporation but also for other defects, for example, phosphorus, which could open up new avenues for diamond-based electronics.

cond-mat.mtrl-sci↗

arXiv2025

Chemical Vapor Deposition of Nitrides by Carbon-free Brominated Precursors

The epitaxial growth of group 13-nitride semiconductors (GaN, AlN, and AlGaN alloys) for the mass production and fabrication of high-frequency and high-power devices relies on metal-organic chemical vapor deposition (MOCVD) using metal-organic molecules, also called precursors. While this growth method ensures high productivity and low operation costs compared to other methods, its most significant disadvantage lies in the presence of carbon atoms in the precursors, which are unavoidably incorporated into the epitaxial layers and hamper the performance of most types of fabricated devices. Carbon-free precursors for the CVD process could enhance the performance of high-frequency and high-power nitride-based devices while maintaining growth capability in industrial equipment. In this work, we implement gallium- and aluminum-brominated precursors, which contain no carbon atoms, to grow GaN and AlN layers in an industrial CVD system. We compare the results of this alternative CVD process with the conventional method using trimethyl precursors through several characterization techniques, indicating a clear reduction in optically active carbon-related defects.

cond-mat.mtrl-sci↗

arXiv2024

Improved Leakage Currents and Polarity Control through Oxygen Incorporation in Ferroelectric Al0.73Sc0.27N Thin Films

This article examines systematic oxygen (O)-incorporation to reduce total leakage currents in sputtered wurtzite-type ferroelectric Al0.73Sc0.27N thin films, along with its impact on the material structure and the polarity of the as-grown films. The O in the bulk Al0.73Sc0.27N was introduced through an external gas source during the reactive sputter process. In comparison to samples without doping, O-doped films showed almost a fourfold reduction of the leakage current near the coercive field. In addition, doping resulted in the reduction of the steady-state leakage currents by roughly one order of magnitude sub-coercive fields. Microstructure analysis using X-ray diffraction 1and scanning transmission electron microscopy (STEM) revealed no significant structural degradation of the bulk Al0.73Sc0.27N. In case of the maximum O-doped film, the c-axis out-of-plane texture increased by only 20% from 1.8° and chemical mapping revealed a uniform distribution of oxygen incorporation into the bulk. Our results further demonstrate the ability to control the as-deposited polarity of Al0.73Sc0.27N via the O-concentration, changing from nitrogen- to metal-polar orientation. Thus, this article presents a promising approach to mitigate the leakage current in wurtzite-type Al0.73Sc0.27N without incurring any significant structural degradation of the bulk thin film quality, thereby making ferroelectric nitrides more suitable for microelectronic applications.

physics.app-ph↗

arXiv2024

Structural and electrical properties of fiber textured and epitaxial molybdenum thin films prepared by magnetron sputter epitaxy

Molybdenum (Mo) due to its optimal structural, physical, and acoustic properties find application as electrode material in aluminum scandium nitride (AlScN) and aluminum nitride (AlN) based bulk acoustic wave (BAW) resonators. Epitaxial Mo thin films exhibiting low resistivity can improve the performance of the BAW resonator by enhancing both the electromechanical coupling coefficient and quality factor. In this study, we systematically vary the growth temperature of Mo grown on fiber-textured and epitaxial wurtzite-aluminum nitride (AlN) to study the changes in structural and electrical properties of the Mo films. Results show that Mo grown at 700°C on epitaxial AlN exhibit low surface roughness, large average grain diameter, low resistivity, and high crystal quality. XRD pole figure and phi-scan reveal that irrespective of the growth temperature, Mo is fiber textured on fiber-textured AlN, and has three rotational domains on epitaxial AlN. The study shows that the resistivity of Mo reduces with increasing growth temperature, which we relate to increasing average grain diameter. Additionally, we show that fiber-textured Mo has more high angle grain boundaries resulting in consistently higher resistivity than its epitaxial equivalent.

physics.app-ph↗

arXiv2024

Sputtered Aluminum Nitride Waveguides for the Telecommunication Spectrum with less than 0.16 dB/cm Loss

We report the fabrication and characterization of photonic waveguides from sputtered aluminum nitride (AlN). The AlN films were deposited on 6" silicon substrates with a 3 $μ$m buried silicon oxide layer using reactive DC magnetron sputtering at a temperature of 700°C. The resulting uncladded polycrystalline waveguides exhibit propagation losses of 0.137 $\pm$ 0.005 dB/cm at wavelengths of 1310 nm and 0.154 $\pm$ 0.008 dB/cm at a wavelength of 1550 nm in the TE polarization. These results are the best reported for sputtered AlN waveguides in the C-band and the first report in the O-band. These performances are comparable to those of the best-reported AlN waveguides, which are epitaxially grown by metal-organic chemical vapor deposition (MOCVD) on sapphire substrates. Our findings highlight the potential of sputtered AlN for photonic platforms working in the telecom spectrum.

physics.optics↗

arXiv2023

Demonstration and STEM Analysis of Ferroelectric Switching in MOCVD-Grown Single Crystalline Al$_{0.85}$Sc$_{0.15}$N

Wurtzite-type Al$_{1-x}$Sc$_x$N solid solutions grown by metal organic chemical vapour deposition are for the first time confirmed to be ferroelectric. The film with 230 nm thickness and x = 0.15 exhibits a coercive field of 5.5 MV/cm at a measurement frequency of 1.5 kHz. Single crystal quality and homogeneous chemical composition of the film was confirmed by X-ray diffraction spectroscopic methods such as time of flight secondary ion mass spectrometry. Annular bright field scanning transmission electron microscopy served to proof the ferroelectric polarization inversion on unit cell level. The single crystal quality further allowed to image the large-scale domain pattern of a wurtzite-type ferroelectric for the first time, revealing a predominantly cone-like domain shape along the c-axis of the material. As in previous work, this again implies the presence of strong polarization discontinuities along this crystallographic axis, which could be suitable for current transport. The domains are separated by narrow domain walls, for which an upper thickness limit of 3 nm was deduced, but which could potentially be atomically sharp. We are confident that these results will advance the commencing integration of wurtzite-type ferroelectrics to GaN as well as generally III-N based heterostructures and devices.

physics.app-ph↗

arXiv2020

Influence of Different Surface Morphologies on the Performance of High Voltage, Low Resistance Diamond Schottky Diodes

Vertical diamond Schottky diodes with blocking voltages $V_{\text{BD}} > 2.4 \text{ kV}$ and on-resistances $R_{\text{On}} < 400 \text{ m}Ω\text{cm}^{2}$ were fabricated on homoepitaxially grown diamond layers with different surface morphologies. The morphology (smooth as-grown, hillock-rich, polished) influences the Schottky barrier, the carrier transport properties, and consequently the device performance. The smooth as-grown sample exhibited a low reverse current density $J_{\text{Rev}} < 10^{-4} \text{ A}/\text{cm}^{2}$ for reverse voltages up to $2.2 \text{ kV}$. The hillock-rich sample blocked similar voltages with a slight increase in the reverse current density ($J_{\text{Rev}} < 10^{-3} \text{ A}/\text{cm}^{2}$). The calculated 1D-breakdown field, however, was reduced by $30 \text{ } \%$, indicating a field enhancement induced by the inhomogeneous surface. The polished sample demonstrated a similar breakdown voltage and reverse current density as the smooth as-grown sample, suggesting that a polished surface can be suitable for device fabrication. However, a statistical analysis of several diodes of each sample showed the importance of the substrate quality: A high density of defects both reduces the feasible device area and increases the reverse current density. In forward direction, the hillock-rich sample exhibited a secondary Schottky barrier, which could be fitted with a modified thermionic emission model employing the Lambert W-function. Both polished and smooth sample showed nearly ideal thermionic emission with ideality factors $1.08$ and $1.03$, respectively. Compared with literature, all three diodes exhibit an improved Baliga Figure of Merit for diamond Schottky diodes with $V_{\text{BD}} > 2 \text{ kV}$.

physics.app-ph↗

arXiv2020

Microstructural and Optical Emission Properties of Diamond Multiply Twinned Particles

Multiply twinned particles, MTPs, are fascinating crystallographic entities with a number of controllable properties originating from their symmetry and cyclic structure. In the focus of our studies are diamond MTPs hosting optically active defects, objects demonstrating a high application potential for emerging optoelectronic and quantum devices. In this work, we discuss the growth mechanisms along with the microstructural and optical properties of the MTPs aggregating high-density of silicon-vacancy complexes on the specific crystal irregularities. It is demonstrated that the silicon impurities incite a rapid growth of MTPs via intensive formation of penetration twins on 100 facets of regular octahedral grains. We also show that the zero-phonon-line emission from the Si color centers embedded in the twin boundaries dominates in photo- and electroluminescence spectra of the MTP-based light-emitting devices defining their steady-state optical properties.

physics.app-ph↗