arXiv subjects

Michael Stiefel

Publications and source records attributed to Michael Stiefel.

6 recordsLinked to original sources

arXiv2026

Elemental Germanium Phase-Change Memory

Phase-change memory (PCM) is a mature technology for fast, scalable, non-volatile data storage, with applications spanning embedded memory, as well as in-memory and neuromorphic computing. PCM predominantly relies on chalcogenide alloys, with $\mathrm{Ge_2Sb_2Te_5}$ (GST) as the industry standard. Yet in these alloys, the individual Ge, Sb, and Te atoms redistribute upon cycling, causing stochastic operation and ultimately device failure. To address this issue, elemental antimony was proposed as a PCM material, but it exhibits a metastable amorphous state that prevents reliable data retention. Moreover, tellurium and antimony can contaminate complementary metal-oxide-semiconductor (CMOS) production lines or act as unintended dopants, restricting manufacturing of PCM to dedicated fabs. Here we introduce elemental germanium (Ge) as a CMOS-native phase-change material that overcomes these fundamental limitations. In a vertical PCM cell architecture, Ge enables sub-nanosecond crystallization (240 ps, 40 times faster than GST), non-volatile data storage with excellent thermal stability ($>$110 {\deg}C for 10 years vs. $\sim$87 {\deg}C for GST), and a resistance drift coefficient approximately 60% lower than in GST. These results establish pure Ge, a standard semiconductor, as an alternative to chalcogenide phase-change materials, achieving superior performance in key metrics and enabling phase-change memory to be fabricated in standard semiconductor facilities.

cond-mat.mtrl-sci↗

arXiv2025

Optimized Synthesis and Device Integration of Long 17-Atom-Wide Armchair Graphene Nanoribbons

Seventeen-carbon-atom-wide armchair graphene nanoribbons (17-AGNRs) are promising candidates for high-performance electronic devices due to their narrow electronic bandgap. Atomic precision in edge structure and width control is achieved through a bottom-up on-surface synthesis (OSS) approach from tailored molecular precursors in ultra-high vacuum (UHV). This synthetic protocol must be optimized to meet the structural requirements for device integration, with ribbon length being the most critical parameter. Here, we report optimized OSS conditions that produce 17-AGNRs with an average length of approximately 17 nm. This length enhancement is achieved through a gradual temperature ramping during an extended annealing period, combined with a template-like effect driven by monomer assembly at high surface coverage. The resulting 17-AGNRs are comprehensively characterized in UHV using scanning probe techniques and Raman spectroscopy. Raman measurements following substrate transfer enabled the characterization of the length distribution of GNRs on the device substrate and confirmed their stability under ambient conditions and harsh chemical environments, including acid vapors and etchants. The increased length and ambient stability of the 17-AGNRs lead to their reliable integration into device architectures. As a proof of concept, we integrate 17-AGNRs into field-effect transistors (FET) with graphene electrodes and confirm that electronic transport occurs through the GNRs. This work demonstrates the feasibility of integrating narrow-bandgap GNRs into functional devices and contributes to advancing the development of carbon-based nanoelectronics.

cond-mat.mes-hall↗

arXiv2022

Tunable quantum dots from atomically precise graphene nanoribbons using a multi-gate architecture

Atomically precise graphene nanoribbons (GNRs) are increasingly attracting interest due to their largely modifiable electronic properties, which can be tailored by controlling their width and edge structure during chemical synthesis. In recent years, the exploitation of GNR properties for electronic devices has focused on GNR integration into field-effect-transistor (FET) geometries. However, such FET devices have limited electrostatic tunability due to the presence of a single gate. Here, we report on the device integration of 9-atom wide armchair graphene nanoribbons (9-AGNRs) into a multi-gate FET geometry, consisting of an ultra-narrow finger gate and two side gates. We use high-resolution electron-beam lithography (EBL) for defining finger gates as narrow as 12 nm and combine them with graphene electrodes for contacting the GNRs. Low-temperature transport spectroscopy measurements reveal quantum dot (QD) behavior with rich Coulomb diamond patterns, suggesting that the GNRs form QDs that are connected both in series and in parallel. Moreover, we show that the additional gates enable differential tuning of the QDs in the nanojunction, providing the first step towards multi-gate control of GNR-based multi-dot systems.

cond-mat.mes-hall↗

arXiv2022

Transparent Josephson Junctions in Higher-Order Topological Insulator WTe2 via Pd Diffusion

Highly transparent superconducting contacts to a topological insulator (TI) remain a persistent challenge on the route to engineer topological superconductivity. Recently, the higher-order TI WTe$_2$ was shown to turn superconducting when placed on palladium (Pd) bottom contacts, demonstrating a promising material system in pursuing this goal. Here, we report the diffusion of Pd into WTe$_2$ and the formation of superconducting PdTe$_x$ as the origin of observed superconductivity. We find an atomically sharp interface in vertical direction to the van der Waals layers between the diffusion crystal and its host crystal, forming state-of-the-art superconducting contacts to a TI. The diffusion is discovered to be non-uniform along the width of the WTe$_2$ crystal, with a greater extend along the edges compared to the bulk. The potential of this contacting method is highlighted in transport measurements on Josephson junctions by employing external superconducting leads.

cond-mat.mes-hall↗

arXiv2021

Synthesis and Characterization of the Ternary Nitride Semiconductor Zn$_2$VN$_3$: Theoretical Prediction, Combinatorial Screening and Epitaxial Stabilization

Computationally guided high-throughput synthesis is used to explore the Zn-V-N phase space, resulting in the synthesis of a novel ternary nitride Zn$_2$VN$_3$. Following a combinatorial PVD screening, we isolate the phase and synthesize polycrystalline Zn$_2$VN$_3$ thin films with wurtzite structure on conventional borosilicate glass substrates. In addition, we demonstrate that cation-disordered, but phase-pure (002)-textured, Zn$_2$VN$_3$ thin films can be grown using epitaxial stabilization on α-Al2O3 (0001) substrates at remarkably low growth temperatures well below 200 °C. The structural properties and phase composition of the Zn$_2$VN$_3$ films are studied in detail using XRD and (S)TEM techniques. The composition as well as chemical state of the constituent elements are studied using RBS/ERDA as well as XPS/HAXPES methods. These analyses reveal a stoichiometric material with no oxygen contamination, besides a thin surface oxide. We find that Zn$_2$VN$_3$ is a weakly-doped p-type semiconductor demonstrating broadband room-temperature photoluminescence spanning the range between 2 eV and 3 eV. In addition, the electronic properties can be tuned over a wide range via isostructural alloying on the cation site, making this a promising material for optoelectronic applications.

cond-mat.mtrl-sci↗

arXiv2019

A combinatorial guide to phase formation and surface passivation of tungsten titanium oxide prepared by thermal oxidation

TiO$_2$ and WO$_3$ are two of the most important earth-abundant electronic materials with applications in countless industries. Recently alloys of WO$_3$ and TiO$_2$ have been investigated leading to improvements of key performance indicators for a variety of applications ranging from photo-electrochemical water splitting to electrochromic smart windows. These positive reports and the complexity of the ternary W-Ti-O phase diagram motivate a comprehensive experimental screening of this phase space. Using combinatorial thermal oxidation of solid solution W$_{1-x}$Ti$_{x}$ precursors combined with bulk and surface analysis mapping we investigate the oxide phase formation and surface passivation of tungsten titanium oxide in the entire compositional range from pure WO$_3$ to TiO$_2$. The system shows a remarkable structural transition from monoclinic over cubic to tetragonal symmetry with increasing Ti concentration. In addition, a strong Ti surface enrichment is observed for precursor Ti-concentrations in excess of 55 at.%, resulting in the formation of a protective rutile-structured TiO$_2$ surface layer. Despite the structural transitions, the optical properties of the oxide alloys remain largely unaltered demonstrating an independent control of multiple functional properties in W$_{1-x}$Ti$_{x}$O$_{n}$. The results from this study provide valuable guidelines for future development of W$_{1-x}$Ti$_{x}$O$_{n}$ for electronic and energy applications, but also novel engineering approaches for surface functionalization and additive manufacturing of Ti-based alloys.

cond-mat.mtrl-sci↗