arXiv subjects

Lado Filipovic

Publications and source records attributed to Lado Filipovic.

13 recordsLinked to original sources

arXiv2026

Surface chemistry as data for level-set simulation of etching and deposition

Level-set simulators of etching and deposition move the surface with a velocity that depends on adsorbate coverages. These coverages are usually computed from balances solved and coded by hand for each chemistry, whereas voxel-based Monte Carlo profile simulators read the reaction mechanism as data. For the open-source level-set simulator ViennaPS, this work presents a surface model whose rate laws, Jacobian and velocity are derived from a reaction file and evaluated with ray-traced fluxes. Steady-state coverages are obtained with Newton's method and transient coverages with a conservative integrator. One compiled model is applied to eight chemistries for chemical vapour deposition, selective epitaxy, plasma etching, sputtering and atomic layer deposition. It reproduces a hand-written silicon etch model to seven significant digits at a single point and within the Monte Carlo noise in a masked trench. In a high-aspect-ratio cavity, alumina conformality is determined by a trimethylaluminium reaction probability within the measured range.

physics.chem-ph↗

arXiv2026

Multiscale ensemble Monte Carlo of transport and gas sensing in monolayer MoS$_2$

Two-dimensional semiconductors such as monolayer MoS2 combine a useful band gap with a thickness of three atomic planes, which places the entire carrier density within a few angstroms of the surface and makes them attractive both as transistor channels and as chemical sensors. We present a self-consistent ensemble Monte Carlo (EMC) framework for monolayer MoS2 built on the ViennaEMC solver, which couples a Boltzmann transport model to Poisson's equation with degenerate free-carrier statistics. The transport model carries wavevector-dependent acoustic, polar-optical, homopolar and intervalley phonon scattering within and between the K valleys and the spin-orbit-split Q valleys, together with remote substrate phonons, screened charged impurities and surface roughness. Every intrinsic input is computed from first principles for this work, the band structure of the K and Q valleys, the phonon energies, the sound velocities and the dielectric response, and the electron-phonon couplings from density-functional perturbation theory matrix elements at 34 explicit phonon wavevectors and from Wannier interpolation with the two-dimensional long-range treatment. The model therefore contains no fitted transport parameter and no coupling taken from another calculation, and its scattering rates are checked against an independent solution of the linearised Boltzmann equation with the same couplings. The EMC velocity-field characteristic feeds a one-dimensional channel model, verified against a direct self-consistent three-dimensional EMC simulation of the gated channel, which reproduces the measured transfer characteristics of a CVD monolayer device in air and vacuum. An ambient and adsorbate extension reproduces the measured conductivity response to oxygen partial pressure and the concentration dependence of NO2 and NH3 sensing of independent monolayer devices.

cond-mat.mes-hall↗

arXiv2026

Faulted loop nucleation and dopant activation in Al-implanted 4H-SiC

We present a molecular dynamics (MD) study of Al implantation in 4H-SiC to determine how implantation temperature and dose affect defect evolution and dopant activation during annealing. Simulations use the Gao-Weber potential with a Morse Al-SiC interaction reparameterized to density functional theory diffusion and kick-in/kick-out barriers. At Al concentrations above the saturation limit of ~2e20/cm3, implantation at 900 K promotes interstitial-rich planar clusters already during implantation. During annealing, these clusters trap Al and evolve into faulted interstitial loops, reducing substitutional Al incorporation. In contrast, lower implantation temperatures preserve stronger local disorder that is consumed during epitaxial regrowth, resulting in higher chemical activation within MD-accessible annealing times. Atomistic trajectories show a thermally activated transition to faulted loops once planar clusters reach about 60 interstitials. Rather than nucleating as a single coherent disk, several locally faulted regions form first and subsequently merge. The activation energy is ~1.1 eV for dislocation nucleation and ~2.1 eV for stacking-fault growth. Frank-type loops dominate at high temperature, while transient Shockley partials occur mainly at early stages and below 2000 K. As large planar defects form and dissolve, stable compensating Al-C complexes also emerge. These findings support the experimental hypothesis that secondary defects contribute to the reduced Al activation observed under supersaturation during annealing.

cond-mat.mtrl-sci↗

arXiv2026

Competition between vacancy creation and filling in defect-engineering of hBN

Hexagonal boron nitride (hBN) has recently become the focus of intense research as a material that can host quantum emitters. It is known that such emission is related to point defects, but in order to conclusively correlate specific defects to their spectra, having control over the defect creation mechanism is required. Here, we prepare freestanding, monolayer hBN samples and irradiate them with ultra-low-energy (150 eV) Ar+ ions. The samples are characterized before and after irradiation via scanning transmission electron microscopy to assess the defect density and distribution. Contrary to what analytical potential molecular dynamics simulations have predicted, we predominantly observe boron single vacancies after ion irradiation, followed by double vacancies at half the count. Moreover, we also observe that vacancy filling with Si and C impurity atoms plays a more significant role in the created defects than previously assumed, potentially posing a problem for selective creation of quantum emitters in hBN.

cond-mat.mtrl-sci↗

arXiv2026

The influence of implantation conditions on dopant activation in Al-implanted 4H-SiC: A MD study applying an Al potential fitted to DFT barriers

The non-monotonic dependence of Al dopant activation on implantation temperature in 4H-SiC has been experimentally observed, but its atomistic origin remains unclear. We present a molecular dynamics (MD) study of Al implantation at $500$,K and $900$,K over seven doses from $1\times10^{13}$ to $7.5\times10^{14}$,cm$^{-2}$, followed by up to $100$,ns of annealing at $1500$--$2500$,K. Using the Gao-Weber potential combined with a reparameterized Morse potential for Al-SiC interactions fitted to DFT migration and kick-in/out barriers, we show that implantation at both temperatures reduces Frenkel-pair formation and extended amorphous pockets compared with room-temperature implantation. Above the Al solubility limit ($>10^{20}$,cm$^{-3}$), however, annealing reveals a non-monotonic temperature dependence. Samples implanted at $900$,K form larger, kinetically stable interstitial clusters that persist throughout annealing and act as sinks and trapping centers for Al, reducing substitutional incorporation. Although the $500$,K samples initially exhibit lower crystallinity, they contain a significantly larger fraction of substitutional Al after annealing. The simulations identify two regimes: a low-dose regime dominated by isolated point defects and small complexes, and a high-dose regime characterized by defect clustering and planar-defect formation with strong implantation-temperature dependence. These results explain the experimentally observed optimal implantation window between $500$ and $900$,K and suggest that controlled nanoscale amorphization at $500$,K enhances activation through regrowth-assisted incorporation while suppressing extended defects. The simulations also identify a new basal-plane diffusion path for Al and an activation mechanism involving kick-out of a carbon antisite; both were confirmed by DFT-NEB calculations.

cond-mat.mtrl-sci↗

arXiv2025

Hexatic Phase in Covalent Two-Dimensional Silver Iodide

According to the Kosterlitz-Thouless-Halperin-Nelson-Young (KTHNY) theory, the transition from a solid to liquid in two dimensions proceeds through an orientationally ordered liquid-like hexatic phase. However, alternative mixed melting scenarios, in which melting proceeds through the hexatic phase with both continuous and discontinuous transitions, have also been observed in some two-dimensional systems. In this study, we imaged silver iodide embedded in multilayer graphene using time- and temperature-resolved in situ atomic-resolution scanning transmission electron microscopy and nanobeam electron diffraction. We observed the hexatic phase and provide evidence supporting a mixed melting scenario.

cond-mat.mtrl-sci↗

arXiv2025

Single photon emitters in hBN: Limitations of atomic resolution imaging and potential sources of error

There is a growing interest in identifying the origin of single-photon emission in hexagonal boron nitride (hBN), with proposed candidates including boron and nitrogen vacancies as well as carbon substitutional dopants. Because photon emission intensity often increases with sample thickness, hBN flakes used in these studies commonly exceed 30 atomic layers. To identify potential emitters at the atomic scale, annular dark-field scanning transmission electron microscopy (ADF-STEM) is frequently employed. However, due to the intrinsic AA' stacking of hBN with vertically alternating boron and nitrogen atoms, this approach is complicated even in few-layer systems. Here, we demonstrate using STEM image simulations and experiments that, even under idealized conditions, the intensity differences between boron- and nitrogen-dominated columns and carbon substitutions become indistinguishable at thicknesses beyond 17 atomic layers (ca. 6 nm). While vacancy-type defects can remain detectable at somewhat larger thicknesses, also their detection becomes unreliable at thicknesses typically used in photonic studies. We further show that common residual aberrations, particularly threefold astigmatism, can lead to artificial contrast differences between columns, which may result in misidentification of atomic defects. We systematically study the effects of non-radially symmetric aberrations on multilayer hBN and demonstrate that even small residual threefold astigmatism can significantly distort the STEM contrast, leading to misleading interpretations.

cond-mat.mtrl-sci↗

arXiv2025

Atomically clean free-standing two-dimensional materials through heating in ultra-high vacuum

Surface contamination not only influences but in some cases even dominates the measured properties of two-dimensional materials. Although different cleaning methods are often used for contamination removal, commonly used spectroscopic cleanliness assessment methods can leave the level of achieved cleanliness ambiguous. Despite two decades of research on 2D materials, the true cleanliness of the used samples is often left open to interpretation. In this work, freestanding monolayer graphene and hexagonal boron nitride are annealed at different temperatures in a custom-built ultra-high vacuum heating chamber, connected to a scanning transmission electron microscope via a vacuum transfer line, enabling atomically resolved cleanliness characterization as a function of annealing temperature, while eliminating the introduction of airborne contamination during sample transport. While annealing at 200 °C already reduces contamination significantly, it is not until 400 °C or higher, where over 90% of the free-standing monolayer areas are atomically clean. At this point, further contamination removal is mainly limited by defects in the material and metal contamination introduced during the sample transfer or growth. The achieved large, atomically clean areas can then be used for further nanoscale engineering steps or device processing, facilitating interaction with the material rather than contamination.

cond-mat.mtrl-sci↗

arXiv2025

Uncovering the atomic structure of substitutional platinum dopants in MoS$_2$ with single-sideband ptychography

We substitute individual Pt atoms into monolayer MoS$_2$ and study the resulting atomic structures with single-sideband (SSB) ptychography supported by ab initio simulations. We demonstrate that while high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) imaging provides excellent Z-contrast, distinguishing some defect types such as single and double sulfur vacancies remains challenging due to their low relative contrast difference. However, SSB with its nearly linear Z-contrast and high phase sensitivity enables reliable identification of these defect configurations as well as various Pt dopant structures at significantly lower electron doses. Our findings uncover the precise atomic placement and highlight the potential of SSB ptychography for detailed structural analysis of dopant-modified 2D materials while minimizing beam-induced damage, offering new pathways for understanding and engineering atomic-scale features in 2D systems.

cond-mat.mtrl-sci↗

arXiv2024

Electron-beam-induced adatom-vacancy-complexes in mono- and bilayer phosphorene

Phosphorene, a puckered two-dimensional allotrope of phosphorus, has sparked considerable interest in recent years due to its potential especially for optoelectronic applications with its layer-number-dependant direct band gap and strongly bound excitons. However, detailed experimental characterization of its intrinsic defects as well as its defect creation characteristics under electron irradiation are scarce. Here, we report on the creation and stability of a variety of defect configurations under 60 kV electron irradiation in mono- and bilayer phosphorene including the first experimental reports of stable adatom-vacancy-complexes. Displacement cross section measurements in bilayer phosphorene yield a value of 7.7 +- 1.4 barn with an estimated lifetime of adatom-vacancy-complexes of 19.9 +- 0.7 s, while some are stable for up to 68 s under continuous electron irradiation. Surprisingly, ab initio-based simulations indicate that the complexes should readily recombine, even in structures strained by up to 3 %. The presented results will help to improve the understanding of the wide variety of defects in phosphorene, their creation, and their stability, which may enable new pathways for defect engineered phosphorene devices.

cond-mat.mes-hall↗

arXiv2023

The role of thermalisation in hot carrier cooling dynamics

The hot carrier solar cell (HCSC) concept has been proposed to overcome the Shockley Queisser limit of a single p-n junction solar cell by harvesting carriers before they have lost their surplus energy. A promising family of materials for these purposes is metal halide perovskites (MHP). MHPs have experimentally shown very long cooling times, the key requirement of a HCSC. By using Ensemble Monte Carlo (EMC) simulations we shed light on why cooling times are found to be extended for these materials. In this manuscript, we concentrate on the role of thermalisation in the cooling process. We specify the role of electron-phonon and electron-electron interactions in thermalisation and cooling, while furthermore showing how these processes depends on several relevant material parameters, such as the dielectric constant and the effective mass. Finally, we quantify how thermalisation can also act as a cooling mechanism via the cold background effect. Here, we stress the importance of a low degree of background doping in order to achieve the observed extended cooling times. This work provides insights into the ongoing discussion on cooling times in MHPs. In addition our results are an important addition to the debate on whether or not tin perovskites are suitable candidates for HCSCs.

cond-mat.mtrl-sci↗

arXiv2022

Modeling Incomplete Conformality during Atomic Layer Deposition in High Aspect Ratio Structures

Atomic layer deposition allows for precise control over film thickness and conformality. It is a critical enabler of high aspect ratio structures, such as 3D NAND memory, since its self-limiting behavior enables higher conformality than conventional processes. However, as the aspect ratio increases, deviations from complete conformality frequently occur, requiring comprehensive modeling to aid the development of novel technologies. To that end, we present a model for surface coverage during atomic layer deposition where incomplete conformality is present. This model combines existing approaches based on Knudsen diffusion and Langmuir kinetics. Our model expands the state-of-the art by (i) incorporating gas-phase diffusivity through the Bosanquet formula as well as reaction reversibility in the modeling framework first proposed by Yanguas-Gil and Elam, and (ii) being efficiently integrated within level-set topography simulators. The model is manually calibrated to published results of the prototypical atomic layer deposition of Al$_2$O$_3$ from TMA and H$_2$O in lateral high aspect ratio structures. We investigate the temperature dependence of the H$_2$O step, thus extracting an activation energy of $0.178\,\mathrm{eV}$ which is consistent with recent experiments. In the TMA step, we observe increased accuracy from the Bosanquet formula and we reproduce multiple independent experiments with the same parameter set, highlighting that the model parameters effectively capture the reactor conditions.

physics.comp-ph↗

arXiv2021

Optimizing the Stability of FETs Based on Two-Dimensional Materials by Fermi Level Tuning

Despite the enormous progress achieved during the past decade, nanoelectronic devices based on two-dimensional (2D) semiconductors still suffer from a limited electrical stability. This limited stability has been shown to result from the interaction of charge carriers originating from the 2D semiconductors with defects in the surrounding insulating materials. The resulting dynamically trapped charges are particularly relevant in field effect transistors (FETs) and can lead to a large hysteresis, which endangers stable circuit operation. Based on the notion that charge trapping is highly sensitive to the energetic alignment of the channel Fermi-level with the defect band in the insulator, we propose to optimize device stability by deliberately tuning the channel Fermi-level. Our approach aims to minimize the amount of electrically active border traps without modifying the total number of traps in the insulator. We demonstrate the applicability of this idea by using two differently doped graphene layers in otherwise identical FETs with Al$_2$O$_3$ as a gate oxide mounted on a flexible substrate. Our results clearly show that by increasing the distance of the Fermi-level to the defect band, the hysteresis is significantly reduced. Furthermore, since long-term reliability is also very sensitive to trapped charges, a corresponding improvement in reliability is both expected theoretically and demonstrated experimentally. Our study paves the way for the construction of more stable and reliable 2D FETs in which the channel material is carefully chosen and tuned to maximize the energetic distance between charge carriers in the channel and the defect bands in the insulator employed.

physics.app-ph↗