arXiv subjects

Cara-Lena Nies

Publications and source records attributed to Cara-Lena Nies.

4 recordsLinked to original sources

arXiv2025

Developing a valence force field model for wurtzite semiconductors by exploiting similarities with [111]-oriented zinc blende systems: The case of wurtzite boron nitride, III-N materials and (B,In,Ga)N alloys

Controlling the crystal phase and lattice mismatch of semiconductors offers a powerful route to engineer electronic and optical properties of heterostructures. As a consequence, semiconductors in the wurtzite phase are increasingly sought after, superseding the thermodynamically favored cubic zinc blende phase. Empirical atomistic modeling, required for large scale simulations of heterostructures and their properties, relies heavily on valence force field (VFF) methods to find the equilibrium atomic positions in an alloy. For zinc blende crystals, VFF models are well-established. In the case of wurtzite, such parameters are frequently adopted without rigorous analysis, despite subtle but consequential differences from the zinc blende structure. Such an approach can compromise accuracy in describing material properties, since the structural differences between zinc blende and wurtzite directly influence electronic and optical characteristics. Based on the analytical VFF model by Tanner et al., and using structural similarities between wurtzite and [111]-oriented zinc blende, we construct a wurtzite VFF without introducing additional parameters. Our framework relies on analytic expressions and minimization routines to project zinc blende models onto wurtzite systems. Beyond elastic tensors, we train the model to reproduce bond length asymmetries and band gaps by using output of the VFF model in density functional theory calculations. Applied to wurtzite III-N compounds and BN, the model accurately reproduces targeted observables but also properties it has not been trained on, including the internal parameter u. We further validate the model on highly mismatched alloys such as (B,Ga)N and (B,In,Ga)N, exhibiting good agreement between VFF and density functional theory results when using identical supercells in these calculations.

cond-mat.mtrl-sci↗

arXiv2024

Control of Cu morphology on TaN barrier and combined Ru-TaN barrier/liner substrates for nanoscale interconnects from atomistic kinetic Monte Carlo simulations

The miniaturization of electronic devices brings severe challenges in the deposition of metal interconnects in back end of line processing due to a continually decreasing volume available for metal deposition in the interconnect via. Cu is currently used as the interconnect metal and requires barrier and liner layers to prevent diffusion into silicon and promote deposition of smooth conducting metal films, respectively. However, these take up a significant volume in the interconnect vias, which are already extremely narrow due to the high aspect ratio required. Designing combined barrier/liner materials provides opportunities to promote deposition of smooth films and avoid islanding and frees up precious volume to deposit the required amount of interconnect metal. We present here the first atomistic kinetic Monte Carlo (kMC) investigation of the potential of Ru-modified TaN as a dual-function barrier/line material to promote 2D morphology of deposited Cu enabling optimization of the limited available interconnect volume. The kMC simulations use Density Functional Theory (DFT) activation barriers to predict the role of the combined barrier/liner material on the morphology of deposited Cu and give, for the first time, a tool to predict metal deposition on technologically important substrates. We study Cu deposition at different partial pressures on a series of substrates, including unmodified TaN and TaN with 25% or 50% surface Ru incorporation, at deposition temperatures of 300 K, 500 K and 800 K. The deposition simulations predict Cu film roughness, island size and number, substrate exposure area (as a measure of 2D vs. 3D morphology), layer occupation rate, compactness (as a measure of film quality) and the impact of annealing. As-deposited Cu on unmodified TaN exhibits significantly higher roughness and substrate exposure areas, demonstrating island formation...

cond-mat.mtrl-sci↗

arXiv2024

Impact of boron atom clustering on the electronic structure of (B,In)N alloys

Tailoring the electronic and optical properties of nitride-based alloys for optoelectronic applications in the ultraviolet and red spectral range has attracted significant attention in recent years. Adding boron nitride (BN) to indium gallium nitride, (In,Ga)N, alloys, can help to control the lattice mismatch between (In,Ga)N and GaN and may thus allow reduction of strain related defect formation. However, understanding of the impact of BN on the electronic properties of III-N alloys, in particular the influence of experimentally observed boron atom clustering, is sparse. This work presents first-principles calculations investigating the electronic properties of (B,In)N alloys with boron contents between 2% and 7%. Special attention is paid to the impact of the alloy microstructure. While the results show that the lattice constants of such alloys largely agree with lattice constants determined from a Vegard approximation, the electronic properties strongly depend on the local boron atom configurations. For instance, if boron atoms are dispersed throughout the structure and are not sharing nitrogen atoms, the band gap of (B,In)N alloys is largely unaffected and stays close to the gap of pristine InN. However, in the case of boron atom clustering, e.g., when boron atoms are sharing nitrogen atoms, the band gap can be strongly reduced, often leading to a metallic state in (B,In)N alloys. These strong band gap reductions are mainly driven by carrier localization effects in the valence band. The calculations thus show that the electronic structure of (B,In)N alloys strongly depends on the alloy microstructure and that boron atom clustering plays an important role in understanding the electronic and optical properties of these emerging materials.

cond-mat.mtrl-sci↗

arXiv2023

Electronic and optical properties of boron containing GaN alloys: The role boron atom clustering

Boron (B) containing III-nitride materials, such as wurtzite (B,Ga)N alloys, have recently attracted significant interest to tailor the electronic and optical properties of optoelectronic devices operating in the visible and ultraviolet spectral range. However, the growth of high quality samples is challenging and B atom clustering is often observed in (B,Ga)N alloys. To date, fundamental understanding of the impact of such clustering on electronic and optical properties of these alloys is sparse. In this work we employ density functional theory (DFT) in the framework of the meta generalized gradient approximation (modified Becke Johnson (mBJ) functional) to provide insight into this question. We use mBJ DFT calculations, benchmarked against state-of-the-art hybrid functional DFT, on (B,Ga)N alloys in the experimentally relevant B content range of up to 7.4%. Our results reveal that B atom clustering can lead to a strong reduction in the bandgap of such an alloy, in contrast to alloy configurations where B atoms are not forming clusters, thus not sharing nitrogen (N) atoms. We find that the reduction in bandgap is linked mainly to carrier localization effects in the valence band, which stem from local strain and polarization field effects. However, our study also reveals that the alloy microstructure of a B atom cluster plays an important role: B atom chains along the wurtzite c-axis impact the electronic structure far less strongly when compared to a chain formed within the c-plane. This effect is again linked to local polarization field effects and the orbital character of the involved valence states in wurtzite BN and GaN. Overall, our calculations show that controlling the alloy microstructure of (B,Ga)N alloys is of central importance when it comes to utilizing these systems in future optoelectronic devices with improved efficiencies.

cond-mat.mtrl-sci↗