arXiv subjects

Luigi Colombo

Publications and source records attributed to Luigi Colombo.

12 recordsLinked to original sources

arXiv2024

Exploring Dielectric Properties in Models of Amorphous Boron Nitride

We report a theoretical study of dielectric properties of models of amorphous Boron Nitride, using interatomic potentials generated by machine learning. We first perform first-principles simulations on small (about $100$ atoms in the periodic cell) sample sizes to explore the emergence of mid-gap states and its correlation with structural features. Next, by using a simplified tight-binding electronic model, we analyse the dielectric functions for complex three dimensional models (containing about $10.000$ atoms) embedding varying concentrations of ${\rm sp^{1}, sp^{2}}$ and ${\rm sp^3}$ bonds between B and N atoms. Within the limits of these methodologies, the resulting value of the zero-frequency dielectric constant is shown to be influenced by the population density of such mid-gap states and their localization characteristics. We observe nontrivial correlations between the structure-induced electronic fluctuations and the resulting dielectric constant values. Our findings are however just a first step in the quest of accessing fully accurate dielectric properties of as-grown amorphous BN of relevance for interconnect technologies and beyond.

cond-mat.mtrl-sci↗

arXiv2024

Amorphous Boron Nitride as an Ultrathin Copper Diffusion Barrier for Advanced Interconnects

This study focuses on amorphous boron nitride ($\rm \alpha$-BN) as a novel diffusion barrier for advanced semiconductor technology, particularly addressing the critical challenge of copper diffusion in back-end-of-line (BEOL) interconnects. Owing to its ultralow dielectric constant and robust barrier properties, $\rm \alpha$-BN is examined as an alternative to conventional low-k dielectrics. The investigation primarily employs theoretical modelling, using a Gaussian Approximation Potential, to simulate and understand the atomic-level interactions. This machine learning-based approach allows the performance of realistic simulations of amorphous structure of $\rm \alpha$-BN, enabling the exploration of the impact of different film morphologies on barrier efficacy. Furthermore, we studied the electronic and optical properties of the films using a simple Tight-Binding model. In addition to the theoretical studies, we performed diffusion studies of copper through PECVD $\rm \alpha$-BN on Si. The results from both the theoretical and experimental investigations highlight the potential of $\rm \alpha$-BN as a highly effective diffusion barrier, suitable for integration in nanoelectronics. This research shows that $\rm \alpha$-BN is a promising candidate for BEOL interconnects but also demonstrates the synergy of advanced computational models and experimental methods in material innovation for semiconductor applications.

cond-mat.mtrl-sci↗

arXiv2023

Impact of Hydrogenation on the Stability and Mechanical Properties of Amorphous Boron Nitride

Interconnect materials with ultralow dielectric constant, and good thermal and mechanical properties are crucial for the further miniaturization of electronic devices. Recently, it has been demonstrated that ultrathin amorphous boron nitride (aBN) films have a very low dielectric constant, high density (above 2.1 g/cm3), high thermal stability, and mechanical properties. The excellent properties of aBN derive from the nature and degree of disorder, which can be controlled at fabrication, allowing tuning of the physical properties for desired applications. Here, we report an improvement in the stability and mechanical properties of amorphous boron nitride upon hydrogen doping. With the introduction of a Gaussian approximation potential (GAP) for atomistic simulations, we investigate the changing morphology of amorphous boron nitride with varying H doping concentrations. We found that for 8 at% of H doping, the concentration of $sp^3$-hybridized atoms reaches a maximum which leads to an improvement of thermal stability and mechanical properties by 20%. These results will be a guideline for experimentalists and process engineers to tune the growth conditions of amorphous boron nitride films for numerous applications.

cond-mat.mtrl-sci↗

arXiv2018

Quantum engineering of transistors based on 2D materials heterostructures

Quantum engineering entails atom by atom design and fabrication of electronic devices. This innovative technology that unifies materials science and device engineering has been fostered by the recent progress in the fabrication of vertical and lateral heterostructures of two-dimensional materials and by the assessment of the technology potential via computational nanotechnology. But how close are we to the possibility of practical realisation of the next generation atomically thin transistors? In this perspective we analyse the outlook and the challenges of quantum- engineered transistors using heterostructures of two-dimensional materials against the benchmark of silicon technology and its foreseeable evolution in terms of potential performance and manufacturability. Transistors based on lateral heterostructures emerge as the most promising option from a performance point of view, even if heterostructure formation and control are in the initial technology development stage.

cond-mat.mtrl-sci↗

arXiv2016

Scaling Properties of Polycrystalline Graphene: A Review

We present an overview of the electrical, mechanical, and thermal properties of polycrystalline graphene. Most global properties of this material, such as the charge mobility, thermal conductivity, or Young's modulus, are sensitive to its microstructure, for instance the grain size and the presence of line or point defects. Both the local and global features of polycrystalline graphene have been investigated by a variety of simulations and experimental measurements. In this review, we summarize the properties of polycrystalline graphene, and by establishing a perspective on how the microstructure impacts its large-scale physical properties, we aim to provide guidance for further optimization and improvement of applications based on this material, such as flexible and wearable electronics, and high-frequency or spintronic devices.

cond-mat.mtrl-sci↗

arXiv2013

Band alignment of two-dimensional transition metal dichalcogenides: application in tunnel field effect transistors

Tunnel field effect transistors (TFETs) based on vertical stacking of two dimensional materials are of interest for low-power logic devices. The monolayer transition metal dichalcogenides (TMDs) with sizable band gaps show promise in building p-n junctions (couples) for TFET applications. Band alignment information is essential for realizing broken gap junctions with excellent electron tunneling efficiencies. Promising couples composed of monolayer TMDs are suggested to be VIB-MeX2 (Me= W, Mo; X= Te, Se) as the n-type source and IVB-MeX2 (Me = Zr, Hf; X= S, Se) as the p-type drain by density functional theory calculations.

cond-mat.mtrl-sci↗

arXiv2010

Graphene films with large domain size by a two-step chemical vapor deposition process

The fundamental properties of graphene are making it an attractive material for a wide variety of applications. Various techniques have been developed to produce graphene and recently we discovered the synthesis of large area graphene by chemical vapor deposition (CVD) of methane on Cu foils. We also showed that graphene growth on Cu is a surface-mediated process and the films were polycrystalline with domains having an area of tens of square microns. In this paper we report on the effect of growth parameters such as temperature, and methane flow rate and partial pressure on the growth rate, domain size, and surface coverage of graphene as determined by Raman spectroscopy, and transmission and scanning electron microscopy. Based on the results, we developed a two-step CVD process to synthesize graphene films with domains having an area of hundreds of square microns. Scanning electron microscopy and Raman spectroscopy clearly show an increase in domain size by changing the growth parameters. Transmission electron microscopy further shows that the domains are crystallographically rotated with respect to each other with a range of angles from about 13 degrees to nearly 30 degrees. Electrical transport measurements performed on back-gated FETs show that overall films with larger domains tend to have higher carrier mobility, up to about 16,000 cm2 V-1 s-1 at room temperature.

cond-mat.mtrl-sci↗

arXiv2010

Large domain graphene

Graphene growth by chemical vapor deposition has received a lot of attention recently owing to the ease with which large area films can be grown, but growth of large domain or equivalently large grain size has not been reported yet. In this brevia, we report on a CVD process that yields graphene with domains of hundreds of micrometers, by very low pressure CVD, less than 50 mTorr, and very low precursor flow rates using methane as the source of carbon on the inside of copper foil enclosures at high temperature, around 1000 oC. The carrier mobility for the large-domain graphene films is up to 21,000 cm2/Vs.

cond-mat.mtrl-sci↗

arXiv2010

Dielectric Thickness Dependence of Carrier Mobility in Graphene with HfO2 Top Dielectric

We investigate the carrier mobility in mono- and bi-layer graphene with a top HfO2 dielectric, as a function of the HfO2 film thickness and temperature. The results show that the carrier mobility decreases during the deposition of the first 2-4 nm of top dielectric and remains constant for thicker layers. The carrier mobility shows a relatively weak dependence on temperature indicating that phonon scattering does not play a dominant role in controlling the carrier mobility. The data strongly suggest that fixed charged impurities located in close proximity to the graphene are responsible for the mobility degradation.

cond-mat.mes-hall↗

arXiv2009

Realization of a High Mobility Dual-gated Graphene Field Effect Transistor with Al2O3 Dielectric

We fabricate and characterize dual-gated graphene field-effect transistors (FETs) using Al2O3 as top-gate dielectric. We use a thin Al film as a nucleation layer to enable the atomic layer deposition of Al2O3. Our devices show mobility values of over 8,000 cm2/Vs at room temperature, a finding which indicates that the top-gate stack does not significantly increase the carrier scattering, and consequently degrade the device characteristics. We propose a device model to fit the experimental data using a single mobility value.

cond-mat.mes-hall↗

arXiv2009

Evolution of graphene growth on Cu and Ni studied by carbon isotope labeling

Large-area graphene is a new material with properties that make it desirable for advanced scaled electronic devices1. Recently, chemical vapor deposition (CVD) of graphene and few-layer graphene using hydrocarbons on metal substrates such as Ni and Cu has shown to be a promising technique2-5. It has been proposed in recent publications that graphene growth on Ni occurs by C segregation2 or precipitation3, while that on Cu is by surface adsorption5. In this letter, we used a carbon isotope labeling technique to elucidate the growth kinetics and unambiguously demonstrate that graphene growth on Cu is by surface adsorption whereas on Ni is by segregation-precipitation. An understanding of the evolution of graphene growth and thus growth mechanism(s) is desired to obtain uniform graphene films. The results presented in this letter clearly demonstrate that surface adsorption is preferred over precipitation to grow graphene because it is a self-limiting process and thus manufacturable.

cond-mat.mtrl-sci↗

arXiv2009

Large-Area Synthesis of High-Quality and Uniform Graphene Films on Copper Foils

Graphene has been attracting great interest because of its distinctive band structure and physical properties. Today, graphene is limited to small sizes because it is produced mostly by exfoliating graphite. We grew large-area graphene films of the order of centimeters on copper substrates by chemical vapor deposition using methane. The films are predominantly single layer graphene with a small percentage (less than 5%) of the area having few layers, and are continuous across copper surface steps and grain boundaries. The low solubility of carbon in copper appears to help make this growth process self-limiting. We also developed graphene film transfer processes to arbitrary substrates, and dual-gated field-effect transistors fabricated on Si/SiO2 substrates showed electron mobilities as high as 4050 cm2V-1s-1 at room temperature.

cond-mat.mtrl-sci↗