Our research is focused on the development and application of theoretical methodologies for calculations of defects and defect related processes in solids and at interfaces and grain boundaries.
- We develop atomistic theories of the geometric and electronic structure of point defects and the mechanisms of electronic and ionic processes in insulators and semiconductors, in particular for microelectronics applications.
- We create models of trapped excitons and electron and hole polarons in solids and at surfaces and develop mechanisms of photo-induced processes at ionic surfaces and in metal nanofilms.
- We develop theoretical methods for modelling of Atomic Force Microscopy imaging of surfaces and the mechanisms of contrast formation in AFM images in vacuum and in liquids.
- We model adsorption and diffusion of organic molecules at insulating surfaces and mechanisms of formation of molecular super-structures and self-assembled monolayers.
Dielectric breakdown in HfO2 dielectrics: Using multiscale modeling to identify the critical physical processes involved in oxide degradation
Strand, J., La Torraca, P., Padovani, A., Larcher, L. and Shluger, A.L.
Journal of Applied Physics, May 2022
We use a multi-scale modeling to study the time-dependent dielectric breakdown of an amorphous (a-) HfO2 insulator in a metal–oxide–metal capacitor. We focus on the role played by electron injection in the creation of oxygen vacancies, which eventually form the percolation path responsible for dielectric breakdown. In this scenario, the electron transport through the dielectric occurs by multi-phonon trap assisted tunnelling (MPTAT) between O vacancies. Energy parameters characterizing the creation of oxygen vacancies and the MPTAT process are calculated using density functional theory employing a hybrid density functional. The results demonstrate that the formation of neutral O vacancies facilitated by electron injection into the oxide represents a crucial step in the degradation process dominating the kinetics at common breakdown fields. We further show the importance of the so-called “energetic correlation” effect, where pre-existing O vacancies locally increase the generation rate of additional vacancies accelerating the oxide degradation process. This model gives realistic breakdown times and Weibull slopes and provides a detailed insight into the mechanism of dielectric breakdown and atomistic and electronic structures of percolation paths in a-HfO2. It offers a new understanding of degradation mechanisms in oxides used in the current MOSFET technology and can be useful for developing future resistive switching and neuromorphic nanodevices.
Atomistic Modeling of the Electrical Conductivity of Single-Walled Carbon Nanotube Junctions
Durrant, T.R., El-Sayed, A.M., Gao, D., Rueckes, T., Gennadi Bersuker, G., Shluger, A.L.
pss–Rapid Research Letters, June 2022
Energies and structures of Cu/Nb and Cu/W interfaces from density functional theory and semi-empirical calculations
Bodlos, R, Fotopoulos, V, Spitaler, J, Shluger, A.L, Romaner, L.
Cu/Me multilayer systems, with Me referring to a body-centered cubic (bcc) metal, such as Nb and W, are widely used for nuclear, electrical, and electronic applications. Despite making up only a small percentage of the volume, interfaces in such systems play a major role in determining their electrical, mechanical, thermal and diffusive properties. Face-centered cubic (fcc) Cu often forms Kurdjumov-Sachs (KS) and Nishiyama-Wassermann (NW) type interfaces with bcc metals or variations thereof. For the Cu/Nb system, these interface relationships have been extensively studied with semi-empirical methods. Surprisingly, the energetics and interface properties of Cu/W have not yet been studied in detail, in spite of extensive applications. In this study, we employ both periodic Embedded Atom Method (EAM) and Density Functional Theory (DFT) simulations to explore the geometric and energetic properties of the KS and NW interfaces of Cu/Nb and Cu/W. To assess the reliability of our approach, the dependence of the results on the size of periodic cells is examined for coherent and incoherent interfaces. We provide the interface energies and the work of separation for the Cu/W and Cu/Nb interfaces at DFT accuracy. The results of calculations with two EAM potentials are in qualitative agreement with those obtained using DFT and allow investigating the convergence of interfacial properties. These key energetic quantities can be used for future thermodynamic and mechanical modeling of Cu/Me interfaces.
The nature of column boundaries in micro-structured silicon oxide nanolayers
Patel, K., Cottom, J., Mehonic, A., Ng, W.H., Kenyon, A.J., Bosman, M. and Shluger, A.L.
Columnar microstructures are critical for obtaining good resistance switching properties in SiOx resistive random access memory (ReRAM) devices. In this work, the formation and structure of columnar boundaries are studied in sputtered SiOx layers. Using TEM measurements, we analyze SiOx layers in Me–SiOx–Mo heterostructures, where Me = Ti or Au/Ti. We show that the SiOx layers are templated by the Mo surface roughness, leading to the formation of columnar boundaries protruding from troughs at the SiOx/Mo interface. Electron energy-loss spectroscopy measurements show that these boundaries are best characterized as voids, which in turn facilitate Ti, Mo, and Au incorporation from the electrodes into SiOx. Density functional theory calculations of a simple model of the SiO2 grain boundary and column boundary show that O interstitials preferentially reside at the boundaries rather than in the SiO2 bulk. The results elucidate the nature of the SiOx microstructure and the complex interactions between the metal electrodes and the switching oxide, each of which is critically important for further materials engineering and the optimization of ReRAM devices.
Electron trapping in ferroelectric HfO2
Roman A Izmailov, Jack W Strand, Luca Larcher, Barry J O'Sullivan, Alexander L Shluger, Valeri V Afanas' ev
Physical Review Materials, March 2021
Charge trapping study at 300 and 77 K in ferroelectric (annealed Al- or Si-doped) and nonferroelectric (unannealed and/or undoped) HfO2 films grown by atomic layer deposition reveals the presence of “deep” and “shallow” electron traps with volume concentrations in the 1019−cm−3 range. The concentration of deep traps responsible for electron trapping at 300 K is virtually insensitive to the oxide doping by Al or Si but slightly decreases in films crystallized by high-temperature annealing in oxygen-free ambient. This behavior indicates that the trapping sites are intrinsic and probably related to disorder in HfO2 rather than to the oxygen deficiency of the film. Electron injection at 77 K allowed us to fill shallow electron traps energetically distributed at ∼0.2 eV. These electrons are mobile and populate states with thermal ionization energies in the range ∼0.6–0.7 eV below the HfO2 conduction band (CB). The trap energy depth and marginal sensitivity of their concentration to crystallization annealing or film doping with Si or Al suggests that these traps are associated with boundaries between crystalline grains and interfaces between crystalline and amorphous regions in HfO2 films. This hypothesis is supported by density functional theory calculations of electron trapping at surfaces of monoclinic, tetragonal, and orthorhombic phases of HfO2. The calculated trap states are consistent with the observed thermal ionization (0.7–1.0 eV below the HfO2 CB) and photoionization energies (in the range of 2.0–3.5 eV below the HfO2 CB) and support their intrinsic polaronic nature.
Effect of electric field on defect generation and migration in HfO2
Jack W Strand, Jonathon Cottom, Luca Larcher, Alexander L Shluger
Understanding the effect of electric fields on defect creation and diffusion in metal oxides is of fundamental importance for developing accurate models of oxide degradation in electronic devices and dielectric breakdown. We use the Berry phase operator method within density functional theory to calculate how an applied electric field affects barriers for the creation of oxygen vacancy-interstitial defect pairs (DPs) and diffusion of interstitial O ions in monoclinic (m-)HfO2. The results demonstrate that even close to breakdown fields, barriers for DP generation exceed 6 eV in the perfect m−HfO2 lattice. Simulated injection of extra electrons from electrodes significantly lowers barriers for the creation of DPs, which are further reduced by the field to around 1 eV. Thus, bias application facilitates the injection of electrons into the oxide; these extra electrons reduce energy barriers for the creation of O vacancies, and these barriers as well as those for O ion diffusion are further lowered by the field. We find that, within a linear regime, the electric field modulates the barrier height by a dot product between the electric field and the electric dipole at the zero-field transition state to good accuracy.
With the current trend in technology to lower fabrication processing temperatures, extreme bonding geometries in the oxide are expected to become more abundant and increase the influence of strain, ranging from ultra-thin oxides sandwiched between electrodes to porous low-k insulators intrinsically strained by re-bonding reactions. Hence this discovery of unexpected reactivity of atomic hydrogen may have significant implications for the future of silica based device processing.