From Jekyll to Hyde and Beyond: Hydrogen's Multifaceted Role in Passivation, H-Induced Breakdown, and Charging of Amorphous Silicon Nitride

In semiconductor devices, hydrogen has traditionally been viewed as a panacea for defects, being adept at neutralizing dangling bonds and consequently purging the related states from the band gap. With amorphous silicon nitride (a-Si3N4)─a material critical for electronic, optical, and mechanical applications─this belief holds true as hydrogen passivates both silicon and nitrogen dangling bonds. However, there is more to the story. Our density functional theory calculations unveil hydrogen’s multifaceted role upon incorporation in a-Si3N4. On the “Jekyll” side, hydrogen atoms are indeed restorative, healing coordination defects in a-Si3N4. However, “Hyde” emerges as hydrogen induces Si–N bond breaking, particularly in strained regions of the amorphous network. Beyond these dual roles, our study reveals an intricate balance between hydrogen defect centers and intrinsic charge traps that already exist in pristine a-Si3N4: the excess charges provided by the H atoms result in charging of the a-Si3N4 dielectric layer

Electrically detected magnetic resonance of carbon dangling bonds at the Si-face 4H-SiC/SiO2_2 interface

SiC based metal-oxide-semiconductor field-effect transistors (MOSFETs) have gained a significant importance in power electronics applications. However, electrically active defects at the SiC/SiO$_2$ interface degrade the ideal behavior of the devices. The relevant microscopic defects can be identified by electron paramagnetic resonance (EPR) or electrically detected magnetic resonance (EDMR). This helps to decide which changes to the fabrication process will likely lead to further increases of device performance and reliability. EDMR measurements have shown very similar dominant hyperfine (HF) spectra in differently processed MOSFETs although some discrepancies were observed in the measured $g$-factors. Here, the HF spectra measured of different SiC MOSFETs are compared and it is argued that the same dominant defect is present in all devices. A comparison of the data with simulated spectra of the C dangling bond (P$_\textrm{bC}$) center and the silicon vacancy (V$_\textrm{Si}$) demonstrates that the P$_\textrm{bC}$ center is a more suitable candidate to explain the observed HF spectra.Comment: Accepted for publication in the Journal of Applied Physic

Combined density functional theory and molecular dynamics study of Sm0.75A0.25Co1−xMnxO2.88 (A = Ca, Sr; x = 0.125, 0.25) cathode material for next generation solid oxide fuel cell

One of the main challenges facing solid oxide fuel cell (SOFC) technology is the need to develop materials capable of functioning at intermediate temperatures (500–800 °C), thereby reducing the costs associated with SOFCs. Here, Sm0.75A0.25MnxCo1−xO2.88 (A = Ca, or Sr) is investigated as a potential new cathode material to substitute the traditional lanthanum–strontium manganate for intermediate temperature SOFCs. Using a combination of density functional theory calculations and molecular dynamics simulations, the crucial parameters for SOFC performance, such as the electronic structure, electronic and ionic conductivity, and thermal expansion coefficient, were evaluated. An evaluation of the results illustrates that the conductivity and thermal match of the materials with the electrolyte is dramatically improved with respect to the existing state-of-the-art

Theoretical Study of Ag Interactions in Amorphous Silica RRAM Devices

In this study, Density Functional Theory (DFT) calculations were used to model the incorporation and diffusion of Ag in Ag/a-Si02/Pt resistive random-access memory (RRAM) devices. The Ag clustering mechanism is vital for understanding device operation and at this stage is unknown. In this paper an O vacancy (Vo) mediated cluster model is presented, where the Vo is identified as the principle site for Ag^{+} reduction. The Ag^{+} interstitial is energetically favored at the Fermi energies of Ag and Pt, indicating that Ag^{+} ions are not reduced at the Pt electrode via electron tunneling. Instead, Ag^{+} ions bind to Vo forming the [Ag/Vo]^{+} complex, reducing Ag^{+} via charge transfer from the Si atoms in the vacancy. The [Ag/Vo]^{+} complex is then able to trap an electron forming [Ag/Vo]^{0} at the Fermi energy of Pt. This complex is then able to act as a nucleation site for of Ag clustering with the formation of [Ag2/Vo]^{+} which is reduced by the above mechanism

Computational study of the mixed B-site perovskite SmBxCo1−xO3−d (B = Mn, Fe, Ni, Cu) for next generation solid oxide fuel cell cathodes

SmCoO3 is a promising perovskite material for the next generation of intermediate temperature solid oxide fuel cells (SOFC), but its potential application is directly linked to, and dependent on, the presence of dopant ions. Doping on the Co-site is suggested to improve the catalytic and electronic properties of this cathode material. Fe, Mn, Ni, and Cu have been proposed as possible dopants and experimental studies have investigated and confirmed the potential of these materials. Here we present a systematic DFT+U study focused on the changes in electronic, magnetic, and physical properties with B-site doping of SmCoO3 to allow cathode optimization. It is shown that doping generally leads to distortion in the system, thereby inducing different electron occupations of the Co d-orbitals, altering the electronic and magnetic structure. From these calculations, the 0 K electronic conductivity (σe) was obtained, with SmMnxCo1−xO3 having the highest σe, and SmFexCo1−xO3 the lowest σe, in agreement with experiment. We have also investigated the impact of dopant species and concentration on the oxygen vacancy formation energy (Ef), which is related to the ionic conductivity (σO). We found that the Ef values are lowered only when SmCoO3 is doped with Cu or Ni. Finally, thermal expansion coefficients were calculated, with Mn-doping showing the largest decrease at low x and at x = 0.75. Combining these results, it is clear that Mn-doping in the range x = 0.125–0.25 would imbue SmCoO3 with the most favorable properties for IT-SOFC cathode applications

Structural, elastic, vibrational and electronic properties of amorphous Sm₂O₃ from Ab Initio calculations

Rare earth oxides have shown great promise in a variety of applications in their own right, and as the building blocks of complex oxides. A great deal of recent interest has been focused on Sm2O3, which has shown significant promise as a high-k dielectric and as a ReRAM dielectric. Experimentally, these thin films range from amorphous, through partially crystalline, to poly-crystalline, dependent upon the synthetic conditions. Each case presents a set of modelling challenges that need to be defined and overcome. In this work, the problem of modelling amorphous Sm2O3 is tackled, developing an atomistic picture of the effect of amorphization on Sm2O3 from a structural and electronic structure perspective

Fabrication and Characterisation of an Adaptable Plasmonic Nanorod Array for Solar Energy Conversion

The surface plasmonic modes of a side-by-side aligned gold nanorod array supported on a gold substrate has been characterised by electron energy loss spectroscopy (EELS). Plasmonic coupling within the array splits the nanorods' longitudinal mode into a bright mode (symmetrically aligned dipoles) and a dark mode (anti-symmetrically aligned dipoles). We support this observation by means of finite element modelling (FEM)

Modelling the interactions of NO in a-SiO2

Nitric oxide (NO) is often used for the passivation of SiC/SiO2 metal oxide semiconductor (MOS) devices. Although it is established experimentally, using XPS, EELS, and SIMS measurements, that the 4H-SiC/SiO2 interface is extensively nitridated, the mechanisms of NO incorporation and diffusion in amorphous (a)-SiO2 films are still poorly understood. We used Density Functional Theory (DFT) to simulate the diffusion of NO through a-SiO2 and correlate local steric environment in amorphous network to interstitial NO (NOi) incorporation energy and migration barriers. Using an efficient sampling technique we identify the energy minima and transition states for neutral and negatively charged NOi molecules. Neutral NO interacts with the amorphous network only weakly with the smallest incorporation energies in bigger cages. On the other hand NOi -1 binds at the intrinsic precursor sites for electron trapping

The nature of column boundaries in micro-structured silicon oxide nanolayers

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