Origin of light instability in amorphous IGZO thin-film transistors and its suppression

Radiating amorphous In–Ga–Zn–O (a-IGZO) thin-film transistors (TFTs) with deep ultraviolet light (λ = 175 nm) is found to induce rigid negative threshold-voltage shift, as well as a subthreshold hump and an increase in subthreshold-voltage slope. These changes are attributed to the photo creation and ionization of oxygen vacancy states (VO), which are confined mainly to the top surface of the a-IGZO film (backchannel). Photoionization of these states generates free electrons and the transition from the neutral to the ionized VO is accompanied by lattice relaxation, which raises the energy of the ionized VO. This and the possibility of atomic exchange with weakly bonded hydrogen leads to metastability of the ionized VO, consistent with the rigid threshold-voltage shift and increase in subthreshold-voltage slope. The hump is thus a manifestation of the highly conductive backchannel and its formation can be suppressed by reduction of the a-IGZO film thickness or application of a back bias after radiation. These results support photo creation and ionization of VO as the main cause of light instability in a-IGZO TFTs and provide some insights on how to minimize the effect.

A Review of Density Functional Models for the Description of Fe(II) Spin Crossover Complexes

Spin crossover (SCO) complexes are in the forefront of image, memory and sensing devices, with applications already established since for thirty years. In order to reach magnetic multistability conditions, the high-spin (HS) and low-spin (LS) states have to be carefully balanced by ligand field stabilization and spin pairing energies. Both of these effects could be effectively modelled by electronic structure theory, if the description would be accurate enough to describe these concurrent influences to within a few kJ/mol. Such a milestone would allow for the in silico-driven development of SCO complexes. However, so far, the ab initio simulation of such systems has been dominated by general gradient approximation density functional calculations. The latter can only provide the right answer for the wrong reasons, given that the LS states are grossly stabilized. In this contribution, we explore different venues for the parameterisation of hybrid functionals. A fitting set is provided on the basis of explicitly correlated coupled cluster calculations, with single- and multi-dimensional fitting approaches being tested to selected classes of hybrid functionals (hybrid, range separated and local hybrid). Promising agreement to benchmark data is found for a rescaled PBE0 hybrid functional and a local version thereof, with a discussion of different atomic exchange factors.

Effect of Atomic Complexes Formation in Grain Boundaries on Grain Boundary Diffusion

The peculiarities of grain boundary diffusion in Cu connected with the effect of atomic pairs formation in grain boundaries (GB) were studied using the molecular dynamics (MD) simulation. In present study Cu GB selfdiffusion was simulated with the use of semi-empirical potential. Besides, the ‘heterodiffusion’ simulation was performed with the artificially addеd energy of interaction (E) between identical atoms in arbitrary chosen pairs. To obtain reliable data on the mean square displacements (MSD) the simulation cell, consisted about three hundreds thousands atoms and two symmetrical GBs Σ5 (001)(012), was used. 70 pairs of identical Cu atoms in GBs, bonded into pairs, were chosen as initial state. Energy of interaction was varied between 0 and - 0.5eV/atomThe results obtained for selfdiffusion are in a good agreement with experimental results and other results of computer simulation. Two main effects for heterodiffusion are under discussion. The first is atomic exchange between GB zone and adjacent lattice zone, where the mobility of the atoms decreases significantly. As a result, the MSD decrease. Another effect is connected with attraction between the “marked” atoms, which leads to formation of relatively stable complexes and the MSD also decreases. The results obtained involve also dependence the number of the stable pairs on time and temperature and show the possibility of pairs to condense into ternary, quarterly and more numerous complexes.

Aspects of the Solid State

Symmetries play an important role in the theory of the solid state. As will be developed in this Chapter, DFT calculations for crystalline materials are commonly performed for the irreducible part of the first Brillouin zone, an approach which relies on the use of translational and point group symmetries. Two central properties that result from a calculation in reciprocal space are the wave vector resolved energy spectra, the so called band structure, and the energy resolved density of states. For magnetic materials, atomic magnetic moment moments can be defined and calculated, as well as effective inter-atomic exchange interactions.