Crystal Structure and Electron Density of α-Silicon Nitride: Experimental and Theoretical Evidence for the Covalent Bonding and Charge Transfer

Better band gaps for wide-gap semiconductors from a locally corrected exchange-correlation potential that nearly eliminates self-interaction errors

This work constitutes a comprehensive and improved account of electronic-structure and mechanical properties of silicon-nitride ([Formula: see text] [Formula: see text]) polymorphs via van Leeuwen and Baerends (LB) exchange-corrected local density approximation (LDA) that enforces the exact exchange potential asymptotic behavior. The calculated lattice constant, bulk modulus, and electronic band structure of [Formula: see text] [Formula: see text] polymorphs are in good agreement with experimental results. We also show that, for a single electron in a hydrogen atom, spherical well, or harmonic oscillator, the LB-corrected LDA reduces the (self-interaction) error to exact total energy to  ∼10%, a factor of three to four lower than standard LDA, due to a dramatically improved representation of the exchange-potential.

Experimental and theoretical rationalization of the growth mechanism of silicon quantum dots in non-stoichiometric SiN x : role of chlorine in plasma enhanced chemical vapour deposition

Silicon quantum dots (Si-QDs) embedded in an insulator matrix are important from a technological and application point of view. Thus, being able to synthesize them in situ during the matrix growth process is technologically advantageous. The use of SiH₂Cl₂ as the silicon precursor in the plasma enhanced chemical vapour deposition (PECVD) process allows us to obtain Si-QDs without post-thermal annealing. Foremost in this work, is a theoretical rationalization of the mechanism responsible for Si-QD generation in a film including an analysis of the energy released by the extraction of HCl and the insertion of silylene species into the terminal surface bonds. From the results obtained using density functional theory (DFT), we propose an explanation of the mechanism responsible for the formation of Si-QDs in non-stoichiometric SiN _x starting from chlorinated precursors in a PECVD system. Micrograph images obtained through transmission electron microscopy confirmed the presence of Si-QDs, even in nitrogen-rich (N-rich) samples. The film stoichiometry was controlled by varying the growth parameters, in particular the NH₃/SiH₂Cl₂ ratio and hydrogen dilution. Experimental and theoretical results together show that using a PECVD system, along with chlorinated precursors it is possible to obtain Si-QDs at a low substrate temperature without annealing treatment. The optical property studies carried out in the present work highlight the prospects of these thin films for down shifting and as an antireflection coating in silicon solar cells.

The effect of bias-temperature stress on Na+ incorporation into thin insulating films

The action of Na(+) incorporation into thin insulating films and transport therein under influence of a bias voltage and temperature (BT stress) is the subject of this work. Deposited onto highly n-doped Si wafers, the insulators get BT stressed and subsequently investigated by means of time-of-flight-secondary ion mass spectrometry (ToF-SIMS). A thin PMMA film, spin-coated onto the insulator, serves as host matrix for a defined amount of Na(+), provided via sodium triflate. Combining BT stress and ToF-SIMS depth profiling enables the unambiguous detection of Na(+), incorporated into the insulating material. The insulators of interest vary in their nitride content: SiO(2), SiO(x)N(y), and Si(3)N(4). For SiO(2), it is shown that once a threshold BT stress is exceeded, Na(+) gets quantitatively incorporated from PMMA into the underlying insulator, finally accumulating at the SiO(2)/Si interface. A quantitative assessment by combination of Butler-Volmer kinetics with hopping dynamics reveals activation energies of E(a) = 1.55 - 2.04 eV for Na(+) transport in SiO(2) with varying thickness. On the other hand, SiO(x)N(y) and Si(3)N(4) films show a different Na(+) incorporation characteristic in this type of experiment, which can be explained by the higher coordination of nitrogen and hence the reduced Na(+) permeability within these insulators.