Low-density silicon allotropes for photovoltaic applications

Silicon materials play a key role in many technologically relevant fields, ranging from the electronic to the photovoltaic industry. A systematic search for silicon allotropes was performed by employing a modified ab initio minima hopping crystal structure prediction method. The algorithm was optimized to specifically investigate the hitherto barely explored low-density regime of the silicon phase diagram by imitating the guest-host concept of clathrate compounds. In total 44 metastable phases are presented, of which 11 exhibit direct or quasi-direct band-gaps in the range of $\approx$1.0-1.8 eV, close to the optimal Shockley-Queisser limit of $\approx$1.4 eV, with a stronger overlap of the absorption spectra with the solar spectrum compared to conventional diamond silicon. Due to the structural resemblance to known clathrate compounds it is expected that the predicted phases can be synthesized

Thermochemical and Mechanical Stabilities of the Oxide Scale of ZrB\u3csub\u3e2\u3c/sub\u3e+SiC and Oxygen Transport Mecha

Refractory diboride with silicon carbide additive has a unique oxide scale microstructure with two condensed oxide phases (solid+liquid), and demonstrates oxidation resistance superior to either monolithic diboride or silicon carbide. We rationalize that this is because the silica-rich liquid phase can retreat outward to remove the high SiO gas volatility region, while still holding onto the zirconia skeleton mechanically by capillary forces, to form a solid pillars, liquid roof scale architecture and maintain barrier function. Basic assessment of the oxygen carriers in the borosilicate liquid in oxygen-rich condition is performed using first-principles calculations. It is estimated from entropy and mobility arguments that above a critical temperature Tc~1500°C, the dominant oxygen carriers should be network defects, such as peroxyl linkage or oxygen-deficient centers, instead of molecular O2* as in the Deal–Grove model. These network defects will lead to sublinear dependence of the oxidation rate with external oxygen partial pressure. The present work suggests that there could be significant room in improving the high-temperature oxidation resistance by refining the oxide scale microstructure as well as controlling the glass chemistry

Diffusive Molecular Dynamics and its Application to Nanoindentation and Sintering

The interplay between diffusional and displacive atomic movements is a key to understanding deformation mechanisms and microstructure evolution in solids. The ability to handle the diffusional time scale and the structural complexity in these problems poses a general challenge to atomistic modeling. We present here an approach called diffusive molecular dynamics (DMD), which can capture the diffusional time scale while maintaining atomic resolution, by coarse-graining over atomic vibrations and evolving a smooth site-probability representation. The model is applied to nanoindentation and sintering, where intimate coupling between diffusional creep, displacive dislocation nucleation, and grain rotation are observed

First-principles calculation of intrinsic defect formation volumes in silicon

We present an extensive first-principles study of the pressure dependence of the formation enthalpies of all the know vacancy and self-interstitial configurations in silicon, in each charge state from -2 through +2. The neutral vacancy is found to have a formation volume that varies markedly with pressure, leading to a remarkably large negative value (-0.68 atomic volumes) for the zero-pressure formation volume of a Frenkel pair (V + I). The interaction of volume and charge was examined, leading to pressure--Fermi level stability diagrams of the defects. Finally, we quantify the anisotropic nature of the lattice relaxation around the neutral defects.Comment: 9 pages, 9 figure

Ubiquitous mechanisms of energy dissipation in noncontact atomic force microscopy

Atomistic simulations considering larger tip structures than hitherto assumed reveal novel dissipation mechanisms in noncontact atomic force microscopy. The potential energy surfaces of realistic silicon tips exhibit many energetically close local minima that correspond to different structures. Most of them easily deform, thus causing dissipation arising from hysteresis in force versus distance characteristics. Furthermore, saddle points which connect local minima can suddenly switch to connect different minima. Configurations driven into metastability by the tip motion can thus suddenly access lower energy structures when thermal activation becomes allowed within the time required to detect the resulting average dissipation

Energy landscape of silicon tetra-interstitials using an optimized classical potential

Mobile single interstitials can grow into extended interstitial defect structures during thermal anneals following ion implantation. The silicon tetra-interstitials present an important intermediate structure that can either provide a chain-like nucleation site for extended structures or form a highly stable compact interstitial cluster preventing further growth. In this paper, dimer searches using the tight-binding (TB) model by Lenosky et al. and density functional calculations show that the compact ground-state I-4(a) and the I-4-chain are surrounded by high-lying neighboring local minima. To furthermore explore the phase space of tetra-interstitial structures an empirical potential is optimized to a database of silicon defect structures. The minima hopping method combined with this potential extensively searches the energy landscape of tetra-interstitials and discovers several new lowenergy I-4 structures. The second lowest-energy I-4 structure turns out to be a distorted ground-state tri-interstitial bound with a single interstitial, which confirms that the ground-state tri-interstitial may serve as a nucleation center for the extended defects in silicon

Energetic and vibrational analysis of hydrogenated silicon m vacancies above saturation

We present a systematic study on hydrogenated silicon m vacancies above saturation. For each system a global geometry optimization search for low-lying local minima is performed using a newly developed SiH tight binding model. Subsequently a large number of low-energy structures are examined by density functional calculations using a minimal basis set. Finally the energetically favorable structures are reexamined using a systematically extendable basis set with local, semilocal, and hybrid exchange-correlation functionals. Particular attention is paid to the divacancy to which the Raman peak at 3822 cm(-1) associated with the H-2 molecule had previously been assigned. Both the energetics and vibrational analysis of divacancy-related stable configurations suggest a revision of the above conclusion

Energy landscape of silicon systems and its description by force fields, tight binding schemes, density functional methods, and quantum Monte Carlo methods

The accuracy of the fundamental properties of the energy landscape of silicon systems obtained from density functional theory with various exchange-correlation functionals, a tight binding scheme, and force fields is studied. Depending on the application, quantum Monte Carlo or density functional theory results serve as quasiexact reference values. In addition to the well-known accuracy of density functional methods for geometric ground states and metastable configurations we find that density functional methods give a similar accuracy for transition states and thus a good overall description of the energy landscape of the silicon systems. On the other hand, force fields give a very poor description of the landscape that are in most cases too rough and contain many spurious local minima and saddle points or ones that have the wrong height