Experimental evidence of the vacancy-mediated silicon self-diffusion in single-crystalline silicon

A simple kinetic parameter indicating the origin of the relaxations induced by point(-like) defects in metallic crystals and glasses

Computer simulation shows that an increase of the volume V due to point defects in a simple metallic crystal (Al) and high entropy alloy (Fe₂₀Ni₂₀Cr₂₀Co₂₀Cu₂₀) leads to a linear decrease of the shear modulus G. This diaelastic effect can be characterized by a single dimensionless parameter K = dln G/dln V. For dumbbell interstitials in single crystals K ≈ -30 while for vacancies the absolute K-value is smaller by an order of magnitude. In the polycrystalline state, K ≈ -20 but its the absolute value remains anyway 5-6 times larger than that for vacancies. The physical origin of this difference comes from the fact that dumbbell interstitials constitute elastic dipoles with highly mobile atoms in their nuclei and that is why produce much larger shear softening compared to vacancies. For simulated Al and high entropy alloy in the glassy state, K equals to -18 and -12, respectively. By the absolute magnitude, these values are by several times larger compared to the case of vacancies in the polycrystalline state of these materials. An analysis of the experimental data on isothermal relaxations of G as a function of V for six Zr-based metallic glasses tested at different temperatures shows that K is time independent and equals to ≈-43, similar to interstitials in single-crystals. It is concluded that K constitutes a important simple kinetic parameter indicating the origin of relaxations induced by point(-like) defects in the crystalline and glassy states.

Deterministic Role of Collision Cascade Density in Radiation Defect Dynamics in Si

The formation of stable radiation damage in solids often proceeds via complex dynamic annealing (DA) processes, involving point defect migration and interaction. The dependence of DA on irradiation conditions remains poorly understood even for Si. Here, we use a pulsed ion beam method to study defect interaction dynamics in Si bombarded in the temperature range from ∼-30 °C to 210 °C with ions in a wide range of masses, from Ne to Xe, creating collision cascades with different densities. We demonstrate that the complexity of the influence of irradiation conditions on defect dynamics can be reduced to a deterministic effect of a single parameter, the average cascade density, calculated by taking into account the fractal nature of collision cascades. For each ion species, the DA rate exhibits two well-defined Arrhenius regions where different DA mechanisms dominate. These two regions intersect at a critical temperature, which depends linearly on the cascade density. The low-temperature DA regime is characterized by an activation energy of ∼0.1  eV, independent of the cascade density. The high-temperature regime, however, exhibits a change in the dominant DA process for cascade densities above ∼0.04 at.%, evidenced by an increase in the activation energy. These results clearly demonstrate a crucial role of the collision cascade density and can be used to predict radiation defect dynamics in Si.

Site-controlled and advanced epitaxial Ge/Si quantum dots: fabrication, properties, and applications

In this review, we report on fabrication paths, challenges, and emerging solutions to integrate group-IV epitaxial quantum dots (QDs) as active light emitters into the existing standard Si technology. Their potential as laser gain material for the use of optical intra- and inter-chip interconnects as well as possibilities to combine a single-photon-source-based quantum cryptographic means with Si technology will be discussed. We propose that the mandatory addressability of the light emitters can be achieved by a combination of organized QD growth assisted by templated self-assembly, and advanced inter-QD defect engineering to boost the optical emissivity of group-IV QDs at room-temperature. Those two main parts, the site-controlled growth and the light emission enhancement in QDs through the introduction of single defects build the main body of the review. This leads us to a roadmap for the necessary further development of this emerging field of CMOS-compatible group-IV QD light emitters for on-chip applications.

The role of Frenkel defect diffusion in dynamic annealing in ion-irradiated Si

The formation of stable radiation damage in crystalline solids often proceeds via complex dynamic annealing processes, involving migration and interaction of ballistically-generated point defects. The dominant dynamic annealing processes, however, remain unknown even for crystalline Si. Here, we use a pulsed ion beam method to study defect dynamics in Si bombarded in the temperature range from -20 to 140 °C with 500 keV Ar ions. Results reveal a defect relaxation time constant of ~10-0.2 ms, which decreases monotonically with increasing temperature. The dynamic annealing rate shows an Arrhenius dependence with two well-defined activation energies of 73 ± 5 meV and 420 ± 10 meV, below and above 60 °C, respectively. Rate theory modeling, bench-marked against this data, suggests a crucial role of both vacancy and interstitial diffusion, with the dynamic annealing rate limited by the migration and interaction of vacancies.

Self-Diffusion in Amorphous Silicon

The present Letter reports on self-diffusion in amorphous silicon. Experiments were done on ^{29}Si/^{nat}Si heterostructures using neutron reflectometry and secondary ion mass spectrometry. The diffusivities follow the Arrhenius law in the temperature range between 550 and 700 °C with an activation energy of (4.4±0.3)  eV. In comparison with single crystalline silicon the diffusivities are tremendously higher by 5 orders of magnitude at about 700 °C, which can be interpreted as the consequence of a high diffusion entropy.

Electronic structure and van der Waals interactions in the stability and mobility of point defects in semiconductors

We study the role of electronic structure (band gaps) and long-range van der Waals (vdW) interactions on the stability and mobility of point defects in silicon and heavier semiconductors. Density functional theory calculations with hybrid functionals that contain part of the Hartree-Fock exchange energy are essential to achieve a reasonable description of defect electronic levels, leading to accurate defect formation energies. However, these functionals significantly overestimate the experimental migration barriers. The inclusion of screened vdW interactions further improves the description of defect formation energies, significantly changes the barrier geometries, and brings migration barrier heights into close agreement with experimental values. These results suggest that hybrid functionals with vdW interactions can be successfully used for predictions in a broad range of materials in which the correct description of both the electronic structure and the long-range electron correlation is essential.

Range-separated approach to the RPA correlation applied to the van der Waals Bond and to diffusion of defects

The random-phase approximation (RPA) is a promising approximation to the exchange-correlation energy of density functional theory, since it contains the van der Waals (vdW) interaction and yields a potential with the correct band gap. However, its calculation is computationally very demanding. We apply a range-separation concept to RPA and demonstrate how it drastically speeds up the calculations without loss of accuracy. The scheme is then successfully applied to a layered system subjected to weak vdW attraction and is used to address the controversy of the self-diffusion in silicon. We calculate the formation and migration energies of self-interstitials and vacancies taking into account atomic relaxations. The obtained activation energies deviate significantly from the earlier calculations and challenge some of the experimental interpretations: the diffusion of vacancies and interstitials has almost the same activation energy.

Defect formation energies without the band-gap problem: combining density-functional theory and the GW approach for the silicon self-interstitial

We present an improved method to calculate defect formation energies that overcomes the band-gap problem of Kohn-Sham density-functional theory (DFT) and reduces the self-interaction error of the local-density approximation (LDA) to DFT. We demonstrate for the silicon self-interstitial that combining LDA with quasiparticle energy calculations in the G0W0 approach increases the defect formation energy of the neutral charge state by approximately 1.1 eV, which is in good agreement with diffusion Monte Carlo calculations (E. R. Batista, Phys. Rev. B 74, 121102(R) (2006)10.1103/PhysRevB.74.121102; W.-K. Leung Phys. Rev. Lett. 83, 2351 (1999)10.1103/PhysRevLett.83.2351). Moreover, the G0W0-corrected charge transition levels agree well with recent measurements.