Session We-B2

Nano-Micro-Poly-Silicon and Multilayers: Transport and Electronic Properties II

Chair: Antonin Fejfar, Academy of Sciences of the Czech Republic

We-B2.1 10:40–11:00

Is the Concept of Electronic Band Structure Valid for Si Nanocrystals of Few nm in Size?

Prokop Hapala, Kateřina Kůsová, Ivan Pelant, and Pavel Jelínek

Institute of Physics, Academy of Sciences of the Czech Republic, Cukrovarnická 10, 162 00 Prague, Czech Republic

There has been a long-standing discussion on whether or not an electronic band structure concept, i.e. energy-to-wavevector dispersion, can be assigned to zero-dimensional objects such as quantum dots (nanocrystals) (see e.g. [1–4]). To answer this question, we introduce a general method [5], which allows reconstruction of electronic band structure of nanocrystals from ordinary real-space electronic structure calculations. We carried out an extensive analysis of band structure of a realistic Si nanocrystals of up to 3 nm in size including full geometric and electronic relaxation with different surface passivating groups including hydrogen, hydroxyl and methyl groups [6]. In particular, we combine this method with large scale Density Functional Theory calculations [7] incorporating more than thousand of atoms to obtain insight into the luminescence properties of silicon nanocrystals, in dependence on their surface passivation and mechanical deformation [8,9]. To demonstrate character of the band structure of Si nanocrystals, we calculate band dispersion along the Γ-X direction to compare it with a bulk counterpart. Based on this comparison, we conclude that the band structure concept is applicable to silicon nanocrystals with diameter larger than ~2 nm with certain limitations. In addition we will discuss impact of polarized surface hydroxyl groups or geometric distortion on momentum space selection rules important for light emission.

[1] B. Delley et al., Phys. Rev. B 47, 1397 (1993)

[2] C. Delerue et al., Phys. Rev. B 48, 11024 (1993)

[3] M. S. Hybertsen, Phys. Rev. Lett. 72, 1514 (1994)

[4] D. Kovalev et al., Phys. Rev. Lett. 81, 2803 (1998)

[5] P. Hapala, et al. , arXiv:1204.0421v2

[6] K. Kůsová et al., ACS Nano 4, 4495 (2010)

[7] J. P. Lewis, et al. , Physica Status Solidi (B) (2007)

[8] D. C. Hannah et al., Nano Lett. 12, 4200 (2012)

[9] K. Kůsová et al., Appl. Phys. Lett. 101, 143101 (2012)

Keywords: electronic band structure, silicon nanocrystals, DFT, surface passivation

We-B2.2 11:00–11:20

Role of Temperature on the Optical and Electronic Properties of Nano-crystals: An Ab-initio Molecular Dynamics and Electronic Structure Study

Nancy C. Forero-Martinez (1), Hans-Ch. Weissker (2), Ning Ning (1), Ha-Linh Thi Le (1), and Holger Vach (1)

1. LPICM, Ecole Polytechnique, CNRS, F-91128 Palaiseau, France

2. CINaM-CNRS, Campus Luminy, F-13288 Marseille, France

Silicon nano-crystals (Si-NC) have been extensively investigated in view of their electronic and optical properties. However, it is not completely understood how these properties change under realistic experimental conditions. In this study, we address this issue by investigating the thermal dependence of the electronic and optical properties of hydrogenated Si-NC. A combination of density functional theory with Born-Oppenheimer molecular dynamics simulations is used to compute HOMO-LUMO gap values, radiative lifetimes and absorption spectra for Si-NC of different sizes at different temperatures. Spectra are calculated within the independent-particle approximation. This approximation is robust enough to provide and discuss trends with size of the Si-NC and temperature.

Two different types of NC structures are considered: surface reconstructed NCs corresponding to spherical sections of the bulk material, where one-fold coordinated Silicon atoms are removed and two-fold coordinated Silicon atoms are bonded to form dimers; and non-reconstructed structures, where the NC is built shell-wise, starting with one atom and adding successively the next shell of neighbours. In both cases, the NC surface is passivated with Hydrogen atoms. For all the structures considered in the present study, HOMO-LUMO gaps decrease linearly with temperature. In general, NCs with non-reconstructed surfaces exhibit larger band gaps than the ones with reconstructed surfaces. Furthermore, even at room temperature, independent-particle spectra show an important broadening which is shifted to lower photon energies as temperature increases. The thermal dependence of the radiative lifetimes is explained in terms of the NC size and reconstruction method.

Keywords: Silicon nano-crystals, temperature dependence, band gap, optical properties

We-B2.3 11:20–11:40

Quasicrystalline Phase of Silicon Thin-Film: New Era in Microelectronics

A. R. Middya (1) and K. Ghosh (2)

1. Department of Physics, Syracuse University, Syracuse, NY 13244 - 1130

2. Physics, Astronomy and Materials Science Department, Missouri State University, Springfield, MO 65804

Polycrystalline silicon (poly-Si) thin-film is the building block in thin-film devices in microelectronics. Recently, hot-wire chemical vapor deposition (hot-wire CVD) technique produced poly-Si thin-film shows doping efficiency is close to 0.1, which is nearly an order of magnitude higher than traditional poly-Si thin-film deposited by low pressure chemical vapor deposition (LPCVD) technique [1]. In this report, authors will present a breakthrough in silicon thin-film, we invented quasicrystalline silicon thin-film at low temperature (250°C–300°C) by taking advantages of power of photons in condensation of atoms (Si and H) from gas phase. We used a hot-wire CVD system built within a RF PECVD system to develop quasicrystalline silicon thin-film. We obtained quasicrystalline silicon thin-film after condensation of hot species (Si, H and SiHn) on glass substrate. The quasicrystalline phase of silicon has been analyzed by scanning electron microscopy (SEM), atomic force microscopy (AFM), Raman spectroscopy and X-ray diffraction. We observed Penrose tiling at the surface of silicon thin-film, i.e. surface is decorated by sixfold symmetry and fivefold symmetry. We also observed spherical balls throughout the surface, these balls serve as a quasi-unit cell in quasicrystalline silicon, similar to unit cells in single-crystal and polycrystalline structure. These balls are not grains since we observed Raman peak at 517 cm-1, clear indication of large grain size (~1 to 2 μm). SEM confirmed beautiful ordering of grains, where grains are arranged along crystallographic planes {(111) and (311)}. AFM phase image shows beautiful internal structure of quasicrystalline silicon that structure is sensitive to hydrogen (H2) to silane (SiH4) gas mixing ratio. The photons emitted from filament get reflected within two electrodes of PECVD and photons multiplication takes place. We will discuss how atomic absorption of light and subsequent emission of species (Si, H and SiHn) during gas phase influence formation of compact silicon on glass substrate. In general, atoms and species become very cool after emission of photons. Author will discuss new possibility in developing quasicrystalline thin-film semiconductor from gas phase taking advantage of power of photons during condensation of matter.

[1] A. R. Middya, J. Guillet, R. Brenot, J. Perrin, J. E. Bouree, C. Longeaud, and J. P. Kleider, Mat. Res. Symp. Proc. 467 (1997) p. 271

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