1 Workshop on Advancement of Group IV Nanostructures Nanophotonics and Nanoelectronics November 18-19, 2014 Takikawa Memorial Hall, Kobe University, Rokkodai, Nada, Kobe 657-8501, Japan (http://www.kobe-u.ac.jp/en/access/rokko/campus.html) Organization: 1. Japan-Czech bilateral joint research project (JSPS) 2. Smart materials team in organization of advanced science and technology, Kobe University.
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Advancement of Group IV Nanostructures …...Advancement of Group IV Nanostructures Nanophotonics and Nanoelectronics November 18-19, 2014 Takikawa Memorial Hall, Kobe University,
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In this presentation we would like to summarize various methods of silicon nanocrystals
preparation and present the pros and cons of each method. Especially we concentrate on
preparation of nanocrystals from hydrogen silsesquioxane (HSQ). The silicon nanocrystals
are formed from HSQ molecules annealed at 1000°C in inert Ar atmosphere with 5% H2. I
this way, we obtain silicon nanocrystals in silicon oxide matrix. The nanocrystals are released
from the matrix by HF etching and need to be stabilized in order to prevent the nanocrystals
from deterioration. In the stabilization process the surface Si-H group is usually replaced
by an organic group. This process can be initiated by light, high temperature,
microwaves, or radicals. The properties of passivated nanocrystals are varying greatly in
quantum efficiency, PL peak position and width.
In cooperation with J. Linnros group we also developed a method for direct
passivation of silicon nanocrystals in the annealing step. The individual
silicon nanocrystals exhibited significantly narrow emission peak at room
temperature (average linewidth ~ 25 meV) compared to silicon nanocrystals
embedded in a silicon oxide shell (150 meV), when studied by single dot
spectroscopy. The luminescence from produced nanocrystals covers a broad
spectral range from 530-720 nm (1.7-2.3 eV). Blinking and spectral hopping
of individual nanocrystals were also detected. The silicon nanocrystals
did not show any deterioration of luminescence for at least 16 months.
Surface and energy band gap engineering of silicon nanocrystals
Vladimir Svrcek1, Mickael Lozach
1, Somak Mitra
2 Davide Mariotti
2,
1 National Institute of Advanced Industrial Science and Technology, Research Center for Photovoltaic
Technologies, Tsukuba, Japan. 2 University of Ulster, Nanotechnology & Integrated Bio-Engineering Centre-NIBEC, Newtownabbey,
UK.
In 1st/2
nd generation devices, every absorbed photon can only generate one electron-hole pair whereby
in devices with carrier multiplication (CM), more than one electron-hole pair can be formed for every
photon absorbed. CM has been demonstrated to occur in PbSe, PbS, PbTe, CdSe, InAs and Si quantum
dots (Si QDs) [1]. In this context, silicon as a raw material represents an attractive solution; however
CM in Si QDs is triggered only for high energy photons (above ~2 eV) [2] and the indirect nature of
silicon still plays a role in limiting the absorption coefficient.
In this talk we discuss how surface engineering and alloying bulk Si with another material offers
another opportunity to challenge the nature of silicon’s indirect energy band gap in Si QDs. In this
respect the atmospheric generated plasmas present unique opportunities. In particular atmospheric
pressure confined plasma (e.g. microplasmas) has allowed substantial promising advances for both QDs
surface engineering and synthesis. Firstly, here we would like to report on recent strategies for
surfactant free surface engineering of Si QDs that have employed microplasma interactions with
colloids [3]. The surface characteristics achieved via microplasma processing contributed to improve
optoelectronic properties, add to our understanding of Si QDs fundamental properties [4-6] and made
possible the fabrication of photovoltaic devices with improved efficiencies [7]. Secondly we discuss the
approaches how to engineer the energy band gap and direct transition in Si QDs by alloying with
environmental friendly and abundant element (e.g, tin). Confined plasmas generated in the liquid by
ns/fs laser pulses allowed the growth of the silicon–tin QDs via kinetic pathways. In particular we
would like to report on the synthetic feasibility of semiconducting alloyed silicon–tin QDs where the
silicon–tin alloys at quantum confinement size have the potential to undergo a real transition from
indirect to direct narrower energy bandgap, compared to corresponding elemental Si QDs [8].
Acknowledgements This work was partially supported by a NEDO and JSPS projects, international
network through the Leverhulme Trust Grant.
References
1. P. V. Kamat J. Phys. Chem. Lett. 2013, 4, 908
2. D. Timmerman et al. Nat. Nanotechnol. 2011, 6, 710L18
3. V. Svrcek et al. Applied Physics Letters 2010, 97, 161502
4. D. Mariotti, et al Adv. Funct. Mat. 2012 ,22, 954.
5. V. Svrcek et al. J. Phys. Chem. C 2013, 117, 10939
6. V. Svrcek et al. Adv. Funct. Mater. 2013, 23, 6051
7. V. Svrcek et al. Appl.Phys.Lett. 2012 , 100, 223904
8. V. Svrcek et al. Nanoscale, 2013, 5, 6725
Characteristics of nano-porous silicon luminescence and electroluminescence, and
effects of various treatments.
Gelloz Bernard
Nano porous silicon can be prepared by electrochemical etching of silicon wafers in
hydrofluoric acid. Luminescent layers can be obtained when the porous structure
contains crystallites whose sizes are lower than the Bohr radius in silicon (~4 nm).
Regarding optoelectronic applications, compared to other nano-silicon materials
obtained by planar technologies, porous silicon has a set of key advantages, such as (i)
the ease and low-cost of manufacture, (ii) the ability to form quickly thick layers
(limited only for the silicon substrate thickness), and (iii) easy modulation of index for
photonic crystal formation. However, it also has critical limitations, such as (i)
mechanical weakness of high porosity layers, (ii) an easily contaminated exposed
surface (it is an opened structure) leading to poor stability, and (iii) a difficult control of
feature size distribution within the porous silicon skeleton.
This presentation reviews the luminescence and electroluminescence properties of
porous silicon. In addition, various treatments used in a view to enhancing their
characteristics (such as efficiency, stability, color) are reviewed.
CMOS-compatible nonlinear optical materials for Si photonics
Kenji Imakita
Graduate School of Engineering, Kobe University
In recent years, nonlinear optical phenomena in Si-based materials have attracted
significant attentions due to the possible optoelectronic applications such as all optical
signal routing, wavelength converter, electro-optic modulator, and so on. It is well
known that Si crystal shows small third order nonlinearity compared to direct bandgap
semiconductors due to the indirect bandgap nature, and exhibits no second order
nonlinearity due to the centrosymmetric structure. These motivate researchers to
explore new CMOS-compatible materials with large second or third order nonlinearity.
This work consists of two parts. The first part is on the third order nonlinear optical
properties of phosphorous(P)- or boron(B)-doped silicon nanocrystals embedded in SiO2
thin films prepared by a sputtering method. A z-scan method and a pump-probe optical
kerr gate method were used to evaluate the nonlinear refractive index (n2), two photon
absorption coefficient (), and the time response of the third order optical nonlinearity.
The values of n2 and of our undoped samples were the order of 10-12 cm2/W and 10-9
cm/W, respectively. Regardless of P- or B-doping, n2 and were found to be enhanced
about 5 times by the doping. The time response of the optical nonlinearity was faster
than our time resolution of 100 femtoseconds. The results indicate that doping of Si
nanocrystals can be a promising tool to improve the nonlinear optical properties.
The second part is on the second order nonlinearity of CMOS-compatible amorphous
thin films. Until recently, it was believed that amorphous materials do not show second
order nonlinearity due to the centrosymmetric structure. However, a few kinds of
amorphous thin films prepared by conventional deposition systems, such as SiN thin
films prepared by plasma enhanced chemical vapor deposition and silicon monoxide
(SiO) thin films prepared by an electron beam deposition, have been reported to exhibit
large second order nonlinearity. In this work, we investigated the second order
nonlinearity of Ge-doped SiO2 amorphous thin films prepared by a sputtering method.
It was found that the second-order nonlinearity of SiO2, which vanishes in the
electric-dipole approximation, can be significantly enhanced by Ge doping. The observed
maximum value of d33 is 8.2 pm/V, which is 4 times larger than d22 of β-BaB2O4 crystal.
Strong correlation was observed between the deff values and the electron spin resonance
signals arising from GePb centers, suggesting that GePb centers are the most probable
origin of the large second-order nonlinearity.
Special techniques of optical spectroscopy applied to Si nanostructures
Jan Valenta
Department of Chemical Physics & Optics, Faculty of Mathematics & Physics, Charles University,
Prague 2, Czechia
[email protected] Light-emitting silicon nanostructures have been studied for more than 25 years, but the full understanding of electronic and optical properties is still not achieved. We believe that application of advanced techniques of optical spectroscopy can move forward our knowledge of Si nanostructures. Namely the following techniques have great potential: single-nano-object spectroscopy, non-linear optical spectroscopy, quantum yield measurement etc. All these techniques require careful calibration and reliable sample
preparation, otherwise severe artefacts could distort the obtained results. In this contribution we shall discuss some critical aspects of these spectroscopy techniques.
Charge transfer dynamics of “Bright” and “Dark” Si-nc’s
Masashi Ishii
National Institute for Materials Science (NIMS), Tsukuba, Ibaraki 305-0047, Japan