Symposium : J
Silicon-based nanophotonics
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| Silicon nanocrystals - Quantum confinement and surface states : Dmitry Kovalev | ||
| 09:00 | Foreword of Symposium J: Silicon-Based Nanophotonics Authors : Jurgen Michel, MIT, USA Resume : Foreword of Symposium J: Silicon-Based Nanophotonics | 1 1 |
| 09:15 | Silicon Photonics: Twenty Years of Porosification Authors : Dr Paul Snow, Centre for Photonics and Photonic Materials University of Bath Bath United Kingdom Resume : 20 years ago, Canham’s report of efficient room-temperature photoluminescence (PL) from porous silicon (pSi) started work that has grown into a wide research field. I will review the history of pSi photonics, showing how ideas and applications evolved from the initial work. An early controversy emerged on the explanation of the PL. This highlighted the fact that the large surface area of the porous media could be a major contributor to the properties, or the pores filled with contaminants. The final consensus was that long-lived confined excitons are the light emitting elements. Alongside the PL work, there was an emerging understanding that the tuneabilty of pSi gives interesting opportunities. Using the porosity dependence of the index allowed Bragg mirrors and microcavities to be etched to help enhance the intrinsic luminescence of pSi, using engineered optical modes. This was followed by work to enhance erbium emission from pSi multilayers for which pSi was now only a host material. There was also the start of considerable work on sensors using pSi. The surface area of the silicon and the pore volume can be exposed to a material and the resulting change in the bulk properties of the pSi used as a sensing mechanism. The photonics of this work started with changes to Fabry-Perot modes but rapidly progressed to Bragg mirrors, microcavities and rugate filters. Finally, I will introduce recent results on acoustic devices in pSi; we have made an acoustic Bragg mirrors at 1GHz. | 1 2 |
| 09:45 | On the role of NBOHC (Non-Bridging Oxygen Hole Centres) in the red photo-luminescence emission from Si nanocrystals Authors : E. Borsella, R. D’Amato, F. Fabbri, M.Falconieri- ENEA (Italy) E. Trave- Univ. Venezia (Italy) V. Bello, G. Mattei-Univ. Padova (Italy) Y. Nie, D. Wang MPIKG-Potsdam (Germany) Resume : The process responsible for visible-near infrared PL (Photo-Luminescence) emission in Si nanostructures has generated significant controversy for years. The debate has focused on wether light emission is originated by recombination of electrons (e) and holes (h) in Si nanostructeres (band to band transitions in quantum confined systems) or by defects located at the surface. It is experimentally difficult to distinguish the two contributions since both are size-dependent. Moreover, all the studies were performed on different systems and consequently the comparisons were not free from ambiguities. Here we relate the spectroscopic properties of crystalline Si nanoparticles, as prepared by laser pyrolysis and after complete conversion to amorphous silica by oxidation-assisted alkali etching. The strong resemblance of the spectral and time-behavior of the red PL emission (in the range 600-1040 nm) in both systems, indicates that this emission is dominated by defects states localized at the SiO2/Si interface, whereas Q. C. (Quantum Confinement) affects the absorption of Si nanocrystals, that results to be the main size dependent effect. Changes in bond angle and bond length in the emitting centre, as well as variation of the chemical environment, influence the emission energies and can give the false impression of a shift due to the size effect. The strongly non-exponential time behaviour of the photo-luminescence emission in both systems (nano-crystalline Si and oxidised amorphous sample), can also be explained as the sum of exponential decays from the emitting non- bridging oxygen hole centres (NBOHC), thus ruling out the interpretation in terms of the so-called “stretched exponential” decay. | 1 3 |
| 10:00 | PHOTO-LUMINESCENCE BLEACHING IN FREE STANDING Si NANOCRYSTALS PREPARED BY LASER ASSISTED SILANE PROLYSIS Authors : Ayse Seyhan1, Urcan Guler1 , Mauro Falconieri2, Rosaria D\'Amato2, Elisabetta Borsella3, Rasit Turan1 1Department of Physics Middle East Technical University, Inonu Blvrd, 06531 Ankara, Turkey 2Italian National Agency for New Technologies, Energy and Sustainable Economic Development (ENEA) C. R. Casaccia, Via Anguillarese 301, 00123 Roma, Italy 3Italian National Agency for New Technologies, Energy and Sustainable Economic Development (ENEA) C. R. Frascati, Via E. Fermi 45, 00044 Frascati, Roma, Italy Resume : Temporal variation of photoluminescence (PL) from free standing Si nanoparticles is studied under a strong laser illumination. Upon exposure to the laser beam, the PL peak intensity decays to a lower level in a time duration of a few seconds. The decay rate for the luminescence bleaching increases with higher laser intensity, and decreases with decreasing temperature. It is shown by a series of experiements that the bleaching is reversible when the laser exposure is switched off. The bleaching behavior is investigated for different laser wavelengths and intensities at different ambient temperatures. Si nanoparticles were prepared by the laser assisted silane pyrolysis technique. The high resolution transmission electron microscopy images confirmed the presence of both crystalline and amorphous nanoparticles with an average diameter of 5 nm. The chemical structure of the nanoparticles were identified by Raman spectroscopy which exhibited two bands corresponding Si-Si bonds in crystalline and amorphous form. The bleaching in Si nanostructures is generally attributed to a blinking process which is observed in a single nanoparticle populated by more than one excitons under a strong laser illumination. It is commonly accepted that the blinking occurs only in connection with and Auger assisted charge trapping. We discuss the observed PL bleaching in terms of exciton trapping at the interface between nanocrystal and the surrounding oxide layer. | 1 4 |
| 10:15 | Atomic scale investigation of Si nanoclusters growth in SiO2 Authors : Etienne TALBOT (1), Manuel ROUSSEL (1), Rodrigue LARDE (1), Fabrice GOURBILLEAU (2), Phillipe PAREIGE (1). (1) Groupe de Physique des Matériaux, Université de Rouen et INSA de Rouen, UMR CNRS 6634, avenue de l\\\'Université, BP 12, 76801 Saint Etienne du Rouvray, France (2) CIMAP, UMR CNRS 6252, ENSICAEN, 6 Bd Maréchal Juin, 14050 Caen Cedex, France Resume : Si-based photonic or memory devices, compatibles with silicon integration technology, is a very fast growing field of nanosciences. A part of this field focuses on the light-emission properties of Si nanoclusters (Si-nc) embedded in silicon oxide in particular for the study of quantum confinement mechanism. The Si-nc properties appear to be very sensitive to their Si/SiO2 interface and their size distribution. In this work, Si nanoclusters embedded in silica/silicon rich silica multilayers were characterized, at the atomic scale using the atom probe tomography (APT). A chemical and structural picture of the nanostructured material is thus given at the atomic scale and in 3D. A discussion about the nature of this interface and the quantification of the substochiometric silicon dioxide interface thickness which is a keypoint for light-emission properties will be given. Furthermore, the growth method and the annealing time and temperature govern the nanostructural characteristics (density, size distribution, phase separation ...), directly linked to the optical properties. The influence of annealing conditions on silicon decomposition (driving forces and kinetics) in these nanolayers will be presented and compared to literature. | 1 5 |
| 11:00 | Direct bandgap-related emission in silicon nanocrystals Authors : W.D.A.M. de Boer, T. Gregorkiewicz Van der Waals—Zeeman Institute, University of Amsterdam Resume : Next to the well-investigated indirect bandgap-related PL, another emission band is frequently observed. It is characterized by higher photon energies and faster decay times (nano- vs. microseconds for the indirect bandgap recombination time). Controversy remains however on the microscopic nature of this emission band – whether it arises from O-related surface state recombinations or transitions between NC core levels. Based on results obtained with time-resolved PL experiments conducted on SiNCs embedded in a SiO2-matrix supported by a theoretical model on the QC effects at the Γ-point [1], we postulate to identify this band with radiative recombinations of electron-hole pairs across the direct bandgap. The theoretical model predicts a QC-induced redshift of the direct bandgap value upon NC size decrease, from the 3.32 eV value of bulk Si, covering the entire visible spectral range. We demonstrate that the proposed model accurately describes the experimental findings conducted on NCs of various sizes. We have found that the efficiency of the direct bandgap recombination can increase in NCs by a factor 10^3 in comparison to bulk Si.
The proposed identification yields far-reaching consequences both on the fundamental side – understanding of QC effects in SiNCs, and application-wise, where use could be made of the enhanced absorption/emission cross-section tunable in the visible.
1.A. A. Prokofiev et al., Pis’ma v ZhtETF 90, iss.12 856-860 (2009) | 2 1 |
| Short wavelength emission and kinetics in Si nanocrystals : Jan Valenta | ||
| 11:15 | Blue emission from silicon nanocrystals studied by ultrafast photoluminescence spectroscopy Authors : K. Žídek, F. Trojánek, and P. Malý Charles University in Prague, Faculty of Mathematics and Physics, Ke Karlovu 3, 121 13, Prague, Czech Republic I. Pelant Institute of Physics, Academy of Sciences of the Czech Republic, v.v.i., Cukrovarnická 10, 162 53, Prague 6, Czech Republic Resume : Silicon nanocrystals (Si NCs) have been widely studied due to their efficient light emission with foreseen application as a light source in optoelectronics. Yet, the identification of processes leading to efficient photoluminescence (PL) is ambiguous. For example, origin of the blue PL (F-band) from oxidized Si NCs is a subject of debate with many proposed mechanisms. The time-resolved PL measurement helps here to study recombination and relaxation pathways step-by-step.
By using the up-conversion technique with a femtosecond excitation, we were able to observe PL evolution in Si NCs with sub-picosecond time resolution, as we reported previously [1] for Si NCs with dominant red emission (S-band) in time-integrated PL. Here we focus on the changes in ultrafast PL accompanying the modification of Si NCs that leads to an enhancement of the F-band.
We have prepared a set of Si NCs samples with SiO2 passivation featuring either major S- or F- PL band by using electrochemical etching with various parameters. For the S-band dominating samples the ultrafast PL decay had a two-exponential character with spectrally dependent PL lifetimes. In case of the F-band dominating samples the PL decay changed to a monoexponential decay with weakly spectrally dependent lifetimes. We also investigated a linear polarization memory and excitation spectra of F-band PL.
We discuss possible recombination mechanisms on basis of our results.
[1] F. Trojánek et al., J. Appl. Phys. 99, 116108 (2006) | 2 2 |
| 11:30 | Spin-flip dynamics between the bright and dark exciton ground states in silicon nanocrystals. Authors : Brian Julsgaard, Ying-Wei Lu, Peter Balling, and Arne Nylandsted Larsen. Department of Physics and Astronomy, University of Aarhus, Ny Munkegade 120, DK-8000 Aarhus C, Denmark. Resume : We present experimental investigation of the spin-flip dynamics between bright and dark exciton ground states in silicon nanocrystals using time-resolved fluorescence spectroscopy. By laser excitation the bright and dark state levels are populated initially at random, but subsequently the fluorescence decay curve reveals a thermalization between these levels. The characteristic thermalization time can be extracted and is found to be approximately 100 ns for temperatures below 100 K and surprisingly increases for higher temperatures. The data analysis employs a simple two-state model for the bright and dark exciton ground states.
Our work represents fundamental studies on silicon nanostructures and is thus within the scope of the silicon-based nanophotonics sessions at the E-MRS meeting. The results are important for the detailed
understanding of nanocrystal fluorescence and of the interaction mechanisms between excitons and the nanocrystal environment. | 2 3 |
| 11:45 | Time-resolved PL measurements of energy transfer in an ensemble of silicon nanocrystals Authors : H. Jayatilleka, M.Wojdak, A.J. Kenyon Department of Electronic and Electrical Engineering University College London Torrington Place London WC1E 7JE UK C. Mokry, P.J. Simpson Department of Physics and Astronomy The University of Western Ontario 1151 Richmond Street London Ontario Canada N6A 3K7 A.P. Knights Department of Engineering Physics McMaster University 280 Main Street West Hamilton Ontario Canada L8S 4L7 I. Crowe, M.P. Halsall School of Electrical and Electronic Engineering University of Manchester PO Box 88, Sackville Street Building Sackville Street Manchester M60 1QD UK Resume : Time-resolved photoluminescence measurements of silicon nanocrystals formed by ion implantation of silicon into silicon dioxide reveal multi-exponential luminescence decays. Three time components are apparent in the rise and decay data, which we associate with different populations of nanocrystals. We assign emission at high energies to small nanocrystals, and that at low energies to large nanocrystals. By deconvolving the decay data over the full emission band it is possible to study the migration of excitation from smaller to larger clusters. Similar effects can be seen in luminescence rise data. We propose a model of diffusion of excitation between neighbouring nanoclusters, with emission being from the largest cluster in the local neighbourhood. | 2 4 |
| 12:00 | Temperature emission quenching of Si clusters and its influence on the excitation mechanism of co-doped Nd ions Authors : A. Podhorodecki , G. Zatryb, J. Misiewicz Institute of Physics, Wroclaw University of Technology, Wybrzeze Wyspianskiego 27, 50-370 Wroclaw, Poland F. Gourbilleau, J. Cardin, O.Debieu and C. Dufour CIMAP, UMR CNRS 6252, Ensicaen 6 Bd Maréchal Juin, 14050 Caen Cedex 4, France Resume : In this work, optically excited carriers relaxation processes within the silicon quantum dots (Si-QD’s) were investigated when the dot structure was changing from amorphous (or even defect like) to more crystalline phase. Silicon reach oxide thin films annealed at different temperatures were investigated by using HRTEM, FTIR, Raman and photoluminescence spectroscopy measured as a function of the external temperature at different excitation wavelengths (800, 488, 360, 266 nm) and additionally at different excitation power densities (1018 up to 1022 ph./s∙cm2). Based on obtained results, it has been found that obtained functional dependences of PL intensities vs. temperature characterized by strong dependence on the excitation power density. It has been concluded that strong out-tunneling of the excited carriers from the dot core is present at low temperatures. Moreover, it has been found that collected in this way carriers could be recover when the temperature increases, increasing the emission intensities at higher temperatures. Additionally, the influence of variety of discussed above processes on the energy transfer from Si-QD’s to Nd ions and related quenching mechanisms will be given. | 2 5 |
| Theory of electronic and optical properties of Si nanocrystals : Christophe Delerue | ||
| 14:00 | First principles electronic and optical properties of free standing and embedded Si nanocrystals Authors : Elena Degoli CNR-INFM-S3 and Dipartimento di Scienze e Metodi dell’Ingegneria, Università di Modena e Reggio Emilia, via Amendola 2 Padiglione Morselli, I-42122 Reggio Emilia, Italy Resume : In this talk I present the work done in last years by our research group concerning the optoelectronic properties of silicon nanocrystals (Si-NC).
During the past decade, several breakthroughs, like the observation of optical gain in a large variety of experimental conditions, have increased the hopes of using nanocrystalline silicon as an optical active material.
Si-NC present unique electronic and optical properties that are intrinsically but not exclusively associated with quantum confinement effect (QCE).
Actually, due to different synthesis techniques, Si-NC presents different properties in size, shape and surface/interface structure. Moreover doping, that is important in order to quench non radiative processes and play a central role in semiconductor technology, is expected to change significantly the electronic band structure of quantum confined Si.
We have performed ab-initio calculations of free standing and silica embedded Si-NC in the framework of Density Functional Theory both in the single-particle approach and beyond it using the GW approximation and the Bethe-Salpeter equation.
We show how the electronic and optical properties of Si-NC depend not only on the size (i.e.QCE), but also on the surface termination, doping and structural features of both the matrix and the NC.
The theoretical characterization of these quantities is, actually, of fundamental importance in order to determine the best condition for light emission and optical gain. | 3 1 |
| 14:30 | Electronic properties of Si nanocrystals embedded in an amorphous SiO2 matrix: A first-principles study Authors : Kaori Seino (1), Friedhelm Bechstedt (1), Peter Kroll (2), (1) Institut für Festkörpertheorie und -optik, Friedrich-Schiller-Universität Jena, Jena, Germany, (2) Department of Chemistry and Biochemistry, University of Texas at Arlington, Arlington, TX, USA Resume : In recent years, there has been considerable interest in nanostructured silicon since it is a promising material for quantum devices of next generations. Although a high control of size and arrangement of Si nanocrystals (NCs) embedded in a SiO2 matrix can be archived experimentally, theoretical investigations for Si NCs embedded in a SiO2 matrix are limited. Here we study electronic properties for Si NCs embedded in a SiO2 matrix by means of first-principles calculations. In our calculations we use realistic structural models: Si nanocrystals with varying diameter in the range from 0.8 to 1.6 nm, which are comparable to those in experimental studies, an embedding matrix of SiO2 in amorphous phase, and intact interface bonds. The density functional calculations demonstrate the influence of the interface between NC and glass phase on the electronic properties. Moreover, we present the evolution of the valence and conduction band edges between Si NCs and amorphous silica along the radial direction. They are discussed in terms of confinement potentials for electrons and holes. | 3 3 |
| 14:45 | Si/Si02 quantum dots and statistical properties of electron levels Authors : I. Filikhin, S. G. Matinyan and B. Vlahovic North Carolina Central University, 1801 Fayetteville St. Durham, NC 27707, Tel: (919) 530-7253, Fax: (919) 530-7472, E-mail: ifilikhin@nccu.edu Resume : We model the Si quantum dots (QDs) embedded into SiO2 substrate. Single sub-band effective mass approach is used to calculate energy levels of electrons and heavy holes. For weak confinement regime (QD size >15 nm), when number of confinement levels is limited by several hundred, we considered statistical properties of the electron confinement. In particular to determine the type of the nearest neighbor spacing (NNS) statistics, the distribution function is calculated. The influence of the QD shape on the NNS distribution is investigated and the conditions for changing type of statistics are determined. The Brody formula [1] is applied and variations of the Brody parameter for different QD shapes are evaluated. Evidence for chaotic properties is discussed including the effect of magnetic field. To test the model, which we are using, we compared our results with available experimental PL exciton data (see for instance [2]). Calculations of low-lying single electron and hole energy levels are performed for spherically shaped QD with diameter D<6nm. For such size QD the number of energy levels is restricted to several levels. The energy dependence of electron effective mass is applied [3] to take into account non-parabolic effect of conduction band, which became important for small size QDs. First order perturbation theory is used to calculate neutral exciton recombination energy taking into account the Coulomb force between electron and heavy hole. The experimental data are reproduced well by our model calculations.
This work is supported by NSF CREST award, HRD-0833184 and NASA award NNX09AV07A.
[1] T. A. Brody, Lett. Nuovo Cimento, 7, 482 (1973).
[2] S. Moskalenko et al. Phys. Rev. B 76, 085427 (2007).
[3] I. Filikhin, V. M. Suslov and B. Vlahovic, Phys. Rev. B 73, 205332 (2006). | 3 4 |
| 15:00 | Effect of hydrocarbon surface saturation on stability and oxidation of silicon nanoparticles Authors : A.P. Mukhtarov, A.B.Normurodov, S.B. Hudayberganov Institute of Nuclear Physics AN RUz, 100214 Tashkent, Uzbekistan and M.T. Swihart University at Buffalo, State University of New York, Buffalo, New York 14260-4200, USA Resume : Recently luminescent silicon nanoparticles have attracted great interest, based on their potential use as light emitters in displays and as fluorescent probes for bioimaging. Photoluminescence (PL) at wavelengths ranging from blue to the near infrared is exhibited by Si particles ranging from 1 to 5 nm in diameter. The origin and characteristics of the luminescence are associated not only with the nanoparticle size but also with the nature of its surface. The structure of these clusters can by stabilized only by termination of the dangling bonds of the surface atoms. H-terminated clusters are rather unstable in air and will be oxidized. The presence of O at the particle surface can introduce states within the band gap that lead to red-shifted emission. An effective means of stabilizing the silicon surface and PL properties is to graft an organic monolayer onto the H-terminated surface. These particles are quite resistant to oxidation.
Here the non-conventional tight-binding method has been used for studying clusters ranging from 20 to 30 silicon atoms. Geometries of fully and partially passivated nanoparticles have been fully optimized by a computational approach combining the above method with molecular dynamics.
On the basis of results for oxygen atom bonded to one or two silicon atoms of the surface, the role of the UV irradiation in oxidation of the silicon nanoparticles, as observed experimentally, was explained. | 3 5 |
| 15:15 | Conditions of High Luminescence in Si/SiO2 Nanocrystals Authors : Roberto Guerra CNR-INFM-S3 and Dipartimento di Fisica - Universita\' di Modena e Reggio Emilia – via Campi 213/A I-41100 Modena Italy. Email: robguerra@unimore.it Stefano Ossicini Address: CNR-INFM-S3 and Dipartimento di Scienze e Metodi dell\'Ingegneria - Universita\' di Modena e Reggio Emilia - via Amendola 2 Pad. Morselli, I-42100 Reggio Emilia Italy. Email: ossicini@unimore.it Resume : In recent years many experiments demonstrated the possibility to achieve efficient photoluminescence (PL) from Si/SiO2 nanocrystals (NCs). While it is widely known that only a small portion of the NCs in the samples contributes to the observed PL, the high complexity of the Si/SiO2 interface and the dramatic sensitivity to the fabrication conditions make the identification of the most active structures at the experimental level not a trivial task [1,2]. Focusing on this aspect we addressed the problem theoretically, by calculating the radiative recombination rates (RRs) for different classes of Si-NCs in the diameter range of 0-1.5 nm, in order to identify the best conditions for optical emission.
We show that the RRs of hydrogenated NCs follow the quantum confinement (QC) picture in which the NC diameter is the principal quantity in determining the system response. A completely different behaviour emerges from the OH-terminated or SiO2-embedded NCs, where the number of oxygens at the
interface results intimately connected to the RRs, resulting the most important quantity for the characterization of the optical yield in such systems. Besides, secondary conditions for the achievement of high RRs are constituted by a high crystallinity of the NCs and high confinement energies (small diameters), respectively.
[1] R. Guerra et al., Phys. Rev. B 79, 155320 (2009).
[2] R. Guerra et al., Phys. Rev. B 80, 155332 (2009). | 3 6 |
| Non-linear optical properties of nanocrystals : Ivan Pelant | ||
| 16:00 | Nonlinear optical properties of impurity-doped silicon nanocrystals Authors : Minoru Fujii, Masahiko Ito, Kenji Imakta, and Shinji Hayashi, Department of Electrical & Electronic Engineering, Graduate School of Engineering, Kobe University Resume : It has been demonstrated experimentally and theoretically that optical properties of silicon nanocrystals are significantly modified by doping very small number of impurities. Therefore, in addition to the size and the shape, impurity doping is a parameter to control the optical properties and by this additional freedom for the material design, the application field of silicon-nanocrystal-based materials may be significantly extended. In this work, we focus on nonlinear optical properties of silicon nanocrystals and study how they are modified by impurity doping. We prepare phosphorus- or boron-doped Si nanocrystals embedded in thin glass films by a co-sputtering method and study the nonlinear refractive index and the nonlinear absorption coefficient by z-scan technique in a femtosecond regime at around 1.6 eV. We show that phosphorus or boron doping enhances the nonlinear optical responses significantly. In order to clarify the mechanism of the enhancement, we characterize the samples by optical absorption, photoluminescence, Raman scattering, and electron spin resonance spectroscopy. Our results suggest that impurity-related states in the bandgap are responsible for the large nonlinear optical response of impurity-doped silicon nanocrystals. | 4 1 |
| 16:30 | Comparative study of the nonlinear optical properties of Si nanocrystals fabricated by e-beam evaporation, PECVD or LPCVD Authors : A. Martinez, S. Hernandez, P. Pellegrino, O. Jambois and B. Garrido MIND-IN2UB, Departament d’Electronica, Universitat de Barcelona, Marti i Franques 1, 08028 Barcelona, Spain. P. Miska, M. Grün, H. Rinnert and M. Vergnat Institut Jean Lamour, UMR CNRS 7198 – Nancy Université – UPV Metz, France. V. Izquierdo-Roca, M-2E/XaRMAE/IN2UB, Departament d’Electronica, Universitat de Barcelona, Marti i Franques 1, 08028 Barcelona, Spain. J. M. Fedeli CEA, LETI, Minatec, 17 rue des Martyrs, 38054 Grenoble Cedex 9, France. Resume : Silicon nanocrystals (Si-nc) embedded in oxide matrices have been proposed as active material for nonlinear photonic applications, as their nonlinear optical properties were found to be larger than the ones in silica or Si. For their fabrication, several approaches have been widely employed, such as ion implantation, evaporation, plasma enhanced or low pressure chemical vapor deposition (PECVD and LPCVD, respectively). However, the different technological steps strongly affect the precipitation of Si-aggregates and their surrounding medium, modifying their structural, and both linear and nonlinear optical properties. Here we present a comparison of the nonlinear optical properties of Si-nc under ns excitation pulses for materials produced by three different techniques.
Si-rich oxides were deposited onto silica substrates either by LPCVD, PECVD or e-beam thermal evaporation, and subsequently annealed at high temperatures in order to precipitate the nanostructures. Energy filtered transmission microscopy was used to determine the size and distribution of Si-nc, finding a mean size of ~5 nm in all the samples. The crystalline and amorphous fractions were estimated from the integrated intensity of Raman spectra performed in cross-section configuration. The nonlinear response was determined at 1064 nm by z-scan experiments and using ns-pulses of a Nd:YAG laser. While a similar nonlinear refractive index was found in all the samples, differences in the nonlinear absorption were found as a function of the deposition method. Furthermore, the nonlinear response was analyzed by varying the duration of the laser pulses, finding different trends for each sample. The nonlinear response will be discussed and presented in correlation with the nanostructure of the Si precipitates and their linear optical response. | 4 2 |
| 16:45 | Field-assisted dissociation of excitons in Si nanocrystals Authors : Thomas Garm Pedersen Department of Physics and Nanotechnology Aalborg University Skjernvej 4A, DK-9220 Aalborg East, Denmark Resume : Excitons in nanocrystals are tightly bound due to the enhanced electron-hole overlap enforced by the confinement. While this is beneficial for radiative recombination within the nanocrystal it is detrimental for solar cells requiring dissociated excitons. An electric field produced by external electrodes or within a pn-junction geometry may facilitate exciton dissociation but the required field strength will depend sensitively on nanocrystal size, shape and composition. We study the effect of external fields on the binding of nanocrystal excitons. Both core- and surface-confined states are considered using a recently developed formalism [1]. We observe pronounced differences between core- and surface-confined excitons cases reflecting the different degrees of confinement. Implications for solar cells base Si nanocrystals are discussed.
[1] T. Garm Pedersen, \"Excitons on the surface of a sphere\", accepted. | 4 3 |
| 17:00 | Stark effect, polarizability and electroabsorption in silicon and germanium nanocrystals Authors : Umit Keles (1), Ceyhun Bulutay (1), Mustafa Kulakci (2), Rasit Turan (2) (1) Department of Physics, Bilkent University, 06800, Ankara, Turkey (2) Department of Physics, Middle East Technical University, 06530, Ankara, Turkey Resume : Achieving quantum-confined Stark effect (QCSE) in silicon nanocrystals (NCs) embedded in oxide has been rather elusive, unlike the other material systems. Here, we report the direct observation of QCSE under an external field in ion-implanted Si NCs with a mean diameter of 5.6 nm that yields as large as a 40 nm redshift at cryogenic temperatures, which remains to be easily detectable at the room temperature. Using an atomistic pseudopotential theory, we unambiguously reproduce this experimental data which reveals that the majority of the Stark shift comes from the valence states that undergo a level crossing leading to a nonmonotonic radiative recombination behavior with respect to the applied field. Our experimental analysis is repeated on different size NCs which are also supported by the same theoretical approach. The polarizability of embedded Si NCs is obtained over a diameter range of 2.5-6.5 nm including the excitonic effects, which can be fitted by a cubic dependence with respect to the NC diameter. Based on intraband electroabsorption analysis, it is predicted that p-doped Si NCs will show substantial voltage tunability, whereas n-doped samples should be almost insensitive. Hence, this offers a viable alternative for electromodulation with p-doped Si NCs which was a major challenge with bulk silicon that lacks the linear electro-optic effect as being a centrosymmetric crystal. Finally, we provide a theoretical comparison of silicon versus germanium NCs. | 4 4 |
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