bluuebunny/crossref_metadata_2025_split huggingface dataset:
Efficient Near Infrared Si/Ge Quantum Dot Photo-Detector Based on a Heterojunction Bipolar Transistor
Authors: Elfving Anders , Larsson Mats , Holtz Per-Olof , Hansson Göran V. , Ni Wei-Xin 
Published: June 2019

AbstractGe dots embedded in Si offer the possibility of Si-based light detection at 1.3-1.55 μm. In this communication, we report a very efficient photo-detector based on a Si/SiGe heterojunction bipolar transistor structure with 10 Ge dot layers (8 ML Ge each) incorporated in the basecollector junction. The device structures were grown using low-temperature molecular beam epitaxy, and fabricated for both normal and edge incidence with no electrical contact to the base. The processed Ge-dot transistor detectors revealed a rather low dark current density, 0.01 mA/cm2 at -2 V. Photoconductivity measurements were performed at room temperature. At 1.31 μm, responsivity values of 50 mA/W at normal incidence have been directly measured at Vce = -4 V, without involving any rescaling factor due to light coupling. This value is a ∼250-fold increase compared to a reference p-i-n diode with the same dot layer structure, due to the current amplification function of the transistor. For a rib waveguide device, a very high responsivity value of about 470 mA/W (Vce = -4V) has been obtained at 1.31 μm. Measurements were also performed at 1.55 μm, and the photo-response of the waveguide phototransistor was 25 mA/W, which is again a large improvement compared with the reference waveguide photodiode (∼1 mA/W). Moreover, time-resolved photoconductivity measurements have been carried out. The results have indicated that the device frequency performance is primarily limited by the emitterbase junction capacitance.
Pump-Probe Measurements Using Silicon Nanocrystal Waveguides
Authors: Smith Nathanael , Lederer Max J. , Samoc Marek , Luther-Davies Barry , Elliman Robert G. 
Published: June 2019

AbstractOptical pump-probe measurements were performed on planar slab waveguides containing silicon nanocrystals in an attempt to measure optical gain from photo-excited silicon nanocrystals. Two experiments were performed, one with a continuous-wave probe beam and a pulsed pump beam, giving a time resolution of approximately 25 ns, and the other with a pulsed pump and probe beam, giving a time resolution of approximately 10 ps. In both cases the intensity of the probe beam was found to be attenuated by the pump beam, with the attenuation increasing monotonically with increasing pump power. Time-resolved measurements using the first experimental arrangement showed that the probe signal recovered its initial intensity on a time scale of 45-70 μs, a value comparable to the exciton lifetime in Si nanocrystals. These data are shown to be consistent with an induced absorption process such as confined carrier absorption. No evidence for optical gain was observed.
Photoluminescence from Silicon Nanocrystals Formed by Pulsed-Laser Deposition
Authors: Chen X. Y. , Lu Y. F. , Wu Y. H. , Cho B. J. , Song W. D. , Hu H. 
Published: June 2019

AbstractSi nanocrystals (NCs) consisting of small crystals from 1 to 20 nm were formed by pulsedlaser deposition (PLD) in inert Ar gas and reactive O2 gas. The oxygen content of the Si NCs increases with increasing O2 ambient pressure and nearly SiO2 stoichiometry is obtained when O2 pressure is higher than 100 mTorr. The optical absorption of the Si NCs shows an indirect band transition. Broad PL spectra are observed from Si NCs. The peak position and intensity of the PL band at 1.8–2.1 eV are dependent on excitation laser intensity, while intensity changes and blue shifts are observed after oxidation and annealing. The PL band at 2.55 eV displays vibronic structures with periodic spacing of 97 ± 9 meV, while no peak shift is found before and after oxidation and annealing. The as-deposited Si NCs show a polycrystal structure and crystallinity improves after annealing. Combined with the PL of Si NCs obtained by crumbling electrochemical-etched porous Si layer, the results give strong evidence that the PL band at 1.8–2.1 eV is due to the quantum confinement effect (QCE) in Si NC core while the PL band at 2.55 eV is related to the localized surface states at SiOx/Si interface.
Two - Color Mid-Infrared Spectroscopy of Isoelectronic Centers in Silicon
Authors: Vinh N.Q. , Gregorkiewicz T. 
Published: June 2019

AbstractOne of the open questions in semiconductor physics is the origin of the small splittings of the excited states of bound excitons in silicon. A free electron laser as a tunable source of the mid-infrared radiation (MIR) can be used to investigate such splittings of the excited states of optical centers created by transition metal dopants in silicon. In the current study, the photoluminescence from silver and copper doped silicon is investigated by two color spectroscopy in the visible and the MIR. It is shown the PL due recombination of exciton bound to Ag and Cu is quenched upon application of the MIR beam. The time-resolved photoluminescence measurements and the quenching effects of these bands are presented. By scanning the wavelength of the free-electron laser ionization spectra of relevant traps involved in photoluminescence are obtained. The formation and dissociation of the bound excitons, and the small splittings of the effective-mass excited states are discussed. The applied experimental method allows correlation of DLTS data on trapping centers to specific channels of radiative recombination. It can be applied for spectroscopic analysis in materials science of semicondutors.
Preparation and Microstructure of Au (Ru)-SiO2 Micro/Nano-particles
Authors: Ma Xicheng , Cai Yuanhua , Ao Qing , Lun Ning , Li Shitong , Li Fengzhao , Wen Shulin , Dou Fan , Zhu Hongwei 
Published: June 2019

AbstractIn this presentation, we described a study on preparation and microstructure of Au (Ru) nano-particles embedded in SiO2 matrices, which were prepared as advanced display materials by using Sol-gel method. Both microstructure and composition of these micro/nano-particles composites were characterized by HRTEM (high resolution transmission electron microscope), EMP (electron microprobe) and nano-probe EDS (energy dispersive X-ray spectroscopy). Experimental results showed that the products have multi-levels in structure like nebulae. The aggregation of Au/Ru nano-particles can be extremely diminished by using SiO2 micro-particles as matrices. Distortions of the atomic lattices of Au and Ru nano-crystallites were often observed in the surface of these nano-particles. Moreover, nano-EDS measurements revealed that Au and Ru nano-particles are always separately embedded in SiO2 matrices, no alloy of them is observed.
Epitaxial Si1-xGex Films and Superlattice Structures Grown by CVD for Infrared Photodetectors
Authors: Maddiona L. , Coffa S. , Lorenti S. , Bongiorno C. 
Published: June 2019

AbstractIntegration of photodetectors with high responsitivity in the near infrared (1.3-1.55 μm) on standard Si electronic circuits is important for a variety of applications in the field of on-chip, local area and long haul optical communications. In this work we report on a detailed structural and optical characterization of epitaxial Si1-xGex films and Si1-xGex /Si multilayers grown by chemical vapor deposition on (100) Si wafers. Cross-sectional transmission electron microscopy analyses show that metastable strained Si1-xGex films of few nanometer with x>40% can be deposited at low growth temperature and pressure. Absorption measurements on these films demonstrate the extension of the photo-response to 1.55 μm. Using these films as active layers Schottky integrated photodetectors have been fabricated.
Tunneling Conductivity in Thermally Oxidized Porous Silicon
Authors: Yarkin Dmitrii G. , Balagurov Leonid A. , Orlov Andrei F. , Zvyagin Igor P. 
Published: June 2019

AbstractMetal/PS/c-Si structures with porous silicon (PS) layers of 55-75% porosity were fabricated on moderately doped p- and n-type c-Si substrates and were thermally oxidized at 400-960°C. The effect of oxidation on the photoluminescence and the transport of charge carriers in these structures were studied. We demonstrated that trap-filled space charge limited current (SCLC) is the dominant transport mechanism at large forward bias. The analysis of the current – voltage characteristics in the SCLC region allowed us to determine the oxidation dependence of the effective thickness of the trap-rich tissue isthmuses, in which space charge is mostly accumulated. The exponential dependence of the ohmic conductance on the thickness of SiOX tissue is explained by tunneling of carriers through potential barriers formed by the tissue surrounding silicon crystallites.
The Nd-nanocluster Coupling Strength and its Effect in Excitation/de-excitation of Nd3+Luminescence in Nd-doped Silicon-rich Silicon Oxide
Authors: Seo Se-Young , Shin Jung H. 
Published: June 2019

AbstractThe Nd-nanocluster Si (nc-Si) coupling strength and its effect in excitation/de-excitation of Nd3+luminescence in Nd-doped silicon-rich silicon oxide (SRSO) is investigated. Nd-doped SRSO thin films, which consist of nc-Si embedded inside a SiO2matrix, were prepared by electron-cyclotron-resonance plasma enhanced chemical vapor deposition (ECR-PECVD) of SiH4and O2with co-sputtering of Nd and subsequent anneal at 950 °C. Efficient Nd3+luminescence with moderate temperature quenching is observed. Based on the temperature dependence of Nd3+luminescence lifetime, a coupling strength between nc-Si and Nd that is strong enough to result in efficient excitation of Nd3+via quantum confined excitons while weak enough to result in a small back-transfer rate is identified as the key to Nd3+luminescence.
Controlled Passivation and Luminescence Blue Shifts of Isolated Silicon Nanocrystals
Authors: Biteen Julie S. , Tchebotareva Anna L. , Polman Albert , Lewis Nathan S. , Atwater Harry A. 
Published: June 2019

AbstractWe have performed a comparative study of oxide- and nonoxide-passivated silicon nanocrystals to probe the role of the silicon/oxygen interface in low coverage, non-interacting silicon nanocrystal systems. Ensembles of Si nanocrystals characterized by a narrow distribution and diameters of 2–5 nm were synthesized by ion implantation into SiO2 films followed by a high-temperature anneal in Ar. The nanocrystals were removed from the SiO2 film matrix and deposited on Si substrates using a chemical etch in HF, leaving a hydrogen-terminated surface. A natural oxide layer grows on these surfaces in air. We characterized the morphology of the samples with atomic force microscopy (AFM) and the spectroscopic properties with photoluminescence (PL) and X-Ray photoelectron spectroscopy. We found that the PL energy of Si nanocrystals can be shifted by particle size reduction and hydrogen or oxygen termination. Further, PL peak energy shifts upon etching and oxidation were consistent with the model of Wolkin et al. that proposes that for very small radii, a silicon-oxygen double bond will produce deep interface states which red shift the luminescence.
Electrochemical characterization of blue-emitting polyfluorene LEP
Authors: Grizzi Ilaria , Foden Clare , Goddard Simon , Towns Carl 
Published: June 2019

AbstractIn an operating LED, electrons are injected into the polymer's LUMO level from the cathode and holes from the anode into the polymer's HOMO level. The value of the electron affinity (EA) is usually inferred (often incorrectly) from the experimental HOMO level energy and the value of the optical band gap. Using carefully controlled experimental conditions we can now directly access the EA position. Reduction events have been observed on poly(9,9-dioctylfluorene) (F8) and blue emitting polymers based on 9,9-dioctylfluorene. This event is consistent and reversible. As predicted by theory, the LUMO level is entirely delocalised over F8 blocks. An EA of approx. –2.3eV, equal to that of F8, is therefore a common feature of blue polymers containing sequences of 9,9-dioctylfluorene. Blue emitting 9,9-dioctylfluorene -triarylamine AB copolymers containing either 4-sec-butylphenyl diphenyl amine (TFB) or N,N'-bis(4-butylphenyl)-N,N'-diphenyl phenylenediamine (PFB) are characterized by an electron affinity of –2.1eV. This result shows that the LUMO wave function in these systems is localized on the Ph-F8-Ph units. In the case of PFB homopolymer the LUMO level is again localized, this time on the biphenyl unit at the junction of each 2 repeating units. The EA for this material is therefore lower than in the previous cases: -1.84eV, as a result of the increased localization of the LUMO wave function Electrochemical characterisation provides a direct probe for the dynamic of the charge injection in a polymer system, as the HOMO and LUMO are the states into which the holes and electrons are injected. Electrochemical measurement together with theoretical modelling gives a more complete understanding of molecular properties and behaviour. However, electrochemical measurements are not necessarily relevant to the optical energy gap, as the optical excitation does not necessarily involve molecular orbitals probed by electrochemical methods.
Study of the MDMO-PPV/metal interface and PCBM/metal interface by electroabsorption spectroscopy
Authors: Reynaert J. , Jin Y.D. , Aernouts T. , Geens W. , Borghs G. , Mertens R. , Heremans P. 
Published: June 2019

AbstractIn this work, we study the workfunction alignment of organic compounds and electrodes evaporated on top of these organic materials by means of electroabsorption (EA). The organic materials of this study are poly[2-methoxy-5-(3',7'-dimethyloctyloxyl)]-1,4-phenylene vinylene (MDMO-PPV) and methanofullerene [6,6]-phenyl C61-butyric acid methyl ester (PCBM). The electrodes are Al, Au, LiF/Au and LiF/Au, the latter two with a 1 nm thick layer of LiF, as well as with co-evaporation of LiF and the metal. In case of the MDMO-PPV, LiF enhances the built-in potential and thus reduces the electron injection barrier. Equal built-in potentials for a co-evaporated LiF:metal contact suggest doping is happening at the MDMO-PPV/metal interface. Unlike predicted by the Au and Al workfunction value, the built-in potentials for the PCBM/Al and PCBM/Au interface are equal. This suggests that Fermi level pinning occurs at these interfaces. The presence of a thin LiF layer screens the charge transfer from the metal to the PCBM.
Focused Microwave-assisted Synthesis of 2,5-Dihydrofuran Derivatives as Electron Acceptors for Highly Efficient Nonlinear Optical Chromophores
Authors: Liu Sen , Haller Marnie A. , Luo Jingdong , Jang Sei-Hum , Ma Hong , Dalton Larry R. , Jen Alex K.-Y. 
Published: June 2019

AbstractA diversified family of 2,5-dihydrofuran derivatives has been synthesized as a new class of highly efficient and tunable electron acceptors using the single-mode focused microwave irradiation. High poling efficiency and very large electro-optic coefficients (r33 values of 128 and 116 pm/V at 1.3 μm) have been demonstrated using 2-dicyanomethylen-3-cyano-4,5,-dimethyl-5-trifluoromethyl-2,5-dihydrofuran (CF3-TCF)-based chromophores as dopant in poly(methyl methacrylate) (PMMA) and a high glass-transition temperature polyquinoline (PQ-100) respectively. Excellent dipole alignment stability has also been demonstrated in the guest/host composite at 85°C. Multi-functionalized NLO chromophores based on hydroxy containing 2,5-dihydrofuran acceptors were also synthesized through microwave methodology for further characterizations.
Energy Level Alignment at Fullerene/Phthalocyanine Interface Studied by Electron Spectroscopies
Authors: Ishii Hisao , Seko Atsushi , Kawakami Akira , Umishita Kazunori , Ouchi Yukio , Seki Kazuhiko 
Published: June 2019

AbstractInterfacial band offset and band bending of organic semiconductors are critical to understand and improve organic photovoltaic cells. In this study, the energy level alignment of fullerene(C60) / metal-free phthalocyanine (H2Pc) interface which is one of the model interfaces of organic photovoltaic cells has been investigated using UV and X-ray photoemissions. For both ‘H2Pc on C60’ and “C60 on H2Pc' interfaces, 0.3 eV downward energy level shift was observed in XPS at the interface formation. This energy shift is quite steep in contrast to the band bending observed for C60/metal interfaces in our previous study, where thickness of 500nm was required to achieve 0.21eV band bending to get Fermi level alignment between metal electrode and C60. To clarify the origin of the band bending, the effect of the insertion of C60-H2Pc co-deposited layer between C60 and H2Pc layers was also investigated. The result suggested that possible doping of H2Pc to C60 is not main origin of the observed energy shift. We also found that the vacuum level shift at H2Pc/C60 interface is strongly dependent on the deposition sequence of the interface formation.
Metal vs. Polymer Electrodes in Organic Devices: Energy Level Alignment, Hole Injection, and Structure
Authors: Koch N. , Nickel B. , Ghijsen J. , Elschner A. , Schwartz J. , Pireaux J.-J. , Kahn A. 
Published: June 2019

AbstractL We have investigated the electronic, structural, and charge injection properties of interfaces formed between three electroactive conjugated organic materials, i.e., N, N'-bis-(1-naphthyl)-N, N'-diphenyl1-1,1-biphenyl1-4,4'-diamine (á-NPD), pentacene, p-sexiphenyl, and two high work function electrode materials, i.e., gold and the conducting polymer poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS). Ultraviolet photoelectron spectroscopy shows that the hole injection barrier between the three organic materials and PEDOT:PSS is lower by 0.6-1.0 eV as compared to Au, despite a similar work function of the pristine electrode material surfaces (ca. 5 eV). This very large difference is due to an effective change of the metal work function due to the deposition of organic molecules, i.e., a decrease of the Au surface dipole due to adsorption. Accordingly, model device structures built from á-NPD and pentacene on the two different electrode materials show much higher current densities for hole injection from PEDOT:PSS than from Au. Hole injection from Au for á-NPD devices is independent of deposition sequence and substrate. Pentacene devices exhibit significant asymmetries in that respect, due to a strong dependence of the morphology and preferred molecular orientation of the crystalline material on the substrate, as shown by atomic force microscopy and X-ray diffraction. Consequently, great care must be taken when modeling current-voltage characteristics of devices comprised of crystalline organic solids, especially when the influence of film thickness or different substrate materials is to be studied.
Trivalent Europium Complexes Using Dendritic â-Diketone Ligands
Authors: Case Patrick J. , Harper Aaron W. 
Published: June 2019

AbstractA series of photon harvesting lanthanide complexes have been synthesized by chelating dendritic â-diketone ligands to Eu+3. The ligands are modified benzoyltrifluoroacetone units (BTFA) with poly(aryl ether) dendron attachments. These ligands impose a very large asymmetric nature to the first coordination sphere of europium compared to previous dendritic compounds. These complexes exhibit europium emission with no residual emission from the ligands as the dendritic subunits increase from G-0 to G-3. This is due to both antenna and site isolation effects. These complexes mimic light harvesting organisms seen in Nature. When these intensities are compared throughout the series, an increase in intensity is seen where [(G2)BTFA]3Eu is the brightest, with [(G3)BTFA]3Eu being less intense than the G-2 complex. However, the lifetime data indicates that the G-3 complex is the longest lived species.
EL Properties of Polyfluorene Copolymers Containing Hole-Transporting Carbazole Group
Authors: Hwang Do-Hoon , Lee Jong-Don , Park Moo-Jin , Lee Ji-Hoon , Lee Chang-Hee 
Published: June 2019

AbstractAn alkoxyphenyl substitutued new polyfluorene derivative, poly(9,9-bis(4'-n-octyloxyphenyl) fluorene) (PBOPF) was synthesized through Ni(0) mediated polymerization. Blue light-emitting PBOPF shows peak PL at 426 nm. No significant excimer emission was observed even after annealing the polymer film at 100 oC for 2h. Aseries of random copolymers of 2,7-dibromo-9,9-bis(4'-n-octyloxyphenyl)) fluorene (BOPF) and 2,7-dibromo-N-(2'-ethylhexyl)carbazole (EHC) were also synthesized through Ni(0) mediated polymerization. Carbazole comonomer was introduced to improve hole-transporting properties of PBOPF. The synthesized poly(BOPF-co-EHC)s showed similar UV-visible absorption and PL emission to PBOPF. EL devices were fabricated in an ITO/PEDOT/polymer/Ca/Al configuration. The EL devices using copolymers showed improved device performance than the device using PBOPF homopolymer due to a more balanced charge transport.
Scanning-tunneling spectroscopy on conjugated polymer films
Authors: Kemerink M. , Alvarado S.F. , Koenraad P.M. , Janssen R.A.J. , Salemink H.W.M. , Wolter J.H. 
Published: June 2019

AbstractScanning-tunneling spectroscopy experiments have been performed on conjugated polymer films and have been compared to a three-dimensional numerical model for charge injection and transport. It is found that field enhancement near the tip apex leads to significant changes in the injected current, which can amount to more than an order of magnitude, and can even change the polarity of the dominant charge carrier. As a direct consequence, the single-particle band gap and band alignment of the organic material can be directly obtained from tip height-voltage (z-V) curves, provided that the tip has a sufficiently sharp apex.
Balanced Charge Injection and Singlet Exciton Quenching in Bilayer Organic Electroluminescent Devices
Authors: Ganzorig Chimed , Fujihira Masamichi 
Published: June 2019

AbstractWe demonstrate here balancing between hole and electron injection in bilayer organic tris(8-hydroxyquinoline)aluminum (Alq3) based electroluminescent (EL) devices by measuring current-voltage (I-V) characteristics of single-carrier devices with different thickness and by comparing the device performance with various chemically modified ITO. We measure I-V characteristics due to holes in hole-only bilayer devices with various modified ITO anodes and those due to electrons in electron-only single-layer devices with C6H5COOLi(2 nm)/Al cathode. Here, hole-only devices are those, in which only holes are carriers and the current due to electrons is negligible. We found that the I-V characteristics observed for the hole-only bilayer devices with UV-ozone and ClC6H4COCl treated ITO were comparable to the I-V characteristics for the electron-only single-layer device of ITO(as-cleaned)/Alq3(150 nm)/C6H5COOLi(2 nm)/Al. In other words, hole and electron injections are well balanced when we use UV-ozone and ClC6H4COCl treated ITO anodes. EL characteristics have shown that luminance decreased clearly for bilayer organic EL devices with an Al cathode where electron injection was not enhanced significantly. The decrease in EL efficiency can be attributed to unbalanced hole and electron injection. The unbalanced injection results in excess of one carrier type that does not contribute to light emission, and results in deactivation of 1Alq3 at high currents due to quenching 1Alq3 by large excess of Alq3+. in the emission zone.