bluuebunny/crossref_metadata_2025_split huggingface dataset:
Nanocrystals of II-VI Semiconductor Materials
Authors: Weller Horst , Vossmeyer Tobias , Eychmüller Alexander , Mews Alf , Katsikas Lynne , Reck Gunter 
Published: April 2011

ABSTRACTCdS nanoclusters ranging in diameter between 1 and 4 nm were prepared in aqueous solution using aliphatic mercaptoalcohols as ligands. The photon energies of the Is Is absorption and the respective oscillator strengths are in accordance with size quantization theory. Some clusters crystallize as macroscopic 3-D superlattices which were investigated by single crystal x-ray analysis. The neutral Cd17S4(RS)26 clusters are covalently linked in the superlattice the structure of which exhibits self similarity to the interior structure of the clusters.Onion-shell-like composite particles from CdS and HgS were prepared by successive substitution and re-precipitation processes. Particles with a core radius of 2 nm, a shell of up to 1 nm HgS followed by a final shell of up to 1.5 nm CdS were obtained. Electrons and holes were localized in the HgS shell giving rise to excitonic fluorescence.
Optical Properties of Lead Sulfide Nanoclusters: Effects of Size, Stoichiometry and Surface Alloying
Authors: Bliss D.E. , Wilcoxon J.P. , Newcomer P.P. , Samara G.A. 
Published: April 2011

ABSTRACTThe optical absorption and photoluminescence properties of nanosize PbS clusters synthesized in inverse micelles are presented. Relative to bulk crystals, the bandgap of PbS nanoclusters is significantly blue shifted from the near-IR into the visible and near-UV region with decreasing cluster size. The optical absorption spectra for clusters with a diameter of ∼2.5-3 nm contain a molecular-like feature at 284 nm (4.37eV) which matches the absorption of diatomic PbS. The main photoluminescence band occurs at 520 nm and is due to recombination at surface states. The PLE spectrum of the 520 nm band has a minimum which coincides with the absorption peak. The absorption line is interpreted to arise from a pre-dissociative excitation of PbS molecules on the surface. The PLE band is interpreted to be an excitation of molecular-like levels in the interior of the cluster. Evidence from size, stoichiometry and surface alloy (Au) dependent properties supports this interpretation.
Combined Optical, Surface and Nuclear Microscopic Assessment of Porous Silicon Formed in HF-Acetonitrile
Authors: Feng Z. C. , Chen Z. , Padmanabhan K. R. , Li K. , Wee A. T. S. , Lin J. , Tan K. L. , Yue K. T. , Bhat A. , Rohatgi A. 
Published: April 2011

ABSTRACTA new type of HF solution, HF-acetonitrile (MeCN), has been employed to produce 10-30 μm thick porous silicon (P-Si) layers by photoelectrochemical etching of different types of Si wafers, Si(100), Si(111) and polycrystalline Si, with different resistivities. A combined optical, surface and nuclear microscopic assessment of these P-Si layers was performed using photoluminescence (PL), Raman scattering, X-ray photoelectron spectroscopy (XPS) and Rutherford backscattering spectroscopy (RBS). The PL emission intensities, Raman line shapes and structural features are strongly dependent on the properties of the substrates such as the crystallinity and resistivity of the Si wafers used for forming P-Si. With increasing resisitivity of the Si(100) wafers, the resulting P-Si layers show a slight blue-shift of their visible light emission peak energy, an up-shift of the peak position and a narrowing of the band width of the dominant Raman band, and a decrease in the amount of residual elemental Si on the surface. Those Si(l 11) wafers, etched in HF-MeCN, showed no porous structures and no visible light emission.
The Effect of Starting Silicon Crystal Structure on Photoluminescence Intensity of Porous Silicon
Authors: Dubbelday W. B. , Russell S. D. , Kavanagh K. L. 
Published: April 2011

ABSTRACTIn previous work we reported that porous silicon (PS) films formed using a dilute HF:HNO3 chemical etch on polycrystalline, implant damaged single crystal, or amorphous starting material have luminescent characteristics that differ from PS fabricated on single crystal silicon1. Polycrystalline and implant damaged porous silicon exhibits brighter luminescence compared to single crystal silicon etched under identical conditions. No photoluminescence is detected from the porous amorphous silicon. In this work these effects are examined using HF:NaNO2 solutions with freely available NO2. The accelerated etching effects from work damage are reduced, and the PS from polycrystalline and implant damaged silicon luminesce with the same intensity as the PS from single crystal silicon. Again, etched amorphous silicon does not luminesce. TEM and EDX porosity measurements are used to determine the differences in structure and etching characteristics between the luminescent and non-luminescent materials.
Chemical Modification of the Porous Silicon Surface
Authors: Lee Eric J. , Ha James S. , Sailor Michael J. 
Published: April 2011

ABSTRACTThe porous silicon (PS) surface is derivatized with ethanol, triethylsilanol and formic acid as well as oxidized with water. The two reactions used to prepare these surfaces are discussed, and FTIR spectra of the products are presented. Surface-modified PS retains 10-40% of its original photoluminescence. PS-derivatives display reversible luminescence quenching by gas phase water, ethanol, acetonitrile and benzene. The extent of quenching varies with different PS-derivatives depending on the interaction of the chemical vapor with the modified PS surfaces.
FTIR Studies of CH3OH on Porous Silicon
Authors: Glass John A. , Wovchko Edward A. , Yates John T. 
Published: April 2011

ABSTRACTFourier transform infrared spectroscopy (FTIR) was used to investigate the reaction of methanol with porous silicon and hydrogen passivated porous silicon. At 300 K methanol adsorbs onto hydrogen free porous silicon by cleavage of the O-H bond. Both of the resulting surface species, Si-H and Si-OCH3, were determined to be stable up to ∼500 K. Above 500 K the Si-OCH3 moiety decomposes by breakage of the C-O and C-H bonds. The resulting carbon and oxygen were incorporated into the porous layer and additional Si-H surface species were detected. Further heating to 900 K removed the Si-H surface species. Adsorption of methanol onto hydrogen-passivated porous silicon did not occur until 600 K. At temperatures beyond 600 K, oxygen and carbon incorporation into the porous layer and Si-OCH3, Si-CH3, and Si-H surface species were seen. The previously unseen Si-CH3 surface species is believed to be stabilized by oxygen incorporation.
Contribution of the Nanocrystallites and Their Interfaces to the Optical Response of Porous Silicon Layers
Authors: Rossow U. , Frotscher U. , Richter W. , Muender H. , Thoennissen M. , Berger M. 
Published: April 2011

ABSTRACTWe discuss the dependence of the dielectric function on the nanocrystalline size of porous silicon layers. The layers were grown by a standard electrochemical process and characterized by spectroscopic ellipsometry. By a lineshape analysis values of the critical point energies and broadening parameters of the Interband critical points were derived. In order to obtain further information about the nanocrystallites, the preparation conditions were varied (HF concentration, NH4F was added) or the layers were further treated by etching and arsenic deposition. The lineshape analysis values indicate that the layers consist mainly of nanocrystallites, or more accurately, that the electrons are confined to regions of a few nanometers in size. Furthermore, there is strong indication that some preparation conditions may leave these nanocrystallites heavily strained .
Electron Spin Resonance Investigations on Porous Silicon
Authors: Meyer B.K. , Hofmann D.M. , Christmann P. , Stadler W. , Nikolov A. , Scharmann A. , Hofstaetter A. 
Published: April 2011

ABSTRACTThe defect properties of as-etched, annealed and thermally oxidized nano-porous Si as well as self supporting macro-porous silicon layers are studied by electron paramagnetic resonance (EPR), photoluminescence (PL), optically detected magnetic resonance (ODMR), and electron nuclear double resonance (ENDOR). The paramagnetic defects observed are dangling bonds closely related to the Pb-center, the Si/SiO2 interfacial defect. In EPR a minimum defect density of 1016 cm−3 is observed for as-etched material, it reaches a maximum of 8 x 1018 cm−3 for samples annealed around 400°C. We quantitatively correlate the defect density with hydrogen desorption data and luminescence efficiency. In the ODMR experiments the same dangling bond centers are observed on the 1.7 eV luminescence band, but with increased sensitivity in the infra-red emission band at 1.15 eV. Electron nuclear double resonance experiments show that the dangling bonds are not solely bonded to Si neighbors but involve hydrogen and fluorine.
Origin of the Infrared Band From Porous Silicon
Authors: Mauckner G. , Hamann J. , Rebitzer W. , Baier T. , Thonke K. , Sauer R. , Halbleiterphysik Abt. 
Published: April 2011

ABSTRACTThe photoluminescence (PL) infrared (IR)-band of p-doped porous Si (PS) films is studied by steady-state and time-resolved PL and by photoluminescence excitation (PLE) in detail. In analogy to the S-band in the visible the IR-band shifts to higher energies with reduced average nanocrystal size. The IR- and S-bands are very different in their decay behavior and in their recombination lifetimes. The temperature-dependent PL intensity shows non-exponential decay with lifetime distributions in the nsec-µsec range in contrast to the stretched exponential decay shape of the S-band corresponding to lifetime distributions in the μsec -msec range. The origin of the IR-band is likely related to radiative recombination at deep defects in Si nanocrystals with quantum-upshifted band gaps.
Quenching and Recovery of the Photoluminescence in Porous Si After Pulse IR Irradiation
Authors: Diener J. , Ganichev S. , Ben-Chorin M. , Kovalev D. , Petrova-Koch V. , Koch F. 
Published: April 2011

ABSTRACTA pulsed, high-power TEA CO2 laser with lines in the region from 9.2 to 10.6 μm has been used to irradiate luminescent porous Si samples. The visible luminescence quenches and then recovers to its initial value on a time scale of one hour. It is found that the quenching is efficient when the IR wavelength is within the Si-O absorption band. We suggest that the resonant excitation of the Si-O bonds results in a metastable reconfiguration of the oxygen together with the creation of dangling bonds. These non-radiative centers are responsible for the PL quenching.
Excitation Time Dependence of Luminescence Decay in Thermally Oxidized Porous Si
Authors: Shiba K. , Miyazaki S. , Hirose M. 
Published: April 2011

ABSTRACTThermally oxidized porous silicon shows stable visible luminescence under laser irradiation. The photoluminescence decay has been measured in the temporal range from microsecond to millisecond. The decay curve is well fitted to a stretched exponential function. The mean lifetime of the luminescence increases with increasing the excitation pulse width. It is also found that the higher energy emission exhibits a faster decay compared with the lower energy emission at 293K, while at 18K it is reversed. The results are explained by a model in which photogenerated carriers are captured by localized radiative centers through a thermally assisted tunneling mechanism
Electrical Transport in Mesoporous Silicon Layers
Authors: Ben-Chorin M. , Grebner S. , Wang F. , Schwarz R. , Nikolov A. , Koch F. 
Published: April 2011

ABSTRACTIn order to clarify the role of the enlarged surface area of porous silicon on the electrical conductivity, we have studied the transport in mesoporous silicon layers, for which quantum confinement effects are negligible. We prepare free standing mesoporous films, from highly doped p-type Si wafers. The dark conductivity of the mesoporous layers is activated with an energy of 0.5 eV. Thermopower measurements show negative sign indicating electron conduction. The exposure of these layers to methanol vapor results in an increased conductivity and change of the thermopower magnitude. Photoconductivity measurements and the Steady-State Photocarrier Grating technique (SSPG) are used to evaluate the density of the surface states and the dynamics of the photo-excited carriers. All these results indicate that a large density of surface states exist, which results in a depletion of the free holes.
Photovoltaic Characterization of Trapping in Porous Silicon
Authors: Boeringer D. W. , Tsu R. 
Published: April 2011

ABSTRACTWe report the first observation of the lateral photovoltaic effect in porous silicon. Contacts placed on either side of a porous silicon region develop a voltage up to several millivolts if the sample is asymmetrically illuminated. If the light spot is closer to one contact, the voltage will have one polarity; if it is closer to the other contact, the polarity will be opposite. In the case of n-type, the contact nearest the light spot is positive; for p-type, the contact nearest the light spot is negative In the region between the contacts, the photovoltage varies almost linearly with the position of the light spot, over a distance 4.5 cm across. The origin of our lateral photoeffect may be explained by the trapping of photoexcited carriers by a pair of dangling bond centers in porous silicon. In the case of p-type, the photogenerated electrons are trapped by the dangling bond states while holes diffuse away in the substrate. The situation for n-type is opposite; holes are trapped by the dangling bond states while electrons diffuse away in the substrate. This differs from the conventional lateral photoeffect, which arises under the nonuniform illumination of a junction between two layers of differing conductivities. Hamamatsu sells silicon-based position-sensitive detectors with a resolution down to 0.1 µm. The possibility of using this lateral photoeffect to characterize these dangling bond states in porous silicon as well as several possible device applications will be discussed.
Spectral Response of Photoelectrochemical Cells Based on Nanocrystalline Semiconductor Films
Authors: Rossi M. C. , Vincenzoni R. , Galluzzi F. 
Published: April 2011

ABSTRACTWide band-gap semiconductors such as TiO2, CdSe and CdS, forming the working electrode in photoelectrochemical cells, show quite different spectral responses depending on semiconductor morphology. On the basis of a theoretical model, which takes into account the collection of both majority and minority carriers, spectral responses have been simulated and compared with experimental data for both nanocrystalline and polycrystalline materials.
Near-Infrared Emission from a Porous Silicon Device
Authors: Penczek J. , Knoesen A. , Lee H. W. H. , Smith R. L. 
Published: April 2011

ABSTRACTVisible to near-infrared emission is produced from a porous silicon device under current injection. The porous silicon emitter is fabricated by selectively under-etching a p-n junction. The device is rectifying when biased across the junction, and exhibits a region of negative differential resistance (NDR) at the higher current levels. Bright red-orange emission is observed along the length of the junction in forward bias, but most of the light is emitted in the near-infrared with a peak near 1.3 µm at 30 mA. The intensity of the visible component has an exponential dependence on photon energy. The optical and electrical properties of this device are presented and possible mechanisms are discussed.
Depositing Metals into Porous Silicon - the Impact on Luminescence
Authors: Steiner P. , Kozlowski F. , Lang W. 
Published: April 2011

ABSTRACTIndium, tin, antimony and aluminum are deposited by an electrochemical process into the pores of n-type porous silicon which is anodized with ultraviolet light applied during formation. The presence of these metal atoms in the porous layer is checked by electron microprobe measurement. As reported previously, UV-light etched material shows red photoluminescence (630 nm) and blue electroluminescence (470 nm) without the metal treatment. After metal deposition the photoluminescence intensity decreases slightly (factor 0.5 - 0.8), whereas the spectral position remains constant. The electroluminescence efficiency is significantly enhanced by indium, aluminum and tin in the pores (factor 5 - 90). The tin and antimony treatment causes a red shift to 580 nm and 740 nm, respectively. The conductivity is slightly increased by all kinds of metals by a factor 2-5.
UV-Visible-IR Electroluminescence from Si and Ge Nanocrystals in a Wider Bandgap Matrix
Authors: Tompa G.S. , Morton D.C. , Sywe B.S. , Lu Y. , Forsythe E.W. , Ott J.A. , Smith D. , Khurgin J. , Khan B.A. , Philips N.A. 
Published: April 2011

ABSTRACTThe demonstration of photoluminescence (PL) and electroluminescence (EL) in nanostructures of Si or Ge, such as those found in porous silicon, has significantly improved the prospects of all Si based photonic devices. While the physical mechanisms at work are still a subject of much study, it is clear that the luminescence is associated with the formation of nanometer or “quantum” sized particles. Further, it is clear that prototype NanoCrystal Displays (NCDs) and communication devices are being fabricated in these material systems. We report here on the electroluminescent properties of nanometer sized particles in an SiO2 host matrix, which were fabricated by LPCVD techniques. The films have demonstrated reproducible emission from well below 400 nm to well above 800 nm. We believe that dispersion effects of the nanocrystals can account for "white" light emission. The films have been characterized using PL, Raman, XRD, TEM, and SIMS. The nanocrystals are primarily in the 2-7 nm range although larger crystal clusters are also observed. The development of stable and efficient Si or Ge nanocrystalline EL based devices could find applications in lamps/LEDs, photonic integrated circuits, and displays.
Size Dependent Electroluminescence from CdSe Nanocrystallites (Quantum Dots)
Authors: Dabbousi B.O. , Onitsuka O. , Rubner M.F. , Bawendi M.G. 
Published: April 2011

ABSTRACTWe obtain spectrally narrow (FWHM < 40 nm) electroluminescence from nearly monodisperse CdSe nanocrystallites (quantum dots) incorporated into thin films of polyvinyl carbazole (PVK) and an oxadiazole derivative (PBD) sandwiched between aluminum and ITO electrodes. The electroluminescence and photoluminescence spectra are nearly identical at room temperature and are tunable from ∼530 nm to ∼650 nm by varying the size of the dots. Voltage studies at 77K indicate that while only the dots electroluminesce at the lower voltages, both the dots and the PVK matrix electroluminesce at higher applied voltages. Variable temperature studies indicate that the electroluminescence efficiency increases substantially as the films are cooled down to cryogenic temperatures.
A Tight-Binding Model for Molecular Dynamics of Carbon-Hydrogen Systems
Authors: Kopidakis G. , Wang C.Z. , Soukoulis C.M. , Ho K.M. 
Published: April 2011

ABSTRACTA model for studying carbon-hydrogen systems with molecular dynamics (MD) is developed based on an empirical tight-binding approach for the calculation of the interatomic forces. The parameters involved are obtained by fitting to the structure of methane. The transferability of the model is tested by reproducing accurately several electronic, structural, and vibrational properties of hydrocarbon molecules. Ab initio results on carbon clusters with hydrogen are compared with the results obtained with our model.
Microcrystalline Silicon Thin film Growth and Simultaneous Etching of Amorphous Material
Authors: Heintze M. , Zedlitz R. , Westlake W. 
Published: April 2011

ABSTRACTAn investigation of silicon plasma deposition and etching using both a VHF plasma in dilute SiH4/H2 and the pulsed silane flow method is presented. It is possible to find preparation conditions at which simultaneous growth of µc-Si:H and etching of amorphous silicon (a-Si:H) on the same substrate is observed. The results clearly demonstrate that microcrystalline silicon growth proceeds via the preferential etching of amorphous tissue during film growth, and that observations of crystallization during hydrogen plasma treatment without etching are due to chemical transport of silicon within the reactor.
Grain Growth in Dispersions of µc-Si IN a-Si:H
Authors: Taguchi M. , Wagner S. 
Published: April 2011

ABSTRACTWe annealed dispersions of microcrystalline Si in amorphous silicon by heating to 550 °C. The dispersions had been deposited by plasma-enhanced CVD from SiF4 and H2 at 250 to 290 °C. By X-ray diffraction and Raman scattering we observe an increase in crystal size and volume fraction. The electron mobility, measured by the Van der Pauw method, also was raised to a highest value of 91 cm2V−1s−1 at the electron density of 7.6×1017 cm−3.
Luminescent Hydrogenated Nanocrystalline Silicon Films
Authors: Wang Y. , Yun F. , Liao X. B. , Pan G. Q. , Kong G.L. , Yang B. 
Published: April 2011

ABSTRACTHydrogenated nanocrystalline silicon (nc-Si:H) films with room temperature luminescence have been prepared in a PECVD system. Heavily H2 diluted silane and large negative bias accompanying low deposition temperature are used to decrease the crystalline size. The films comprise nanocrystallites surrounded by an interfacial phase with wurtzite structure characterized by 495cm−1 feature in the Raman spectrum. The X-ray diffraction spectrum shows the average grain size is about 4-5nm. The room temperature photoluminescence (PL) spectrum consists of two peaks, one at 2.21eV which may be induced by the transitions of the quantum subbands in the nanocrystalline phase, the other at 2.84eV which may be attributed to some kinds of localized centers in the interfacial phase. The photoluminescence excitation (PLE) spectrum also shows two peaks, a low one at 3.4eV approaching the bulk like transitions between Г25'V-Г15c while the high energy envelope around 5.0eV has a complicated configuration, and might be related to both the bulk like transition between Λ3v-Λ3cand band transitions of (Si-H2)n chains.
Deposition of Microcrystalline Si,Ge (µc-Si,Ge) Alloys by Reactive Magnetron Sputtering
Authors: Cho S.M. , Wolfe D. , He S.S. , Christensen K. , Maher D.M. , Lucovsky G. 
Published: April 2011

ABSTRACTSixGei1−x:H alloys which span the transition from amorphous to microcrystalline structures have been prepared by reactive magnetron sputtering (RMS) from pure crystalline Si and Ge targets in different partial pressures of hydrogen, using argon as the sputtering gas. Film properties were studied as a function of H2 flow and partial pressure. X-ray diffraction (XRD), Raman scattering, Fourier transform infrared spectroscopy (FTIR), reflection high-energy electron diffraction (RHEED), and high resolution transmission electron microscopy (HRTEM) have been used for microstructural characterization. Films prepared by RMS at a partial pressure of hydrogen (PH2) < ∼ 4 mTorr were amorphous, while those prepared with PH2 > ∼ 6 mTorr were microcrystalline.
The Structure and Composition of Doped Silicon Oxycarbide Microcrystalline Layers Produced by Spatial Separation Techniques
Authors: Martins R. , Vieira M. , Ferreira I. , Fortunato E. 
Published: April 2011

ABSTRACTThis work presents experimental data concerning the role of the oxygen partial pressure used during the preparation process, on the structure, composition and optoelectronic properties of wide band gap doped microcrystalline silicon oxycarbide films produced by a TCDDC system [1].