bluuebunny/crossref_metadata_2025 huggingface dataset:
Real-Time Monitoring for Laser Surface Cleaning
Authors: Lu Y.F. , Hong M.H. , Aoyagi Y.
Published: April 2011

Real-time monitoring of surface cleanness in laser dry cleaning was investigated by monitoring the acoustic waves emitted from the substrate surface under pulsed laser irradiation. It is found that the acoustic wave from the surface under laser irradiation can reflect the surface condition, i.e., the surface cleanness. Besides the surface condition, the acoustic waveforms also depend on the substrate materials and laser pulse energies (intensities). The frequency spectrum obtained from the Fourier transform can provide clear indication for different surface cleanness. Removing of surface contaminants can generally result in weaker laser-induced acoustic wave and change in the frequency spectrum. The surface cleanness, therefore, can be monitored in real-time by detecting the amplitude and frequency of the acoustic wave during the laser cleaning process. This phenomenon provides the laser cleaning technique a real-time feedback during the cleaning process.
Exafs Studies of the Difference in Local Structure of Various Tantalum Oxide Capacitor Films
Authors: Kimura H. , Mizuki J. , Kamiyama S. , Suzuki H.
Published: April 2011

The extended x-ray absorption fine structure (EXAFS) above the Ta L3-edge on tantalum oxide capacitor films has been measured. Four kinds of tantalum oxide films were studied: as-deposited (amorphous), dry 02 annealed (crystalline), 02-plasma annealed (amorphous) and 2-step (02-plasma + dry O2) annealed (crystalline). From EXAFS analysis, differences in the local structures of tantalum oxide capacitor films, in terms of oxygen deficiency around Ta, were observed in the various annealed films. The leakage current characteristics of tantalum oxide capacitors correspond with the differences in the local structures around Ta. The discussion looks at the relationship between the leakage current characteristics and the local structures around Ta.
Ion Beam Sputter Deposition of Refractory Metal Oxides
Authors: Kempf B.E. , Dinges H.W. , PÖcker A.
Published: April 2011

Oxides of hafnium, niobium, tantalum, and zirconium are deposited by ion beam sputtering of the pure metal targets using CO2 as working gas. The resulting thin films are amorphous, featureless smooth and of excellent adherence to semiconductor substrates. Despite a certain content of carbon they are highly transparent in the visible and near infrared wavelength range as determined by spectroscopic ellipsometry. Their wide range of refractive indices makes them suitable for multilayer optical filter design.
Gamma Ray Processing of ZnGeP2: A Nonlinear Optical Material for the Infrared
Authors: Schunemann P.G. , Drevinsky P.J. , Ohmer M.C.
Published: April 2011

Zinc germanium phosphide, an important frequency-conversion material for producing mid-infrared lasers, is plagued by a defect-related absorption band extending from the fundamental edge (0.62 microns) to ∼3 microns. The level of absorption varies with melt composition, and can be reduced by post-growth annealing treatments. In these experiments, further reduction of the near-band-edge absorption was achieved by irradiating with 1.173 MeV and 1.3325 MeV gamma rays. A 40 kiloCurie “Co source was used to irradiate a series of crystals at an average flux of 4.8x10s rads/hr to cumulative doses up to 3.91xl010 rads. The absorption coefficient in the near-infrared was reduced by nearly a factor of two, and the penetration depth of the ionizing radiation was sufficient to uniformly reduce the absorption in the largest samples tested (up to 7x8x23mm3). The loss coefficients achieved at 1 micron (4.28cm∼l) and at 2.05 microns (0.19cm1) are lower than the best results achievable by conventional thermal annealing alone. No saturation effects or absorption increases at longer wavelengths were observed.
Laser assisted molecular beam deposition of thin films of polymeric copperphthalocyanine and their characterization
Authors: Wijekoon W.M.K.P. , Xia Ping , Prasad P.N. , Garvey J.F.
Published: April 2011

Thin films of polymeric copperphthalocyanine were fabricated via laser assisted molecular beam deposition by reacting the laser ablated plasma flume of a copper target with 1,2,3,5-tetracyanobenzene entrained within a molecular beam expansion. 1,2,4,5-Tetracyanobenzene entrained within a helium carrier gas was supersonically expanded to intercept a second expansion containing gas-phase aggregates of metallic copper. These two molecular beams met at a point between the target and the substrate producing gas phase polymeric copperphthalocyanine which could be then directly deposited as a thin film coating on room temperature substrates. Such films were characterized by scanning electron microscopy and uv-visible, infrared and x-ray photoelectron spectroscopic techniques. Optical spectra of films dissolved in H2S04 solutions reveal that small amounts of monomeric copperphthalocyanine is also present in the grown films. All the copper in these films is found to be in Cu+2 ionization state.
Fabrication and Modification of Metal Nanocluster Composites Using Ion and Laser Beams
Authors: Haglund R.F. , Osborne D.H. , Magruder R.H. , White C.W. , Zuhr R.A. , Townsend P.D. , Hole D.E. , Leuchtner R.E.
Published: April 2011

Metal nanocluster composites have attractive properties for applications in nonlinear optics. However, traditional fabrication techniques — using melt-glass substrates — are severely constrained by equilibrium thermodynamics and kinetics. This paper describes the fabrication of metal nanoclusters in both crystalline and glassy hosts by ion implantation and pulsed laser deposition. The size and size distribution of the metal nanoclusters can be modified by controlling substrate temperature during implantation, by subsequent thermal annealing, or by laser irradiation. We have characterized the optical response of the composites by absorption and third-order nonlinear-optical spectroscopies; electron and scanning-probe microscopies have been used to benchmark the physical characteristics of the composites. The outlook for controlling the structure and nonlinear optical response properties of these nanophase materials appears increasingly promising.
A Comparative Study of Pulsed Laser And Electron Beam Irradiation Effects In Fe81B13.5Si3.5C2 Glass
Authors: Sorescu Monica , Knobbe E. T. , Barb D.
Published: April 2011

Samples of Fe81B13.5Si3.5C2 metallic glass were irradiated with a pulsed excimer laser (λ=308 nm, τ=10 ns), with a high-energy electron beam (W=7 MeV), and with low-energy electron beams (W=30 and 50 keV). Irradiation-driven changes in the magnetic anisotropy and phase equilibrium of alloy samples were studied by Mössbauer spectroscopy and scanning electron microscopy. Pulsed-excimer-laser irradiation was found to induce controlled magnetic anisotropy without onset of bulk crystallization in the Fe81B13.5Si3.5C2 amorphous system. High-energy electron-beam irradiation determined an out-of-plane magnetic anisotropy due to changes in the chemical short-range order. Low-energy electron-beam irradiation resulted in the formation of crystalline regions, in which α-Fe, Fe-Si, Fe3B, Fe2B, and clusters of γ-Fe were identified. Interpretation of these results is given in terms of radiation-enhanced diffusion.
Sintering of Textured YBa2Cu307-x Under Intensive 60Co Gamma Irradiation
Authors: Ibragimova Elvira M. , Gasanov Eldar M. , Kalanov Makhmud , Kirk Marquis A. , Goretta Ken C.
Published: April 2011

A study of structural and superconducting characteristics of YBa2Cu307-x ceramics sintered after and in the course of gamma-irradiation is reported. Using X-difraction and SEM analyses and transport measurements, it has been shown that mainly the subsurface layer of crystallites and intergrain contacts are affected by the irradiation by means of sorption/desorption of oxygen and ordering/disordering in oxygen sublattice, which depends on gamma dose rate and dose. The irradiation provides the sintering process with an additional superthermal energy to form the orthorhombic well-ordered structure and to obtain dense ceramics possessing strong intergrain contacts and improved and stable superconducting properties
Interaction of Pt Films With ZnO(0001) and ZnO(0001)
Authors: Petrie W.T. , Vohs J.M.
Published: April 2011

The growth, structure, and electronic properties of Pt films supported on the (0001) and (0001) surfaces of ZnO were investigated using high-resolution electron energy-loss spectroscopy (HREELS) and low-energy electron diffraction (LEED). Vapor-deposited Pt films were found to grow in a layer-by-layer fashion on both surfaces and exhibited hexagonal LEED patterns. HREELS results indicate that there are only weak electronic interactions at the Pt/ZnO(0001) interface, while charge transfer and Schottky barrier formation occurs at the Pt/ZnO(0001) interface.
Epitaxial Growth and Characterization of Lattice-Matched MgO/Cr0.7Mo0.3/MgO(001) Layered Structures
Authors: Chambers S.A. , Liang Y. , Gao Y.
Published: April 2011

We have grown by molecular beam epitaxy (MBE) structurally coherent layers of MgO and a random bcc alloy of 70% Cr and 30% Mo to which MgO is lattice matched. Specimens werecharacterized by means of reflection high-energy electron diffraction (RHEED), x-ray photoelectron spectroscopy and diffraction (XPS/XPD), transmission electron microscopy (TEM), and atomic force microscopy (AFM). These systems exhibit excellent long- and shortrange crystallographic order, as well as nearly perfect structural coherence across the interface.
A Study of the MBE Growth of MGO on The FE(001) Surface
Authors: Vassent J.L. , Dynna M. , Patrat G. , Gilles B. , Marty A.
Published: April 2011

The deposition of MgO on the Fe(001) surface has been carried out using electron beam evaporation. MgO is observed to grow epitaxially with a 45° rotation between the Fe(001) and MgO(001) unit cell axes. Oscillations in the intensity of the RHEED specular beam are observed at room temperature showing that growth is two-dimensional. The relaxation of the in-plane lattice parameter during the growth at room temperature has been investigated in-situ by measuring the position of the RHEED streaks and ex-situ with GIXD experiments. Pseudomorphic growth is observed up to about five monolayers. Then the in-plane lattice parameter starts to evolve towards the MgO bulk parameter. HREM shows that the relaxation of MgO occurs via the generation of 1/2<011> misfit dislocations which are non-uniformly spaced at the Fe/MgO interface.
Interfacial Electronic Structure and Full Spectral Hamaker Constants of Si3N4 Intergranular Films from VUV and Sr-Veel Spectroscopy
Authors: French R. H. , Scheu C. , Duscher G. , Müllejans H. , Hoffmann M. J. , Cannon R. M.
Published: April 2011

The interfacial electronic structure, presented as the interband transition strength Jcv(ω) of the interatomic bonds, can be determined by Kramers Kronig (KK) analysis of vacuum ultraviolet (VUV) reflectance or spatially resolved valence electron energy loss (SR-VEEL) spectra. For the wetted interfaces in Si3N4, equilibrium thin glass films are formed whose thickness is determined by a force balance between attractive and repulsive force terms. KK analysis of Jcv(ω) to yield ξ(ξ) for the phases present, permits the direct calculation of the configuration-dependent Hamaker constants for the attractive vdW forces from the interfacial electronic structure.

Interband transition strengths and full spectral Hamaker constants for Si3N4 samples containing a SiYA1ON glass have been determined using SR-VEELS from grains and grain boundaries and compared with results from bulk VUV spectroscopy on separate samples of glass and nitride. The At2, Hamaker constant for Si3N4 with glass of the bulk composition is 8 zJ (zJ = 10−21 J) from the more established optical method. The EELS method permits the determination of vdW forces based upon actual local compositions and structure, which may differ noticeably from bulk standards. Current results show that full spectral Hamaker constants determined from VUV and SR-VEEL measurements of uniform bulk samples agree, but care must be taken in the single scattering and zero loss subtraction corrections, and more work is ongoing in this area. Still the results show that for the grain boundary films present in these polycrystalline Si3N4 samples the glass composition is of lower index of refraction. This can arise from increased oxygen content in the intergranular glass and leads to an increased value of the Hamaker constant (24 zJ) determined in situ from the SR-VEELS of a particular grain boundary film.
Sreels Analysis of Oxygen-Rich Inversion Domain Boundaries in Aluminum Nitride
Authors: Bruley J. , Westwood A.D. , Youngman R. A. , Zhao J.-C. , Notis M.R.
Published: April 2011

Spatially resolved electron energy loss spectroscopy analysis has been conducted on planar inversion domain boundaries in aluminum nitride. The defects were found to contain 1.5 monolayers of oxygen, in agreement with the most recent structural model of Westwood. From variations in near-edge structure, the local atomic environments of both oxygen and aluminum are compared with α-A1203, γ-A1203 and γ-AION standards. Based upon this study the stnrcture of the inversion domain boundary is found to resemble that of the cubic γ-AION spinel, and eliminates from consideration those structural models based upon ai-Al203. Furthermore, quantification of the shape resonances provided Al-O bond-length data from the inversion domain boundary interface. These distances closely agree with the Youngman Model that has recently been further refined by Westwood et al.
The “Solidification” of Grain Boundaries with Increasing Temperature
Authors: Lojkowski Witold , Palosz Bogdan
Published: April 2011

The aim of the paper is to explain the recently observed de-wetting grain boundary transition with increasing temperature. On the example of a bicrystal from the Fe-6at.%Si alloy, it was found recently that as temperature is increased, the following GB transitions take place: “solid” (or regular) GB-→“premelted” GB →“solid” GB. At the same time the wetting/de-wetting transitions have taken place. Another example of such GB behavior was discovered during sintering of alumina. The inverse melting behavior is explained as follows: low melting point impurities cause GB premelting at low temperatures, However de-segregation of impurities at high temperatures causes return of the GB structure to its regular “solid” state.
Theory of the Physical Properties of Si Nanocrystals
Authors: Lannoo M. , Delerue C. , Allan G. , Martin E.
Published: April 2011

This paper reviews calculations concerning several aspects of silicon crystallites and their relevance for porous silicon. This begins with the optical properties of perfect crystallites: gap versus size, radiative recombination time, relative importance of phonon assisted transitions. A second part is devoted to the determination of the excitonic exchange splitting and of the Stokes shift which are found to bring a similar contribution (∼10 to 20 meV). The effect of surface defects like dangling bonds is then investigated with their contribution to the recombination time. The Auger non radiative recombination time is also calculated and found to be short (∼1 nsec). This is confirmed by some experiments on porous silicon which show a saturation effect of the photoluminescence under intense optical excitation or under cathodic polarization in aqueous solution, Auger recombination preventing the existence of more than one electron-hole pair per crystallite. Donor and acceptor impurities are studied in detail (screening of Coulomb potential, notion of ionization energy) with the conclusion that they are ionized. A final discussion shows the present level of understanding and identifies problems remaining to be solved.
Compound Semiconductor Nanocrystals formed by Sequential Ion Implantation
Authors: White C. W. , Budai J. D. , Zhu J. G. , Withrow S. P. , Zuhr R. A. , Chen Y. , Hembree D. M. , Magruder R. H. , Henderson D. O.
Published: April 2011

Ion implantation and thermal processing have been used to synthesize compound semiconductor nanocrystals (SiGe, GaAs, and CdSe) in both SiO2 and (0001) Al2O3. Equal doses of each constituent are implanted sequentially at energies chosen to give an overlap of the profiles. Subsequent annealing results in precipitation and the formation of compound nanocrystals. In SiO2 substrates, nanocrystals are nearly spherical and randomly oriented. In Al2O3, nanocrystals exhibit strong orientation both in-plane and along the surface normal.
Nanocrystals of II-VI Semiconductor Materials
Authors: Weller Horst , Vossmeyer Tobias , Eychmüller Alexander , Mews Alf , Katsikas Lynne , Reck Gunter
Published: April 2011

CdS 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

The 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

A 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

In 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

The 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

Fourier 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

We 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

The 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

The 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

A 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

Thermally 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

In 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

We 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

Wide 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

Visible 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

Indium, 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

The 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

We 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

A 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

An 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

We 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

Hydrogenated 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

SixGei1−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

This 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].
Microcrystalline β-SiC Growth on Si by ECR-CVD at 500°C
Authors: Cheng Kuan-Lun , Liu Chih-Chien , Cheng Huang-Chung , Lee Chiapyng , Yew Tri-Rung
Published: April 2011

Microcrystalline β-SiC films were deposited on silicon substrates by electron cyclotron resonance chemical vapor deposition (ECR-CVD) at 500°C utilizing a SiH4-CH4-H2 gas mixture. The effects of two important parameters on film growth, SiH4/CH4 flow ratio and microwave (MW) power, were investigated using X-ray photoelectron spectroscopy (XPS) along with the Fourier transform infrared spectra (FTIR). Results showed that the optimum flow ratio is about 0.5. Under the optimum flow ratio, a large MW power is favorable for the growth of high quality films with an ideal film stoichiometry. Surface morphology inspected by the contact mode atomic force microscopy (AFM) reveals that high MW powers not only improve the film crystallinity but also increase its surface roughness as well.
X-Ray Diffraction Study of Clusters in a-tC Films
Authors: Martínez-Miranda L. J. , Friedmann T. A. , Sullivan J. P. , Siegal M. P. , Mercer T. W. , DiNardo N. J. , Fang F.
Published: April 2011

We performed an X-ray diffraction study of amorphous-tetrahedrally-coordinated carbon (a-tC) films prepared by pulsed laser deposition (PLD). The samples' properties were analyzed as a function of laser energy and thickness. For all thicknesses and laser energies, films were made up of clusters with a basic unit size of 7 -11 nm. Thicker films, as well as films prepared at higher laser densities exhibit larger clusters, in the tens of nanometers. The clusters are not readily observable by AFM, which may indicate the presence of a flat (graphitized) top film surface.
Early Stage of Polycrystalleve Growth of Ge and SiGe by Reactive Thermal Cvd from GeF4 and Si2H6
Authors: Hanna Jun-Ichi , Ohuchi Takayuki , Yamamoto Masaji
Published: April 2011

We have investigated the structure and crystallinity of poly SiGe and Ge films in the early stage of their growth prepared at < 450°C by means of a new type of thermal CVD from GeF4 and Si2H6. Raman study and SEM observation of the films revealed that the nucleation of crystallites took place directly on the substrate, followed by the grain growth in a manner of island growth. Thus, the crystallinity of the films is well established even in the very early stage of the growth, probably < 10 nm in thickness, in contrast to the poly Si growth in the low-temperature CVD precesses.
Thin Film Polycrystalline Si by Cs Solution Growth Technique
Authors: Wallace Richard L. , Anderson Wayne A. , Jones K. M.
Published: April 2011

A deposition process has been developed which allows the growth of large grain (20+ μm) polysilicon films on SiO2 substrates at a growth temperature of 650° C. A thin layer of liquid Si-metal solution is formed on the substrate surface as the growth medium. This layer is kept saturated by Si flux from a DC magnetron sputter gun. XRD analysis of the deposited films show a strong (111) preferred orientation, with increasing integrated peak intensities with increasing depositon temperature and solution layer thickness. Films deposited using an In-Si solution are p-type, with carrier concentrations in the mid 1016 cm−3 range. Conductivities of ∼.2 (Ω cm)−1 were measured, with activation energies for both carrier generation and conductivity of about 135meV. The hole mobility was found to be ∼ 30 cm2 V−1s−1. A wetting layer is used which may have a detrimental effect on the minority carrier lifetime.
The Crystalline Quality of Epitaxial Si Layers Solution Grown on Polycrystalline Si Substrates
Authors: Albrecht M. , Steiner B. , Bergmann Th. , Voigt A. , Dorsch W. , Strunk H. P. , Wagner G.
Published: April 2011

We investigate the crystalline and electrical quality of thin layers epitaxially grown on polycrystalline substrates from metallic solution by the method of electron beam induced current, transmission electron microscopy and etching experiments. We observe a reduced recombination strength of dislocations and small angle grain boundaries, i.e. an improved electrical quality of the epitaxial layer compared to the substrate. The improved quality can be attributed (i) to an altered structure of grain boundaries and dislocations and (ii) to a reduced defect density in the epitaxial layer.
High-Quality Polycrystalline Silicon Thin Film Prepared by a Solid Phase Crystallization Method
Authors: Baba T. , Matsuyama T. , Sawada T. , Takahama T. , Wakisaka K. , Tsuda S.
Published: April 2011

We succeeded, for the first time, in depositing a silicon film which features 1000Å-wide single-crystalline grains embedded in a matrix of amorphous tissue. The deposition was done by plasma-enhanced CVD from silane diluted with hydrogen at a considerably high temperature (550°C). 5pm-thick undoped amorphous silicon film was deposited on the above film and was crystallized by a solid phase crystallization method. The polycrystalline silicon film which was obtained has a columnar structure and shows an extremely high electron mobility of 808 cm2/Vs.
Effect of Deposition Temperature on the Photoresponse of Crystallized Hydrogenated Amorphous Silicon Films
Authors: Sridhar Nagarajan , Chung D. D. L. , Anderson W. A. , Coleman J.
Published: April 2011

The deposition temperature of hydrogenated amorphous silicon films deposited by dc glow discharge was found to affect the photoresponse (ratio of the photo to dark conductivity) after crystallization of the film. This effect depended on the crystallization technique. For crystallization by laser annealing, the photoresponse (0.15 - 1.5) increased with increasing deposition temperature (150 - 300 °C) due to the increase in SiH and SiH2 bonding, as shown by infrared spectroscopy. For crystallization by furnace annealing (e.g. 650 °C, 50 h), the photoresponse (0.08 - 0) decreased with increasing deposition temperature (150 - 300 °C) due to the decrease in grain size and crystallinity as shown by x-ray diffraction; the complete loss in hydrogen during furnace annealing made the photoresponse low and the silicon-hydrogen bonding effect immaterial. Thus, laser crystallization at the highest deposition temperature gave the highest photoresponse.
Diffusion Behavior of B-, As- and Sb-Dopants in Thin Epitaxial Layers
Authors: Krautschneider W.H. , Lau F. , Gossner H. , Schaefer H.
Published: April 2011

Advanced CVD- and MBE-epitaxy allows processing of very thin epitaxial layers with extremely steep dopant gradients. As steep dopant profiles are very sensitive to temperature treatments, the impact of thermal processing from 800°C to 1000°C on the dopant redistribution in epitaxial layers was investigated.
Inorganic Fullerenes of MX2 (M=W,Mo;X=S,Se)
Authors: Tenne R. , Margulis L. , Feldman Y. , Homyonfer M.
Published: April 2011

The gas-phase reaction between MoO3-x and H2S in a reducing atmosphere and at elevated temperatures (800° to 950°C) has been used to synthesize large quantities of an almost pure nested inorganic fullerenes (IFs) phase of MoS2. A uniform IF phase with a relatively narrow size distribution was obtained. The x-ray spectra of the different samples show that as the average size of the IF decreases the van der Waals gap along the c axis increases, largely because of the strain involved in folding of the lamella. Large amount of quite uniform nanotubes were obtained under modified preparation conditions.

In the second part of this work MoS2 nested fullerenes were grown on Ti and Nb and Mo oxide substrates. In one set of conditions hollow nested fullerenes of MoS2 were collected on a titanium oxide matrix, and analyzed by local area energy dispersive analysis. Under different flow conditions of the reacting gasses nested fullerenes with endohedral metal oxides were obtained and analyzed by electron diffraction.
Inter-Cage Orbital Interactions in [2+2] and [4+4] Dimers of Buckminsterfullerene
Authors: Ōsawa Shūichi , Ōsawa Eiji
Published: April 2011

Geometry optimization of [2+2]- and [4+4]-dimers of C60 by semiempirical AMI MO method reveals usual length for the bridge bonds of the former but unusually long bonds for the latter. Perturbational analysis of molecular orbitals confirm the presence of orbital interaction through bond (OITB) between the cages in the [2+2] dimer but no interaction in the [4+4] dimer. These results contradict with previous examples of OITB wherein elongation of the mediating σ bond is usually observed.