bluuebunny/crossref_metadata_2025_split huggingface dataset:
Electronic Properties of Porous Silicon
Authors: Beltran M. R. , Tagüeña-Martinez J. , Cruz M. , Wang C. 
Published: April 2011

ABSTRACTPorous silicon (PS) has been extensively studied in recent years. The origin of its luminescence has been a subject of debate. This work attempts to give some insight towards the understanding of this phenomenon, studying the behaviour of the energy band structure of PS as a function of the pore morphology and distribution, for a given porosity. The porous structure is modeled as empty columns of different sizes and shapes, produced into san otherwise silicon perfect crystal. The columns are passivated with hydrogen atoms. A tight-binding Hamiltonian on an ; sp3:s* basis set is applied onto a supercell. Due to the simplicity of the model, morphology effects can be analyzed. The results show that the band :gap and the nature of the 'States at the top of the valence band depend on the morphology. Furthermore, we also discuss the shift of the conduction band minimum towards the gamma point, producing an almost direct band gap, as the pore distribution changes.
Extended Quantum Model for Porous Silicon Formation
Authors: Münder H. , Frohnhoff St. , Berger M.G. , Marso M. , Thönissen M. , Arens-Fischer R. , Lüth H. 
Published: April 2011

ABSTRACTThe formation of porous silicon (PS) by electrochemical dissolution of bulk Si is described by a new model involving quantum mechanical calculations of the tunneling probability of holes through small crystallites (< 60 Å) into the electrolyte. This tunneling probability shows oscillations as a function of crystallite size. The presented model calculations are in agreement to the microstructure of p-PS — deduced from Raman measurements — as a function of etching parameters and substrate doping level.
Non-Destructive Characterization of Porous Silicon Using X-Ray Reflectivity
Authors: Chason E. , Guilinger T.R. , Kelly M.J. , Headley T.J. , Howard A.J. 
Published: April 2011

ABSTRACTUnderstanding the evolution of porous silicon (PS) layers at the early stages of growth is important for determining the mechanism of PS film growth and controlling the film properties. We have used X-ray reflectivity (XRR) to determine the evolution of layer thickness and interfacial roughness during the growth of thin PS layers (< 200 nm) prepared by electrochemical anodization. The porous layer grows at a constant rate for films as thin as 15 nm indicating a very short incubation period during which the surface may be electropolished before the PS structure begins to form. Interface roughness measurements indicate that the top surface of the film remains relatively smooth during growth while the roughness of the PS/silicon interface increases only slightly with film thickness. The XRR results are compared with results obtained from the same films by cross-sectional transmission electron microscopy (XTEM), atomic force microscopy (AFM) and gravimetry.
Formation and Properties of Porous Si Superlattices
Authors: Berger M.G. , Arens-Fischer R. , Frohnhoff St. , Dieker C. , Winz K. , Münder H. , Lüth H. , Arntzen M. , Theiss W. 
Published: April 2011

ABSTRACTPorous silicon superlattices (PS-SL) were investigated by transmission electron microscopy (TEM), scanning electron microscopy (SEM), spectroscopic ellipsometry and reflectance spectroscopy. Type I superlattices were formed by periodically changing the current density during the etch process. The quality of the interface between layers of different porosity depends on the quality of the silicon substrate. Optical components such as Bragg reflectors or Fabry-Perot filters were designed using the optical data of single porous layers. A good long term stability of the layers is achieved by using thermal oxidation steps. Type II superlattices were formed on substrates with layers of alternating doping level. The more complex layer structure of these superlattices is explained by the selectivity of the etch process on the doping level.
Preparation, Properties and Applications of Free-Standing Porous Silicon Films
Authors: Von Behren J. , Tsybeskov L. , Fauchet P. M. 
Published: April 2011

ABSTRACTUsing special electrochemical etching and lift-off steps, we have fabricated large-area freestanding porous silicon films in the thickness range from 0.1 μm to 50 μm. Their transmission is near 100% in the near infrared which is indicative of very high porosity/low index of refraction films. These optically flat and homogeneous films exhibit no surface and bulk scattering, despite the fact that they did not undergo supercritical drying. The relationship between the absorption coefficient, the luminescence spectrum, and the chemical and structural properties is examined as a function of preparation and post-treatment conditions. Because of their superior optical properties, these films are suitable for many device applications.
Effective Dielectric Function of Porous Silicon: the Transverse Component
Authors: Gumbs G. 
Published: April 2011

ABSTRACTA self-consistent many-body theory is developed to study the effect of temperature and electron density on the interband absorption coefficient and the frequency-dependent refractive index for an array of isolated quantum wires. The peaks in the absorption coefficient correspond to interband transitions resulting in the resonant absorption of light. The oscillations in the derivative spectrum are due to the quantization of the energy levels related to the in-plane confining potential for such reduced dimensional systems. There are appreciable changes in the absorption spectrum when the electron density or temperature is increased. One interband transition peak is suppressed in the high electron density limit and the thermal depopulation effect on the electron subbands can be easily seen when the temperature is high. We also find that the exciton coupling weakens the shoulder features in the absorption spectrum. This study is relevant to optical characterization of the confining potential and the areal density of electrons using photoreflectance. By using incident light with tunable frequencies in the interband excitation regime, contactless photoreflectance measurements may be carried out and the data compared with our calculations. By fitting the numerical results to the peak positions of the photoreflectance spectrum, the number of electrons in each wire may be extracted.
Localized States and Porous Silicon Luminescence
Authors: Dubin V. M. , Ozanam F. , Chazalviel J.-N. 
Published: April 2011

ABSTRACTElectronic states of photocarriers in porous silicon have been investigated by photomodulated infrared and photoluminescence spectroscopies. Wet porous silicon is only weakly yellow-green luminescent, and contains photocarriers which exhibit a Drude-like absorption characteristic of free carriers. On the contrary, once dried or anodically oxidized, porous silicon becomes strongly red luminescent and photocarriers exhibit a broad Gaussian-like absorption, characteristic of localized carriers, which correlates in frequency, response time and intensity with the photoluminescence. This behavior does not appear to be related to surface chemistry. Instead, non-wetting of the surface seems to be mandatory in order to obtain localization of the photocarriers and strong red luminescence. This suggests that the driving effect for switching from green to red luminescence could be the dielectric screening of the Coulombic interactions by the embedding medium in which porous silicon is maintained. The electronic states bound to the potential generated by a charge in a cylindrical silicon wire have then been computed for various values of the dielectric constant of the embedding medium. It is shown that, when the dielectric constant is low, one may account for the red luminescence in terms of recombination through such shallow states, and that the finite time needed for the dielectric relaxation in electrolytes may also account for an inefficient trapping in such states in a wet environment. Preliminary luminescence results are consistent with the predictions of the model.
Experiments and Monte Carlo Simulations on the Recombination Dynamics in Porous Silicon
Authors: Pavesi L. , Roman H. Eduardo 
Published: April 2011

ABSTRACTWe present a detailed study of the time-resolved photo-luminescence of porous Silicon samples with different porosities providing clear evidence of anomalous relaxation behaviour of the luminescence, which follows stretched exponential decay for a variety of experimental conditions. In addition, a numerical study of the underlying transport behaviour in these disordered materials by means of Monte-Carlo simulations has been performed. Nanometer sized particles, characterised by a distribution of radiative and non-radiative recombination times, are randomly placed at the sites of a cubic lattice forming a single three dimensional percolation cluster. Charge carriers are allowed to hop between nearest-neighbour occupied sites. The competing effect between radiative and non-radiative transitions in a single nanometer particle, as well as the effects of geometrical constraints on transport due to the complex topology, are discussed and compared to experiments.
Temperature Dependence of Stresses and H Desorption in Porous Silicon
Authors: Diawara Y. , Currie J.F. , Yelon A. , Petrova-Koch V. , Nikolov A. 
Published: April 2011

ABSTRACTThe mechanical stress induced in as-prepared porous silicon is generally compressive. Upon thermal annealing, the stress shows hysteresis. Isothermal annealing reveals second-order kinetic processes for both the stress and hydrogen effusion. The correlations between the changes in stress and porous silicon composition and structure are discussed.
Gas Sensor Using an Aluminium-Porous Silicon Junction Application to the Detection of Non-Zero Molecular Dipole Moment
Authors: Stievenard D. , Deresmes D. 
Published: April 2011

ABSTRACTPorous silicon is known to be sensitive to moisture. Using an aluminium-porous p+ silicon junction, we have realized a sensor which dc current increases up to two orders of magnitude in the presence of ammoniac. We have tested a series of various gases and we show that if the dipole moment of the molecule is zero, there is no effect on the dc current. To interpret quantitatively this phenomenon, we assume that the conductivity is governed by the width of a channel resulting from the partial depletion of silicon located between two pores. This depleted region is due to the charges trapped on surface states associated with the Si-SiO2 interface where SiO2 is the native silicon oxide. When some gas is adsorbed, we propose there is a passivation of the interface states (mainly dangling bonds), leading to a decrease of the depleted region, i.e. an increase of the width of the channel and thus an increase of the current. The adsorbed gas gives a dipole layer at the surface of the pore. This layer has no influence on the depleted region. It stabilizes electrons or holes at the porous Si surface, allowing a stable charge state of the dangling bonds.
Photoluminescence and Electroluminescence in Partially Oxidized Porous Silicon
Authors: Tsybeskov L. , Duttagupta S. P. , Fauchet P. M. 
Published: April 2011

ABSTRACTThe results of photoluminescence (PL) and electroluminescence (EL) studies from partially oxidized porous silicon (POPS) layers are presented. The PL from POPS is stable, peaks at 600-570 nm and its temperature dependence can be fitted by an exponential law with an activation energy Ea « 10 meV. The current-voltage characteristics of Au-(POPS)-crystalline silicon (c-Si) structures follow a power law I = Vn. When the index n becomes higher than 3, electroluminescence (EL) is found. The EL peaks at 760 nm and is stable for more than 100 hours of operation. The intensity of the EL is a linear function of current for all measured structures up to current density J ≈ 1 A/cm2. Our results suggest that partially oxidized porous silicon is more useful for device applications than freshly anodized porous silicon which has unstable properties or than fully oxidized porous silicon in which transport is poor.
Preparation of Nanocrystalline Silicon by Pulsed Plasma Processing
Authors: Oda S. , Otobe M. 
Published: April 2011

ABSTRACTWe have proposed digital plasma processing for the fabrication of silicon quantum dots with grain size less than l0nm. By using the pulsed gas supply of SiH4 and H2 in the very-high frequency (VHF) plasma, we have clarified the role of atomic hydrogen in the nucleation, crystallization of nanocrystalline Si (nc-Si) as well as in the selective etching of amorphous Si to nc-Si. Recently, we have prepared nc-Si by employing an ultra-high-vacuum (UHV) chamber equipped with VHF plasma cells of SiH4 and H2. Flux rate of Si cluster depends significantly on the pressure of the plasma cell and VHF power. Spherical shaped nc-Si clusters less than 6nm in diameter have been observed by transmission electron microscopy (TEM). Infrared absorption measurements have clarified that the surface of nc-Si is covered by hydrogen. In an attempt to control the position of nuclei, we have prepared nc-Si on SiO2 with micro trenches, 40nm wide and 20nm deep, fabricated by electron beam exposure and electron cyclotron resonance (ECR) etching. It has been revealed by TEM observation that nc-Si are formed preferentially along micro trenches.
Thin Films of Semiconducting SnSi Alloys Grown by Pulsed Laser Deposition
Authors: Treece Randolph E. , Horwitz J. S. , Chrisey D. B. , Tang J. , Williams R. S. 
Published: April 2011

ABSTRACTSemiconducting SnxSi1−x (0≤x≤0.6) thin-film alloys have been grown by pulsed laser deposition (PLD). These new materials are amorphous to X-rays and display small positive optical band gaps, suggesting potential applications in solar cells. The tin silicide films were grown by depositing very thin (1–30 Å) alternating atomic layers from individual Sn and Si targets utilizing an automated multi-target holder coupled to a conventional PLD system. The value of x was selected by controlling the thickness of the atomic layers. The films were characterized by X-ray diffraction, optical absorption, Rutherford backscattering spectroscopy, temperature-dependent resistivity, and X-ray photoelectron spectroscopy. Tin segregation is prevented by keeping the Sn layer thickness below a critical value. Compositions beyond x > 0.6 led to semimetallic SnxSi1−x films with tin crystallites.
Transmission Electron Microscopy Observation of Constrained Crystallization in a-Si:H/a-SiNx:H Multilayer Film
Authors: Huang Xinfan , Shi Weihua , Chen Kunji , Yu Shidong , Feng Duan 
Published: April 2011

ABSTRACTThe constrained crystallization in a-Si:H/a-SiNx:H multilayer structures by Ar ion laser annealing treatment has been studied by high-resolution transmission electron microscopy (HRTEM) and Raman scattering. HRTEM photograph shows that the a-Si:H layers crystallize without disturbing the multilayer structures and that the interfaces after the crystallization are atomically smooth and uniform. The lattice image of the Si crystallites arrayed one by one can be seen clearly in Si layers and the average size is roughly equal to the thickness of Si layer. The thermodynamics of constrained crystallization within multilayer structures has been discussed.
Nd-YAG Laser Induced Crystallization on a-Si:H Thin Films
Authors: Carvalho J. , Ferreira I. , Fernandes B. , Fidalgo J. , Martins R. 
Published: April 2011

ABSTRACTIn this paper we present results concerning the influence of laser energy and shot density on the electrical resistance, X-ray diffraction pattern, and structure obtained by SEM, on recrystallized a-Si:H thin films produced by using a Nd-YAG laser, working in a wavelength of 532 nm. The base material (undoped and doped a-Si:H) was obtained by Plasma Enhanced Chemical Vapour Deposition (PECVD). The structure and electrical characteristics of the recrystallized thin films are dependent on the laser beam energy density, beam spot size and the number of shots applied to the base a-Si:H thin film used. Overall, the data show recrystallized material with grain sizes larger than lµm, where the electrical resistance of both, undoped and doped materials, can be varied up to 5 orders of magnitude, by the proper choice of the recrystallization conditions.
Structural and Electrical Characterization of Undoped Poly-Si Oxides
Authors: Sakamoto T. , Tokioka H. , Takanabe S. , Kubota T. , Niwano Y. , Goto Y. , Namizaki H. , Wada O. , Kurokawa H. 
Published: April 2011

ABSTRACTWe evaluated structural and electrical characteristics of undoped poly-Si oxide films. Poly-Si films made by solid phase crystallization at 600-900°C from undoped amorphous Si films were oxidized to form oxide layers of 140nm thickness. We observed protuberances on the surface of poly-Si layers after oxidation. Poly-Si oxide layers also generated protuberances above the protuberances of poly-Si films. The number of protuberances per unit area is larger in the case of high temperature crystallization. The measurement of current through the poly-Si oxide films shows that the conductivity of poly-Si oxide films depends on crystallization temperature of poly-Si films in the case of positive gate bias. When the gate is biased negatively, current through the poly-Si oxide films remained almost constant regardless of crystallization temperature. We find that poly-Si crystallized at lower temperatures offers poly-Si oxide films of lower leakage current in the case of electron injection from undoped poly-Si layers. The lower leakage current is due to highness of energy barrier for electron at undoped poly-Si/poly-Si oxide interface.
Control of the Crystallite Size and Dielectric Tissue in nc-Si/SiO2 Plasma CVD Films: Origin of the Green/Blue and the Efficiency of the Red Photoluminescence
Authors: Vepřek S. , Wirschem Th. , Rückschloß M. , Tamura H. , Oswald J. 
Published: April 2011

ABSTRACTPlasma CVD allows one a good control of the crystallite size and dielectric tissue in the nc-Si/SiO2 compact films. It is shown that the green/blue PL originates from SiO2 (s) ≡Si-OH groups whereas the red PL is associated with states at the nc-Si/SiO2 interface. With ≥ 1 nm thick SiO2 tissue the efficiency of the red PL increases by more than three orders of magnitude when the crystallite size decreases from about 8 to 2 nm in agreement with the theoretical predictions. If, at a constant crystallite size, the thickness of the SiO2 tissue is decreased to 0.2-0.3 nm, the PL intensity decreases by two orders of magnitude. Possible reasons of this effect are discussed together with the question of optimizing the electroluminescence. Microwave absorption is used to distinguish between free and localized (emitting PL) photogenerated carriers.
Production of Fullerenes from Solar Energy
Authors: Laplaze D. , Bernier P. , Barbedette L. , Flamant G. , Lebrun M. , Brunelle A. , Della-Negra S. 
Published: April 2011

ABSTRACTThe high intensity of solar radiation, obtained with the Odeillo (France) solar furnace facilities, is used to vaporize graphite in inert gas atmosphere. The soot obtained contains C60, C70 and other heavier fullerenes. We discuss the possibility of increasing the evaporation rate of graphite and the yield of soot with this technique. From our last experiments, we obtain a first estimate of the soluble fullerene yield Y ( greater than 12%) and we have shown that 13C enriched fullerenes can be easily produced by this process.
Smaller Carbon Clusters: Linear, Cyclic, Polyhedral
Authors: Slanina Z. , Lee S.-L. , Smigel M. , Kurtz J. , Adamowicz L. 
Published: April 2011

ABSTRACTThe MP2 perturbation treatment with the 6-31G* basis set has been applied to linear, cyclic, and polyhedral structures of Cn, n = 6-13, esp. to C12 in this report. While the linear and cyclic species can co-exist, polyhedral ones are too high in energy. Preliminary results on C8H (related to diffuse interstellar bands) are reported, too.
Constraints on Small Fullerene Helices
Authors: Dunlap Brett. I. 
Published: April 2011

ABSTRACTA single heptagon together with a single pentagon can join two graphene semitubules together at a 30° angle so that all carbon atoms are three-fold coordinated and all other carbon rings are hexagons. This bend connects tubules of complementary classes. A tubule class is the set of all tubules having the same helicity. A tubule class has a uniform density of radii, which is the number of different tubules in the class per unit change in tubule radius. The classes that are joined by a heptagon and pentagon have smallest members whose radii differ by a factor of . Thus tubule segments joined by a heptagon and pentagon cannot have exactly equal circumferences. There are only a finite number of twist angles allowed between sequential bends along a tubule. Pentagons and heptagons should be isolated and separated as far as possible. These requirements particularly constrain the bends and twists that can occur in the smallest tubules. These considerations favor long-range order along a helix, given a driving force for curling during the formation process.
The Fullerene Neighbours
Authors: Slanina Z. , Sun M.-L. , Lee S.-L. , Adamowicz L. 
Published: April 2011

ABSTRACTSemiempirical quantum-chemical calculations are reported for the fullerenic structures C60, Si60, Ge60, N60; B36N24, B36P24, A136N24, A136P24; and various BnNn A new route towards B/N clusters is considered, being based on squares and hexagons. The pattern always requires six squares. The route can produce species of similar or even higher stability comparing to the conventional pentagon/hexagon pattern. Four particular stoichiometries emerge from the available AM1 computations: B12N12, B28N28, B36N36, and B36 N24.
Electronic Structure Calculations of Defect C60 with One or Two Vacancies
Authors: MorÁn-lÓpez J.L. , Dorantes-DÁvila J. , Cabrera-Trujillo J.M. 
Published: April 2011

ABSTRACTThe electronic properties of defect C60 with one or two vacancies, are calculated by using a Hubbard-like Hamiltonian for sp-electrons in the unrestricted Hartree-Fock approximation. Results are given for the cohesive energy and local charge distribution of the different non-equivalent sites. These results might support a possible mechanism to encapsulate atoms in the internal cavities of C60. This mechanism involves the production of C60 molecules with two carbon isotopes AC and BC (A, B = 12,13,14). The molecules AC59BC1 and AC58BC2 are separated from the total production and collected in a chamber under partial pressure of the element to be inserted.
Thermodynamic Properties of the Fcc Modification of Solid Fullerene C60
Authors: Zubov V.I. , Tretiakov N.P. , Rabelo J.N. Teixeira , Ortiz J.F.Sanchez 
Published: April 2011

ABSTRACTWe have applied the correlative method of unsymmetrized self-consistent field to study thermodynamic properties of the FCC phase of solid fullerene C60 which is stable at T≥261 K. We have calculated the temperature dependences of the intermolecular distance, cohesive energy, bulk moduli, thermal expansion coefficient, elastic constants and sound velocities. The central intermolecular potential of Girifalco and its approximation by Yakub have been used. We have infered the crucial role of anharmonicity of lattice vibrations at high temperatures. It has been noted also that unlike other Van der Waals crystals, for the considered modification of C60 the quantum effects are small at any temperature. Our results agree closely with available experimental data. Some properties near the spinodal point are discussed.