Improvement of electrical properties of silicon-based thin-film transistors by modifying technological fabrication processes

This paper is a review of technological process evolution associated to electrical performance improvement of silicon-based thin-film transistors (TFTs) that were performed mainly in the GM/IETR laboratory. The main objective in agreement with the fields of applications is to fabricate TFTs at a temperature low enough to be compatible with the substrates, glass substrates in a first place and flexible substrates in a second one, which implies several approaches. In fact, the electrical properties of the TFTs, mainly field-effect mobility of carriers in the channel, I _on/I _off drain current ratio, and subthreshold slope, are strongly dependent on the quality and the nature of the channel material, on the material quality and thus on the density of states at the interface with the gate insulator, and on the quality of the gate insulator itself. All the improvements are directly linked to all these aspects, which means an actual combination of the efforts. For the glass substrate, compatible technology processes such as deposition techniques, or solid phase, or laser crystallizations of active layers were studied and compared. The paper details all these approaches and electrical performances. In addition, some results about the use of a silicon–germanium compound as channel active layer and airgap transistors for which the insulator is released, complete the presentation of the evolution of the silicon-based TFTs during the last twenty years.     

Simulation of random packing of spherical particles with different size distributions

A numerical model for a loose packing process of spherical particles is presented. The simulation model starts with randomly choosing a sphere according to a pregenerated continuous particle-size distribution, and then dropping the sphere into a dimension-specified box, and obtaining its final position by using dropping and rolling rules which are derived from a similar physical process of spheres dropping in the gravitational field to minimize its gravity potential. Effects of three different particle-size distributions on the packing structure were investigated. Analysis on the physical background of the powder-based manufacturing process is additionally applied to produce optimal packing parameters of bimodal and Gaussian distributions to improve the quality of the fabricated parts. The results showed that higher packing density can be obtained using bimodal size distribution with a particle-size ratio from 1.5 to 2.0 and the mixture composition around n ₂:n ₁=6:4. For particle size with a Gaussian distribution, the particle radii should be limited in a narrow range around 0.67 to 1.5.     

Phenomenological theory of structural relaxation based on a thermorheologically complex relaxation time distribution

The aim of this work is to explore the consequences on the kinetics of structural relaxation of considering a glass-forming system to consist of a series of small but macroscopic relaxing regions that evolve independently from each other towards equilibrium in the glassy state. The result of this assumption is a thermorheologically complex model. In this approach each relaxing zone has been assumed to follow the Scherer-Hodge model for structural relaxation (with the small modification of taking a linear dependence of configurational heat capacity with temperature). The model thus developed contains four fitting parameters. A least-squares search routine has been used to find the set of model parameters that fit simultaneously four DSC thermograms in PVAc after different thermal histories. The computersimulated curves are compared with those obtained with Scherer-Hodge model and the model proposed by Gómez and Monleón. The evolution of the relaxation times during cooling or heating scans and also during isothermal annealing below the glass transition has been analysed. It has been shown that the relaxation times distribution narrows in the glassy state with respect to equilibrium. Isothermal annealing causes this distribution to broaden during the process to finally attain in equilibrium the shape defined at temperatures above T _g .     

Structural,optical and electrical properties of sulfur-incorporated amorphous carbon films

Amorphous carbon–sulfur (a-C:S) composite films were prepared by vapor phase pyrolysis technique. The structural changes in the a-C:S films were investigated by electron microscopy. A powder X-ray diffraction (XRD) study depicts the two-phase nature of a sulfur-incorporated a-C system. The optical bandgap energy shows a decreasing trend with an increase in the sulfur content and preparation temperature. This infers a sulfur incorporation and pyrolysis temperature induced reduction in structural disorder or increase in sp ² or π-sites. The presence of sulfur (S 2p) in the a-C:S sample is analyzed by the X-ray photoelectron spectroscopy (XPS). The sp ³/sp ² hybridization ratio is determined by using the XPS C 1s peak fitting, and the results confirm an increase in sp ² hybrids with sulfur addition to a-C. The electrical resistivity variation in the films depends on both the sulfur concentration and the pyrolysis temperature.     

Kinetics of nonisothermal crystallization in Sn<Subscript>10</Subscript>Sb<Subscript>20−<Emphasis Type="Italic">x</Emphasis></Subscript>Bi<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Se<Subscript>70</Subscript> glassy semiconductors

The crystallization kinetics of Sn₁₀Sb_20−x Bi _x Se₇₀ (x=0,2,4,6,8) chalcogenide system has been studied. Crystallization studies using differential scanning calorimetry under nonisothermal conditions with different heating rates are reported. The glass transition temperature is found to increase with the increase in heating rate as well as with the addition of bismuth. The apparent activation energy for glass transition and that for crystallization have been determined using the Kissinger equation. Thermal stability and glass forming tendency have also been studied.     

Pulse shaping and diffraction properties of multi-layers reflection volume holographic gratings

The multi-layers coupled-wave theory is extended to systematically investigate the pulse shaping and diffraction properties of a system of multi-layers reflection volume holographic gratings (MRVHG) under ultrashort laser pulse (ULP) readout. The combined effects that the grating parameters such as the number and thickness of layers and gaps between them and the pulse duration of the input ULP have on the pulse shaping properties are considered. The pulse profiles of the diffracted and transmitted beams, the diffraction bandwidth, and the total diffraction efficiency are presented. The calculated results we have derived permit an optimal choice of grating parameters for the pulse shaping and process applications.     

An evidence for a microscopic phase separation in AsSeAg glasses revealed by thermal and structural investigations

Alternating Differential Scanning Calorimetric (ADSC) studies show that Se rich As₂₀Se_80-xAg_x (0 ≤ x ≤ 15) glasses exhibit two endothermic glass transitions and two exothermic crystallization peaks; the observed thermal behavior has been understood on the basis that As₂₀Se_80-xAg_x glasses are microscopically phase separated, containing Ag₂Se phases embedded in an As-Se backbone. With increasing silver concentration, the Ag₂Se phase percolates in the As-Se matrix, with a well-defined percolation threshold at x = 8. A signature of this percolation transition is shown up in the thermal behavior, as sudden jumps in the composition dependence of non-reversing enthalpy, ΔH_nr obtained at the second glass transition reaction. Scanning Electron Microscopic (SEM) studies also confirm the microscopic phase separation in these glasses. The super-ionic conduction observed earlier in these glasses at higher silver proportions, is likely to be associated with the silver phase percolation.     

Ultrafast dynamic ellipsometry measurements of early time laser ablation of titanium thin films

The dynamics of laser ablated titanium thin films are investigated using a recently developed technique that measures time-resolved and one-dimensional spatially-resolved ablation dynamics in a single shot. Ultrafast dynamic ellipsometry, a technique based on space-shifted spectral interferometry, uses the time-dependent frequency of a chirped laser pulse to provide time encoding, allowing the picosecond probing of material dynamics in a single shot. With this technique, the sample is probed at two different incident angles with both s- and p-polarized light, which measures the motion of the material and any change in its complex refractive index. Ultrafast dynamic ellipsometry is applied to study the mechanism of initiation by laser-based optical detonators that employ the ablation of titanium thin films. The resulting data indicate that the titanium is ablated as a fragmented flyer and not as an expanding plasma.     

Structural and electronic properties of Ren (${\sf n} \leq$ 8) clusters by density-functional theory

The structures, stabilities and electronic properties of small-sized Re_n (n ≤ 8) clusters have been systematically investigated by density-functional theory. The lowest-energy structures of Re_n clusters favor 3-dimensional configuration. The results of second-order difference of energies indicate that Re₄ and Re₆ possess relatively higher stability in structure. Importantly, our theoretical results of electron affinity are in agreement with experimental values, which can be responsible for the reliability of the structures.     

Transparence and electrical properties of ZnO-based multilayer electrode

Three-layered ZnO/Ag–Ti/ZnO structures were prepared using both the sol-gel technique and DC magnetron sputtering. This study focuses on the electrical and optical properties of the ZnO/Ag–Ti/ZnO multilayers with various thicknesses of the Ag–Ti layer. The ZnO thin film prepared by the sol–gel method was dried at 300°C for 3 minutes, and a fixed thickness of 20 nm was obtained. The thickness of the Ag–Ti thin film was controlled by varying the sputtering time. The Ag–Ti layer substantially reduced the electrical resistivity of the sol–gel-sprayed ZnO thin films. The sheet resistance of the Ag–Ti layer decreased dramatically and then became steady beyond a sputtering time of 60 s. The sputtering time of Ag–Ti thin film deposition was determined to be 60 s, taking into account the optical transmittance. Consequently, the transmittance of the ZnO/Ag–Ti/ZnO multilayer films was 71% at 550 nm and 60% at 350 nm. The sheet resistance was 4.2 Ω/sq.     

Microstructure and electrical conductivity of YSZ thin films prepared by pulsed laser deposition

Yttria-stabilized zirconia (YSZ) is the most common solid electrolyte material used e.g. in ceramic fuel cells. Thin films of YSZ were deposited on c-cut sapphire single crystals by pulsed laser deposition using a KrF excimer laser focused on a polycrystalline 8 mol% Y₂O₃-stabilized ZrO₂ target. Depending on the substrate temperature and the oxygen background pressure during deposition, different microstructures are obtained. XRD and high-resolution SEM revealed the formation of dense amorphous films at room temperature. At 600°C preferentially (111) oriented polycrystalline films consisting of densely agglomerated nm-sized grains of the cubic phase resulted. Grain size and surface roughness could be controlled by varying the oxygen background pressure. RBS and PIXE evidenced congruent transfer only for a low number of pulses, indicating a dynamical change of the target stoichiometry during laser irradiation. The in-plane ionic conductivity of the as-deposited crystalline films was comparable to bulk YSZ. The conductivity of initially amorphous YSZ passes a maximum during the crystallization process. However, the relative changes remain small, i.e. no significant enhancement of ionic conductivity related to the formation of a nanocrystalline microstructure is found.     

Control of electrical resistance of TiO2 films by short-pulse laser irradiation

Titanium dioxide (TiO₂) films were irradiated with a femtosecond laser beam to alter their electrical resistances. The TiO₂ film was produced by aerosol beam deposition. The wavelength, pulse duration, and repetition rate of the femtosecond laser scanned across the sample surface were 800 nm, 100 fs, and 1 kHz, respectively. By attenuating the laser fluence on the TiO₂ film, a range was found in which the electrical resistance of the TiO₂ film was varied even though the morphology of the film surface was not changed.     

Analysis of serum from type II diabetes mellitus and diabetic complication using surface-enhanced Raman spectra (SERS)

In this paper, we show surface-enhanced Raman spectra (SERS) of serums from type II diabetes mellitus and diabetic complication (coronary disease, glaucoma and cerebral infarction), and analyze the SERS through the multivariate statistical methods of principal component analysis (PCA). In particular, we find that there exist many adenines in these serums, which maybe come from DNA (RNA) damage. The relative intensity of the band at 725±2 cm⁻¹ assigned to adenine is higher for patients than for the healthy volunteers; therefore, it can be used as an important ‘fingerprint’ in order to diagnose these diseases. It is also shown that serums from type II diabetes mellitus group, diabetic complication group and healthy volunteers group can be discriminated by PCA.     

Structural phenomena in glassy carbon induced by cobalt ion implantation

Glassy carbon (GC) was implanted by 150 keV Co⁺ ions to the doses of 1×10¹⁶ (low dose) and 1×10¹⁷ ions/cm² (high dose). The low dose implantation results in GC structure disordering with formation of amorphous carbon (a-C). Analysis of Rutherford backscattering (RBS) and Raman spectra has revealed 15 at.% of sp³-bonded C atoms in the a-C structure. The in-pane size of sp² clusters was estimated to be 1.1 nm. On the contrary, the high dose ion implantation results in ordering of the a-C structure. Content of the sp³ atoms in a-C was reduced to about 5% and, respectively, the in-plane sp² cluster size was increased up to 2.8 nm. Together with the a-C structure ordering the Raman spectra identifies formation of transpolyacetylene (TPA)-like chains after the high-dose Co⁺ implantation. In parallel, RBS suggests an enhanced diffusion of the implanted cobalt within the modified carbon layer. Correlation of the RBS and Raman results argues a driving role of cobalt diffusion in the TPA-like chains formation and a-C ordering. Great surface roughening observed after the high dose Co⁺ implantation suggests also the pronounced cobalt clustering causing large flux of “free volume” to the surface.     

Nanostructuring of FePt thin films by plasma focus device: pulsed ion irradiation dependent phase transition and magnetic properties

The effects of the different number (1, 2 and 3) of H⁺ ion irradiation shots on pulsed laser deposited FePt thin films, using pulsed plasma focus device, are investigated. The FePt thin films were exposed to energetic H⁺ ions in a plasma focus device at a fixed distance of 4 cm from the top of central electrode. It was deduced that single shot ion irradiation based transient thermal treatment induces an effect similar to the conventional annealing at 400°C. Well-separated nanoparticles are formed, and the significant enhancement of the coercivity, by about two orders of magnitude, at a lower annealing temperature of 400°C has been observed in the single shot ion irradiated samples. The increase of plasma focus ion irradiation shots lead to the amorphorization in irradiated FePt samples due to excessive energy transfer causing more defects and lattice distortion, and a decreasing coercivity trend in irradiated and annealed samples are observed due to reduction in the texture coefficient of magnetic easy axis (001) orientation fct phase.     

Square-pyramidal iron coordination modules as potential spin switches for the chemisorption on gold

Octahedral iron(II) complexes of a unique pyridine-derived tetrapodal pentadentate polyamine ligand, 2,6-C₅H₃N(CMe[CH₂NH₂]₂)₂, show temperature-dependent spin crossover (SCO) depending on the nature of a sixth monodentate ligand L (imidazol or pyridine derivative). For L = 1-methylimidazol, the redox behaviour of the complex, as determined by cyclic voltammetry, suggests an accompanying ligand exchange. Pyridine-4-thiol and the disulphides: 4-(2-methyldisulphanyl)pyridine, 4-(2-hexadecyldisulphanyl)pyridine and 1,2-bis([pyridine-4-yl]methyl)disulphane, were studied as mono-dentate ligands L, with a view to enable chemisorption of iron(II) complexes on a gold surface. In the case of pyridine-4-thiol, the participation of the thiolate functional group in iron coordination is difficult to suppress, whereas the disulphides enter into yet unrecorded redox chemistry with iron(II), yielding a di-iron(III) complex containing a persulphide bridge (S ₂²⁻).     

Momentum distributions of projectile fragments produced in the cold and hot fragmentation of relativistic136Xe and197Au projectiles

The longitudinal-momentum distributions of projectile fragments from 0.8 A GeV¹³⁶Xe and 1 A GeV¹⁹⁷Au projectiles impinging on targets of beryllium and aluminium, respectively, have been measured using the projectile-fragment separator FRS at GSI. Different momentum distributions have been found for two different classes of fragmentation processes: the abundant hot fragmentation with several nucleons evaporated from the prefragments, and the rare cold fragmentation with only protons removed from the projectile, but no nucleons evaporated. The data are compared to model calculations.This article comprises part of the Ph.D. thesis of B. Voss     

Refined interpretation of Christiansen-filter experiments and neutron scattering lengths of the lead-isotopes

A refined interpretation of Christiansen filter experiments is described, which allows for the effects of inhomogeneities in the powder column of the filter. Using this procedure the evaluation of experiments on enriched samples of lead isotopes provided the neutron coherent scattering lengths (in fm) for the separated isotopes:b (204) =10.6 ± 2.0;b (206)=9.23 ± 0.05;b (207)=9.28 ± 0.04 andb (208)=9.50 ± 0.06. The corresponding potential scattering radius R was obtained by taking account of resonance contributions as earlier used in the determination of the neutron's electric polarizability. The found R=9.74 ± 0.07 fm is in good argreement with the literature. This confirms the correctness of the used resonance contributions.Work partially supported by the Bundesministerium für Forschung und Technologie     

On the competiting role of mean-field and residual interactions in flow observables

Theoretical analyses of heavy-ion reactions are performed in the framework of the semi-classical Landau-Vlasov approach. The incident energies are investigated in the range from intermediate to low energy regimes, where transverse collective motion has been experimentally evidenced. The influence of the equation of state (E.O.S.) parameters on various collective observables is studied in relation with the action of the residual interactions. From the sensitivity to both aspects, and taking into account the experimental biases limitations, our investigation indicates that E.O.S. signatures should be more expected at energies below 100 MeV per nucleon.