Grain boundary diffusion of Cu in TiN film by X-ray photoelectron spectroscopy

In this study, the grain boundary diffusion of Cu through a TiN layer with columnar structure was investigated by X-ray photoelectron spectroscopy (XPS). It was observed that Cu atoms diffuse from the Cu layer to the surface along the grain boundaries in the TiN layer at elevated temperature. In order to estimate the grain boundary diffusion constants, we used the surface accumulation method. The diffusivity of Cu through TiN layer with columnar structure from 400 °C to 650 °C is D_b≈6×10⁻¹¹exp(−0.29/(k_BT )) cm²/s. Received: 18 May 1999 / Accepted: 8 September 1999 / Published online: 23 February 2000     

Enhancement of the electrochemical performance of silicon/carbon composite material for lithium ion batteries

The silicon/carbon (Si/C) composite material was prepared, and the electrochemical performance was investigated as a promising anode material for lithium ion batteries. The results show that the binder in the electrode acts as an important role for improving the reversible capacity of the Si/C materials during cycling. The Si/C electrode with CMC/SBR binder possesses a better cycle performance than that with PVDF binder. The Si/C composite material shows an initial reversible capacity of more than 700 mAh∙g⁻¹ and remains a reversible capacity of 597 mAh g⁻¹ after 40 cycles.     

Plasma-based nitrogen tail pulse shutter for CO2-TEA lidar dial systems

This paper presents the construction, use and characterisation of a laser-induced sealed plasma shutter to clip off the nitrogen pulse tail of a CO₂-TEA laser-based lidar dial system. Investigation of the optimum gas filling pressure, temporal profile of the clipped pulse, and the laser threshold power intensities to achieve ionisation growth and breakdown in helium, argon, and nitrogen are also presented. Values of these power density thresholds lie between 3×10¹¹ W cm^-2–5×10¹² W cm^-2, 2×10¹¹ W cm^-2–2×10¹² W cm^-2 and 3×10¹³ W cm^-2–2×10¹⁴ W cm^-2 for helium, argon, and nitrogen, respectively. The range resolution attainable with the present clipped pulses is 15 m, which is 30 times better than that readily obtained with the nitrogen-tailed pulses. Field measurements of the lidar returns with the clipped pulse from a co-operative target are presented. Received: 27 December 1999 / Revised version: 11 February 2000 / Published online: 27 April 2000     

Aluminum-assisted crystallization and <Emphasis Type="Italic">p</Emphasis>-type doping of polycrystalline Si

Aluminum-doped p-type polycrystalline silicon thin films have been synthesized on glass substrates using an aluminum target in a reactive SiH₄+Ar+H₂ gas mixture at a low substrate temperature of 300 °C through inductively coupled plasma-assisted RF magnetron sputtering. In this process, it is possible to simultaneously co-deposit Si–Al in one layer for crystallization of amorphous silicon, in contrast to the conventional techniques where alternating metal and amorphous Si layers are deposited. The effect of aluminum target power on the structural and electrical properties of polycrystalline Si films is analyzed by X-ray diffraction, Raman spectroscopy, scanning electron microscopy and Hall-effect analysis. It is shown that at an aluminum target power of 100 W, the polycrystalline Si film features a high crystalline fraction of 91%, a vertically aligned columnar structure, a sheet resistance of 20.2 kΩ/□ and a hole concentration of 6.3×10¹⁸ cm⁻³. The underlying mechanism for achieving the semiconductor-quality polycrystalline silicon thin films at a low substrate temperature of 300 °C is proposed.     

Power factor of very thin thermoelectric layers of different thickness prepared by laser ablation

Room temperature conductivity and the Seebeck coefficient of thin layers prepared by laser ablation from Bi₂Te₃ target were explored. The power factor was calculated for samples prepared at substrate temperature of 360°C with the density of the laser beam 5 J cm⁻² and at substrate temperature of 410°C with the density of the laser beam 2 J cm⁻² during the deposition. Oscillations of the conductivity and the power factor with the layer thickness were observed at room temperature. The oscillations of conductivity were also verified at the temperature of 77 K. The period of oscillations depends on the preparation conditions. This behavior has been theoretically explained by the quantum size effect in the layers containing different phases and in addition, it was demonstrated by the X-ray Diffraction measurement. The behavior of the power factor of the layers is compared to the behavior of the figure of merit of the layers published earlier.     

Visualization of focusing–refocusing cycles during filamentation in BaF<Subscript>2</Subscript>

Filamentation occurs within a 1.5 cm-long crystal of BaF₂ during the propagation of intense, ultrashort (40 fs) pulses of 800 nm light; a systematic study as a function of incident power enables us to extract quantitative information on laser intensity within the condensed medium, the electron density and the six-photon absorption cross section. At low incident power, a single filament is formed within the crystal; two or more filaments are observed along the direction transverse to laser propagation at higher incident powers. Further, due to fluorescence from six-photon absorption (6PA), we are able to map the intensity variation in the focusing–refocusing cycles along the direction of laser propagation. At still higher incident powers, we observe splitting of multiple filaments. By measuring the radius (L _min) of single filament inside BaF₂, we obtain estimates of peak intensities (I _max) and electron densities (ρ _max) to be 3.26×10¹³ W cm⁻² and 2.81×10¹⁹ cm⁻³, respectively. Use of these values enables us to deduce that the 6PA cross-section in BaF₂ is 0.33×10⁻⁷⁰ cm¹² W⁻⁶ s⁻¹.     

Laser forward transfer of silver electrodes for organic thin-film transistors

Organic thin-film transistors (OTFTs) with top- and bottom-contact configurations were fabricated using silver nano-inks printed by laser forward transfer for the gate and source/drain electrodes with pentacene and poly-4-vinylphenol as the organic semiconductor and dielectric layers, respectively. The volume of the laser-printed Ag pixels was typically in the subpicoliter (0.2–0.4 pl) range. The top-contact OTFTs resulted in lower contact resistance compared to those obtained from the bottom-contact OTFTs, and showed improved overall device performance. The top-contact OTFTs exhibited field-effect mobilities of ∼0.16 cm² V⁻¹ s⁻¹ and on/off current ratios of ∼10⁵.     

Electrochemical behavior of silicon compound in LiF–NaF–KF–Na<Subscript>2</Subscript>SiF<Subscript>6</Subscript> molten salt

The electrochemical reduction and nucleation process of Si⁴⁺ on an electrical steel electrode in the eutectic LiF–NaF–KF molten salt were investigated at 750 °C, by means of cyclic voltammetry and chronoamperometry technique. Silicon was electrodeposited on steel, and Fe₃Si was formed by the diffusivity of silicon on the electrode surface. The electrochemical reduction of Si⁴⁺ process in single-step charge transfer and the cathode process was reversible. The electrocrystallization process of silicon is controlled by progressive three-dimensional mechanism. The diffusion coefficient was calculated to be 5.42 × 10⁻⁷ cm²/s by chronopotentiometry at experimental conditions.     

Conversion electron Mössbauer spectrometric study of β-FeOOH formation on iron in sodium chloride solution

CEMS was applied to the study on the formation of β-FeOOH on iron immersed in a NaCl solution. It was found that the control of supply of dissolved oxygen to the initial corrision layers under the presence of chloride ions is very important to produce β-FeOOH, and that β-FeOOH formed as rust layers has large stress. The restriction of diffusion of dissolved oxygen into the intermediate rust layers through the top γ-FeOOH layers formed at the initial stage of corrosion is considered to facilitate in the formation of iron oxyhydroxide complex containing Cl⁻.     

Effects of deposition temperature and post-annealing on structure and electrical properties in (La<Subscript>0.5</Subscript>Sr<Subscript>0.5</Subscript>)CoO<Subscript>3</Subscript> films grown on silicon substrate

(La_0.5Sr_0.5)CoO₃ (LSCO) thin films have been fabricated on silicon substrate by the pulsed laser deposition method. The effects of substrate temperature and post-annealing condition on the structural and electrical properties are investigated. The samples grown above 650°C are fully crystalline with perovskite structure. The film deposited at 700°C has columnar growth with electrical resistivity of about 1.99×10⁻³ Ω cm. The amorphous films grown at 500°C were post-annealed at different conditions. The sample post-annealed at 700°C and 10⁻⁴ Pa has similar microstructure with the sample in situ grown at 700°C and 25 Pa. However, the electrical resistivity of the post-annealed sample is one magnitude higher than that of the in situ grown sample because of the effect of oxygen vacancy. The temperature dependence of resistivity exhibits semiconductor-like character. It was found that post-annealing by rapid thermal process will result in film cracks due to the thermal stress. The results are referential for the applications of LSCO in microelectronic devices.     

Fabrication and characterisation of microscale air bridges in?conductive gallium nitride

Fabrication and electrical characterisation of microscale air bridges consisting of GaN heavily doped with silicon is described. These were made from GaN–AlInN–GaN epitaxial trilayers on sapphire substrates, in which the AlInN was close to the composition lattice matched to GaN at ∼17% InN fraction. The start of the fabrication sequence used inductively coupled plasma etching with chlorine chemistry to define mesas. In situ monitoring by laser reflectometry indicated an AlInN vertical etch rate of 400 nm/minute, ∼70% of the etch rate of GaN. Processing was completed by lateral wet etching of the AlInN in hot nitric acid to leave GaN microbridges supported between anchor posts at both ends. Deposition of Ti–Au contact pads onto the anchor posts allowed study of the electrical characteristics. At low applied voltages, vertical conduction through the undoped AlInN layers was minimal in comparison with the current path through the Si:GaN bridges. Typical structures showed highly linear current-voltage characteristics at low applied voltages, and had resistances of 1050 Ω. The observed resistance values are compared with the predicted value based on materials parameters and an idealised geometry. The microbridges showed damage from Joule heating only at current densities above 2×10⁵ A cm⁻².     

Nanocrystalline silicon thin-film transistors with 50-nm-thick deposited channel layer, 10 cm2V-1s-1 electron mobility and 108 on/off current ratio

Thin-film transistors were made using 50-nm-thick directly deposited nanocrystalline silicon channel layers. The transistors have a coplanar top gate structure. The nanocrystalline silicon was deposited from discharges in silane, hydrogen and silicon tetrafluoride. The transistors combine a high electron field effect mobility of ∼10 cm² V^-1s^-1 with a low ‘off’ current of ∼10^-14 A per μm of channel length and an ‘on’/‘off’ current ratio of ∼10⁸. This result shows that transistors made from directly deposited silicon can combine high mobility with low ‘off’ currents. Received: 28 May 2001 / Accepted: 30 May 2001 / Published online: 30 August 2001     

Surface oxides produced during discharge in water ambient iron surface: A conversion electron Mössbauer spectroscopic study

Oxide layers are produced on iron surface, under discharge in water ambient, applying both anodic and cathodic potentials to iron foil. Non-stoichiometric Fe_1−xO containing different ratios of Fe³⁺ and Fe²⁺ has been detected by using CEMS. Corrosion of these samples in 0.5 M H₂SO₄ shows that, cathodically discharge treated iron foil has higher corrosion resistance as compared to the anodically treated sample. This is attributed to the microstructural differences in the Fe_1−xO_x films formed during the discharge.     

ESCA studies of Si-Fe alloys

ESCA examination of films formed on Si-Fe alloys after immersion in 0.1 M NaCl for 24 h has hown that the thickness of passive films decreased with an increase in silicon content. A thick passive film containing oxidized silicon and oxidized iron was formed on Fe-20 wt% Si and the oxidized iron was about three times higher than the oxidized silicon in the passive film. However, an obvious reduction in the oxidized iron in the film on Fe-30 wt % Si was observed. Oxidized iron was detected up to a depth of 1.0 nm and at a depth greater than 1.0 nm from the surface, the film was exclusively in oxidized silicon. The film was exclusively silicon oxide when the silicon content was increased to 50 wt %. Electrochemical techniques according to ASTM G59 and ASTM G5 were used for the determination of the relative corrosion rate. Fe-50 wt % Si was found to have a corrosion rate smaller than those lower silicon alloys. This relates to the surface film composition and structure as determined by ESCA.     

MeV Al+ and Al2+ ions implantation in Si(100): surface roughness and defects in the bulk

This paper deals with the implantation of high-energy (1.0–3.0 MeV) atomic and molecular Al⁺ ions in Si(100) to a fluence of 5×10¹⁴ Al atoms/cm² at room temperature. The molecular effect, i.e. the increase of the displacement yield compared with the sum of the atomic yields, and the damage formation as well as defect behaviour after annealing have been investigated. A detailed experimental study has been made of the evolution of extended secondary defects which form during thermal anneals of Al⁺ or Al₂ ⁺ irradiated silicon. The samples have been examined using combined Rutherford backscattering and channeling experiments together with transmission electron microscopy observations. The surface structure of the implanted wafers has been measured by atomic force microscopy. The results for the implantation-induced roughness at the Si surface, resulting from Al⁺ or Al₂ ⁺ irradiation at the same energy/atom, total atomic fluence, flux rate, and irradiation temperature, are presented and discussed. Received: 19 August 1999 / Accepted: 20 October 1999 / Published online: 23 February 2000     

Production and characterization of Nd,Cr:GSGG thin films on Si(001) grown by pulsed laser ablation

Nd,Cr:Gd₃Sc₂Ga₃O₁₂ (GSGG) thin films have been produced for the first time. They were grown on Si(001) substrates at 650 °C by pulsed laser ablation at 248 nm of a crystalline Nd,Cr:GSGG target rod. The laser plume was analyzed using time-of-flight quadrupole mass spectroscopy, and consisted of elemental and metal oxide fragments with kinetic energies typically in the range 10 to 40 eV, though extending up to 100 eV. Although films deposited in vacuum using laser fluences of 0.8±0.1 J cm⁻² reproduced the Nd,Cr:GSGG bulk stoichiometry, those deposited using fluences above ≈3 J cm⁻² resulted in noncongruent material transfer and were deficient in Ga and Cr. Attempts to grow films using synchronized oxygen or oxygen/argon pulses yielded mixed oxide phases. Under optimal growth conditions, the films were heteroepitaxial, with GSGG(001)[100]∥Si(001)[100], and exhibited Volmer–Weber-type growth. Room-temperature emission spectra of the films suggest efficient non-radiative energy transfer between Cr³⁺ and Nd³⁺ ions, similar to that of the bulk crystal. Received: 1 October 1999 / Accepted: 15 October 1999 / Published online: 23 February 2000     

Resonance photoacoustic spectroscopy and gas analysis of gaseous flow at reduced pressure

The results of theoretical and experimental studies of sensitivity of a resonant photoacoustic Helmholtz resonator detector for gas flowing through a photoacoustic cell under reduced pressure are presented. The measurements of the sensitivity and ultimate sensitivity of the differential photoacoustic cell were performed with a near-IR room-temperature diode laser using the well-known H₂O absorption line (12496.1056 cm^-1) as a reference. The measured value of the sensitivity (6–17 Pa W m^-1) is in satisfactory agreement with the calculated one, which equals 6–35 Pa W m^-1. The obtained value of the ultimate sensitivity ((3–5)×10^-7 W m^-1 Hz^-1/2) provides measurements of the concentration of molecules at the ppb–ppm level. Received: 19 April 2001 / Revised version: 18 September 2001 / Published online: 7 November 2001     

Preparation and characterizations of PVAc/P(VdF-HFP)-based polymer blend electrolytes

A novel group of polymer blend electrolytes based on the mixture of poly(vinyl acetate) (PVAc), poly(vinylidene fluoride-hexafluoropropylene) (PVdF-HFP), and the lithium salt (LiClO₄) are prepared by solvent casting technique. Ionic conductivity of the polymer blend electrolytes has been investigated by varying the PVAc and PVdF-HFP content in the polymer matrix. The maximum ionic conductivity has been obtained as 0.527 × 10⁻⁴ Scm⁻¹ at 303 K for PVAc/PVdF-HFP ((25/75) wt.%)/LiClO₄ (8 wt.%). The complex formations ascertained from XRD and FTIR spectroscopic techniques and the thermal behavior of the prepared samples has been performed by DSC analysis. The surface morphology and the surface roughness are studied using SEM and AFM scanning techniques respectively.     

Formation of high-conductivity NiSi2 layers on Si at zero-mismatch temperature by high-current Ni-ion implantation

2 and Si lattices at 380 °C, which was defined as zero-mismatch temperature. The implantation was conducted with a metal vapor vacuum arc (MEVVA) ion implanter at an extraction voltage of 45 kV. Based on a thermal conduction estimation, a temperature rise of 380 °C required the Ni-ion current density to be 35 μA/cm². For the Si(111) wafers, the high conducting NiSi₂ layers were indeed directly formed after Ni-ion implantation with this specific current density to a normal dose of 2×10¹⁷ ions/cm² and the resistivity was as low as 9 μΩ cm. For the Si(111) wafers pre-covered with a 10-nm Ni overlayer, the resistivity of the NiSi₂ layers obtained under the same conditions decreased down to about 6 μΩ cm. The superior electrical property of the NiSi₂ was thought to be related to its formation temperature, i.e. at a zero-mismatch temperature of 380 °C, which resulted in minimizing the stress and stress-induced defects involved in its formation as well as cooling process. Received: 27 April 1998 / Accepted: 26 October 1998     

Structure,thermal properties,conductivity and electrochemical stability of di-urethanesil hybrids doped with LiCF<Subscript>3</Subscript>SO<Subscript>3</Subscript>

Variable chain length di-urethane cross-linked poly(oxyethylene) (POE)/siloxane hybrid networks were prepared by application of a sol-gel strategy. These materials, designated as di-urethanesils (represented as d-Ut(Y′), where Y′ indicates the average molecular weight of the polymer segment), were doped with lithium triflate (LiCF₃SO₃). The two host hybrid matrices used, d-Ut(300) and d-Ut(600), incorporate POE chains with approximately 6 and 13 (OCH₂CH₂) repeat units, respectively. All the samples studied, with compositions ∞ > n ≥ 1 (where n is the molar ratio of (OCH₂CH₂) repeat units per Li⁺), are entirely amorphous. The di-urethanesils are thermally stable up to at least 200 °C. At room temperature the conductivity maxima of the d-Ut(300)- and d-Ut(600)-based di-urethanesil families are located at n = 1 (approximately 2.0 × 10⁻⁶ and 7.4 × 10⁻⁵ Scm⁻¹, respectively). At about 100 °C, both these samples also exhibit the highest conductivity of the two electrolyte systems (approximately 1.6 × 10⁻⁴ and 1.0 × 10⁻³ Scm⁻¹, respectively). The d-Ut(600)-based xerogel with n = 1 displays excellent redox stability.