Phase formation at rapid thermal annealing of Al/Ti/Ni ohmic contacts on 4H-SiC

Ohmic contacts to the top p-type layers of 4H-SiC p⁺–n–n⁺ epitaxial structures having an acceptor concentration lower than 1×10¹⁹ cm⁻³ were fabricated by the rapid thermal anneal of multilayer Al/Ti/Pt/Ni metal composition. The rapid thermal anneal of multilayer A1/Ti/Pt/Ni metal composition led to the formation of duplex cermet composition containing Ni₂Si and TiC phases. The decomposition of the SiC under the contact was found to be down to a depth of about 100 nm. The contacts exhibited a contact resistivity R_c of 9×10⁻⁵ Ω cm⁻² at 21°C, decreasing to 3.1×10⁻⁵ Ω cm⁻² at 186°C. It was found that thermionic emission through the barrier having a height of 0.097 eV is the predominant current transport mechanism in the fabricated contacts.     

Growth and spectroscopy of Dy doped in ZnWO4 crystal

Single-crystal ZnWO₄:Dy³⁺ was grown by Czochralski technique. The XRD, absorption spectra as well as fluorescence spectrum are investigated and the Judd–Ofelt intensity parameters Ω₂, Ω₄, Ω₆ are obtained to be 7.76 × 10⁻²⁰ cm², 0.57 × 10⁻²⁰ cm², 0.31 × 10⁻²⁰ cm², respectively. Calculated radiative transition rate, branching ratios and radiative lifetime for different transition levels of ZnWO₄:Dy³⁺ crystals are presented. Fluorescence lifetime of ⁴F_9/2 level is 158 μs and quantum efficiency is 66%.The most intense fluorescence line at 575 nm correlative with transition ⁴F_9/2 → ⁶H_13/2 is potentially for application of yellow lasers.     

Effects of reduction treatment on the photorefractive properties of Pb0.5Ba0.5Nb2O6

Lead barium niobate is a new photorefractive material of high interest for a variety of applications including holographic storage. Pb_0.5Ba_0.5Nb₂O₆ crystals have been grown by the Bridgman method, and the effects of heat treatments on their photorefractive properties were investigated using Ar ion laser at λ=514.5 nm. The color and absorption spectrum of the crystals varied depending on the oxygen partial pressure during heat treatment. The oxygen diffusivity was estimated to be in the order of 10⁻⁶ and 10⁻⁵ cm²/h at 425 and 550 °C, respectively. Reduction treatment at an oxygen pressure of 215 mTorr increased the effective density of photorefractive charges about three times from 8.0×10¹⁵ to 2.2×10¹⁶ cm⁻³ and made the charge transport more electron-dominant. As a result, the maximum gain coefficient improved from 5.5 to 13.8 cm⁻¹. A diffraction efficiency as high as 70% was achieved in a reduced crystal.     

Dielectric properties of AlN films prepared by laser-induced chemical vapour deposition

The dielectric properties and electrical conductivity of AlN films deposited by laser-induced chemical vapour deposition (LCVD) are studied for a range of growth conditions. The static dielectric constant is 8.0 ± 0.2 over the frequency range 10²−10⁷ Hz and breakdown electric fields better than 10⁶ V cm⁻¹ are found for all films grown at temperatures above 130°C. The resistivity of the films grown under optimum conditions (substrate temperature above 170°C, NH₃/TMA flow rate ratio greater than 300 and a deposition pressure of 1–2 Torr) is about 10¹⁴ Ω cm and two conduction mechanisms can be identified. At low fields, F < 5 × 10⁵ V cm⁻¹ and conductivity is ohmic with a temperature dependence showing a thermal activation energy of 50–100 meV, compatible with the presumed shallow donor-like states. At high fields, F > 1 × 10⁶ V cm⁻¹, a Poole-Frenkel (field-induced emission) process dominates, with electrons activated from traps at about 0.7–1.2 eV below the conduction band edge. A trap in this depth region is well-known in AlN. At fields between 4 and 7 × 10⁵ V cm⁻¹ both conduction paths contribute significantly. The degradation of properties under non-ideal growth conditions of low temperature or low precursor V/III ratio is described.     

Charge trapping in polymer diodes

We report studies focusing on the nature of trap states present in single layer ITO/poly(phenylene vinylene)/Al light emitting diodes. At high applied bias the IV characteristics from 11 to 290 K can be successfully modelled by space charge limited current (SCLC) theory with an exponential trap distribution, giving a trap density H_t of 4(±2) × 10¹⁷ cm⁻³, μ_p, between 10⁻⁶ and 5 × 10⁻⁸ cm² V⁻¹ s⁻¹ and a characteristic energy E_t of 0.15 eV at high temperatures. The transient conductance follows a power-law relationship with time whose decay rate decreases with decreasing temperature. This can be directly related to the emptying of the trap distribution found in the SCLC analysis. Due to variations in structure, conformation and environment, the polymer LUMO and HOMO density of states form Gaussian distributions of chain sites. The deep sites in the tail of the distributions are the observed traps for both positive and negative carriers. The same sites dominate the photo- and electroluminescence emission. This implies that the emissive layer in organic LED's should be made as structurally disordered as possible.     

Quantitative analysis of tungsten, oxygen and carbon concentrations in the microcrystalline silicon films deposited by hot-wire CVD

In order for hot-wire chemical vapor deposition to compete with the conventional plasma-enhanced chemical vapor deposition technique for the deposition of microcrystalline silicon, a number of key scientific problems should be cleared up. Among these points, the concentration of tungsten (nature of the filament), as well as the concentration of oxygen and carbon (elements issued when vacuum is broken between two runs), should not exceed threshold values, beyond which electronic properties of the films could be degraded, as in the case of monocrystalline silicon. Quantitative chemical analysis of these elements has been carried out using the secondary ion mass spectrometry technique through depth profiles. It has been shown that for a high effective filament surface area (S_f=27 cm²), the W content increases steadily from 5×10¹⁴ to 2×10¹⁸ atoms cm⁻³ when the filament temperature T_f increases from 1500 to 1800 °C. For a fixed T_f, the W content increases with the effective surface area S_f. Thus, considering our reactor geometry, the W content does not exceed the detection limit (5×10¹⁴ atoms cm⁻³) when T_f and S_f are limited to 1600 °C and 4 cm², respectively. For O and C elements, under deposition conditions of high dilution of silane in hydrogen (96%), O and C concentrations approaching 10²⁰ atoms cm⁻³ have been obtained. The introduction of an inner vessel inside the reactor, the addition of a load-lock chamber and a decrease in substrate temperature to 300 °C have led to a drastic decrease in these contents down to 3×10¹⁸ atoms cm⁻³, compatible with the realization of 6% efficiency HWCVD μc-Si:H solar cells.     

Structure-composition-property dependence in reactive magnetron sputtered ZnO thin films

Thin films of zinc oxide (ZnO) were prepared by dc reactive magnetron sputtering on glass substrates at various oxygen partial pressures in the range 1×10⁻⁴–6×10⁻³ mbar and substrate temperatures in the range 548–723 K. The variation of cathode potential of zinc target on the oxygen partial pressure was explained in terms of target poisoning effects. The stoichiometry of the films has improved with the increase in the oxygen partial pressure. The films were polycrystalline with wurtzite structure. The films formed at higher substrate temperatures were (0 0 2) oriented. The temperature dependence of Hall mobility of the films formed at various substrate temperatures indicated that the grain boundary scattering of charge carriers was predominant electrical conduction mechanism in these films. The optical band gap of the films increased with the increase of substrate temperature. The ZnO films formed under optimized oxygen partial pressure of 1×10⁻³ mbar and substrate temperature of 663 K exhibited low electrical resistivity of 6.9×10⁻² Ω cm, high visible optical transmittance of 83%, optical band gap of 3.28 eV and a figure of merit of 78 Ω⁻¹ cm⁻¹.     

Reactions in Au/Nb bilayer thin films

The interdiffusion and intermetallic compound formation of Au/Nb bilayer thin films annealed at 200–400 °C have been investigated. The bilayer thin films were prepared by electron beam deposition. The Nb film was 50 nm thick and the Au film was 50–200 nm thick. The interdiffusion of annealed specimens was examined by measuring the electrical resistance and depth-composition profile and by transmission electron microscopy. Interdiffusion between the thin films was detected at temperatures above 325 °C in a vacuum of 10^-4 Pa. The intermetallic compound Au₂Nb₃ and other unknown phases form during annealing at over 400 °C. The apparent diffusion constants, determined from the penetration depth for annealing at 350 °C, are 3.5 × 10⁻¹⁵ m² s⁻¹ for Nb in Au and 8.6 × 110^7minus;15 m² s⁻¹ for Au in Nb. The Au surface of the bilayer films becomes uneven after annealing at over 400 °C due to the reaction.     

Al2O3 formation on Si by catalytic chemical vapor deposition

Catalytic chemical vapor deposition (Cat-CVD) has been developed to deposit alumina (Al₂O₃) thin films on silicon (Si) crystals using N₂ bubbled tri-methyl aluminum [Al(CH₃)₃, TMA] and molecular oxygen (O₂) as source species and tungsten wires as a catalyzer. The catalyzer dissociated TMA at approximately 600 °C. The maximum deposition rate was 18 nm min⁻¹ at a catalyzer temperature of 1000 °C and substrate temperature of 800 °C. Metal oxide semiconductor (MOS) diodes were fabricated using gates composed of 32.5-nm-thick alumina film deposited at a substrate temperature of 400 °C. The capacitance measurements resulted in a relative dielectric constant of 7.4, fixed charge density of 1.74×10¹² cm⁻², small hysteresis voltage of 0.12 V, and very few interface trapping charges. The leakage current was 5.01×10⁻⁷ A cm⁻² at a gate bias of 1 V.     

Preparation of PbO nanoparticles by microwave irradiation and their application to Pb(II)-selective electrode based on cellulose acetate

Nanosized lead oxide particles were prepared by thermal decomposition of lead hydroxycarbonate synthesized under microwave irradiation. Urea and lead nitrate were used as the starting materials. Microstructure and morphology of the products were investigated by means of XRD, AFM, TEM, and IR absorption spectra. The results indicated that well crystallized, finely dispersed and spherical -PbO nanoparticles with a size of ca. 30 nm were obtained. Meanwhile, an orthogonal phase β-PbO with a size of ca. 38 nm was also obtained when the calcinations temperature was up to 600 °C. In addition, a Pb(II)-selective electrode based on cellulose acetate was prepared using nanosized -PbO powders synthesized. The electrode exhibited a Nernstian slope of 29±1 mV per decade in a linear range of 2.5×10⁻⁵ mol L⁻¹ to 1.0×10⁻¹ mol L⁻¹ for Pb²⁺ ion. The detection limit of this electrode is down to 8.0×10⁻⁶ mol L⁻¹. This sensor has a short response time of about 10 s and could be used in a pH range of 2.0–8.0. High selectivity was obtained over a wide variety of metal ions.     

Chemical effect of CH4 implantation into iron, nickel and aluminum thin films

Polycrystalline thin films of iron, nickel and aluminium were bombarded with CH₄⁺ ions in the dose range from 1 × 10¹⁶ to 1.2 × 10¹⁸ ions cm⁻² at room temperature and with energies between 15 and 50 keV. The formation of carbides was indicated in the cases of both iron and nickel from high voltage transmission electron micrographs and selected area diffraction patterns. No such compound formation in the case of aluminium could be detected. The carbides of iron and nickel were found to be stable on annealing up to 350 °C for 2 h.     

The optical and electrical properties of copper indium di-selenide thin films

Thin films of copper indium di-selenide (CIS) with a wide range of compositions near stoichiometry have been formed on glass substrates in vacuum by the stacked elemental layer (SEL) deposition technique. The compositional and optical properties of the films have been measured by proton-induced X-ray emission (PIXE) and spectrophotometry (photon wavelength range of 300–2500 nm), respectively. Electrical conductivity (σ), charge-carrier concentration (n), and Hall mobility (μ_H) were measured at temperatures ranging from 143 to 400 K. It was found that more indium-rich films have higher energy gaps than less indium-rich ones while more Cu-rich films have lower energy gaps than less Cu-rich films. The sub-bandgap absorption of photons is minimum in the samples having Cu/In ≈ 1 and it again decreases, as Cu/In ratio becomes less than 0.60. Indium-rich films show n-type conductivities while near-stoichiometric and copper-rich films have p-type conductivities. At 300 K σ, n and μ_H of the films vary from 2.15 × 10⁻³ to 1.60 × 10⁻¹ (Ω cm)⁻¹, 2.28 × 10¹⁵ to 5.74 × 10¹⁷ cm⁻³ and 1.74 to 5.88 cm² (V s)⁻¹, respectively, and are dependent on the composition of the films. All the films were found to be non-degenerate. The ionization energies for acceptors and donors vary between 12 and 24, and 3 and 8 meV, respectively, and they are correlated well with the Cu/In ratios. The crystallites of the films were found to be partially depleted in charge carriers.     

Transparent conductive In2O3:Mo thin films prepared by reactive direct current magnetron sputtering at room temperature

An amorphous transparent conductive oxide thin film of molybdenum-doped indium oxide (IMO) was prepared by reactive direct current magnetron sputtering at room temperature. The films formed on glass microscope slides show good electrical and optical properties: the low resistivity of 5.9 × 10^− 4 Ω cm, the carrier concentration of 5.2 × 10²⁰ cm^− 3, the carrier mobility of 20.2 cm² V^− 1 s^− 1, and an average visible transmittance of about 90.1%. The investigation reveals that oxygen content influences greatly the carrier concentration and then the photoelectrical properties of the films. Atomic force microscope evaluation shows that the IMO film with uniform particle size and smooth surface in terms of root mean square of 0.8 nm was obtained.     

Influence of conventional furnace and rapid thermal annealing on the quality of polycrystalline β-FeSi2 thin films grown from vapor-deposited Fe/Si multilayers

Thin films of polycrystalline β-FeSi₂ were grown on (100) Si substrates of high resistivity by electron beam evaporation of Si/Fe ultrathin multilayers and subsequent annealing by conventional vacuum furnace (CVF) and rapid thermal annealing (RTA) for 1 h and 30 s, respectively, in the temperature range from 600 to 900°C. X-ray diffraction, Raman spectroscopy, spectroscopic ellipsometry, resistivity and Hall measurements were employed for characterization of the silicide layers quality in terms of the annealing conditions. For the silicide layers prepared by CVF annealing, although the grain size increase with increasing the annealing temperature, the optimum temperature to obtain the higher material quality (carrier mobility of the order of 100 cm² Vs⁻¹ and carrier concentration of about 1 × 10¹⁷ cm⁻³) is about 700°C. At higher annealing temperatures, the quality of the material is degraded due to the presence of the oxide Fe₂O₃. In the case of the silicides prepared by RTA, the quality of the material is improved progressively with increasing the annealing temperature up to 900°C.     

Excited-state absorption at 1.57 μm in U:CaF2 and Co:ZnSe saturable absorbers

Appreciable excited-state absorption (ESA) in U²⁺:CaF₂ and Co²⁺:ZnSe saturable absorbers was measured at λ=1.573 μm by optical transmission versus light fluence curves of 30–40 ns long pulses. The ground- and excited-state absorption cross-sections obtained were (9.15±0.3)×10⁻²⁰ and (3.6±0.2)×10⁻²⁰ cm², respectively, for U²⁺:CaF₂, and (57±4)×10⁻²⁰ and (12.5±1)×10⁻²⁰ cm² for Co²⁺:ZnSe. Thus, ESA is not negligible in U²⁺:CaF₂ and Co²⁺:ZnSe, as previously estimated.     

Mechanical stability of Ca65Mg15Zn20 bulk metallic glass during deformation in the supercooled liquid region

Ca₆₅Mg₁₅Zn₂₀ bulk metallic glass (BMG) samples of dimensions 3.2 mm × 7 mm × 125 mm were prepared using a low-pressure die casting technique. These samples were ground to produce tensile test pieces in compliance with ASTM E8-04. This work is the first reported study of the tensile behaviour of Ca₆₅Mg₁₅Zn₂₀ BMG in the supercooled liquid region (105–120 °C). Two deformation conditions were used for testing: (i) constant strain rate testing from 10⁻³ to 10⁻⁴ s⁻¹ and (ii) constant load testing using loads of 20–50 N applied to a tensile sample during heating at a constant rate of 5 °C s⁻¹. The maximum elongation to failure in the BMG was in excess of 850% for constant load testing although, under isothermal testing conditions, most samples failed after 200% elongation. It is concluded that large superplastic elongations (>500%) during isothermal tensile straining is difficult in this alloy due to the onset of crystallization.     

Effect of silver addition on the properties of combustion synthesized nanocrystalline LiCoO2

Nanocrystalline (50 nm) LiCoO₂ powders containing 0–10 mol% of Ag have been prepared by combustion synthesis using citrate–nitrate combustion route. Thermal analyses show a sharp decomposition of the gel at 177 °C for pristine LiCoO₂. With addition of silver, the decomposition becomes sluggish and it completes only above 430 °C. X-ray powder diffraction analyses show an increase in lattice parameter, c, with increasing Ag content suggesting the occupation of Ag within LiCoO₂ interlayer spacings. Transmission electron microscopy indicates diffusion of Ag into LiCoO₂ grains. It has been observed that adding 1.0 mol% silver increases the room temperature electrical conductivity by more than two orders of magnitude (1.5 × 10⁻³ S cm⁻¹). Galvanostatic charge–discharge profiles of coin cells fabricated with the synthesized powders show a two-fold enhancement in the discharge capacity for 1.0 mol% Ag-added LiCoO₂ cathode (140 mAh g⁻¹) compared to that for pristine LiCoO₂ (70 mAh g⁻¹).     

Control of Al-implantation doping in 4H–SiC

We report results of high-dose Al-ion implantation in 4H–SiC. Using multiple energy implantation techniques, box profiles were realized with targeted concentrations: 3.33×10¹⁸ to 10²¹ cm⁻³. The depths were 190 and 420 nm. The implantation energies ranged from 30 to 200 keV. The implantation and annealing temperatures were 650 and 1670°C, respectively. First, infrared investigations were done to assess the surface quality of the samples before and after annealing. Next, the conduction mechanism was investigated. Performing Hall measurements, we found that the room temperature free hole concentration varies like p_H=C_t/105 (cm⁻³), where C_t is the targeted Al-concentration, with a high level of electronic mobility. For the targeted concentration 10²¹ cm⁻³, this resulted in an active layer with 95 mΩ cm resistivity and, at room temperature, a free hole concentration of 10¹⁹ cm⁻³.     

Molecular dynamics study of the mechanical behavior of nickel nanowire: Strain rate effects

We present the analysis of uniaxial deformation of nickel nanowires using molecular dynamics simulations, and address the strain rate effects on mechanical responses and deformation behavior. The applied strain rate is ranging from 1 × 10⁸ s⁻¹ to 1.4 × 10¹¹ s⁻¹. The results show that two critical strain rates, i.e., 5 × 10⁹ s⁻¹ and 8 × 10¹⁰ s⁻¹, are observed to play a pivotal role in switching between plastic deformation modes. At strain rate below 5 × 10⁹ s⁻¹, Ni nanowire maintains its crystalline structure with neck occurring at the end of loading, and the plastic deformation is characterized by {1 1 1} slippages associated with Shockley partial dislocations and rearrangements of atoms close to necking region. At strain rate above 8 × 10¹⁰ s⁻¹, Ni nanowire transforms from a fcc crystal into a completely amorphous state once beyond the yield point, and hereafter it deforms uniformly without obvious necking until the end of simulation. For strain rate between 5 × 10⁹ s⁻¹ and 8 × 10¹⁰ s⁻¹, only part of the nanowire exhibits amorphous state after yielding while the other part remains crystalline state. Both the {1 1 1} slippages in ordered region and homogenous deformation in amorphous region contribute to the plastic deformation.     

Anomalous elastic response of amorphous alloys suggesting collective motions of many atoms

The dynamic Young’s modulus, E, of amorphous (a-) Zr₆₀Cu₃₀Al₁₀ (numbers indicate at.%) alloy was measured as a function of frequency, f, with a strain amplitude, _t, of 10⁻⁶, E(10⁻⁶,f), and also as a function of _t for f near 10² Hz, E(_t,10² Hz), by means of the vibrating reed methods. The elasticity study under the passing of electric current (PEC) was carried out too. E(10⁻⁶,f) is lower than E₀ for f between 10 and 10⁴ Hz showing local minima near 5×10, 5×10² and 5×10³ Hz, which are indicative of the resonant collective motion of many atoms, where E₀ is the static Young’s modulus. E(_t,10² Hz) increases showing saturation with increasing _t. Qualitatively, the outlines of E(10⁻⁶,f) and E(_t,10² Hz) observed for a-Zr₆₀Cu₃₀Al₁₀ are similar to those reported for various a-alloys. Quantitatively, a change in E(_t,10² Hz) for a-Zr₆₀Cu₃₀Al₁₀ is smallest among that reported for various a-alloys, presumably reflecting that the crystallization volume, (ΔV/V)_x, is smallest for a-Zr₆₀Cu₃₀Al₁₀. The effective charge number, Z^*, estimated from the change in E(10⁻⁶,10² Hz) due to PEC is 3.0×10⁵, which is comparable with Z^* reported for various a-alloys. We surmise that the number of atoms in the collective motions excited near 10² Hz is similar among various a-alloys. The E(10⁻⁶,f) data suggest that the spatial sizes of the density fluctuations may show a distribution.