Showing papers in "Journal of Materials Science: Materials in Electronics in 1992"

Determination of the effective mass for highly degenerate copper selenide from reflectivity measurements

Abstract: An investigation was made of the plasma reflection of light and the results were used to find the effective mass of highly degenerate copper selenide,p>1020 cm−3, as a function of carrier concentrationp. The study of the temperature dependence of effective mass, electrical conductivity, thermoelectric power and Hall mobility carried out in the present work gives an idea of the nature of the valence bands in these materials.

Nodular defect growth in thin films

Abstract: The formation and microstructure of nodular defects in thin films of sputtered Ti and evaporated MgF2 are studied and modelled. Unintentional nodular defects in Ti occurred from growth onto uncleaned substrates whereas nodule growth in MgF2 was initiated by seeding the substrate with polystyrene spheres. Nodules grew in a characteristic shape consisting of a cone with a domed top, and with the nodule separated from the bulk of the film by a low density boundary. Simulation of nodule growth by a ballistic deposition model in two and three dimensions confirmed this nodular shape, but also predicted that a low density region extends into the nodule from the boundary. Further predictions of the minimum seed size required to initiate nodule growth lead to suggestions for a technique for film adatom mobility measurement.

Synthesis of fine-particle titanates by the pyrolysis of oxalate precursors

Abstract: Fine-particle MTiO3, where M=Ca, Sr, Ba or Pb, were prepared by the pyrolysis of MTiO(C2O4)2·4H2O in the presence of a redox mixture containing NH4NO3 and oxalic acid dihydrazide at 350 °C. The redox mixture, having a low ignition temperature, not only pyrolyses the oxalate precursor but also produces large quantities of gases, which results in a fine, foamy oxide product. The exothermicity of the redox reaction provides the heat required for the decomposition of the oxalate, and the gases evolved create the foamy nature of the product and help to dissipate the heat, which inhibits the sintering of MTiO3. Metal titanate powders obtained by the pyrolysis of the oxalates are of submicrometre size and have a high surface area (18–42 m2g−1). The sintered BaTiO3 and PbTiO3 showed spontaneous polarization of 12 and 26 µC cm−2, respectively, for a field strength of 120 kV cm−1.

Understanding doped V 2 O 3 as a functional positive temperature coefficient material

Abstract: Doped V2O3 is examined to understand its useful properties as a positive temperature coefficient (PTC) material. The effects of different dopant species and combinations of dopant species and the effects of microstructure are examined in terms of the properties of transition temperature, transition width, resistivity ratio, and transition hysteresis size. In addition to the previously used dopants of Cr and Al, the dopants In, Al+Cr, Al+In, and Al+Ga are also found to be effective at producing a PTC phase transition. The combination of Al+In produced the best resistivity ratio for its transition temperature. Higher sintering temperatures are found to produce larger grains and a sharper transition. The sharper transition is interpreted as the combined effect of heterogeneous nucleation and self-catalysation within a single grain. The hysteresis decreases from over 100 K between the first cooling and the following heating to around 50 K for subsequent thermal cycles. This is most likely a result of the formation of microcracks which are observed in the ceramic. An additional five cycles, though, shows no measurable decrease in the hysteresis size indicating no further growth of the microcracks.

Effects of the reaction parameters on the deposition characteristics in ZrO 2 CVD

Abstract: Zirconium dioxide (ZrO2) films have been deposited on to silicon wafers by the chemical vapour deposition (CVD) technique involving the application of gas mixtures of ZrCl4, CO2, and H2. The relationships between the deposition rate and various reaction parameters, such as the gas flow rate, the deposition temperature, and the composition of reactant gases, were studied. The film was identified as nearly stoichiometric monoclinic ZrO2 by using X-ray photoelectron spectroscopy, infrared transmission, and X-ray diffraction. Zirconium tetrachloride (ZrCl4) is the only species acting as zirconium donor which results from thermodynamic calculations in the present system. The CVD of ZrO2 is a thermally activated process and the activation energy is about 80 kJ mol−1 at the surface chemical reaction controlled region. The deposition mechanism, initially a kinetic process controlled by diffusive mass transfer, becomes a kinetic process governed by the surface chemical reactions with increasing total flow rate above 700°C. The dependence of the deposition rate on the reactant gas composition is mainly affected by the relative contents of the zirconium donor and the oxygen donor. At ZrCl4 mole fractions lower than 2.0 × 10−3, the deposition rate increases with the ZrCl4 mole fraction; however, at ZrCl4 mole fractions higher than that the deposition rate is mainly influenced by the H2O-forming reaction between CO2 and H2.

Mechanism of the oxidative hydrolysis of iron(II) sulphate

Abstract: The kinetics of the oxidative hydrolysis of iron(II) sulphate at temperature 30°C and pH 4.5 in the concentration range 40–300 gl−1 were studied. The change in the kinetic equations in the presence of inhibitor and catalysts gives additional information about the mechanism of the process. In the presence of manganese(II) ions we observed two parallel mechanisms of oxidation (homogeneous and heterogeneous), in agreement with the obtained kinetic data. The change in the α-FeOOH precipitate structure with time, detected by Mossbauer spectroscopy, allows the suggesting of a mechanism of heterogeneous oxidation related to the crystal growth.

Experimental studies on substituted copper phthalocyanine Langmuir-Blodgett films

Abstract: Using the Langmuir-Blodgett (LB) technique, novel copper bis(3-hydroxypropyloxy-hexaisopentyloxy)phthalocyanine molecules were deposited on quartz substrates. It is argued on the basis of pressure-area isotherms that films are deposited as monolayers. Optical absorption spectroscopy suggests that the deposited films were uniform. A.c. measurements were performed on LB films in a planar configuration at different temperatures. The power-law dependence of the conductivity on frequency is explained in terms of electronic conduction through hopping over a coulombic barrier 0.66 eV high. The Debye relaxation time is estimated to be of the order 811 µs at room temperature.

Preparation of superconducting YBa2Cu3O7-x powders by a solution technique

Abstract: Bulk superconducting YBa2Cu3O7−x powder has been synthesized by a solution technique using a mixture of Ba-ethylenediaminetetra-acetic acid (EDTA) and [Y, Cu]-citric acid complexes. A light-blue, molecular-level, homogeneously mixed precursor was prepared, and transferred to powder form through vacuum drying. The vacuum-dried powder was decomposed at 800 °C for 4 h under flowing oxygen, then heat treated at high temperature from 850 to 950 °C for 6–12 h. The results ofTc measurements and X-ray analysis show that the orthorhombic, superconducting phase can be formed at temperatures above 850 °C following low-temperature annealing. A sharp transition (ΔT≃2 K) and high density can be achieved after 930 °C heat treatment. The 930 °C heat treated sample shows aJc value of 510 A cm−2. It is concluded that this solution technique provides better stoichiometric control and lower reaction temperature than the conventional solid-state sintering process.

Photoconductivity of chemically deposited rare-earth-doped CdS films

Abstract: Highly photosensitive films with photocurrent to dark current ratios of the order of 105 have been prepared by doping CdS with Nd and Pr. The conditions for preparing such films with different fluxes are described and results of excitation spectra, transmission spectra and rise-decay curves are presented. Values of trap depths are also calculated.

Applicability of discontinuous palladium films as strain gauges

Abstract: Discontinuous palladium films were prepared by evaporation and sublimation. The variation of the direct current resistance of the as-deposited films with time was investigated in air at room temperature. Films prepared by sublimation exhibited better stability as compared with that of films prepared by evaporation. The piezoresistance of the discontinuous palladium films was measured at different strains and the gauge factor, v, was deduced. It was found that v of films prepared by sublimation was higher than that of films prepared by evaporation. Palladium films prepared by sublimation are thus good candidates as sensors for measuring extremely small strain.

Preparation of a transparent alumina film doped with CdS and its nonlinear optical properties

Abstract: We have examined the preparation of transparent alumina film and CdS-doped alumina film prepared by the sol-gel process using aqueous sol starting from AlCl3. The resulting alumina films obtained by sol-gel transformation are transparent in the visible and near infrared up to 1000 °C. Homogeneous solution of alumina sol and Cd(CH3COO)2 was converted into gel film by dehydration. Then, on exposure to H2S stream at 250–600 °C, a CdS-doped film was obtained. It shows relatively sharp blue-shifted visible absorption that is attributed to size quantization effect. Large nonlinear absorption and refractive index have been obtained, because high concentration of CdS particles can be achieved by using this method.

CaZrO3 additives to enhance capacitance properties in BaTiO3 ceramic capacitors

Abstract: The effects on the dielectric properties of BaTiO3 of addition of CaZrO3 (0–17 wt%) has been investigated. Dielectric constants ranging between 2500 and 15 500 have been obtained by using commercial grade raw materials with the starting dielectric constant of the undoped BaTiO3 being 800. The capacitor dielectrics have also been analysed for their electrical properties, crystalline phases and microstructural features.

Deformation characteristics and real structure around indents on {111}A-surfaces of II–VI solid solutions

Abstract: The microhardness of II–VI compounds and their solid solutions has been measured at temperatures from 0.2 to 0.44Tm. Deformation effects around indents on {111}A-surfaces have been studied by means of selective etching. Changes in the real structure, reflected in the hardness-temperature slope, are discussed in terms of different rate-controlling mechanisms, e.g. dislocation slipping and climbing, and of solid solution hardening effects. Deformation by indentation is characterized, for the investigated temperature range, by an activation energy ofQ≈0.1 eV in the case of (Cd, Zn) (Te, Se) and ofQ=0.12 ... 0.23 eV for Hg-containing compounds. A slightly different hardness between {111}A- and {111}B-faces occurs in Hg0.32Cd0.68Te at temperatures between 360 K and 500 K. Critical stresses for α-dislocation motion, derived from dislocation configurations around the indents, yield values ranging from 12.5 MPa at 295 K to 1.2 MPa at 500 K. Micro-twinning is one of the deformation modes of (Cd, Zn) (Te, Se) below 400 K.

Structural and optical properties of Bi x Se1−x films

Abstract: BixSe1−x thin films have been studied because of their structural and optical properties with a view to judging their suitability as the recording medium in phase-change type optical recording. Amorphous films deposited at room temperature were crystallized by thermal annealing. X-ray diffraction analysis and surface morphological studies are reported. A maximum reflectivity difference of 25% at λ=830 nm was obtained upon amorphous-to-crystalline transition. The optical constants calculated by the Newton-Raphson method using the experimental transmittance, reflectance and thickness data are reported.

Use of AgInTe2 for optoelectronic switching at higher frequencies

Abstract: A possible application of AgInTe2 for optoelectronic switching is reported. Alternating and direct current (a.c. and d.c.) response of the material has been studied over a wide frequency range (5 Hz to 1 MHz) through measurements of conductance, capacitance and photocurrent at different temperature and illumination levels. The switching/recovery time has been analysed in terms of time constant τ and found to be of the order of microseconds for this material. Higher frequencies of operation (>40 kHz) are preferable because, at such frequencies, τ becomes almost independent of illumination level and temperature. Furthermore, the switching speed can be conveniently controlled with d.c. bias applied across the sample. Elliott's CBH model has been satisfactorily used to explain the switching behaviour for AgInTe2.

Relation between thermal conductivity, sintering mechanism and microstructure of AIN with yttrium aluminate grain boundary phases

Abstract: High thermal conductivity, polycrystalline, AIN ceramics are being considered as microelectronic packaging materials. Careful microstructural characterization of AIN with various Y2O3 contents has been used to determine the particular yttrium aluminate second phases formed on sintering. The presence and morphology of the aluminates explains the variation of thermal conductivity with Y2O3 content and gives an indication of the sintering mechanism.

Some properties of the first non-aqueous electro-codeposited InP, In and P thin films

Abstract: A preparatory technique for InP films is described. The composition, surface feature and structural characterizations of the as-deposited films indicated the formation of InP but with poor surface morphology. Annealing appears to change the surface topography and shows no improvement in the quality of the film. The optical and electrical studies of the as-deposited films showed them to be an n-type semiconductor with a band gap of 1.35 eV.

TEM studies of RF magnetron-sputtered thin films

Abstract: Lead scandium tantalate (PST) thin films sputtered onto sapphire substrates have been studied by using transmission electron microscopy. Samples in transverse section were used to characterize the microstructure of the thin films as a function of distance from the PST-sapphire interface whereas samples in plan section allowed a more detailed, structural investigation of the PST. A liquid nitrogen cold stage was used to induce the paraelectric-ferroelectric phase transitionin situ and to perform heating and cooling experiments on the thin films.

Structural and electrical properties of cadmium-sulpho-selenide solid solutions

Abstract: Complete solid solutions of CdS x Se1−x (0≤x≤1) were synthesized by vacuum fusion of stoichiometric proportions of CdS and CdSe X-ray diffraction data revealed that they possess the hexagonal wurtzite structure The unit cell lattice constants vary linearly with the composition parameter,x, following Vegard's law Thin films of CdS x Se1−x (0≤x≤1) solid solutions could be deposited onto glass substrates by thermal evaporation of the bulk material in 10−4 Pa vacuum Structural investigation showed that the films are polycrystalline with predominant appearance of the (002) reflecting plane On annealing at 250°C in 10−2 Pa vacuum atmosphere, aggregation and rearrangement of the as-deposited tiny crystallite occurred, preserving the same crystal structure The dark electrical resistivity of the films is independent on the film thickness, but it varies appreciably with the composition parameter,x, showing a minimum resistivity of 002 Θ cm atx=055 The temperature dependence of the resistivity follows the semiconducting behaviour with an extrinsic and an intrinsic conduction below and above 70°C, respectively The determined activation energies 02 eV and 08 eV, respectively, correspond to shallow and deep trap levels, respectively

Anodic etching of p-type cubic silicon carbide

Abstract: p-Type cubic silicon carbide was anodically etched using an electrolyte of HF:HCl:H2O The etching depth was determined versus time with a fixed current density of 964 mA/sq cm It was found that the etching was very smooth and very uniform An etch rate of 227 nm/s was obtained in a 1:1:50 HF:HCl:H2O electrolyte

Large stable order parameter in an azo dye-attached polymer oriented by corona poling

Abstract: Spectroscopic absorption measurements were used to study the non-linear optical thin-film properties of an azo dye-attached polymer in which the dye chromophores were oriented by corona poling. Details of the polymer synthesis, poling configuration and poling procedure are presented. A rigid oriented gas model was used to describe the poled polymer. The stabilized order parameter of 0.51 found is, to the authors' knowledge, larger than any reported for guest-host or chromophore-attached polymeric systems.

Effect of an increase in the nitrogen content of the working gas on the microstructure of reactive sputtered TiN thin films

Abstract: TiN films were deposited on (100)-Si substrates by RF-reactive sputtering. The effects of processing parameters and substrate orientation on the stoichiometry, resistivity and microstructure of reactively sputtered TiN thin films were investigated. The RF-power was fixed at 50 W, and the nitrogen content in the working gas was adjusted so that target nitridation occurred at a relatively low nitrogen content, 2.6% N2, in a fixed total flow rate of gases of 46.25 standard cubic centimetres (sccm). The N2 percentage was varied from 2.6% to 15.4%. The films were continuous and ∼200 nm thick. Films deposited with the substrate facing the target exhibited 111-texturing, while films on substrates lying in the same plane of the target surface had 100-texturing. Both X-rays photoelectron spectroscopy and high-resolution detailed scans of the Auger electron energy spectrum were used for the stoichiometry determination. On the 100-textured TiN films, the N/Ti ratio was ∼1.0 at low N2 flow rates; the N/Ti ratio rapidly increased with N2 flow rate, and then levelled off. The films had resistivities ranging from 85 to 1340 µΩ cm, and the functional dependence of both N/Ti and the resistivity values behaved similarly with increasing N2. X-ray single-line profile analysis of the 200-reflection indicated that the average crystallite size decreased and the average strain increased with increasing nitrogen content in the working gas. The Si/TiN structures were heat treated in the temperature range from 300 to 600°C in a quartz tube under 1 atm (∼105Pa) of flowing high purity Ar gas. Heat treatment at 300°C did not affect the TiN film integrity, while treatment at 400–600°C resulted in void-type defects.

Heat treatment effect on the structural and optical properties of Ga2Se3-Ga2S3 thin films

Abstract: The structural properties of the 50 mol% Ga2Se3-50 mol% Ga2S3 system in thin-film form were studied using an X-ray diffraction technique. As-deposited films had an amorphous nature, whereas films heat treated for 2 h at 400 °C in a vacuum of 10−2 Pa had a single-phase (cubic) polycrystalline nature with lattice constanta=0.532 nm. The optical properties of Ga2Se3-Ga2S3 thin films as-deposited and as-heat treated were also studied. It was found that heat treatment strongly affects the optical constants as well as the energy gap, which can be attributed to compositional as well as structural changes.

Cooling rate effects in ZnO varistors

Abstract: Cooling rate effects on ZnO-Bi2O3 varistors are presented. In particular, the oxygen vacancies (V o + ) concentration was determined by electron paramagnetic resonance (EPR) spectroscopy and related to the conductivity. It was found that a higher cooling rate provokes an increase of the V o + concentration and a higher conductivity. Current density versus temperature curves are fitted with a modified Schottky barrier model which reflects a higher active donor concentration near the grain interface.

Dislocation generation in silicon during CMOS device processing

Abstract: Dislocation generation in silicon at ion implants in complementary metal oxide semiconductor (CMOS) devices has been studied by using transmission X-ray topography. The dislocations are shown to arise through the interaction between relatively low level thermomechanical stresses and ion implantation damage during processing. These observations are compared with the more common situation of dislocation generation via purely thermomechanical effects. The results provide an illustration of the importance of interactions between the various sources of stress and damage which occur during device processing. Furthermore, they demonstrate that substantial defect generation can occur even when such stresses are controlled at levels which are individually below the threshold for dislocation generation.

Effect of the addition of Fe2O3 and heat treatment duration on the magnetic susceptibility of V2O5-P2O5-B2O3 glass system

Abstract: The effect of the addition of Fe2O3 and heat treatment duration on the magnetic susceptibility of vanadium borophosphate glass were studied The magnetic susceptibility of glass samples was found to increase with increasing Fe2O3 content, which may be explained by the formation of the FeO6 group and the change of Fe2+ to Fe3+ which has higher paramagnetic properties No detectable changes in the magnetic susceptibility with heat treatment for the samples containing 00, 05 and 10 mol% Fe2O3 was observed The magnetic susceptibility for the heat treated samples containing 25, 50 and 75 mol% Fe2O3 decreases sharply with increasing duration of heat treatment up to 6 h and then remains almost constant The sharp decrease in magnetic susceptibility of 25 mol% Fe2O3 is attributed to the increase in the number of ferrous ions The sharp decrease for samples containing 50 and 75 mol% Fe2O3 is attributed to the increase in the number of Fe3+ in tetrahedral co-ordination The rate of crystallization owing to the heat treatment was calculated and was found to increase with increasing iron oxide content The geometry of crystallization was found to be in three-, two-and one-dimension(s) for samples containing 25, 50 and 75 mol% Fe2O3, respectively

Temperature dependence of the electrical conductivity of Salicylaldazinate metal chelates

Abstract: A study of the temperature dependence of the electrical conductivity of Salicylaldazine and its metal complexes with the d-block elements Ni2+, Cu2+ and Zn2+, is correlated with molecular spectral data. The values of the activation energies obtained from the electrical conductivity measurements, as well as the energy gaps corresponding to the maximum absorption wavelengths in the region of ultraviolet and visible spectra, indicate the important role of chelation in producing the Salicylaldazine semiconducting properties.

Influence of sintering and annealing temperature on grain boundary barrier layer capacitors in a modified reduction-reoxidation method

Abstract: To produce grain boundary barrier layer capacitors with a simpler fabrication process and more-stable dielectric characteristics, a modified reduction-reoxidation method was developed. Nb2O5-doped (Ba0.8 Sr0.2)(Ti0.9 Zr0.1)O3 (BSTZ) was calcined at 1170 °C for complete formation of ABO3 phases. After calcination CuO was added to BSTZ as a liquid-phase promoter and insulating boundary layer material. The ceramics were sintered in a reducing atmosphere, and then the fired samples were annealed in air to reoxidize the reduced CuO to form insulating layers. The dielectric constant of the fabricated capacitors was decreased for higher annealing temperature, longer annealing time and smaller grain size. The loss tangent of the fabricated capacitors was increased for BSTZ with more CuO added, and was almost unchanged with annealing temperature, annealing time and grain size because of the existence of an insulating boundary layer material (CuO).

Electrical conduction in some gamma irradiated and unirradiated two-dimensional compounds, (CH2)8(NH3)2Co1−x Zn x Cl4

Abstract: The electrical conductivity of γ-irradiated and unirradiated complexes of (CH2)8(NH3)2Co1−xZnxCl4 (x=0, 0.2, 0.4, 0.6, 0.8 and 1) was measured in the temperature range from 300 K to a temperature near the melting points of the samples. The results showed an increase in the conductivity values with increasing concentration of zinc ions in the samples. Both differential scanning calorimeter (DSC) and conductivity measurements showed the presence of many phase transitions in each of the complexes investigated. The results of irradiated compounds with a composition of 0≤x≤0.8 showed an increase in the conductivity values, at lower temperatures, compared with the values of unirradiated ones. For compositionx=1 no change in the conductivity data has been observed after irradiation. The effect of change in the chemical composition and irradiation on the conductivity data are discussed.

Reaction sequences in the systems Ti(OiPr)4 + Ba(CH3COO)2 and TiO2 + Ba(CH3COO)2

Abstract: The reaction sequences during BaTiO3 formation from the systems Ti(OiPr)4 + Ba(CH3COO)2 and TiO2 + Ba(CH3COO)2 are studied, using a sol-gel method for the former system and solid-state reaction for the latter. The sol-gel system exhibits greater reactivity, forming BaTiO3 at temperatures as low as 500°C; also, the formation of the intermediate Ba2TiO4 occurs to a much lesser extent than in the case of solid-state reaction. An explanation of the structure and behaviour of the gel system is proposed from X-ray diffraction, infrared and thermogravimetric examinations. Under appropriate treatments, both systems yield microstructures with fine and homogeneous grain size; however, the sol-gel method facilitates a complete reaction at temperatures appreciably lower than those needed with the solid-state reaction system.