Showing papers in "Journal of Materials Research in 1996"

Thermodynamic stability of binary oxides in contact With silicon

Abstract: Using tabulated thermodynamic data, a comprehensive investigation of the thermo-dynamic stability of binary oxides in contact with silicon at 1000 K was conducted. Reactions between silicon and each binary oxide at 1000 K, including those involving ternary phases, were considered. Sufficient data exist to conclude that all binary oxides except the following are thermodynamically unstable in contact with silicon at 1000 K: Li2O, most of the alkaline earth oxides (BeO, MgO, CaO, and SrO), the column IIIB oxides (Sc2O3, Y2O3, and Re2O3, where Re is a rare earth), ThO2, UO2, ZrO2, HfO2, and Al2O3. Of these remaining oxides, sufficient data exist to conclude that BeO, MgO, and ZrO2 are thermodynamically stable in contact with silicon at 1000 K. Our results are consistent with reported investigations of silicon/binary oxide interfaces and identify candidate materials for future investigations.

Influences of stress on the measurement of mechanical properties using nanoindentation: Part I. Experimental studies in an aluminum alloy

Abstract: The influence of applied stress on the measurement of hardness and elastic modulus using nanoindentation methods has been experimentally investigated using special specimens of aluminum alloy 8009 to which controlled stresses could be applied by bending. When analyzed according to standard methods, the nanoindentation data reveal changes in hardness with stress similar to those observed in conventional hardness tests. However, the same analysis shows that the elastic modulus changes with stress by as much as 10%, thus suggesting that the analysis procedure is somehow deficient. Comparison of the real indentation contact areas measured optically to those determined from the nanoindentation data shows that the apparent stress dependence of the modulus results from an underestimation of the contact area by the nanoindentation analysis procedures.

Influences of stress on the measurement of mechanical properties using nanoindentation: Part II. Finite element simulations

Abstract: The finite element method has been used to study the behavior of aluminum alloy 8009 during elastic-plastic indentation to establish how the indentation process is influenced by applied or residual stress. The study was motivated by the experiments of the preceding paper which show that nanoindentation data analysis procedures underestimate indentation contact areas and therefore overestimate hardness and elastic modulus in stressed specimens. The NIKE2D finite element code was used to simulate indentation contact by a rigid, conical indenter in a cylindrical specimen to which biaxial stresses were applied as boundary conditions. Indentation load-displacement curves were generated and analyzed according to standard methods for determining hardness and elastic modulus. The simulations show that the properties measured in this way are inaccurate because pileup is not accounted for in the contact area determination. When the proper contact area is used, the hardness and elastic modulus are not significantly affected by the applied stress.

Status of and prospects for organic electroluminescence

Abstract: We review the device and materials science behind organic electroluminescent diodes made both using discrete evaporable molecules and spin-cast organic polymers. A great deal of progress has been made in improving the efficiencies and spectral properties of organic light-emitting diodes, and these are now adequate for many applications. More work is necessary to understand the stability and degradation of emissive and charge-transporting organics, but some systems have been shown to be stable for 104 hours at display brightness. Major challenges still face the community in terms of developing satisfactory systems design and processing techniques if organic electroluminescence is to realize either performance or economic advantages over technologies and significantly penetrate the display market. We present an analysis of the suitability of organic light-emitting diodes for various applications, and consider the materials and manufacturing obstacles that must be overcome.

The dynamic compressive behavior of beryllium bearing bulk metallic glasses

Abstract: In 1993, a new beryllium bearing bulk metallic glass with the nominal composition Zr41.25Ti13.75Cu12.5Ni10Be22.5 was discovered at Caltech. This metallic glass can be cast as cylindrical rods as large as 16 mm in diameter, which permitted specimens to be fabricated with geometries suitable for dynamic testing. For the first time, the dynamic compressive yield behavior of a metallic glass was characterized at strain rates of 102 to 104/s by using the split Hopkinson pressure bar. A high-speed infrared thermal detector was also used to determine if adiabatic heating occurred during dynamic deformation of the metallic glass. From these tests it appears that the yield stress of the metallic glass is insensitive to strain rate and no adiabatic heating occurs before yielding.

Growth defects in GaN films on sapphire: The probable origin of threading dislocations

Abstract: Single crystal GaN films with a wurtzite structure were grown on the basal plane of sapphire A high density of threading dislocations parallel to the {ital c}-axis crossed the film from the interface to the film surface They were found to have a predominantly edge character with a 1/3{l_angle}11{bar 2}0{r_angle} Burgers vector In addition, dislocation half-loops, elongated along the {ital c}-axis of GaN, was also found on the prism planes These dislocations had a mostly screw character with a [0001] Burgers vector Substrate surface steps with a height of 1/6{ital c}{sub Al{sub 2}O{sub 3}}, were found to be accommodated by localized elastic bending of GaN (0001){sub GaN} planes in the vicinity of the film/substrate interface Observations show that the region of the film, with a thickness of {approximately}100 nm, adjacent to the interface is highly defective This region is thought to correspond to the low-temperature GaN {open_quote}{open_quote}buffer{close_quote}{close_quote} layer which is initially grown on the sapphire substrate Based on the experimental observations, a model for the formation of the majority threading dislocations in the film is proposed The analysis of the results leads us to conclude that the film is under residual biaxial compression {copyright} {ital 1996 Materials Research Society}

An investigation of grain boundaries in submicrometer-grained al-mg solid solution alloys using high-resolution electron microscopy

Abstract: High-resolution electron microscopy was used to examine the structural features of grain boundaries in Al–1.5% Mg and Al–3% Mg solid solution alloys produced with submicrometer grain sizes using an intense plastic straining technique. The grain boundaries were mostly curved or wavy along their length, and some portions were corrugated with regular or irregular arrangements of facets and steps. During exposure to high-energy electrons, grain boundary migration occurred to reduce the number of facets and thus to reduce the total boundary energy. The observed features demonstrate conclusively that the grain boundaries in these submicrometer-grained materials are in a high-energy nonequilibrium configuration.

Glass-ceramic sealants for solid oxide fuel cells: Part I. Physical properties

Abstract: A family of sealant materials has been developed for use in the solid oxide fuel cell (SOFC) and in other applications in the temperature range of 800–1000 °C. These materials are based on glasses and glass-ceramics in the SrO–La2O3–Al2O3–B2O3–SiO2 system. The coefficients of thermal expansion (CTE) for these materials are in the range of 8–13 × 10−6/°C, a good match with those of the SOFC components. These sealant materials bond well with the ceramics of the SOFC and, more importantly, form bonds that can be thermally cycled without failure. At the fuel cell operating temperature, the sealants have viscosities in the range of 104–106 Pa-s, which allow them to tolerate a CTE mismatch of about 20% among the bonded substrates. The gas tightness of a sample seal was demonstrated in a simple zirconia-based oxygen concentration cell.

Macrosegregation of Y2Ba1Cu1O5 particles in Y1Ba2Cu3O7−δ crystals grown by an undercooling method

Abstract: Macrosegregation of Y2Ba1Cu1O5 (Y211) particles was observed in Pt-added Y1Ba2Cu3O7−δ (Y123) crystals grown by an undercooling method. It was found that the macrosegregation of Y211 particles depended on the growth direction and the growth rate (R) as a function of undercooling (ΔT). The amount of Y211 particles in Y123 crystals grown at large R was larger than at small R. Also, the amount of Y211 in Y123 growing along the a-direction was larger than that along the c-direction. Further, it was noted that the smaller Y211 particles in size were distributed in Y123 grown at large R. These phenomena could be at least qualitatively explained by the prevalent trapping/pushing theory. In the direct observation of magnetic flux with the Faraday effect of iron garnet film, the flux pinning force was found to be in good agreement with the macrosegregation of Y211 particles.

Electrically active organic and polymeric materials for thin-film-transistor technologies

Abstract: Organic and polymeric materzials have seen a tremendous growth in research in the last five years as potential electroactive elements in thin-film-transistor (TFT) applications. These are driven by the increasing interest in flat-panel-display applications, for which organic and polymeric materials offer strong promise in terms of properties, processability, cost, and compatibility with eventual lightweight, flexible plastic displays. In this review we summarize the current status of our knowledge on the science of these organic and polymeric semiconducting materials. Most of these are based on linear thiophenes, especially a-hexathienyl, which has elicited by far the most attention. Mobility values in the 10−2–10−1 cm2/Vs and especially source-drain current on/off ratios of up to 106 make this a highly promising potential alternative to amorphous silicon. Other thienyl compounds are also discussed, as are polymeric analogues. A brief discussion of technological potential, limitations, and problems that need to be overcome is given at the end.

Magnetoelastic anisotropy distribution in glass-coated microwires

Abstract: Amorphous microwires, obtained by the glass-coated melt-spinning method having diameters in the range of micrometers, can exhibit perfectly square (single and large Barkhausen jump) or quasi-anhysteretic hysteresis loops, depending on the easy magnetization direction determined by the intrinsic magnetoelastic anisotropy. The thermoelastic internal stressed frozen-in during the fabrication that model the domain structure are here calculated by considering the classical theory of elasticity. A complex stress distribution is obtained having magnitude of 103 MPa. Circular stresses turn out to be predominant, which arises from the composite nature of the microwire (metallic nucleus and insulating glass coating having different mechanical and thermal properties).

Highly conducting transparent thin films based on zinc oxide

Abstract: Doped zinc oxide thin films were prepared by rf magnetron sputtering using the dopants Al, Ga, In, and Ge. The best results were obtained with Al and Ga doping where room temperature conductivities were as high as 1600 and 1800 ohm-1 cm-1, respectively. Hall measurements were performed at 77 K and 298 K. The Hall mobility as in the range of 9 to 22 cm2/Vs, and there was generally very little temperature dependence of the mobility or conductivity. Cation doping levels were as high as 10 at. %, but the conductivities did not increase beyond 3 at. % doping level. For films with high conductivity, electron carrier concentrations from Hall measurements were significantly lower than the concentrations of dopants. Optical measurements on the films showed that the average transmittance though the visible range is higher than 85%. The measurements also indicated a blueshift of the absorption edge with doping.

Kinetic and structural studies of oxygen availability of the mixed oxides Pr1–xMxOy (M = Ce, Zr)

Abstract: One composition of Pr–Ce mixed oxide and a range of compositions of Pr–Zr mixed oxide were prepared by coprecipitation methods and characterized by x-ray powder diffraction, thermogravimetric analysis, and x-ray photoelectron spectroscopy. Based on phases formed, the PrOy—ZrO2 system in an oxygen-containing atmosphere at moderate temperatures (up to 800–1000 °C) is analogous to that of CeO2–ZrO2. Addition of either Ce or Zr to pure Pr oxide affects both the total amount of oxygen that can be reversibly exchanged between oxide and gas phase and the kinetics of the redox processes. Ce dramatically increases the amount (per Pr atom) and lowers the temperature of exchange, Zr slightly decreases the amount and also lowers the temperature of exchange, and both modifiers speed up the rate. These observations are rationalized in terms of bulk and surface structural features of the mixed oxides.

Formation of srbi2ta2o9 : part i. synthesis and characterization of a novel sol-gel solution for production of ferroelectric srbi2ta2o9 thin films

Abstract: We have developed a simple and rapid method for the synthesis of a precursor solution used in the production of SBT powders and thin films of the layered-perovskite phase SrBi2Ta2O9 (SBT). Precursor solution preparation takes less than 30 min and involves the generation of two solutions: (a) Bi(O2CMe)3 dissolved in pyridine and (b) Ta(OCH2Me)5 added to Sr(O2CMe)2 and then solubilized by HO2CMe. After stirring separately for 10 min, these solutions are combined, stirred for an additional 10 min, and used without any further modifications. The individual solutions and ternary mixture were studied using a variety of analytical techniques. Films of the layered-perovskite phase were formed at temperatures as low as 700 °C. Ferroelectric testing of SBT films, fired at 750 °C, reveals standard hysteresis loops with no fatigue for up to 4 × 109 cycles.

Damage-resistant alumina-based layer composites

Abstract: A new philosophy for tailoring layer composites for damage resistance is developed, specifically for alumina-based ceramics. The underlying key to the approach is microstructural control in the adjacent layers, alternating a traditional homogeneous fine-grain alumina (layer A) for hardness and wear resistance with a heterogeneous alumina : calcium-hexaluminate composite (layer C) for toughness and crack dispersion, with strong bonding between the interlayers. Two trilayer sequences, ACA and CAC, are investigated. Hertzian indentation tests are used to demonstrate the capacity of the trilayers to absorb damage. In the constituent materials, the indentation responses are fundamentally different: ideally brittle in material A, with classical cone cracking outside the contact; quasi-plastic in material C, with distributed microdamage beneath the contact. In the ACA laminates, shallow cone cracks form in the outer A layer, together with a partial microdamage zone in the inner C layer. A feature of the cone cracking is that it is substantially shallower than in the bulk A specimens and does not penetrate to the underlayer, even when the applied load is increased. This indicates that the subsurface microdamage absorbs significant energy from the applied loads, and thereby “shields” the surface cone crack. Comparative tests on CAC laminates show a constrained microdamage zone in the outer C layer, with no cone crack, again indicating some kind of shielding. Importantly, interlayer delamination plays no role in either layer configuration; the mechanism of damage control is by crack suppression rather than by deflection. Implications for the design of synergistic microstructures for damage-resistant laminates are considered.

An elastic-plastic indentation model and its solutions

Abstract: An analytical model of hardness has been developed. Four major indentation tests, namely indentation by cones, wedges, spheres, and flat-ended, axisymmetric cylinders have been analyzed based on the model. Analytical relationships among hardness, yield stress, elastic modulus, Poisson's ratio, and indenter geometries have been found. These results enable hardness to be calculated in terms of uniaxial material properties and indenter geometries for a wide variety of elastic and plastic materials. These relationships can also be used for evaluating other mechanical properties through hardness measurements and for converting hardness from one type of hardness test into those of a different test. Comparison with experimental data and numerical calculations is excellent.

Epitaxial growth and magnetic behavior of NiFe2O4 thin films

Abstract: Thin films of NiFe2O4 were deposited on SrTiO3 (001) and Y0.15Zr0.85O2 (yttria-stabilized zirconia) (001) and (011) substrates by 90°-off-axis sputtering. Ion channeling, x-ray diffraction, and transmission electron microscopy studies reveal that films grown at 600 °C consist of ∼300 A diameter grains separated by thin regions of highly defective or amorphous material. The development of this microstructure is attributed to the presence of rotated or displaced crystallographic domains and is comparable to that observed in other materials grown on mismatched substrates (e.g., GaAs/Si or Ba2YCu3O7/MgO). Postdeposition annealing at 1000 °C yields films that are essentially single crystal. The magnetic properties of the films are strongly affected by the structural changes; unannealed films are not magnetically saturated even in an applied field of 55 kOe, while the annealed films have properties comparable to those of bulk, single crystal NiFe2O4. Homoepitaxial films grown at 400 °C also are essentially single crystal.

Microwave-hydrothermal processing of layered anion exchangers

Abstract: We have compared the microwave-hydrothermal (M-H) processing with conventional hydrothermal (C-H) processing in the preparation of two layered anion exchangers, i.e., Mg3Al(OH)8NO3 · nH2O and Ni1-xZn2x(OH)2(CH3COO)2x · nH2O. Both these phases can be crystallized more rapidly (an order of magnitude) under M-H processing compared to C-H processing. The above layered mixed basic salt of Ni and Zn was found to exhibit very high selectivity for PO4= (Kd = 15,000). Its order of selectivity for various anions in the presence of 0.1 N NaC1 (ratio of C1- to anion in question is 100) increases as follows: PO4= ≫ SO4 = > NO3-.

Fabrication of silicon carbide nanoceramics

Abstract: Ultrafine silicon carbide powder with an average particle size of 90 nm was densified by hot-processing with the addition of Al2O3, Y2O3, and CaO at 1750 °C. Silicon carbide nanoceramics with an average grain size of 110 nm were prepared by liquid phase sintering at low temperature. The materials showed superplastic deformation at a strain rate of 5.0 × 10-4/s at 1700 °C, which is the lowest temperature published. The microstructure and deformation behavior of materials from a submicrometer powder were also investigated as a reference.

Characteristics of titanium nitride films grown by pulsed laser deposition

Abstract: Laser physical vapor deposition (LPVD) has been used to grow titanium nitride films on hydrogen-terminated silicon(100) substrates at deposition temperatures ranging from room temperature to 600 °C. A pulsed KrF excimer laser (λ = 248 nm, τ = 25 ns) was used with the deposition chamber maintained at a base pressure of 10−7 Torr prior to deposition. Different properties of the films were investigated by x-ray diffraction, Auger electron spectroscopy, Raman spectroscopy, optical, scanning, and high resolution transmission electron microscopy, and measurement of electrical resistivity. When the substrate temperature was low (at and below 500 °C), oxygen atoms from the residual gases were incorporated in the films. The microstructures and resistivities of TiN films were found to be strongly dependent on the temperature of the silicon substrates. The TiN films deposited at 600 °C were oxygen-free, as observed from Auger analysis, and the room temperature resistivity was found to be 14–15 μΩ-cm. Raman spectroscopy of the films showed that the nitrogen-related optical phonon peak increased with deposition temperature in comparison with the titanium-related acoustic peak. Transmission electron microscopy and x-ray diffraction analyses showed that the films were polycrystalline at low temperature with grain size ranging from 300–600 A, depending on the temperature of the substrate. At 600 °C, the films were found to be single crystals with occasional presence of dislocation loops. The spacing of Moire fringes in TiN/Si samples deposited at 600 °C established the nearly periodic elastic strain field extending into the TiN and Si at the interface. Although there exists a large misfit between TiN and Si (24.6%), the epitaxial growth of TiN films on Si(100) substrates was explained by means of domain-matched epitaxy with a 4-to-3 match in unit cells for TiN/Si structure, giving rise to a residual lattice misfit of only 4%.

Microstructural development in sol-gel derived lead zirconate titanate thin films: The role of precursor stoichiometry and processing environment

Abstract: The role of precursor stoichiometry and local firing environment on the microstructural development of sol-gel derived lead zirconate titanate (PZT) thin films was investigated. Typically, excess Pb is added to films to compensate for PbO volatilization during heat treatment. Here, it is shown that the use of stoichiometric precursors with either a PbO atmosphere powder or a PbO overcoat during the crystallization heat treatment is an attractive and viable alternative method for control of film stoichiometry. Using these approaches, we have fabricated single phase perovskite thin films with microstructures and electrical properties (Pr ∼ 36 μC/cm2 and Ec ∼ 45 kV /cm) comparable to those of films using optimized solution chemistries and excess Pb additions. The potential advantage of increasing PbO partial pressure, or activity, during firing versus excess Pb additions is discussed from the standpoint of a proposed crystallization scenario based on the kinetic competition between Pb loss and the nucleation and growth rates of the perovskite phase.

Fabrication of large grain YBCO by seeded peritectic solidification

Abstract: The ability to process large grain, uniform high temperature superconducting ceramics that exhibit high critical current densities at 77 K is essential if the enormous potential of these materials for a range of permanent magnet-type applications is to be realized. We report a study of the fabrication of large grain YBa2Cu3O7−δ by seeded peritectic solidification in which key processing parameters such as the peritectic melting process, the seed-YBCO reaction, and the YBCO solidification kinetics are investigated in detail. Evolution of the sample microstructure during various stages of the growth process, in particular, has been studied extensively. The superconducting properties of specimens cut from different regions of large grain samples have been measured using vibrating sample magnetometry, and the results correlated with the microstructure of the materials.

In situ characterization of vapor phase growth of iron oxide-silica nanocomposites : part i. 2-d planar laser-induced fluorescence and mie imaging

Abstract: Planar laser-based imaging measurements of fluorescence and particle scattering have been obtained during flame synthesis of iron-oxide/silica superparamagnetic nanocomposites. The theory and application of laser-induced fluorescence, the spectroscopy of FeO(g), and the experimental approach for measurement of gas phase precursors to particle formation are discussed. The results show that the vapor phase FeO concentration rapidly rises at the primary reaction front of the flame and is very sensitive to the amount of precursor added, suggesting nucleation-controlled particle growth. The FeO vapor concentration in the main nucleation zone was found to be insensitive to the amount of silicon precursor injected, indicating that nucleation occurred independently for the iron and silicon components. Light scattering measurements indicate that nanocomposite particles sinter faster than single component silica, in agreement with TEM measurements.

Microstructural instability in single-crystal thin films

Abstract: Epitaxial PbTiO3 thin films were produced from a mixed Pb–Ti double-alkoxide precursor by spin-coating onto single crystal (001) SrTiO3 substrates. Heat treatment at 800 °C produces a dense and continuous, epitaxial lead titanate film through an intermediate Pb-Ti fluorite structure. A microstructural instability occurred when very thin single crystal films were fabricated; this instability caused the films to become discontinuous. Scanning electron microscopy and atomic force microscopy observations show that single crystal films with a thickness less than ∼80 nm developed holes that expose the substrate; thinner films broke up into isolated, single crystal islands. The walls of the holes were found to be (111) perovskite planes. A free energy function, which considered the anisotropic surface energies of different planes, was developed to describe the microstructural changes in the film and to understand the instability phenomenon. The function predicted that pre-existing holes greater than a critical size are necessary to initiate hole growth, and it predicted the observed morphological changes in the current system. Morphological stability diagrams that explain the stability fields for different film configurations, i.e., either completely covered, with holes, or single crystal islands, can be calculated for any film/substrate system.

Formation of SrBi2Ta2O9: Part II. Evidence of a bismuth-deficient pyrochlore phase

Abstract: A bismuth-deficient pyrochlore phase has been observed in both powder and film samples fired at 775{degree}C. The estimated stoichiometry of this pyrochlore (based on calculated diffraction patterns) was Sr{sub 0.2}(Sr{sub 0.5}Bi{sub 0.7})Ta{sub 2}O{sub 6.75}. This bismuth-deficient pyrochlore phase may be considered deleterious to the formation of the SrBi{sub 2}Ta{sub 2}O{sub 9} {open_quote}{open_quote}SBT{close_quote}{close_quote} ferroelectric compound since a significant presence of this pyrochlore compound implies a large deviation from the desired cation ratios. Additionally, films prepared on platinized silicon substrates indicate the SBT phase formation may be encouraged by the substrate; there appears to be some 00{ital l} preferential orientation for stoichiometric SBT thin films. {copyright} {ital 1996 Materials Research Society.}

Solid-state processing and phase development of bulk (MgO) w /BPSCCO high-temperature superconducting composite

Abstract: The inherently weak mechanical properties associated with monolithic high-temperature superconductors (HTS) can be improved by introducing properly selected strong ceramic whiskers into the HTS materials. In this research, processing and superconducting properties of monolithic Pb-doped Bi-2223 (BPSCCO) and MgO whisker-reinforced BPSCCO HTS composite materials have been systematically studied. A solid-state processing method is successfully developed to fabricate the (MgO)w/BPSCCO composite. The HTS composite contains a dense and highly pure BPSCCO matrix phase with a preferred grain orientation, which is reinforced by MgO whiskers randomly oriented in the plane perpendicular to the hot-pressing direction. The HTS composite material is shown to exhibit excellent superconducting properties. For example, a transport J c measured at 77 K in a zero field has been obtained to exceed 5000 A/cm2 in a (MgO)w/BPSCCO composite with 10% MgO whiskers by volume. Relationships among solid-state processing variables, HTS phase development, and superconducting properties of the monolithic BPSCCO and the HTS composite are established in the paper.

Processing and characterization of Pb(Zr, Ti)O3 films, up to 10 μm thick, produced from a diol sol-gel route

Abstract: A recently developed diol sol-gel route has been modified in order to produce multilayer PbZr 0.53 Ti 0.47 O 3 films on platinized sapphire substrates. Up to 20 depositions of a 1.1 M sol were carried out leading to a final film thickness of 10 μm. A similar thickness could be achieved from 12 coatings of a more concentrated 1.6 M sol. Decomposition and crystallization of the multilayer coatings were performed using a two-stage prefiring sequence, at 350 °C and 600 °C, followed by a final firing step at 700 °C. Ferroelectric remanant polarization increased with increasing film thickness to a value of 40 μC cm −2 for a 10 μm film, with a corresponding coercive field of 30 kV cm −1 ; the relative permittivity of this film was ∼1000 and the dissipation factor 0.04. The thickness dependence of relative permittivity could be modeled on a simple series capacitor circuit representing the ferroelectric Pb(Zr, Ti)O 3 (PZT) film and low-permittivity interface layers; but other possible contributory factors are also discussed.

Microstructure, electrical properties, and thermal stability of au-based ohmic contacts to p-gan

Abstract: Single Ti layers, single TiN layers, and thin Ti films overlayered with Au were investigated as ohmic contacts to n-type (n 4.5 × 1017 to 7.4 × 1018 cm−3) single-crystal GaN (0001) films. Transmission line measurements (TLM) revealed the as-deposited TiN and Au/Ti contacts on n = 1.2 − 1018 cm−3 to be ohmic with room-temperature specific contact resistivities of 650 and 2.5 × 107minus;5 Ω cm2, respectively. Single Ti layer contacts had high resistance and were weakly rectifying in the as-deposited condition. The three contact/GaN systems exhibited a substantial decrease in resistivity after annealing; the value of ρc was also a function of the carrier concentration in the GaN. The Au/Ti contacts exhibited the lowest resistivity values yet observed in these contact studies, particularly for the more lightly doped n-GaN. The ρc for n = 1.2 × 1018 cm−3 reached 1.2 × 1026 Ω cm2; for n = 4.5 × 1017 cm−3, ρc = 7.5 × 1025 Ω cm2 after annealing both samples through 900 °C. X-ray photoelectron spectroscopy (XPS) and high-resolution cross-sectional transmission electron microscopy (X-TEM) analysis revealed the formation of TiN at the interface of annealed Ti layers in contact with GaN, which is believed to be beneficial for ohmic contact performance on n-GaN.

Surface and optical properties of porous silicon

Abstract: Although silicon is the material of choice in the semiconductor industry, it has one serious disadvantage: it is an extremely poor optoelectronic material. This is because it is an indirect gap semiconductor, in which radiative transition results in extremely weak light emission in the infrared part of the spectrum. Thus, the discovery of strong visible luminescence from a silicon-based material (porous silicon) has been quite surprising and has generated significant interest, both scientific and technological. This material differs from bulk silicon in one important way, in that it consists of interconnected silicon nanostructures with very large surface to volume ratios. Although the first mechanism proposed to explain this emission process involved carrier recombination within quantum size silicon particles, more recent work has shown that the surface chemistry appears to be the controlling factor in this light emission process. Thus, the aim of this work is to outline the data and arguments that have been presented to support the quantum confinement model, along with the shortcomings of such a model, and to examine more recent models in which the chemical and structural properties of the surface regions of the nanostructures have been incorporated.

Effects of lanthanum modification on rhombohedral pb(zr1-xtix)o3 ceramics : part i. transformation from normal to relaxor ferroelectric behaviors

Abstract: The interruption of long-range polar order in rhombohedral ferroelectricPb(Zr1−xTix)O3 (PZT) ceramics has been systematically studied by incorporating La onto the A-site of the perovskite (ABO3) structure for Zr/Ti ratios of 65/35 and 80/20 and various La contents. Studies have been performed by hot-stage transmission electron microscopy, dielectric spectroscopy, and Sawyer–Tower polarization (P-E) techniques. The evolution of a polar nanodomain state from a normal micron-sized domain state with increasing La content was observed. The emergence of this polar cluster state was characterized by the onset of strong frequency dispersion in the dielectric response, indicative of relaxor behavior. The La content that drives the structure into the relaxor state was found to be related to the lattice distortion of the undoped base composition.