Showing papers in "Materials Science and Engineering B-advanced Functional Solid-state Materials in 2001"
TL;DR: Wurtzitic ZnO is a widebandgap semiconductor which has many applications, such as piezoelectric transducers, varistors, phosphors, and transparent conducting films as discussed by the authors.
Abstract: Wurtzitic ZnO is a wide-bandgap (3.437 eV at 2 K) semiconductor which has many applications, such as piezoelectric transducers, varistors, phosphors, and transparent conducting films. Most of these applications require only polycrystalline material; however, recent successes in producing large-area single crystals have opened up the possibility of producing blue and UV light emitters, and high-temperature, high-power transistors. The main advantages of ZnO as a light emitter are its large exciton binding energy (60 meV), and the existence of well-developed bulk and epitaxial growth processes; for electronic applications, its attractiveness lies in having high breakdown strength and high saturation velocity. Optical UV lasing, at both low and high temperatures, has already been demonstrated, although efficient electrical lasing must await the further development of good, p-type material. ZnO is also much more resistant to radiation damage than are other common semiconductor materials, such as Si, GaAs, CdS, and even GaN; thus, it should be useful for space applications.
2,573 citations
TL;DR: The ionic conductivity studies have been conducted for polyvinyl chloride polymer electrolytes (PVC:LiCF 3 SO 3 :LiBF 4 :PC:EC) over a wide frequency regime as discussed by the authors.
Abstract: The ionic conductivity studies have been conducted for poly (vinyl chloride) polymer electrolytes (PVC:LiCF 3 SO 3 :LiBF 4 :PC:EC) over a wide frequency regime. It was found that the addition of plasticizers significantly improved the ionic conductivity. The dielectric data were analyzed using complex impedance Z *, complex admittance A *, complex permittivity e * and complex electric modulus M * of the sample with the highest ionic conductivity at various temperatures. The conductivity–temperature data obeys the classical Arrhenius relationship.
237 citations
TL;DR: In this paper, the use of reactive molecular beam epitaxy (MBE) to synthesize layered oxide heterostructures, including new compounds and metastable superlattices, involving monolayer-level integration of the dielectric and ferroelectric oxides, was described.
Abstract: Molecular beam epitaxy (MBE) has achieved unparalleled control in the integration of semiconductors at the nanometer level; its use for the integration of oxides with similar nanoscale customization appears promising. This paper describes the use of reactive MBE to synthesize layered oxide heterostructures, including new compounds and metastable superlattices, involving monolayer-level integration of the dielectric and ferroelectric oxides SrO, SrTiO 3 , BaTiO 3 , PbTiO 3 , and Bi 4 Ti 3 O 12 . The controlled synthesis of such layered oxide heterostructures offers great potential for tailoring the dielectric and ferroelectric properties of materials. Oxide nano-engineering is accomplished by supplying the incident species in the desired layering sequence with submonolayer composition control. Comparisons between the growth of compound semiconductors and oxides by MBE are made.
186 citations
TL;DR: A comprehensive review on the recent literature of boron carbide CVD can be found in this article, where the status of the theoretical modeling and the developments on the experimental processes are reported.
Abstract: Boron carbide is an important non-metallic material with outstanding hardness, excellent mechanical, thermal and electrical properties. Its low density, high chemical inertness and neutron capture section make boron carbide an attractive material for micro-electronic, nuclear, military, space and medical applications. Boron carbide based materials are widely deposited by chemical vapor deposition methods (CVD). This paper provides a comprehensive review on the recent literature of boron carbide CVD. Structure, properties and potential application areas of this material are also reviewed. The status of the theoretical modeling of boron carbide deposition and the developments on the experimental processes are reported. It was evident from this review that extensive research still remains to be done on the modeling of CVD boron carbides. Majority of the reviewed published research papers deals with characterization, and growth rate of deposited boron carbide phases. Some thermodynamic modeling based on Gibbs free energy minimization were attempted in classical CVD systems. However, these models were often not able to represent the actual growth mechanism. No significant modeling work has been reported in other CVD systems such as plasma enhanced chemical vapor deposition (PECVD), hot filament chemical vapor deposition (HFCVD), synchrotron radiation chemical vapor deposition (SRCVD). Reliable thermo-chemical data for boron carbides with various stoichiometries are also needed to study and model actual deposition reaction mechanisms of such complex systems.
176 citations
TL;DR: In this paper, the influence of the substrate-to-target distance and deposition temperature on the resistivity of ZnO films has been studied, and it was shown that the visible transmittance and IR reflectance of ZAO films deposited on polyester at room temperature are comparable to those of ITO films.
Abstract: Al-doped ZnO (ZAO) is an attractive material, as it is cheap and abundant, it is nontoxic, has comparable electrical, optical and infrared (I-R) properties to ITO, is cost-effective and is easy to fabricate. In this paper, ZAO films have been prepared by reactive magnetron sputtering. Their electrical conductivity is shown to be in the region of 10(-4) Omega cm. The influence of the substrate-to-target distance and deposition temperature on the resistivity of ZAO films has been studied. Their IR and optical properties have been investigated with UV-VIS and Fourier transformation IR spectrometry. It was shown that the visible transmittance and IR reflectance of ZAO films deposited on polyester at room temperature are comparable to those of ITO films. ZAO films have n-type conductivity. It was found that by adding Al to ZnO the variation in resistivity of ZAO films was decreased, and was usually no more than 1% at 50-400 degreesC in vacuum and 5% at 50-400 degreesC in air. (C) 2001 Elsevier Science B.V. All rights reserved.
126 citations
TL;DR: In this article, X-ray diffraction patterns indicated that the products were pure orthorhombic Bi2S3 phase, cubic phase HgS, hexagonal phase CuS, cubic phases ZnS and cubic phase PbS, respectively.
Abstract: Nanocrystalline metal (Cu, Hg, Zn, Bi, Pb) sulfides with different shapes and different particle sizes have been successfully prepared in a formaldehyde solution of metal salt and thioacetamide by microwave irradiation. Powder X-ray diffraction patterns indicated that the products were pure orthorhombic Bi2S3 phase, cubic phase HgS, hexagonal phase CuS, cubic phase ZnS, cubic phase PbS, respectively. The products were also characterized by transmission electron microscopy. Ultraviolet reflection spectrum clearly indicates the presence of quantum size effects in ZnS.
107 citations
TL;DR: In this article, the authors used DTA, XRD, BET, SEM, Mossbauer spectroscopy and tested for the catalytic combustion of methane using perovskite catalysts, assuming that the excess of iron oxide in the e > 0 samples corresponds to the presence of surface of layers of FeO 5 polyhedra, coordinatively unsaturated.
Abstract: La (1− e ) FeO (3−1.5 e ) perovskite catalysts ( e =0, 0.1, 0.2, 0.3) are synthetized by low-temperature thermal decomposition of La and Fe nitrates and further heating to 873 K in air. They are characterized by DTA, XRD, BET, SEM, Mossbauer spectroscopy and tested for the catalytic combustion of methane. All oxides are monophasic; for e =0 the structure is orthorhombic, whereas for 0 e ≤0.3 a pseudocubic symmetry is evident. The Mossbauer spectra are characterized by antiferromagnetic sextuplets; for e =0.3 a doublet is also present. The catalytic activity toward CH 4 oxidation appears to be strongly enhanced in the samples with e >0; the most active composition corresponds to e =0.1, while for e =0.2 and 0.3 the catalytic activity is slightly decreased. The results are explained assuming that the excess of iron oxide in the e >0 samples corresponds to the presence of surface of layers of FeO 5 polyhedra, coordinatively unsaturated, according to proposed structural model.
98 citations
TL;DR: In this article, GaN:Er TFEL devices have exhibited a brightness of 500-1000 cd:m 2 at 540 nm. But the performance of these devices is limited by the bias voltage and bias polarity.
Abstract: Rare earth (RE) doping of GaN has led to a new full color thin film electroluminescent (TFEL) phosphor system. GaN films doped with Eu, Er, and Tm dopants emit pure red, green, and blue emission colors, respectively. As a host for RE luminescent centers, GaN possesses many properties which are ideal for bright multiple color TFEL. Specifically, GaN has excellent high field transport characteristics, is chemically and thermally rugged, and incorporates well the RE dopants. X-ray absorption measurements have shown that even at RE dopant levels exceeding 0.1 at.% the majority of RE dopants occupy a strongly bonded substitutional site on the Ga sublattice. According to RE crystal field theory this tetrahedrally bonded site allows optical activation and emission from RE 4f‐4f inner-shell electronic transitions. Monte Carlo calculations of GaN carrier transport have shown that at 2M V cm 1 applied field the average electron possesses 2.6 eV energy which is adequate for exciting blue emission. GaN:Er TFEL devices have exhibited a brightness of 500‐1000 cd:m 2 at 540 nm. In addition to pure colors, mixed colors can be achieved by doping with a combination of REs. For example, co-doping with Er and Tm results in an emission spectrum which is perceived by the human eye as a blue‐green (turquoise) hue. Multiple color capability in a single device has also been demonstrated by adjusting the bias voltage (in a co-doped GaN:Er,Eu layer) or by switching the bias polarity (in a stacked two layer GaN:Er:GaN:Eu structure). The combination of pure or mixed color emission, the availability of bias controlled color, and the potential for white light emission indicate that GaN:RE TFEL devices have enormous potential for display applications. © 2001 Elsevier Science B.V. All rights reserved.
98 citations
TL;DR: In this article, the currentvoltage and capacitance-voltage characteristics of Au/n-GaAs Schottky diodes on n-Ge substrates are investigated and compared with characteristics of GaAs/GaAs substrates.
Abstract: The current–voltage (I–V) and capacitance–voltage (C –V) characteristics of Au/n-GaAs Schottky diodes on n-Ge substrates are investigated and compared with characteristics of diodes on GaAs substrates. The diodes show the non-ideal behavior of I– V characteristics with an ideality factor of 1.13 and barrier height of 0.735 eV. The forward bias saturation current was found to be large (3 ×10 −10 A vs. 4.32 ×10 − 12 A) in the GaAs/Ge Schottky diodes compared with the GaAs/GaAs diodes. The energy distribution of interface states was determined from the forward bias I–V characteristics by taking into account the bias dependence of the effective barrier height, though it is small. The interface states density was found to be large in the Au/n-GaAs/n-Ge structure compared with the Au/n-GaAs/n + -GaAs structure. The possible explanation for the increase in the interface states density in the former structure was highlighted. © 2001 Published by Elsevier Science B.V.
97 citations
TL;DR: In this article, the mechanical, piezoelectric and dielectric properties of PZT ceramics (Pb 0.98 M 0.02 [(Zr 0.535 Ti 0.465 ) 0.995 ]O 3, M=Er, Gd, Eu, Nd, La and Bi) have been studied.
Abstract: Mechanical, piezoelectric and dielectric properties of PZT ceramics (Pb 0.98 M 0.02 [(Zr 0.535 Ti 0.465 ) 0.995 ]O 3 , M=Er, Gd, Eu, Nd, La and Bi) have been studied. The fraction of A sites occupied by the different additives is estimated and is found to increase with the ionic radius of the additive ion. The density is high in Bi-, Gd- and Eu-PZT samples. Small amounts of the pyrochlore phase are present in Gd- and Eu-PZT. A maximum strength of 80–100 MPa and K IC of ∼1.5 MPa m 1/2 are obtained in Gd- and Eu-PZT. The R -curve behavior becomes more pronounced in Gd- and Eu-PZT, and it is observed that the strength and K IC increase with the slope of the K IC versus c 1/2 plot. Very good piezoelectric properties, i.e. d 33 of 410 PCN −1 , dielectric constant of 1005, κ p of 0.49 and the hydrophone figure of merit, d h g h , of 2760 are obtained for Nd-PZT. The piezoelectric properties in Bi-PZT are also higher than for the undoped composition, while for the Gd-PZT, these are slightly inferior. For all other compositions, the piezoelectric properties are not affected much.
95 citations
TL;DR: In this article, the carrier concentration of ZnO thin films was found to be dependent upon the doping of Al, and varied in the range from 10 19 to 10 20 cm −3.
Abstract: Undoped and Al-doped ZnO thin films have been deposited on Si substrates using metalorganic vapor phase epitaxy at atmospheric pressure. The as-deposited ZnO films showed good crystalline character and exhibited (002) orientation with the c axis perpendicular to the substrate surface. The carrier concentration of ZnO films was found to be dependent upon the doping of Al, and varied in the range from 10 19 to 10 20 cm −3 . The resistivity of ZnO films was in the order of magnitude of 10 −3 Ωcm. The Hall mobility decreased with the doping of Al and was in the range 5–53 cm 2 (V·s) −1 . Raman spectra indicated the observed A 1 (LO) and E 2 (high) bands shifted towards the low-frequency side.
TL;DR: In this article, high-textured NaCo2O4 ceramics were prepared and the thermoelectric properties were examined in the preferred {001} plane and the textured ceramic specimen had in-plane electrical conductivity: 600-300 S cm−1, Seebeck coefficient, 60-120 μV K−1 and thus power factor: 2-5×10−4 W mK−2 in the temperature range from room temperature to 700°C.
Abstract: Highly-textured NaCo2O4 ceramics were prepared and the thermoelectric properties were examined in the preferred {001} plane. Plate-like Co3O4 particles were synthesized and used as a reactive template for c-axis oriented NaCo2O4 ceramics. Tape casting of the template particles with Na2CO3 and the following in-situ reaction produced grain-oriented ceramics topotaxially as designed. The textured ceramic specimen had in-plane electrical conductivity: 600–300 S cm−1, Seebeck coefficient, 60–120 μV K−1 and thus power factor: 2–5×10−4 W mK−2 in the temperature range from room temperature to 700°C. A large excess amount of Na and inhomogeneity in its distribution could be attributed to the lower transport properties than expected from those of a single crystal.
TL;DR: In this article, the authors used micro-Raman spectroscopy to characterize the surface of the thin Al 2 O 3 films deposited on Si(010) substrate through pulsed laser ablation.
Abstract: Successful use of micro-Raman spectroscopy to characterize the surface of the thin Al 2 O 3 films deposited on Si(010) substrate through pulsed laser ablation is presented. The micro-Raman spectra have been recorded on several spots (sized ≈1 μm) over the length and the breadth of the films in a systematic fashion. The as-deposited films are inhomogeneous from spot-to-spot over the surface. The variation in characteristic properties of 521 cm −1 line of crystalline Si (c-Si) have been used to estimate the thickness of the Al 2 O 3 films. The thickness of the films has been found to depend on the position of the substrate with respect to the plume boundaries produced by the laser pulse after hitting the Al target. The thickness also depends on the position of the spot on a film placed at a fixed distance from the target surface. The thickness profiling of the deposited films on the basis of micro-Raman spectroscopic investigations corroborates the theoretical prediction of the model of the plume formation and the formation of the shock-front region in the pulsed laser deposition technique. All the seven Raman active modes (2A 1g +5E g ) observable in single crystals of sapphire (c-Al 2 O 3 ) powder have been identified in the complex spectra obtained from deposited film surfaces under varying experimental conditions. The intensity variation of an isolated line (750 cm −1 ) of Al 2 O 3 corroborates the deposition profiling obtained from c-Si line parameters. The most intense Raman lines in the micro-Raman spectra of the deposited films are two infrared active modes around 450 and 600 cm −1 . The strong presence of these infrared active modes in the micro-Raman spectra suggests a highly distorted micro-crystalline structure of the films. The local micro-stress created by the micro-crystals on the Si-substrate is maximum near the stopping distance of the plume. Largest micro-crystal of Al 2 O 3 are also seen over the spot near the stopping distance of the plume.
TL;DR: In this paper, the authors used a silica particle to intentionally create some predictable pinhole defects on the protective layer of an OLED display, and found a linear growth of all dark spots and a linear correlation between growth rate and area.
Abstract: We present our in-situ experimental observations of dark spot growth in OLED devices using optical microscopy. In order to understand the formation mechanism of dark spots, we employed silica particle to intentionally create some predictable pinhole defects on the protective layer. We found a linear growth of all dark spot as well as a linear correlation between growth rate and area. These results indicate that dark spot formation is related to corrosion of the calcium or other materials with water and oxygen through pinhole defects. Furthermore, we found a correlation between the degradation of entire devices and growth of all dark spots, which allows us to predict the lifetime of entire devices.
TL;DR: A brief review of recent progress in fabrication of high voltage GaN and AlGaN rectifiers, GaN/AlGaN heterojunction bipolar transistors and GaN metaloxide semiconductor field effect transistors is given in this article.
Abstract: A brief review is given of recent progress in fabrication of high voltage GaN and AlGaN rectifiers, GaN/AlGaN heterojunction bipolar transistors and GaN metal-oxide semiconductor field effect transistors. Improvements in epitaxial layer quality and in fabrication techniques have led to significant advances in device performance.
TL;DR: In this article, a novel gel auto-combustion method was used to synthesize Mg-Zn-Cu ferrite powders with compositions of (Mg 0.5− x Cu x Zn 0.98)O(Fe 2 O 3 ) 0.20, 0.25,0.35, 040).
Abstract: A novel gel auto-combustion method was used to synthesize Mg–Zn–Cu ferrite powders with compositions of (Mg 0.5− x Cu x Zn 0.5 )O(Fe 2 O 3 ) 0.98 (where x =0.20, 0.25, 0.30, 0.35, 0.40). X-ray diffraction results showed that the dried gels, synthesized from metal nitrites and citric acid, transformed directly into nano-sized ferrite particles after an auto-combustion process in air. The synthesized powders exhibited high-sintering activity, and can be sintered at temperature less than 950 °C. The low-temperature sintered Mg–Zn–Cu ferrites prepared possess good electromagnetic properties, as well as fine-grained microstructures, making them become good materials for multilayer chip inductors with high-performance and low-cost. Copper content has significant influence on the electromagnetic properties, such as initial permeability, quality factor, DC resistivity, dielectric constant and dielectric loss tangent as well as their frequency dispersion for Mg–Zn–Cu ferrites. The possible reasons that are responsible for the composition dependence of main electromagnetic properties were discussed.
TL;DR: In this paper, a polaron hopping mechanism was used to obtain the dielectric constant (e′), loss tangent (tan δ), and AC conductivity ( σ AC ) of magnetoelectric composites in the frequency range 100 Hz-1 MHz and also in the temperature range 30-400°C.
Abstract: Magnetoelectric composites with composition x BaTiO 3 +(1− x )MnFe 1.8 Cr 0.2 O 4 in which x varies as 1, 0.85, 0.7, 0.55 and 0 were prepared by a standard ceramic technique. X-Ray analysis confirms single-phase formation in x =1 and x =0 compositions whereas the presence of both phases in x =0.85, 0.7 and 0.55 compositions. The dielectric constant (e′), loss tangent (tan δ) and AC conductivity ( σ AC ) of the composites have been investigated in the frequency range 100 Hz–1 MHz and also in the temperature range 30–400 °C. The results obtained are interpreted in terms of a polaron hopping mechanism. Static magnetoelectric conversion factor (d E /d H ) as a function of magnetic field was measured in x =0.85, 0.70 and 0.55 compositions. The maximum value of d E /d H obtained was 150 μV/(cm −1 ×Oe) for x =0.85 composition.
TL;DR: The present combined work of transport measurement, neutron scattering, and theoretical calculation provides evidence that the gigantic anomalous Hall effect observed in Nd2Mo2O7, a pyrochlore ferromagnet with geometrically frustrated lattice structure, is mostly due to the spin chirality and the associated Berry phase originating from the Mo spin tilting.
Abstract: Quantum transport is very sensitive to the interference of the quantal phase. The Aharanov–Bohm effect is one of the most prominent examples of it. In ferromagnets, the non-coplanar spin structure gives rise to the analogous effect due to the spin chirality, i.e. the solid angle subtended by the spins. This contributes to the anomalous Hall effect at low temperatures. We present here the theoretical results on the two-dimensional Kagome lattice, which is a typical example of the flat-band ferromagnet. The Berry phase induced by the tilting of the spins opens the band gap, and quantized Hall conductance σxy=±e2/h is realized without external magnetic field. We also discuss the anomalous Hall effect observed in the three-dimensional metallic pyrochlore ferromagnets R2Mo2O7 (R=Nd, Sm, Gd).
TL;DR: In this article, the effects of hydrothermal ageing on the dielectric properties of epoxy-silica composites used for microelectronic packaging were investigated. But the study was carried out for samples with various degrees of curing.
Abstract: We report on the effects of hydrothermal ageing on the dielectric properties of epoxy-silica composites used for microelectronic packaging. The study was carried out for samples with various degrees of curing. Epoxy compounds were subjected to moisture exposure (standard JEDEC procedures) and consecutive thermal stress (240°C). Changes in the dielectric constant and in the loss factor were measured in the 100 Hz–100 kHz frequency range. It is found that water absorption increases with curing. After ageing the dielectric constant is decreased and the loss factor increased. Cured materials have a higher resistance against hydrothermal ageing.
TL;DR: In this paper, an Er-doped SiO2 layer was implanted with Er to peak concentrations ranging from 0.015 to 1.8 at. 458 nm and two sharp Er luminescence lines at 982 and 1536 nm were observed.
Abstract: Silicon nanocrystals were formed in SiO2 using Si ion implantation followed by thermal annealing. The nanocrystal-doped SiO2 layer was implanted with Er to peak concentrations ranging from 0.015 to 1.8 at.%. Upon 458 nm excitation, a broad nanocrystal-related luminescence spectrum centered around 750 nm and two sharp Er luminescence lines at 982 and 1536 nm are observed. By measuring the temperature-dependent intensities and luminescence dynamics at a fixed Er concentration, and by measuring the Er concentration dependence of the nanocrystal and Er photoluminescence intensity, the nanocrystal excitation rate, the Er excitation and decay rate, and the Er saturation with pump power we conclude that: (1) the Er is excited by excitons recombining within Si nanocrystals through a strong coupling mechanism; (2) the exciton‐Er energy transfer rate is \10 6 s 1 ; (3) the exciton‐Er energy transfer efficiency is\60 %; (4) each nanocrystal can have at most 1‐2 excited Er ions in its vicinity, which is attributed to either an Auger de-excitation or a pair-induced quenching mechanism; (5) at a typical nanocrystal concentration of 10 19 cm 3 , the maximum optical gain at 1.54 mm of an Er-doped waveguide amplifier based on Si nanocrystal-doped SiO2 is 0.6 dB cm 1 ; (6) the effective Er excitation cross-section using this nanocrystal sensitization scheme is seff:10 15 cm 2 at 458 nm, which is a factor 10 5 ‐10 6 larger than the cross-section for direct optical pumping of Er. This enables the fabrication of an Er-doped nanocrystal waveguide amplifier that can be pumped using a white light source. © 2001 Elsevier Science S.A. All rights reserved.
TL;DR: In this paper, the KPb2Cl5 single crystals, doped by different rare earth ions were grown and the RE segregation coefficient was found to decrease from 10 to 015 in the set from Nd to Yb Spectroscopic properties and luminescence decay were studied, stimulated emission was demonstrated at 106 μm (Nd3+) and at 243 μm(Dy3+)
Abstract: The KPb2Cl5 single crystals, doped by different Rare Earth ions were grown The RE segregation coefficient was found to decrease from 10 to 015 in the set from Nd to Yb Spectroscopic properties and luminescence decay were studied, stimulated emission was demonstrated at 106 μm (Nd3+) and at 243 μm (Dy3+)
TL;DR: In this paper, the effect of the presence of metal acetates and fluorides at an aluminum/tris(8-hydroxyquinoline) aluminum (Alq 3 ) interface on performance of organic electroluminescent (EL) devices is described.
Abstract: The effect of alkali metal (Li + , Na + , K + , Rb + , and Cs + ) acetates and fluorides at an aluminum/tris(8-hydroxyquinoline) aluminum (Alq 3 ) interface on performance of organic electroluminescent (EL) devices is described. We also studied the effect of acetates with different metals, such as Mg 2+ and Al 3+ , on the performance of the EL devices. EL characteristics of the devices with Al/alkali metal acetates were compared with those of devices with Al/alkali metal fluorides or an Al cathode itself. We found that the bilayer cathodes with alkali metal acetates exhibited better device performance than the bilayer cathodes with alkali metal fluorides. The improvement can be attributed to lowering of the work function of the Al cathode and doping of Alq 3 with alkali metals that were formed by the higher reactivity of alkali metal acetates than alkali metal fluorides with hot Al atoms impinging on these salts during the vapor deposition. The enhanced electron injection resulted in more balanced charge-carrier injection and thus higher EL efficiencies.
TL;DR: The optical properties of these films were investigated using both transmission and reflection spectra in the wavelength range of 200-900 nm in this paper, and it was found that the refractive index and packing fraction values decrease with La doping.
Abstract: The La modified lead titanate Pb 1− x La x Ti 1− x /4 O 3 (PLT) ( x =0.0, 0.05, 0.10, 0.15, 0.20, 0.25 and 0.30) thin films were deposited on sapphire (0001) substrates by the sol–gel process. The X-ray diffraction data and micro-Raman analysis show that with the increase in La content the crystal quality of the PLT films undergoes a tetragonal-to-cubic transformation. X-ray photoelectron spectroscopy analysis shows an excellent surface stoichiometry for all compositions under study. The optical properties of these films were investigated using both transmission and reflection spectra in the wavelength range of 200–900 nm. The appearance of interference fringes is an indication of the thickness uniformity of the film. The low value of extinction coefficient (in the order of 10 −2 ) as observed in our films is a qualitative indication of excellent surface smoothness of the films. Absorption coefficient ( α ) and the band-gap energy ( E g ) are obtained for undoped and La doped films with varying La concentrations. It has been found that the refractive index and packing fraction values decrease with La doping. Lanthanum doping was found to decrease the grain size of the films and improve the densification of individual grains. Increased La content lead to clusterification of smaller grains. The observed variation of band-gap energy with La doping has been correlated with the observed microstructure of these films.
TL;DR: In this paper, the current status of SiC bulk sublimation growth for the production of these substrates is reviewed from an industrial point of view, and the effect of micropipe densities and their characteristic lateral distribution in SiC wafers on achievable device yields is discussed.
Abstract: The production of large diameter, high quality SiC substrates is essential to realize the full potential of this important semiconductor material. The current status of SiC bulk sublimation growth for the production of these substrates is reviewed from an industrial point of view. Specific efforts towards larger diameter high quality substrates have led to the production of 50 and 75 mm diameter 4H and 6H wafers and the demonstration of high quality 100-mm wafers. We present thermal conductivity data for material of different doping levels, relevant for device applications. In SiC, micropipes are the most harmful defects for SiC device production. By continuous optimization of the growth process we were able to steadily decrease the micropipe density over the past several years, down to densities as low as 1.1 cm −2 for an entire 50-mm wafer, indicating that micropipes may be totally eliminated in the next few years. In order to achieve this goal for SiC substrates of increasing diameter, a thorough understanding of the growth process is essential. We will summarize results of modeling the growth process and its experimental verification. The effect of micropipe densities and their characteristic lateral distribution in SiC wafers on achievable device yields will be discussed, using large area Schottky diodes as an example.
TL;DR: The effect of combined Pd and Cu doping on microstructure, electrical and gas sensor properties of nanocrystalline tin dioxide was studied in this article, where the authors found that Pd clusters increased resistivity increase accompanied by the appearance of conductivity activation process and showed that CO adsorption may be a factor neutralizing the Pd doping action on the films conductivity.
Abstract: The effect of combined Pd and Cu doping on microstructure, electrical and gas sensor properties of nanocrystalline tin dioxide was studied. SnO2, SnO2(PdO), SnO2(CuO), and SnO2(PdO+CuO) films thickness of 0.8–1 μm with doping metal content 0.5–1.6 at.% were synthesized by aerosol pyrolysis. An average SnO2 grain size decreased with the addition of both Pd and Cu. The resistance measurements at 77–373 K showed that all types of doping induce resistivity increase accompanied by the appearance of conductivity activation process. Conductivity transients in the presence of CO were studied at 323–523 K. For the samples doped with Pd the sensor response to CO was found to be comparable with the resistivity increment induced by Pd incorporation into SnO2 matrix. To reveal the effect of CO on the conductivity the low temperature resistance was measured for the films in non-equilibrium state reached by cooling down the film exposed to CO at T=523 K. Experimental data proved that CO adsorption may be regarded as a factor neutralizing the Pd doping action on the films conductivity. The catalytic effect of Pd clusters was found in the interaction of SnO2(PdO+CuO) films with CO.
TL;DR: In this article, a technique for the synthesis of bismuth nanometer-sized particles using an inverse microemulsion method was presented, where poly(vinylpyrrolidone) was used as protecting agent.
Abstract: Bismuth (Bi) is classic semi-metal with a low carrier density, small carrier effective masses, very long mean free path and highly anisotropic Fermi surface. In this work, we present a technique for the synthesis of bismuth nanometer-sized particles using an inverse microemulsion method. To prevent air-oxidation in the characterization stage, poly(vinylpyrrolidone) was used as protecting agent. Our characterizations reveal that single rhombohedral phase of bismuth can be obtained with ultrafine particle with ∼20 nm in size by following this synthetic route. The optical absorption spectrum of the aqueous nanoparticle colloids shows an absorption band at ∼268 nm.
TL;DR: In this paper, a matched emitter material for thermophotovoltaic (TPV) energy conversion is described, where the emissive power spectrum is matched very efficiently to the response of GaSb photovolta cells that convert the infrared radiation into electricity.
Abstract: A novel, thermally excited emitter material for thermophotovoltaic (TPV) energy conversion is described in this work. The new emitter is called a ‘matched emitter’ because its emissive power spectrum is matched very efficiently to the response of GaSb photovoltaic cells that convert the infrared radiation into electricity. It has been shown that doping concentrations of 2–4 wt.% Co 3 O 4 or NiO within a low infrared emissivity MgO host will produce matched emitters with continuous, strong radiant emissions in the optimal energy range between 1 and 2 μm and minimal radiation at non-convertible wavelengths. The experimental measurements show that power densities of almost 9 W cm −2 , in the convertible wavelength range between 1 and 1.9 μm, and temperatures of 1400°C or more are easily achieved with NiO-doped MgO tape cast emitters in an air–propane flame. The first reported emittance measurements for NiO-doped MgO tape cast emitters show that remarkably high peak emissivities, up to 0.9, at near optimal wavelengths for TPV energy conversion are achievable. Both the measured power density (9 W cm −2 ) and peak emissivity (nearly 0.9) appear to greatly exceed all previous values reported for any selective emitter at the given temperatures and wavelengths.
TL;DR: In this paper, the physicochemical, dielectric and piezoelectric properties of anionic collagen and collagen-hydroxyapatite (HA) composites, considering the development of new biomaterials which have potential applications in support for cellular growth and in systems for bone regeneration, were reported.
Abstract: In this paper we report a study of the physicochemical, dielectric and piezoelectric properties of anionic collagen and collagen–hydroxyapatite (HA) composites, considering the development of new biomaterials which have potential applications in support for cellular growth and in systems for bone regeneration. The piezoelectric strain tensor element d14, the elastic constant s55, and the dielectric permittivity e11 were measured for the anionic collagen and collagen–HA films. The thermal analysis shows that the denaturation endotherm is at 59.47 °C for the collagen sample. The collagen–HA composite film shows two transitions, at 48.9 and 80.65 °C. The X-ray diffraction pattern of the collagen film shows a broad band characteristic of an amorphous material. The main peaks associated to the crystalline HA is present in the sample of collagen–HA. In the collagen–HA composite, one can also notice the presence of other peaks with low intensities which is an indication of the formation of other crystalline phases of apatite. The scanning electron photomicrograph of anionic collagen membranes shows very thin bundles of collagen. The scanning electron photomicrography of collagen–HA film also show deposits of hydroxyapatite on the collagen fibers forming larger bundles and suggesting that a collagenous structure of reconstituted collagen fibers could act as nucleators for the formation of apatite crystal similar to those of bone. The piezoelectric strain tensor element d14 was measured for the anionic collagen, with a value of 0.062 pC N−1, which is in good agreement compared with values reported in the literature obtained with other techniques. For the collagen–HA composite membranes, a slight decrease of the value of the piezoelectricity (0.041 pC N−1) was observed. The anionic collagen membranes present the highest density, dielectric permittivity and lowest frequency constant f.L.
TL;DR: In this paper, the effect of substitution of In 3+, Al 3+ and Cr 3+ ions on the electrical and magnetic properties of Mg-Mn ferrites has been investigated.
Abstract: The effect of substitution of In 3+ , Al 3+ and Cr 3+ ions on the electrical and magnetic properties of Mg–Mn ferrites has been investigated. The substitution of In 3+ ions in place of Fe 3+ ions, resulted in an increase of lattice parameter, owing to the larger size of the substituted ions, whereas lattice parameter decreased on substituting Al 3+ and Cr 3+ ions in place of the Fe 3+ ions, owing to the smaller sizes of these substituted ions. D.C. resistivity was found to increase with the substitution of In 3+ , Al 3+ or Cr 3+ ions in the Mg–Mn ferrite system. The improvement in the d.c. resistivity has been observed at the expense of deterioration in the magnetic properties of Al 3+ and Cr 3+ substituted Mg–Mn ferrites. A significant reduction in the value of initial permeability, saturation magnetization and Curie temperature were observed with successive increase of Al 3+ and Cr 3+ ions. The saturation magnetization and initial permeability were found to increase with incorporation of In 3+ ions. A marked increase in the value of initial permeability was found for the Mg 0.9 Mn 0.1 In 0.5 Fe 1.5 O 4 ferrites. The Curie temperature was found to decrease with an increase of In 3+ ion concentration. These changes in the properties are explained on the basis of a modified cation distribution and their magnetic interactions and various models.