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We demonstrate a high-pressure soft sputtering technique that can grow large area 1T′ phase MoTe2 sheets on HOPG and Al2O3 substrates at temperatures as low as 300 °C. The results show that a single Mo/Te co-sputtering step on heated substrates produces highly defected films as a result of the low Te sticking coefficient. The stoichiometry is significantly improved when a 2-step technique is used, which first co-sputters Mo and Te onto an unheated substrate and then anneals the deposited material to crystalize it into 1T′ phase MoTe2. A MoTe2−x 1T′ film with the lowest Te vacancy content (x = 0.14) was synthesized using a 300 °C annealing step, but a higher processing temperature was prohibited due to MoTe2 decomposition with an activation energy of 80.7 kJ mol−1. However, additional ex situ thermal processing at ∼1 torr tellurium pressure can further reduce the Te-vacancy (VTe) concentration, resulting in an improvement in the composition from MoTe1.86 to MoTe1.9. Hall measurements indicate that the films produced with the 2-step in situ process are n-type with a carrier concentration of 4.6 × 1014 cm−2 per layer, presumably from the large VTe concentration stabilizing the 1T′ over the 2H phase. Our findings (a) demonstrate that large scale synthesis of tellurium based vdW materials is possible using industrial growth and processing techniques and (b) accentuate the challenges in producing stoichiometric MoTe2 thin films.

Herein we show the terahertz (THz) time-domain ellipsometry investigation of the InSb semiconductor. At room temperature, the carriers of InSb are thermally excited to the conduction band due to its narrow bandgap. We, nevertheless, show that THz ellipsometry can detect subtle differences between carrier densities of undoped and lightly doped InSb bulk semiconductors. The advantage of THz ellipsometry over other THz time-domain spectroscopy techniques is that it does not require reference measurements through an aperture or standard mirrors.

A series of pyrite thin films were deposited in-situ in a monolayer-by-monolayer fashion using sequential evaporation of iron under high vacuum, followed by sulfidation at a sulfur pressure of 133 Pa, as a function of substrate temperature. The stoichiometry, crystallinity, topographic smoothness, and phase purity of the deposited pyrite thin films improve with increasing substrate temperature up to 420 °C, the highest temperature studied. Characterization of the films deposited at 420 °C using selected-area precession electron diffraction, Raman Spectroscopy, and conventional X-ray diffraction indicated that the pyrite layer is phase pure, with no evidence of a secondary marcasite phase, and grew in columnar grains with a preferential (100) orientation on the (100) silicon substrates. This novel sequential evaporation technique has the potential to make useful low-cost semiconducting pyrite materials for large-area electronic applications.

We demonstrate a high-pressure soft sputtering technique that can grow large area 1T' phase MoTe

We produced a large uniaxial anisotropy $(H_{k})$ in ultrathin (1.2–2.4 nm) soft ferromagnetic layers through the oblique deposition of a nanometer-thick metallic underlayer. Fe, Ni, and Ni80Fe20 (Permalloy) films deposited on a 5 nm thick Nb layer grown at an oblique angle (up to 60 ${}^{\circ}$ ) exhibit an $H_{k}\approx 40-500$ Oe and have properties that are nearly ideal for use as a fixed layer in magnetic devices, such as for magnetic memory. A much higher induced $H_{k}$ is observed in Ni (500 Oe) and Fe (300 Oe) compared to Ni-Fe (40 Oe). We used this technique to increase the switching field ( $H_{{\rm{sw}}}$ ) by 3–5 times in an array of pattered Fe or Ni-Fe bits, and achieved an excellent relative switching distribution of about 0.05–0.06. We also controllably enhanced the $H_{k}$ in the fixed layer of patterned spin-valve devices for superconducting Josephson-junction magnetic random-access memory and produced a large difference in $H_{{\rm{sw}}}$ between the fixed and free layers. Different spacer layers between the Nb and the magnetic layers either propagate (for Ru) or reduce (for Cu, Ru/Al, ion-milled Ru) the effect of the oblique Nb. This technique was also effectively used in synthetic antiferromagnet structures and, therefore, has the potential to eliminate the need for an antiferromagnetic pinning layer when it degrades magnetic device performance.

In this study, we show that deposited Ge and Si dielectric thin-films can exhibit low microwave losses at single-photon powers and sub-Kelvin temperatures (≈40 mK). This low loss enables their use in a wide range of devices, including coplanar, microstrip, and stripline resonators, as well as layers for device isolation, interwiring dielectrics, and passivation in microwave and Josephson junction circuit fabrication. We use coplanar microwave resonator structures with narrow trace widths and minimal over-etch to maximize the sensitivity of loss tangent measurements to the interface and properties of the deposited dielectrics, rather than to optimize the quality factor. In this configuration, thermally evaporated ≈1 µm thick amorphous germanium (a-Ge) films deposited on Si (100) have effective single-photon loss tangents of 4–5 × 10−6 and 9 μm-thick chemical vapor deposited homoepitaxial single-crystal Si has effective single-photon loss tangents of 4–14 × 10−6. Material characterization suggests that interface contamination could be the limiting factor for the loss.

In lithium–sulfur batteries (LSBs), soluble long-chain polysulfide intermediates can easily shuttle between the cathode and the anode, causing rapid performance degradation. Although significant progress, through rational cathode structure and composition design, has been made to solve the polysulfide shuttling problem, this challenging issue still exists. Considering the function of a separator in a cell is to isolate the cathode and anode materials, the transport properties of species across the separator should be investigated. Using bacterial cellulose (BC) as an example of a functional separator, we hypothesize that grafting anionic function groups on the cellulose chains could create an energy barrier that will block the diffusion of polysulfides across the separator. In our study, BC is functionalized by oxidizing hydroxyl groups on cellulose chains into carboxylate groups. Physicochemical and electrochemical studies confirm polysulfide shuttling is effectively suppressed. As a result, functionalized BC separator equipped LSB cells with a sulfur load of 4 mg/cm2 delivers ~1,300 mAh/g of specific capacity at 0.1°C, which can be maintained after 100 cycles above 1,000 mAh/g at 0.3°C, demonstrating its superior performance over commercial polyolefin-based separators.

Despite the practical importance of Ba-based perovskite dielectrics in microwave applications, the literature does not consistently agree which structural phases form. To provide insight, we use density functional methods to calculate Gibbs energies of structural polymorphs of 1:1 ordered Ba-based perovskites. Ba(In1/2Ta1/2)O3 has a ground state Fm 3¯m cubic phase at 0 K, while Ba(Y1/2Ta1/2)O3, Ba(Gd1/2Ta1/2)O3, and Ba(Nd1/2Ta1/2)O3 form lower symmetry ground state phases. With increasing temperature, the structural phases change from monoclinic I2/m to trigonal R 3¯ and finally to cubic Fm 3¯m. For the zero and higher temperature conditions, the energy differences between the ground state and next higher energetic polymorphs are below 50 meV/atom and arise predominantly from changes in the octahedral tilts. The resulting coexistence of two or more polymorphs at room temperature and similar x-ray diffraction patterns may explain why so many papers report different phases for the same compound.

The microwave dielectric loss of stoichiometric and non-stoichiometric Ba(Co1/3Nb2/3)O3 ceramics have been measured from 2-300 K in magnetic fields ranging from 0 to 5 T using a dielectric resonator (DR) technique. The microwave absorption from spin excitations of unpaired d-electrons in exchange coupled Co2+ ions dominate the loss of the Ba(Co1/3Nb2/3)O3 ceramics at cryogenic temperatures. Two peaks in the loss tangent (tan δ) versus temperature relation from a distinctly different origin occur at 25-30 K and 90 K, which increase in magnitude with increasing Co content in the bulk dielectric samples. Evidence that these peaks result from polaron conduction from hopping between Co2+ and Co3+ ions includes (1) the peak's observed temperature range, (2) the decrease in peak intensity of approximately a factor of two in a large applied magnetic fields (5 T), and (3) a strong correlation between the peak's magnitude and both the fraction of the minority Co3+ in the dominant Co2+ matrix and D.C. conductivity at elevated temperatures. A magnetic-field independent high temperature peak with a maximum at 250 K dominates the room temperature microwave loss whose magnitude correlates with those of the low temperature peaks. This suggests that the defects responsible for carrier conduction play an important role in establishing the loss tangent at room temperature. This article is protected by copyright. All rights reserved.

Copper-permalloy [Cu 1−x (Ni 80 Fe 20 ) x ] alloy films were deposited by co-sputtering and their chemical, structural, magnetic, and electrical properties were characterized. These films are found to have favorable weak ferromagnetic properties for low temperature magnetoelectronic applications. Our results show that by varying the composition, the saturation magnetization (M s ) can be tuned from 700 emu/cm 3 to 0 and the Curie temperature (T c ) , can be adjusted from 900 K to 0 K. The M s and T c are found to scale linearly between x = 25% and 100%. Electronic structure calculations are used to provide a strong fundamental understanding of the mechanisms responsible for establishing the observed electrical and magnetic properties. The theoretical results also show that the introduction of Cu into the permalloy lattice results in very strong spin scattering in the minority spin channel, with only moderate interactions in the majority channel.

The structural, vibrational, densification, and microwave properties of Ba(Co1/3Nb2/3)O3 ceramics with small compositional variations along several tie lines in the ternary BaO CoO Nb2O5 diagram were studied. The results showed that very small deviation from stoichiometric Ba(Co1/3Nb2/3)O3 composition has profound effect on Q × f, degree of ordering, densification, and phase assemblage. The 0.94 Ba(Co1/3Nb2/3)O3–0.06 Ba5Nb4O15 ceramic has the highest Q × f value (71 THz) – a value two times larger than that of stoichiometric Ba(Co1/3Nb2/3)O3 (36 THz). Transformation from the (partial) disordered distribution of Co and Nb cations to 1:2 ordered arrangement in the octahedral sites was found to increase the Q factor of the high density and single phase ceramics. It was also observed that formation of very small amount of Ba9CoNb14O45 second phase degraded Q × f value severely for the dense and highly ordered Nb-rich and Ba-deficient ceramics.

In 2012, we demonstrated that microwave loss in practical microwave dielectrics is dominated by electron paramagnetic resonance transitions at cryogenic temperatures. We later used this understanding to develop “smart” materials that switch Fe-doped Al2O3 (er = 9.8) dielectric ceramics between a low-loss “on state” and a high-loss “off state” at frequencies of ∼12 and ∼19 GHz with a small magnetic field (

Low-frequency electron paramagnetic resonance (EPR) is used to extract the EPR parameter A-mid and support the approximate X-band value of g-mid for Ba(CoyZn1/3−yTa2/3)O3. Although the cobalt hyperfine structure for the |±1/2〉 state is often unresolved at X-band or S-band, it is resolved in measurements on this compound. This allows for detailed analysis of the molecular orbital for the |±1/2〉 state, which is often the ground state. Moreover, this work shows that the EPR parameters for Co substituted into Zn compounds give important insight into the properties of zinc binding sites.

Nanoporous structures of a p-type semiconductor, delafossite CuAlO(2), with a high crystallinity have been fabricated through an inorganic/polymer double-gel process and characterized for the first time via Mott-Schottky measurements. The effect of the precursor concentration, calcination temperature, and atmosphere were examined to achieve high crystallinity and photoelectrochemical properties while maximizing the porosity. The optical properties of the nanoporous CuAlO(2) are in good agreement with the literature with an optical band gap of 3.9 eV, and the observed high electrical conductivity and hole concentrations conform to highly crystalline and well-sintered nanoparticles observed in the product. The Mott-Schottky plot from the electrochemical impedance spectroscopy studies indicates a flat-band potential of 0.49 V versus Ag/AgCl. It is concluded that CuAlO(2) exhibits band energies very close to those of NiO but with electrical properties very desirable in the fabrication of photoelectrochemical devices including dye-sensitized solar cells.

Microwave resonator measurements were performed on high-performance microwave ceramics Ba(Zn1/3Ta2/3)O3 (BZT) and Ba(Zn1/3Nb2/3)O3 (BZN) containing additives commonly used by commercial manufacturers (i.e., Co, Mn, and Ni). We find that the loss tangent, even in ambient magnetic fields, is dominated by electron paramagnetic resonance (EPR) absorption by exchange-coupled 3d electrons in transition metal clusters at cryogenic temperatures. The large orbital angular momentum in Co2+ and Ni2+ ions of L = 3 causes strong anisotropic-broadened dipolar interactions that extend EPR losses to zero applied field. This effect is greatest in BZN with Co concentrations greater than 0.5 mol%, dominating the losses at liquid nitrogen temperatures (77 K) and below. In samples containing Mn2+ ions with L = 0, the dipolar interactions and associated EPR losses in ambient fields are smaller. We show the magnetic-field-dependent changes in the EPR losses (i.e., tan δ) and magnetic reactive response (i.e., μr) are from the same mechanism, as they follow the Kramers–Kronig relation. Finally, we note that these materials can make ultra-high Q passive microwave devices with externally controlled transfer functions, as the quality factor (Q) of the composition Ba(Co1/15Zn4/15Nb2/3)O3 at 77 K can be tuned from 1 100 to 12 000 at 10 GHz by applying practical magnetic fields.

The prevalence of androgenic alopecia (AGA) increases with age and it affects both men and women. Patients diagnosed with AGA may experience decreased quality of life, depression, and feel self-conscious. There are a variety of therapeutic options ranging from prescription drugs to non-prescription medications. Currently, AGA involves an annual global market revenue of US$4 billion and a growth rate of 1.8%, indicating a growing consumer market. Although natural and synthetic ingredients can promote hair growth and, therefore, be useful to treat AGA, some of them have important adverse effects and unknown mechanisms of action that limit their use and benefits. Biologic factors that include signaling from stem cells, dermal papilla cells, and platelet-rich plasma are some of the current therapeutic agents being studied for hair restoration with milder side effects. However, most of the mechanisms exerted by these factors in hair restoration are still being researched. In this review, we analyze the therapeutic agents that have been used for AGA and emphasize the potential of new therapies based on advances in stem cell technologies and regenerative medicine., Video abstract

The temperature coefficient of resonant frequency ((f)) of a microwave resonator is determined by three materials parameters according to the following equation: (f)=-(1/2 (epsilon) + 1/2 + (L)), where (L), (epsilon), and are defined as the linear temperature coefficients of the lattice constant, dielectric constant, and magnetic permeability, respectively. We have experimentally determined each of these parameters for Ba(Zn1/3Ta2/3)O-3, 0.8 at.% Ni-doped Ba(Zn1/3Ta2/3)O-3, and Ba(Ni1/3Ta2/3)O-3 ceramics. These results, in combination with density functional theory calculations, have allowed us to develop a much improved understanding of the fundamental physical mechanisms responsible for the temperature coefficient of resonant frequency, (f).

Originally published in Microwave Opt Technol Lett 56: 371–375, 2014. © 2014 Wiley Periodicals, Inc. View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.28113. © 2014 Wiley Periodicals, Inc. Microwave Opt Technol Lett 56:1727–1728, 2014

Chromium silicide (CrSi 2 ) thermoelectric thin films with two different thicknesses, 1 μm and 0.1 μm, were deposited using radio frequency magnetron sputtering on glass substrates. These films were characterized after deposition and then after 300–600 °C anneals using X-ray diffraction and Energy dispersive X-ray spectroscopy. The Seebeck coefficient and electrical resistivity were measured. The compositions of the sputtered films were found to be close to the sputtering target stoichiometry. The annealing conditions and variations of thickness had a great influence on the thermoelectric performance of the films. The 0.1 μm p-type films annealed in an argon atmosphere at 400 °C exhibited the largest power factor of 1.0 × 10 − 3 W/(K 2 ·m).

We have synthesized stoichiometric Ba(Cd 1/3 Ta 2/3 )O 3 [BCT] (100) dielectric thin films on MgO (100) substrates using Pulsed Laser Deposition. Over 99% of the BCT film was found to be epitaxial [BCT (100) || MgO (100) and BCT (010) || MgO (010)] when grown with an elevated substrate temperature of 635 °C, an enhanced oxygen pressure of 53 Pa and a Cd-enriched BCT target with a 1 mol BCT: 1.5 mol CdO composition. A dielectric constant of 32 was inferred from low-frequency capacitance measurements of a planar interdigital metal pattern. Analysis of ultra violet optical absorption results indicates that BCT has a bandgap of 4.9 eV; while the interference pattern in the visible range is consistent with a refractive index of 2.1. Temperature-dependent electrical measurements indicate that the BCT films have a room temperature conductivity of 3 × 10 − 12 Ω − 1 cm − 1 with a thermal activation energy of 0.7 eV. A mean particle size of ~ 100 nm and a root mean square surface roughness of 5 to 6 nm were measured using Atomic Force Microscopy.

We explore the potential utility of the II-IV-V semiconductor ZnGeAs2 as the absorber material in solar cells. As-deposited ZnGeAs2 films prepared by pulsed laser deposition are amorphous because of the limited substrate temperature that can be used without the rapid loss of volatile Zn and As. Thermal processing above 450 °C results in crystallization and improved electrical properties with hole mobilities as high as 58 cm2/V s. The annealed films were used to fabricate p-type ZnGeAs2: n-type CdS cells on SnO2-buffered borosilicate glass substrates in the so-called superstrate geometry. Light-induced currents of up to ~2 mA/cm2 and open-circuit voltages of up to 470 mV were observed using backside illumination, indicating that these nascent devices hold potential for realizing high performance solar cells from earth-abundant elements. The performance of the devices fabricated to-date is degraded by conduction through shorts resulting from the presence of micron-sized pinholes in the absorber layer. Copyright © 2012 John Wiley & Sons, Ltd.

To understand the thermochemistry and determine the rate limiting steps of ZnGeAs 2 thin-film synthesis, experiments were performed to measure the (a) thermal decomposition rate and (b) elemental composition and deposition rate of films produced with pulsed laser deposition (PLD). The decomposition rate is kinetically limited with an activation energy of 1.08±0.05 eV and an evaporation coefficient of ∼10 −3 . We show that ZnGeAs 2 thin film synthesis is a metastable process with the kinetically-limited decomposition rate playing a dominant role at the elevated temperatures needed to attain epitaxy. Our conclusions are in contrast to those of earlier reports that assumed the growth rate is limited by desorption and the resulting low reactant sticking coefficient. The thermochemical analysis presented here can be used to predict optimal conditions for ZnGeAs 2 film physical vapor deposition and thermal processing.

We have experimentally measured the current–voltage and capacitance–voltage characteristics of Au/amorphous AsxSe1 − x (x ≤ 0.05)/Zr trilayer structures at temperatures from 4 to 295 K. The observed capacitance of structures with an amorphous AsxSe1 − x (a-AsxSe1 − x) thickness of ~ 0.4 to ~ 2.8 μm does not significantly change over the entire range of applied bias (− 5 V to 5 V), indicating that the a-AsxSe1 − x films are fully depleted and thus the structures are Mott barriers. The current–voltage (I–V) characteristics of the a-As0.03Se0.97 device at low ( 104 V/cm) increases rapidly with applied field as a result of carrier emission from localized states and is consistent with transport by the Poole–Frenkel mechanism. A permanent transition to a high conductance state (~ 10− 3 S) is observed after exposure to very high electric fields (~ 4 × 105V/cm).

In 0.14 Ga 0.86 N/GaN multiple quantum-wells (MQWs) with well widths of 7, 5 and 2.5 nm were grown using metal–organic chemical vapor deposition on (0 0 0 1) GaN/sapphire composites. The as-grown MQWs were investigated by high-resolution X-ray diffraction (HRXRD), scanning transmission electron microscopy and photoluminescence (PL). The HRXRD ω –2 θ scans and high-angle annular dark-field (HAADF) images show that the In 0.14 Ga 0.86 N/GaN interfaces are flat. Furthermore, the atomic-resolution HAADF images and HRXRD reciprocal space mappings indicate that interfaces between different layers in MQWs are pseudomorphic. The calculations of stresses in MQWs show that the compressive stress in the In 0.14 Ga 0.86 N wells decreases as the well width increases. Electron energy loss spectroscopy line scans reveal the occurrence of phase separation in the lateral direction only in 7 nm thick wells and inhomogeneous In distribution in the vertical direction as well. PL shows a red-shift of the In 0.14 Ga 0.86 N peak and decreased peak emission with increased well widths. Arguments will be developed to rationalize these observations.

Electrical, thermal and Li transport properties have been measured for polyester polyol and isocyanate-based polyurethanes doped with lithium trifluoromethanesulfonimide (LiTFSI) and lithium perchlorate (LiClO 4 ). Electrical conductivities are estimated at 10 −5 –10 −6 S/cm near 300 K. The conductivities show a Vogel–Tammann–Fulcher (VTF) behavior over wide temperature ranges, characteristic of segmental polymer chain motions, and are approximately an order of magnitude larger for LiTFSI-doped than for perchlorate-doped samples. Differential scanning calorimetry (DSC) shows that T g does not significantly depend on doping type or concentration. Room-temperature 7 Li diffusivities, measured by pulsed gradient NMR, show an unexpected strong, linear increase with LiTFSI doping, but only a weak increase with LiClO 4 content. These findings may indicate substantial Li clustering in the LiTFSI-doped polymers, but may also reflect the effects of doping upon interface conduction between hard and soft polymer domains. Charge carrier densities estimated from the Nernst–Einstein relation, using measured NMR diffusivity values and ionic conductivities, range from approximately 8% to 29% of total Li densities for LiTFSI, indicating that a significant fraction of Li is involved in room-temperature ionic conduction in this material. For LiCLO 4 the carrier fraction is smaller, implying that Li is more tightly bound to its anion site.

The effect of $\alpha$-particle irradiation on a NdFeAs(O,F) thin film has been investigated to determine how the introduction of defects affects basic superconducting properties, including the critical temperature $T_c$ and the upper critical field $H_{c2}$, and properties more of interest for applications, like the critical current density $J_c$ and the related pinning landscape. The irradiation-induced suppression of the film $T_c$ is significantly smaller than on a similarly damaged single crystal. Moreover $H_{c2}$ behaves differently, depending on the field orientation: for H//c the $H_{c2}$ slope monotonically increases with increasing disorder, whereas for H//ab it remains constant at low dose and it increases only when the sample is highly disordered. This suggests that a much higher damage level is necessary to drive the NdFeAs(O,F) thin film into the dirty limit. Despite the increase in the low temperature $H_{c2}$, the effects on the $J_c$(H//c) performances are moderate in the measured temperature and field ranges, with a shifting of the pinning force maximum from 4.5 T to 6 T after an irradiation of $2\times10^{15} cm^{-2}$. On the contrary, $J_c$(H//ab) is always suppressed. The analysis demonstrates that irradiation does introduce point defects acting as pinning centres proportionally to the irradiation fluence but also suppresses the effectiveness of c-axis correlated pinning present in the pristine sample. We estimate that significant performance improvements may be possible at high field or at temperatures below 10 K. The suppression of the $J_c$(H//ab) performance is not related to a decrease of the $J_c$ anisotropy as found in other superconductors. Instead it is due to the presence of point defects that decrease the efficiency of the ab-plane intrinsic pinning typical of materials with a layered structure., Comment: 12 pages, 9 figures

We report the leakage-current characteristics of bulk vanadium (donor)- and scandium (acceptor)-doped Ba0.7Sr0.3TiO3 ceramic structures measured using gold electrical contacts. Vanadium doping reduces the ohmic leakage current that dominates the transport characteristics by up to 5 kV cm−1. The Arrhenius activation energy is 0.18, 0.20 and 0.23 eV for 1, 2 and 4 at.% V-doped samples, respectively. Above this field, the current–voltage characteristics exhibit discontinuous current transitions associated with trap filling by electronic carriers. At higher fields, trap-controlled, space-charge-limited conduction (SCLC) is observed with an effective mobility of 4 ± 1 × 10−7 cm2 V−1 s−1, characteristic of an electronic transport process that involves quasi-equilibrium between conduction in the band and trapping. In contrast, the leakage current of Sc-doped samples increases with impurity concentration and exhibits a 0.60 eV activation energy. In this case, the limiting current conduction mechanism is the transport of holes over the electrostatic barrier at grain boundaries. Comparison of these results to those on similarly doped homoepitaxial SrTiO3 thin films deposited on single-crystal and bicrystal substrates helped to identify the characteristics of transport in the bulk and across grain boundaries for this class of materials. The Sc- and V-doped thin films and ceramics did not exhibit electrical properties characteristic of shallow-level impurity doping, suggesting that V and Sc impurities form deep-level point defects and /or complexes, or are strongly compensated.

Cd13-xInySb10 (x ≈ 2.7, y ≈ 1.5) was synthesized in the form of mm-sized crystals from reaction mixtures containing excess cadmium. The intermetallic compound crystallizes in the rhombohedral space ...

Samples of the solid solution [ZnSnSb2]1-x[2(InSb)]x have been prepared over the whole range of composition by tin flux synthesis. The lattice parameter of the sphalerite-type average structure var ...

We have synthesized and characterized epitaxial and stoichiometric Ba(Zn1/3Ta2/3)O3 (1 0 0) dielectric thin films grown on MgO (1 0 0) substrates by pulsed laser deposition. Advanced electronic structure calculations were used to guide the interpretation of the experimental data. Zn-enriched targets and high oxygen pressures were used to compensate for Zn loss during film growth. The Ba(Zn1/3Ta2/3)O3 films had an indirect optical band gap of ∼3.0 eV and a refractive index of 1.91 in the visible spectral range. Zn–Ta B-site ordering was not observed in the Ba(Zn1/3Ta2/3)O3 thin film X-ray diffraction data. A dielectric constant of 25 and dissipation factor of 0.0025 at 100 kHz were measured using the interdigital capacitor method. The Ba(Zn1/3Ta2/3)O3 films exhibited a small thermally activated ohmic leakage current at high fields (

The alloy Zn41Sb59 was synthesized by high-pressure techniques and its bulk amorphous nature at ambient conditions confirmed by X-ray powder diffraction and electron microscopy studies. Electrical resistivity, thermopower, and thermal conductivity of Zn41Sb59 were measured in a temperature range from 6 to 350 K. Zn41Sb59 is a semiconductor and resistivity data suggest an activation energy of about 0.25 eV above 250 K. The thermopower is positive and increases with increasing temperature. Room-temperature values for the resistivity and thermopower are 8 Ω cm and 300 μV/K, respectively. The temperature dependence of the thermal conductivity is characteristic for amorphous solids and continuously increasing from 0.08 W/(m K) at 6 K to 0.19 W/(m K) at room temperature. Comparison of the thermal conductivity of Zn41Sb59 with the thermal lattice conductivity of the crystalline thermoelectric material β-Zn4Sb3 reveals a remarkable resemblance, justifying the classification of the latter as a phonon-glass material.

We studied the occurrence of phase separation and atomic ordering in quaternary InxAlyGa1−x−yN layers by transmission electron microscopy. Three layers of different compositions were examined: one lattice-mismatched In0.10Al0.02Ga0.88N layer together with two lattice-matched In0.12Al0.29Ga0.59N and In0.06Al0.18Ga0.76N layers. The composition modulations were seen in all the layers. The wavelengths (λ) of composition modulations resulting from phase separation were calculated using selected-area electron diffraction patterns. The smaller λ (9 nm) in In0.10Al0.02Ga0.88N layer in comparison to that in In0.12Ga0.88N layer (λ = 20 nm) suggests that the driving force for phase separation in InxAlyGa1−x−yN layers is greater than that in InxGa1−xN layers with similar In contents. Energy dispersive spectroscopy line profiles across InxAlyGa1−x−yN/GaN interfaces revealed a gradual increase in Al and In incorporation. Additional (0 0 0 1) diffraction spots in an SADP taken on an 〈 1 1 ¯ 0 0 〉 zone from the In0.06Al0.18Ga0.76N layer suggests that atomic ordering can occur in quaternary layers. Arguments are developed to rationalize these experimental observations.

The metastable binary intermetallic compound Cd4Sb3 was obtained as polycrystalline ingot by quenching stoichiometric Cd-Sb melts and as mm-sized crystals by employing Bi or Sn fluxes. The compound crystallizes in the monoclinic space group Pn with a = 11.4975(5) A, b = 26.126(1) A, c = 26.122(1) A, beta = 100.77(1) degrees, and V = 7708.2(5) A(3). The actual formula unit of Cd4Sb3 is Cd13Sb10 and the unit cell contains 156 Cd and 120 Sb atoms (Z = 12). Cd4Sb3 displays a reversible order-disorder transition at 373 K and decomposes exothermically into a mixture of elemental Cd and CdSb at around 520 K. Disordered beta-Cd4Sb3 is rhombohedral (space group R3c, a approximately = 13.04 A, c approximately = 13.03 A) with a framework isostructural to beta-Zn4Sb3. The structure of monoclinic alpha-Cd4Sb3 bears resemblance to the low-temperature modifications of Zn4Sb3, alpha- and alpha'-Zn4Sb3, in that randomly distributed vacancies and interstitial atoms of the high-temperature modification aggregate and order into distinct arrays. However, the nature of aggregation and distribution of aggregates is different in the two systems. Cd4Sb3 displays the properties of a narrow gap semiconductor. Between 10 and 350 K the resistivity of melt-quenched samples first increases with increasing temperature until a maximum value at 250 K and then decreases again. The resistivity maximum is accompanied with a discontinuity in the thermopower, which is positive and increasing from 10 to 350 K. The room temperature values of the resistivity and thermopower are about 25 mohms cm and 160 microV/K, respectively. Flux synthesized samples show altered properties due to the incorporation of small amounts of Bi or Sn (less than 1 at. %). Thermopower and resistivity appear drastically increased for Sn doped samples. Characteristic for Cd4Sb3 samples is their low thermal conductivity, which drops below 1 W/mK above 130 K and attains values around 0.75 W/mK at room temperature, which is comparable to vitreous materials.

Quaternary In0.12Al0.29Ga0.59N and In0.10Al0.02Ga0.88N layers ∼200 nm thick were grown on (0 0 0 1) GaN/sapphire composites using metalorganic chemical vapor deposition. The layers were studied using transmission electron microscopy (TEM). TEM results indicate that the quaternary layers contain high-density stacking faults (SFs). Weak-beam dark-field analysis coupled with high-resolution electron microscopy reveal that SFs have zinc-blende structures bounded by Shockley partials. Compared with In0.10Ga0.90N layers, SF density increases substantially in the In0.10Al0.02Ga0.88N layer with only 2% additional Al. Z-contrast annular dark-field images showed that SFs are Al-rich in the In0.12Al0.29Ga0.59N layer, but not in the In0.10Al0.02Ga0.88N layer. Two reference AlxGa1−xN layers were grown using identical conditions except the carrier gases. Using H2 carrier gas resulted in a ∼400 nm thick Al0.45Ga0.55N layer with no SFs, while using N2 carrier gas resulted in a ∼250 nm thick Al0.25Ga0.75N layer with SFs. It is suggested that the low surface mobility of (CH3)2Al:NH2 species in the N2 environment led to SF formation in the quaternary layers.

The low-temperature structural phase transitions of Bi, Pb, In and Sn-doped samples of thermoelectric Zn4Sb3 have been characterized on crystals grown from molten metal fluxes, using electrical resistance and single crystal X-ray diffraction measurements. Room temperature stable, disordered, beta-Zn4Sb3 undergoes two phase transitions at 254 and 235 K to the consecutively higher ordered phases a and alpha', respectively. The ideal crystallographic composition of alpha-Zn4Sb3 is Zn13Sb10. The alpha-alpha' transformation is triggered by a slight and homogenous Zn deficiency with respect to this composition and introduces a compositional modulation in the alpha-Zn4Sb3 structure. When preparing beta-Zn4Sb3 in the presence of metals with low melting points (Bi, Sn, In, Pb) the additional metal atoms are unavoidably incorporated in small concentrations (0.04-1.3 at%.) and act as dopants. This incorporation alters the subtle balance between Zn disorder and Zn deficiency in Zn4Sb3 and has dramatic consequences for its low-temperature structural behavior. From molten metal flux synthesis it is possible to obtain (doped) Zn4Sb3 samples which (1) only display a P-a transition, (2) only display a beta-alpha' transition, or (3) do not display any low-temperature phase transition at all. Case (2) provided diffraction data with a sufficient quality to obtain a structural model for highly complex, compositionally modulated, alpha'-Zn4Sb3. The crystallographic composition of this phase is Zn84Sb65.

This paper reports the results of an investigation of the structure–property relationship of vanadium (donor) and scandium (acceptor) doped Ba 0.7 Sr 0.3 TiO 3 . While V doping produces a fully miscible Ba 0.7 Sr 0.3 Ti 1− x V x O 3 alloy, Sc doping results in an inhomogeneous microstructure with grains containing a Sc-doped Ba-rich shell and an undoped Sr-rich core. This results from a solution–precipitation process that works in combination with a thermodynamic driving force for the preferential segregation of Sc into Ba-rich regions. The Curie temperature ( T c ) of Ba 0.7 Sr 0.3 TiO 3 decreases with increasing dopant concentration from a T c of 40 °C for undoped material to 18 °C for 4 mol.% V and 22 °C for 4 mol.% Sc (i.e., Ba 0.7 Sr 0.3 Ti 0.96 V 0.04 O 3 , Ba 0.7 Sr 0.3 Ti 0.96 Sc 0.04 O 3 ). Sc- and V-doped materials are found to have significantly reduced dielectric constants at their Curie temperature than their undoped counterpart. This is most evident for Sc doping, where a 4 mol.% concentration has a dielectric constant of 1000 at T c , a factor of 12 times smaller than undoped Ba 0.7 Sr 0.3 TiO 3 .

We investigated atomic ordering in InxGa1−xN layers for x between 0.03 and 0.28 using transmission electron microscopy. We demonstrate that {1 0 −1 1} facets play a role in 1:1 ordering. Deviations in composition from x = 0.5 are accommodated by the formation of small ordered regions embedded in a short-range ordered or disordered matrix.