LaAlO<Subscript>3</Subscript>/SrTiO<Subscript>3</Subscript> Epitaxial Heterostructures by Atomic Layer Deposition

Thin films of LaAlO₃ were deposited on TiO₂-terminated (100) SrTiO₃ crystals by atomic layer deposition (ALD), using tris(iso-propylcyclopentadienyl)lanthanum and trimethyl aluminum precursors. Water was used as the oxidizer. The film composition was shown to be controlled by the ratio of La/Al precursor pulses during ALD, with near-stoichiometric LaAlO₃ resulting at precursor pulse ratios of 4/1 to 5/1. Films near the stoichiometric LaAlO₃ composition were shown to crystallize on subsequent annealing to form epitaxial LaAlO₃/SrTiO₃ heterostructures. Electrical characterization of these structures was done by two-terminal direct-current (DC) current–voltage scans at room temperature and under high-vacuum conditions. The results show electrical conductivity for the ALD-deposited epitaxial LaAlO₃/SrTiO₃ heterostructures, which turns on for thickness above four unit cells for the LaAlO₃ film.     

Preparation and Thermoelectric Properties of La-Doped SrTiO<Subscript>3</Subscript> Ceramics

Single-phase polycrystalline La _x Sr_1−x TiO₃ (x = 0, 0.04, 0.06, 0.08, and 0.12) ceramics were prepared by the conventional solid-state reaction method using high-activity hydroxides as the raw materials. The electrical conductivity of all the samples increased with increasing x value and decreased with measurement temperature, while the thermal conductivity decreased with increasing x value and measurement temperature. The La_0.12Sr_0.88TiO₃ sample showed the lowest thermal conductivity of 2.45 W m⁻¹ K⁻¹ at 873 K and the largest ZT of 0.28 at 773 K. The present work revealed that hydroxides with high activity as raw materials are beneficial to improve the thermoelectric properties, especially to decrease the thermal conductivity.     

Low Fatigue in Epitaxial Pb(Zr<Subscript>0.2</Subscript>Ti<Subscript>0.8</Subscript>)O<Subscript>3</Subscript> on Si Substrates with LaNiO<Subscript>3</Subscript> Electrodes by RF Sputtering

Epitaxial PZT (001) thin films with a LaNiO₃ bottom electrode were deposited by radio-frequency (RF) sputtering onto Si(001) single-crystal substrates with SrTiO₃/TiN buffer layers. Pb(Zr_0.2Ti_0.8)O₃ (PZT) samples were shown to consist of a single perovskite phase and to have an (001) orientation. The orientation relationship was determined to be PZT(001)[110]∥LaNiO₃(001)[110]∥SrTiO₃ (001)[110]∥TiN(001)[110]∥Si(001)[110]. Atomic force microscope (AFM) measurements showed the PZT films to have smooth surfaces with a roughness of 1.15 nm. The microstructure of the multilayer was studied using transmission electron microscopy (TEM). Electrical measurements were conducted using both Pt and LaNiO₃ as top electrodes. The measured remanent polarization P _r and coercive field E _c of the PZT thin film with Pt top electrodes were 23 μC/cm² and 75 kV/cm, and were 25 μC/cm² and 60 kV/cm for the PZT film with LaNiO₃ top electrodes. No obvious fatigue after 10¹⁰ switching cycles indicated good electrical endurance of the PZT films using LaNiO₃ electrodes, compared with the PZT film with Pt top electrodes showing a significant polarization loss after 10⁸ cycles. These PZT films with LaNiO₃ electrodes could be potential recording media for probe-based high-density data storage.     

Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>-Sb<Subscript>2</Subscript>Te<Subscript>3</Subscript> Superlattices Grown by Nanoalloying

In this work, Bi₂Te₃-Sb₂Te₃ superlattices were prepared by the nanoalloying approach. Very thin layers of Bi, Sb, and Te were deposited on cold substrates, rebuilding the crystal structure of V₂VI₃ compounds. Nanoalloyed super- lattices consisting of alternating Bi₂Te₃ and Sb₂Te₃ layers were grown with a thickness of 9 nm for the individual layers. The as-grown layers were annealed under different conditions to optimize the thermoelectric parameters. The obtained layers were investigated in their as-grown and annealed states using x-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive x-ray (EDX) spectroscopy, transmission electron microscopy (TEM), and electrical measurements. A lower limit of the elemental layer thickness was found to have c-orientation. Pure nanoalloyed Sb₂Te₃ layers were p-type as expected; however, it was impossible to synthesize p-type Bi₂Te₃ layers. Hence the Bi₂Te₃-Sb₂Te₃ superlattices consisting of alternating n- and p-type layers showed poor thermoelectric properties.     

Electron Density Distribution in Mn<Subscript>4</Subscript>Si<Subscript>7</Subscript>

Single crystal diffraction measurements were successfully carried out for spherical fine grains grown as single crystals of 0.05–0.2 mm in diameter. Local modulations in the silicon layers were also observed by means of high-resolution electron microscopy. The metallic tin–flux technique was used for crystal growth. The Fourier synthesis and maximum entropy method (MEM) were applied to x-ray diffraction data to obtain electron density distribution maps. Mn₄Si₇ is one of the most promising p-type thermoelectrics useable from 400 K to 700 K. The crystal structure is described in terms of a chimney-ladder structure. The doping effect, by which the system becomes n-type and a structure modulation occurs, was reported by our group previously. The resultant electron density maps were compared with those from the band calculation. The MEM calculation shows the displacement of silicon positions.     

Conduction Mechanisms in Multiferroic Multilayer BaTiO<Subscript>3</Subscript>/NiFe<Subscript>2</Subscript>O<Subscript>4</Subscript>/BaTiO<Subscript>3</Subscript> Memristors

Memristive devices and materials are extensively studied as they offer diverse properties and applications in digital, analog and bio-inspired circuits. In this paper, we present an important class of memristors, multiferroic memristors, which are composed of multiferroic multilayer BaTiO₃/NiFe₂O₄/BaTiO₃ thin films, fabricated by a spin-coating deposition technique on platinized Si wafers. This cost-effective device shows symmetric and reproducible current–voltage characteristics for the actuating voltage amplitude of ±10 V. The origin of the conduction mechanism was investigated by measuring the electrical response in different voltage and temperature conditions. The results indicate the existence of two mechanisms: thermionic emission and Fowler–Nordheim tunnelling, which alternate with actuating voltage amplitude and operating temperature.     

Growth of Epitaxial (110) 0.7Pb(Mg<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>)O<Subscript>3</Subscript>-0.3PbTiO<Subscript>3</Subscript> Thin Films on r-Plane Sapphire Substrates by RF Magnetron Sputtering

The role of substrate temperature and substrate surface geometry in determining the crystal structure and crystallinity of 0.7Pb(Mg_1/3Nb_2/3)O₃-0.3PbTiO₃ (PMN-PT) thin films grown on r-plane sapphire substrates is examined. A 30-nm-thick amorphous PMN-PT seed layer deposited at 250°C and subjected to rapid thermal annealing at 850°C results in the formation of an epitaxial (110) perovskite PMN-PT growth template that can be used for subsequent growth of single-crystal (110) perovskite PMN-PT films at elevated temperature. The data show that single-crystal perovskite is promoted when the films nucleate with the \( \langle \overline{1} 11 \rangle \) PMN-PT direction parallel to the \( \langle 0\bar{2}21 \rangle \) Al₂O₃ direction.     

Thermoelectric Properties of Mo<Subscript>3</Subscript>Sb<Subscript>5.4</Subscript>Te<Subscript>1.6</Subscript> and Ni<Subscript>0.06</Subscript>Mo<Subscript>3</Subscript>Sb<Subscript>5.4</Subscript>Te<Subscript>1.6</Subscript>

Mo₃Sb₇, crystallizing in the Ir₃Ge₇ type structure, has poor thermoelectric (TE) properties due to its metallic behavior. However, by a partial Sb-Te exchange, it becomes semiconducting without noticeable structure changes and so achieves a significant enhancement in the thermopower with the composition of Mo₃Sb₅Te₂. Meanwhile, large cubic voids in the Mo₃Sb₅Te₂ crystal structure provide the possibility of filling the voids with small cations to decrease the thermal conductivity by the so-called rattling effect. As part of the effort to verify this idea, we report herein the growth as well as measurements of the thermal and electrical transport properties of Mo₃Sb_5.4Te_1.6 and Ni_0.06Mo₃Sb_5.4Te_1.6.     

Thermoelectric Properties of Spark Plasma-Sintered In<Subscript>4</Subscript>Se<Subscript>3</Subscript>-In<Subscript>4</Subscript>Te<Subscript>3</Subscript>

We report the thermoelectric properties of spark plasma-sintered In₄Se₃-In₄Te₃ materials. For comparison, pure In₄Se₃ and In₄Se₃ (80 wt.%)/In₄Te₃ (20 wt.%) mixture samples were prepared. In₄Se₃ and In₄Te₃ powders were synthesized by a conventional melting process in evacuated quartz ampoules, and a spark plasma method was used for the sintering of the pure In₄Se₃ and mixture samples. Thermoelectric and structural characterizations were carried out, and the mixing effect of In₄Se₃ and In₄Te₃ on the thermoelectric properties was investigated.     

Synthesis and Characterization of In<Subscript>2</Subscript>S<Subscript>3</Subscript> Thin Films Deposited by Chemical Bath Deposition on Polyethylene Naphthalate Substrates

Indium sulfide (In₂S₃) thin films were deposited on polyethylene naphthalate (PEN) by chemical bath deposition (CBD). The materials were characterized by ultraviolet (UV)–visible spectroscopy, x-ray photoelectron spectroscopy (XPS), energy-dispersive x-ray spectroscopy (EDX), scanning electron microscopy (SEM), and x-ray diffraction (XRD) to investigate the influence of the polymeric substrate on the resulting thin In₂S₃. The films showed polycrystalline (cubic and tetragonal) structure. A reduction of the ordering of the polymeric chains at the surface of the PEN was also observed, demonstrated by the appearance of two infrared bands at 1094 cm⁻¹ and 1266 cm⁻¹. Presence of oxygen during the early stages of In₂S₃ growth was also identified. We propose a reaction mechanism for both the equilibrium and nucleation stages. These results demonstrate that In₂S₃ can be deposited at room temperature on a flexible substrate.     

Magnetic susceptibility of Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript>-Sb<Subscript>2</Subscript>Te<Subscript>3</Subscript> alloys

The magnetic susceptibility of Czochralski-grown single crystals of Bi₂Te₃-Sb₂Te₃ alloys containing 0, 10, 25, 40, 50, 60, 65, 70, 80, 90, 99.5, or 100 mol % Sb₂Te₃ has been investigated. The magnetic susceptibility of these crystals was determined at the temperature T = 291 K and the magnetic field H oriented parallel (χ_‖) and perpendicularly (χ_⊥) to the trigonal crystallographic axis C ₃. A complicated concentration dependence of the anisotropy of magnetic susceptibility χ_‖/χ_⊥ has been revealed. The crystals with the free carrier concentration p ≈ 5 × 10¹⁹ cm^?3 do not exhibit anisotropy of magnetic susceptibility. The transition to the isotropic magnetic state occurs for the compositions characterized by a significantly increased (from 200 to 300 meV) optical bandgap.     

Comparative studies of p-type InP layers formed by Zn<Subscript>3</Subscript>As<Subscript>2</Subscript> and Zn<Subscript>3</Subscript>P<Subscript>2</Subscript> diffusion

The Zn₃As₂ and Zn₃P₂ were used as Zn-diffusion sources to form a p-region in undoped-InP wafers. The p-type InP formed by Zn diffusion from a Zn₃P₂ source has higher transmittance over the testing-spectrum range 1,000–1,700 nm versus Zn diffusion from a Zn₃As₂ source. In the case of a p-type region formed from a Zn₃As₂ source, x-ray photoelectron spectroscopy (XPS) showed As atoms were reduced from the oxide state and formed an InAs composition, which introduces more absorption loss.     

Crystallization behavior and ferroelectric properties of PbTiO<Subscript>3</Subscript>/Ba<Subscript>0.85</Subscript>Sr<Subscript>0.15</Subscript>TiO<Subscript>3</Subscript>/PbTiO<Subscript>3</Subscript> sandwich thin film on Pt/Ti/SiO<Subscript>2</Subscript>/Si substrates

A PbTiO₃/Ba_0.85Sr_0.15TiO₃/PbTiO₃ (PT/BST15/PT) sandwich thin film has been prepared on Pt/Ti/SiO₂/Si substrates by an improved sol-gel technique. It is found that such films under rapid thermal annealing at 700°C crystallize more favorably with the addition of a PbTiO₃ layer. They possess a pure, perovskite-phase structure with a random orientation. The polarization-electric field (P-E) hysteresis loop and current-voltage (I-V) characteristic curves reveal that a PT/BST15/PT film exhibits good ferroelectricity at room temperature. However, no sharp peak, only a weak maximum, is observed in the curves of the dielectric constant versus temperature. The dielectric constant, loss tangent, leakage current density at 20 kV/cm, remnant polarization, and coercive field of the PT/BST15/PT film are 438, 0.025, 1.3 × 10⁻⁶ Acm⁻², 2.46 μCcm⁻², and 41 kVcm⁻¹, respectively, at 25°C and 10 kHz. The PT/BST15/PT film is a candidate material for high sensitivity elements for uncooled, infrared, focal plane arrays (UFPAs) to be used at near ambient temperature.     

Optical Study of Filled Tetrahedral Compounds Li<Subscript>3</Subscript>AlN<Subscript>2</Subscript> and Li<Subscript>3</Subscript>GaN<Subscript>2</Subscript>

A detailed analysis of the optical properties of filled tetrahedral semiconductors Li₃AlN₂ and Li₃GaN₂ has been performed, using the full potential linearized augmented plane wave method within the density functional theory. The real and imaginary parts of the dielectric function ε(ω), the optical absorption coefficient I(ω), the reflectivity R(ω), and the electron energy loss function are calculated within the random phase approximation. The interband transitions responsible for the structures in the spectra are specified. Looking at optical matrix element, we note that the major peaks are dominated by transition from metal s, N 2p states to N 2p, Ga 3d states. The theoretical calculated optical properties and electron energy loss spectrum yield a static dielectric constant of 5.34 and a plasmon energy of 19.47 eV for Li₃GaN₂. In the Li₃AlN₂ compound, the static dielectric constant decreases to 4.75 and yields a plasmon energy of 18.5 eV. The effect of spin–orbit coupling on the optical properties is also investigated and found to be quite small, especially in the low-energy region. In order to check the reliability of our calculations, analogous results obtained for Be₃N₂ in the same structure [space group Ia3(206)] are included in this work.     

High-Temperature Capacitor Based on Ca-Doped Bi<Subscript>0.5</Subscript>Na<Subscript>0.5</Subscript>TiO<Subscript>3</Subscript>-BaTiO<Subscript>3</Subscript> Ceramics

High-temperature capacitors were prepared by the conventional oxide method based on Bi_0.5Na_0.5TiO₃-BaTiO₃-CaTiO₃ (BNT-BT-CT) lead-free piezoelectric ceramics. BNT-BT is one of the promising candidates as a high-temperature relaxor, and has a high Curie temperature and broadened dielectric constant. The addition of CT increases the dielectric constant at lower temperatures and decreases the dielectric constant at higher temperatures, so that the variation of capacitance is decreased. The effect of BT on the temperature characteristic of dielectric constant is contrary to that of CT. A single-phase rhombohedral perovskite and square grains were obtained in this study. With the proper amount of BT and CT additions, the high-temperature specification can be met: from −55°C to 200°C, the variation of capacitance is within ±15% of room-temperature capacitance.     

Properties of CuIn<Subscript>3</Subscript>Se<Subscript>5</Subscript> crystals and In/CuIn<Subscript>3</Subscript>Se<Subscript>5</Subscript> structures

Using the method of planar crystallization from the melt with deviations from the stoichiometric composition, p-CuIn₃Se₅ single crystals are grown. The electrical properties of the homogeneous crystals are studied. It is found that the resistivity of the p-CuIn₃Se₅ crystals depends on the excess Se content in the melt. It is established that the voltaic photosensitivity of the In/CuIn₃Se₅ structures is enhanced with an increasing excess of Se content in the melt. The energy spectrum and the character of interband transitions in the CuIn₃Se₅ crystals are discussed. It is concluded that the CuIn₃Se₅ ternary compound can be used in high efficiency photoelectric converters of solar radiation.     

Thermoelectric Properties of Heavily Doped <Emphasis Type="Italic">n</Emphasis>-Type SrTiO<Subscript>3</Subscript> Bulk Materials

Three Ta-doped strontium titanates were prepared as potential candidates for n-type thermoelectric oxides. The purity of the polycrystalline samples of SrTi_1−x Ta _x O₃ (x = 0.05 to 0.14) were characterized by means of powder x-ray diffraction and electron probe micro analysis (EPMA). We present the results of Seebeck coefficient, electrical conductivity, and thermal conductivity measurements performed at high temperatures.     

Nanotribological Characteristics of Cu<Subscript>6</Subscript>Sn<Subscript>5</Subscript>, Cu<Subscript>3</Subscript>Sn,and Ni<Subscript>3</Subscript>Sn<Subscript>4</Subscript> Intermetallic Compounds

Nanotribological characteristics, including the coefficient of friction, wear coefficient, and wear resistance, of Cu₆Sn₅, Cu₃Sn, and Ni₃Sn₄ intermetallic compounds developed by the annealing of Sn–Cu or Sn–Ni diffusion couples were investigated in this work. The scratch test conditions combined a constant normal load of 10 mN, 20 mN, or 30 mN and a scratch rate of 0.1 μm/s, 1 μm/s, or 10 μm/s. Experimental results indicated that, as the normal load increases, the pile-up grows taller and the scratch deepens, leading to a greater coefficient of friction and wear coefficient, and reduced wear resistance. Moreover, the scratch rate does not have a significant effect on the nanotribological characteristics except for those of Cu₆Sn₅ and Cu₃Sn under a normal load of 10 mN. Though the hardness of Cu₆Sn₅, Cu₃Sn, and Ni₃Sn₄ is similar, Ni₃Sn₄ appears to be more prone to wear damage.     

Structure and Spectral Dependences of the Raman Scattering of Photosensitive CuIn<Subscript>0.95</Subscript>Ga<Subscript>0.05</Subscript>Se<Subscript>2</Subscript> Films on Glass,Aluminum, and Nanoporous Al/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Substrates

Photosensitive polycrystalline CuIn_0.95Ga_0.05Se₂ thin films have been formed on glass, aluminum, and nanoporous Al/Al₂O₃ substrates by means of two-step selenization in a gas (nitrogen) flow carrying a reaction component (selenium). The structural properties and the Raman scattering spectral dependences have been investigated. The dependence of the main lattice parameters and intensities of the Raman scattering lines on the substrate material is demonstrated.     

Fabrication and Characterization of Pb(Zr<Subscript>0.53</Subscript>,Ti<Subscript>0.47</Subscript>)O<Subscript>3</Subscript>-Pb(Nb<Subscript>1/3</Subscript>,Zn<Subscript>2/3</Subscript>)O<Subscript>3</Subscript> Thin Films on Cantilever Stacks

0.9Pb(Zr_0.53,Ti_0.47)O₃-0.1Pb(Zn_1/3,Nb_2/3)O₃ (PZT–PZN) thin films and integrated cantilevers have been fabricated. The PZT–PZN films were deposited on SiO₂/Si or SiO₂/Si₃N₄/SiO₂/poly-Si/Si membranes capped with a sol–gel-derived ZrO₂ buffer layer. It is found that the membrane layer stack, lead content, existence of a template layer of PbTiO₃ (PT), and ramp rate during film crystallization are critical for obtaining large-grained, single-phase PZT–PZN films on the ZrO₂ surface. By controlling these parameters, the electrical properties of the PZT–PZN films, their microstructure, and phase purity were significantly improved. PZT–PZN films with a dielectric constant of 700 to 920 were obtained, depending on the underlying stack structure.