Pyroelectric properties of BST/PLT composite thick films with addition of xPbO–(1 − x)B2O3 glass

The Ba_xSr_1−xTiO₃ (BST)/Pb_1−xLa_xTiO₃ (PLT) composite thick films (20 μm) with 12 mol% amount of xPbO–(1 − x)B₂O₃ glass additives (x = 0.2, 0.35, 0.5, 0.65 and 0.8) have been prepared by screen-printing the paste onto the alumina substrates with silver bottom electrode. X-ray diffraction (XRD), scanning electron microscope (SEM) and an impedance analyzer and an electrometer were used to analyze the phase structures, morphologies and dielectric and pyroelectric properties of the composite thick films, respectively. The wetting and infiltration of the liquid phase on the particles results in the densification of the composite thick films sintered at 750 °C. Nice porous structure formed in the composite thick films with xPbO–(1 − x)B₂O₃ glass as the PbO content (x) is 0.5 ≥ x ≥ 0.35, while dense structure formed in these thick films as the PbO content (x) is 0.8 ≥ x ≥ 0.65. The volatilization of the PbO in PLT and the interdiffusion between the PLT and the glass lead to the reduction of the c-axis of the PLT phase. The operating temperature range of our composite thick films is 0–200 °C. At room temperature (20 °C), the BST/PLT composite thick films with 0.35PbO–0.65B₂O₃ glass additives provided low heat capacity and good pyroelectric figure-of-merit because of their porous structure. The pyroelectric coefficient and figure-of-merit F_D are 364 μC/(m² K) and 14.3 μPa^−1/2, respectively. These good pyroelectric properties as well as being able to produce low-cost devices make this kind of thick films a promising candidate for high-performance pyroelectric applications.     

Dielectric and piezoelectric properties of sol–gel derived Ca doped PbTiO3

Synthesis of Ca doped PbTiO₃ powder by a chemically derived sol–gel process is described. Crystallization characteristics of different compositions Pb_1−xCa_xTiO₃ (PCT) with varying calcium (Ca) content in the range x = 0–0.45 has been investigated by DTA/TGA, X-ray diffraction and scanning electron microscopy. The crystallization temperature is found to decrease with increasing calcium content. X-ray diffraction reveals a tetragonal structure for PCT compositions with x ≤ 0.35, and a cubic structure for x = 0.45. Dielectric properties on sintered ceramics prepared with fine sol–gel derived powders have been measured. The dielectric constant is found to increase with increasing Ca content, and the dielectric loss decreases continuously. Sol–gel derived Pb_1−xCa_xTiO₃ ceramics with x = 0.45 after poling exhibit infinite electromechanical anisotropy (k_t/k_p) with a high d₃₃ = 80 pC/N, ′ = 298 and low dielectric loss (tan δ = 0.0041).     

Epitaxial grown K1−xRbxTiOPO4 films with extremely flat surfaces for waveguiding

We report on epitaxial {1 0 0} K_1−xRb_xTiOPO₄ waveguide films for the visible spectral range grown on KTiOPO₄ substrates by liquid phase epitaxy. Using the m-line technique a refractive index increase of Δn_x≈0.007 and Δn_z≈0.004 for TM and TE polarisation has been determined for a K_0.78Rb_0.22TiOPO₄ film. Optical transmission and nearfield distribution are comparable to conventional ion-exchanged waveguides. Typical attenuation of about 1 dB/cm for both TM and TE polarisation was obtained at λ=532 and 1064 nm. Energy-dispersive X-ray spectrometry reveals solid-solution films with graded rubidium composition profiles. X-ray rocking curve analyses confirm the epitaxial growth process and indicate perfect and relaxed K_1−xRb_xTiOPO₄ films. Atomic force microscopy investigations reveal regular step structures with step heights Δh<1.3 nm resulting in rms-roughness values of ≈0.4 nm.     

Preparation of (Ti1−xAlx)N films from mixed alkoxide solutions by plasma CVD

(Ti_1−xAl_x)N films were prepared on a Si wafer at 700°C from toluene solution of alkoxides (titanium tetraetoxide and aluminum tri-butoxide) in an Ar/N₂/H₂ plasma by the thermal plasma chemical vapor deposition (CVD) method. The films were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, electrical resistivity, and Vickers micro-hardness. Single phase TiN formed at an Al atomic fraction of 0–0.2, with a mixed TiN and AlN phase occurring up to 0.6 and single phase AlN forming above 0.8. The films had relatively sooth surfaces, 0.4 μm thick at an Al atomic fraction of 0.2, and thickened with increasing Al fraction. The atomic concentration of Ti, Al, N, O, and C determined from their respective XPS areas showed that the Ti and Al contents of the films changes with the solution composition in a complementary way. The impurities were about 10 at.% oxygen and carbon. The electrical resistivity was almost unchanged from the value of 10³ μΩ cm at 0–0.6 Al but then suddenly increased to 10⁴ μΩ cm at higher Al contents. The hardness showed a synergic maximum of about 20 GPa at an Al fraction of 0.6–0.8.     

Characterization of GaN epitaxial layers on SiC substrates with AlxGa1−xN buffer layers

High quality GaN epitaxial layers were obtained with Al_xGa_1−xN buffer layers on 6H–SiC substrates. The low-pressure metalorganic chemical vapor deposition (LP-MOCVD) method was used. The 500 Å thick buffer layers of Al_xGa_1−xN (0≤x≤1) were deposited on SiC substrates at 1025°C. The FWHM of GaN (0004) X-ray curves are 2–3 arcmin, which vary with the Al content in Al_xGa_1−xN buffer layers. An optimum Al content is found to be 0.18. The best GaN epitaxial film has the mobility and carrier concentration about 564 cm² V⁻¹ s⁻¹ and 1.6×10¹⁷ cm⁻³ at 300 K. The splitting diffraction angle between GaN and Al_xGa_1−xN were also analyzed from X-ray diffraction curves.     

YNixMn1−xO3 thin films by pulsed laser deposition: Structure and magnetic properties

Highly oriented YNi_xMn_1−xO₃ thin films on SrTiO₃ (100) substrates were achieved by using pulsed laser deposition for x = 0.33 and x = 0.50. We used a combination of X-ray diffraction, scanning electron microscopy, atomic force microscopy, and magnetic-property measurements. The magnetic transition temperatures (T_c) of the as-grown films are higher than the corresponding bulk values (typically 85 K instead of 80 K, for x = 0.5, and 60 K instead of 50 K, for x = 0.33). Our magnetic measurements also suggest a spin-glass characteristic in the x = 0.33 films, while a cluster glasslike behavior is observed for the films with x = 0.5, which is quite different from that of the bulk samples. Finally, the influence of post-deposition heat treatment on the magnetic properties of the as-grown films is discussed.     

Optical and electrical properties of p-type AgInSnxS2−x (x = 0–0.04) thin films prepared by spray pyrolysis

AgInSn_xS_2−x (x = 0–0.2) polycrystalline thin films were prepared by the spray pyrolysis technique. The samples were deposited on glass substrates at temperatures of 375 and 400 °C from alcoholic solutions comprising silver acetate, indium chloride, thiourea and tin chloride. All deposited films crystallized in the chalcopyrite structure of AgInS₂. A p-type conductivity was detected in the Sn-doped samples deposited at 375 °C, otherwise they are n-type. The optical properties of AgInSn_xS_2−x (x < 0.2) resemble those of chalcopyrite AgInS₂. Low-temperature PL measurements revealed that Sn occupying an S-site could be the responsible defect for the p-type conductivity observed in AgInSn_xS_2−x (x < 2) thin films.     

Ferroelectric/superconductor heterostructures

This review covers the fabrication and characterization of ferroelectric/superconductor heterostructures such as Pb(Zr_xTi_1−x)O₃/YBa₂Cu₃O_7−δ (YBCO), BaTiO₃/YBCO and Ba_xSr_1−xTiO₃/YBCO etc. on various single crystal substrates. Pulsed laser deposition, laser molecular beam epitaxy, and magnetron-sputtering methods are compared. This report shows that pulsed laser deposition equipped with in situ reflection high-energy electron diffraction is a good method to control the growth mode of YBCO thin films. Furthermore, laser molecular beam epitaxy is a superb method for research of complex oxide films and their superlattices. Atomic force microscopy and transmission electron microscopy showed the ferroelectric films grown on the rough surface of the YBCO films produced high-density planar defects in the film and is detrimental to the ferroelectric/dielectric properties of the heterostructures. Therefore, for device usage, it is more advantageous to use SrRuO₃ than YBCO as the bottom electrode material. For growing atomically smooth surface films step-flow mode is highly recommended. Prospects of microwave device application of the ferroelectric/superconductor heterostructures are discussed, and proposed the BSTO films as the best candidate for passive microwave components.     

High-resolution and analytical transmission electron microscopy of epitaxial YBa2Cu3O7−x thin films and YBa2Cu3O7/PrBa2Cu3O7 superlattices

Transmission electron microscopy (TEM) studies of epitaxial YBa₂Cu₃O_7−x thin films and YBa₂Cu₃O₇/PrBa₂Cu₃O₇ superlattices are summarized. High-resolution imaging of cross-sections and plan views and energy-dispersive X-ray microanalysis and electron energy loss spectroscopy in the transmission electron microscope were the methods applied. In the first section results on YBa₂Cu₃O_7−x thin films With varying oxygen stoichiometry deposited onto SrTiO₃ are discussed. Then, YBa₂Cu₃O₇/PrBa₂Cu₃O₇ superlattices deposited onto SrTiO₃ and MgO are investigated. Finally, an interface analysis of high-quality YBa₂Cu₃O_7−x thin films deposited onto sapphire with yttrium-stabilized zirconia buffer layers is presented.     

Synthesis, characteristics and oxygen-sensitive properties of SrMgxTi 1 − xO3 nanocrystals

SrMg_xTi_{1 - x}O₃ nanocrystals (x = 0.1–0.6) were synthesized by the stearic acid gel method. Powder samples were characterized by X-ray diffraction and X-ray photoelectron (XP) spectroscopy. The results showed that the lattice parameter a and the O 1s XP spectrum changed not only with the Mg content x but also with the grain size d of the samples. The conductivity of a thick film specimen fabricated on an aluminium oxide wafer was investigated in a nitrogen—oxygen atmosphere.     

Molecular beam epitaxy synthesis of Si1−yCy and Si1−x−yGexCy alloys and Ge islands using an electron cyclotron resonance argon/methane plasma

We report the growth of Si_1−yC_y and Si_1−x−yGe_xC_y alloys on Si(001) by electron cyclotron resonance plasma-assisted Si molecular beam epitaxy using an argon/methane gas mixture. Various Si/Si_1−yC_y and Si/Si_1−x−yGe_xC_y multilayers have been grown and characterized principally by X-ray diffraction and Raman spectroscopy. The influence of growth parameters and electron cyclotron resonance plasma source operating conditions on the C substitutional incorporation was studied. Under optimum growth conditions the structures show good structural properties and sharp interfaces with carbon being essentially substitutionally incorporated up to concentrations of 1%. No significant carbon incorporation was measured in films grown under a high methane partial pressure without plasma excitation. Si_1−x−yGe_xC_y layers grown with this technique exhibit the strain compensation and enhanced thermal stability expected for these ternary alloys. Carbon pre-deposition of Si through surface exposure to the argon/methane plasma is shown to act as an antisurfactant on the growth of Ge islands by suppressing the formation of a Ge wetting layer on the surface.     

Phase formation process of IrxSi1−x thin films Structure and electrical properties

Experimental data on the phase formation process of amorphous Ir_xSi_1−x thin films with 0.30 ≤ x ≤ 0.41 are presented and discussed in relation to electric transport properties. The structure formation process at temperatures from 300 K up to 1223 K was investigated by means of X-ray diffraction. Distinct phases were observed in the final stage in dependence on the initial composition: Ir₃Si₄, Ir₃Si₅, and IrSi₃. An unknown metastable phase was found in films with a silicon concentration of 61 at.% to 64 at.% after annealing above the crystallization temperature T = 970 K. The crystal structure of this phase was determined by the combined use of X-ray diffraction and electron diffraction. It was found to be monoclinic, basic-face centred with lattice constants a = 1.027 nm, b = 0.796 nm, c = 0.609 nm, and γ = 113.7°. Additionally, microstructure and morphology of the films were investigated by transmission electron microscopy (TEM). The annealing process was studied by means of mechanical stress investigations as well as by electrical resistivity and thermopower measurements. Correlations between the structure, the phase formation and the electrical transport behaviour are discussed on the basis of conduction mechanism.     

Physical properties of (La,Sr)Ti(O,N)3 thin films grown by pulsed laser deposition

Thin films of La_xSr_1−xTiO_3+x/2 (x = 0, 0.25, 0.5, 0.75, 1) were grown by laser ablation on two different kinds of substrates (SrTiO₃ (STO) and MgO) and were subsequently ammonolysed to yield the corresponding oxynitrides La_xSr_1−xTi(O,N)₃. For both substrates all films were found to grow epitaxially to the (1 0 0) direction of the cubic perovskite structure, except for x = 0.5 that grew parallel to the (1 1 0) direction. For some of the films TiN was detected as impurity phase. Scanning electron microscopy revealed that the films are dense and homogeneous with thicknesses around 350 nm. Atomic force microscopy showed that the surface roughness of the films varied between 4.2 and 14.1 nm. The employed substrate had a strong influence on the electrical properties. Films grown on STO exhibited a metallic behaviour, in contrast to the films grown on MgO, which were insulating.     

Crystallization kinetics of SbxSe100−x thin films

The crystallization kinetics in Sb_xSe_100−x films with 39≤x≤58is studied by monitoring the optical transmission of the films during both isothermal and constant rate heatings. The structure of the films upon crystallization and at certain intermediate stages is studied by electron microscopy techniques. The results are analyzed in the frame of the Johnson-Mehl-Avrami theory in order to determine the kinetic parameters (Avrami exponent, activation energy and frequency factor) in addition to the crystallization temperature. The results show that film crystallization is always preceeded by a relaxation process which modifies substantially the optical properties of the amorphous material. Amorphous films with compositions close to the stoichiometric compound (Sb₂Se₃) are found to show the highest activation energy for crystallization.     

Physical and electrical properties of thin films produced by Low Energy Cluster Beam Deposition of mixed InxSb1−x clusters

We present a new gas-aggregation cluster source with two independent crucibles, one for indium and another one for antimony. This source was used to produce mixed In_xSb_1−x clusters in the nanometer range size (typically 4 nm), which were deposited at room temperature on amorphous carbon or glass substrates by low energy cluster beam deposition technique (LECBD). The film composition was analysed by energy dispersive X-ray (EDX) spectroscopy. The morphology and structure of the films were studied by transmission electron microscopy (conventional and high resolution) and selected area electron diffraction (SAED) at different compositions. Semiconducting InSb clusters could be produced by controlling the temperature of the two crucibles. The electrical properties of the films were studied at a film thickness of 20 nm. The conductivity versus temperature appeared to be thermally activated for all compositions.     

Preparation of B-doped a-Si1−xCx:H films and heterojunction p–i–n solar cells by the Cat-CVD method

B-doped a-Si_1−xC_x:H films for a window layer of Si thin film solar cells have been prepared by the Cat-CVD method. It is found that C is effectively incorporated into the films by using C₂H₂ as a C source gas, where an only little C incorporation is observed from CH₄ and C₂H₆ under similar deposition conditions. Using a-Si_1−xC_x:H films grown from C₂H₂, heterojunction p–i–n solar cells have been prepared by the Cat-CVD method. The cell structure is (SnO₂ Asahi-U)/ZnO/a-Si_1−xC_x:H(p)/a-Si:H(i)/μc-Si:H(n)/Al. The obtained conversion efficiency was 5.4%.     

Tungsten–carbon films prepared by reactive sputtering from argon–benzene discharges

Tungsten–carbon thin films have been deposited by reactive (Ar+C₆H₆) DC magnetron sputtering onto various substrates. Deposition onto glass, monocrystalline silicon, tantalum and stainless steel at room temperature yielded W–C films, having XRD patterns corresponding to the structure of heavily disordered W₂C or WC_1−x carbides. The samples deposited upon the Au or Cu foils were nanocrystalline cubic WC_1−x with the grain size of 2.9 nm. Disordered tungsten–carbon films were stable up to 1200°C. Microhardness of the films with disordered W₂C phase was about 5–6 GPa while that of the films with disordered WC_1−x phase was about 17 GPa. The characteristics of films can be understood considering the effects of the incorporation of free carbon and/or carbon–hydrogen fragments into the tungsten carbide layer.     

Highly conductive microcrystalline silicon carbide films deposited by the hot wire cell method and its application to amorphous silicon solar cells

Microcrystalline silicon carbide (μc-Si_1−xC_x) films were successfully deposited by the hot wire cell method using a gas mixture of SiH₄, H₂ and C₂H₂. It was confirmed by Fourier transform infrared and X-ray diffraction analyses that the films consisted of μc-Si grains embedded in a-Si_1−xC_x tissue. The p-type μc-Si_1−xC_x films were deposited using B₂H₆ as a doping gas. A dark conductivity of 0.2 S/cm and an activation energy of 0.067 eV were obtained. The p-type μc-Si_1−xC_x was used as a window layer of a-Si solar cells, in which the intrinsic layer was deposited by photo-chemical vapor deposition, and an initial conversion efficiency of 10.2% was obtained.     

Structural and optical properties of sintered ZnSxSe1−x films

Sintered ZnS_xSe_1−x films have been prepared in the entire composition range from ZnSe to ZnS by using the screen printing method. To deposit good quality films, optimum conditions have been determined. Wide band gap ternary films have wide applications in solar cells. The band gap of these films are determined by reflection spectra in the wavelength range of 325–600 nm using the Tauc relation. These films have a direct band gap, which varies from 3.50 eV for ZnS to 2.66 eV for ZnSe films. The wurtzite structure of ZnSSe films was confirmed by X-ray diffraction analysis of these films.     

In situ co-precipitation synthesis and photoluminescence of YxGd1−xVO4:Tm microcrystalline phosphors by hybrid precursors

Y_xGd_1−xVO₄:Tm³⁺ (5 mol%) phosphors were prepared by in situ co-precipitation technology with the different content ratio of Y/Gd (x = 0.2, 0.3, 0.4, 0.5, 0.6, 0.8, respectively). During the process, rare earth coordination polymers with o-hydroxylbenzoate were used as precursors, composing with polyethylene glycol (PEG) as dispersing media. After heat-treatment of the resulting multicomponent hybrid precursors at 900 °C, the samples were obtained. SEM indicated the particles present good crystalline state, whose crystalline grain sizes were about 0.2–2 μm. Under the excitation of 257 nm, all the materials show the characteristic emission of Tm³⁺ which is the strong blue emission centered at 475 nm originating from ¹G₄ → ³H₆ of Tm³⁺. Besides this, concentration quenching appears in the system of YVO₄:Tm³⁺ and GdVO₄:Tm³⁺. And when x reaches 0.5, the system of Y_xGd_1−xVO₄:Tm³⁺ shows the strongest blue emission.