Computer simulation of heat distribution processes in W/(La,Ce)B<Subscript>6</Subscript>/W sensor of thermoelectric detector

The results are presented of the computer simulation of heat distribution in the W/(La,Ce)B₆/W multi-layer sensor of the thermoelectric detector after the absorption of single photons with the energies 1–100 eV. The influence of the choice of computer simulation parameters on the revealing of the peculiarities of heat transmission processes arising in the sensor of detector depending on the photon energy, the sensor geometry, the absorption area of the absorber surface and the depth of photon thermalization is investigated in details. The energy resolution and the count rate of sensor are evaluated. It is shown that the multi-layer sensor with the thermoelectric (La,Ce)B₆ is capable to register a single photon in a wide range of the electromagnetic spectrum at 0.5 K; it has an advantages as compared with the sensor based on the CeB₆ with the operating temperature 9 K, and has perspectives to be used in the science and technology.     

Investigation of the processes of heat propagation in W/FeSb<Subscript>2</Subscript>/W detection pixel of the single photon thermoelectric detector

The results are presented of the computer modeling of the processes of heat propagation in the single layer detection pixel of the thermoelectric single photon detector after the absorption of photon with the energy of 1–1000 eV. The different geometries of the detection pixel consisting of the tungsten absorber and thermoelectric sensor from the strongly correlated FeSb₂ semiconductor are considered. The differences of temporal dependence of the signal appearing on the sensor for various size of the sensitive pixel elements are studied in detail. The energy resolution and the count rate of the detector are estimated. It is shown that the FeSb₂ sensor of thermoelectric detector provides the generation of significantly higher signal as compared with the CeB₆ sensor and has the perspectives of application in the single photon detectors with the high energy resolution.     

Structural and electrical properties of 0.7Pb(Mg<Subscript>1/3</Subscript>Nb<Subscript>2/3</Subscript>)O<Subscript>3</Subscript>–0.3PbTiO<Subscript>3</Subscript> thin films grown on Ir/MgO buffered Si(100) substrates

Thin films of 0.7Pb(Mg_1/3Nb_2/3)O₃–0.3PbTiO₃ (PMN-PT) have been grown on Ir/MgO buffered Si(100) substrates at different substrate temperatures by pulsed laser deposition. Crystalline phases as well as preferred orientations in the PMN-PT films were investigated by X-ray diffraction analysis (XRD). The microstructure, dielectric and ferroelectric properties of PMN-PT film prepared at 650 °C were studied. The results show that the film prepared at 650 °C exhibits pure perovskite phase and single c-axis orientation. The dielectric constant and dissipation factor of the single c-axis oriented film are 1000 and 0.04 at a frequency of 1 kHz, while the remnant polarization and coercive field are about 13.0 μC/cm² and 100 kV/cm under an electric field of 480 kV/cm, respectively. PACS 81.15.Fg; 77.80.-e; 77. 22.Ej; 77.55.+f; 85.50.Gk     

Nonlinear dielectric response of epitaxial Ba<Subscript>0.6</Subscript>Sr<Subscript>0.4</Subscript> TiO<Subscript> 3</Subscript> thin films

Based on Landau-Devonshire (LD)-type phenomenological thermodynamic theory, the electric field dependence of the dielectric properties of tetragonal single-domain barium strontium titanate(Ba_1-xSr_xTiO₃) films on cubic substrates is theoretically investigated by taking into account the high order terms of the polarization. At room temperature, the nonlinear dielectric responses of epitaxial Ba_0.6Sr_0.4TiO₃ films are provided by adjusting the film thickness and growth temperature. The strong nonlinearity of relative dielectric constant and pyroelectric coefficient are attained around critical film thickness on MgO (69 nm) and LaAlO₃ (132 nm) substrates or critical growth temperature on MgO (337 °C) substrate with respect to epitaxy-induced lattice misfit and thermal stresses during deposition. This can be explained that small compressive stresses are effective to support high nonlinearity of dielectric constant and pyroelectric coefficient for Ba_0.6Sr_0.4TiO₃ films irrespective of whether they are on compressive substrate or tensile substrate. It is also predicted that a large tunability may be achieved by altering processing conditions, such as the film thickness and growth temperature for different substrates. Our theoretical results are in good agreement with the experimental data reported in literature.     

Electrical properties of Sr<Subscript>3</Subscript>Bi<Subscript>4</Subscript>Ti<Subscript>6</Subscript>O<Subscript>21</Subscript> thin films

Highly c-axis-oriented Sr₃Bi₄Ti₆O₂₁ (SBTi) thin films were fabricated on Pt-coated Si substrates by pulsed laser deposition (PLD). The structures were characterized by X-ray diffraction (XRD), atomic force microscopy (AFM) and scanning electron microscopy (SEM). No peaks of SrTiO₃ (STO) could be detected in the XRD pattern, indicating the existence of the SBTi single phase. Good ferroelectric hysteresis loops of the films with Pt electrodes were obtained. With an applied field of 400 kV/cm, the measured remanent polarization (P_r) and coercive field (E_c) values were 4.1 C/cm² and 75 kV/cm respectively. The films showed little fatigue after 2.22×10⁹ switching cycles: the nonvolatile polarizations decreased by less than 5% of the initial values. The dielectric constant and the loss tangent of the films were measured to be 363 and 0.04 at 100 kHz. These results might be advantageous for nonvolatile ferroelectric random access memory (NVFRAM) and dynamic random access memory (DRAM). PACS 77.84.Dy; 77.22.-d; 68.55.Jk     

An investigation into ultra-thin pseudobinary oxide (TiO<Subscript>2</Subscript>)<Subscript>x</Subscript>(Al<Subscript>2</Subscript>O<Subscript>3</Subscript>)<Subscript>1-x</Subscript> films as high-<Emphasis Type="Italic">k</Emphasis> gate dielectrics

As potential gate dielectric materials, pseudobinary oxide (TiO₂)_x(Al₂O₃)_1-x (0.1≤x≤0.6) films (TAO) were deposited on Si (100) substrates by pulsed-laser deposition method and studied systematically via various measurements. By a special deposition process, including two separate steps, the TAO films were deposited in the form of two layers. The first layer was deposited at room temperature and the second layer was completed at the substrate temperature of 400 °C. Detailed data show that the properties of the TAO films are closely related to the ratio between TiO₂ and Al₂O₃. The existence of the first layer deposited at room temperature can effectively restrain the formation of the interfacial layer. And according to the results of X-ray photoelectron spectroscopy and high-resolution transmission electron microscopy performed on the films, no other information belonging to the silicon oxide could be observed. For the (TiO₂)_0.4(Al₂O₃)_0.6 film, the best result has been achieved among all samples and its dielectric constant is evaluated to be about 38. It is valuable for the amorphous TAO film as one of the promising dielectric materials for high-k gate dielectric applications. PACS 77.55.+f; 73.40.Qv; 81.15.Fg     

Nanostructured TiO<Subscript>2</Subscript> Films with a Mixed Phase for Perovskite Solar Cells

A series of thin films made with TiO₂ nanoparticles with a varied anatase/rutile phase ratio is prepared on conducting glass substrates using a spin-coating method. The structure, morphology, and optical properties of TiO₂ nanopowders and thin films fabricated are studied using powder X-ray diffraction, scanning electron microscopy, and optical spectroscopy. The TiO₂ nanostructured films created are used as photoelectrodes for the fabrication of perovskite solar cells (PSCs). The photovoltaic characteristics of PSCs under AM1.5 light illumination (1000 W/m²) under ambient conditions are studied. It is shown that the best efficiency of solar-to-electrical energy conversion, namely, 9.3%, is obtained for the PSC with a photoelectrode based on a TiO₂ film with an anatase/rutile mixed phase ratio of 86/14%.     

Dielectric properties of thin ferroelectric Sn<Subscript>2</Subscript>P<Subscript>2</Subscript>S<Subscript>6</Subscript> films deposited by thermal evaporation

The structure, microstructure, and temperature and field dependences of the dielectric properties of thin (0.5–8.0μm) Sn₂P₂S₆ ferroelectric films deposited onto glass and aluminum foil substrates by thermal vacuum evaporation in a quasi-closed volume are studied. The film-thickness and frequency dependences of the dielectric properties and the unipolarity of the C–V characteristics are explained by the presence of near-surface Schottky-type barrier layers.     

Hexaborides of rare earths as a sensor material for thermoelectric single-photon detectors

We consider the prospects of crystals of cerium hexaboride CeB₆ and lanthanum-cerium hexaborides (La,Ce)B₆ as a sensitive element in the so-called QVD thermoelectric single-photon detectors, operating at cryogenic temperatures. We have collected and analyzed the values of thermoelectric parameters of CeB₆ and (La,Ce)B₆ available in the literature. On this basis we calculated the energy resolution and photon count rate of the future thermoelectric QVD detectors. We conclude that the detectors on CeB₆ can register a single X-ray photon, and detectors on (La,Ce)B₆ — a single UV photon.     

X-ray spectroscopic examination of thin HfO<Subscript>2</Subscript> films ALD- and MOCVD-grown on the Si(100) surface

HfO₂ films 5 nm thick grown on Si(100) substrates by the methods of MOCVD hydride epitaxy and atomic layer deposition (ALD) are studied using X-ray photoelectron spectroscopy combined with Ar⁺ ion etching and X-ray reflectometry. It is found that (i) the ALD-grown HfO₂ films are amorphous, while the MOCVD-grown films show signs of a crystal structure; (ii) the surface of the ALD-grown films is more prone to contamination and/or is more reactive; and (iii) the amount of interfacial silicon dioxide in the case of the MOCVD-grown film is greater than in the case of the films synthesized by ALD. It is also shown that the argon ion etching of the HfO₂ film results in the formation of a metallic hafnium layer at the interface. This indicates that HfO₂ can be used not only as a gate dielectric but also as a material suitable for fabricating nanodimensional conductors by direct decomposition.     

Crystallographic orientation dependence of the dielectric constant in polymorphic BaNb<Subscript>2</Subscript>O<Subscript>6</Subscript> thin films deposited by laser ablation

Well-crystallized barium metaniobate (BaNb₂O₆) thin films were fabricated on fused quartz substrates by pulsed laser deposition. The influence of substrate temperature and oxygen pressure on the crystal structure and preferred orientation were studied to understand the growth mechanism of BaNb₂O₆ thin films. The films formed at 600 °C at an oxygen pressure of 100 mTorr exhibited predominantly the orthorhombic (040) orientation, and turned to the orthorhombic (230) orientation at 800 °C. It was found that (220)-oriented hexagonal thin films were formed at 600 °C at an oxygen pressure less than 50 mTorr. The dielectric constant of the BaNb₂O₆ thin films was measured by scanning microwave microscopy (SMM). Preferentially (230)-oriented orthorhombic and (220)-oriented hexagonal BaNb₂O₆ thin films were shown to have significantly enhanced dielectric constants of 47.8 and 56.7, respectively. This could be attributed to the dependence of the dielectric constant on crystallographic orientation. PACS 77.55.+f; 77.84.Dy     

Structure and dielectric properties of ultra-thin ZrO<Subscript>2</Subscript> films for high-<Emphasis Type="Italic">k</Emphasis> gate dielectric application prepared by pulsed laser deposition

ZrO₂ thin films have been prepared on Pt-coated silicon substrates and directly on n-Si(100) substrates by the pulsed laser deposition (PLD) technique using a ZrO₂ ceramic target under different deposition conditions. X-ray diffraction showed that the films prepared at 400 °C in 20 Pa oxygen ambient remained amorphous. Differential thermal analysis was carried out to study the crystallization behavior of ZrO₂. The dielectric constant of ZrO₂ was determined to be around 24 by measuring a Pt/ZrO₂/Pt capacitor structure. Sputtering depth profile X-ray photoelectron spectroscopy was used to investigate the interfacial characteristics of ZrO₂/n-Si stacks. A Zr silicate interfacial layer was formed between the ZrO₂ layer and the silicon substrate. The equivalent oxide thickness (EOT) and leakage current densities of the films with 6.6 nm physical thickness post-annealed in O₂ and N₂ ambient were investigated. An EOT of 1.65 nm with a leakage current of 36.2 mA/cm² at 1 V gate voltage for the film post-annealed in N₂ has been obtained. ZrO₂ thin films prepared by PLD have acceptable structure and dielectric properties required by a candidate material of high-k gate dielectrics. PACS 77.55.+f; 81.15.Fg; 73.40.Qv     

Highly (100) oriented Pb(Zr<Subscript>0.52</Subscript>Ti<Subscript>0.48</Subscript>)O<Subscript>3</Subscript>/LaNiO<Subscript>3</Subscript> films grown on amorphous substrates by pulsed laser deposition

Pb(Zr_0.52Ti_0.48)O₃ (PZT)/LaNiO₃ (LNO) thin films with highly (100) out of plane orientation were produced on SiO₂/Si(100) and alkaline earth aluminosilicate glass substrates by pulsed laser deposition (PLD). Orientations of both PZT and LNO films were evaluated using X-ray diffraction. The pure (100)-oriented PZT/LNO films were obtained under optimized deposition conditions. Time of flight-secondary ion mass spectrometry analysis showed that LNO could effectively block interdiffusion between the PZT films and the substrates. Fairly smooth surfaces of the PZT films with roughness of about 4 nm were observed using an atomic force microscope. Cross sectional examination revealed that the films grew in columnar grains. The PZT films grown on both SiO₂/Si and glass substrates demonstrated very good ferroelectric characteristic at room temperature with remnant polarization of up to 26 μC/cm². PACS 79.20.DS; 77.84.DY; 78.70.Ck     

Effects of BaTiO<Subscript>3</Subscript> and SrTiO<Subscript>3</Subscript> as the buffer layers of epitaxial BiFeO<Subscript>3</Subscript> thin films

BiFeO₃ (BFO) thin films with BaTiO₃ (BTO) or SrTiO₃ (STO) as buffer layer were epitaxially grown on SrRuO₃-covered SrTiO₃ substrates. X-ray diffraction measurements show that the BTO buffer causes tensile strain in the BFO films, whereas the STO buffer causes compressive strain. Different ferroelectric domain structures caused by these two strain statuses are revealed by piezoelectric force microscopy. Electrical and magnetical measurements show that the tensile-strained BFO/BTO samples have reduced leakage current and large ferroelectric polarization and magnetization, compared with compressively strained BFO/STO. These results demonstrate that the electrical and magnetical properties of BFO thin films can be artificially modified by using a buffer layer.     

Structural and optical properties of Bi<Subscript>3.25</Subscript>La<Subscript>0.75</Subscript>Ti<Subscript>3</Subscript>O<Subscript>12</Subscript> ferroelectric thin films prepared by chemical solution methods

Ferroelectric Bi_3.25La_0.75Ti₃O₁₂ (BLT) thin films have been grown on Pt/Ti/SiO₂/Si substrates by chemical solution methods. X-ray diffraction analysis shows that BLT thin films are polycrystalline with (171)-preferential orientation. Atomic force microscopy investigation shows that they have large grains about 120 nm in size. A Pt/BLT/Pt capacitor has been fabricated and showed excellent ferroelectricity, with a remnant polarization and coercive field of 24 μC/cm² and 116 kV/cm, respectively. The capacitor shows no polarization fatigue up to 10⁹ switching cycles. The optical constants (n,k) of the BLT thin films in the wavelength range 0.35–1.7 μm were obtained by spectroscopic ellipsometry measurements, and the band-gap energy was found to be about 3.25 eV. Received: 16 October 2001 / Accepted: 6 January 2002 / Published online: 3 June 2002 RID="*" ID="*"Corresponding author. Fax: +86-21/65830-734, E-mail: gswang@mail.sitp.ac.cn     

La<Subscript>9.33</Subscript>Si<Subscript>6</Subscript>O<Subscript>26</Subscript> electrolyte thin films for IT-SOFC application deposited by a HIPIMS/DC hybrid magnetron sputtering process

Apatite-type La_9.33Si₆O₂₆ thin films were elaborated by co-sputtering of two metallic La and Si targets powered, respectively, by high power impulse magnetron sputtering and direct current sources, in pure Ar atmosphere, followed by a subsequent high temperature oxidation treatment in air. The structural and chemical features of these films have been assessed by X-ray diffraction and scanning electron microscopy (SEM). The film with near lanthanum silicate La/Si atomic ratio deposited on a porous Ni-YSZ cermet substrates was initially amorphous. After thermal oxidation at 1,173 K in air, the coating crystallised under the expected apatite structure. SEM observation revealed that both film compactness and thickness increased after thermal oxidation. The conductivity evolution with temperature of the pure apatite-like lanthanum silicate coatings, as measured by complex impedance spectroscopy, showed that the activation energy of is quite low compared to the literature data.     

Effect of substitution of vanadium on the structure and electrical properties of Bi<Subscript>3.25</Subscript>La<Subscript>0.75</Subscript>Ti<Subscript>3</Subscript>O<Subscript>12</Subscript> thin films

The ferroelectric and dielectric properties of Bi_4-xLa_xTi₃O₁₂ (BLT) and Bi_4-xLa_xTi_2.97V_0.03O₁₂ (BLTV) thin films deposited on (111)Pt/Ti/SiO₂/Si substrates using a chemical solution method were investigated. The BLTV thin films showed a larger remanent polarization (9.6 C/cm²) than the BLT thin films (6.5 C/cm²), while the coercive field for both thin films was nearly the same. The capacitance of the films as a function of a small ac driving field was measured, and the data were processed using Rayleighs law. The results show that the Rayleigh constant of the BLT films was smaller than that of the BLTV films, indicating that the defect concentration was lower in the latter case. The superior ferroelectricity of the BLTV films was attributed to a decrease of both the (001) orientation and the defect concentration. PACS 77.80.Bh; 77.55.+f     

Dielectric investigations in Sr<Subscript>0.75</Subscript>Ba<Subscript>0.25</Subscript>Nb<Subscript>2</Subscript>O<Subscript>6</Subscript> relaxor ferroelectric thin films

The dielectric properties of Sr_0.75Ba_0.25Nb₂O₆ relaxor ferroelectric thin films were carefully analyzed. In contrast to bulk samples which present three distinct dielectric relaxation phenomena Sr_0.75Ba_0.25Nb₂O₆ thin films present only two of them. The suppression of the third anomaly can be mainly attributed to the narrow grain size distribution of nanograins and weak tensile strains imposed to the film from the substrate. The whole set of results point to the interpretation of a dielectric response characteristic of mesoscopic structure, which is composed of clusters and nanodomains.     

<Emphasis Type="Bold">(Ba,Sr)TiO</Emphasis><Subscript>3</Subscript> thin films grown by pulsed laser deposition with low dielectric loss at microwave frequencies

We report a method for producing BST films with consistently high figures of merit for tunable microwave applications. (Ba_1-x,Sr_x)TiO₃ (x=0.4, target doped with 1% W) thin films have been deposited using pulsed laser deposition onto (100)MgO substrates. Films were deposited at low partial pressures of oxygen (50 mTorr) at a substrate temperature of 730 °C. An analysis of the X-ray diffraction data indicates that the film has a nearly cubic structure, with the overall lattice parameter enlarged relative to the bulk material due to the presence of oxygen vacancies. A post-deposition anneal of the film in flowing oxygen (1000 °C for 6 h) resulted in a decrease in the lattice parameter while remaining nearly cubic. An analysis of the microwave dielectric properties (1–20 GHz) showed that the annealed film exhibited about 10% tunability for an applied bias field of 67 kV/cm with a dielectric Q(1/tan)>600. Investigation of the films by time-resolved confocal scanning optical microscopy (CSOM) has revealed that there is an out-of-plane polarization at zero applied field (E_DC=0). The results show that the paraelectric response is relatively insensitive to applied field, while the ferroelectric response is correlated with the growth of in-plane nanodomains. We find these results to be consistent with a large number of studies that show that strain-relief is of paramount importance if ferroelectric films are to be developed as microwave circuit components. PACS 81.15.Fg; 85.50.-n     

Preparation and dielectric properties of the lead-free BaFe<Subscript>1/2</Subscript>Nb<Subscript>1/2</Subscript>O<Subscript>3</Subscript> ceramics obtained from mechanically triggered powder

In the paper the influence of mechanical activation of the powder on the final dielectric properties lead-free Ba(Fe_1/2Nb_1/2)O₃ (BFN) ceramic was examined. The BFN ceramics were obtained by 3-steps route. Firstly, the substrates were pre-homogenized in a planetary ball mill. Than, the powder was activated in vibratory mill (the shaker type SPEX 8000 Mixer Mill) for different duration between 25 h and 100 h. The influence of the milling time on the BFN powder was monitored by X-ray diffraction. The diffraction data confirmed that the milling process of the starting components is accompanied by partial synthesis of the BFN materials. The longer of the high-energy milling duration the powders results in increasing the amount of amorphous/nanocrystalline content. The mechanically activated materials were sintered in order to obtain the ceramic samples. During this temperature treatment the final crystallisation of the powder appeared what was confirmed by XRD studies. The performed dielectric measurements have revealed the reduction of the dielectric loss of the BFN ceramics compared to materials obtained by classic methods.