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In this contribution, the influence of different substrates and textures on the reversible and irreversible polarization in Pb(Zr,Ti)O3 (PZT) thin films will be presented. One possible scenario to explain the origin of the ferroelectric hysteresis is the notion that the domain walls move through a potential generated by their interaction with randomly distributed defects of the matrix. This potential then gives rise to reversible and irreversible changes in the ferroelectric polarization. The exact features of the interaction potential also depend on the stress state of the material which can be influenced by a suitable choice of the substrate.
To study the substrate influence, PZT thin films have been deposited on commercial Si wafers, MgO and SrTiO3 single crystals. Electrical characterization methods (hysteresis and small signal capacitance measurements) have been used to extract information on reversible and irreversible polarization contributions.
Two conducting oxides, La0.5Sr0.5CoO3(LSCO) and SrRuO3, were deposited by pulsed laser ablation onto silicon substrates coated with biaxially textured MgO on an amorphous silicon nitride isolation layer. Comparison is made between templates using just 10 nm of ion-beam assisted deposited (IBAD) MgO and substrates with an additional 100 nm of homoepitaxial MgO. Both of these conducting oxide layers exhibited in-plane and out-of-plane texture, on the order of that obtained by the underlying MgO. The SrRuO3 was c-axis oriented on both substrates, but exhibited a slightly sharper out-of- plane texture when the homoepitaxial MgO layer was included. On the other hand, the LSCO showed only (100) orientation when deposited directly on the IBAD-MgO templates, whereas a significant (110) peak was observed for films on the homoepitaxial MgO. A simple calculation of the distribution of grain boundary angles, assuming a normal distribution of grains, is also presented.
Vanadium dioxide (VO2) is one of the most attractive thermochromic materials, which shows large changes in optical and electrical properties at around 68°C, nearly room temperature. This thermochromic behavior has been explained in terms of the Mott-Hubbard transition from a high-temperature rutile structure (metal phase) to a low-temperature monoclinic structure (semiconductor phase). We already reported that rf magnetron sputtering using V2O3 or V2O5 targets enable us to deposit polycrystalline thermochromic VO2 films with high reproducibility by introduction of oxygen gas (O2/(Ar+O2)=1∼1.5%) or hydrogen gas (H2/(Ar+H2)=2.5∼10%), respectively, as reactive gases [see ref.1]. In this study, ZnO polycrystalline films were deposited as a buffer layer between the VO2 film and glass substrate also by rf magnetron sputtering, which have been known to exhibit <001> preferred orientation in the wide range of the deposition conditions. Very thin thermochromic VO2 films with thickness of 50nm were successfully deposited on the ZnO coated glass substrate because of the heteroepitaxial relationship of VO2(010)//ZnO(001),,.
We propose a materials design to fabricate the transparent and half-metallic ferromagnets in V-, Cr-, Mn+hole, Fe-, Co-, and Ni-doped ZnO based upon ab initio electronic structure calculation. Mn-doped ZnO is anti-ferromagnetic spin glass state, however, it becomes half-metallic ferromagnets upon hole doping. The ferromagnetic state becomes more stable by electron doping in Fe-, Co- or Ni-doped ZnO. From the point of practical applications, it is feasible to realize the half-metallic ferromagnets with high Curie temperature, because n-type ZnO is easily available. We propose the design of new functional devices, such as spin-FET, photo-induced ferromagnets, and spin-injection devices using negative electron affinity in the wide band gap semiconductors.
Medium-k dielectric Y2O3 films were directly grown on (100) Si substrates by the pulsed laser deposition (PLD) technique. X-ray photoelectron spectroscopy, variable angle spectroscopic ellipsometry, current-voltage, capacitance-voltage, and high-resolution transmission electron microscopy were used to investigate the composition, thickness, and electrical properties of the grown structures. It has been found that at the interface between the Si substrate and the grown dielectric layer, a SiOx interfacial layer, whose thickness depended on the oxygen pressure used during the PLD growth, was always formed. The main oxygen source for this interfacial layer formation is the physisorbed oxygen trapped inside the grown layer during the laser ablation-deposition process. When trying to reduce the thickness of this low-k interfacial layer by decreasing the oxygen pressure during laser ablation, a marked degradation of the electrical properties of the structures was noticed.
Thin films of intermixed layers of In2O3:Sn (denoted ITO) and silver were made by reactive dc magnetron sputtering. The silver to indium atomic ratio was 0 ≤ x ≥ 0.09 Films with x = 0.01 showed increased luminous transmittance Tlum after annealing at 100 or 200°C, whereas x > 0.01 yielded lowered Tlum. The optical properties could be reconciled with the Maxwell-Garnett effective medium theory assuming that a well-defined portion of the silver was occluded as spheroidal particles. Films with x ≤ 0.06 had enhanced electrical conductivity after annealing at 200 or 300°C. Transmission electron microscopy displayed columnar features whose character depended on x.
In this paper we investigated a feasibility of Y2O3 films as a buffer layer of MFIS (metal ferroelectric insulator semiconductor) type capacitor. Buffer layers were prepared by two-step process of a low temperature film growth and subsequent RTA treatment. Investigated parameters are given as substrate temperature, O2 partial pressure, post-annealing temperature, and suppression method of interfacial SiO2layer generation. By employing an ultra thin Y pre-metal layer, unwanted SiO2 layer generation was successfully suppressed at an interface between the buffer layer and Si substrate. By using two-step process, we improved the leakage current density of Y2O3 films by 2 orders and the Dit as low as 8.72×1010 cm−2eV−1. For a substrate temperature above 400°C and O2 partial pressure of 20%, we observed cubic Y2O3 phase domination in XRD spectra. We achieved 1.75% lattice mismatch between Y2O3 film and silicon substrate. Y2O3 buffer layer for a single transistor FRAM exhibited optimal properties when it was grown at 400°C with 20% O2 partial pressure then RTA treatment at 900°C in oxygen ambient.
Indium-tin-oxide films were grown hetero-epitaxially on YSZ surface at a substrate temperature of 900 °C, and their surface microstructures were observed by using atomic force microscopy. ITO films grown on (111) surface of YSZ exhibited very high crystal quality; full width at half maximum of out-of-plane rocking curve was 54 second. The ITO was grown spirally, with flat terraces and steps corresponding to (222) plane spacing of 0.29 nm. Oxygen pressure during film growth is another key factor to obtain atomically flat surfaced ITO thin film.
Zinc Oxide is a potentially valuable semiconductor with photoconducting, piezoelectric, optoelectronic, and optical waveguide applications. A technique for the growth of ZnO epitaxial layers is necessary for improved device fabrication. Previously, attempts at ZnO epitaxial growth were constrained to sapphire substrates using sputtering as the preferred preparation technique. However, with the present availability of single crystal ZnO substrates the hope of high quality epitaxial layers exists. The authors will discuss several possibilities for growth precursors, and the resulting chemical reactions required for the formation of ZnO. This presentation discusses the growth of ZnO epitaxial layers by chemical vapor deposition without using metal-organic compounds. The thermodynamics and predicted growth rates for several reactions are presented as well as the preliminary growth results. The preliminary growths are characterized with the aid of Hall Effect measurements, providing information on resistivity, Hall mobility, and carrier concentration.
We have been investigating the potential for a channel transistor which utilizes a perovskite oxide capable of undergoing the Mott metal-insulator transition as the channel material. Our experiments have identified three limitations to the performance of the oxide devices: contact resistance to the channel, mobility limitations due to polycrystalline channels, and inadequate field induced surface charge density. In this paper we review progress we have made in oxide electrodes and in improving channel growth conditions which have mitigated the limitations due to contact resistance and polycrystalline channels. We conclude with an outline of our approach to improving the field induced surface charge density.
This paper reports the microstructure and physical properties of ferroelectric capacitors formed from SrBi2Ta2O9(SBT) layers on Si with various buffer layers including jet-vapor deposited silicon nitride, zirconium oxide, hafnium oxide and thermally grown silicon oxide. Results from cross-sectional transmission electron microscopy (X-TEM), energy dispersive spectroscopy (EDS), X-Ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and non-contact atomic force microscopy (nc-AFM) data coupled with capacitance-voltage (C-V) and current- voltage (I-V) data indicate that both the microstructure and physical properties of SBT films deposited on silicon are dependent on the buffer layer material employed.
Materials design for transparent p-type conducting oxides was extended to oxysulfide system. LaCuOS was selected as a candidate for a transparent p-type semiconductor. It was found that the electrical conductivity of LaCuOS was p-type and controllable from semiconducting to semi-metallic states by substituting Sr2+ for La3+. LaCuOS films showed high transparency in the visible region, and the bandgap estimated was approximately 3.1 eV. Moreover, it was revealed that LaCuOS showed sharp excitonic absorption and emission at the bandgap edge, which is advantageous for optical applications. A layered oxysulfide, LaCuOS, was proposed to be a promising material for optoelectronic devices.
Transparent p-n heterojunction diodes are fabricated using p-type CuYO2:Ca and n-type ZnO:Al thin films on a glass substrate coated with indium-tin oxide (ITO). The contact between the n-ZnO:Al / p-CuYO2:Ca heterojunction is found to be rectifying, while the ITO / ZnO:Al contact is ohmic. The typical ratio of forward to reverse current is 15 in the range -3 to 3V. The diode current-voltage characteristics are dominated by the flow of space charge limited current, which is ascribed to the existence of an insulating ZnO interfacial layer. The diode structure has a total thickness of 0.85 μm and an optical transmission of 40%-50% in the visible region.
Tin oxide thin films were deposited by plasma-enhanced chemical vapor deposition (PECVD) for applications as a transparent conductor. X-ray diffraction (XRD) and atomic force microscopy (AFM) were used to quantify the crystal structure and morphology of these films both as-deposited and after annealing conditions. Annealing was performed in an argon environment as a function of time and temperature. Although annealing was found to significantly improve electrical properties, the structure as measured by XRD remained largely unchanged. Hall effect measurements show that the improvements in resistivity are due to increases in carrier concentration. XRD did reveal that films deposited on the powered electrode had a film orientation that was distinctly different than films deposited on the grounded electrode. These changes suggest the importance of ion bombardment energy. The structural changes were correlated with improved electrical properties.
Sensitivity tests to reductive gases such as methane, hydrogen and ethane were performed on zinc oxide (ZnO) thin films. The highest value of sensitivity was obtained for the film with a high electrical resistivity and a low thickness. The variation of the operating temperature of the film leads to a significant change in the sensitivity of the sensor with an ideal operating temperature dependence of the gas used. The sensitivity of the ZnO thin films changes linear with the increase of the gas concentration. However these films seem to be more appropriated for the detection of hydrogen following by methane and than for ethane since the value of sensitivity obtained are higher and its variation with the gas concentration more pronounced.
In this paper, we describe the underlying theory along with experiments concerning the electrical conductivity of transparent conducting ZnO films with a carrier concentration of 1019-1021 cm−3. The experimentally determined mobility as a function of carrier concentration in the range of 1019-1021 cm−3 could be quantitatively referenced to a theoretically calculated mobility that is dominated by not only grain boundary scattering but also ionized impurity scattering using the Brooks-Herring-Dingle theory with both degeneracy and nonparabolicity of the conduction band taken into account. Concerning nonparabolicity, the conduction band effective mass as a function of carrier concentration was theoretically analyzed and experimentally determined.
We report optical and structural properties of ZnO films deposited by pulsed laser deposition technique on silicon (100) n-type, quartz, sapphire, and corning glass substrates. We have studied the influence of the deposition parameters, such as substrate temperature, oxygen pressure, and laser fluence on the properties of the grown films. Dependence of nanocrystallites on temperature of substrate and ambient gas pressure is investigated. ZnO plasma created at varying fluence of KrF laser (248 nm) on the target was investigated using optical emission spectroscopy and 2-d images of the expanding plumes at various pressures of the ambient gas. X-ray diffraction, atomic force microscopy, electron probe micro-analyzer, and spectro-photometry were used to characterize as grown films.
High-quality thin films of BaTiO3 and SrTiO3 on SrTiO3 substrate were obtained by shutting off the oxygen supply during growth. Epitaxial growths were carried out with molecular-beam epitaxy (MBE) under extremely low oxygen partial pressure (pO2 < 1×10−8 Pa). Although only Ba(Sr) and Ti metals were supplied without introducing oxidant during the growth, clear reflection high-energy electron diffraction (RHEED) intensity oscillations from layer-by-layer growth were observed. The deposited films were found to have approximately stoichiometric compositions of BaTiO3 and SrTiO3. Oxygen was automatically fed from the substrate during the growths. It was found that the BaTiO3/SrTiO3 interface was abrupt without intermixing, despite a considerable amount of oxygen seems to have moved from the substrate to the film through the interface.
Using an RF magnetron sputtering technique, thin layers (∼500 nm) of amorphous silicon suboxides (a-SiOx) were deposited, with oxygen/silicon ratios x ranging from 0 to 1.8. These layers contain a large density (1020−1021 cm−3) of, mostly silicon dangling bond, defect states. The level of conduction decreases several orders of magnitude with increasing x. The temperature dependence of the DC conductivity showed that the variable range hopping conduction mechanism is dominant for all x, over the temperature range 30- 330 K. In this mechanism the extent of localization and density of states around the Fermi level determine the conductance. We conclude that the decrease in conductance with increasing oxygen content must, for a large part, be due to a variation in the localization, since Electron Spin Resonance (ESR) measurements showed no decrease in defect density with increasing x. We performed DC conduction measurements at both low and high electric field strengths, showing phenomena, which are consistently desribed within the variable range hopping (VRH) model. These measurements allow the extraction of quantitative information, concerning both the localization and the density of the states involved in the hopping process.
We report on a ferroelectric film pressure sensor fabricated on the top of 4 mm long and 1.4 mm in diameter Pt80Ir20 (PtIr) rod-shaped tip. It consists of a PZT(0.5μm)/LSMO(0.1μm) film heterostructure, deposited by pulsed laser ablation of stoichiometric ceramic targets PbZr0.52Ti0.48O3 and La0.67Sr0.33MnO3, and a circular, ø= 1.2 mm, Au electrode on the top of the PZT film. The Au/PZT/LSMO/PtIr thin-film capacitor demonstrates good ferroelectric properties: dielectric constant of 762 and loss tanσ =0.008 @ 5 kHz, induced polarization as high as 32 μC/cm2 at electric field of 250 kV/cm. Piezoelectric test, performed in a hydrostatic pressure chamber, exhibits the piezoelectric constant to be as high as 67 pC/N. This is 20% higher than 56 pC/N shown by a polarized bulk PZT sensor fabricated from the ceramics used as the target in the pulsed laser deposition process. Such an increase of the piezoelectric constant we attribute to the preferential (001) orientation of the PZT film grown on the PtIr bulk substrate. The resolution of the thin PZT film pressure microsensor was found to be about 1 mbar.