Effect of bias field on the dielectric properties of 0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 single crystals with different orientations

Microdomain-macrodomain transformations and phase transitions in 0.7Pb(Mg_1/3Nb_2/3)O₃-0.3PbTiO₃ single crystals were investigated by studying their relative permittivity under various dc bias at constant heating and cooling rates. The orientation dependence of the bias field effect was revealed by examining the temperature dependence of relative permittivity as a function of crystal orientation (the 111, 011 and 001 directions) and dc bias field. The crystals have a microdomain rhombohedral ferroelectric state in the ferroelectric phase under zero dc bias. External bias field could modulate the domain state and induce a macrodomain state in the crystals. Also, it is proposed that the dc bias applied along the 001 or 011 direction could induce a tetragonal ferroelectric phase or an orthorhombic ferroelectric phase, respectively, in an intermediate temperature range. PACS 77.80.-e; 77.22.-d; 81.40.-z     

Effect of Pb2+ ions on the photoluminescence characteristics of Mn2+-doped Zn2SiO4

Zn₂SiO₄ single-doped and co-doped with Mn²⁺ ions and Pb²⁺ ions has been prepared by a sol–gel process. The luminescent properties of the samples have been investigated. From the excitation and emission spectral analysis, it was found that the emission peaks vary with the change of the excitation wavelength. An enhanced luminescent characteristic of Mn²⁺ ions (in blue and green spectral zones) has been observed, due to the energy transfer from the Pb²⁺ ion to the Mn²⁺ ion. The emission peaks originate from the d–d transitions of the Mn²⁺ ion. However, the relative intensities of the peaks show a dependence on the concentration of the Pb²⁺ ion. PACS 81.05.Je; 78.55.Hx; 61.72.Ww     

Barium ferrite (BaFe12O19) thin films prepared by pulsed laser deposition on MgO buffered Si substrates

Barium ferrite (BaFe₁₂O₁₉) thin films have been deposited by pulsed laser deposition (PLD) on Si substrates with MgO underlayers. The films were deposited in oxygen atmosphere by excimer laser (=248 nm, pulse duration=23 ns) in the temperature range 750–900 °C. The experiments showed that the substrate temperature has remarkable effect on the films magnetic and structural properties. The BaFe₁₂O₁₉ films deposited at 900 °C in 200 mTorr oxygen showed some perpendicular orientation, with a perpendicular squareness of 0.5 and an in-plane squareness of 0.3. Such thin BaFe₁₂O₁₉ films have platelet grains with a size of about 300 nm. The perpendicular saturation magnetization and coercivity are 185 emu/cm³ and 1.4 kOe, respectively. PACS 81.15.Fg.; 75.70.Ak; 75.50.Pp; 68.55.j     

Influence of sp 3 fraction on the field emission properties of tetrahedral amorphous carbon films formed by magnetic filtered plasma stream

Electron field emission properties of tetrahedral amorphous carbon films (ta-C) with various sp³ fractions, [sp³]/([sp²]+[sp³]), prepared by magnetic filtered plasma deposition system, were investigated. The ta-C films were deposited on (100) n-Si wafer with a resistivity of 0.01–0.02 cm in a substrate bias voltage V_b range from +20 V to -80 V. The relative fraction of sp³-bonded carbon in these films was qualitatively and quantitatively estimated by a fitting of the Raman and XPS spectra, respectively. Results show that ta-C films of high sp³ fraction, more than 80%, can be formed with a substrate bias voltage V_b in the range from -10 to -50 V. A remarkably low turn-on field of about 1.7 V/m was observed for these samples. For V_b outside this range, the sp³ fraction is lower. The surface of such ta-C films was found to be smooth and uniform from the images of atomic force microscopy. The sp³ fraction of the sample is believed to be the main factor affecting field emission properties of ta-C films. PACS 79.70; 78.30; 73.90.+f     

Characterization and field-emission property of aligned porous carbon nanotube film by hydrogen-ion implantation

To enhance the field emission of carbon nanotube (CNT) films, a novel technique that combines hydrogen-ion implantation used in the silicon-on-insulation smart-cut process and plasma-enhanced chemical vapor deposition was developed to produce an aligned porous carbon nanotube (AP-CNT) film on a Si substrate. All steps in the AP-CNT synthesis were carried out in vacuum, which reduced possible contamination. The morphology and the field-emission properties of the CNT films were investigated and results show that CNT holes with a diameter of 5 m and a depth of 30 m were produced in the AP-CNT film. The alignment of the CNTs is visibly improved. Due to the implantation treatment, the turn-on field of the CNT films decreased from 1.5 to 0.8 V/m, and the emission-current (and dot) density obviously increased. This field-emission improvement may mainly arise from the holes formed in the AP-CNT films. The edges of these holes not only intensified the electron emission but also increase the emission site density of the CNT films. PACS 81.05.Uw; 61.72.Tt; 85.45.Db     

Morphology and microstructure of all-epitaxial ferroelectric tri-layered (Bi,La)4Ti3O12/Pb(Zr0.4Ti0.6)O3/ (Bi,La)4Ti3O12 thin films on SrTiO3(011)

The morphology and microstructure of all-epitaxial (Bi,La)₄Ti₃O₁₂/Pb(Zr_0.4Ti_0.6)O₃/(Bi,La)₄Ti₃O₁₂ (BLT/PZT/BLT) tri-layered ferroelectric films, grown on (011)-oriented SrTiO₃ (STO) substrates by pulsed laser deposition, are investigated by transmission electron microscopy (TEM). X-ray diffraction and electron diffraction patterns demonstrate that the epitaxial relationship between BLT, PZT and STO can be described as ; . Cross-sectional TEM images show that the growth rate of BLT is nearly two times that for PZT at the same growth conditions, and 90° ferroelectric domain boundaries lying on {110} planes are observed in the PZT layer. The 90° ferroelectric domains in the PZT layer extend up to 600 nm in length. Long domains penetrate into the neighboring columnar grain through the columnar grain boundary, whereas others are nucleating at the columnar grain boundaries. The roughness of the PZT/BLT interfaces appears to depend on the viewing direction, i.e., it is different for different azimuthal directions. Planar TEM investigations show that the grains in the top BLT layer have a rod-like morphology, preferentially growing along the [110]_BLT direction. The grain width is rather constant at about 90 nm, whereas the length of the grains varies from 150 to 625 nm. These morphological details point to the important role the crystal anisotropy of BLT plays for the growth and structure of the tri-layered films. PACS 81.15-z; 68.37.Lp; 77.84.-s     

Improved interfacial properties of SiO2 grown on 6H-SiC in diluted NO

The interface quality and reliability of gate oxides grown on n-/p-type 6H-SiC in diluted NO gas at 1150 °C are investigated. As compared to conventional 100%-NO oxidation, the diluted-NO (50% and 23%) oxidations lead to lower interface-state, border-trap and oxide-charge densities. This is attributed to the fact that carbon-accumulation and carbon-removal rates are closer when oxidation is performed in diluted NO, giving a smoother, less disordered and strained interface. Moreover, less degradation of the diluted-NO samples than 100%-NO samples is observed during high-field stressing (±7 MV/cm), indicating that stronger SiN bonds are created near/at the SiC/SiO₂ interface for oxide grown in diluted NO ambient. PACS 85.30.Tv; 81.65.Mq; 81.05.HD; 85.30.De; 73.20.At     

Vertebral cancellous bone turn-over: microcallus and bridges in backscatter electron microscopy

Backscatter electron microscopy (BSE) is a powerful technique for investigating cancellous bone structure. Its main function is to offer information regarding the degree of mineralization of the tissue within individual trabeculae.

To illustrate the qualitative information that can be drawn from BSE imaging technique, we present a study on human vertebral cancellous bone. This tissue is continuously remodeled through osteoclastic resorption and osteoblastic new bone apposition. It is thought that osteoclastic resorption pits are especially deleterious for vertebral bone architecture since they often perforate the thin trabeculae; the osteoblasts being unable to repair the gap. In addition, excessive stress may also disrupt the architecture in case of trabecular fracture or damage accumulation.

Waves of new bone formation were easy to identify in BSE. Often these waves were connecting both edges of a perforation and called bridges. Additionally, we present a few images of microcallus formations. A microcallus is described as a small mass of woven bone that generally repairs a trabecula. The microstructural aspects of different microcalluses are presented and discussed. Both bridges and microcallus should be considered as examples of the repair porcess since they obviously preserve the connectivity of the trabeculae. However, bridges were much more frequent than microcallus (396 vs 15). Both mechanisms probably illustrate the normal response to different local stimuli.     

Structural characteristics of ultra-low k SiO<Subscript>2</Subscript> thin films prepared using a molecular template

Nanoporous SiO₂ thin films with ultra-low dielectric constants were synthesized using a molecular template method. Uniform films with pore size between 10 and 20 nm were obtained as observed by N₂ adsorption/desorption isotherms and transmission electron microscopy. Fourier transform infrared spectroscopy (FTIR) and differential thermal analysis were carried out to investigate the effect of n-hexane washing on structural properties before and after the surface modification process. The results showed that –OH bonds were substituted with –CH₃ bonds in the films as a result of modification of trimethylchlorosilane (TMCS)/n-hexane solution. Four kinds of model were used to analyze the relationship between porosity and dielectric constant of the films, where the dielectric constant was determined from capacitance-voltage measurements. The investigation indicated that the corresponding relationship was in accord with that estimated by the Rayleigh model.     

Contactless gating,surface charging and illumination effects in a buried Al<Subscript>0.24</Subscript>Ga<Subscript>0.76</Subscript>As/GaAs quantum well structure

The conductivity of an Al_0.24Ga_0.76As/GaAs quantum well was studied as a function of the surface charge generated by electron bombardment of the sample in the absence of an externally applied surface electric field. Under a suitable rate of electron irradiation, it was possible to completely shut off the conductive channel, implying a surface density . Light illumination quenches the increase of the resistivity, apparently due to photoemission from the metastable surface states. Upon turning off the electron bombardment the surface charge on adsorbed layers of xenon and water at 8 K decays in room temperature darkness with a lifetime τ= 0.30 ±0.02 s. The average charging efficiency, is μ₀ ≃0.001. Surface charging is shown to be an effective method for contactless gating of field effect devices.