Residual strain in cadmium telluride/gallium arsenide (001) heterostructures as a function of temperature

Optical studies of residual strain in cadmium telluride (CdTe) films grown using molecular beam epitaxy on gallium arsenide (GaAs) substrate have been performed using photoreflectance techniques. Measurements have been conducted to determine the fundamental transition energy, heavy-hole and light-hole transition energy critical-point parameters in a range of temperatures between 12 and 300 K. There are problems inherent in the fabrication of optoelectronic devices using high-quality CdTe films, due to strain effects resulting from both the lattice mismatch (CdTe: 14.6%) and the thermal expansion coefficient difference. The CdTe film exhibits compressive stress causing valence-band splitting for light and heavy holes. We have used different models to fit the obtained experimental data and, although the critical thickness for the CdTe has been surpassed, the strain due to the lattice mismatch is still significant. However, the strain due to the thermal expansion is dominant. We have found that the fundamental transition energy, E₀, is affected by the compressive strain and the characteristic values are smaller than those reported. In addition, the total strain is compressive for the full measured range, since the strain due to the lattice mismatch is one order of magnitude higher than that calculated from the thermal expansion.     

Influence of growth rate on the structural and morphological properties of TiN, ZrN and TiN/ZrN multilayers

The effect of the deposition rate on the structural and morphological properties of TiN and ZrN single layers and TiN/ZrN multilayers deposited by radiofrequency reactive magnetron sputtering has been studied. The total pressure was kept constant and the growth rate variation was obtained by small difference of nitrogen concentration in the fed gas. The decreasing deposition rate results in a structural change in the thin films from (111) orientation to (100) one. As consequence the surface morphology becomes smoother. Films roughness is strongly related with texture and it decreases with an increase in the (100) X-ray diffraction line intensity. In order to achieve a clear interpretation of our experimental results, the ratio between the N⁺ ions of the plasma and the atoms number reaching the substrate was considered. At high deposition rate with respect to the N⁺ concentration, the chemical potential of transition metal on (100) growth surface is higher than (111) one favouring the (111) orientation of the films. On the contrary, when the growth rate is low with respect to the nitrogen concentration, the chemical potential of transition metal on (111) growth surface is higher than the (100) one leading to a preferential growth in the (100) direction.     

Progress in multilayer devices as X-ray optical elements

A new kind of mirror based on multilayering techniques gives the possibility of preparing artificial Bragg reflectors over all the X-UV range. The possibility to choose most of the parameters governing the reflectivity opens the possibility to get a very good efficiency, even under normal incidence. Main parameters, evaporating techniques and tests are described. Some example of recent results are compared to the theoretical predictions.     

Enhancement of remnant polarization in multilayered Bi4Ti3O12/(Bi3.25La0.75)Ti3O12 films obtained by chemical solution deposition

Enhancement of remnant polarization was observed in artificially multilayered Bi₄Ti₃O₁₂ (BT)/(Bi_3.25La_0.75)Ti₃O₁₂ (BLT) films_. The multilayer were prepared on platinum coated silicon substrate by chemical solution deposition and compared with the single-phase BT, BLT and (Bi_3.5La_0.5)Ti₃O₁₂ films_. The multilayered film with a stacking periodicity of 60 nm BLT/30 nm BT shows a remnant polarization (2Pr) of about 61 μC/cm², which is much higher than those of the single-phase films. In addition, the multilayered films show a good fatigue-endurance character. After post-annealing the multilayered films at 700 °C for a long time (20 h), its remnant polarization decreased to a value close to the corresponding uniform film. Some possible mechanisms behind the polarization enhancement were proposed.     

Interfacial reactions in Mo-Zr multilayer structure: an X-ray reflectivity study

The effect of elevated temperature on the structural stability and alloy formation in Mo-Zr multilayers is investigated. Mo-Zr multilayers deposited by the electron beam evaporation technique under ultra-high vacuum conditions are annealed up to 650 °C. The changes induced due to thermal treatment are observed using X-ray reflectivity (both specular and off-specular) and X-ray diffraction techniques. The Mo-Zr multilayers remained as an insoluble layered structure even after annealing as revealed from X-ray reflectivity measurements. The interfacial roughness is found to be very similar at all interfaces and decreases on annealing. The multilayer structure remains intact on annealing with expansion of the multilayer period and a marginal increase in X-ray reflectivity.     

Growth mechanisms of multilayered anodic-titanium-oxide nanotube membranes

Anodic titanium oxide TiO₂ nanotube membranes with one, two and three layers have been prepared by the anodic oxidation method via adjusting applied bias during the oxidation process in glycol electrolytes containing 0.25 wt% or 0.50 wt% NH₄F. The morphologies of the as-prepared samples were presented in detail using scanning and transmission electron microscopy. The results indicated that the glycol electrolyte containing 0.25 wt% NH₄F yielded smooth double-walled nanotubes, and their tubes remained open in multilayers; while the glycol electrolyte containing 0.50 wt% NH₄F yielded single-wall nanotubes with ripples, and most of the nanotubes have closed bottoms. In the case of nanotubes with closed bottoms, it was proposed that the newly grown nanotubes on the succeeding layer formed in the gap between the nanotubes of the previous layer. In order to keep track of the growth of the nanotubes, samples were anodized for different periods of time from 1 to 90 min to study their formation process. Growth mechanisms were proposed for different morphologies.     

Fabrication of evacuated glazing at low temperature

An indium-based seal augmented with an adhesive, developed to maintain a vacuum between two sheets of glass, avoids the high temperatures required to produce a seal in evacuated glazings to date. An experimentally-validated three-dimensional transient model has been used to predict heat transfer for an indium/adhesive sealed 1 m² area evacuated window with a highly insulating frame. An overall heat loss coefficient of 0.9 W m⁻² K⁻¹, with a midplane value of 0.36 W m⁻² K⁻¹, can be achieved with 0.72 visible transmittance for two 6 mm panes separated by 0.2 mm diameter pillars 40 mm apart. The conduction through a 3 mm edge-seal was 1.14 W m⁻² K⁻¹. Detailed three-dimensional isothermal contour plots through the system are presented.     

Observation and formation mechanism of individual graphene foils inside multi-walled carbon nanotubes

The formation of individual graphene foils (GF) has been evidenced in the present study inside multi-walled carbon nanotubes. The graphene foils are attached on one side to a catalyst nanoparticle and on the other side to the internal walls of the nanotube. Moreover, results suggest that a necking process occurs in which internal carbon walls are deformed until formation of the graphene foil. A possible mechanism for the GF formation is then proposed.     

Synthesis of zirconium tungstate–zirconia core–shell composite particles

In this work, ZrW₂O₈–ZrO₂ core–shell composite particles were synthesized. ZrW₂O₈ that was used in the core is a material with negative coefficient of thermal expansion, and it was synthesized from a high-pH precursor based on use of tungstic acid and zirconium acetate. Shell layer was composed of ZrO₂ nanocrystallites and precipitated from an aqueous solution by urea hydrolysis. While volume of the shell was effectively controlled by the initial zirconium ion concentration in the solutions, the rate of precipitation was a function of the ratio of initial concentrations of urea to zirconium ions. It is hypothesized that isolation of the ZrW₂O₈ within a layer of ZrO₂, will be a key element in solving problems associated with reactivity of ZrW₂O₈ towards other components in sintering of ceramic–ceramic composites with tuned or zero thermal expansion coefficient.