The origin of tetrapods is a complex question that webs together genetic, paleontological, developmental and physical facts. Basically, the development of embryos is described by a complex mix of mechanical movements and biochemical inductions of genetic origin. It is difficult to sort out in this scientific question what are the fundamental features imposed by conservation laws of physics, and by force equilibria, and what can be ascribed to successive, very specific, stop-and-go inductions of genetic nature. A posteriori, evolution selects the parameters of this process as found in the observed species. Whether there is a general law to animal formation seems out of the question. However, several concepts developed in biology, like the concept of “organizer” seem questionable from a physics point of view, since the entire deformation and force field should be the “organizer” of development, and one can hardly ascribe such a role to a single small area of the embryo body. In the same spirit, the concept of “positional information” encapsulated in concentration of chemicals seems questionable since the deformation and force fields in embryonic tissues are tensors. Finally, the concept of a development organized in space along three orthogonal (“Cartesian”) axes associated to chemical gradients seems also questionable, since early embryo development is driven by complex vortex fields, with hyperbolic trajectories which span the entire embryo. Such hyperbolic trajectories are best understood by a description in terms of dipolar components of the morphogenetic forces, whose projections along orthogonal axe have no specific meaning except as a mathematical tool. I review here the present state of description of several aspects of tetrapods morphogenesis and evolution, from the point of view of physics. It is getting clear that several basic features of tetrapods body are a direct consequences of fundamental laws of physics. Several lines of work reviewed here show that the topology of the tetrapods may be directly related to the structure of the earliest movements in embryos. The bio-mechanical approach leads to important consequences for the constraints on evolution of the craniates. Such consequences have received a controversial welcome in the last decade, although they may encapsulate the true origin of craniates, esp. simians, and eventually homo.

IR photodetectors based on GaAs/Al0.25Ga0.75As multiquantum wells (QWIP) grown by molecular beam epitaxy (MBE) are studied. The envelop function formalism is used to determine the theoretical intersubband transition energies. The electronic states are calculated in both parabolic and non parabolic cases. IR spectroscopy transmission is used as the experimental technique to evaluate the optical absorption. The measures are made at 77 K for incidence at both 45° and Brewster angles geometries. The last experimental results compare well with the theoretical ones and correspond to 10–12 $\mu $m operating wavelength.

Electrical conductivity was performed on amorphous thin films of Ge1–xSe2Pbx (with x = 0, 0.2, 0.4, 0.6 and 0.8) as a function of temperature in the range 300–450 K. The experimental results indicate that the conduction is through thermally activated process having two conduction mechanisms. In the first region at high-temperatures range, the values of $\sigma _{o}$ suggest that the dominant conduction of charge carriers changes from the extended states to the localized states in the band tails at composition x = 0.8. The experimental results have also been analyzed using Meyer-Neldel Rule. The other one appears in the low temperatures region and the conductivity has been analyzed using Mott's variable range hopping conduction. Mott's parameters were calculated for Ge1–xSe2Pbx films.

We have reported a study of the I–V characteristics of Ni/n-GaAs Schottky barrier diodes (SBDs) in a wide temperature range of 60–320 K by a step of 20 K, which are prepared by magnetron DC sputtering. The experimental I–V data of the device quite well obey the thermionic emission model at 300 and 320 K, respectively. The ideality factor and barrier height values have changed by change of the sample temperature, the case has been attributed to the presence of the lateral inhomogeneities of the barrier height. The barrier inhomogeneity has been explained by the Gaussian distribution models of barrier heights suggested by some authors, Y.-L. Jiang et al. [Chin. Phys. Lett. 19, 553 (2002)]; Y.-L. Jiang et al. [J. Appl. Phys. 93, 866 (2003)], and S. Chand, J. Kumar [Appl. Phys. A 65, 497 (1997)]. It has been seen that the SBH inhomogeneity of our Ni/n-GaAs SBD can be well described by Gaussian distribution model suggested by Y.-L. Jiang et al. [Chin. Phys. Lett. 19, 553 (2002)]; Y.-L. Jiang et al. [J. Appl. Phys. 93, 866 (2003)] over whole measurement temperature range. Moreover, the modified ln($I_{0}/T ^{2}$) versus $1/k(T+T_0^\ast)$ plot is obtained using a method developed for T0 anomaly in the literature. Richardson constant value of 3.37 A cm-2 K-2 for n-type GaAs was obtained from the modified Richardson plot.

A simple Monte Carlo (MC) model is proposed to study the avalanche characteristics of heterojunction avalanche photodiode (HAPD). This model is capable to simulate the avalanche multiplication and excess noise factor in HAPDs by including the dead-space effect, hole to electron ionization ratio and heterointerface probability. The dead-space effect showed a vital role in reducing noise in single junction HAPDs based on the statistical determination in our model. It is shown that the dead-space effect reduces the avalanche noise in heterojunction device due to the localized ionization events. We found that the dead-space effect and the number of hole feedback impact ionizations are still the dominant effects to improve the excess noise factor especially in the injection layer of the device. In addition, the probability of electron and hole to cross the heterointerface will eliminate the secondary impact ionizations in the device.

($1-x$)Pb(Fe0.5Nb0.5)O3-xPb(Zr0.2Ti0.8)O3 (PFN-PZT, x = 0.75, 0.80, 0.85) ferroelectric ceramics were prepared by conventional solid-state reaction method via the wolframite precursor route. XRD measurements confirmed that the synthesized PFN-PZT ceramics are of pure tetragonal perovskite structure. With the increase of Pb(Zr0.2Ti0.8)O3 (PZT) content, tetragonality (defined as the ratio of cell parameter c/a) increases slightly accompanied by the variation of cell volume. At the optimized sintering condition of 1175 °C for 2 h, the 0.20PFN-0.80PZT ceramics exhibit the largest value of relative density (93.77%). The PFN-PZT ceramics exhibit first-order ferroelectric phase transition of typical normal ferroelectrics, where the dielectric response peaks are narrow, sharp and without frequency dispersion, and the dielectric constant above Curie temperature (TC) can be fitted well by Curie law. The sintered PFN-PZT ceramics exhibit high-TC, and with the increase of PZT content, TC increases and reaches 368, 394 and 401 °C, respectively. With the increase of PZT content, the P-E ferroelectric hysteresis loops of the PFN-PZT ceramics become narrower accompanied by the decrease of remanent polarization (Pr) and coercive field (EC). Piezoelectric constant d33 of all the PFN-PZT ceramics is small, less than 10 pC/N.

In this work, we present experimental evidence and analytical studies of dark current noise spectral density in CdTe detectors for a low and mid frequency range. The measured noise level in CdTe detectors indicates shot noise suppression at 10 KHz; this suppression can occur in CdTe detectors when the diffusion time is greater than the transit time between electrodes. Moreover we show thermal noise is negligible in CdTe resistors, taking into account their high resistance: R # 150 G${\rm \Omega} $.

Dy-doped CdO films were prepared and post-annealed in hydrogen atmosphere for different durations (15 min, 30 min and 45 min). They were characterised by measuring their structural, electrical, and optical properties. The results indicate that annealing in H2 gas does not change the crystalline CdO structure of the films. The electrical behaviours of Dy-doped CdO films show that they are degenerate semiconductors with bandgap 1.75 eV. The bandgap of the hydrogenated Dy-doped CdO films changes with H2-annealing time following the changing in the carrier concentration. These results were found to be in agreement with the available models for bandgap widening and bandgap narrowing. The Dy-doping of CdO films enhances their electrical conduction. An additional enhancement was obtained with pre-annealing in H2 gas so that the greatest enhancement occurs with annealing for 30 min. when the conductivity increased by about 40% and the free-electrons concentration increased by about 260% relative to the non-hydrogenated CdO:Dy film. From transparent-conducting-oxide point of view, Dy is sufficiently effective for CdO doping especially when including a pre-annealing in H2 atmosphere.    

The materials under consideration are binary aluminium-copper alloys (10 at% to 90.3 at%Cu) produced by HF melting and RF magnetron sputtering. The resulting micro structures have been observed by standard metallographic techniques, X-ray powder diffraction, scanning electron microscopy and transmission electron microscopy. Vickers microhardness of bulk Al–Cu alloys reaches a maximum of 1800 MPa at 70.16 at%Cu. An unexpected metastable $\theta '$ phase has been observed within aluminium grain in Al-37 at%Cu. The mechanical properties of a family of homogeneous Al$_{1-x}$Cux (0 < x < 0.92) thin films made by radiofrequency (13.56 MHz) cathodic magnetron sputtering from composite Al–Cu targets have been investigated. The as-deposited microstructures for all film compositions consisted of a mixture of the two expected face-centred-cubic (fcc) Al solid solution and tetragonal θ (Al2Cu) phases. The microhardness regularly increases and the grain size decreases both with copper concentration. This phenomenon of significant mechanical strengthening of aluminium by means of copper is essentially due to a combination between solid solution effects and grain size refinement. This paper reports some structural features of different Al–Cu alloys prepared by HF melting and RF magnetron on glass substrate sputtering.

ZnIn2Se4 has a polycrystalline structure in as synthesized condition. It transforms to nanostructure grains of ZnIn2Se4 upon thermal deposition as thin films. The optical properties of as deposited ZnIn2Se4 films are studied using spectrophotometer measurements of transmittance and reflectance at normal incidence of light in wavelength range (450–2500) nm. The refractive index and absorption index are calculated and it is found that they are independent of film thickness in the thickness range (500–790) nm. Absorption analysis showed two types of electronic transitions; indirect allowed transition with energy gap 1.76 eV accompanied by phonon of energy 25 meV and direct allowed transition with energy gap 2.3 eV. The dispersion analysis showed that the oscillator energy, dispersion energy, dielectric constant at infinite frequency and lattice dielectric constant are 2.49 eV, 14.36 eV, 6.84 and 8.17, respectively.

We investigate theoretically a mode of heating layers using a laser beam. In this mode thermal energy, the temperature of the layer, propagates in a steady-state self sustained fashion from the bottom of the layer towards the surface and may exhibit a very steep front. The propagation exhibits a constant speed, related to the intensity of the power flux. To achieve this heating mode the absorption coefficient has to remain low in low temperatures and increase rapidly as a function of temperature in higher temperatures. Additionally, a significant temperature increase must be generated to trigger this propagation mode, for example through the presence of a strongly absorbing layer beneath the transparent layer. The mode is well suited to semiconductors, especially silicon. A few examples of applications are proposed.

The preparation and properties of CdTe thin films is of a primary interest for the CdTe thin film solar cells in both research and technology. In our work, polycrystalline CdTe thin films were deposited on pretreated glass substrates in Ar/O2 atmosphere by closed-space sublimation (CSS) technology. Structural property was studied by X-ray diffraction (XRD), surface morphology was observed by scanning electron microscopy (SEM). The optical and electrical properties of CdTe films were investigated, as well as the effects of deposition temperatures, the ratio of gas (Ar/O2) and post-treatment on the properties. The high quality CdTe layer was prepared based on the above studies. These layers were used to prepared CdS/CdTe/ZnTe:Cu solar cells. Efficiency of 13.38% and fill factor of 70.3% (0.501 cm2 area) for CdTe solar cells have been achieved.

Strontium titanate (SrTiO3) or ST thin films have been prepared using strontium 2-ethylhexanoate [ Sr[ CH3(CH$_{2})_{3}$CH(C2H5)CO2] 2] and titanium(IV) isopropoxide Ti[ OCH(CH$_{3})_{2}$] 4 as precursors using a modified sol-gel processing technique. The precursor [ Sr[ CH3(CH$_{2})_{3}$CH(C2H5)CO2] 2] was synthesised in the laboratory. Transparent and crack-free films were fabricated on pre-cleaned quartz substrates by spin coating. The structural and optical properties of films annealed at different temperatures have been investigated. The as-fired film was found to be amorphous while crystalline, cubical phase structure film was obtained after annealing at 550 °C for one hour in air. The films are polycrystalline in structure with the lattice constants of a = 3.90 Å. The grain sizes of the films annealed at 450, 500 and 550 °C were found to be 26.4, 31.8 and 35.2 nm, respectively. The optical constants of the films (refractive index, extinction coefficient, optical conductivity, complex dielectric constant, and energy band gap) were determined from the transmittance spectra. The refractive indexes of the films were found to increase while the optical band gaps were found to decrease with annealing temperature. The optical dispersion data are also analysed in the light of single oscillator model and are discussed.

The aim of this work is to compare several methods for the determination of very thin films Young's modulus and stress state: the nanoindentation test, the bulge test and the point-deflection method. The tested structures were silicon nitride and silicon nitride/silicon oxide bilayer membranes with different shapes (square or rectangular) and dimensions (from 1 mm to 3 mm). We report new experimental results on submicron thick dielectric membranes with thicknesses down to 100 nm. A Young's modulus of 217 ± 14 GPa have been found for silicon nitride membranes with a residual stress of 411 ± 30 MPa using the bulge test. Using nanoindentation experiments, a Young's modulus higher than 190 GPa has been estimated. The bulge test is still valid for the studied high dimension to thickness ratio membranes and more appropriate to determine the Young's modulus. A mixture law was shown to be possibly applied for Si3N4/SiO2 bilayer membranes for the Young's modulus and stress determination. The point deflection method is limited by the very low stiffness of these structures and only the residual stress can be accurately extracted. As the Young's modulus and membrane geometry have no significant influence on the stress determination by means of the point deflection method for the studied membranes (with a high lateral dimension to thickness ratio), more reliable results have been obtained such as 487 ± 40 MPa using an AFM cantilever for load-deflection experiments, for Si3N4 thin films.

We report the growth of indium-rich InGaN alloys by metal-organic vapour phase epitaxy, using ammonia, trimethylgallium and trimethylindium as precursors. Compared to indium nitride, our alloy samples present substantial photoluminescence robustness with temperature. The analysis of the optical properties of these samples versus temperature indicates the existence of two non radiative recombination channels: one with a thermal activation temperature of 77 K and another one with an activation temperature ranging from 300 K for InN up to 640 K for In0.72Ga0.28N. The latter competes with the former at low temperatures whilst the former rules the optical properties at ambient conditions.

Spectroscopic ellipsometry in the mid infrared spectral range, Raman scattering and TEM measurements on (100) oriented p+ and n+-type porous silicon (PS) samples were carried out. Porosities of 68% and 48% for p+ and n+ wafers, respectively, and thicknesses of 27.6 $\mu $m and 14 $\mu $m with the same extinction coefficient k = 0.1 were determined from spectroscopic ellipsometry. Raman scattering measurements show that the resultant surface morphology of the PS layers consists of irregular and randomly distributed nanocrystalline Si structures. Using the phonon confinement model, the diameters of Si nanocrystallites have been estimated as 8 and 3 nm for p+ PS type and 12 and 5 nm for n+ PS type. Transmission electron microscopy shows clearly defined pores with sizes ranging from 15 to 35 nm, inhomogeneously distributed along the PS surface. We demonstrate that the filling of the PS pores by organic material (Rhodamine 6G) brings about important enhancement on photoluminescence intensity.

The present paper focuses on magnetoelectric (ME) interactions under ferromagnetic resonance (FMR) in a bilayer of a ferrite and a piezoelectric. Applying a dc electric field perpendicular to the sample plane induces a uniaxial magnetic anisotropy. The effects of flexural deformation and substrate clamping have been considered in determining the electric field induced FMR line shift. The obtained model is applied to a specific case of yttrium iron garnet/lead magnesium niobate-lead titanate. Our studies indicate that flexural deformation gives rise to a decrease in the strength of ME interaction for a free standing bilayer. The substrate thickness dependence of FMR line shift reveals a maximum for the substrate thickness that is approximately equal to that of the bilayer.

In this paper we report the nano-phase separation structure in the polymer blend film and its optical properties. Polystyrene (PS)/polymethyl methacrylate (PMMA) blend film is spin-coated on substrate. A sandwiched structure consisting of a depleted PMMA layer, a PS/PMMA blend layer and a PS-rich top layer has been formed. By selectively dissolving the PS-rich phase, a nanoporous film is generated, and the nanoporous structure can be tuned by changing the weight ratio of PS/PMMA in the blend solution in fabrication. The optical properties of the nanoporous thin films are determined. Our results show that by introducing the nanoporous structure, the refractive index can be effectively modified. By selecting proper film thickness, the maximum optical transmission can be achieved in the specific waveband.

A simple chemical route has been deployed to synthesis highly monodispersed and controllable ZnS and CdS nanoparticles (quantum dots) doped with Mn. XRD investigations shows the cubic structure of both ZnS and CdS nanoparticles and doping has no effect on the structure. The size measurement gives an approximate size of 1.5 nm and 2.5 nm for ZnS:Mn and CdS:Mn quantum dots respectively by Deby-Shreer. UV-Visible is used to study the Optical absorption of ZnS and CdS pure as well as doped nanoparticles. Furthermore as the chemical parameters change, the size of quantum dots change systemically. Of course although doping percentage is a significant parameter in photoluminescence properties of quantum dots, but it has no effect on the size. Photoluminescence spectra obtained at 6% molar of the Mn dopant in CdS:Mn (Ex = 315, Em = 495 nm) and ZnS:Mn (Ex = 285, Em = 572 nm). Microscopic images (SEM, TEM) show the 1–3 nm size for ZnS and CdS nanoparticles.

The ΔE effect is often presented as the dependency of the Young's modulus of a material on its state of magnetization. Nevertheless, the elastic properties of a magnetic material do not depend on the magnetization state. Actually, the sensitivity of the magnetostriction strain to the application of a stress explains the ΔE effect. According to this statement, a semi-analytical model for the ΔE effect is proposed, in which magnetization rotation is not considered. An experimental procedure to measure the ΔE effect in magnetic materials is then built-up. Experimental and modeling results are finally compared, with satisfying agreement.