Deep centers introduced by argon ion bombardment in n-type silicon

Platinum-silicon Schottky diodes on epitaxial layers have been subjected to argon ion bombardment, prior to metal evaporation, with doses in the 10¹⁴ ions/cm² range and with energies between 1 and 3.5 KeV. By transient capacitance technique four electron traps have been found. The oxygen-vacancy pair (O-V) located at 0.17 eV below the conduction band, the double negatively charged divacancy (V-V⁼) at 0.23 eV and the phosphorous-vacancy pair (P-V) at 0.44 eV are observed. One more unknown level at 0.36 eV which could be oxygen-related is also found.     

Argon-ion bombardment effects on the electrical characteristics of platinum-silicon Schottky diodes

Before barrier metal evaporation, silicon Schottky diodes were cleaned by argon-ion bombardment. Some device series were evaporated just after the ion cleaning, whereas others were annealed beforehand. The electrical characteristics of the different series were checked by means of standard I/V and C/V measurements. Whereas the ideality coefficient and the barrier height obtained from I/V characteristics showed nearly complete recovery after heat treatment for 1 h at 700°C, the barrier height from C/V measurements did not recover.     

Microindentation technique as a tool for investigating the development of order in PET under uniaxial stress

The development of microhardness during annealing was used as a tool to follow the development of the liquid crystalline transient mesophase during the crystallization of uniaxially oriented polyethylene terephthalate (PET). 2D X-ray diffraction was used to characterize the different stages of the crystallization process. We were able to separate those stages by quenching into air. Microindentation hardness experiments were done in real time where samples were heated with their ends fixed on a specially developed stage and microhardness was measured simultaneously. The oriented samples examined exhibit a clear difference in behavior from isotropic samples that mainly lack the existence of such an ordered mesophase. The mesophase clearly has a reinforcement effect on the whole polymer matrix that leads to an increase in hardness value with annealing of the oriented PET films. Microindentation hardness is shown to be a versatile tool to detect the existence of the liquid crystalline transient mesophase. It is also efficient in comparing and explaining results obtained by wide angle X-ray scattering.     

Mesostructured SiO2-doped TiO2 with enhanced thermal stability prepared by a soft-templating sol–gel route

Mesostructured SiO₂–TiO₂ mixed oxides have been prepared by a soft-templating sol–gel route, using a non-ionic triblock copolymer as structure-directing agent. Tetraethylorthosilicate (TEOS) and titanium tetraisopropoxide (TTIP) have been employed as Si and Ti sources, respectively. Using a prehydrolysis TEOS step allows mixed oxides to be produced with a homogeneous porosity and with no phase segregation, in a wide range of Si/Ti compositions. Both the hydrolysis molar ratio and the silicon content have been found to be important factors determining the final properties of these materials. For instance, mixed oxides containing low silicon concentrations exhibit N₂ physisorption isotherms typical of mesoporous materials, although with an important contribution of microporosity. On the other hand, increasing the hydrolysis molar ratio makes more difficult to reach a total dispersion of SiO₂ through the TiO₂ matrix. Even with low SiO₂ loadings, the thermal stability is effectively enhanced, when compared to the equivalent pure TiO₂ materials, as a consequence of a delay in the titania crystallization to anatase. Thus, after calcination at 300 °C for 3 h, mixed oxides containing low Si/Ti ratios (20/80) show BET surface area in the range 290–346 m²/g, while pure TiO₂ materials largely collapse under the same treatment and their BET surface area drop strongly to values around 125 m²/g. This synthesis route, therefore, provides mesoporous TiO₂-rich materials with enhanced stability and textural properties, which is of high interest for applications as catalysts and supports.     

The role of the compatibilizer on the microindentation hardness of iPP/PC blends

Blends of isotactic polypropylene (iPP) and polycarbonate (PC) with and without a compatibilizer were prepared using a Brabender Haake Rheocord at 260°C and 32 rpm. Maleic anhydride grafted styrene‐ethylene/butylene‐styrene (SEBS‐g‐MAH) and maleic anhydride grafted ethylene–propylene diene (EPDM‐g‐MAH) were chosen as compatibilizers and their proportion was set to 5, 10, and 15 wt%, respectively. The thermal properties and crystallization behavior were determined by differential scanning calorimetry (DSC) and wide angle X‐ray scattering (WAXS). Micromechanical properties were also investigated using a Vickers microindentation tester. The DSC analysis indicates that the melting temperature of iPP in the all the blends, compatibilized and uncompatibilized ones, remains constant and is almost the same as those of the pure component. On the other hand, it is shown that the degree of crystallinity of iPP in the blends calculated by DSC and WAXS is dependent of the composition of the polymeric mixture. However the hardness (H) decreases with increasing PC content until the composition of iPP/PC (75/25) is reached, whereas for larger PC content values, H increases. The same trend was obtained with the addition of both compatibilizers. POLYM. ENG. SCI., 56:1138–1145, 2016. © 2016 Society of Plastics Engineers     

Challenges for nanomechanical sensors in biological detection

Nanomechanical biosensing relies on changes in the movement and deformation of micro- and nanoscale objects when they interact with biomolecules and other biological targets. This field of research has provided ever-increasing records in the sensitivity of label-free detection but it has not yet been established as a practical alternative for biological detection. We analyze here the latest advancements in the field, along with the challenges remaining for nanomechanical biosensors to become a commonly used tool in biology and biochemistry laboratories.     

On the dynamic mechanical behavior of poly(methyl methacrylate) reinforced with Kevlar fibers

The dynamic mechanical relaxation behavior of poly(methyl methacrylate) reinforced by continuous parallel Kevlar-49 fibers is investigated here on samples with several fiber volume fractions. The shifts in the temperature of the main relaxation of the matrix are interpreted according to free volume considerations and with the help of a thermomechanical block model. The dependence of the storage and loss moduli both on temperature and fiber content found experimentally can be reproduced by the model. It is not necessary to rely on the existence of an interphase to account for the modifications evidenced by the spectrum of the matrix, which can be explained on the basis of the two phase model.     

Room temperature performance of low threshold 1.34-1.44-/spl mu/m GaInNAs-GaAs quantum-well lasers grown by molecular beam epitaxy

Room temperature lasing emission at 1.338 and 1.435 /spl mu/m with threshold current densities of 1518 and 1755 A/cm/sup 2/, respectively, is obtained in broad area GaInNAs-GaAs laser diodes (LDs) grown by molecular beam epitaxy. The 1.338-/spl mu/m LDs show a power output per facet up to 0.20 W/A, a characteristic temperature (T/sub 0/) of 78 K, and an external transparency current density (J/sub tr/) of 0.77 kA/cm/sup 2/. Increasing the lasing wavelength to 1.435 /spl mu/m results in a larger J/sub tr/ of 1.16 kA/cm/sup 2/ and a lower T/sub 0/ of 62 K, due to larger nonradiative recombination. However, the 1.435-/spl mu/m LDs still display a power output per facet up to 0.15 W/A, and a high internal quantum efficiency of 52%. These improved performances are achieved without the need to use strain compensation layers, Sb as a surfactant during the quantum-well growth, or a postgrowth thermal anneal cycle.     

Optical properties of non-stoichiometric SiO2 as a function of excess silicon content and thermal treatments

The infrared (IR) absorption spectra and the behavior of the refraction index of a two-phase non-stoichiometric SiO₂ film with excess Si have been studied as a function of the excess of Si and post-deposition thermal treatment. The oxides were deposited by low-pressure chemical vapor deposition using SiH₄ and N₂O as reactant gases at a substrate temperature in the range of 650 to 750 °C. Some of the films were given a final annealing treatment at temperatures ranging from 700 to 1100 °C in N₂ for 30 min. Both annealed and as-deposited oxides have IR absorption peaks associated with the bending, rocking and stretching modes of the Si-O-Si bonds in SiO₂, although the exact location of these peaks is different for different contents of excess silicon and it also depend on the post-deposition thermal treatment given to the oxides. Unannealed samples present a shift of the stretching peak towards low wavenumbers as the excess of Si is increased. The samples annealed at 1000 °C on the other hand do not present this shift. Unannealed samples with large content of Si also present an absorption peak at 890 cm⁻¹ that could be associated with partially oxidized Si. It is suggested that at least part of the excess Si in the as-deposited samples is present in the form of an SiO_x phase while in the annealed samples a clear separation occurs between a Si and a SiO₂ phase. The behavior of the refraction index is similar for both types of sample, increasing as the excess silicon is increased.     

X-ray diffraction study of poly-p-phenylene doped with SbF5 and SbCl5

X-ray diffraction experiments have been performed on SbF₅ and SbCl₅ doped PPP prepared by the Kovacic method. Results are discussed in terms of two structural models which account for the regular intercalation of dopants within the polymer crystal lattice.