Reduction Of Defect Concentration Research Articles

Effect of ZnCl2 on structural and optical features of TeO2–B2O3–Bi2O3–ZnCl2 glasses

Tellurium dioxide-based quaternary glass system 60TeO2–15B2O3-(25-x) Bi2O3-xZnCl2 (0 ≤ x ≤ 20, in the step of 5 mol%) was synthesized by melt quenching technique. The XRD analysis was utilized to confirm that the glass samples were amorphous. The resultant values of molar volume, density, oxygen packing density, and glass transition temperature were shown to decrease with rise in ZnCl2 content. By employing vibrational spectroscopy, TeO2 endures as structural units trigonal pyramidal TeO3, TeO3+1, trigonal bipyramidal TeO4, together with TeO6 polyhedral. The optical characterization of the current glass system shows the presence of indirect transitions with optical band gap range from 2.16 eV to 3.02 eV. Along with the rise in ZnCl2 content, the Urbach energy value shows a tendency towards lowering values (0.436 eV–0.203 eV), which denotes a reduction in defect concentration. Metallization criterion values fall between 0.329 and 0.389 indicates that the current glass system can be utilizes for non-linear applications.

Nickel substituted oxygen deficient nanoporous lithium ferrite based green energy device hydroelectric cell

In green energy generation, recently Hydroelectric cell (HEC) by dissociating the water molecules at room temperature has taken a big stride among other alternative green energy sources. In this work, another unique novel material Ni substituted lithium ferrite (LNFO) for the fabrication of hydroelectric cell to generate green electricity has been reported. Oxygen deficient nanoporous LNFO has been synthesised by the Solid-State reaction method. Special processing steps were taken to control oxygen defect’s concentration in the ferrite by varying pre-sintering temperature during its synthesis to deliver more power output. Phase formation of nickel substituted lithium ferrite has been confirmed by X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). The porous microstructure of LNFO has been analysed by Field-emission scanning electron microscope (FESEM) micrographs, BET and DFT techniques. The presence of defects and decrease in their concentration with the increase in pre-sintering temperature has been confirmed by analysing X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL) measurements. Hydroelectric cell fabricated using LNFO pellets pre-sintered at 750 °C and 800 °C, delivered output current densities of 3.8 mA/cm2 and 3.6 mA/cm2 respectively. The decrease in output current is attributed to reduction in defect concentration as confirmed by PL and XPS spectrum. Generated current densities are two times higher than reported in lithium substituted magnesium ferrite based hydroelectric cell (1.7 mA/cm2).

Physical, structural and optical characterizations of borate modified bismuth–silicate–tellurite glasses

Quaternary bismuthate glasses with compositions xB2O3–(80−x) Bi2O3–15SiO2–5TeO2 have been prepared by melt-quench technique. X-ray diffraction studies were performed to ascertain the amorphous nature of samples. The density, molar volume and crystalline volume decrease with increase in B2O3 content whereas the glass transition temperature shows the reverse trend. The Raman and FTIR spectra of the studied glasses indicate that B2O3 has been found to exist in the form of BO3 trigonal and BO4 tetrahedral structural units and vibrations corresponding to these structural units increase with increase in B2O3 content. SiO2 is present in the form of SiO4 tetrahedral structural units and TeO2 in the form of TeO3 structural units. Bismuth plays the role of network modifier [BiO6 octahedra] as well as network former [BiO3 pyramids] for all the glass compositions. The optical band gap energy has been calculated from the fitting of both Mott and Davis’s model and Hydrogenic excitonic model with the experimentally observed absorption spectra. A good fitting of experimental data with HEM indicates the excitonic formation in the studies glass system. The values of optical band gap energy show nonlinear behavior due to the structural changes that take place in the present glass samples. The Urbach energy calculated using Urbach empirical formula for studied glass samples suggest the possibility of reduction in defect concentrations. The metallization criterion of the presently studied samples suggests that the prepared glasses may be potential candidates for nonlinear optical applications.

Effect of EBPVD coated nanoceria thickness on the isothermal oxidation of AISI 304 stainless steel

Stainless steels such as AISI 304 are widely used as structural material but are prone to degradation at high operating temperature due to thermal coefficient mismatch and thermal stress between the oxide scale and alloy. Coating of rare earth oxides such as cerium oxide on AISI 304 prevents the high temperature degradation by the formation of protective oxide layer on the surface. In the present work we report the effect of cerium oxide with thickness varied from 10 to 2200nm on the high temperature oxidation protection. Cerium oxide nanoparticles synthesized by precipitation method and sintered at 1473K was used as a target in electron beam physical vapour deposition. Structural and surface properties of the coatings were analysed prior and post-oxidation with respect to thickness using X-ray diffraction, Raman spectroscopy and scanning electron microscopy. Independent of coating thickness, the resultant films had a crystallite size of less than 8nm which increased to about 40nm upon oxidation at 1243K. The increase in cerium oxide coating thickness resulted in a decrease in oxidation rate constant by 3–4 orders. Though bare AISI 304 exhibited severe oxidation, the presence of cerium oxide with 10nm thickness reduced the spallation on exposure to high temperature. Oxygen vacancy concentration calculated from Raman spectroscopic analysis showed one order reduction in defect concentration upon oxidation but the changes were found to be minimal with the increase in coating thickness. Surface studies by scanning electron microscopy showed a flake like morphology for the bare AISI 304 on oxidation. But the size of the flakes reduced in the presence of 10nm thick coating which indicates the beneficial effect of cerium oxide even at lower thickness towards the protection against high temperature oxidation.

The improvement of Si0.5Ge0.5/Si interface quality by using low energy hydrogen plasma cleaning process and positron annihilation spectroscopy

Positron Annihilation Spectra (PAS), Raman, and photoluminescence spectroscopy reveal that Si 0.5 Ge 0.5 /Si interface quality can be dramatically improved through a low energy plasma cleaning process using hydrogen. In the PAS, the particularly small values of lifetime and intensity near the Si 0.5 Ge 0.5 /Si interface in the treated sample indicate a 2.25 times reduction in defect concentration. Fewer defects were found in the interface, resulting in a high compressive strain of about 0.36 % in the top layer, which can be observed in Raman spectra, and a 1.39 times increase to the radiative recombination rate for the infrared emission, which can be observed in the photoluminescence spectra. Improved Si 0.5 Ge 0.5 /Si interface quality leads to improved optical and electrical characteristics in SiGe-based devices in a broader range of photovoltaic applications. The PAS is also shown to be a useful metrology tool for quantifying defects in SiGe-based devices.

VUV Spectroscopy of Ca3Sc2Si3O12:Pr3+: Scintillator Optimization by Co-Doping with Mg2+

Single phase Ca3Sc2Si3O12 co-doped with Pr3+ and Mg2+ was synthesized by high temperature solid state reaction. The optical spectroscopy of this composition was investigated using time-resolved VUV spectroscopy upon excitation with synchrotron radiation. It was demonstrated that the addition of Mg2+ leads to a significant modification of the luminescence properties in terms of emission intensity, dynamics and reduction of defect concentration in comparison to Mg-free Ca3Sc2Si3O12:Pr3+. The peculiarities of the host-to-impurity energy transfer are analyzed and discussed.

Properties investigation of GaN films implanted by Sm ions under different implantation and annealing conditions

GaN films grown by MOCVD were implanted by Sm ions under different implantation and annealing conditions, in order to optimize the implantation parameters. The structural and magnetic measurements indicated a reduction of defect concentration and an increase of saturation magnetization when samples were implanted at 400°C, most probably due to the increased substitutional fraction of Sm ions. While the subsequent annealing process further decreased the damage in GaN lattice, but reduced the saturation magnetization on the contrary, caused by the decomposition of the surface layer and the formation of Sm-defect complexes during high-temperature annealing.

Synthesis, characterization and optical properties of Zn 1− xTi xO nanoparticles prepared via a high-energy ball milling technique

Zn 1− x Ti x O ( x = 0, 0.01, 0.03 and 0.05) nanoparticles were prepared by high-energy ball milling at 400 rpm. The milled powders were characterized by X-ray diffractometer (XRD) and the results exhibited that Ti-doped ZnO nanoparticles consisted of single phase with hexagonal structure when the mixtures of ZnO and TiO 2 powders were milled for 20 h. The crystallite size reduced as a function of the doping content and milling time from 1 to 10 h then increased after milling for 20 h and when the annealing temperature increased. The strain changed inversely to the crystallite size. A wider band-gap was obtained by increasing the doping content and annealing temperature because of a reduction in defect concentration. Both ZnO- and Ti-doped ZnO nanoparticles caused damage to S. aureus, E. coli, P. mirabilis, S. typhi and P. aeruginosa.

Defect-induced trap-assisted tunneling current in GaInNAs grown on GaAs substrate

We present the reverse-bias current-voltage and deep-level transient spectroscopy (DLTS) characteristics of a Ga0.90In0.10N0.033As0.967∕GaAs positive-intrinsic-negative photodiode (Eg=0.92 eV) and a trap-assisted tunneling model which considers generation-recombination and tunneling mechanisms. Using trap parameters obtained from the DLTS measurement, the model generates current-voltage characteristics of the photodiode, which were found to be in good agreement with experimental current-voltage curves at different temperature. The model also suggests that high dark current at low reverse-bias voltage is caused by the presence of traps which have low activation energy. Furthermore, it is predicted that approximately ten times reduction in the dark current can be achieved when the trap concentration of type H-1 (Ea=0.15 eV) is reduced by one order. On the other hand, a similar reduction in defect concentration of type H-2 (Ea=0.40 eV), which is nearer to midgap does not produce the same effect.

In-situ IR measurements of OH species in quartz at high temperatures

The nature of OH species in natural clear quartz was investigated by means of in-situ IR measurements over the temperature range –185 to 1000 °C. Reversible thermal behavior of OH species was examined for a sample pre-heated to 1000 °C for 1 hour. At room temperature, the IR spectrum of the quartz sample examined includes an intense absorption peak at 3379 cm–1 which has been assigned to an OH stretching vibration associated with Al substituting for Si (OH(Al)). The major spectral changes of the OH(Al) bond involve a systematic shift of its peak position and a decrease in its integral absorbance with temperature. A quasi-linear increase of the peak position from –185 to 400 °C is interpreted to be due to the change in the vibrational frequency of OH(Al) with hydrogen bond (H bond) distance. At higher temperatures, the IR frequency shows only a slight change, indicating a small influence of the H bond. On the other hand, the gradual decrease of the integral absorbance of OH(Al) with temperature indicates a decrease of this defect’s molar absorptivity without any reduction in defect concentration. This is interpreted to result from a decrease in dipole moment of OH(Al) with temperature. A sudden shift of the vibrational frequency from 3396 to 3386 cm–1 between 550 and 560 °C and a constant value of the integral absorbance from 535 to 570 °C were considered to be related to the change in H bond distance during the structural transformation of α-quartz to its β-form. The local environment of OH(Al) begins to change at temperatures below 570 °C, where the crystallographic α–β transition occurs.

Vacancy-type defects in crystalline and amorphous SiO2

Positron lifetime spectroscopy and two-dimensional angular correlation of annihilation radiation have been used to investigate grown-in vacancy structures in synthetic crystalline α-SiO2, synthetic fused quartz, and in a 60-μm-thick chemical-vapor-deposited amorphous SiO2 film. For α-SiO2 a ∼300 ps lifetime component suggests trapping by either silicon monovacancies or by oxygen divacancies (or both). The vacancies are neutral and present at a concentration level of 1017/cm3. The positron bulk lifetime for α-SiO2 is estimated to be ∼238 ps in good agreement with semiempirical predictions. In the fused quartz significant positronium formation is found (80%) and the remaining positrons annihilate in voids yielding a lifetime of ∼500 ps. The amorphous SiO2 film contains a mixture of small vacancy clusters and voids and ∼30% of the positrons form positronium. Heat treatment above 950 °C results in a substantial reduction in defect concentration, but up to 1100 °C a small vacancy cluster contribution persists. The positron data indicate that positronium formation in the fused quartz and in the amorphous film takes place in the voids.