Second-harmonic generation in the thermal/electrical poling (100−x)GeS2·x(0.5Ga2S3·0.5CdS) chalcogenide glasses

Utilizing Maker fringe (MF) method, second-harmonic generation (SHG) has been observed within the GeS₂-Ga₂S₃-CdS pseudo-ternary glasses through thermal/electrical poling technique. The SHG phenomenon was considered to be the result of breakage of the glassy macroscopic isotropy originated from the reorientations of dipoles during the thermal/electrical poling process. Under the same poling condition conducted with 5 kV and 280 °C for 30 min, the maximum value of second-order nonlinear susceptibility χ⁽²⁾ of the poled (100−x)GeS₂·x(0.5Ga₂S₃·0.5CdS) glasses was obtained to be ≈4.36 pm/V when the value of x is equal to 30. Nonlinear dependence of χ⁽²⁾ on compositions of these glasses can be well explained according to the theory related to the reorientation of dipoles.     

Influence of hydroxyl content on second-order nonlinearity in thermally poled type-III fused silica

Effects of high (∼1000 ppm) and low (∼200 ppm) hydroxyl (OH) content on second-order optical nonlinearity (χ⁽²⁾) in thermally poled type-III fused silica (KU-1) are studied. Type-III fused silica with low OH content poled under 275 °C with 6 kV for various poling durations shows much stronger intensity of second-harmonic generation (SHG) signals compared with that with high OH content poled under the same condition. A near-anodic uniform surface and uniform bulk χ⁽²⁾ profiles which are opposite in sign are calculated to be formed in the fused silica poled longer than the optimum poling duration. A two-charge-carrier model with charge injection and non-blocking electrodes is given to explain the creation of the χ⁽²⁾ profile. More OH is believed to decrease the mobility of alkali ions during poling.     

Dynamics of CO2 molecules confined in the micropores of solids as studied by C NMR

The pressure and temperature dependence of ¹³C NMR of CO₂ adsorbed in several porous materials was measured. For CO₂ in activated carbon fiber (ACF), the spectrum observed in the pressure range from 0 to 10 MPa consisted of two lines. A very sharp peak at δ = 126 ppm was attributed to free CO₂ gas and a broad peak at δ = 123 ppm was attributed to confined CO₂ molecules in the micropores of ACF, although CO₂ in microporous materials such as zeolites and mesoporous silica, gave only a single peak attributed to free CO₂ gas. In the low-pressure region, the peak at δ = 123 ppm shifted to 118 ppm and a very broad peak with a line width of about 200 ppm appeared. This indicates that there are two kinds of CO₂ molecules confined in ACF with different rates of molecular motion: one is undergoing isotropic rotation and the other is undergoing anisotropic motion, which rotates around an axis tilted by 30° from the molecular axis. This implies that small pockets with a characteristic diameter exist on the surface of the ACF micropore.     

Enhancement of second harmonic intensity in thermally poled ferroelectric nanocrystallized glasses in the BaO-TiO2-SiO2 system

The transparent nanocrystallized (heat-treatment: 750 ^°C, 1 h) glasses consisting of ferroelectric Ba₂TiSi₂O₈ nanocrystals (size: 100-200 nm) have been prepared in 40BaO-20TiO₂-40SiO₂ glass, and the effect of a thermal poling (DC electric voltage: 8.8 kV/cm, temperature: 110-300 ^°C, time: 1 h) on the second harmonic (SH) intensity has been examined. It is found that the formation behavior of nanocrystals at the surface differs from that in the interior of glass, giving a new insight into the well-known concept for nanocrystallization or homogeneous nucleation in glass. The Maker fringe patterns with fine structures are observed, indicating a high orientation of the polarization axes of Ba₂TiSi₂O₈ crystals formed at the surface. The prominent enhancement in the SH intensity is observed due to thermal poling, demonstrating that thermal poling is an effective method in enhancing anisotropic polarization of ferroelectric Ba₂TiSi₂O₈ nanocrystals in crystallized glasses. The present study proposes that the Maker fringe pattern for SH intensity of nanocrystallized glasses is very sensitive to the anisotropic polarization of nanocrystals at the surface, indicating the importance of the Maker fringe technique for the characterization of nanocrystals in materials.     

Room temperature fabrication Oxide TFT with Y2O3 as a gate oxide and Mo contact

In this investigation, an operating voltage as low as 5 V has been achieved for Oxide TFT with Y₂O₃ as a gate oxide and a-IGZO as an active layer. The OTFT has been fabricated at room temperature using RF sputter. The mobility and threshold voltages are 11.3 cm²/V s and 3.4 V for the device with W/L = 0.8, respectively. The annealing at 400 °C in N₂ containing 5% H₂ ambient has been utilized to improve the electrical performance of TFT. The on-off current which is determined by gate dielectric has been observed to be 10⁴. It has also been observed that the dielectric properties of gate oxide deteriorate on annealing. The dielectric constant of Y₂O₃ is observed in the range between 5.1 and 5.4 measured on various devices.     

Side-hole to anti-hole conversion in time-resolved spectral hole burning of ruby: Long-lived spectral holes due to ground state level population storage

We report time-resolved transient spectral hole burning of Verneuil-grown 20 ppm and ca. 0.6 ppm ruby (Al₂O₃:Cr³⁺) in zero field and low magnetic fields B∥c at 4 K. The hole-burning spectroscopy of the 20 ppm sample implies relatively rapid cross relaxation in the ⁴A₂ ground state on the ∼1 ms timescale both in zero field and in low magnetic fields, B∥c, up to 0.2 T. In the 0.6 ppm sample, side-hole to anti-hole conversion is observed both in zero field and in low magnetic fields. This conversion is caused by population storage in ⁴A₂ ground state levels. Spin-lattice relaxation, on the 200 ms timescale, is directly observed from the time dependence of the resonant hole and anti holes in B∥c, consistent with a very low cross-relaxation rate. However, in zero field cross relaxation in the ⁴A₂ ground state is still a significant relaxation mechanism for the 0.6 ppm sample resulting in hole decay in ∼50 ms.     

The barrier height inhomogeneity in Al/p-Si Schottky barrier diodes with native insulator layer

The current-voltage (I-V) characteristics of Al/p-Si Schottky barrier diodes (SBDs) with native insulator layer were measured in the temperature range of 150-375 K. The estimated zero-bias barrier height Φ_B0 and the ideality factor n assuming thermionic emission (TE) theory show strong temperature dependence. Evaluation of the forward I-V data reveals an increase of zero-bias barrier height Φ_B0 but decrease of ideality factor n with increase in temperature. The conventional Richardson plot exhibits non-linearity below 250 K with the linear portion corresponding to activation energy of 0.41 eV and Richardson constant (A^*) value of 1.3 × 10⁻⁴ A cm⁻² K⁻² is determined from intercept at the ordinate of this experimental plot, which is much lower than the known value of 32 A cm² K² for holes in p-type Si. Such behavior is attributed to Schottky barrier inhomogene ties by assuming a Gaussian distribution of barrier heights (BHs) due to barrier height inhomogeneities that prevail at interface. Also, Φ_B0 versus q/2kT plot was drawn to obtain evidence of a Gaussian distribution of the BHs, and values of Φ_B0 = 1.055 eV and σ₀ = 0.13 V for the mean BH and zero-bias standard deviation have been obtained from this plot, respectively. Thus, the modified versus q/kT plot gives Φ_B0 and A^* as 1.050 eV and 40.08 A cm⁻² K⁻², respectively, without using the temperature coefficient of the barrier height. This value of the Richardson constant 40.03 A cm⁻² K⁻² is very close to the theoretical value of 32 A K⁻² cm⁻² for p-type Si. Hence, it has been concluded that the temperature dependence of the forward I-V characteristics of the Al/p-Si Schottky barrier diodes with native insulator layer can be successfully explained on the basis of TE mechanism with a Gaussian distribution of the barrier heights.     

The influence of pre-adsorbed water on adsorption of methane on fumed and nanoporous silicas

Co-adsorption of water and methane onto fumed (A-300, A-380) and micro/mesoporous (Gasil 200DF) silicas was studied. FTIR and ¹H NMR spectroscopy with layer-by-layer freezing-out of bound water were used at different levels of hydration (h = 0.005-1.0 g of water per gram of silica). Methane adsorption was largest (1-2 wt% at T < 280 K) for nanosilica A-300 (S_BET = 337 m²/g) at hydration h = 0.1 g of water per gram of silica for a non-equilibrated system. This sample was characterised by a large amount of weakly associated water (δ_H ≈ 1 ppm), and maximal clustering of all bound water. These conditions provide the increased microporosity necessary for enhanced methane adsorption. Heating and subsequent wetting, or long equilibration of nanosilica, decreased the adsorption of methane. The adsorption of methane on silica 200DF decreased with increasing amounts of pre-adsorbed water, characterised by significant associativity (δ_H ≈ 5 ppm) at h ≥ 0.005 g/g.     

Residual surface stress measurements in YBa2Cu3Ox superconductors

XRD and residual surface stress (sin² ψ) measurements were carried out on YBa₂Cu₃O_x superconductors with varying oxygen stoichiometry (6.3 < x < 7.0). Slopes of the surface strain versus sin² ψ were plotted against oxygen content for certain reflections. Compressional surface stress has been found along the c-axis, while a tensile surface stress has been observed along the ab-plane. Both surface stresses were found to vary slightly with oxygen content. These findings qualitatively agree with a very small hydrostatic pressure effect on T_c for fully oxygenated YBa₂Cu₃O_x (x = 7) compared to oxygen deficient material at the surface.     

Surface nano-structural modifications and characteristics in nitrogen ion implanted W as a function of temperature and N energy

The surface modifications of tungsten massive samples (0.5 mm foils) made by nitrogen ion implantation are studied by SEM, XRD, AFM, and SIMS. Nitrogen ions in the energy range of 16-30 keV with a fluence of 1 × 10¹⁸ N⁺ cm⁻² were implanted in tungsten samples for 1600 s at different temperatures. XRD patterns clearly showed WN₂ (0 1 8) (rhombohedral) very close to W (2 0 0) line. Crystallite sizes (coherently diffracting domains) obtained from WN₂ (0 1 8) line, showed an increase with substrate temperature. AFM images showed the formation of grains on W samples, which grew in size with temperature. Similar morphological changes to that has been observed for thin films by increasing substrate temperature (i.e., structure zone model (SZM)), is obtained. The surface roughness variation with temperature generally showed a decrease with increasing temperature. The density of implanted nitrogen ions and the depth of nitrogen ion implantation in W studied by SIMS showed a minimum for N⁺ density as well as a minimum for penetration depth of N⁺ ions in W at certain temperatures, which are both consistent with XRD results (i.e., I_{W (2 0 0)}/I_{W (2 1 1)}) for W (bcc). Hence, showing a correlation between XRD and SIMS results.     

Tolerance and tuning properties of the optical parametric processes using periodically poled RbTiOAsO4

The maximal tolerance parameters of poling period and phase-matching, temperature in second harmonic generation (SHG) using periodically poled RbTiOAsO₄(PPRTA) as a function of the fundamental wavelength are investigated theoretically. The tolerance of the poling period ΔΛ of PPRTA is found larger than that of PPLN and PPKTP when the fundamental wavelength is beyond 2 μm, which reaches its maximum ΔΛ_max for PPRTA at a fundamental wavelength of 2.7433 μm. However, the tolerance for the phase-matching temperature ΔT of PPRTA is found smaller than that of PPLN and PPKTP with an exception that PPRTA has a larger tolerance of the temperature or a larger temperature phase-matching bandwidth at fundamental wavelength of 2.2474 μm, where the maximum of ΔT(ΔT_max) is obtained. Furthermore, the tuning characteristics of the optical parametric processes using PPRTA for collinear quasi-phase-matching (CQPM) is analyzed. The combination of temperature tuning and poling period tuning enables a quasi-continuous wavelength tuning range of 1493.2-1593.7 nm for the signal and 3201.8-3699.2 nm for the idler, where poling period of 39 μm, 39.5 μm and 40 μm and a temperature between 20 and 120° have been employed in the corresponding theoretical analysis.     

Electromechanical properties and dielectric behavior of (Bi1/2Na1/2)(1−1.5x)BixTiO3 lead-free piezoelectric ceramics

Bismuth doped bismuth sodium titanate ceramics [(Bi_1/2Na_1/2)_(1−1.5x)Bi_xTiO₃, x=0 to 0.06] were prepared, and the resulting effects on the microstructure and dielectric properties were examined. All of the Bi-doped ceramics exhibited a single phase of perovskite structure with rhombohedral symmetry. The poling leakage current was significantly reduced by the doping of Bi, facilitating the poling process of the ceramics. The doping with Bi enhances the piezoelectric properties and increases the dielectric constant and the dielectric loss of the ceramics. At 2 mol% Bi-doping level, the ceramics exhibit a large remanent polarization of 47 μC/cm² and a relatively low coercive field of 71 kV/cm, while their d₃₃ and k_p reach a maximum value of 95 pC/N and 21%, respectively.     

Reflectivity measurements of superionic conductors for Li ion secondary battery materials

The reflection measurements of superionic conductors LiCoO₂ and Li_1-xCoO₂, which are already in use for the positive electrode material of 4 V rechargeable lithium batteries, have been performed in the millimeter wave region from 6 to 60 cm⁻¹ using the electron storage ring facilities of Institute of Molecular Science in Okazaki. The increase of the reflectivity has been observed in the low wavenumber region below 10 cm^-1 above 300 K in Li_1-xCoO₂ for the first time, while the reflectivity of LiCoO₂ has almost the constant value in all observed temperature region between 77 to 380 K. The results will be discussed in connection with our previous results of LiNiO₂.     

Characterization of crystal lattice constant and dislocation density of crack-free GaN films grown on Si(1 1 1)

GaN have sphalerite structure (Cubic-GaN) and wurtzite structure (hexagonal GaN). We report the H-GaN epilayer with a LT-AlN buffer layer has been grown on Si(1 1 1) substrate by metal-organic chemical vapor deposition (MOCVD). According to the FWHM values of 0.166° and 14.01 cm⁻¹ of HDXRD curve and E₂ (high) phonon of Raman spectrum respectively, we found that the crystal quality is perfect. And based on the XRD spectrum, the crystal lattice constants of Si (a = 5.3354 ?) and H-GaN (a_epi = 3.214 ?, c_epi = 5.119 ?) have been calculated for researching the tetragonal distortion of the sample. These results indicate that the GaN epilayer is in tensile strain and Si substrate is in compressive strain which were good agreement with the analysis of Raman peaks shift. Comparing with typical values of screw-type (D_screw = 7 × 10⁸ cm⁻²) and edge-type (D_edge = 2.9 × 10⁹ cm⁻²) dislocation density, which is larger than that in GaN epilayers growth on SiC or sapphire substrates. But our finding is important for the understanding and application of nitride semiconductors.     

Resonantly enhanced three-photon excitation spectra of lithium

We present the first measurement on the resonantly enhanced three-photon excitation spectra of natural lithium using a Nd:YAG laser pumped dye laser in conjunction with a thermionic diode ion detector. Exploiting the linear and circular polarizations, the np ²P_3/2(8 ? n ? 11) and nf  ²F_7/2 (8 ? n ? 38) series have been observed via three-photon excitation from the ground state. The measured level energies reveal a dynamic shift from calculated values, which increases with an increase of the principal quantum number n. The ac stark shift and line broadening mechanisms are studied as a function of laser intensity. It is noted that the width increases and the line center shifts towards the higher energy side as the laser intensity is increased. The maximum observed shift for the 12f ²F_7/2 line is 0.33 cm⁻¹ corresponding to the laser intensity variation from 1.34 × 10¹² W/m² to 1.03 × 10¹³ W/m², whereas its width increases from 0.36 cm⁻¹ to 0.82 cm⁻¹.     

Microstructuring of fused silica by laser-induced backside wet etching using picosecond laser pulses

The laser-induced backside wet etching (LIBWE) is an advanced laser processing method used for structuring transparent materials. LIBWE with nanosecond laser pulses has been successfully demonstrated for various materials, e.g. oxides (fused silica, sapphire) or fluorides (CaF₂, MgF₂), and applied for the fabrication of microstructures. In the present study, LIBWE of fused silica with mode-locked picosecond (t_p = 10 ps) lasers at UV wavelengths (λ₁ = 355 nm and λ₂ = 266 nm) using a (pyrene) toluene solution was demonstrated for the first time. The influence of the experimental parameters, such as laser fluence, pulse number, and absorbing liquid, on the etch rate and the resulting surface morphology were investigated. The etch rate grew linearly with the laser fluence in the low and in the high fluence range with different slopes. Incubation at low pulse numbers as well as a nearly constant etch rate after a specific pulse number for example were observed. Additionally, the etch rate depended on the absorbing liquid used; whereas the higher absorption of the admixture of pyrene in the used toluene enhances the etch rate and decreases the threshold fluence. With a λ₁ = 266 nm laser set-up, an exceptionally smooth surface in the etch pits was achieved. For both wavelengths (λ₁ = 266 nm and λ₂ = 355 nm), LIPSS (laser-induced periodic surface structures) formation was observed, especially at laser fluences near the thresholds of 170 and 120 mJ/cm², respectively.     

Microfabrication and magnetoelectric properties of amorphous magnetic-tunnel-junctions with Co–Fe–B/Al–O/Co–Fe–B hardcore structure

Both single-barrier magnetic tunnel junctions (SBMTJs) and double-barrier magnetic tunnel junctions (DBMTJs) with an amorphous hardcore structure of Co₆₀Fe₂₀B₂₀/Al–O/Co₆₀Fe₂₀B₂₀ were microfabricated. A high TMR ratio of 102.2% at 4.2 K was observed in the SBMTJs after annealing at 265 °C for 1 h. High TMR ratio of 56.2%, low junction resistance-area product RS of 4.6 kΩ μm², small coercivity H_C=25 Oe, and relatively large bias-voltage-at-half-maximum TMR with the value V_1/2 greater than 500 mV at room temperature (RT) had been achieved in such Co–Fe–B SBMTJs. Whereas, high TMR ratio of 60% at RT and 89% at 30 K, low junction resistance-area product RS of 7.8 kΩ μm² at RT and 8.3 kΩ μm² at 30 K, low coercivity H_C=8.5 Oe at RT and H_C=14 Oe at 30 K, and relatively large bias-voltage-at-half-maximum TMR with the value V_1/2 greater than 1150 mV at RT had been achieved in the Co–Fe–B DBMTJs. Temperature dependence of the TMR ratio, resistance, and coercivity from 4.2 K to RT, and applied voltage dependence of the TMR ratio and resistance at RT for such amorphous MTJs were also investigated.     

Annealing induced microstructural evolution of electrodeposited electrochromic tungsten oxide films

A significant influence of microstructure on the electrochromic and electrochemical performance characteristics of tungsten oxide (WO₃) films potentiostatically electrodeposited from a peroxopolytungstic acid (PPTA) sol has been evaluated as a function of annealing temperature. Powerful probes like X-ray diffractometry (XRD), transmission electron microscopy (TEM), UV-vis spectrophotometry, multiple step chronoamperometry and cyclic voltammetry have been employed for the thin film characterization. The as-deposited and the film annealed at 60 °C are composed of nanosized grains with a dominant amorphous phase, as well as open structure which ensues from a nanoporous matrix. This ensures a greater number of electroactive sites and a higher reaction area thereby manifesting in electrochromic responses superior to that of the films annealed at higher temperatures. The films annealed at temperatures ≥250 °C are characterized by a prominent triclinic crystalline structure and a hexagonal phase co-exists at temperatures ≥400 °C. The deleterious effect on the electrochromic properties of the film with annealing is ascribed to the loss of porosity, densification and the increasing crystallinity and grain size. Amongst all films under investigation, the film annealed at 60 °C exhibits a high transmission modulation (ΔT ∼ 68%) and coloration efficiency (η ∼ 77.6 cm² C⁻¹) at λ = 632.8 nm, charge storage capacity (Q_ins ∼ 21 mC cm⁻²), diffusion coefficient (6.08 × 10⁻¹⁰ cm² s⁻¹), fast color-bleach kinetics (t_c ∼ 275 s and t_b ∼ 12.5 s) and good electrochemical activity, as well as reversibility for the lithium insertion-extraction process upon cycling. The remarkable potential, which the film annealed at 60 °C has, for practical “smart window” applications has been demonstrated.     

Current-voltage characteristics of Au/GaN/GaAs structure

Au/GaN/n-GaAs structure has been fabricated by the electrochemically anodic nitridation method for providing an evidence of achievement of stable electronic passivation of n-doped GaAs surface. The change of the electronic properties of the GaAs surface induced by the nitridation process has been studied by means of current-voltage (I-V) characterizations on Schottky barrier diodes (SBDs) shaped on gallium nitride/gallium arsenide structure. Au/GaN/n-GaAs Schottky diode that showed rectifying behavior with an ideality factor value of 2.06 and barrier height value of 0.73 eV obeys a metal-interfacial layer-semiconductor (MIS) configuration rather than an ideal Schottky diode due to the existence of GaN at the Au/GaAs interfacial layer. The formation of the GaN interfacial layer for the stable passivation of gallium arsenide surface is investigated through calculation of the interface state density N_ss with and without taking into account the series resistance R_s. While the interface state density calculated without taking into account R_s has increased exponentially with bias from 2.2×10¹² cm⁻² eV⁻¹ in (E_c−0.48) eV to 3.85×10¹² cm⁻² eV⁻¹ in (E_c−0.32) eV of n-GaAs, the N_ss obtained taking into account the series resistance has remained constant with a value of 2.2×10¹² cm⁻² eV⁻¹ in the same interval. This has been attributed to the passivation of the n-doped GaAs surface with the formation of the GaN interfacial layer.     

Enhancement of luminescence intensity and increase of emission lifetime in Eu-doped 3CdO-Al2O3-3SiO2 amorphous system

Intense photoluminescence at room temperature was observed in amorphous cadmium aluminum silicate doped with europium prepared by the sol-gel method. The structure of the 3CdO-Al₂O₃-3SiO₂:Eu³⁺ system (CAS:Eu³⁺) has been determined by transmission electron microscopy (TEM) and X-ray diffraction (XRD). The excitation and the emission spectra indicated that the red characteristic emission (611 nm) of CAS:Eu³⁺ under UV excitation due to ⁵D₀→⁷F₂ electric dipole transition is the strongest. Both XRD data and the emission ratio of (⁵D₀→⁷F₂)/(⁵D₀→⁷F₁) reveal that the Eu³⁺ is in a site without inversion symmetry. The maximum photoluminescence intensity has been obtained for 25 mol% concentration of Eu³⁺ in CAS, and the intensity enhancement and lifetime increase of Eu³⁺ with increasing sintering temperature were observed due to the less OH-content in the samples.