TEM study of defects in AlxGa1−xN layers with different polarity

Transmission electron microscopy (TEM) studies of defects in Al_xGa_1?xN layers with various Al mole fractions (x=0.2, 0.4) and polarities were carried out. The samples were grown by ammonia molecular beam epitaxy on sapphire substrates and consisted of low-temperature AlN (LT-AlN) and high-temperature AlN (HT-AlN) buffer layers, a complex AlN/AlGaN superlattice (SL) and an Al_xGa_1?xN layer (x=0.2, 0.4). It was observed that at the first growth stages a very high density of dislocations is introduced in both Al-polar and N-polar structures. Then, at the interface of the LT-AlN and HT-AlN layers half-loops are formed and the dislocation density considerably decreases in Al-polar structures, whereas in the N-polar structures such a behavior was not observed.The AlN/AlGaN superlattice efficiently promotes the bend and annihilation of threading dislocations and respectively the decrease of the dislocation density in the upper Al_xGa_1?xN layer with both polarities.The lattice relaxation of metal-polar Al_0.2Ga_0.8N was observed, while N-polar Al_0.2Ga_0.8N did not relax. The dislocation densities in the N-polar Al_0.2Ga_0.8N and Al_0.4Ga_0.6N layers were 5.5×10⁹ cm^?2 and 9×10⁹ cm^?2, respectively, and in metal-polar Al_0.2Ga_0.8N and Al_0.4Ga_0.6N layers these were 1×10¹⁰ cm^?2 and 6×10⁹ cm^?2, respectively.Moreover, from TEM images the presence of inversion domains (IDs) in N-polar structures has been observed. The widths of IDs varied from 10 to 30 nm. Some of the IDs widen during the growth of the AlN buffer layers. The IDs formed hills on the surface of the N-polar structures.     

Characterization of Eu3+-doped fluorophosphate glasses for red emission

Fluorescence spectra and decay curves of the ⁵D₀ level for different concentrations of Eu³⁺ (4f⁶) ions in K–Ba–Al fluorophosphate glasses have been measured at room temperature and are analyzed. The Judd–Ofelt intensity parameters Ω₂ and Ω₄ have been determined from the intensity ratios of emission peaks corresponding to ⁵D₀ → ⁷F_J (J = 2 and 4) to ⁵D₀ → ⁷F₁ transitions for 1.0 mol% glass. The intensity parameters thus obtained are in turn used to calculate the radiative properties of the fluorescent levels of Eu³⁺ ions. Second and fourth rank crystal-field parameters have been evaluated by assuming a C_2V site symmetry for the local environment of Eu³⁺ ions to estimate the crystal-field strength experienced by Eu³⁺ ions in the present host. The decay profiles of the ⁵D₀ → ⁷F₂ transition of Eu³⁺ ions in the present glasses are found to be single exponential for all the studied Eu³⁺ ion concentrations. A marginal increase in lifetime of the ⁵D₀ level has been noticed with Eu³⁺ ion concentration up to 2.0 mol% and then the lifetime marginally decreases for higher Eu³⁺ ion concentrations.     

Melting enthalpy ΔHm for describing glass forming ability of bulk metallic glasses

A new melting enthalpy ΔH_m criterion for the prediction of glass forming ability (GFA) of alloys is proposed and five Zr–Al–Ni–Cu bulk metallic glasses (BMG) with critical dimension Z_max up to ? 7.5 mm are also developed by us in the light of the optimum ΔH_m of Zr–Al–Ni–Cu alloy system. And then, we researched the relationships between ΔH_m and two GFA parameters (critical cooling rate R_c and Z_max) of five bulk metallic glass (BMG) systems, such as Mg–Ni–Nd, Pd–Cu–Si, La–Al–Ni–Cu, Zr–Al–Ni–Cu and Zr–Ti–Ni–Cu–Be, respectively. The results show that the relationships between ΔH_m and R_c are all concave upward parabolas, and the optimum ΔH_ms for Mg–Ni–Nd, Pd–Cu–Si, Zr–Al–Ni–Cu, Zr–Ti–Ni–Cu–Be and La–Al–Ni–Cu are 10.3960 kJ mol^?1, 21.2202 kJ mol^?1, 19.7146 kJ mol^?1, 18.1455 kJ mol^?1 and 13.1558 kJ mol^?1, respectively. On the contrary, the relationships between ΔH_m and Z_max are all concave downward parabolas, and the optimum ΔH_ms for Mg–Ni–Nd, Pd–Cu–Si, Zr–Al–Ni–Cu, Zr–Ti–Ni–Cu–Be and La–Al–Ni–Cu are 10.5530 kJ mol^?1, 21.0830 kJ mol^?1, 19.6603 kJ mol^?1, 19.7231 kJ mol^?1 and 13.1173 kJ mol^?1, respectively. Furthermore, other BMGs’ R_cs or Z_maxs predicted by above-mentioned relationships satisfactorily agree with the tested results, which indicates that these relationships are reliable. However, the predicted results are reliable only if the main components are similar with the fitted BMGs or the additive is sparkle enough that the alloy’s character does not change. On the whole, the ΔH_m can act as a criterion for quickly predicting the alloy’s GFA and be helpful for the development of new BMGs.     

Kinetics of crystal growth of germanium disulfide in Ge0.38S0.62 chalcogenide glass

The crystal growth kinetics of GeS₂ in Ge_0.38S_0.62 glass has been studied by Differential Scanning Calorimetry (DSC) and microsopy. The linear crystal growth kinetics of both high temperature α-GeS₂ and low temperature β-GeS₂ polymorphs has been observed over a relatively broad range of temperatures, i.e. 420 < T < 494 °C that correspond to viscosity of supercooled melt: 3 × 10⁹ > η > 8 × 10⁵ Pa s. It seems that 2D nucleated growth is the most probable mechanism of crystallization for high temperature α-GeS₂ under these conditions. However, there are significant deviations for this model for the crystallization of low-temperature β-GeS₂. This might indicate some changes in crystal-melt interfacial energy or break down of Stokes–Einstein relation in that particular case. At temperatures below 500 °C the temperature range of directly observed crystal growth overlaps with isothermal DSC measurements. In this case overall crystallization kinetics can be described by the Johnson–Mehl–Avrami (JMA) nucleation-growth model for kinetic exponent n ≅ 4. The value of activation energy of nucleation estimated from these experiments E_N = 434 kJ mol⁻¹ is comparable with the activation energy of viscous flow in supercooled Ge_0.38S_0.62 melt (E_η = 478 kJ mol⁻¹). A more complex eutectic crystallization involving both GeS₂ and GeS phases has been observed at higher temperatures. This process is probably associated with secondary nucleation and cannot be described by a simple JMA model.     

Upconversion luminescence in Ho3+/Yb3+- and Tb3+/Yb3+-codoped fluorogermanate glass and glass ceramic

Energy-transfer excited upconversion luminescence in Ho³⁺/Yb³⁺- and Tb³⁺/Yb³⁺-codoped PbGeO₃–PbF₂–CdF₂ glass and glass–ceramic under infrared excitation is investigated. In Ho³⁺/Yb³⁺-codoped samples, green (545 nm), red (652 nm), and near-infrared (754 nm) upconversion emission corresponding to the ⁵S₂(⁵F₄) → ⁵I₈, ⁵F₅ → ⁵I₈, and ⁵S₂(⁵F₄) → ⁵I₇ transitions, respectively, was observed. Blue (490 nm) emission assigned to the ⁵F_2,3 → ⁵I₈ transition was also detected. In the Tb³⁺/Yb³⁺-codoped system, bright UV–visible emission around 384, 415, 438, 473–490, 545, 587, and 623 nm, identified as due to the ⁵D₃(⁵G₆) → ⁷F_J(J = 6, 5, 4) and ⁵D₄ → ⁷F_J(J = 6, 5, 4, 3) transitions, was measured. The comparison of the upconversion process in glass ceramic and its glassy precursor revealed that the former samples present much higher upconversion efficiencies. The dependence of the upconversion emission upon pump power, and doping contents was also examined. The results indicated that successive energy-transfer between ytterbium and holmium ions and cooperative energy-transfer between ytterbium and terbium ions followed by excited-state absorption are the dominant upconversion excitation mechanisms herein involved. The viability of using the samples for three-dimensional solid-state color displays is also discussed.     

Structural and optical spectroscopy of LiNbO3:Tm nanocrystals embedded in a SiO2 glass matrix

Transparent SiO₂:Li₂O:Nb₂O₅ glass doped with Tm³⁺ has been prepared by the sol–gel method, and heat-treated in air (HT) at temperatures between 500 and 800 °C. X-ray diffraction (XRD) patterns and Raman spectroscopy show SiO₂ and LiNbO₃ phases in samples HT above 650 °C, and a NbTmO₄ phase for T > 750 °C. The XRD SEM analysis show increasing particle size and number with the increase of HT temperature. Intra-4f¹² transitions due to Tm³⁺ ion dispersed in the matrix are observed in samples with T > 650 °C. The luminescence is dominated by the ¹G₄ → ³F₄ (～650 nm), ¹D₂ → ³F₃ (～780 nm), ³H₄ → ³H₆ (～800 nm), ³H₅ → ³H₆ (～1200 nm) and ³H₄ → ³F₄ (～1500 nm) transitions under resonant excitation to the ion levels.     

Judd Ofelt calculation of quantum efficiencies and branching ratios of Nd3+ doped glasses

This paper presents the optical characterization of Nd³⁺ lead silicate glasses (SiO₂–Na₂CO₃–PbO–ZnO), synthesized by means of the fusion method. Absorption, luminescence, lifetime and Raman spectroscopy measurements were performed in order to determine the radiative properties of the glasses. The near infrared luminescence exhibited the typical Nd³⁺ bands, particularly the band at 1060 nm (⁴F_3/2 → ⁴I_11/2). The calculated branching ratios for the ⁴F_3/2 level are: β (9/2) = 35%, β (11/2) = 55%, β(13/2) = 9.5% and β (15/2) = 0.5%. The estimated quantum efficiency was about 90%, based on comparison with the Judd Ofelt theory and experimental lifetime measurements.     

Intense luminescence of amorphous Eu2O3 prepared by aqueous sol–gel method

Amorphous Eu₂O₃ was prepared by an aqueous sol–gel method. Emission due to the ⁵D₀ → ⁷F_J (J = 0, 1, 2) transitions of Eu³⁺ ions were observed. The dominant transition was the ⁵D₀ → ⁷F₂ red emission of Eu³⁺. The properties of the as-prepared samples were different with changes in the annealing temperature. To investigate the luminescence properties of the amorphous Eu₂O₃, the temperature-dependent photoluminescence (PL) spectra of samples annealed at 600 °C were measured in the temperature range 77–300 K. PL peak positions were unchanged with the change of temperature.     

Validation of the mechanism of nitrogen/oxygen substitution in Li–Na–Pb–P–O–N oxynitride phosphate glasses

As evidenced by O_1s X-ray photoelectron spectroscopy, there are three kinds of oxygen atoms in Li_0.25Na_0.25Pb_0.25PO_3−3x/2N_x oxynitride phosphate glasses (0 < x ⩽ 0.67): (i) bridging oxygens (BO) which interconnect the P(O, N)₄ tetrahedral units of the glass network (ii) non-bridging oxygens (NBO) which coordinate the modifier cations and (iii) oxygen atoms, also non-bridging, which belong to PO₄ tetrahedra involved in the coordination sphere of Pb²⁺. The variation of their relative proportions as the nitrogen content increases is explained according to the nitridation mechanism previously proposed for this series of glasses. Moreover, the variation of the BO/NBO ratio confirms that the nitrogen/oxygen substitution taking place during nitridation obeys the simple chemical equivalences already stated.     

Influence of hydrogen on the thermomechanical properties of a-CNx:H and a-CNx films deposited by glow discharge and ion beam assisted deposition

The coefficient of thermal expansion (CTE), Young’s modulus, Poisson’s ratio, stress and hardness of a-CN_x and a-CN_x:H were investigated as a function of nitrogen concentration. Hydrogenated films were prepared by glow discharge, GD, and unhydrogenated films were prepared by ion beam assisted deposition, IBAD. Using nanohardness measurements and the thermally induced bending technique, it was possible to extract separately, Young’s modulus and Poisson’s ratio. A strong influence of hydrogen, in a-CN_x:H films, was observed on the CTE, which reaches about ∼9 × 10⁻⁶ C⁻¹, close to that of graphite (∼8 × 10⁻⁶ C⁻¹) for nitrogen concentration as low as 5 at.%. On the other hand, the CTE of unhydrogenated films increases with nitrogen concentration at a much lower rate, reaching 5.5 × 10⁻⁶ C⁻¹ for 33 at.% nitrogen.     

Heterophase inclusions and dissolved impurities in Ge25Sb10S65 glass

The impurity content and microhomogeneity of Ge₂₅Sb₁₀S₆₅ glass samples, prepared by direct synthesis from elements, were investigated. It was shown that the increase in temperature of synthesis of the glass-forming melt resulted in the increase of the content of impurities of H, Na, Al, Si, K, Ca and transition metals in the prepared glasses. The glasses from the melt, subjected to chemical-distillation purification, were characterized by the low content of gas-forming impurities and the increased content of Al, Si and Cl. The glasses contained heterophase impurity inclusions mainly consisting of SiO₂, and their concentration and size depended on the conditions of glass preparation. The impurity content in the purest glasses was as follows: oxygen – <0.5 ppm wt, carbon – <5 ppm wt, hydrogen – 0.1 ppm wt, Si – <1 ppm wt, transition metals – <0.25 ppm wt, heterophase impurity inclusions with sizes larger than 80 nm – <10² cm³. It was shown that heterophase impurity inclusions behaved as the centers of glass crystallization.     

Investigation of glass forming ability in Ce–Al–Ni alloys

The compositional dependence of the glass forming ability (GFA), the correlation between their GFA and the GFA related parameters, and the thermal stability of the Ce–Al–Ni alloys were investigated. Rapidly quenched Ce₆₅Al_xNi_35 ? x (x = 2, 5, 10, 17, 20) and Ce₇₀Al_xNi_30 ? x (x = 2, 5, 10, 15, 20) ribbons were prepared by melt spinning, and their phase transformations were studied by X-ray diffraction (XRD), transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). The experimental results indicated that the GFA of Ce₆₅Al_xNi_35 ? x (x = 2, 5, 10, 17, 20) and Ce₇₀Al_xNi_30 ? x (x = 2, 5, 10, 15, 20) alloys increased firstly and then decreased with the increasing of the Al content up to 20 at.%, respectively. It was found that only one parameter, F₁, in evaluated currently available empirical GFA parameters searching for metallic glasses with a good GFA, can reflect the GFA of the Ce–Al–Ni alloys. It was indicated that the thermal stability of alloy with fully amorphous maybe lower than that of alloy with partial amorphous.     

Growth of highly oriented crystalline α-Fe/AlN/Fe3N trilayer structures on Si(1 1 1) substrates by molecular beam epitaxy

We have realized highly oriented nitride-based α-Fe/AlN/Fe₃N ferromagnetic hybrid structures on Si(1 1 1) substrates by molecular beam epitaxy using AlN/SiC intermediate layers. A two-step hysteresis loop, typically observed in magnetic tunneling junctions, was clearly observed in magnetization versus magnetic field measurements. This result indicates the formation of ferromagnetic α-Fe and Fe₃N layers separated by 8-nm-thick AlN layers over approximately 1 cm² area, and also shows the difference in coercive field between the two ferromagnetic layers.     

Tb3+-impregnated,non-porous silica glass possessing intense green luminescence under UV and VUV excitation

A colorless transparent luminescence material was successfully prepared by impregnation of leached, porous glass with Tb³⁺ ions followed by reductive sintering in a CO atmosphere. Tb³⁺ emissions under ultraviolet (UV) and vacuum ultraviolet (VUV) excitation clearly showed the most intense emission band to be situated at 543 nm, which corresponds to the ⁵D₄ → ⁷F₅ transition. Sintering of the Tb glass in a reducing atmosphere resulted in a significant enhancement of Tb³⁺ emission intensity in comparison with sintering in air. The presence of traces of cerium ions was verified in Tb glasses, and more Ce³⁺ ions were produced as a result of the reductive sintering. The increase in Ce³⁺ ions was believed to be mainly responsible for the enhancement of ⁵D₃ → ⁷F_j and ⁵D₄ → ⁷F_j transitions from Tb³⁺ ions owing to an energy transfer channel. A clearly defined difference in the spectral energy distribution of Tb³⁺ emissions was found for 231 nm UV and 160 nm VUV excitation of the Tb glass. The phenomenon of cross relaxation was only observed under 231 nm UV excitation. Different excitation mechanisms were taken into account. Direct excitation of Tb³⁺ ions together with Ce³⁺ ions occurred in the Tb glass under the 231 nm UV light, whereas indirect excitation consisting of host absorption of energy and transfer from host to Tb³⁺ ions occurred under the 160 nm VUV light.     

Preparation and ionic conductivity of transparent glass−ceramics containing a large quantity of lithium−mica

In order to crystallize a large quantity of the lithium?mica in glass?ceramics, 5.1 mass% MgF₂ was added to the starting materials of the parent glasses having chemical compositions of Li_(1+x)Mg₃AlSi_3(1+x)O_10+6.5xF₂ (x = 0.5 and 1.0). Transparent glass?ceramics, in which a large quantity of lithium?mica with particle size of <50 nm was separated, could be prepared from the MgF₂-added parent glass with x = 0.5. While the parent glass, which had a binodal phase separation structure, did not exhibit electrical conductivity, the transparent glass–ceramic was given conductivity by the formation of an interlocking structure of mica. As the separated mica formed a tighter interlocking structure, the conductivity increased and reached a value of 2.0 × 10^?3 S/cm at 600 °C. The MgF₂-added parent glass with x = 1.0 was not transparent because of coarse spinodal phase separation. The conductivity was 4.3 × 10^?4 S/cm at 600 °C but was significantly decreased by the separation of mica.     

Structure–property correlations in lead borate and borosilicate glasses doped with aluminum oxide

Glass samples from four systems: xPbO–(100?x)B₂O₃ (x = 30, 40, 50 and 60 mol%), 50PbO–yAl₂O₃–(50?y)B₂O₃ (y = 2, 4, 6, 8 mol%), 50PbO–ySiO₂–(50?y)B₂O₃ (y = 5, 10, 20, 30 mol%) and 50PbO–5SiO₂–yAl₂O₃–(45?y)B₂O₃ (y = 2, 4, 6, 8 mol%) were prepared by a melt-quench technique. Characterization of these systems was carried out using density measurements, UV–visible spectroscopy, differential scanning calorimetry (DSC), and ¹¹B and ²⁷Al magic-angle spinning (MAS) solid-state nuclear magnetic resonance (NMR). Our studies reveal an increase in glass density with increasing lead(II) oxide concentration in pure lead borates and also with addition of silica into 50PbO–50B₂O₃ glass. ¹¹B MAS NMR measurements determine that the fraction of tetrahedral borons (N₄) reaches a maximum for the glass containing 50 mol% of PbO in the PbO–B₂O₃ glass series and that N₄ is sharply reduced upon adding small amounts of Al₂O₃ into lead borate and lead borosilicate systems. ²⁷Al MAS NMR experiments performed on glasses doped with aluminum oxide show that the Al³⁺ are tetra-, penta- and hexa-coordinated with oxygen, even without any excess concentration of Al³⁺ over charge-balancing Pb²⁺ cations. ^[5]Al and ^[6]Al concentrations are found to have unusually high values of up to 30%. The results of UV–visible absorption spectroscopy, DSC and density measurements support the conclusions drawn from the NMR studies, providing a consistent picture of structure–property relations in these glass systems.     

Thermal lens study of PbO–Bi2O3–Ga2O3–BaO glasses doped with Yb3+

The aim of this paper is to present a study of the thermal lens technique in quantifying the thermo optical coefficients: ds/dT (optical path change with temperature), thermal diffusivity and conductivity of PbO–Bi₂O₃–Ga₂O₃–BaO glasses doped with Yb³⁺. The thermal lens results indicate that the heat generation, as a function of the incident wavelength, resembles the absorption band ²F_7/2 → ²F_5/2 of Yb³⁺. Thermal diffusivity of 2 × 10⁻³ cm²/s and thermal conductivity of 4.5 × 10⁻³ W/K cm were obtained and are similar to other glasses already reported in previous literature. The results emphasize that the thermal lens technique can be a powerful tool to study the heat generation of new glassy systems.     

Photoluminescence lifetime spectrum in erbium doped Ge–Ga–S glasses

Ge₃₀Ga₄S_65.5:Er_0.5 glasses with stoichiometric composition have been prepared using a conventional melt-quenching technique. The PL lifetime distribution corresponding to ⁴I_13/2 → ⁴I_15/2 transition in Er³⁺ ion has been measured experimentally using quadrature frequency-resolved PL spectroscopy (QFRS) from 2 ns to 160 s at room temperature and at 3.7 K. The distribution seems to be principally single-peaked at around τ_QFRS ≈ 3–4 ms with a half width <3 ms irrespective of PL energy and temperature. The Judd–Ofelt analysis based on optical transitions from ⁴I_15/2 level to ⁴I_13/2, ⁴I_11/2, ⁴I_9/2, ⁴F_9/2, ⁴S_3/2, ²H_11/2 and ⁴F_7/2 levels in Er³⁺ ions gives the radiative lifetime of ⁴I_13/2 → ⁴I_15/2 transition τ_JO ≈ 2.6 ms. The closeness of τ_JO and τ_QFRS implies the dominant role of the radiative relaxation in ⁴I_13/2 → ⁴I_15/2 transitions. The shape of steady-state PL spectra can be predicted by using a modified McCumber theory taking into account the possibility of absorption and re-emission of PL quanta in heavily doped materials under weak excitation. The latter calculations used Monte Carlo simulations taking into account the re-absorption and re-emission of the PL radiation. The non-radiative lifetime for the ⁴I_13/2 → ⁴I_15/2 transition has been estimated to be more than 100 times larger than the radiative lifetime.     

Through-bond and through-space connectivities of amorphous aluminophosphate by two-dimensional 27Al–31P heteronuclear NMR

The connectivities between Al and P through chemical bond and internuclear distance have been studied for an amorphous aluminophosphate (a-AlPO₄) using two-dimensional (2D) solid-state ²⁷Al–³¹P correlation NMR (MAS J-HMQC and CP HETCOR). Whereas the conventional ³¹P MAS spectrum provides less informative results because of poor resolution caused by large distributions of the nucleus surroundings, the 2D HETCOR shows much better resolution and at least four non-equivalent P sites in the a-AlPO₄. These P sites are found to be correlated with one ^[4]Al, two ^[5]Al and one ^[6]Al species, and have different chemical shifts. This result might indicate that the mean P–O–^[n]Al (n = 4, 5, 6) bond angles are different each other, and they are estimated using the relationship with the ³¹P chemical shifts in crystalline AlPO₄ previously reported.     

Phosphate speciation in sodium borosilicate glasses studied by nuclear magnetic resonance

The structure of RNa₂O · B₂O₃ · KSiO₂ · xP₂O₅ (0.5 < R < 2; 0.86 < K < 3) borosilicate glasses has been studied by nuclear magnetic resonance (NMR). ³¹P magic angle spinning (MAS), double quantum-magic angle spinning (DQ-MAS) and ³¹P–¹¹B transfer of populations under double resonance magic angle spinning (TRAPDOR MAS) NMR were used to determine the phosphate speciation in the glasses and their connectivity with the borosilicate network. The structure of the glass network was characterized with ¹¹B, ²⁹Si and ²³Na MAS NMR. Ab initio calculations of the ³¹P chemical shielding were carried out in order to confirm the connectivity between phosphorus and the structural units of the borosilicate glass network. Na₃PO₄ (monophosphate), Na₄P₂O₇ (diphosphate) and P–O–B species (mono- and diphosphate groups with borate units as the next nearest neighbors) are found all along the compositional range studied. The proportion of the P–O–B groups increases as the glass optical basicity decreases, while the proportions of mono- and diphosphate species decrease. The change in the glass transition temperature of the phospho-borosilicate glasses with respect to that of the borosilicate ones is discussed in terms of the structural characterization. The formation of phosphate species gives rise to the increase in T_g, which is attributed to the re-polymerization of the silicate network, while the formation of P–O–B bonds weakens the glass network and produces a decrease in the glass transition temperature.