Studies on crystal growth,optical and electrical characterization of pure and Dy-doped bismuth silicate single crystals

(0.5 mol%) Dysprosium (Dy) doped bismuth silicon oxide (BSO) single crystals were grown by the Czochralski technique under air atmosphere. Detailed analysis of Dy-doped BSO with pure BSO has been studied through optical analysis. The absorption edges of pure and Dy-doped BSO crystals are found to be 405 nm and 415 nm, respectively. The shift in the absorption edge is contributed to the defect centers created in the crystal with Dy-doping. The shifts observed in the Raman spectra on doping Dy are found to be lower, when compared with the pure BSO crystal. This effect can be correlated to the lattice distortion induced by the Dy doping. The oxide formation and intrinsic defects in the BSO crystal have been identified by photoluminescence analysis. Dielectric measurements reveal that higher permeability value in the BSO sample is due to the presence of charged defects, which can be related to the space charge polarization. There is a slight decrease in dielectric constant on doping with Dy. The piezoelectric value explains the defects formed in the crystal. On poling, d₃₃ value of BSO and Dy-doped BSO are 32 pC/N and 40 pC/N, respectively.     

Hydrothermal growth and optical properties of RbBe2BO3F2 crystals

Crystals of RbBe₂BO₃F₂ up to a size of 22×18×5 mm³ have been grown by the hydrothermal method. The growth conditions were optimized mainly by adjusting the mineralizers and growth temperature. The crystals linear and nonlinear optical properties, including its transmittance spectrum and second harmonic generation from 532 to 266 nm, were measured and compared with those of flux grown crystals. A maximum conversion efficiency of 20% was obtained with a 25 ps, 10 Hz laser system. The results indicate that RbBe₂BO₃F₂ crystals grown by the hydrothermal method show similar capability for frequency conversion compared with flux grown crystals and are promising for future deep-UV harmonic generation.     

Effect of annealing and gamma irradiation on undoped and Eu3+-doped Y2SiO5 single crystals

The absorption spectra of the undoped Y₂SiO₅ and Eu³⁺-doped Y₂SiO₅ crystals grown by the Czochralski technique were compared before and after annealing and, similarly, the unannealed and annealed crystals after γ-ray irradiation. The absorption bands of Eu²⁺ ions with peaks at 300 and 390 nm were observed in the as-grown Y₂SiO₅:Eu³⁺ crystal. These peaks were more intense in H₂-annealed and irradiated Y₂SiO₅:Eu³⁺ crystals. The additional absorption peaks at 260 and 320-330 nm which were attributed to F color centers and O⁻ hole centers were observed in irradiated undoped Y₂SiO₅ and Y₂SiO₅:Eu³⁺ crystals, respectively.     

Chemical durability and weathering resistance of canasite based glass and glass-ceramics

Classic composition 8.4Na₂O·5K₂O·10.8CaO·64SiO₂·10.5CaF₂·1.3Al₂O₃ (G1/GC1) and high silicon composition 7.6Na₂O·4K₂O·8.4CaO·71SiO₂·8CaF₂·1.0Al₂O₃ (G2/GC2) canasite-based glass and glass-ceramics were prepared, and the chemical durability and weathering of samples were studied with XRD, ICP-AES, SEM and optical microscopy. Interestingly, a kind of color fringe pattern caused by the acid leaching was directly observed on the glass ceramic surface under optical microscopy. The 20 day weight losses of glass and glass ceramic in acid (1 M HCl) and alkali (1 M NaOH/1 M Na₂CO₃) solution were measured. Accelerated weathering was used to demonstrate that increasing silicon content contributes to the weathering performance of glass and glass-ceramics. For different micro-structures and compositions, the weight loss of each glass and glass-ceramic is quite different. In general, through increasing the network interconnectivity of residual glass network and suppressing the crystallization of the less durable canasite phase, the addition of SiO₂ (from 60 mol% to 71 mol%) enhanced the chemical durability of canasite-based glass and glass ceramic relatively under acid, alkali and weathering conditions.     

Growth and characterization of the chalcopyrite LiGaTe2: A highly non-linear birefringent optical crystal for the mid-infrared

LiGaTe₂ crystals have been grown by the Bridgman–Stockbarger technique. The clear transparency range of LiGaTe₂ extends from 2.5 to 12 μm and its band-gap at room temperature is at 2.41 eV (515 nm). LiGaTe₂ is a positive uniaxial crystal which possesses sufficient birefringence for phase-matching. Its non-linear coefficient d₃₆ estimated by phase-matched second harmonic generation is 43 pm/V ± 10% at 4.5 μm. The properties of LiGaTe₂ are compared to those of other mid-IR chalcopyrite non-linear optical crystals with special emphasis on the frequency doubling potential for CO₂ lasers operating at 10.6 μm.     

Single crystal growth from the melt and magnetic properties of hexaferrites–aluminates

Single crystals of aluminum substituted barium hexaferrite were grown by the floating zone method with optical heating. Single crystals were produced from a melt of stoichiometric composition. The process was carried out under a pressure of 50 atm of oxygen. In the system BaO–(x)Al₂O₃–(6?x)Fe₂O₃ the region of single phase crystal growth from the melt is limited by the value x=3. For higher substitutions single-phase crystallization is not observed. The grown single crystals are cylindrical boules with a diameter of 4–5 mm and with lengths up to 50 mm. To avert cracking the crystals have been annealed during the process of growth at 1100 °C. The content of FeO in the composition of single crystals of barium hexaferrite, grown by zone melting under an oxygen pressure of 50 atm, is approximately 0.3 wt%. In the system of hexaferrite–aluminates the macroscopic magnetic moment of the material disappears at x=3.     

Vertical Bridgman growth and characterization of large ZnGeP2 single crystals

The growth of ZnGeP₂ crystals by seeded Vertical Bridgman method was studied. High-quality near-stoichiometric ZnGeP₂ single crystals obtained were of 20–30 mm in diameter and 90–120 mm in length. By selection of the seed crystallographic orientation the single crystal ingots without cracks and twins were grown, as shown by X-ray diffraction. The infrared transmission property of the ZnGeP₂ crystals was studied by the calculated optical absorption coefficient spectra. The results showed that after thermal annealing of the crystals the optical absorption coefficient was ～0.10 cm^?1 at 2.05 μm, and ～0.01 cm^?1 at 3–8 μm. The rocking curves patterns of the (4 0 0) reflection demonstrated that the as-grown single crystals possessed a good structural quality. The composition of the crystals was close to the ideal stoichiometry ratio of 1:1:2. The low-loss typical ZnGeP₂ samples of 6 mm×6 mm×15 mm in sizes were cut from the annealed ingots for optical parametric oscillation experiments. The output power of 3.2 W was obtained at 3–5 μm when the incident pumping power of 2.05 μm laser was 9.4 W, and the corresponding slope efficiency and the conversion efficiency were 44% and 34%, respectively.     

Flux growth of β-Mn2V2O7 single crystals

Large single crystals of β-Mn₂V₂O₇ are successfully grown at a slow cooling rate using flux method in a closed crucible. The grown crystals exhibit a characteristic morphology with natural (1 1 0) and (3 5 0) facets. X-ray diffraction and chemical analyses show that the grown crystals have high quality.     

Effect of Al2O3 addition on the formation of perovskite-type NaNbO3 nanocrystals in silicate-based glasses

The crystallization behavior of 30Na₂O–25Nb₂O₅–(45 ? x) SiO₂–xAlO_1.5 (x = 0, 2.5, and 5) (mol%) glasses was examined and the effect of Al₂O₃ addition on the formation of perovskite-type NaNbO₃ crystals was clarified. It is found from X-ray diffraction analyses and transmission electron microscope observations that NaNbO₃ nanocrystals are formed in all glasses and the size of NaNbO₃ crystals decreases with the substitution of Al₂O₃ for SiO₂. A crystallized (heat-treated at 684 °C for 5 h) glass with x = 5, which contains NaNbO₃ nanocrystals with an average size of 50 nm, shows good optical transparency in the wavelength region of 500–2000 nm and a small hysteresis loop in the polarization–electric field curve. The lines containing NaNbO₃ crystals were patterned on the surface of NiO-doped glass with x = 5 by irradiations (power: 1.3–1.4 W, scanning speed: 10 μm/s) of Yb:YVO₄ fiber laser (wavelength: 1080 nm). The formation mechanism of NaNbO₃ nanocrystals in aluminosilicate glasses was also discussed.     

Optical constants and band gap expansion of size controlled silicon nanocrystals embedded in SiO2 matrix

Silicon nanocrystals (Si-NCs) with different sizes embedded in SiO₂ matrix were synthesized by phase separation and thermal crystallization of SiO_x/SiO₂ supperlattice approach. The optical constants and band gap expansion of Si-NCs have been investigated by spectroscopic ellipsometry, based on the Maxwell–Garnett effective medium approximation and the Forouhi–Bloomer optical dispersion model. Similar spectra shapes but smaller values of Si-NCs optical constants with respect to bulk crystalline Si is observed. With the size of Si-NCs decreasing from 6 nm to 2 nm, the band gap increases from 1.64 eV to 2.56 eV. The band gap expansion, as compared to bulk crystalline Si, which agrees with the prediction of first-principles calculations based on quantum confinement effect, is presented in this paper.     

Characterization of thermo-optical and mechanical properties of calcium aluminosilicate glasses

In this work several different compositions of CaO:Al₂O₃:SiO₂ were prepared under vacuum atmosphere to study the glass forming ability of this system as a function of the SiO₂ content. Samples containing 25–45 wt% of Al₂O₃, 31–44 wt% of CaO, 14–39 wt% of SiO₂ and 4.1 wt% of MgO were prepared in graphite crucibles, for approximately 2 h at ∼ 1600 °C. The influence of silica content is discussed in terms of the mechanical properties, glass transition temperature, crystallization temperature, transmittance spectrum, refractive index, mass density, specific heat, thermal diffusivity, thermal conductivity and the temperature coefficient of optical path length change. The results reinforce the idea that these glasses are strong materials, having useful working-temperature range, good combination of thermal, mechanical and optical properties that could be exploited in many optical applications, in particular, as glass laser materials.     

Influence of Ag nanoparticles on luminescent performance of SiO2:Tb3 + nanomaterials

Tb^3 + single-doped SiO₂ (SiO₂:Tb^3 +) and Tb^3 +, Ag co-doped SiO₂ (SiO₂:Tb^3 +–Ag) nanostructured luminescent materials were prepared by a modified Stöber method. Their microstructure and optical property were investigated using scanning electron microscopy, ultraviolet visible absorption and photoluminescence spectrophotometry. Results show that the samples are composed of well-dispersed near-spherical particles with a diameter about 50 nm, the highest fluorescence intensity is obtained when the doping concentration of Tb^3 + is 4.86 mol%, the corresponding internal quantum efficiency is 13.6% and the external quantum efficiency is 8.2%. The experimental results indicate that Ag nanoparticles can greatly enhance the light absorption at 226 nm and the light emission at 543 nm of SiO₂:Tb^3 +–Ag, and the fluorescence lifetime reduces with increasing Ag concentration in SiO₂:Tb^3 +–Ag. Additionally, the present results indicate that fluorescence enhancement is attributed to the local field enhancement and the increased radiative decay rates induced by Ag nanoparticles.     

Growth and characterization of l-prolinium picrate single crystal: A promising NLO crystal

l-Prolinium picrate was synthesized and its solubility in the mixed solvent of acetone and water was estimated. Employing temperature reduction method crystals of size 22×4×3 mm³ were grown. The cell dimensions were obtained by single-crystal X-ray diffraction study. FT-IR, UV–vis–NIR and fluorescence spectral analyses were carried out for the grown crystals. The optical band gap estimated for this crystal is about 2.4 eV. Thermogravimetric study was carried out to determine the thermal properties of the grown crystal. Kurtz powder second harmonic generation (SHG) efficiency measurement revealed that the SHG efficiency is about 52 times that of potassium dihydrogen orthophosphate. Mechanical strength was tested by Vicker's microhardness test.     

Low-K factor of SiO2 layer on Si irradiated by YAG:Nd laser

The change of optical and electrical properties of SiO₂ layer on Si single crystal exposed to YAG:Nd laser radiation has been found experimentally. The second harmonic of YAG:Nd laser was used as a source of light. Before irradiation the SiO₂ layer with thickness 0.75 μm had red color in reflecting light due to the interference. After irradiation with the laser with intensity of more than 3.5 MW/cm² red color changed to yellow. However, samples with thickness 0.21 μm did not change color after irradiation. We explain such peculiarities of optical properties by change of optical path. Capacity (C) measurements of SiO₂ layer with thickness 0.21 μm by the method of capacity–voltage characteristics have shown a decrease of C to more than 40%. It is possible if real part of dielectric permittivity (K) decreases or thickness of the SiO₂ layer increases. Atomic force microscope and profilemeter measurements did not show any change of surface roughness for the SiO₂ layer with thickness 0.21 μm. We suppose that after irradiation of the SiO₂ layer decrease of K takes place due to the formation of nanopores in SiO₂ or/and generation of the charged point defect at the interface of Si–SiO₂. Particularly the first is in agreement with measurements of micro hardness and capillary effect.     

Active planar waveguides based on sol–gel Er3+-doped SiO2–ZrO2 for photonic applications: Morphological,structural and optical properties

Er³⁺-doped glass-ceramic SiO₂–ZrO₂ optical planar waveguides were prepared by the sol–gel route using different SiO₂:ZrO₂ molar ratios (90:10, 85:15, 80:20 and 75:25). Multilayered films were deposited onto Si(1 0 0) substrates by the dip-coating technique. Structural characterization was performed using vibrational spectroscopy and X-ray diffraction. Some optical properties, densification and surface morphology of these films were investigated as a function of the SiO₂:ZrO₂ ratio, annealing temperature and time. Optical properties such as refractive index, number of propagating modes and attenuation coefficient were measured at 632.8, 543.5 and 1550 nm, by the prism coupling technique. Uniform surface morphology with roughness less than 0.5 nm. Low losses, less than 0.9 dB/cm at 612.8 nm in the TE₀ mode, were measured for the planar waveguides containing up to 25 mol% zirconium oxide. Luminescence of Er³⁺ in the near infrared was observed for the active nanocomposite.     

Growth and scintillation properties of Pr doped Gd3(Ga,Al)5O12 single crystals

Pr:Gd₃(Ga,Al)₅O₁₂ single crystals were grown by the micro-pulling down (μ-PD) method. All grown crystals were greenish and transparent with 3.0 mm in diameter, 15–30 mm in length. Neither visible inclusions nor cracks were observed. Luminescence and scintillation properties were measured. The substitution at the Al³⁺ sites by Ga³⁺ in garnet structure has been studied. In these crystals, Pr³⁺ 5d–4f emission is observed with 340 nm wavelength. Pr1%:Gd₃Ga₃Al₂O₁₂ shows highest emission intensity. The light yield of Pr:Gd₃Ga₃Al₂O₁₂ sample with 3 mmφ×1 mm size was around 4500 photon/MeV. Scintillation decay time was 7.9 ns (0.5%), 46 ns (0.7%) and 214 ns (98.8%).     

Growth improvement and quality evaluation of ZnGeP2 single crystals using vertical Bridgman method

ZnGeP₂ single crystals were grown using two-temperature zone vertical Bridgman method. The effect of crucible material, crucible shape, and cooling program on the growth of the ZnGeP₂ crystal was investigated. The qualities of the crystals were evaluated by high resolution X-ray diffraction, X-ray fluorescence spectrometry, and IR transmittance spectra. The results show that the full width at half maximum of the rocking curves for (200), (004), and (220) faces are 45″, 37″, and 54″, respectively. The concentration of the P, Zn and Ge are almost homogeneous along the growth axis, but P and Zn are slightly deficient compared with Ge in the as-grown ZnGeP₂ crystals. The increase of annealing temperature from 600 °C to 700 °C has little effect on the reduction of the absorption losses in ZnGeP₂ powders, and has negative effect on the reduction of the absorption losses in ZnP₂ powders. Annealed in ZnP₂ powders at 600 °C for 300 h, the optical absorption loss at 2.05 μm reduce by 37%, compared with that of 27% reduction annealed in ZnGeP₂ powders.     

The local atomic environment of oxygen in silicate glasses from molecular dynamics

Recent molecular dynamics (MD) results for (Na₂O)_x(SiO₂)_1−x and (CaO)_x(SiO₂)_1−x glasses show that co-ordination of bridging oxygen (O_b) by modifiers (M) is a normal structural feature. This can be explained as a consequence of the limitation on oxygen co-ordination in network oxides, a common rule of thumb being that total co-ordination of oxygen by (Si + M) is ⩽4. This gives an upper limit on co-ordination of non-bridging oxygen (O_nb) by modifiers of N_OnbM ⩽ m with m = 3, corresponding to N_MOnb ⩽ mv, where v is modifier valence. If modifier co-ordination exceeds this limit, i.e. N_MO > mv, then there is bonding of O_b to modifiers, i.e. N_ObM > 0. This is the case in alkali and alkaline earth silicate crystals (apart from Be and Mg), and is predicted to be a feature of glasses in these systems. An illustration of the influence of oxygen co-ordination is given from MD models of (CaO)_0.33 (SiO₂)_0.67 glass at pressures of 5 and 10 GPa. The main effect of densification is to increase the co-ordination of Ca by O_b. This can be understood because at 0 GPa the co-ordination of O_nb by Ca is already high, with N_OnbCa ∼ 2.7, but the co-ordination of O_b by Ca is less high, with N_ObCa ∼ 1, and so can more easily increase.     

Comparison of growth methods for Si/SiO2 nanostructures as nanodot hetero-emitters for photovoltaic applications

Two different growth mechanisms are compared for the fabrication of Si/SiO₂ nanostructures on crystalline silicon (c-Si) to be used as hetero-emitter in high-efficiency solar cells: (1) The decomposition of substoichiometric amorphous SiO_x (a-SiO_x) films with 0 < x < 1.3 and (2) the dewetting of thin amorphous silicon (a-Si) layers.The grown layers are investigated with regard to their structural properties, their passivation quality for c-Si wafer substrates and their electrical properties in order to evaluate their suitability as a nanodot hetero-emitter. While by layer decomposition, no passivating nanodots could be formed, the dewetting process allows fabricating nanodot passivation layers at temperatures as low as 600 °C. The series resistance through Ag/[Si-nanodots in SiO₂]/c-Si/Al structures for dewetting is similar to nanostructured silicon rich SiO_x films. Still, a nanodot hetero-emitter which exhibits both a satisfying passivation of the substrate and induces a high band bending by doping at the same time could not be fabricated yet.     

Photosensitivity of SiO2–Al and SiO2–Na glasses under ArF (193 nm) laser

Photosensitivity of SiO₂–Al and SiO₂–Na glass samples was probed by means of the induced optical absorption and luminescence as well as by electron spin-resonance (ESR) after irradiation with excimer-laser photons (ArF, 193 nm). Permanent visible darkening in the case of SiO₂–Al and transient, life time about one hour, visible darkening in the case of SiO₂–Na was found under irradiation at 290 K. No darkening was observed at 80 K for either kind of material. This investigation is dedicated to revealing the electronic processes responsible for photosensitivity at 290 and 80 K. The photosensitivity of both materials is related to impurity defects excited directly in the case of SiO₂–Na and/or by recapture of self-trapped holes, which become mobile at high temperature in the case of SiO₂–Al. Electrons remain trapped on the localized states formed by oxygen deficient defects.