Directional solidification experiments have been carried out in eutectic and off eutectic composition in Al
O
-YAG-ZrO
system using the micro-pulling down (μ-PD) technique. The formation of different ...microstructures of stable phases, over a range of velocities studied, is discussed. During the solidification process, coupled growth microsctrucure can occur at off eutectic composition. Colonies and dendrites were analysed as a function of the composition and velocity. Increasing the solidification rate resulted in a smaller grain size and instability of the crystallization interface with respect to ZrO
segregation at the solidification front. Increasing ZrO
in the eutectic composition showed enhanced tendency for colony and dendrite formation. When the velocity was extended to the rapid solidification regime (>1.75 mm min
), the geometrical pattern microstructure was found to grow and a change in the growth orientation of Al
O
corresponding to
-axis (0 0 01) was observed. Whatever the pulling rate, during the eutectic solidification the Al
O
and ZrO
:Y phases' growth competition was observed, and 〈100〉 YAG orientation was grown along the pulling direction.
•The evolutions of crystallization and surface morphology of the as-deposited GaN films were studied as a function of process conditions systematic variation.•E2-high Raman peak red shifts toward the ...unstrained 567.6cm−1 location were observed indicating a clear reduction in film tensile stress.•An increase in texture coefficient of (0002) orientation was recorded.•An improvement in average surface roughness to values less than 3nm was recorded.•The threading dislocations density dropped to ∼3×109cm−2.
Epitaxial GaN thin films are grown on conventional sapphire (0001) substrates using pulsed laser deposition in five different sets. The evolution of crystallization and surface morphology of the as-deposited films is studied to evaluate the influence of growth conditions such as: substrate heating, background N2 pressure, target-substrate distance, laser energy density, and substrate location, which were systematically varied. Upon the optimization of process conditions, E2-h Raman peak red-shifts toward the unstrained 567.6cm−1 location are observed, accompanied with an increase in texture coefficient of (0002) orientation, a decrease in average surface roughness to values less than 3nm, and threading dislocations density in the order of ∼3×109cm−2. This study accounts for the physics behind the careful tuning of PLD parameters as an approach to optimize the structural quality of GaN grown films on conventional sapphire substrates. The results could be a reference for further growth complications in PLD.
Tm3+-doped sesquioxides Y2O3, Lu2O3 and Sc2O3 are well known for their emissions around 2 μm (3F4 →3H6 transition). We study here their absorption and emission properties, both at 2 μm and at 1.5 μm ...(3H4 → 3F4) and 2.3 μm (3H4 → 3H5), by considering that the totality of the Tm3+ ions or only part of them are contributing to the line intensities. The study is also extended to the Tm3+-doped mixed compounds LuYO3, LuScO3 and YScO3. The results obtained in terms of radiative lifetimes and branching ratios, as derived from a Judd-Ofelt analysis of the absorption data, thus appear to be more consistent with the measured fluorescence decays, by assuming that only 75% of the Tm3+ ions, those sitting into C2 symmetry sites, are involved in the absorption and the emission spectra. The registered emission spectra are then calibrated in cross section unit and some of them compared between each other depending on the method (reciprocity, modified reciprocity or Füchtbauer method) which is used. Calculations are finally carried out to predict the cross section of important excited-state transitions which are involved in the two-step excitation pumping of the 3H4 emitting level around 1.05 μm (3F4 → 3F2,3) and 1.45 μm (3F4 → 3H4).
•Absorption and emission properties (cross section spectra, fluorescence lifetimes) of Tm-doped sesquioxides are almost exclusively governed by Tm3+ ions sitting in C2 symmetry sites.•3H4.→3F4 emission transitions in the Tm3+-doped sesquioxides as well as in their mixed compounds should deserve more attention for 1.55 μm laser operation under two-step diode pumping around 1.06 μm and 1.45 μm•3F4.→3H6 emission transitions around 2 μm in the mixed sesquioxides present both broad and high cross section spectra favorable for tunable and short-pulse laser operation•3H4.→3H5 emission transitions around 2.3 μm are too weak to be exploited as laser transitions
Gadolinium aluminium garnet (Gd
3
Al
5
O
12
) nanopowders, with 30 at% lutetium substitution have been successfully synthesized using the co-precipitation method. Several sources, such as aluminium ...nitrate or sulphate, have been used to develop the (Gd
0.7
Lu
0.3
)
3
Al
5
O
12
(GdLuAG) compound via normal and reverse add titration using ammonium hydrogen carbonate (AHC) as a precipitant. Different techniques such as X-ray diffraction, FTIR, Raman scattering, BET and SEM/EDS analysis were performed in this study. The X-ray results confirmed that the garnet compound crystallized in the cubic structure with group space Ia
3
¯
d. The crystallite sizes of the powders were determined by using Rietveld refinement analyses. The FTIR results of vibrational characteristics exhibit the internal and external bonds of GdLuAG. The He–Ne laser 633 nm Raman spectra showed a response in the second phonon region exhibiting strong fluorescence in the near-infrared (NIR). The GdLuAG internal Raman active mode in the visible range and excited by a 785 nm diode laser were measured. The X-ray, BET and SEM results indicated that aluminium nitrate reverse titration compound calcined at 1150°C is the best homogeneous and uniform sample.
Undoped and Ce3+-doped Lu3Al5O12 (LuAG) fibers were grown to evaluate their potential use in new particle physics experiments, such as dual-readout calorimeters. The choice of grown crystals was made ...to detect scintillation (doped LuAG) and Cherenkov radiation (undoped LuAG). Growth conditions for obtaining fibers with improved quality were found based on measurements of attenuation length of the fibers and cathodoluminescence measurements. The effect of annealing on attenuation length for LuAG and LuAG:Ce was also studied. In addition, we also evaluated a possibility to substitute LuAG by the cheaper mixed and (Lu,Y)3Al5O12 (LuYAG:Ce) fibers.
•Undoped and Ce-doped LuAG fibers were grown by micro-pulling down technique.•Substitution of Lu3+ by Y3+ in (Lu1−xYx)3Al5O12 fibers improve Ce distribution.•Annealing of Ce-doped LuAG fibers improve the light propagation through the fibers.•Attenuation length over 1m can be achieved in LuAG:Ce after thermal annealing.
Directional solidification experiments have been carried out in eutectic and off eutectic composition in Al2O3–YAG–ZrO2 system using the micro-pulling down (μ-PD) technique. The formation of ...different microstructures of stable phases, over a range of velocities studied, is discussed. During the solidification process, coupled growth microsctrucure can occur at off eutectic composition. Colonies and dendrites were analysed as a function of the composition and velocity. Increasing the solidification rate resulted in a smaller grain size and instability of the crystallization interface with respect to ZrO2 segregation at the solidification front. Increasing ZrO2 in the eutectic composition showed enhanced tendency for colony and dendrite formation. When the velocity was extended to the rapid solidification regime (>1.75 mm min−1), the geometrical pattern microstructure was found to grow and a change in the growth orientation of Al2O3 corresponding to c-axis (0 0 01) was observed. Whatever the pulling rate, during the eutectic solidification the Al2O3 and ZrO2:Y phases' growth competition was observed, and 〈100〉 YAG orientation was grown along the pulling direction.
Under a stationary stable regime undoped and Ce-doped LuAG (Lu3Al5O12) single-crystal fibers were grown by a micro-pulling-down technique. The meniscus length corresponding to the equilibrium state ...was <200μm. Fluctuations in the fiber composition and pulling rate were found to have a significant effect on the properties of the fibers grown. A great improvement in the performance was found in samples containing low Ce concentrations (⩽0.1at.%) and produced using pulling rates <0.5mmmin−1. Under such conditions a good lateral surface fiber quality was obtained and light propagation was significantly improved. Conversely, a high Ce concentration and a high pulling rate resulted in a strong degradation of the fiber surface quality causing defects to appear and a decrease in light output.
•Growth interface shape in Cz growth of piezoelectric langatate crystals is numerically investigated.•Interface shape is mainly determined by the internal radiative effect in the semitransparent ...crystal.•Numerical results are in good agreement with experimental visualization of the growth interface.•The interface becomes almost flat when high rotation rate of the crystal (50rpm) is applied.•Applying crystal and crucible counter rotation could have deleterious effects on the growth interface.
Single La3Ga5.5Ta0.5O14 (LGT) crystals have been grown by using the Czochralski technique with inductive heating. Some ingots exhibit imperfections such as cracks, dislocations and striations. Numerical modeling is applied to investigate the factors affecting the shape of the crystal-melt interface during the crystallization of ingots having 3cm in diameter. It was found that the conical shape of the interface depends essentially on the internal radiative exchanges in the semi-transparent LGT crystal. Numerical results are compared to experimental visualization of the growth interface, showing a good agreement. The effect of the forced convection produced by the crystal and crucible rotation is numerically investigated at various rotation rates. Increasing the crystal rotation rate up to 50rpm has a significant flattening effect on the interface shape. Applying only crucible rotation enhances the downward flow underneath the crystal, leading to an increased interface curvature. Counter rotation between the crystal and the crucible results in a distorted shape of the interface.
PDF
