Optical birefringence images of dislocations in large gallium phosphide crystals

The optical birefringence induced by the strain field of a dislocation has been used to follow dislocation lines through large gallium phosphide crystals. Dislocations could be seen whether their lines were parallel to the imaging light beam or were steeply inclined to it. The images obtained so far suggest that it would be possible to resolve individual dislocations in crystals that contain ?5 × 10⁵ dislocations per cm².     

The stability and surface reactivity of gallium phosphide nanocrystals

The stability and surface reactivity of Gal? nanocrystals were characterized by TG-DTA, XRD and XPS measurements. The samples were heated in O₂ and N₂ atmospheres respectively. The experimental curve which describes the mass change as a function of temperature shows that the mass decreases under 330°C but increases in two stages between 33°C and 550°C. The curves obtained in O₂ and N₂ are obviously different. The results of XPS indicate that the density of oxygen atoms bonded chemically to the gallium atoms on the surface increases when the sample is heated in O₂ up to 200°C, whereas the density of nitrogen atoms increased when heated to 330°C in N₂. It can be concluded that the N₂ could be activated on the surface of GaP nanocrystals at a rather low temperature, and highly reactive nitrogen atoms formed. Our result obtained makes it possible to synthesize a series of nitrogen-containing compounds in N₂ gas under moderate conditions.     

On the initial stage of plastic deformation of sulphur-doped gallium phosphide

The overall deformation behaviour between room temperature and 1000 K has been investigated by bending and uniaxial compression at small strains ≦ 0.1%. Ductile-to-brittle transition temperature was found at 44% of the melting point. Dislocations are generated mainly at surface irregularities. Whereas screw dislocations predominate at the beginning 30° and 60° segments become more important after cross slip and interaction of different slip systems.     

Revealing of lattice defects on (111) faces of gallium phosphide and indium phosphide by chemical etching

A solution is presented suitable for revealing lattice defects on (1 1 1 ) phosphorous faces of GaP and InP. It is possible to distinguish between pits originated by grown-in dislocations and microdefects such as perfect loops, faulted loops and precipitates and/or inclusions. Moreover it is also possible to reveal large stacking faults of Shockley type and microtwin lamellae. One to one correlations have been given by means of transmission electron microscopy.     

Coupled substitution of gallium by magnesium and zirconium in single crystals of gadolinium gallium garnet

Single crystals of gadolinium gallium garnet in which the gallium ions on the octahedral sites were partially substituted by coupled substitution of magnesium and zirconium have been grown using the Czochralski technique. Single crystals of 36 mm in diameter and 100 mm in length corresponding to the formula Gd₃Ga_5-x-yMg_xZr_yO₁₂ have been obtained from melt compositions in which 0.1 ? x = y ? 0.7. The dislocation and inclusion densities of the single crystals are below 5 cm^-2. The lattice parameters increase linearly from 12.382 Å for gadolinium gallium garnet to 12.489 Å for x = y ≈ 0.54. The distribution coefficient increases in this concentration range from K_eff = 0.58 to K_eff = 0.89. Boules grown from melt compositions in which x = y ? 0.7 appear cloudy through precipitation of Gd₂Zr₂O₇ as a second phase.     

Morphology of gallium orthophosphate crystals grown under hydrothermal conditions

Gallium orthophosphate crystals with sizes of 5 mm and more have been obtained by spontaneous nucleation from hydrothermal 5.6 – 15 M orthophosphoric acid solutions. Preferable concentrations of solvent were found in the range of 11 – 12 M, the temperature difference should not be over 6–10°C at the heating rate of 4 – 5 °C/day. Morphological investigations are carried out using optical and polarizing microscopies. GaPO₄ crystals, like quartz and berlinite, tends to grow with well developed {1 1}, {0 1}, {1 0}, {1 2}, and {0 2} faces, and they were divided into three habit types. Effect of orthophosphoric acid concentration on the crystal habit has been studied.     

Growth and investigation of substituted gadolinium gallium garnet laser crystals

Gadolinium gallium garnets substituted with (Ca,Zr), (Mg,Zr) and (Ca,Mg,Zr) are considered to be potential new laser materials. Such substituted GGG boules double-doped with (Nd,Cr) and single-doped with Cr have been grown by the Czochralski method. Color center investigation and lattice constant determination have been carried out. Laser output slope efficiency for a GGG(Ca,Mg,Zr) : (Nd,Cr) rod of dimensions 5 mm diameter by 40 mm long is greater than 0.7%.     

Growth spirals of gadolinium gallium garnet (GGG) crystals

Slices cut from various locations in two GGG spirals grown by the Czochralski method have been studied using double crystal X-ray topography. In selected regions of the slices the lattice parameter has been measured by a modified Bond method. An increase of lattice parameter has been found in the outer part of the crystal where growth bands have smaller periodicity than those in the central region of the spiral. An analysis of crystallization front changes allowed us to suppose that the asymmetry of the heat field around the growing crystal, and the forced convection of the melt, are responsible for the crystal's growing in the shape of a spiral.     

Growth and raman scattering of aluminium gallium orthophosphate piezoelectric crystals

A new series of non-ferroelectric crystals, the piezoelectric solid solution aluminium gallium orthophosphate Al_1-xGa_xPO₄ (AGP, 0 < x < 1) system has been successfully developed by a hydrothermal method. The AGP crystal with X = 0.10 shows a phase transition temperature T_−β = 587 ± 3 °C with a cell parameters A = b = 0.49386(3) and C = 1.09563(2) nm. Raman spectra including directional dispersion in AGP are similar to those from -AIPO4 and -quartz crystals, indicating all of them belong to an isomorphic family, the short-range interaction being dominant in AGP. Comparison with -AIPO4 crystals, there exists an apparent red shift of the mode frequency and a smaller TO/LO mode splitting of the E species in the AGP crystals.     

X-ray topographic studies of the melting of nearly perfect gallium crystals

The temperature dependence of the concentration and shape of long dislocation lines in pure, nearly perfect gallium crystals has been measured by X-ray topography in anomalous transmission (Borrmann-topography). The dislocation structure did not change appreciably between temperatures of 22°C and at least 0.1 mK below the melting point T_m = 29.75°C. A few dislocations decreased their length due to tempering at the melting point. Pre-melting effects could not be detected.     

Characterization of SHG candidate crystals in barium aluminum (gallium) borates

Crystals listed in the JCPDS card in BaO‐B₂O₃‐Al₂O₃ and BaO‐B₂O₃‐Ga₂O₃ systems were synthesized by means of a solid‐state reaction method and a liquid‐state reaction method. Synthesized crystals were characterized from a viewpoint of Second Harmonic Generation (SHG) and crystal growth possibility. Ba₅Al₄B₁₂O₂₉ and Ba₅Ga₄B₁₂O₂₉ are promising new candidate crystals from the viewpoint of SHG activity and congruent melting behavior. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Preparation and characterization of Nd doped gadolinium gallium garnet nanopowders and crystals

The structural and optical features of gallium gadolinium garnet (GGG) nanopowders doped with neodymium were investigated. Nanopowders of GGG:Nd were prepared by modified sol‐gel method using acetic acid as complexing agent. This way permitted to incorporate large amounts of dopant (up to 10 %) without destroying garnet structure. Small single crystals of Nd:GGG were grown by a μ‐pulling down method and spectroscopic features of nanopowders and their single crystal counterparts were compared. It has concluded that the Nd³⁺ ions are located preferentially in the same type of sites in crystal lattices of GGG:Nd nanopowders and their single crystal counterparts. In addition, it follows from the perfect agreement of emission wavelengths and line width recorded for nanopowders and single crystal samples that the crystal lattice of GGG in nanopowders is not distorted. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Investigation of the variations in the crystallization front shape during growth of gadolinium gallium and terbium gallium crystals by the Czochralski method

Numerical investigation of the variations in the crystallization front shape during growth of gadolinium gallium and terbium gallium garnet crystals in the same thermal zone and comparison of the obtained results with the experimental data have been performed. It is shown that the difference in the behavior of the crystallization front during growth of the crystals is related to their different transparency in the IR region. In gadolinium gallium garnet crystals, which are transparent to thermal radiation, a crystallization front, strongly convex toward the melt, is formed in the growth stage, which extremely rapidly melts under forced convection. Numerical analysis of this process has been performed within the quasistationary and nonstationary models. At the same time, in terbium gallium garnet crystals, which are characterized by strong absorption of thermal radiation, the phase boundary shape changes fairly smoothly and with a small amplitude. In this case, as the crystal is pulled, the crystallization front tends to become convex toward the crystal bulk.     

Effect of small copper additives on the growth of gallium crystals from a melt

It is shown in the present work that copper additives influence substantially the growth of Ga crystals from a melt which is expressed in different ways, depending on the impurity concentration. With rising copper concentrations we observed a substantial retarding in the growth rates as compared with deformed pure Ga crystals, and a change of the mechanism of growth. The results can be explained by a rejection of the impurity by the growing crystal which causes retarding of the growth and with the impurity inclusion like CuGa₂ particles, which gives rise to dislocations, intersecting the growing faces.     

The role of structural defects in diffusion of elements of group VI in semi-insulating gallium arsenide crystals

The combined effect of the changes in the number and type of vacancies and dislocation density on selenium and sulfur diffusion in single crystals of undoped semi-insulating gallium arsenide has been studied. The differences in the diffusion mechanisms in the subsurface region of samples with an initial deficiency in gallium or arsenic are established as well as the dependence of the effective radius of arsenic trapping by dislocations on the ratio of the concentrations of gallium and arsenic vacancies.     

The role of dislocations in the formation of mechanical stresses during annealing of gallium arsenide single crystals

The effect of dislocations on the change of mechanical stresses in undoped semi-insulating gallium arsenide single crystals has been studied during their annealing in vacuum and in the arsenic atmosphere. The phenomena observed are explained by the effect of dislocations playing the role of channels for arsenic diffusion on the concentration of intrinsic point defects in the crystal regions surrounding dislocations. The mechanism of arsenic diffusion over dislocations allows one to consider dislocations as sources and sinks of arsenic without the translational and twinning processes and, thus, makes the well-known data on the reactions of dislocation interaction with point defects and the experimental structural data for single crystals more consistent.     

Specific features of the formation of dislocation structure in gallium arsenide single crystals obtained by the Czochralski method

The influence of the deviation of seed orientation from the [100] direction on the formation of a dislocation structure of gallium arsenide single crystals grown by the Czochralski method has been revealed. The intensive multiplication of dislocations and formation of a block structure occur at deviation by an angle of more than 3° in the region that is radially shifted to one of crystal sides. The linear density of dislocations in the walls changes from 1 × 10⁴ cm^–1 in low-angle boundaries to 6 × 10⁴ cm^–1 in subboundaries.     

Synthesis of gallium nitride by ammonia injection into gallium melt

Gallium nitride (GaN) was synthesized by injecting ammonia gas into molten gallium at 900–980°C under atmospheric pressure. A large amount of GaN powder was reproducibly obtained using a simple apparatus. The synthesized powder was characterized by scanning electron microscopy, X-ray diffraction, photoluminescence and energy dispersive X-ray spectroscopy, and was found to consist of fine crystals of hexagonal GaN of good quality. The total of GaN obtained was far more than the amount calculated from expected saturation solubility in the Ga melt at that temperature. We speculate that the GaN crystals were largely formed by direct reaction between Ga and the gaseous N source at the surface of the NH₃ bubbles in the melt. GaN synthesized by this method may be useful as a starting material for bulk growth.