Shape evolution of detached Bridgman crystals grown in microgravity

Detached (or dewetted) Bridgman crystal growth defines that process in which a gap exists between a growing crystal and the crucible wall. In microgravity, the parameters that influence the existence of a stable gap are the growth angle of the solidifying crystal, the contact angle between the melt and the crucible wall, and the pressure difference across the meniscus. During actual crystal growth, the initial crystal radius will not have the precise value required for stable detached growth. Beginning with a crystal diameter that differs from stable conditions, numerical calculations are used to analyze the transient crystal growth process. Depending on the initial conditions and growth parameters, the crystal shape will either evolve towards attachment at the crucible wall, towards a stable gap width, or inwards towards eventual collapse of the meniscus. Dynamic growth stability is observed only when the sum of the growth and contact angles exceeds 180°.     

Study of crystalline perfection and thermal analysis of zinc cadmium thiocyanate single crystals grown in silica gel

Zinc cadmium thiocyanate ZnCd(SCN)₄ abbreviated as ZCTC is a bimetallic thiocyanate complex that exhibits excellent nonlinear optical property. Single crystals of ZCTC have been grown in silica gel by the process of diffusion. Colorless transparent crystals of size 12 mm x 2 mm x 1.3 mm have been obtained. High resolution X‐ray diffraction study was carried out to investigate the crystalline perfection of the grown crystal and the quality of the crystal was found to be quite good. Thermal stability of the grown crystal was investigated by thermogravimetric and differential thermal analyses. Fourier Transform Infrared spectrum was recorded to confirm the functional groups. Microhardness of the crystal is also studied. Being a nonlinear optical material, a comparative study of its second harmonic generation efficiency with urea has been made. (© 2007 WILEY ‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Polar dependence of impurity incorporation and yellow luminescence in GaN films grown by metal-organic chemical vapor deposition

We have investigated the unintentional impurities, oxygen and carbon, in GaN films grown on c-plane, r-plane as well as m-plane sapphire by metal-organic chemical vapor deposition. The GaN layer was analyzed by secondary ion mass spectroscopy. The different trend of the incorporation of oxygen and carbon has been explained in the polar (0 0 0 1), nonpolar (1 1 2¯ 0) and semipolar (1 1 2¯ 2) GaN by a combination of the atom bonding structure and the origin direction of the impurities. Furthermore, it has been found that there is a stronger yellow luminescence (YL) in GaN with higher concentration of carbon, suggesting that C-involved defects are originally responsible for the YL.     

On the variation of the impurity composition in LaF3 crystals grown by the Bridgman-Stockbarger method

Single crystals of the composition LaF₃ with CeF₃, YF₃, and SrF₂ impurities have been grown by the Bridgman-Stockbarger method in a graphite crucible with six cells. The CeF₃ concentration has been measured in the LaF₃ charge prior to the growth experiment and in the grown LaF₃ crystals upon it. It is shown that, depending on the growth conditions, the CeF₃ concentration can change during crystallization from 25% of its initial concentration in the charge up to 550%.     

Effect of rhodium incorporation in KTiOPO4 single crystals grown from self‐flux

The presence of an impurity like rhodium in the platinum crucible used for the growth of KTiOPO₄ (KTP) single crystals can have severe consequences in the performance of devices made from these crystals. In the present study the effect of rhodium incorporation has been investigated. Rhodium‐incorporated KTP crystals have a lower ionic conductivity (1.3 × 10^–7 S/cm at 100 kHz) than pure KTP crystals (3.5 × 10^–6 S/cm at 100 kHz) along the c‐axis. And the optical absorption in the green‐wavelength regime leads to a detrimental effect on their SHG performance. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Complex formation and background impurity of Oxygen in the PbTe:Eu doped crystals grown from melt by the Bridgman method

The magnetic field dependence of magnetization M(B) at the temperature 1.72 K in magnetic fields up to 5 T and the temperature dependence of magnetic susceptibility (M_S) χ(T) in the temperature range 1.7–400 K of six PbTe:Eu samples with the concentration of Eu impurity of the order of 1×10¹⁹–1×10²⁰ cm^?3, prepared from the doped crystals grown from the melt by the Bridgman method, have been investigated. It is shown that the dependence of M(B) and χ(T) can be quantitatively explained by the contribution of the single centers of Eu ions, their pairs, and the matrix of the doped crystals using the same set of parameters for each sample. This is true provided we use in our analysis the values of the exchange integrals between Eu ions in EuO normalized with the lattice constant of PbTe, i.e., J₁/k_B=0.056 K for the ferromagnetic interaction of the NN (nearest neighbor) pairs and J₂/k_B=?0.13 K for the antiferromagnetic interaction of the NNN (next nearest neighbor) pairs, as well as different values of the M_S of crystal χ_matrix. It is revealed that the probability of the formation of complexes based on the magnetic impurity pairs is higher in the incipient section of a doped ingot, and it decreases towards the ingot end where the single centers of Eu ions become the only centers of the impurity. We conclude that the pairs of Eu²⁺ ions, which are formed during the growth of the PbTe:Eu ingots from melt by the Bridgman method, are the constituents of the complexes of the magnetic impurities with the background Oxygen impurities in the crystal matrix of the doped lead telluride. It is shown that the formation of the complexes leads to an increase of the M_S of crystal matrix χ_matrix and can even cause the change of its sign from minus to plus, i.e., it can convert the crystal matrix from the diamagnetic to paramagnetic state. The possible causes of this effect are analyzed.     

On the incorporation mechanism and distribution mode of impurities in potassium acid phthalate crystals grown from aqueous solutions

Mechanism of impurity structure formation in crystals grown from aqueous solutions has been studied on the example of potassium acid phtalate (abbreviated hereafter as KAP) single crystals. Gold decoration technique at an electronmicroscopic scale has been applied to the study of the distribution of uncontrolled impurities on KAP cleavage face (010) after 10, 20 and 30 days of growth, taking into consideration different growth rates in 〈001〉 and 〈001 〈 directions. A technique for visualization of impurities in water, based on the adsorption of these impurities by the surface of amorphous film of nitrocellulose (parlodion) and the vacuum decoration with gold of these impurities, has been developed. Differences in the impurity structure of KAP regions located in 〈001〉 and 〈001〉 directions from the seed have been established. In 〈001〉 direction after 20 days of growth impurity assemblies 0.1—0.4 μm in size are revealed, and in 〈001〉 direction heterogeneous impurity structure is revealed only after 30 days of growth. The real (impurity) structure of KAP outside impurity assemblies is quite homogeneous and is the same throughout the whole crystal volume, the impurities incorporating mainly into complex active centres. From comparison of the changes in time of the impurity structure of water used for crystallization solutions and the impurity structure of KAP crystals a conclusion is made that the impurity structure of crystals is “programmed” in the impurity structure of crystallization solutions which regularly changes with time, i. e. impurities from different kinds of assemblies which are selectively adsorbed by the growing crystal faces. The role of the adjacent to the growing face interfacial layers which control the growth rate and have a complex impurity structure is stressed.     

The influences of O/Zn ratio and growth temperature on carbon impurity incorporation in ZnO grown by metal-organic chemical vapor deposition

The strong correlations between the O/Zn ratio and carbon impurity incorporation have been observed on the ZnO films grown using N₂O or O₂ as oxygen source in metal-organic chemical vapor deposition (MOCVD). From in-situ mass spectrometric measurements, the O/Zn ratio in the MOCVD reactor is found to decrease to a minimum value as the growth temperature increased till a critical growth temperature T_c, and then increased above T_c due to different dissociation rates of the oxygen and Zn sources. The strongest D and G modes, which are ascribed to carbon clusters sp² related modes, have been observed in Raman scattering spectroscopy for the ZnO samples grown at T_c, indicating the highest incorporation rate of carbon impurity in the samples grown at T_c. Compared with O₂, N₂O has a low dissociation rate and that leads to a lower value of O/Zn ratio, resulting in much stronger D and G modes and higher incorporation rate of carbon impurities in the samples grown at T_c. It is interesting to note that the lowest specific resistances from Hall effect measurements were also obtained on the samples grown at T_c, indicating possible electrical contributions from the formation of carbon clusters, which should be highly conductive regions in ZnO. Furthermore, ionization or addition of H₂ in the case of N₂O can significantly enhance the dissociation of N₂O, with film quality improved significantly. This study shows that a high O/Zn ratio is critical to suppress carbon impurity incorporation and to grow high quality ZnO by MOCVD, especially at low growth temperature.     

A comparative study of pure and potassium doped cadmium mercury thiocyanate single crystals grown in silica gel

Pure and potassium doped cadmium mercury thiocyanate single crystals have been obtained from silica gel by the process of diffusion. The X‐ray diffraction studies reveal the crystal lattice of both pure and doped crystals to be tetragonal. The crystalline perfection of the grown crystals were investigated by high resolution X‐ray diffraction analysis and the quality of the crystals are found to be extremely good. Transmission and Fourier transform infrared spectra were recorded for the grown crystals. The TG/DTA analyses show that the crystals are highly thermally stable. The mechanical strength of the crystals were studied by Vickers microhardness test and a study of their second harmonic generation efficiency in comparison with urea has been made by performing Kurtz powder test. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Low-temperature growth and doping of InP:S single crystals from melt-solution in microgravity

The effect of microgravity on the growth of bulk InP:S single crystals from a melt with an initial equilibrium composition (84 at % In, 16 at % P, and ～2.2 × 10¹⁸ at cm^?3 of S) on board the Foton-11 satellite was investigated. The growth of crystals on board the satellite and on Earth (a reference crystal) was carried out by the traveling heater method. The samples of the grown crystals were investigated by metallography, double-crystal X-ray diffractometry, single-and double-crystal X-ray topography, and secondary-ion mass spectrometry. It is shown that the mass transfer in the melt in microgravity is similar to the diffusion mode. Hence, the mass transfer in the melt results in the following: the formation of a nonstationary boundary layer, depleted in phosphorus; the constitutional supercooling at the crystallization front accompanied with the development of a cellular substructure in the early growth stage; and the hypothetical phase structurization of the transition layer with the formation of In-based associates (clusters), which were found in the grown crystals in the form of spherical defects 10–20 μm in diameter. The coefficients of sulfur distribution k₀ = 0.274 and k_eff = 0.43, the sulfur diffusivity in the melt D_S = 4.2 × 10^?7 cm²/s, and the effective thickness of the transition layer δ = 0.07 cm in terrestrial gravity are determined. The data obtained are necessary to develop a mathematical model of crystallization in zero gravity.     

The influence of melt stirring intensity on the axial impurity distribution in proustite single crystals grown by the Stockbarger method using ACRT

The influence of modulated crucible rotation on the axial distribution of Cu, Mg and Si impurities in proustite single crystals grown by the Stockbarger method using ACRT is studied in a wide range of Taylor numbers (1.9·10⁵ < Ta < 7.12·10⁷). The impurity content in the upper part and in the tail portion of the grown crystals is measured using X‐ray‐phase analysis. Micro and macrostriations are observed in the grown crystals. The wavelengths of impurity content fluctuations have been determined. The microfluctuations of axial impurity content are caused by modulated crucible rotation. The studies have revealed that the ACRT provides an effective removal of impurities from the main part of the grown crystal at high intensity of melt stirring, and consequently, the ACRT can be applied validly to decrease the impurity content during the growing of high‐quality proustite single crystals. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The role of impurity dispersion and hardness anisotropy studies in NaCl:Sm crystals

The effect of the distributed impurities and their state of dispersion on hardness of crystal is studied from microhardness measurements on NaCl crystals doped with different concentrations of samarium both in as-grown state as well as after quenching them from various elevated temperatures. It is found that the small additions of impurity decrease the hardness. This has been explained on the basis of formation and dissolution of precipitates     

The influence of the intensity of melt stirring on the radial impurity distribution in proustite single crystals grown by the Stockbarger method using ACRT

The influence of the intensity of melt stirring on the radial impurity distribution and optical quality of proustite single crystals grown by the Stockbarger method using ACRT is studied by the example of Cu, Sb and Mg impurities. We report results obtained in a wide range of Taylor numbers (1.9×10⁵<Ta<7.12×10⁷). The studies revealed that the ACRT can be applied validly to decrease the impurity content during the growing of high-quality single crystals.     

A study of structural perfection of interfaces in Si/SiGe superlattices

A complex investigation into the structural perfection of the Si/SiGe superlattices grown by molecular-beam epitaxy at different temperatures of the Si substrate has been carried out by high-resolution X-ray diffraction analysis, secondary ion mass spectrometry (SIMS), and transmission electron microscopy (TEM). It is demonstrated that the combination of these methods makes it possible to describe in sufficient detail the distributions of the strains and Ge concentrations in the elastically strained superlattices and also to evaluate the sharpness of the layer interfaces. It is shown that the densitometry of electron microscope images of the superlattice cross-sections permits characterization of the relative sharpness of the layer interfaces and a qualitative representation of the Ge distribution throughout the thickness of the SiGe layers.     

State of dispersion of impurity on formation of Z1-centres in NaCl:Sr2+ crystals

Thermoluminescence (TL) and Optical absorption studies are carried out to study the bleaching kinetics of F-centre and formation of Z₁-centres in X-irradiated strontium doped NaCl crystals, both in as-grown state and in quenched state. An additional TL peak is observed in this crystal system after F-bleaching of irradiated samples which is attributed to thermal annealing of Z₁-centres. Various models proposed for Z₁-centres are reviewed in the light of these results and it is concluded that Z₁-centres are associated with impurity-vacancy dipoles.     

Study of microdefects in GaAs:Si single crystals grown by the vertical gradient freeze method

Microdefects in Si-doped GaAs single crystals grown by the vertical gradient freeze method have been studied with X-ray diffuse scattering. In the case of doping to majority carrier concentrations n ～ 1 × 10¹⁸ cm^?3, large microdefects with positive dilatation that accompany the initial stage of arsenic precipitation at high temperatures were observed. It is shown that GaAs samples heavily doped with silicon (n ～ 3 × 10¹⁸ cm^?3) contain large (several micrometers) interstitial microdefects, which can play the role of nucleation regions for new SiAs and SiAs₂ phases.