Vertical Bridgman Growth Research Articles

High-pressure vertical Bridgman growth of lead magnesium niobate–lead titanate single crystal

A modified high-pressure vertical Bridgman (HPVB) method has been used to grow single crystals of lead magnesium niobate–lead titanate (PMN–PT) using self-seeding. The starting charge was prepared close to stoichiometric composition of (1− x)Pb(Mg 1/3Nb 2/3)O 3– xPbTiO 3, ranging from x=0.29 to x=0.32. The ingots grown had a top diameter of ∼114 mm and weighing ∼1.9 kg with a volume of ∼230 cc. Standard form off-the-shelf platinum crucibles from vendors with a wall thickness ∼0.34±0.01 mm were used. The thinner platinum helped to attain a larger radial thermal gradient as well as enhancing the cost effectiveness of the process by reducing usage of precious metals. An oxygen partial pressure of 8000–10 000 ppm was used and controlled during the entire growth process. Self-seeding technique was used in order to attempt to grow large diameter single crystals of PMN–PT. The composition variation in the ingot is dependent on the growth rate, thermal gradients and oxygen partial pressure. The segregation coefficient of Ti was 0.86±0.008 for the starting composition of x=0.32.

Interface shape control using localized heating during Bridgman growth

Numerical calculations were performed to assess the effect of localized radial heating on the melt–crystal interface shape during vertical Bridgman growth. System parameters examined include the ampoule, melt and crystal thermal conductivities, the magnitude and width of localized heating, and the latent heat of crystallization. Concave interface shapes, typical of semiconductor systems, could be flattened or made convex with localized heating. Although localized heating caused shallower thermal gradients ahead of the interface, the magnitude of the localized heating required for convexity was less than that which resulted in a thermal inversion ahead of the interface. A convex interface shape was most readily achieved with ampoules of lower thermal conductivity. Increasing melt convection tended to flatten the interface, but the amount of radial heating required to achieve a convex interface was essentially independent of the convection intensity.

Single-Crystal Growth of the Ternary BaFe2As2 Phase Using the Vertical Bridgman Technique

Ternary Ba–Fe–As system has been studied to determine a primary solidification field of the BaFe2As2 phase. We found that the BaFe2As2 phase most likely melts congruently and primarily solidifies either in the FeAs excess or BaxAs100-x excess liquid. Knowing the primary solidification field, we have performed the vertical Bridgman growth using the starting liquid composition of Ba15Fe42.5As42.5. Large single crystals of the typical size 10×4×2 mm3 were obtained and their quality was confirmed by X-ray Laue and neutron diffraction.

Thermosolutal convection and solute segregation during the vertical Bridgman growth of Hg 1−xCd xTe single crystals

The mechanism for thermosolutal convection and the coupling effects of flow, temperature and solute fields during the vertical Bridgman growth of Hg 1− x Cd x Te single crystals have been numerically analyzed. The calculations take into account the thermophysical properties and their dependence on temperature and composition. The results show that there are two main thermal convection cells in the melt caused by two temperature gradients during the growth of Hg 1− x Cd x Te single crystals. The stabilizing axial solute gradient in the melt will significantly damp the thermal convection cells when the absolute value of the solute Rayleigh number is close to the value of the thermal Rayleigh number. When there is a large solute gradient in the melt, the thermosolutal convection will become unstable, and the upper flow cell will evolve into a two-cell flow pattern. During the growth of Hg 1− x Cd x Te single crystals, the radial solute segregation in the melt develops non-monotonically with two minima values. Thus, the methods that solely aim at either damping or enhancing the thermosolutal convection are not always able to improve the radial solute segregation.

Segregation control of vertical Bridgman growth of Ga-doped germanium crystals by accelerated crucible rotation: ACRT versus angular vibration

The control of dopant segregation by accelerated crucible rotation was investigated for the vertical Bridgman growth of Ga-doped germanium crystals. The accelerated crucible rotation technique (ACRT) and the angular rotation technique (AVT) were considered. Both techniques were found effective in radial segregation control; however, ACRT generated more global mixing and thus slightly larger axial segregation. Computer simulation was also carried out for the experiments. The results were also consistent with previous simulation predictions and low-temperature transparent experiments.

Numerical investigation of non-isothermal phase change phenomena in vertical Bridgman crystal growth

The non-isothermal phase change phenomena during the vertical Bridgman growth process for HgCdTe are numerically investigated using an interface capturing finite element scheme. The influence of the growth parameters such as Bi, Ste, U and the flow parameters including Gr T and Gr S on the non-isothermal phase change phenomena are obtained. Some new features about the melt/crystal interface shape, the temperature field near the interface and the flow field are revealed by comparing the non-isothermal phase change with the isothermal phase change. Furthermore, the comparison of the non-isothermal interfacial characteristics between the pure diffusion, the natural convection and the double-diffusive convection is made and the obvious differences are presented.

Influence of rotational vibrations on heat and mass transfer during vertical bridgman growth F germanium crystals

The effect of high-frequency vibrations with small amplitude on the directional solidification of binary alloys is studied numerically. The calculations are made in the framework of a quasi-static approach for gallium doped germanium. The finite volume method is used. It is shown that in terrestrial conditions the influence of rotational vibrations on the radial dopant segregation is rather weak: quite high intensity of vibrations is needed to achieve a significant decrease in the dopant segregation. Under reduced gravity conditions, the effect of vibrations is considerable: the radial dopant segregation becomes uniform even at low-intensity vibrations.

Effects of cycle patterns of accelerated crucible rotation technique (ACRT) on the flows, interface, and segregation in vertical Bridgman crystal growth

The accelerated crucible rotation technique (ACRT) has been used extensively in crystal growth for the control of flow and segregation. However, the amelioration of the spin-up and spin-down flows for a favorable growth has not yet been fully understood. In this study, we investigate several ACRT schemes and discuss their effects on the flow, interface morphology, and solute segregation during the vertical Bridgman growth of succinonitrile (SCN) containing acetone. Both numerical simulation and visualization experiments are conducted. In addition to the triangular and trapezoidal patterns, stair-up, stair-down, and pulse schemes are proposed for a better flow and mixing control without much interface deformation. For all the ACRT schemes investigated here, the simulated results are in good agreement with the observed ones.

Boron oxide fully encapsulated CdZnTe crystals grown by the vertical Bridgman technique

One of the main problems affecting the vertical Bridgman growth of CdZnTe crystals is the crystal–crucible contact. In this work, we show that it is possible to avoid this interaction by inserting a stable boron oxide liquid layer between the crucible and the melt. This layer improves the structural properties of the grown crystals, reducing the etch pit density to the 10 3 cm −3 range. The possible origins for the formation of a stable boron oxide layer are discussed.

Electronic and optical properties of bulk crystals of semiconducting orthorhombic BaSi 2 prepared by the vertical Bridgman method

BaSi 2 crystals were grown from a near-stoichiometric melt containing mixtures of Ba (99.9%) and Si (99.99999%) with Ba:Si at.% ratios of 36:64 (Ba-rich) and 30:70 (Si-rich) by the vertical Bridgman growth method. X-ray diffraction analysis revealed that the grown crystals were semiconducting orthorhombic BaSi 2. The samples produced were polycrystalline consisting of single crystal grains. Optical absorption measurements were carried out to identify the different types of optical transition at room temperature. All the bulk orthorhombic BaSi 2 grown exhibited an indirect transition with an indirect band-gap energy ( E g indir ) of 1.13 eV. Emissions from photoluminescence measurements at 15 K, that seem to be due to indirect transitions, revealed that there were impurities and crystal defects. To understand the experimental results, we calculated the electronic-band structure from first principles using a local density approximation (LDA) in the all-electron full-potential linearized augmented plane wave (FLAPW) method. The calculation is consistent with the experimental indirect band gap results giving an indirect band gap of 0.72 eV and a direct gap of 0.81 eV. The band gap arises from the Si 3p orbitals, while Ba 5d orbitals are strongly hybridized with the Si 3p orbitals in the conduction band.

Decreasing lateral segregation in cadmium zinc telluride via ampoule tilting during vertical Bridgman growth

We employ a 2D, planar computational model to study the effects of ampoule tilting on zinc segregation during the growth of cadmium zinc telluride in a low-gradient vertical Bridgman process. Tilting the axis of the ampoule away from the direction of gravity strongly affects the structure and strength of melt convection, which, in turn, modifies heat transfer and the shape of the solid–liquid interface. While these effects have relatively little direct effect on axial segregation, the pattern of lateral segregation is significantly changed by tilting. Optimal tilt angles are found that locally flatten lateral compositional profiles and that decrease a measure of overall segregation, compared to the case of growth in a perfectly vertical ampoule.

Forced convection induced thermal fluctuations at the solid–liquid interface and its effect on the radial alloy distribution in vertical Bridgman grown Ga 1−xIn xSb bulk crystals

The mode of redistributing the excess solute species at the melt–solid interface accumulated during the growth of bulk ternary semiconductor crystals is a key factor dictating the crystalline quality and spatial alloy composition. Since the solidification temperatures vary with alloy composition, a control over both heat and mass transport during crystal growth is necessary for achieving spatial compositional homogeneity in the grown crystals. The effects of forced and natural convections, as well as diffusion (in the liquid) on the radial alloy distribution in vertical Bridgman growth Ga 1− x In x Sb bulk polycrystals (20–50 mm diameter) have been investigated. In the absence of forced convection, the shape of the solid–liquid interface has been found to be highly curved under growth conditions necessary for avoiding constitutional supercooling (high axial temperature gradients and low growth rates). Lowering the axial temperature gradient does flatten the interface shape, but extremely slow growth rate (below 0.1 mm/h) is necessary for avoiding constitutional supercooling. Forced convection helps in accelerating the removal and re-distributing the excess solute from the growth interface to the entire melt volume thus leading to a 4–5 fold increase in the growth rate without constitutional supercooling and flattening the solid–liquid interface necessary for uniform radial composition in the crystal. However, the mechanism of forced convection coupled with axial temperature gradient in the melt leads to thermal fluctuation at the growth interface. As a consequence, fluctuation in radial alloy composition is observed. A comparative study of various melt stirring schemes with different axial temperature gradients and the resulting alloy distribution in the crystals has been presented in this paper.

Control of growth interface shape using vibroconvective stirring applied to vertical Bridgman growth

The ability of vibroconvective stirring to control the growth interface shape during vertical Bridgman growth was demonstrated using the coupled vibrational stirring (CVS) technique. CVS involves the application of low-frequency vibrations to the outside of the growth ampoule and produces strong flows emanating from the fluid surface. Previous studies showed that the growth rate and interface shape fluctuations occurred under growth with a constant vibrational frequency. In order to eliminate these fluctuations, this paper examines methods to lower the vibrational frequency as growth progressed to maintain a constant fluid flow rate in the vicinity of the growth interface. These flow control methods were tested on two growth systems: NaNO 3 and lead magnesium niobate–lead titanate (PMNT). The previously observed fluctuations were greatly reduced when flows were appropriately controlled. Using this control method, CVS flows were then applied to reduce the concavity of the growth interface in the NaNO 3 system at fast growth rates (>10 mm/h).

Thermal convection and related instabilities in models of crystal growth from the melt on earth and in microgravity: Past history and current status

AbstractThe paper presents a comparative study of a number of theoretical/experimental/numerical results concerning the dynamics of natural (gravitational), Marangoni and related mixed convection in various geometrical models of widely‐used technologies for the production of single‐crystalline materials (Horizontal and vertical Bridgman growth, Czochralski method, Floating Zone Technique). Emphasis is given to fundamental knowledge provided over the years by landmark analyses as well as to very recent contributions. Such a knowledge is of paramount importance since it is validating new, more complex models, accelerating the current trend towards predictable and reproducible phenomena and finally providing an adequate scientific foundation to industrial processes which are still conducted on a largely empirical basis. A deductive approach is followed with fluid‐dynamic systems of growing complexity being treated as the discussion progresses. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Thermoelectric properties of polycrystalline Si1-xGex grown by die-casting vertical Bridgman growth technique

ABSTRACTThe die-casting growth process combined with an advanced version of the Bridgman method was employed for manufacturing the multicrystalline bulk crystal of Si1−xGex. This process provides a form of phase transformation which is completely different from that predicted by the Si-Ge phase diagram. By combining this growth with subsequent heat treatment of the precipitated sample, the variation in the germanium content obtained was within ± 4 % for Si0.65Ge0.35 sample with a carrier concentration in the mid-1018 cm−3. The power factor obtained exhibited a quite flat characteristic over the temperature range of room temperature to 800 K. However, there was a drop in the Seebeck coefficient at about 800 K, which corresponded to a rise in the electrical conductivity. The value of the thermal conductivity was about 0.04 W/cmK at temperatures ranging from 600 to 900 K. The maximum value of the figure of merit obtained for the grown Si0.65Ge0.35 sample was 0.19 at 773 K.

Vertical Bridgman growth and optical characterization of LiNbO 3:Cu:Ce crystals

In this paper we grew the LiNbO 3:Cu:Ce crystals with the size of φ 25 mm×65 mm along the Z axis by the vertical Bridgman method. The UV-visible absorption spectra and OH − absorption spectra of 2-mm thick crystal samples cut from the bottom, middle and top parts of the as-grown crystal were measured with a Shimadzu spectrophotometer. It was found that the absorption edge at the absorption coefficient of 20 cm −1 increased along the growth axis. The oxidized samples exhibited a difference in the absorbance of OH − peaks at 3480 cm −1, while the reduced samples showed almost the same absorbance. These optical inhomogeneities were explained in terms of the nonunity segregations of Cu and Ce ions in the LiNbO 3 crystal. The light-induced scattering effects of the poled crystal samples were also studied and compared using a green laser at the wavelength of 532 nm. Both oxidized and reduced X-cut samples with e-light showed stronger scattering as compared with the Z-cut samples. The reduced samples exhibited faster scattering effects than the oxidized ones. It was also found that the Z-cut sample with the light incident onto its + Z face had almost no scattering. In the case of low incident power density, the reduced samples exhibited stronger scattering than the oxidized ones at the same power density.

Comparative study on residual strain and thermal stress in 4-in GaAs crystals grown by LEC and VB techniques

Residual strains of 4-in diameter GaAs crystals grown by liquid encapsulated Czochralski (LEC) and vertical Bridgman (VB) techniques have been measured by scanning infrared polariscope (SIRP). It was found that the residual strain of the substrate cut at the shoulder portion of a LEC-grown crystal was remarkably larger than that cut at the middle portion of the crystal. It was also found that the residual strains of the substrates cut at the shoulder and middle portions of a VB-grown crystal were both smaller than those of the LEC-grown crystal, and showed the similar values and profiles with each other. Von Mises stress and excess shear stress according to the Jordan's criterion were calculated to understand these experimental results. It was found that large thermal stress was caused during the LEC growth especially in the early stage, while thermal stresses caused during the VB growth were considerably smaller than those caused during the LEC growth from the shoulder to the middle. These experimental and analytical results suggest that the difference in the residual strain along the growth axis between LEC and VB crystals is due to the difference of the thermal stress caused during the crystal growth between LEC and VB techniques.

Flow and segregation control by accelerated rotation for vertical Bridgman growth of cadmium zinc telluride: ACRT versus vibration

The control of flow and segregation by accelerated crucible rotation is investigated by computer simulation of vertical Bridgman crystal growth of cadmium zinc telluride. In view of the cycle frequency, two extremes, i.e., the accelerated crucible rotation technique (ACRT) and angular vibration technique (AVT), are investigated and compared. For a simple benchmark system, it is clear that ACRT generates significant oscillatory growth and segregation, while increasing slightly the global mixing. On the other hand, AVT provides, essentially, a steady control and can reverse easily the radial segregation without enhancing the global mixing. Although both techniques generate similar averaged flow structures and reduce radial segregation, AVT is more effective for growth control.

Vertical Bridgman growth and annealing effect of Bi doped ZnSe single crystal

In order to prepare p-type ZnSe, bismuth (Bi) as an acceptor dopant was doped into ZnSe single crystal by vertical Bridgman method. Photoluminescence (PL) spectra measured at 4.2 K on as-grown crystal showed the strong shallow donor–acceptor pair (DsAP) emission, deep donor–acceptor pair (DdAP) emission and weak I1Bi emission line. To enhance the activation ratio of Bi in ZnSe single crystals, ZnSe:Bi was annealed in saturated zinc and selenium atmosphere at different temperatures. In the case of annealing in the range of 700–800 °C, DdAP emission disappeared and the intensities of DsAP and I1Bi emission relatively increased. From the dependence of PL spectra on excitation light intensity and the temperature, the acceptor activation energy and deep donor level were estimated about 103 meV and 34 meV, respectively. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Vertical Bridgman growth of Hg1−xMnxTe with variational withdrawal rate

Based on the solute redistribution models, Vertical Bridgman growth of Hg1−xMnxTe with variational withdrawal rate is studied. Both theoretical analysis and experimental results show that the axial composition uniformity is improved and the crystal growth rate is also increased at the optimized variational method of withdrawal rate.