EFG Process Research Articles

On the growth and structure of Al2O3-Y3Al5O12-ZrO2:Y solidified eutectic

Structural features of Al2O3-YAG-ZrO2:Y eutectic plates grown by the EFG process have been studied. X-ray tomography shows that all three phases are continuous along the whole sample, suggesting a fully coupled ternary eutectic growth. However the growth of alumina and YAG is well described by a classical binary coupled growth, in spite of the facetted structure of their growth interface. Colonies are observed at high growth rate and have been related to the chemical rejection of zirconium ions at the solid-liquid interface, possibly due to a slight off-stoechiometry of the raw material.

High speed growth of SrI2 scintillator crystals by the EFG process

Strontium iodide (SrI2), an important new scintillator crystal having a high light yield and excellent energy resolution, was grown for the first time by the edge-defined film-fed (EFG) growth method. Using high purity starting materials and floating dies made of graphite, fused quartz or AlN, large cylindrical, planar or square cross-section single crystals (12–15mm across and >7cm long) were produced at growth rates up to 15mm/h, significantly faster than the current Bridgman growth technology. Details on the equipment used to grow this deliquescent material and on its growth behavior are given along with some discussion of crystalline quality.

Working point of the EFG process

A theoretical investigation of the EFG process working point based on the meniscus height control is carried out. The link between the two control parameters, which are the pulling rate and the upper die temperature, is found from the thermal equilibrium at the crystallization interface. Using the pressure equilibrium in the film and considering the meniscus shape, the change in meniscus height depending on the crystal radius is analyzed. Limited to small crystal radii, an algebraic formulation of the temperature gradient at the interface is established. Some die design parameters are taken into account and their impacts on the process working point are discussed.

Simulation of the crystallisation of silicon ribbons on substrate

Today's photovoltaic market is divided into multicrystalline silicon wafers from ingot casting and monocrystalline wafers from Czochralski crystals. In both cases large crystals have to be cut into thin wafer geometry. As an alternative approach the EFG process and the string ribbon process are in industrial use for the production of silicon ribbons directly out of the melt. One future alternative for a high production process is the Ribbon Growth on Substrate (RGS) process which is now in the final development stage for industrial production of silicon ribbons. In this paper we will report simulation results for the crystallisation of silicon melt in contact with a substrate. This is the basis not only for the RGS process but also for thin film processes like laser remelting of amorphous Si. The results show the general solidification behaviour at the tip region of a growing Si sheet. Different crystal growth modes are predicted resulting in different grain structures of the silicon sheet as observed experimentally.

Grain boundary structure and electrical activity in shaped silicon

Crystalline structure, recombination activity, and electron transport properties have been investigated in boron-doped, thin-walled crystals of silicon obtained from hexagons grown by the EFG process (a modification of Stepanov's method). All grain boundaries (GBs) can be divided into special (Σ3, Σ9), near coincidence, highly deviated (Σ3, Σ9, Σ13), and general (i.e., non-special) GBs. Special GBs are electrically inactive to both minority and majority carriers. Near coincidence GBs are active to minority but inactive to majority carriers. General and highly deviated GBs are very active to both minority and majority carriers. The observed classification of GB activity is explained on the basis of a phenomenological model.

Zeta-potential and radiotracer adsorption measurements on EFG α-Al 2O 3 single crystals in NaBr solutions

Radiotracer adsorption measurements ( 24Na 82Br tracer) and ζ-potential measurements were performed on α-Al 2O 3 single crystals, grown by the EFG process, in 0.01 and 0.05 mole dm −3 NaBr and with varying pH. Evidence was obtained for the adsorption of sodium as well as of bromide ions in the compact part of the electrical double layer. A gel layer is not formed. Surface charge density curves were derived from the adsorption and ζ-potential data. The point of zero charge is at pH ≈ 4.5, whereas the isoelectric point (iep) occurs at pH 3.1–3.5, depending on the NaBr concentration. This difference between iep and pzc is attributed to a greater complexation strength of Al 2O 3 for Br − ions than for Na + ions. This explanation is corroborated by the radiotracer adsorption measurements and the low iep therefore cannot be attributed to silica contamination. An acceptable description of the experimental data can be given in terms of the site-binding model, following the approach of Davis et al. [ J. Colloid Interface Sci. 63, 480 (1978)]. The inner layer capacitance, required to describe the pH region of predominating Br − adsorption, is about twice as large as that required to describe the pH region of predominating Na + adsorption.

The growth of silicon tubes by the EFG process

The growth of silicon tubes by the EFG technique is reported. Tubes were grown with a diameter of 0.95 cm, wall thickness in the range of 0.005 to 0.1 cm at rates up to 12 cm/min. The theory of tube growth by the EFG process is developed to show the dependence of tube wall thickness on the growth variables. Experimental apparatus and procedure are described as well as the application of the theory in controlling the wall thickness of the growing crystal. The experimental and theoretical results are combined to form a predictive model for wall thickness yield.

Dislocations in { [formula omitted]012} sapphire ribbons grown by the EFG process

Dislocations in as-grown { 1 012} EFG sapphire ribbons were investigated by etching in fused KOH in conjunction with X-ray topography. The etch pit density in the bulk along the ribbon thickness, measured on a fracture-cleaved { 1 012} surface lying at 86° from the surface was ≈1 × 10 5 cm −2. At the subsurface region, ≈50 μm from the surface, a high etch pit density of ≈3 × 10 6 cm −2 was measured. The measured variation of the etch pit densities across the ribbon thickness was in good agreement with the results of the X-ray section topography. The etch pit density on the as-grown ribbon surface varied from ≈1 × 10 5 to ≈2 × 10 6 cm −2.

EFG, the invention and application to sapphire growth

It has been over a decade since the invention of the EFG process. The experimental goals and problems which led to the EFG concept are briefly described. In addition, the growth of sapphire crystal for commercial use is mentioned with specific attention given to the manufacture of sapphire ribbon for use as ultraviolet windows.

Dynamics and control of meniscus height in ribbon growth by the EFG method

In the growth of ribbon by the EFG method, the geometry of the grown crystal is largely controlled by the meniscus height. The need to control meniscus height automatically during growth (in a closed-loop control system) has led us to a review of the possible control systems and a study of the dynamics of the process itself. The complete dynamic process has been broken down into two components: a heater-die thermal system, and the physics of the EFG process itself. A lumped parameter model has been developed for the thermal system, and the physics of the EFG process has been reviewed and extended so that it might be easily interpreted in the light of experimental data. Frequency response data has been obtained through the use of an instrumented crystal growth system which included an anamorphic optical-video system (enlarges in one direction more than in the other) and video tape recorder. Data have been obtained on the response of meniscus height, ribbon thickness, and die temperature to heater power level, as well as the response of meniscus height and ribbon thickness to pulling speed changes. In all cases, theory and experiment agreed well. To summarize, it has been found that the heater-die thermal system behaves as a second-order system; e.g., die temperature and heater power level are related by a second-order differential equation. Further, it has been found that the EFG system per se behaves as a second-order system. That is, meniscus height or ribbon thickness and pulling speed or die temperature are related by a second-order differential equation. In the case of ribbon thickness response, the terms involving pulling speed or die temperature involve only constants, while in the case of meniscus height response these terms involve first derivatives as well. The dynamic response is identical whether the input is pulling speed or die temperature. Finally, it has been found that the complete, composite system behaves as a fourth-order system; e.g., meniscus height or ribbon thickness is related to heater power level by a fourth-order differential equation. The implications of these results on the design of meniscus height measurement-based automatic control systems are discussed.

Electroosmotic zeta potential measurements on single crystals

A technique by which the ζ potential of rodlike materials can be measured is described. The electroosmosis of the system consisting of a rod placed in a cylindrical hole drilled in a piece of PTFE is measured. The method was tested with vitreous silica. The isoelectric point of a synthetic sapphire, grown by the Tyco EFG process, was situated at pH 3.3. The ζ potential of PTFE, required in the calculation, was measured similarly on a rod of PTFE. The method can be applied to any material in which a cylindrical hole can be drilled and from which an appropriate rod can be formed; the method can be applied also to materials from which only rodlike pieces are available.