Edge-defined Film-fed Growth Process Research Articles

Tuning the sapphire EFG process to the growth of Al2O3/YAG/ZrO2:Y eutectic

In this work, a model is proposed, in order to analytically study the working point of the Edge defined Film-fed Growth (EFG) pulling of crystal plates. The model takes into account the heat equilibrium at the interface and the pressure equilibrium across the meniscus. It is validated on an industrial device dedicated to the pulling of sapphire ribbons. Then, the model is applied to pulling ceramic alloy plates, of the ternary eutectic Al2O3/YAG/ZrO2:Y. This allowed understanding the experimental difficulties of pulling this new material and suggested improvements of the control software. From these results, pulling net shaped ceramic alloy plates was successful in the same industrial equipment as used for sapphire.

Structural evaluation of defects in β-Ga2O3 single crystals grown by edge-defined film-fed growth process

We have structurally evaluated β-Ga2O3 crystals grown by edge-defined film-fed growth process using etch pitting, focused ion beam scanning ion microscopy, transmission electron microscopy, and related techniques. We found three types of defects: arrays of edge dislocations corresponding to etch pit arrays on -oriented wafers, platelike nanopipes corresponding to etch pits revealed on the (010)-oriented wafers, and twins including twin lamellae.

Silicon EFG process development by multiscale modeling

An overview of simulation models in use for optimizing the edge-defined film-fed growth (EFG) process of thin-walled hollow silicon tubes at WACKER SCHOTT Solar is presented. The simulations span the length scales from complete furnace models over growth simulations with a mesoscopic description of the crystalline character of silicon down to solidification simulations with atomic resolution. Results gained from one model are used as input parameters or boundary conditions on other levels. Examples for the application of these models and their impact on process design are given. These include the reduction of tube thickness variations, the control of tube deformations, residual stresses and dislocation densities and the identification of twin formation processes typical for EFG silicon.

3D coupled electromagnetic and thermal modelling of EFG silicon tube growth

The edge-defined film-fed growth (EFG) process is mainly used to grow silicon ribbons and hollow cylinders as well as hollow polygons of various geometries. The development of the EFG process is primarily focused on technology improvements and cost reduction of wafers for photovoltaic applications. In case of the growth of hollow polygons, three-dimensional (3D) numerical analysis is extensively used because the system of polygonal geometry cannot be adequately described in axisymmetric model. Electromagnetic simulation predicts a 3D distribution of electromagnetic energy in all the system. The calculated three-dimensional temperature fields allow the analysis of the temperature profiles along and across the growing silicon tubes and the investigation of thermally induced stress and strain in different cases. The non-linear model consisting on electromagnetic, thermal and structural simulations has been adjusted and successfully validated by experimental tests with industrial installations.

Edge-Defined Film-Fed (EFG) Growth of Rare-Earth Orthovanadates REVO4 (RE=Y, Gd): Approaches to Attain High-Quality Shaped Growth

In spite of their superior laser and polarizer properties rare-earth orthovanadates (REVO4) single crystals have not been adopted yet into extensive industrial applications because of crystal growth difficulties. The main problems of CZ technique are compositional change and diameter instability. This work presents the first attempt to apply the edge-defined film-fed growth (EFG) technique by which well-shaped REVO4crystals have been grown directly. The capillary properties of YVO4 and GdVO4 melt have been measured. The applicability of shaped growth for rare-earth orthovanadate family was approved by successful EFG growth of transparent rod-like macro-defect-free single crystals of YVO4 and GdVO4. We address two main approaches to enhance the quality of EFG crystals: (i) meniscus and crystal shape stability dependence on die top shape and (ii) the strategy of effective operating control. Concave die top was found to be the best choice for high-quality EFG growth of REVO4 along [001] direction. The spectral analysis of weight signal from growing crystal was shown to be a useful feedforward clue to prevent crystallinity degradation at a very early stage. A reasonable stability of the EFG process was achieved using [211], [101], [001] and [100] pulling directions.

Effect of dynamic perturbation and contact condition on edge-defined fiber growth characteristics

Sapphire fibers for use in both optical sensors and structure composites have been manufactured using the edge-defined film-fed growth (EFG) process. A thermocapillary model based on a combined Lagrangian/Eulerian method has been developed to simulate the dynamic characteristics of the EFG process, subject to the pull speed perturbations. The meniscus behavior is governed by the Young-Laplace equation subject to a specified contact condition at the trijunction point. Two models have been investigated at the trijunction point, including a conventionally adopted static model which fixes the contact angle, and a dynamic model which regulates the contact angle according to the speed and direction of the instantaneous movement of the trijunction point. It has been predicted, and observed experimentally, that the fiber diameter responds to the pull speed perturbation at the corresponding frequency, but the sensitivity of the response decays as the frequency increases. While both the static and dynamic models at trijunction points cause the crystal to vary in size in response to the external fluctuations, their effects are noticeably different, indicating that the conventional models are not adequate to yield accurate predictions in the solidification characteristics dynamically.

Growth of TiO 2 single crystals using the edge-defined, film-fed growth technique

TiO 2 boule single crystals, 15 mm in diameter and 40 mm in length, have been grown successfully using the edge-defined, film-fed growth (EFG) method. When the Ir die top temperature is a little high, the boule crystal shape is difficult to control and we find that spiral growth occurs regardless of crystal rotation in the EFG process. By observing the boule crystals with a polarizing microscope, stress is observed to correspond to the slot pattern on the die top. The stress is suppressed by crystal rotation during growth. The growth characteristics and their relation to the growth conditions are reported.

Hydrogen diffusion along passivated grain boundaries in silicon ribbon

Using the electron beam induced current mode of the scanning electron microscope, a technique has been developed to study the extent of hydrogen passivation and diffusion along grain boundaries in silicon ribbon grown by the edge-defined film-fed growth process. Passivation and diffusion depths x, ranging from a few microns to more than 200 microns have been found. Grain boundary diffusivities of 10−8–10−9 cm2/s have been measured. A finite width to the spatial distribution of recombination centers along the grain boundaries has been found and measurements of the surface recombination velocity S, indicate that, for S≥2×104 cm/s, ln S∝x.

Hydrogen passivation of defects in silicon ribbon grown by the edge-defined film-fed growth process

Electrically active defects in edge-defined film-fed grown silicon ribbon solar cells have been passivated using a hydrogen plasma from a Kaufman ion source. Significant improvements in solar cell efficiency for both low diffusion length starting material (∼20 μm) and high diffusion length (∼50 μm) material have been obtained. For the former, passivation has produced solar cell efficiency improvements as high as 41% and in the case of the latter, solar cell efficiencies as high as 14.5% (AM1) have been obtained. Electron beam induced current micrographs are shown which indicate near total defect passivation for at least the top 9 μm of the solar cell.

Surface Morphology and Shape Stability in Silicon Ribbons Grown by the Edge‐Defined, Film‐Fed Growth Process

The morphology and shape stability of silicon ribbons grown by the edge‐defined, film‐fed growth (EFG) technique are described. A theoretical analysis of shape stability is presented for controlled‐shape growth (where the ribbon shape is determined by the shape of the meniscus); experimental evidence is introduced to show the inherent stability of the ribbon side and the relative instability of the ribbon edges, in agreement with the theory. The characteristic meniscus angle for constant‐dimension growth is shown to depend on the crystallographic orientation of the ribbon surface; the anisotropy is associated with the anisotropy in the crystal‐vapor surface free energy in silicon. Growth features which result from faceted growth on peripheral octahedral planes are described, and the competition between faceted growth and controlled‐shape growth is analyzed. Various morphological features associated with variations and fluctuations in the EFG process variables are described.

The EFG process applied to the growth of silicon ribbons*

The development of edge-defined, film-fed growth (EFG) techniques for silicon ribbons has prompted analyses of appropriate die materials, die shapes, meniscus shapes and thermal gradients. The requirement for high electronic quality of the ribbons, in conjunction with the high effective solute distribution coefficients expected for EFG, narrows the choice of die materials primarily to graphite and fused silica. The shape of the meniscus between crystal and die has been calculated. This shape, together with contact angle information, is used to determine design criteria for both graphite and silica dies. The effect of crystal growth on the temperature gradient ahead of the growth interface has been calculated in terms of the ribbon thickness and an empirical parameter characterizing the radiating environment of the growth apparatus. Numerous 25 mm wide × 0.3 mm thick ribbons from carbon dies and a few small ribbons from silica dies have so far been grown. The ribbon quality is discussed.

Edge-defined, film-fed growth (EFG) of silicon ribbons

The edge-defined, film-fed growth (EFG) technique was applied to the growth of silicon ribbons. An investigation of various graphite and silicon carbide coated graphite dies indicated that high density (>1.9 gm/cm 3), small grain size (<20 μ) graphite was physically suitable for the EFG process. Resultant silicon ribbons contained approximately 10 ppma carbon, 6–40 ppma oxygen, and lesser amounts of other impurities. Dislocation-free growth was demonstrated; however, in general, crystallographic defects were a significant problem in this crystal growth investigation.