On the void distribution and size in shaped sapphire crystals

Abstract

Ribbon and rod sapphire pulling has been performed in three different crystal growth equipments in order to study the effect of the installation, of the atmosphere, of the die shape, of the feed material and of the pulling rate on the distribution, number and diameter of the characteristic voids (micro-bubbles) in the crystals. The location of the bubbles in the crystals depends on the die geometry; however, in most cases they are essentially located close to the crystal periphery and then can be efficiently removed by lapping. After statistical analysis of the results, it is demonstrated that the number of gas moles incorporated in the crystals, inside the voids, is totally independent of any growth parameter. It is also shown that the bubble diameter depends only on the pulling rate. Consequently, for a given pulling rate, the number of bubbles auto-adjusts in order to satisfy the constant molar gas incorporation.

Voids engulfment in shaped sapphire crystals

“…The flow field near the meniscus-free surface for the two types of die differs by the velocity values [13,[15][16][17][18]. For ACC die the V max is higher than for CCC die; the minimum value of the velocity appears in two positions near the crystallization interface: on the capillary outlet and another one on the symmetry axis of the shaper.…”

“…The influence of the ambient temperature distribution [10,12] and of the pulling rate [12,13,[15][16][17][18] on the void distribution in the rod-shaped sapphire crystals have been studied by numerical simulation of the heat, momentum and mass transfer, using the STHAMAS 3D software [10,12] and the FIDAP software [13,[15][16][17][18]. The influence of the melt flow on the impurity distribution in various growth methods has been studied by many investigators, both experimentally and theoretically.…”

“…The model for fluid flow and mass transfer, the boundary conditions used for the two shaper geometries (a central capillary channel shaper-CCC and an annular capillary channel shaper-ACC) are described in Refs. [13,[15][16][17][18]. The influence of the die design on the fluid flow and on the impurity distribution is studied for various meniscus heights, Marangoni numbers and segregation coefficients.…”

“…In this paper, we present a review of our, more than 20 years long research on void engulfment in shaped sapphire crystals [6,8,[10][11][12][13][14][15][16][17][18]. Because the gas bubbles are mobile with respect to the crystallization interface, the study of gas engulfment in crystals involves a number of physical processes.…”

“…The experimental results on the void distribution in crystals, published in Refs. [6,8,18] and some unpublished observations, are compared with the numerical simulation of the heat, fluid flow and mass transfer [10][11][12][13][15][16][17][18]. The purpose of the numerical simulation of the physical processes in the meniscus are to obtain additional information about the melt flow and the impurity (gases) distribution in order to explain the void distribution observed in shaped sapphire crystals obtained by the EFG method.…”