Solid-Liquid Interface Reconstructions Using Laser Ultrasonic Fan-Beam Projection Data

Abstract

Many single crystal semiconductors are grown by variants of the Bridgman technique in which a cylindrical ampoule containing a molten semiconductor is translated through a thermal gradient, resulting in directional solidification and the growth of a single crystal. During crystal growth, the shape and location of the solid-liquid interface together with the local temperature gradient control the mechanism of solidification (i.e. planar, cellular or dendritic), the likelihood of secondary grain nucleation/twin formation (i.e. loss of single crystallinity), solute (dopant) segregation, dislocation generation, etc. and thus determine the crystals’ quality [1]. For crystals grown by the vertical Bridgman (VB) technique, optimum properties are obtained with a low (∼1–5mm/hr) constant solidification velocity and a planar or near planar (slightly convex towards liquid) interface shape maintained throughout growth [2,3]. The solidification rate and the interface shape are both sensitive functions of the internal temperature gradient (both axial and radial) during solidification, which is governed by the heat flux distribution incident upon the ampoule, the latent heat release at the interface, and heat transport (by a combination of conduction, buoyancy surface tension driven convection and radiation) within the ampoule [4,5]. The solid-liquid interface’s instantaneous location, velocity and shape during crystal growth are therefore difficult to predict and to control, especially for those semiconductor materials with low thermal conductivity (i.e. CdZnTe alloys) [6]. Thus the development of ultrasonic technologies to non-invasively sense the interface location and shape throughout VB crystal growth processes has become a key step in developing a better understanding of the growth process and for enabling eventual sensor-based manufacturing.