For certain semiconductors with important applications, the existing bulk crystal growth technique from the melt usually results in poor-quality multi-crystalline ingots that cause the typically low yield of the commercial growth process. The low-quality, multi-grained crystal growth is mainly caused by the large supercool of the melt, which prohibits the ideal growth condition that a small, single-crystal nucleus forms at the very tip and grows into a large single crystal. For instance, semi-insulating cadmium zinc telluride (CdZnTe) crystal is a highly promising material for room-temperature x-ray and gamma ray detectors. However, the major hurdle in using the CdZnTe crystals is its cost. The ability to pack many data acquisition channels (hundreds) with the stopping power for high-energy radiation requires large single crystals of CdZnTe.

The long solid section of spontaneous nucleation has also been reported on another family of the ternary telluride semiconductor, mercury zinc telluride. To promote nucleation under the condition of small supercooling, a mechanical device was employed to induce nucleation. The crystalline quality of the crystals grown with the mechanically induced nucleation shows, most of the time, single nucleation at the tip, and continued to grow as single crystals throughout most of the ingot.

To prevent the undesired formation of a large, multi-grained, spontaneous nucleation, i.e., to promote nucleation under the condition of small supercooling, a mechanical tapping was applied to the growth ampoule when the melt at the ampoule tip was just below the liquidus’ temperature. The high-frequency shaking of the ampoule causes local inhomogeneity in the supercooled melt, which promotes the nucleation in the melt and, consequently, a small section of solid, usually single grain, was formed at the growth tip, which grows continuously throughout the length of the ingot. In the crystal growth setup, there was a 15-cm-long fused silica rod attached to the tip of the growth ampoule. The ampoule was held vertically inside the furnace by inserting the rod in a fused silica tube (about 60 cm long), which was held vertically by a lathe chuck sitting on the floor. The tapping was applied at the bottom of the fused silica tube by a solenoid AC vibrator, which was bound together with the tube by a plastic strap. The frequency of the vibration was 60 Hz and its magnitude was adjusted by varying its power input from 0 to 120 volts.

The novel feature of the innovation is that with the induced nucleation employed by the mechanical tapping, the physical nature of the large supercooling of the homogeneous nucleation in the melt, which results in the undesirable multi-crystalline solid, has been modified into significantly lower supercooling, which results in the ideal growth condition of single crystal nucleation.

This work was done by Ching-Hua Su of Marshall Space Flight Center. This invention has been patented by NASA. For more information, contact Ronald C. Darty, Licensing Executive in the MSFC Technology Transfer Office, at This email address is being protected from spambots. You need JavaScript enabled to view it.. Refer to MFS-32715-1.

NASA Tech Briefs Magazine

This article first appeared in the February, 2015 issue of NASA Tech Briefs Magazine.

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