Available Technology

Improving Melt-Grown Semiconductor Crystal Quality

Enables improved quality and yield of melt-grown semiconductor crystals
This technology is particularly relevant for preparation of single-crystal II-VI wafers, and has been demonstrated for CdTe and CdZnTe compositions. The problem with II-VI semiconductors is the large degree of supercooling of the melt that occurs before nucleation and solidification. This supercooling causes spontaneous nucleation to occur throughout the solidification front, with the result being a large number of crystal grains with varying orientation in the crystallized boule. However, because the devices for which these materials are used, such as x-ray and gamma detectors, the yield from these multi-crystalline boules is very low. The process improvement provided by this innovation involves controlled crystal in the liquid melt. Essentially the method involves a slight mechanical perturbation of the supercooled liquid to promote nucleation and growth of a single crystal versus simultaneous nucleation and growth of multiple crystals. The mechanical perturbation is provided by a slight mechanical tapping on the fixture supporting the growth ampoule, achieved by a simple set-up of a 60 Hz solenoid and variable power supply. This simple arrangement can be readily adapted and implemented on existing melt crystal growth systems. The improvement in crystal quality offered by this innovation has been demonstrated via single crystal wafers obtained from melt-grown boules of the CdZnTe.
Patent Abstract: 
NASA Marshall Space Flight Center has developed a method for melt growth of II-VI semiconducting materials that offers improved yield of high-quality, bulk single-crystals for use in making various semiconductor devices. A number of semiconductor single crystal materials are prepared via directional solidification from the melt. Such single crystals, often referred to as ingots or boules, are then sliced into wafers for further processing to make a full range of optoelectronic and semiconductor devices. Preparation of II-VI compositions, in particular CdZnTe or CdTe, is problematic due to the spontaneous nucleation and formation of multi-crystalline boules. The yield from these boules of the single-crystal material required for device fabrication is low. By using the process described by this innovation, substantial improvement in the yield of high-quality single-crystal materials during crystal growth from the melt can be achieved.
Benefits 

The innovation significant improvements in yield and crystal quality of II-VI semiconductor materials, thereby providing significant cost savings in production.

applications 

Applications of the NASA innovation are directed specifically at the growth of high-quality semiconductor single-crystal boules within the II-VI family of semiconductor compositions. Such semiconductors are important for a range of high performance electronic and optoelectronics applications.

Reps: 
Patent Number: 
8,535,440
Internal Laboratory Ref #: 
MFS-TOPS-86
Patent Status: 
Patent Issue Date: 
March 21, 2018
Agency
NASA
Region
Southeast
State: 
Alabama
Phone: 
256-544-9151
Email: 
terry.taylor@nasa.gov
Lab Representatives
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