Magneto-controlled Czochralski method for single crystal growth

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Introduction

Magnetically-controlled single crystals are of great interest in the area of semiconductor materials due to their superior properties compared to polycrystalline materials. This type of single crystal growth has been traditionally achieved through the time-consuming, expensive Bridgman-Stockbarger technique that relies on the growth of the crystals under a gravitational force.

However, this technique has its limitations when it comes to controlling the uniformity of the crystals over a larger area. In such cases, the enhanced controllability and uniformity of the growth are desirable and this can be achieved through the use of a magnetic field. This technique is known as magnetically-controlled single crystal growth or, more specifically, the magnetically-controlled vertical axial pulling method (MCVAP).

Theory & Operation

The MCVAP is based on the use of a continuous magnetic field to manipulate the growth direction of the crystal. This magnetic field is generated from a series of Helmholtz coils and is used to control the direction and rate of the crystal growth.

The MCVAP process begins with the formation of a polycrystalline rod in a vacuum chamber. This polycrystalline rod is then placed in a vertical axis inside the vacuum chamber and is connected to a power supply. A strong magnetic field is then generated by the Helmholtz coils and is used to control the direction of the crystal growth.

The crystal is then pulled up at a slow and constant rate while continuously exposed to the magnetic field. This pulling is known as ‘vertical axial pulling’ and is used to remove the polycrystalline material and induce the single crystal growth.

The use of the vertical axial pulling method allows for the formation of uniform single crystals over larger areas as compared to the traditional Bridgman-Stockbarger method. This is due to the enhanced controllability offered by the magnetic field as it allows for the elimination of the uneven pull generated by gravity.

In addition to the improved uniformity of the single crystals, the MCVAP also offers advantages over the traditional method in terms of cost and efficiency. The MCVAP eliminates the need for using larger and more expensive vacuum chambers as well as allowing for faster growth rates.

Advantages & Disadvantages

The advantages of the MCVAP are numerous and include improved controllability, uniformity, cost efficiency, and faster growth rates.

However, there are some disadvantages of the technique that should be taken into consideration. First, the use of the magnetic field may cause some contamination of the single crystals if it is not properly controlled and monitored. Second, the uniformity of the single crystals may vary depending on the strength of the magnetic field and this can lead to quality issues.

Conclusion

The MCVAP is a viable option for the growth of high-quality single crystals due to its increased controllability and uniformity compared to the traditional Bridgman-Stockbarger technique. It also offers a cost-efficient and efficient alternative to traditional crystal growth methods and is being shown to be an increasingly popular choice for the fabrication of semiconductor wafers. However, care should be taken to ensure that the uniformity and contamination levels of the single crystals is as desired as these can vary depending on the strength of the magnetic field.

The single-crystal growth of magnetically-controlled-pulling (MC-pulling) method is the most advanced technique for producing high-purity single crystal of semiconductors, magnetic materials and superconductors. In the process of crystal growth, two large magnets with opposite poles are placed at the two ends of the furnace, with another small magnet in the center of the furnace. The strong interaction between the two opposite poles produces a powerful magnetic field force along the axis of the furnace. As the substrate is lowered, the material is then pulled up along the magnetic field direction, achieving the goal of controlling the direction of crystal growth.

Different from the traditional vertical directional solidification process, there is no need to warm up the furnace during the process of magnetically-controlled-pulling. The process occurs at room temperature. Moreover, this method can provide a very intense magnetic field, which is several orders of magnitude higher than traditional methods, enabling better control of the crystal growth direction and microstructure, as well as an effective defect control.

In addition, this technique can prevents the formation of disconnected crystals, or the generation of additional grains in the sample. This can be attributed to the fact that the magnetic field forces the charged particles to stay in the appropriate channels for long periods of time and maintain the integrity of the sample.

On the other hand, the process of magnetically-controlled-pulling carries a number of disadvantages as well. Firstly, the process can only be conducted inside a dedicated machine, which is costly. Also, since this is a process at very low temperatures, some materials may not be suitable to be grown using this method. Finally, the process is less efficient than the traditional vertical directional solidification process, and the crystal growth rate is relatively lower as well.