Crystal Growth Furnaces Bridgman furnaces for high-quality crystals

Bridgman method crystal growing tube furnaces

BV-HTRV 40-500/18: Bridgman crystal growth furnace with a heated length of 500 mm up to 1800°C. The pulling device is mounted above the furnace.

BV-HTRV 70-250/18: Bridgman crystal growth furnace with a heated length of 250 mm up to 1800°C. The system is equipped with a pre-vacuum pump.

The vertical Bridgman furnace (i.e. BV-HTRV 40-500/18) is designed with a single zone high temperature tube furnace mounted at the bottom, and the pulling device mounted on top. The base frame can be utilized with almost all of our tube furnaces, so a lot of different lengths, diameters and temperatures are available. Multi-zone Bridgman furnaces are available as well, permitting a better influence on the temperature profile.

An inverse design is equally possible. In this case, the vertical Bridgman furnace (i.e. BV-HTRV 70-250/18) is designed with a single zone tube furnace mounted at the top, and the pulling device mounted below. The tube is equipped with vacuum tight flanges, and a water-cooled shaft for pulling. All movements are controlled via a potentiometer. The programming controller specifies the pulling speed, and fast movement is possible.
 

Stockbarger method crystal growing furnace

Crystal growing system for the Stockbarger method. A five zone furnace constructed of graphite whose cool down rate is precisely controlled to grow crystals.
 

Background information Bridgman-Stockbarger Method

Bridgman-Stockbarger is the most common and widely used method in crystal growth furnaces. The process involves slowly moving a polycrystalline melt, in a crucible or an ampoule, across a stable temperature gradient from a hot zone to a cold zone in the furnace. The crucible containing the melt is rotated whilst translating to create a homogenous temperature profile. The principle behind this technique is based on directional solidification.

A single crystal seed is brought into contact with the melt to ensure single crystal growth is governed along a certain crystallographic orientation. This consequently also provides an interface for growth. As the temperature decreases from the hot zone, the polycrystalline melt solidifies. The seed initiatesthe process until the entire melt converts into a solid single crystal ingot with a uniform composition. The crystals are grown under a slow and directional cooling environment which minimises the likelihood of defects within the structure.

A gradient freezing modification can also be applied in this method using a multiple-zone furnace. This does not require the movement of a crucible or furnace. Instead, the temperature gradient is controlled by changing the heat supply, so the melt-crystal interface is maintained.

Key parameters such as pulling speed and rotation speed are at standard of 0.03-50 mm/h and 1-5 rotation/min, respectively. A display is used to show the absolute position of the crucible/ampoule in the furnace’s length, in comparison to its starting point. The thermal gradient in the Bridgman furnace can be controlled as it plays a vital role in producing highly crystalline and homogenous single crystals.

The method can be implemented in a vertically or horizontally configured Bridgman furnace, depending on the process being done and the type of crystals grown. A melt growth crystallisation technique can be carried out in a vacuum, neutral (nitrogen, helium, argon, etc) or oxidising environment (air, oxygen).

Advantages

• Produces high-quality and large single crystals
• Grows different crystals such as ferroelectric, piezoelectric, optical and semiconductor crystals
• Crystal shape and orientation can be modified by changing parameters such as growth rate, rotation speed, temperature gradient and crucible shape

Disadvantages

• This method cannot be used to grow hydrated and anhydrous salts and most organic crystals
• It is challenging to control and ensure uniform temperature distribution within the furnace
• It is challenging to ensure the mechanical stability of the system whilst smoothly running the pulling mechanism
• This method is time-consuming and expensive as it can take days or weeks to grow a single crystal
• Specialised equipment and trained personnel are required to grow crystals

Application: Bridgman furnace used to grow crystals for photovoltaic cells

One example within the broad range of applications of chrystal growing is be the production of cadmium telluride (CdTe) single crystals using the Bridgman-Stockbarger method. Cadmium telluride is a semiconducting material that is employed to create P-N junctions for their use within applications such as radiation detectors, sensors, and photovoltaics.

In practice, PN junctions are formed within the single crystal by doping. Single crystal P-N junctions have higher efficiency than its polycrystalline and amorphous counterparts. Fewer defects and impurities are found in single crystals causing lesser resistance to electron flow. Defects and irregularities disrupt the arrangement of atoms in the crystal, hence altering the number and mobility of charge carriers.

Analysing the crystal structure

A crystal can be classified into seven different crystal systems. Each crystal system consists of a regular array of atoms. By using X-ray diffraction, it is possible to determine a crystal's structure. The principal behind this technique stems from Braggs Law, which describes the interaction of x-rays with the crystal structure.