Comparison of Several Sapphire Crystal Growth Techniques
Since the first synthetic gemstone was produced by the flame fusion method in 1902, various technologies for growing synthetic sapphire crystals have continuously evolved. Over the years, more than a dozen crystal growth methods have emerged, including flame fusion, Czochralski (CZ) method, and Kyropoulos (KY) method, among others. Each method has its own advantages and disadvantages, and they are used in different application fields. Currently, the main industrialized techniques include the Kyropoulos method, Czochralski method, Edge-Defined Film-Fed Growth (EFG) method, and the Vertical Horizontal Gradient Freeze (VHGF) method. The following section will introduce the typical sapphire crystal growth techniques in more detail.
Flame Fusion Method (Verneuil Process)
The Verneuil Process, also known as the Flame Fusion method, was named after the renowned French chemist Auguste Victor Louis Verneuil. He is best known for inventing the first commercially viable method for synthesizing gemstones. In 1902, he developed the "flame fusion" technique, which is still widely used today as an affordable method for producing synthetic gemstones.
As one of the most common methods for producing synthetic gemstones in the market, the flame fusion method is not only used to synthesize rubies and sapphires, but also applies to the production of synthetic spinel, synthetic rutile, synthetic star rubies and star sapphires, and even artificial strontium titanate, among others.
Working Principle
The flame fusion method, in simple terms, utilizes the high temperature generated by the combustion of hydrogen and oxygen. A loose powder of aluminum oxide (Al₂O₃) is fed through the oxyhydrogen flame. As the raw powder passes through the flame, it instantly melts into tiny droplets, which then fall onto a cooled seed rod, where they solidify and form a single crystal.
The following diagram shows a simplified schematic of the flame fusion crystal growth apparatus.
A key prerequisite for successfully synthesizing gemstones is the use of highly pure raw materials, with a minimum purity of 99.9995%. To synthesize rubies or sapphires, aluminum oxide (Al₂O₃) is the primary material. Efforts are typically made to reduce the sodium content, as sodium impurities can cause cloudiness and reduce the clarity of the gemstone. Depending on the desired color, small amounts of different oxide impurities can be added. For example, chromium oxide is added to produce rubies, while iron oxide or titanium oxide is added to produce blue sapphires. For other types, rutile is formed by adding titanium dioxide, and strontium titanate is formed by adding titanium oxalate. Other lower-value crystals can also be mixed into the starting materials.
High Efficiency and Low Cost!The flame fusion method is a highly efficient and low-cost approach to synthesizing artificial gemstones. It is considered the fastest crystal growth method among all synthetic gemstone techniques, enabling the production of large crystals in a short time—approximately 10 grams of crystal can be grown per hour. The crystal size of corundum-based gemstones varies, typically forming boule-shaped crystals ranging from 150 to 750 carats (1 carat = 0.2 grams), with diameters reaching 17–19 mm.
Compared to equipment used in other synthetic gemstone methods, flame fusion devices are the simplest in structure. This makes the flame fusion process especially suitable for industrial-scale production and gives it the highest yield among all synthetic methods.
However, crystals produced by the flame fusion method typically exhibit curved growth striations or color bands resembling the texture of a phonograph record, as well as characteristic bead-like or tadpole-shaped bubbles. These features limit their application in fields such as optics and semiconductors. Therefore, the flame fusion technique is mainly suitable for producing items with relatively small diameters, such as jewelry, watch components, and precision instrument bearings.
In addition, due to its low cost, sapphire crystals grown by the flame fusion method can also be used as seed or starting materials for other melt-based crystal growth methods.
Kyropoulos Method (KY Method)
The Kyropoulos method, abbreviated as the KY method, was first proposed by Kyropoulos in 1926 and initially used for the growth of large halide crystals, hydroxides, and carbonates. For a long time, this technique was mainly applied to the preparation and study of such crystals. In the 1960s and 1970s, the method was improved by the Soviet scientist Musatov and successfully adapted for the growth of sapphire single crystals. Today, it is considered one of the most effective solutions to the limitations of the Czochralski method in producing large crystals.
Crystals grown by the Kyropoulos method are of high quality and relatively low cost, making the technique well-suited for large-scale industrial production. Currently, around 70% of the sapphire substrates used globally for LED applications are grown using the Kyropoulos method or its various modified versions.
The single crystals grown by this method typically have a pear-shaped appearance (see the figure below), and the crystal diameter can reach sizes just 10–30 mm smaller than the inner diameter of the crucible. The Kyropoulos method is currently one of the most effective and mature techniques for growing large-diameter sapphire single crystals. Large-sized sapphire crystals have already been successfully produced using this method.
A recent news report highlighted a breakthrough in this area:
On December 22, the Crystal Growth Laboratory of Jing Sheng Crystals, in collaboration with its subsidiary Jinghuan Electronics, successfully produced the first ultra-large sapphire crystal weighing approximately 700 kg—marking a major innovation milestone.
Kyropoulos Crystal Growth Process
In the Kyropoulos method, the raw material is first heated to its melting point to form a molten solution. A single crystal seed (also known as a seed crystal rod) is then brought into contact with the surface of the melt. At the solid–liquid interface between the seed and the melt, a single crystal with the same lattice structure as the seed begins to grow. The seed crystal is slowly pulled upward for a short period to form a crystal neck.
Once the solidification rate at the interface between the melt and the seed becomes stable, the pulling stops and the seed is no longer rotated. From this point on, the crystal continues to grow downward by gradually controlling the cooling rate, allowing the melt to solidify from top to bottom. This results in the formation of a complete single-crystal ingot.
Characteristics of the Kyropoulos Method
The Kyropoulos method relies heavily on precise temperature control to grow crystals (temperature control is absolutely critical!). Its biggest difference from the Czochralski method lies in the fact that only the crystal neck is pulled; the main body of the crystal grows through controlled temperature gradients, without the additional disturbance of pulling or rotating. This makes the process more stable and easier to control.
While the crystal neck is being pulled, the power of the heater is carefully adjusted to bring the molten material into the optimal temperature range for crystal growth. This helps achieve an ideal growth rate, ultimately producing high-quality sapphire single crystals with excellent structural integrity.
Czochralski Method – CZ Method
The Czochralski method, also known as the CZ method, is a technique where a crystal is grown by slowly pulling and rotating a seed crystal from the molten solution contained in a crucible. This method was first discovered in 1916 by Polish chemist Jan Czochralski. In the 1950s, Bell Laboratories in the United States developed it for growing single-crystal germanium, and it was later adopted by other scientists for growing semiconductor single crystals such as silicon, as well as metal single crystals and synthetic gemstones.
The CZ method is capable of producing important gemstone crystals such as colorless sapphire, ruby, yttrium aluminum garnet (YAG), gadolinium gallium garnet (GGG), alexandrite, and spinel.
As one of the most important techniques for growing single crystals from the melt, the Czochralski method has been widely adopted, particularly the variant involving induction heating crucibles. Depending on the type of crystal being grown, the crucible material used in the CZ method can be iridium, molybdenum, platinum, graphite, or other high-melting-point oxides. From a practical standpoint, iridium crucibles introduce the least contamination to sapphire but are extremely expensive, resulting in higher production costs. Tungsten and molybdenum crucibles, while more affordable, tend to introduce higher levels of contamination.
Czochralski (CZ) Method Crystal Growth Process
First, the raw material is heated to its melting point to form a molten solution. A single crystal seed is then brought into contact with the surface of the melt. Due to the temperature difference at the solid–liquid interface between the seed and the melt, supercooling occurs. As a result, the melt begins to solidify on the seed surface and grows a single crystal with the same crystal structure as the seed.
At the same time, the seed crystal is slowly pulled upward at a controlled speed while being rotated at a certain rate. As the seed is gradually pulled up, the molten solution continues to solidify at the solid–liquid interface, eventually forming a rotationally symmetric single crystal ingot.
The main advantage of the Czochralski method is that the crystal growth process can be easily observed. The crystal grows on the surface of the melt without contacting the crucible, which significantly reduces crystal stress and prevents unwanted nucleation on the crucible walls. The method also conveniently allows the use of oriented seed crystals and “necking” techniques, which greatly reduce dislocation density.
As a result, sapphire crystals grown by the CZ method exhibit high structural integrity, and their growth rate and crystal size are quite satisfactory. Overall, sapphire crystals produced by this method have relatively low dislocation density and high optical uniformity. The main drawbacks are higher cost and limitations on the maximum crystal diameter.
Note: Although the CZ method is less commonly used for commercial sapphire crystal production, it is the most widely used crystal growth technique in the semiconductor industry. Because it can produce large-diameter crystals, approximately 90% of single-crystal silicon ingots are grown by the CZ method.
Melt Shape Method – EFG Method
The Melt Shape Method, also known as the Edge-defined Film-fed Growth (EFG) method, was independently invented in the 1960s by Harold LaBelle in the UK and Stepanov in the Soviet Union. The EFG method is a variation of the Czochralski technique and is a near-net-shape forming technology, meaning it grows crystal blanks directly from the melt in the desired shape.
This method not only eliminates the heavy mechanical machining required for synthetic crystals in industrial production but also effectively saves raw materials and reduces production costs.
A key advantage of the EFG method is its material efficiency and the ability to grow crystals of various special shapes. However, reducing defect levels remains a challenge. Therefore, it is more commonly used for growing shaped or complex materials. With recent advances in technology, the EFG method has also begun to be applied to produce substrates for MOCVD epitaxy, accounting for a growing share of the market.
Heat Exchange Method – HEM Method
In 1969, F. Schmid and D. Viechnicki invented a novel crystal growth technique known as the Schmid-Viechnicki method. In 1972, it was
Principle
The Heat Exchange Method utilizes a heat exchanger to remove heat, creating a vertical temperature gradient in the crystal growth zone with cooler temperatures at the bottom and hotter temperatures at the top. By controlling the gas flow inside the heat exchanger (usually helium) and adjusting the heating power, this temperature gradient is precisely managed, allowing the melt inside the crucible to solidify gradually from the bottom upward into a crystal.
Compared to other crystal growth processes, a notable feature of HEM is that the solid-liquid interface is submerged below the melt surface. Under these conditions, thermal and mechanical disturbances are suppressed, resulting in a uniform temperature gradient at the interface, which promotes uniform crystal growth and facilitates the production of crystals with high chemical uniformity. Additionally, because in-situ annealing is part of the HEM solidification cycle, the defect density is often lower than that of other methods.
Comparison of Several Sapphire Crystal Growth Techniques
Since the first synthetic gemstone was produced by the flame fusion method in 1902, various technologies for growing synthetic sapphire crystals have continuously evolved. Over the years, more than a dozen crystal growth methods have emerged, including flame fusion, Czochralski (CZ) method, and Kyropoulos (KY) method, among others. Each method has its own advantages and disadvantages, and they are used in different application fields. Currently, the main industrialized techniques include the Kyropoulos method, Czochralski method, Edge-Defined Film-Fed Growth (EFG) method, and the Vertical Horizontal Gradient Freeze (VHGF) method. The following section will introduce the typical sapphire crystal growth techniques in more detail.
Flame Fusion Method (Verneuil Process)
The Verneuil Process, also known as the Flame Fusion method, was named after the renowned French chemist Auguste Victor Louis Verneuil. He is best known for inventing the first commercially viable method for synthesizing gemstones. In 1902, he developed the "flame fusion" technique, which is still widely used today as an affordable method for producing synthetic gemstones.
As one of the most common methods for producing synthetic gemstones in the market, the flame fusion method is not only used to synthesize rubies and sapphires, but also applies to the production of synthetic spinel, synthetic rutile, synthetic star rubies and star sapphires, and even artificial strontium titanate, among others.
Working Principle
The flame fusion method, in simple terms, utilizes the high temperature generated by the combustion of hydrogen and oxygen. A loose powder of aluminum oxide (Al₂O₃) is fed through the oxyhydrogen flame. As the raw powder passes through the flame, it instantly melts into tiny droplets, which then fall onto a cooled seed rod, where they solidify and form a single crystal.
The following diagram shows a simplified schematic of the flame fusion crystal growth apparatus.
A key prerequisite for successfully synthesizing gemstones is the use of highly pure raw materials, with a minimum purity of 99.9995%. To synthesize rubies or sapphires, aluminum oxide (Al₂O₃) is the primary material. Efforts are typically made to reduce the sodium content, as sodium impurities can cause cloudiness and reduce the clarity of the gemstone. Depending on the desired color, small amounts of different oxide impurities can be added. For example, chromium oxide is added to produce rubies, while iron oxide or titanium oxide is added to produce blue sapphires. For other types, rutile is formed by adding titanium dioxide, and strontium titanate is formed by adding titanium oxalate. Other lower-value crystals can also be mixed into the starting materials.
High Efficiency and Low Cost!The flame fusion method is a highly efficient and low-cost approach to synthesizing artificial gemstones. It is considered the fastest crystal growth method among all synthetic gemstone techniques, enabling the production of large crystals in a short time—approximately 10 grams of crystal can be grown per hour. The crystal size of corundum-based gemstones varies, typically forming boule-shaped crystals ranging from 150 to 750 carats (1 carat = 0.2 grams), with diameters reaching 17–19 mm.
Compared to equipment used in other synthetic gemstone methods, flame fusion devices are the simplest in structure. This makes the flame fusion process especially suitable for industrial-scale production and gives it the highest yield among all synthetic methods.
However, crystals produced by the flame fusion method typically exhibit curved growth striations or color bands resembling the texture of a phonograph record, as well as characteristic bead-like or tadpole-shaped bubbles. These features limit their application in fields such as optics and semiconductors. Therefore, the flame fusion technique is mainly suitable for producing items with relatively small diameters, such as jewelry, watch components, and precision instrument bearings.
In addition, due to its low cost, sapphire crystals grown by the flame fusion method can also be used as seed or starting materials for other melt-based crystal growth methods.
Kyropoulos Method (KY Method)
The Kyropoulos method, abbreviated as the KY method, was first proposed by Kyropoulos in 1926 and initially used for the growth of large halide crystals, hydroxides, and carbonates. For a long time, this technique was mainly applied to the preparation and study of such crystals. In the 1960s and 1970s, the method was improved by the Soviet scientist Musatov and successfully adapted for the growth of sapphire single crystals. Today, it is considered one of the most effective solutions to the limitations of the Czochralski method in producing large crystals.
Crystals grown by the Kyropoulos method are of high quality and relatively low cost, making the technique well-suited for large-scale industrial production. Currently, around 70% of the sapphire substrates used globally for LED applications are grown using the Kyropoulos method or its various modified versions.
The single crystals grown by this method typically have a pear-shaped appearance (see the figure below), and the crystal diameter can reach sizes just 10–30 mm smaller than the inner diameter of the crucible. The Kyropoulos method is currently one of the most effective and mature techniques for growing large-diameter sapphire single crystals. Large-sized sapphire crystals have already been successfully produced using this method.
A recent news report highlighted a breakthrough in this area:
On December 22, the Crystal Growth Laboratory of Jing Sheng Crystals, in collaboration with its subsidiary Jinghuan Electronics, successfully produced the first ultra-large sapphire crystal weighing approximately 700 kg—marking a major innovation milestone.
Kyropoulos Crystal Growth Process
In the Kyropoulos method, the raw material is first heated to its melting point to form a molten solution. A single crystal seed (also known as a seed crystal rod) is then brought into contact with the surface of the melt. At the solid–liquid interface between the seed and the melt, a single crystal with the same lattice structure as the seed begins to grow. The seed crystal is slowly pulled upward for a short period to form a crystal neck.
Once the solidification rate at the interface between the melt and the seed becomes stable, the pulling stops and the seed is no longer rotated. From this point on, the crystal continues to grow downward by gradually controlling the cooling rate, allowing the melt to solidify from top to bottom. This results in the formation of a complete single-crystal ingot.
Characteristics of the Kyropoulos Method
The Kyropoulos method relies heavily on precise temperature control to grow crystals (temperature control is absolutely critical!). Its biggest difference from the Czochralski method lies in the fact that only the crystal neck is pulled; the main body of the crystal grows through controlled temperature gradients, without the additional disturbance of pulling or rotating. This makes the process more stable and easier to control.
While the crystal neck is being pulled, the power of the heater is carefully adjusted to bring the molten material into the optimal temperature range for crystal growth. This helps achieve an ideal growth rate, ultimately producing high-quality sapphire single crystals with excellent structural integrity.
Czochralski Method – CZ Method
The Czochralski method, also known as the CZ method, is a technique where a crystal is grown by slowly pulling and rotating a seed crystal from the molten solution contained in a crucible. This method was first discovered in 1916 by Polish chemist Jan Czochralski. In the 1950s, Bell Laboratories in the United States developed it for growing single-crystal germanium, and it was later adopted by other scientists for growing semiconductor single crystals such as silicon, as well as metal single crystals and synthetic gemstones.
The CZ method is capable of producing important gemstone crystals such as colorless sapphire, ruby, yttrium aluminum garnet (YAG), gadolinium gallium garnet (GGG), alexandrite, and spinel.
As one of the most important techniques for growing single crystals from the melt, the Czochralski method has been widely adopted, particularly the variant involving induction heating crucibles. Depending on the type of crystal being grown, the crucible material used in the CZ method can be iridium, molybdenum, platinum, graphite, or other high-melting-point oxides. From a practical standpoint, iridium crucibles introduce the least contamination to sapphire but are extremely expensive, resulting in higher production costs. Tungsten and molybdenum crucibles, while more affordable, tend to introduce higher levels of contamination.
Czochralski (CZ) Method Crystal Growth Process
First, the raw material is heated to its melting point to form a molten solution. A single crystal seed is then brought into contact with the surface of the melt. Due to the temperature difference at the solid–liquid interface between the seed and the melt, supercooling occurs. As a result, the melt begins to solidify on the seed surface and grows a single crystal with the same crystal structure as the seed.
At the same time, the seed crystal is slowly pulled upward at a controlled speed while being rotated at a certain rate. As the seed is gradually pulled up, the molten solution continues to solidify at the solid–liquid interface, eventually forming a rotationally symmetric single crystal ingot.
The main advantage of the Czochralski method is that the crystal growth process can be easily observed. The crystal grows on the surface of the melt without contacting the crucible, which significantly reduces crystal stress and prevents unwanted nucleation on the crucible walls. The method also conveniently allows the use of oriented seed crystals and “necking” techniques, which greatly reduce dislocation density.
As a result, sapphire crystals grown by the CZ method exhibit high structural integrity, and their growth rate and crystal size are quite satisfactory. Overall, sapphire crystals produced by this method have relatively low dislocation density and high optical uniformity. The main drawbacks are higher cost and limitations on the maximum crystal diameter.
Note: Although the CZ method is less commonly used for commercial sapphire crystal production, it is the most widely used crystal growth technique in the semiconductor industry. Because it can produce large-diameter crystals, approximately 90% of single-crystal silicon ingots are grown by the CZ method.
Melt Shape Method – EFG Method
The Melt Shape Method, also known as the Edge-defined Film-fed Growth (EFG) method, was independently invented in the 1960s by Harold LaBelle in the UK and Stepanov in the Soviet Union. The EFG method is a variation of the Czochralski technique and is a near-net-shape forming technology, meaning it grows crystal blanks directly from the melt in the desired shape.
This method not only eliminates the heavy mechanical machining required for synthetic crystals in industrial production but also effectively saves raw materials and reduces production costs.
A key advantage of the EFG method is its material efficiency and the ability to grow crystals of various special shapes. However, reducing defect levels remains a challenge. Therefore, it is more commonly used for growing shaped or complex materials. With recent advances in technology, the EFG method has also begun to be applied to produce substrates for MOCVD epitaxy, accounting for a growing share of the market.
Heat Exchange Method – HEM Method
In 1969, F. Schmid and D. Viechnicki invented a novel crystal growth technique known as the Schmid-Viechnicki method. In 1972, it was
Principle
The Heat Exchange Method utilizes a heat exchanger to remove heat, creating a vertical temperature gradient in the crystal growth zone with cooler temperatures at the bottom and hotter temperatures at the top. By controlling the gas flow inside the heat exchanger (usually helium) and adjusting the heating power, this temperature gradient is precisely managed, allowing the melt inside the crucible to solidify gradually from the bottom upward into a crystal.
Compared to other crystal growth processes, a notable feature of HEM is that the solid-liquid interface is submerged below the melt surface. Under these conditions, thermal and mechanical disturbances are suppressed, resulting in a uniform temperature gradient at the interface, which promotes uniform crystal growth and facilitates the production of crystals with high chemical uniformity. Additionally, because in-situ annealing is part of the HEM solidification cycle, the defect density is often lower than that of other methods.
