rus
Issue #4/2022
A. V. Naumov, V. V. Startsev
Preparation of Some Bulk Photonics Crystals by the Melt Crystallization Methods in Russia. Part I
Preparation of Some Bulk Photonics Crystals by the Melt Crystallization Methods in Russia. Part I
DOI: 10.22184/1993-7296.FRos.2022.16.4.272.286
The paper presents an overview of the state-of-the-art methods for melt preparation of some bulk photonics crystals. The first part of the review analyzes the current situation in Russia for some industrially significant photonics crystals. The factors that are important for modern production, as well as the determinant factors for control of the composition, structure, morphology, and other properties of industrial optical materials, are indicated.
The paper presents an overview of the state-of-the-art methods for melt preparation of some bulk photonics crystals. The first part of the review analyzes the current situation in Russia for some industrially significant photonics crystals. The factors that are important for modern production, as well as the determinant factors for control of the composition, structure, morphology, and other properties of industrial optical materials, are indicated.
Теги: crystal growth crystallization family czochralski and bridgman methods gaas single crystal выращивание кристаллов кристаллизация монокристалл семейство методов чохральского и бриджмена
Preparation of Some Bulk Photonics Crystals by the Melt Crystallization Methods in Russia. Part 1
A. V. Naumov, V. V. Startsev
Mechano-optical Design Bureau “Astrohn” JSC, Lytkarino, Moscow region, Russia
The paper presents an overview of the state-of-the-art methods for melt preparation of some bulk photonics crystals. The first part of the review analyzes the current situation in Russia for some industrially significant photonics crystals. The factors that are important for modern production, as well as the determinant factors for control of the composition, structure, morphology, and other properties of industrial optical materials, are indicated.
Keywords: crystal growth, crystallization, single crystal, family Czochralski and Bridgman methods, GaAs
Article received: 12.05.2022
Article accepted: 01.06.2022
Despite any progress in the creation of nanoscale 0D‑2D structures, the growth of bulk (3D) crystalline materials remains one of the most comprehensive and amazing achievements in the field of materials science. For example, the Czochralski-grown electronic grade silicon is one of the purest and most advanced materials ever made by the mankind. Modern production technology for the single-crystal ingots with a diameter of more than 300 mm and a weight of more than 200 kg can achieve an impurity level of ≤0.05 ppba. Such crystals are completely free from dislocations, and distribution of the microdefects, such as vacancies and interstitial atoms, is efficiently controlled. Due to their optical and electronic properties, single crystals of semiconductors АIII–BV, (GaAs, GaN, InP, InSb), AII–BVI (CdTe, ZnCdTe, ZnSe), etc., are applicable for various electronic and optoelectronic devices and components, such as LEDs, photoelectric detectors, high-power lasers, applications in the field of fiber-optic communications, wireless and satellite communications, and much more [1] (Table 1). The high demand for various single-crystal materials has stipulated the improvement of growth technologies that are currently used, as well as the development of new or improved single-crystal growth technologies. This paper is devoted to a review of the current situation in Russia in relation to a certain, very small part of optoelectronics and photonics crystals obtained by the directed melt crystallization methods. However, in our opinion, the given developmental problems are typical for preparation of other photonics materials [1, 2].
Single crystal preparation methods – general classification
The main crystal growing methods are as follows (Fig. 1):
Melt growth – status and problems
At present, about 70% of technically significant crystals are grown from a melt. Such crystals primarily include the inorganic functional materials with a relatively simple composition: elementary semiconductors and metals, oxides, halides, chalcogenides, silicates, germanates, borates, molybdates, tungstates, vanadates, niobates, etc. The main condition is their congruent melting, absence of polymorphic transitions. The sufficient chemical inertness is desirable. To date, various techniques have been developed for growing crystals from melts:
a) under the conditions of temperature change with a stationary crucible (Kyropoulos technique, etc.);
b) when moving a crystal in a temperature gradient (Czochralski method);
c) when moving the crucible or furnace in a temperature gradient (Bridgman method);
d) crucibleless methods (Verneuil technique, float zone melting method, etc.);
e) zone crystallization methods.
The Czochralski and Bridgman methods (Fig. 2) are the most used melt growth methods. The growth of single crystals from a melt makes it possible to produce the large high-quality single crystals in a relatively short period of time compared to other growth methods [1–4]. However, the melt growth method also has a number of disadvantages. Such disadvantages include the difficulties in maintaining a stable temperature during the crystal growth process and in achieving very high melting points for some materials, the difficulties in obtaining chemical homogeneity (this is especially noticeable in the case of a lot of elements in the crystal), reactivity of the molten material with the crucible, as well as high production and equipment costs.
Family of Czochralski Methods
The Czochralski method (Cz) is important for the single crystal production for electronic and optical applications, such as single crystals of silicon and germanium, АIII–BV (GaAs, GaN, InP, InSb), AII–BVI (CdTe, ZnCdTe, ZnSe), as well as some other single crystals of fluorides and oxides. The method belongs to the crucible directional crystallization methods and consists in pulling the stub from the melt together with the single crystal growing on it. The melt is located in the crucible. The resistance-type or RF heater is used, the crucible support is made of graphite, and the heat shields are made of graphite-based materials [1,2]. One of the advantages of this method is ability to obtain the dislocation-free single crystals with a given orientation, an ordered crystalline structure, certain optical and electrical parameters, and a high purity of a single crystal. These features have ensured the continuous development of the method over the entire period of its industrial application (Fig. 2) [3, 4].
The pulling speed depends on the physical and chemical specifications of the crystallized substance and on the crystal diameter (most often it ranges from 1 to 80 mm / hour). The upper limit of the growth rate is limited by the maximum allowable intensity of heat extraction through the crystal into the external environment. The traditional Czochralski method is characterized by the temperature gradients of tens and hundreds of degrees per cm, when the crystallization front-line shape almost follows the shape of isotherms. In the case of such gradients, the crystal-melt interface becomes atomically rough, the growth process is performed according to the standards mechanism, and the occurrence of planes on the rounded crystallization front-line leading to the inhomogeneities in the crystal is suppressed [6].
Another advantage is the absence of direct crystal contact with the crucible walls that makes it possible to obtain more perfect samples; if necessary, extraction of a crystal at any stage of growth; the ability to control the crystal geometric shape when changing the melt temperature and the pulling speed.
All these qualities have contributed to the widespread use of the Czochralski method in the single crystal growth of silicon, germanium, leucosapphire, yttrium-aluminum garnet, lithium niobate, gallium phosphide and arsenide, and many other materials. At present, the following substances are obtained in Russia by the Czochralski method (non-exhaustive list):
There are a number of modifications of the Czochralski method depending on the tasks to be solved. In order to achieve a more uniform distribution of impurities along the crystal length and cross section, the floating-crucible technique is used. In this case, a smaller container is placed in the main crucible with the melt, from which the crystal is grown. A small melt volume is connected with the main melt volume, providing the additional portions of the melt with a given concentration of dope additives.
To increase efficiency, the volume of melt consumed during crystallization can be replenished by feeding using the gradual melting in the peripheral area of the crucible (or outside the floating crucible) of a polycrystalline rod or by filling granules. The intermediate additional loads can also be applied. The grown single crystals are removed through the special linking devices, and the next furnace-charge portion is added to the crucible to grow the following crystal. In this paper, the Czochralski method, together with all its versions (Table 2), is considered as a whole.
Family of Bridgman (Bridgeman-Stockbarger) Methods
In the case of directed crystallization, the front-line slowly moves along the molten system, and a single crystal is grown behind the front-line. As a result of preferential heat removal in one direction, the directional crystallization of the melt occurs. This method is technically relatively simple and makes it possible to grow the crystals with a given diameter by selecting an appropriate crucible. The process can be performed in vertical and horizontal versions. It belongs to the group of techniques that are characterized by only one available interface between the liquid and solid phases during the crystallization process.
The Bridgman method is most often used to obtain metallic, organic, and also a number of dielectric single crystals, such as oxides, fluorides, sulfides, halides, etc. However, the method has a number of disadvantages:
the method is based on a crucible; therefore, the single crystals take the form of a crucible.
The authors of the vertical directional crystallization method (VGF) are L. V. Shubnikov and I. V. Obreimov (1924), as well as Tammann (1914) and Stöber (1925). In 1923, P. Bridgman made changes to the VGF method. The container is moved relative to a fixed temperature gradient (as the crystal grows, the container is descended and gradually exits the heated furnace, being cooled by the environmental air). The difference between the Bridgman method and the Obreimov-Shubnikov method is that in the first case the capsule with the melt is moved in a temperature gradient, while in the Obreimov-Shubnikov technique the melt is cooled in a temperature gradient. D. Stockbarger (1936) proposed new changes in the VGF process. In the Stockbarger technique, the heater is divided into two areas. A heat shield is installed between these areas to increase the temperature gradient. At present, the VGF method, together with all its versions, is considered as a whole [5, 6].
At present, the following substances are obtained in Russia by the Bridgman method (non-exhaustive list):
Some important considerations of the current state-of-the-art in Russia
The critical moment being a pressing challenge for the Russian industrial production of photonics crystals, is the provision with initial domestic high-purity materials at the level of 7N‑8N with the appropriate analytics.
In Russia, there are fairly powerful production capacities for the manufacture of the current-level growth equipment. The production facilities are available in Chernogolovka on the basis of the Experimental Plant of Scientific Instrumentation, in Bryansk on the basis of the Scientific and Production Association “Group of Companies for Mechanical Engineering and Instrumentation Engineering” LLC, in Saint-Petersburg on the basis of Apex LLC, in Volgograd on the basis of Cristars LLC, etc. It seems possible to produce the growth facilities in a fairly short period of time and in the required quantity in accordance with the individual specifications of the required photonic materials. The timeliness of the Listopad project should be noted. On the instructions of the Ministry of Industry and Trade of the Russian Federation, Lassard LLC has been developing a GaAs production facility by the VGF method since 2021.
There are companies that produce materials for the manufacture of various equipment.
The high-purity graphite is produced by Rosatom (Research Institute “Graphite”, Moscow), Etalon-detal LLC, Donkarb Grafit LLC (Chelyabinsk), KRIT LLC (Moscow). The purity level is sufficient for the manufacture of the heating unit elements of growth setups (heaters, shields), but is often insufficient for the manufacture of crucibles being in contact with the melt.
The high-purity quartz for the manufacture of crucibles, boats, pipes (especially with the precision dimensions), etc. on a commercial scale is produced only from natural quartz grits with a purity level of 2N‑4N. There is no production of synthetic high-purity quartz for optics with a purity level of 7N and higher. The synthetic quartz production technology, developed in the Soviet Union (Podolsky Chemico-Metallurgical Plant) and based on the high-temperature hydrolysis of silicon tetrachloride, makes it possible to obtain this material with a total impurity content of no more than 10–4 –I0–5 (by weight). Such quartz is about one or two orders cleaner than quartz obtained from the natural raw materials. It is necessary to develop the silica production processes by the vapor-phase high-temperature hydrolysis technique to provide for the industrial production of devices made of the quartz glass with a purity of 8N‑10N.
There are no domestic crucibles made of high-purity pyrolytic boron nitride for GaAs growth.
Operation of crucibles for growing the hard-melting crystals from platinum, iridium, and other precious metals is rather a purely economic issue for the relatively small private enterprises, especially when developing the growth techniques for the large-diameter crystals. In this case, the state support may consist in establishment of a rental mechanism for precious metals or increase in the working capital for enterprises [7].
Current situation in the production of AIIIBV single crystals in Russia on the example of GaAs
The crystal growth from a melt causes a number of difficulties related to the high melting point of AIIIBV compounds and significant dissociation pressures of these compounds at the melting point, forcing to use the high inert gas pressures or component in the growth chamber in order to suppress the compound dissociation, the complicated selection of the container material, not contaminating the grown single crystals with uncontrolled impurities. It requires an application of comprehensive process equipment, such as the high-pressure chambers, rotation and displacement mechanisms, high-temperature crucibles and heating elements. The advantages of melt growth methods include the high growth rates that make it possible to obtain the large bulk single crystals of almost any AIIIBV compounds. The commonly used growth rates are a few mm / hour.
As the temperature rises, the growing vapor pressure of the compound being grown above the melt prevents its further dissociation. The reactor movement in the temperature gradient to the cold area provides the conditions for melt crystallization [7, 8].
During the industrial production of GaAs single crystals, two growing methods are used: the Czochralski method with liquid encapsulation of the melt with a boric anhydride layer (Liquid Encapsulated Czochralski – LEC) and the vertical directional crystallization method (VDC) or Vertical Gradient Freeze (VGF), also the flux growth.
The most important feature of the LEC method is that a single crystal is grown at the sufficiently large axial and radial temperature gradients near the crystallization front-line, i. e. in the area of maximum material plasticity. In the case of LEC technique, a consequence of crystal growth at the high temperature gradients is a high dislocation density. Typical ND values in the undoped LEC-GaAs single crystals are up to (1–2) · 105 cm–2 for the ingot diameters of 100–200 mm.
The LEC material has a more uniform impact resistance distribution across the plate area. The material obtained by the VGF method has a lower dislocation density, however, their distribution over the plate area is more inhomogeneous. The availability of dislocations in the active areas of light-emitting structures is undesirable, since it leads to a rapid degradation of device specifications. Accordingly, the requirement for a low dislocation density (ND) is a basic requirement for the material used as a substrate. In practice, the following gradation has developed: the crystals with ND < 5.103–1.104 cm–2 are used in the production of LEDs, and the crystals with ND < 5.102 cm–2 are used in the production of lasers.
The production feature of the optoelectronic devices in comparison with the production of SHF integrated circuits is that the predominant part of the device cost falls on operations performed after the structure is divided into individual chips. Accordingly, it is not so important to increase the plate area during the production of optoelectronic devices. As a result, the plates with a diameter of up to 100 mm are still heavily used in the world production of LEDs and lasers, despite the fact that the industry has mastered the production of single crystals with a low dislocation density and a diameter of 200 mm. It is important to note that the single crystals grown by the VDC method have a higher cost than those grown by the LEC method. This is due to a 4–5 times lower crystallization rate and prevention of the repeated etching operation. When comparing the set of characteristics inherent in various growth methods, it can be seen that it is preferable (at least economically) to use LEC-GaAs for the most SHF applications, while the use of GaAs obtained by the VDC method has no alternative for production of LEDs, as well as for all optoelectronic applications. Therefore, both methods are available on the market, but with a significant predominance of VDC. The situation in Russia with indication of the process chain elements is shown in Table 4.
Conclusion
The international labor differentiation in the field of crystal growing is highly developed that is not surprising, since the high costs of process development and arrangement of new material production often do not allow the projects to be implemented by a separate commercial entity. Such projects are now being implemented by major corporations or research centers with the use of a number of benefits and tax preferences from the national governments supporting the projects.
At present, the general situation in the field of industrial production of bulk traditional photonics crystals in Russia can be characterized as “satisfactory with some significant shortcomings” (we do not discuss the lack of growth of electronic grade silicon with a diameter of more than 100 mm in this article). On a practical level, all methods for preparing bulk crystals developed in the 60s‑80s of the last century have been preserved and developed at a fairly modern level that indicates the high level of the scientific and engineering school built up in those years.
As for the process development for new bulk photonics materials (for example, Ga2O3, as a material for manufacturing the solar-blind UV sensors), the current development speed of such technologies is completely insufficient.
The industrial conditions occurred since February 24, 2022 require decisive actions. Despite the efforts made in recent years to restore the Russian optoelectronic industry, there are obvious scientific and technical issues with a number of important materials for micro- and optoelectronics. The main market participants in relation to the photonics materials are the Shvabe State Corporation, the Rosatom Atomic Energy Corporation (Giredmet JSC), institutes of the Russian Academy of Sciences, private entities that have an extensive database and significant competencies in the field of optical and photosensitive materials, equipment, technologies and their production and are able to solve the problem of arranging the small-tonnage production of optical quality materials. However, this problem can be solved only with the establishment of a technological capability that allows the domestic enterprises to produce the world-class materials.
Part 2 will provide the analysis of advantages and disadvantages of certain growing methods. The task of achieving parity with the world level, as well as doing the scientific, technical and technological groundwork for the bulk crystal preparation technology, can be solved by performance of a set of technical, financial and organizational measures within the framework of a more daring use of the public-private partnership mechanism.
AUTHORS
Naumov Arkady Valerievich, engineer-analyst, ASTROHN Technology Ltd,
https://astrohn.ru, Lytkarino, Moscow region, Russia.
ORCID: 0000-0001-6081-8304
Startsev Vadim Valerievich, Cand. of Technical Sciences,
ASTROHN Technology Ltd,https://astrohn.ru, Lytkarino, Moscow region, Russia.
ORCID ID: 0000-0002-2800-544X
A. V. Naumov, V. V. Startsev
Mechano-optical Design Bureau “Astrohn” JSC, Lytkarino, Moscow region, Russia
The paper presents an overview of the state-of-the-art methods for melt preparation of some bulk photonics crystals. The first part of the review analyzes the current situation in Russia for some industrially significant photonics crystals. The factors that are important for modern production, as well as the determinant factors for control of the composition, structure, morphology, and other properties of industrial optical materials, are indicated.
Keywords: crystal growth, crystallization, single crystal, family Czochralski and Bridgman methods, GaAs
Article received: 12.05.2022
Article accepted: 01.06.2022
Despite any progress in the creation of nanoscale 0D‑2D structures, the growth of bulk (3D) crystalline materials remains one of the most comprehensive and amazing achievements in the field of materials science. For example, the Czochralski-grown electronic grade silicon is one of the purest and most advanced materials ever made by the mankind. Modern production technology for the single-crystal ingots with a diameter of more than 300 mm and a weight of more than 200 kg can achieve an impurity level of ≤0.05 ppba. Such crystals are completely free from dislocations, and distribution of the microdefects, such as vacancies and interstitial atoms, is efficiently controlled. Due to their optical and electronic properties, single crystals of semiconductors АIII–BV, (GaAs, GaN, InP, InSb), AII–BVI (CdTe, ZnCdTe, ZnSe), etc., are applicable for various electronic and optoelectronic devices and components, such as LEDs, photoelectric detectors, high-power lasers, applications in the field of fiber-optic communications, wireless and satellite communications, and much more [1] (Table 1). The high demand for various single-crystal materials has stipulated the improvement of growth technologies that are currently used, as well as the development of new or improved single-crystal growth technologies. This paper is devoted to a review of the current situation in Russia in relation to a certain, very small part of optoelectronics and photonics crystals obtained by the directed melt crystallization methods. However, in our opinion, the given developmental problems are typical for preparation of other photonics materials [1, 2].
Single crystal preparation methods – general classification
The main crystal growing methods are as follows (Fig. 1):
- gas phase;
- melt;
- solution;
- solid phase.
Melt growth – status and problems
At present, about 70% of technically significant crystals are grown from a melt. Such crystals primarily include the inorganic functional materials with a relatively simple composition: elementary semiconductors and metals, oxides, halides, chalcogenides, silicates, germanates, borates, molybdates, tungstates, vanadates, niobates, etc. The main condition is their congruent melting, absence of polymorphic transitions. The sufficient chemical inertness is desirable. To date, various techniques have been developed for growing crystals from melts:
a) under the conditions of temperature change with a stationary crucible (Kyropoulos technique, etc.);
b) when moving a crystal in a temperature gradient (Czochralski method);
c) when moving the crucible or furnace in a temperature gradient (Bridgman method);
d) crucibleless methods (Verneuil technique, float zone melting method, etc.);
e) zone crystallization methods.
The Czochralski and Bridgman methods (Fig. 2) are the most used melt growth methods. The growth of single crystals from a melt makes it possible to produce the large high-quality single crystals in a relatively short period of time compared to other growth methods [1–4]. However, the melt growth method also has a number of disadvantages. Such disadvantages include the difficulties in maintaining a stable temperature during the crystal growth process and in achieving very high melting points for some materials, the difficulties in obtaining chemical homogeneity (this is especially noticeable in the case of a lot of elements in the crystal), reactivity of the molten material with the crucible, as well as high production and equipment costs.
Family of Czochralski Methods
The Czochralski method (Cz) is important for the single crystal production for electronic and optical applications, such as single crystals of silicon and germanium, АIII–BV (GaAs, GaN, InP, InSb), AII–BVI (CdTe, ZnCdTe, ZnSe), as well as some other single crystals of fluorides and oxides. The method belongs to the crucible directional crystallization methods and consists in pulling the stub from the melt together with the single crystal growing on it. The melt is located in the crucible. The resistance-type or RF heater is used, the crucible support is made of graphite, and the heat shields are made of graphite-based materials [1,2]. One of the advantages of this method is ability to obtain the dislocation-free single crystals with a given orientation, an ordered crystalline structure, certain optical and electrical parameters, and a high purity of a single crystal. These features have ensured the continuous development of the method over the entire period of its industrial application (Fig. 2) [3, 4].
The pulling speed depends on the physical and chemical specifications of the crystallized substance and on the crystal diameter (most often it ranges from 1 to 80 mm / hour). The upper limit of the growth rate is limited by the maximum allowable intensity of heat extraction through the crystal into the external environment. The traditional Czochralski method is characterized by the temperature gradients of tens and hundreds of degrees per cm, when the crystallization front-line shape almost follows the shape of isotherms. In the case of such gradients, the crystal-melt interface becomes atomically rough, the growth process is performed according to the standards mechanism, and the occurrence of planes on the rounded crystallization front-line leading to the inhomogeneities in the crystal is suppressed [6].
Another advantage is the absence of direct crystal contact with the crucible walls that makes it possible to obtain more perfect samples; if necessary, extraction of a crystal at any stage of growth; the ability to control the crystal geometric shape when changing the melt temperature and the pulling speed.
All these qualities have contributed to the widespread use of the Czochralski method in the single crystal growth of silicon, germanium, leucosapphire, yttrium-aluminum garnet, lithium niobate, gallium phosphide and arsenide, and many other materials. At present, the following substances are obtained in Russia by the Czochralski method (non-exhaustive list):
- silicon: Solar Silicon Technologies (Podolsk), KRIT (Moscow), Kremniy (Zelenograd);
- germanium: Germanium (Krasnoyarsk), Germanium and Applications (Moscow), Mechano-optical Design Bureau “Astron” (Lytkarino);
- indium antimonide, gallium arsenide, other AIIIBV: Giredmet (Moscow);
- yttrium-aluminum garnets YAG: Exciton (Stavropol);
- GGG, YAG, GSGG and AIIIBV : Research Institute of Materials Science (Zelenograd);
- La3Ga5SiО14, lithium tantalate and niobate: Fomos Materials (Moscow);
- various non-linear optical materials: Sobolev Institute of Geology and Mineralogy of the Siberian Branch of the RAS (Novosibirsk).
There are a number of modifications of the Czochralski method depending on the tasks to be solved. In order to achieve a more uniform distribution of impurities along the crystal length and cross section, the floating-crucible technique is used. In this case, a smaller container is placed in the main crucible with the melt, from which the crystal is grown. A small melt volume is connected with the main melt volume, providing the additional portions of the melt with a given concentration of dope additives.
To increase efficiency, the volume of melt consumed during crystallization can be replenished by feeding using the gradual melting in the peripheral area of the crucible (or outside the floating crucible) of a polycrystalline rod or by filling granules. The intermediate additional loads can also be applied. The grown single crystals are removed through the special linking devices, and the next furnace-charge portion is added to the crucible to grow the following crystal. In this paper, the Czochralski method, together with all its versions (Table 2), is considered as a whole.
Family of Bridgman (Bridgeman-Stockbarger) Methods
In the case of directed crystallization, the front-line slowly moves along the molten system, and a single crystal is grown behind the front-line. As a result of preferential heat removal in one direction, the directional crystallization of the melt occurs. This method is technically relatively simple and makes it possible to grow the crystals with a given diameter by selecting an appropriate crucible. The process can be performed in vertical and horizontal versions. It belongs to the group of techniques that are characterized by only one available interface between the liquid and solid phases during the crystallization process.
The Bridgman method is most often used to obtain metallic, organic, and also a number of dielectric single crystals, such as oxides, fluorides, sulfides, halides, etc. However, the method has a number of disadvantages:
the method is based on a crucible; therefore, the single crystals take the form of a crucible.
- Therefore, the stresses in them are inevitable that sometimes lead to their decrepitation;
- the thermal-expansion coefficients of materials of the crystal and the container in the crystal may lead to the significant internal stresses;
- it is impossible to directly observe the shape and position of the crystallization front-line;
- it is difficult to grow the crystals with large diameter in a tubular container, for example, more than 200 mm;
- the problem of crystal orientation control.
The authors of the vertical directional crystallization method (VGF) are L. V. Shubnikov and I. V. Obreimov (1924), as well as Tammann (1914) and Stöber (1925). In 1923, P. Bridgman made changes to the VGF method. The container is moved relative to a fixed temperature gradient (as the crystal grows, the container is descended and gradually exits the heated furnace, being cooled by the environmental air). The difference between the Bridgman method and the Obreimov-Shubnikov method is that in the first case the capsule with the melt is moved in a temperature gradient, while in the Obreimov-Shubnikov technique the melt is cooled in a temperature gradient. D. Stockbarger (1936) proposed new changes in the VGF process. In the Stockbarger technique, the heater is divided into two areas. A heat shield is installed between these areas to increase the temperature gradient. At present, the VGF method, together with all its versions, is considered as a whole [5, 6].
At present, the following substances are obtained in Russia by the Bridgman method (non-exhaustive list):
- CaF2, BaF2, MgF2, etc.: Electrosteklo (Moscow);
- cadmium-zinc-tellurium: Giredmet (Moscow);
- АII–BVI: Institue of Solid State Physics (Chernogolovka);
- LICAF-crystals (Kazan University);
- GaAs: Lassard (Obninsk);
- crystals of halides KPb2Hgl5 – KRS‑5, etc.: Giredmet (Moscow);
- nonlinear optical materials, double halide crystals (Sobolev Institute of Geology and Mineralogy of the Siberian Branch of the Russian Academy of Sciences, Novosibirsk).
Some important considerations of the current state-of-the-art in Russia
The critical moment being a pressing challenge for the Russian industrial production of photonics crystals, is the provision with initial domestic high-purity materials at the level of 7N‑8N with the appropriate analytics.
In Russia, there are fairly powerful production capacities for the manufacture of the current-level growth equipment. The production facilities are available in Chernogolovka on the basis of the Experimental Plant of Scientific Instrumentation, in Bryansk on the basis of the Scientific and Production Association “Group of Companies for Mechanical Engineering and Instrumentation Engineering” LLC, in Saint-Petersburg on the basis of Apex LLC, in Volgograd on the basis of Cristars LLC, etc. It seems possible to produce the growth facilities in a fairly short period of time and in the required quantity in accordance with the individual specifications of the required photonic materials. The timeliness of the Listopad project should be noted. On the instructions of the Ministry of Industry and Trade of the Russian Federation, Lassard LLC has been developing a GaAs production facility by the VGF method since 2021.
There are companies that produce materials for the manufacture of various equipment.
The high-purity graphite is produced by Rosatom (Research Institute “Graphite”, Moscow), Etalon-detal LLC, Donkarb Grafit LLC (Chelyabinsk), KRIT LLC (Moscow). The purity level is sufficient for the manufacture of the heating unit elements of growth setups (heaters, shields), but is often insufficient for the manufacture of crucibles being in contact with the melt.
The high-purity quartz for the manufacture of crucibles, boats, pipes (especially with the precision dimensions), etc. on a commercial scale is produced only from natural quartz grits with a purity level of 2N‑4N. There is no production of synthetic high-purity quartz for optics with a purity level of 7N and higher. The synthetic quartz production technology, developed in the Soviet Union (Podolsky Chemico-Metallurgical Plant) and based on the high-temperature hydrolysis of silicon tetrachloride, makes it possible to obtain this material with a total impurity content of no more than 10–4 –I0–5 (by weight). Such quartz is about one or two orders cleaner than quartz obtained from the natural raw materials. It is necessary to develop the silica production processes by the vapor-phase high-temperature hydrolysis technique to provide for the industrial production of devices made of the quartz glass with a purity of 8N‑10N.
There are no domestic crucibles made of high-purity pyrolytic boron nitride for GaAs growth.
Operation of crucibles for growing the hard-melting crystals from platinum, iridium, and other precious metals is rather a purely economic issue for the relatively small private enterprises, especially when developing the growth techniques for the large-diameter crystals. In this case, the state support may consist in establishment of a rental mechanism for precious metals or increase in the working capital for enterprises [7].
Current situation in the production of AIIIBV single crystals in Russia on the example of GaAs
The crystal growth from a melt causes a number of difficulties related to the high melting point of AIIIBV compounds and significant dissociation pressures of these compounds at the melting point, forcing to use the high inert gas pressures or component in the growth chamber in order to suppress the compound dissociation, the complicated selection of the container material, not contaminating the grown single crystals with uncontrolled impurities. It requires an application of comprehensive process equipment, such as the high-pressure chambers, rotation and displacement mechanisms, high-temperature crucibles and heating elements. The advantages of melt growth methods include the high growth rates that make it possible to obtain the large bulk single crystals of almost any AIIIBV compounds. The commonly used growth rates are a few mm / hour.
As the temperature rises, the growing vapor pressure of the compound being grown above the melt prevents its further dissociation. The reactor movement in the temperature gradient to the cold area provides the conditions for melt crystallization [7, 8].
During the industrial production of GaAs single crystals, two growing methods are used: the Czochralski method with liquid encapsulation of the melt with a boric anhydride layer (Liquid Encapsulated Czochralski – LEC) and the vertical directional crystallization method (VDC) or Vertical Gradient Freeze (VGF), also the flux growth.
The most important feature of the LEC method is that a single crystal is grown at the sufficiently large axial and radial temperature gradients near the crystallization front-line, i. e. in the area of maximum material plasticity. In the case of LEC technique, a consequence of crystal growth at the high temperature gradients is a high dislocation density. Typical ND values in the undoped LEC-GaAs single crystals are up to (1–2) · 105 cm–2 for the ingot diameters of 100–200 mm.
The LEC material has a more uniform impact resistance distribution across the plate area. The material obtained by the VGF method has a lower dislocation density, however, their distribution over the plate area is more inhomogeneous. The availability of dislocations in the active areas of light-emitting structures is undesirable, since it leads to a rapid degradation of device specifications. Accordingly, the requirement for a low dislocation density (ND) is a basic requirement for the material used as a substrate. In practice, the following gradation has developed: the crystals with ND < 5.103–1.104 cm–2 are used in the production of LEDs, and the crystals with ND < 5.102 cm–2 are used in the production of lasers.
The production feature of the optoelectronic devices in comparison with the production of SHF integrated circuits is that the predominant part of the device cost falls on operations performed after the structure is divided into individual chips. Accordingly, it is not so important to increase the plate area during the production of optoelectronic devices. As a result, the plates with a diameter of up to 100 mm are still heavily used in the world production of LEDs and lasers, despite the fact that the industry has mastered the production of single crystals with a low dislocation density and a diameter of 200 mm. It is important to note that the single crystals grown by the VDC method have a higher cost than those grown by the LEC method. This is due to a 4–5 times lower crystallization rate and prevention of the repeated etching operation. When comparing the set of characteristics inherent in various growth methods, it can be seen that it is preferable (at least economically) to use LEC-GaAs for the most SHF applications, while the use of GaAs obtained by the VDC method has no alternative for production of LEDs, as well as for all optoelectronic applications. Therefore, both methods are available on the market, but with a significant predominance of VDC. The situation in Russia with indication of the process chain elements is shown in Table 4.
Conclusion
The international labor differentiation in the field of crystal growing is highly developed that is not surprising, since the high costs of process development and arrangement of new material production often do not allow the projects to be implemented by a separate commercial entity. Such projects are now being implemented by major corporations or research centers with the use of a number of benefits and tax preferences from the national governments supporting the projects.
At present, the general situation in the field of industrial production of bulk traditional photonics crystals in Russia can be characterized as “satisfactory with some significant shortcomings” (we do not discuss the lack of growth of electronic grade silicon with a diameter of more than 100 mm in this article). On a practical level, all methods for preparing bulk crystals developed in the 60s‑80s of the last century have been preserved and developed at a fairly modern level that indicates the high level of the scientific and engineering school built up in those years.
As for the process development for new bulk photonics materials (for example, Ga2O3, as a material for manufacturing the solar-blind UV sensors), the current development speed of such technologies is completely insufficient.
The industrial conditions occurred since February 24, 2022 require decisive actions. Despite the efforts made in recent years to restore the Russian optoelectronic industry, there are obvious scientific and technical issues with a number of important materials for micro- and optoelectronics. The main market participants in relation to the photonics materials are the Shvabe State Corporation, the Rosatom Atomic Energy Corporation (Giredmet JSC), institutes of the Russian Academy of Sciences, private entities that have an extensive database and significant competencies in the field of optical and photosensitive materials, equipment, technologies and their production and are able to solve the problem of arranging the small-tonnage production of optical quality materials. However, this problem can be solved only with the establishment of a technological capability that allows the domestic enterprises to produce the world-class materials.
Part 2 will provide the analysis of advantages and disadvantages of certain growing methods. The task of achieving parity with the world level, as well as doing the scientific, technical and technological groundwork for the bulk crystal preparation technology, can be solved by performance of a set of technical, financial and organizational measures within the framework of a more daring use of the public-private partnership mechanism.
AUTHORS
Naumov Arkady Valerievich, engineer-analyst, ASTROHN Technology Ltd,
https://astrohn.ru, Lytkarino, Moscow region, Russia.
ORCID: 0000-0001-6081-8304
Startsev Vadim Valerievich, Cand. of Technical Sciences,
ASTROHN Technology Ltd,https://astrohn.ru, Lytkarino, Moscow region, Russia.
ORCID ID: 0000-0002-2800-544X
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