Methods for Bulk Growth of Inorganic Crystals: Crystal Growth

Methods for Bulk Growth of Inorganic Crystals: Crystal Growth

Methods for Bulk Growth of Inorganic Crystals: Crystal Growth$ J Friedrich, Fraunhofer Institute IISB, Erlangen, Germany r 2016 Elsevier Inc. All righ...

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Methods for Bulk Growth of Inorganic Crystals: Crystal Growth$ J Friedrich, Fraunhofer Institute IISB, Erlangen, Germany r 2016 Elsevier Inc. All rights reserved.

1 2 2.1 2.1.1 2.1.2 2.1.3 2.2 2.2.1 2.2.2 2.3 2.4 3 3.1 3.2 3.3 3.4 3.5 4 4.1 4.2 5 References

1

Introduction Melt Growth Crystal Pulling Czochralski Technique Nacken–Kyropulous Method Shaped Crystal Growth Directional Solidification Gradient Freeze and Bridgman−Stockbarger Techniques Zone Melting in Crucibles Floating Zone (Crucible-Free Zone Melting) Verneuil Technique Solution Growth Principle of Solution Growth Low Temperature Solution Growth High Temperature Solution Growth Hydrothermal Growth Growth under Extreme High Pressure Vapor Growth Sublimation Technique (Physical Vapor Transport) Chemical Vapor Transport and Chemical Vapor Deposition Conclusions

1 1 1 1 4 5 5 6 8 10 10 11 12 12 12 13 14 14 15 16 16 16

Introduction

Many modern technological systems would not exist without the availability of synthetic single crystals. Therefore, the technology to prepare and to produce bulk single crystals – which is called ‘crystal growth’ – is one of the current key technologies. This article gives a brief overview about the techniques which are used to grow bulk single crystals of inorganic materials. More detailed information on the different technologies and materials can be found in Nishinaga.1 The fundamental physical and physicochemical mechanisms which govern the crystal growth processes are treated in Rudolph.2 The basics and the methods for growing epitaxial films are described in Kuech.3 The status of the growth of organic materials is given in Nishinaga.1 The structure of the article is sketched in Figure 1.

2

Melt Growth

The most frequently used and most important method of producing bulk single crystals is by solidifying the material from its melt (melt growth). In a few cases melt growth is not feasible for certain reasons, for example, if the crystal to be grown has no congruent melting point or if the melting point or vapor pressure are too high. In these cases the crystals have to be grown either from solutions or from the vapor phase.

2.1 2.1.1

Crystal Pulling Czochralski Technique

The Czochralski method (Cz) is the most important method for the production of bulk single crystals of a wide range of electronic and optical materials (Figure 2). At the beginning of the process, the feed material is put into a cylindrically shaped crucible and melted by resistance or radio-frequency heaters. After the feed material is completely molten a seed crystal with a diameter of ☆

Change History: July 2015. J. Friedrich added the following new figures; 5, 10, 13, 14 and 22. Figures 16 and 18 are modified; Figures 2 and 4 are replaced with new figures.

Reference Module in Materials Science and Materials Engineering

doi:10.1016/B978-0-12-803581-8.01010-9

1

2

Methods for Bulk Growth of Inorganic Crystals: Crystal Growth

Bulk Growth

Melt Growth

Solution Growth

Crystal Pulling

Vapor Growth

Growth from Aqueous Solutions

Physical Vapor Transport

Flux Growth

Chemical Vapor Transport

Czochralski Technique Hydrothermal Growth Liquid Encapsulated Czochralski

High Pressure Synthesis

Vapor Controlled Czochralski Magnetic Field Czochralski Other Czochralski Variants Nacken−Kyropulous Shaped Crystal Growth Directional Solidification Gradient Freeze and Bridgman Technique Heat Exchanger Method Skull Melting Detached Solidification Zone Melting Floating Zone Verneuil Technique

Figure 1 Overview of methods for bulk crystal growth.

typically a few mm is dipped from top into the free melt surface and a small portion of the dipped seed is melted. A melt meniscus is formed at the contact interface between seed and melt. Then, the seed is slowly withdrawn from the melt (often under rotation) and the melt crystallizes at the interface by forming a new crystal portion. During the further growth process, the shape of the crystal, especially the diameter, is controlled by carefully adjusting the heating power, the pulling rate and the rotation rate of the crystal. Usually an automatic diameter control is applied. This diameter control is based, either on the control of the meniscus shape (e.g., for silicon) or on the weighing of the crystal (e.g., for GaAs, InP) or of the melt (e.g., for oxides). In order to control the convective heat and species transport in the melt including the shape of the solid–liquid interface, which is playing one of the most dominant roles in terms of the crystal quality, a proper combination of crystal and crucible rotation is used during the whole process. The most important technical application of the Cz method is the growth of dislocation – free silicon crystals with diameters up to 300 mm and a weight up to 300 kg in industrial production (see Figure 3). Silicon crystals with diameters of 450 mm and a weight exceeding 300 kg were already demonstrated. Also several technically important oxide and fluoride crystals like garnets, niobates, tantalates, silicates, vanadates, aluminates, germanates are grown by the Cz method. The heat and species transport in the melt has a very strong influence on the crystal properties as they are responsible for the uniformity of dopants on the micro- and macro-scale as well as for the shape of the solid–liquid interface and, therefore, for the thermal stress generated in the crystal. During crystal growth the use of stationary or time dependent electromagnetic fields enables the control of the flow in electrically conducting melts (e.g., semiconductors). A Lorentz force is generated in the melt which depends on the magnetic field configuration and leads either to a damping of the flow or to a stimulation of a certain flow pattern. Today, magnetic fields (Figure 4) are quite common in the industrial production of silicon crystals with 300 mm diameter. In order to improve the axial uniformity of the dopant distribution in the grown crystal, the crystal can be grown in such way that the melt volume is kept constant by supplying the solidified portion from a source. Such a source can be realized either by continuously supplying the melt with a feed (Continuous Czochralski method (CCz)) or by placing an inner crucible with holes in a larger outer crucible (Double Crucible Czochralski technique). Although the feasibility of these methods was demonstrated, its

Methods for Bulk Growth of Inorganic Crystals: Crystal Growth

a)

b)

c)

d)

e)

g)

h)

d)

j)

3

Seed

Crystal Heater Melt Crucible

f)

Figure 2 Schematic of the principle of the Czochralski method (left) and illustration of the different steps (a–j) of the Cz process for growing a Si crystal. (a) The polycrystalline feedstock is melted (b) in a crucible. (c, d) Seeding procedure: The seed crystal is dipped into the melt, followed by Dash necking (e), shouldering (f), cylindrical growth (g), growth of end cone (h), lift off (i), cooling down and removing of the crystal (j).

Figure 3 Silicon crystal with a diameter of 300 mm and a weight exceeding 250 kg, grown by the Cz method (by courtesy of Siltronic AG4).

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Methods for Bulk Growth of Inorganic Crystals: Crystal Growth

a

b

c

Figure 4 Examples of typical configurations for magnetic field in the MCz method. (a) transversal; (b) axial; (c) cusp. Gray areas are sections of magnetic coils, the curved lines denote the magnetic field lines.

polysilicon rod or chunks

silicon melt

melting

pulling

removal

recharging

seed

reseeding

repeat Figure 5 Sketch of a Cz process with repeated use of the crucible by recharging and without cooling down the crucible between growth runs.

use for production of electronic silicon is limited owing to technical and material problems preventing a high yield. However, for growth of solar silicon it is of interest, as well as the so-called ‘multiple pulling‘ technique (see Figure 5). In the multiple pulling Cz method the crucible is kept at melting temperature after the growth process and filled again with silicon after removal of the grown crystal. The advantage of this method is cost saving by multiple using of the crucible and saving the time period for cooling down and heating up periods. If the material to be grown has a high partial pressure of one or more components at the melting point, a modified Cz-set-up can be used which is called the Liquid Encapsulated Czochralski method (LEC). In this LEC-set-up a liquid, usually boric oxide, is applied to encapsulate the melt and the crystal in order to prevent the evaporation of a volatile component from the melt and crystal. In this case the pressure of the inertial gas in the growth chamber must exceed the partial pressures of the volatile components. The LEC-technique is used for the growth of the compound semiconductors GaAs, GaP, InP, PbSe, and PbTe. However, the low thermal conductivity of the liquid encapsulant causes large temperature gradients and large temperature nonlinearities in the growing crystal. This is unfavorable with respect to the crystal quality as a relatively large number of structural defects (dislocations) are generated in the crystal. LEC is therefore often replaced by the Vertical Gradient Freeze (VGF) technique (see below).

2.1.2

Nacken–Kyropulous Method

The Kyropulous (or Nacken-Kyropulous) method is rather similar to the Cz method. The main difference is that the crystal is not growing at the top of the melt as in the Cz method but is partly immersed in the melt. In order to extract efficiently the heat from the growing crystal usually a seed with a large diameter is used. Growth is achieved by either lowering continuously the heating power or by pulling out the growing crystal carefully from the melt. This method is widely used to produce sapphire boules for cover glass and epitaxial substrates, alkali halide crystals for optical components (NaCl, KBr) and alkali iodides for scintillators (NaI, CsI). Crystals with dimensions over 500 mm have been achieved.

Methods for Bulk Growth of Inorganic Crystals: Crystal Growth

5

seed

crystal

die

meniscus

heater

melt

crucible Figure 6 Schematic of the principle of the Edge defined Film fed Growth (EFG) method.

2.1.3

Shaped Crystal Growth

For a variety of technological applications, crystals of specified size and shape like plates, rods, tubes, fibers are required. Often it is more cost efficient to grow shaped crystals instead of preparing the needed shape from cylindrical boules as they are grown by for example, the Cz method. The diameter of a crystal in the Cz pulling techniques is controlled by the shape and position of the melt meniscus at the triple contour crystal-melt-gas (or encapsulant). This control can be improved by using a die which is in contact to the melt. The melt raises in a narrow channel within the die due to capillary effects. When the seed crystal is dipped onto the melt portion at the top of the die, the meniscus is formed and liquid solidifies at the seed while the seed is pulled away (Figure 6). This causes new liquid to raise up in the die. The heat of crystallization is removed from the solid–liquid interface very efficiently by conduction and by radiation. The method for growing shaped crystals as described above is commonly known as Stepanov method (non wetting die-melt system) or Edge-defined Film-fed Growth (EFG) (wetting die-melt system). In the case of fiber crystals the method is also called Micro Pulling Down method (mPD) because a crystal fiber with a diameter of only some hundred microns can be grown from a die by pulling the crystal downwards (Figure 7(a)). Variants of the EFG method are the Non-capillary Shaping (NCS) method where the diameter of the die channel is greater than the value of the capillary constant (Figure 7(b)). The NCS method offers advantages in order to avoid the generation of bulk micro defects in the growing crystal caused by gaseous or solid inclusions. Applying a continuous displacement to the growing crystal or the die allows one to grow crystals with more complex shapes (Figure 7(c)), like for example domes or hollow cones. This method is named Growth from an Element of Shape (GES). Nowadays, shaped crystal growth is mainly used for industrial production of sapphire. Sapphire crystals in form of ribbons up to 150 mm in width, tubes up to 85 mm in diameter, fibers, near-net-shaped domes (up to 80 mm in diameter), rods of various cross-sections, rods with capillary channels, etc., were grown by the shaped growth techniques described above (Figure 8). For photovoltaic applications thin silicon ribbons or thin wall (octagonal) tubes have been industrially produced by EFG like techniques. But, most of these technologies were stopped due to cost and productivity reason a few years ago.

2.2

Directional Solidification

Opposite to crystal pulling, the growth of single crystals in directional solidification is achieved by melting a charge in a crucible and controlled freezing of the melt inside this crucible from one end (seed) to the other (tail). Directional solidification is the basic principle of controlled solidification of a melt. It has been used for many materials for a long time and has its origin and greatest importance in the field of metallurgical casting. Directional solidification has several advantages compared to the Cz methods. It operates usually under stable hydrodynamic conditions, it is well suited for computer modeling and for automatic process control, the crystal grows under low thermal stress conditions, cylindrical or square shaped crystals are grown without any diameter control, and the equipment is less expensive.

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Methods for Bulk Growth of Inorganic Crystals: Crystal Growth

crucible

outer heater

melt die

seed die crystal

crystal

after heater

heater

seed

seed melt

a)

crystal heater

crucible

die die

b)

melt crucible c) Figure 7 Schematic of the principle of the mPD (a), NCS (b), GES (c) methods.

Figure 8 Shaped sapphire crystals grown by the EFG, NCS, and GES techniques. Source: Sapphire: V. N. Kurlov, Properties, Growth, and Applications in Encyclopedia of Materials: Science and Technology, Elsevier 2001.

However, a crucial limitation of directional solidification is that a strong interaction between the growing crystal and the crucible material might occur, resulting in crystal imperfections which limits the yield and hence the industrial applicability. Furthermore, the failures generated in the crystal during growth for example, by twinning or polycrystal formation cannot be observed in situ during growth. A correction by re-melting as it is done in the Cz methods is, therefore, not possible.

2.2.1

Gradient Freeze and Bridgman–Stockbarger Techniques

The crystal growth configuration consists typically of a tube furnace which provides a temperature profile with a negative gradient parallel to the growth direction (Figure 9). The method can be carried out by moving the growth interface in horizontal or vertical direction. The single-crystal seed is positioned at one end of the horizontal boat or the lower end of the vertical crucible. Then, the crystal is grown by a controlled shifting of the temperature profile relatively to the solid–liquid interface. Three possibilities exist: moving the crucible relatively to the fixed furnace (Bridgman–Stockbarger method); moving the furnace relatively to the fixed crucible; and without any mechanical movement, only by shifting the temperature profile by a controlled change of the heating powers of the furnace (Tamann–Stoeber or Gradient Freeze Method). Based on one of these principles one calls the technique the horizontal Bridgman (HB) or the vertical Bridgman (VB) or the Vertical Gradient Freeze (VGF) method. From an industrial point of view, the vertical configurations (VGF, VB) are preferred because they result in a higher yield of appropriate shapes of the wafers compared to the HB method. The increasing interest in the use of directional solidification results from the fact that it uses the simplest principle of melt growth and that the structural perfection of the single crystals with respect to thermal stress and dislocation formation is better than of crystals produced by Cz methods.

crucible

t3

t

Position

Position

Methods for Bulk Growth of Inorganic Crystals: Crystal Growth

7

crucible

melt

melt

adiabatic zone

crystal

crystal

2

t1

Temperature

seed

seed

Tm

Tm heaters

Temperature heaters

Figure 9 Schematic of the principle of directional solidification methods; (a) Vertical Gradient Freeze Method; (b) Vertical Bridgman Method.

Figure 10 GaAs crystals and wafers grown by the VGF technique with 2", 3", 4", and 6" diameter (by of courtesy of Freiberger Compound Materials5).

Nowadays, the VGF or VB method is mainly used for the production of compound semiconductors like high quality GaAs (Figure 10) and InP single crystals to be used as substrate for laser diodes or CdZnTe crystals for infrared detectors. A variety of oxide and fluoride crystals are grown by VGF/VB methods for high value applications, too. As one example might serve CaF2 crystals (Figure 11) utilized as lenses for the deep ultra violet wavelength in the semiconductor lithography technology. In the field of metallurgy, directional solidification is used to grow single crystalline turbine blades made from nickel-based superalloys (Figure 12). In the last decade the directional solidification (DS) method has gained much attention for the industrial production of silicon ingots for photovoltaic applications. The DS method is usually a combination of the classical VB and VGF method (Figure 13). In this case the crucibles have square cross sections with dimensions of multiple the dimensions of solar cell wafers (today 156 mm  156 mm). Three DS variants are in use. DS without seed crystals resulting in standard ‘multi-crystalline‘ ingots (see Figure 14(a)), DS with small sized silicon particles at the flat bottom resulting in so-called ‘high performance multi‘ silicon ingots (Figure 14(b)), and DS with single crystalline seed plates at the flat bottom resulting in ‘quasi-mono-crystalline‘ ingots (Figure 14 (c)). At present more that 50% of solar silicon is grown by DS. Today, often an active cooling below the crucible bottom is used during DS of solar silicon to extract the heat more efficiently. Originally, a gas-cooled heat exchanger was applied. Therefore, this special variant of the DS technique is also named Heat Exchanger Method (HEM). HEM was developed mainly for the growth of large sapphire crystals with diameters up to 500 mm. Today, large BGO (Bismuth Germanate) crystals are also grown industrially by HEM in addition to sapphire and silicon. For refractory materials with high chemical reactivity and melting points above 2000 1C often no suitable crucible material exists. This problem can be overcome by using the Skull Melting technique, where the material to be grown serves as an inner cover of a cooled crucible. Radio-frequency heating is used to melt the charge contained in a water cooled crucible (e.g., copper). As a result of the water cooling, a skull of sintered material is formed and acts as a non-contaminating inner crucible wall for the melt. Crystals are typically grown by directional solidification of the molten charge without a seed crystal. The method is used, for example, to grow several hundreds of tons of cubic ZrO2 per month for gemstones. Sometimes this technique is also named Cold Crucible Technology especially when the melt is levitated by a RF field in such way that no contact exists between the molten charge and the water cooled crucible. One interesting variant of the directional solidification method is the so-called detached solidification. In detached solidification, which is also called de-wetting growth process, a small gap between the growing crystal and the crucible wall exists. The formation and stability of such a gap depends on the wetting behavior between the melt and the crucible material, the contact

8

Methods for Bulk Growth of Inorganic Crystals: Crystal Growth

Figure 11 CaF2 crystals as lense material for the microlithography are grown by modified Bridgman–Stockbarger methods (by of courtesy of Hellma Materials6).

Figure 12 Single crystalline turbine blades made by directional solidification of nickel-based superalloys (by of courtesy of Institute of Science and Technology of Metals, University Erlangen-Nuremberg7).

angle, the growth angle, and external forces (especially a certain gas pressure difference between the gap and the top of the melt). The experimental results achieved so far for CdTe, Ge, GeSi, and GaSb on earth and also under microgravity demonstrate that the structural perfection of the crystals is generally improved when the de-wetting effect occurred. However, there remains still a lot of research to be done until this technology can be transferred into an industrial production.

2.2.2

Zone Melting in Crucibles

In zone melting only a part of the solid is molten in a crucible or a boat by using a resistance or induction zone heater; Figure 15 shows the horizontal arrangement of zone melting. Growth is achieved by moving the heat source relatively to the axis of the container. Nowadays, the importance of zone melting for bulk crystal growth is mainly limited to its use for purification of the feed material. Impurities with a segregation coefficient k0o1 can be accumulated very efficiently by multiple passes of melt zones

axial Position

Methods for Bulk Growth of Inorganic Crystals: Crystal Growth

Top heater Side heater

Melt t3 mc-crystal

t2 Tm

Crucible s/l interface

t1

Temperature Bottom heater Figure 13 Schematic sketch of a DS set-up (right) for the crystallization of mc–Si. The sketched axial temperature profiles (left) are related to different time steps (t1, t2, t3) of the process; t1 affiliates the beginning of crystallization, t2 and t3 intermediate steps. Tm denotes the melting temperature. The dashed horizontal lines mark the position of the s-l interface at the corresponding time steps (t1, t2, t3).

b

a)

c) ~100 mm

Figure 14 Scan images revealing different grain orientations of vertical cuts (parallel to the growth direction) of silicon crystals grown without seeding (a: standard multi), with polycrystalline silicon particles as seeds (b: high performance multi), and with single crystalline seed (c: quasimono). The blue lines mark the seed surface before the melting process. The dotted lines mark the shape and position of the interface at beginning of growth (by of courtesy of Fraunhofer IISB, Erlangen8).

heater

seed

crystal

melt

polycrystalline feedstock

boat

Figure 15 Schematic of the principle of zone melting.

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Methods for Bulk Growth of Inorganic Crystals: Crystal Growth

poly rod “needle eye” induction coil melt

crystal seed

Figure 16 Schematic principle (left) and photography (right) of the Floating Zone method for growing Si crystals.

towards the end of the ingot. Therefore, zone melting is still the industrial standard technology for purification of metals and for example, germanium feedstock.

2.3

Floating Zone (Crucible-Free Zone Melting)

The floating zone (FZ) technique is a crucible-free crystal growth method. In FZ growth, the molten zone is kept between two vertical solid rods by its own surface tension (Figure 16). The FZ process is started by dipping a seed crystal into one end of the moltenzone. Then the molten zone is moved towards the feed rod and at the other zone end the crystal is growing. The main advantage of the FZ technique is the absence of a container which avoids any contamination by the crucible material. Furthermore the generation of crystal defects caused by an interaction between the growing crystal and the container is avoided. Therefore, the technique is especially useful for growth of highly reactive materials, intermetallic compounds, and refractory materials. The heating of the molten zone can be achieved by several methods including radio frequency induction, optical, electron beam, laser and resistance heating. The industrial application of the FZ technique is rather limited because the maximum sample diameter is relatively small due to the difficulties to maintain a stable molten zone. Usually the maximum height of the molten zone is only a few mm. It depends mainly on the ratio between the surface tension and the density of the melt. In the industrial FZ growth of silicon the so-called needle-eye technique is used to stabilize the shape of the molten zone by an electromagnetic pressure field generated with a specially shaped rf-induction coil. This enables the production of silicon crystals with diameters up to 200 mm and extremely low oxygen contamination (o1016 cm 3). Such pure silicon crystals are needed for the fabrication of power electronic devices. Crystals of other materials with higher density and lower surface tension like for example, GaAs or GaSb can only be grown with larger diameters under microgravity conditions (Figure 17).

2.4

Verneuil Technique

The Verneuil technique (Figure 18), also called the flame fusion technique, is one of the oldest methods for growing refractory oxide crystals. The feed consists of a fine powder of the crystal material which is transported into the flame of an oxygen-hydrogen burner. The fused particles are falling down on top of the growing crystal and form a thin melt film on top of it. A continuous growth process is achieved by slowly pulling down the crystal. It is important to balance the rate of powder feed and the rate of pulling to maintain a constant growth rate and crystal diameter. Otherwise, the thermal field in the vicinity of the solid–liquid interface is changed which results in poor crystal quality. The advantage of this process is the low equipment costs. The main disadvantage is the extremely large temperature gradient causing a high density of stress-induced crystal defects. Therefore, often an after heater is used in order to prevent the crystal from cracking.

Methods for Bulk Growth of Inorganic Crystals: Crystal Growth

11

Figure 17 GaAs crystals grown by the Floating Zone technique; left: sample grown in space under microgravity during the German Spacelab mission D2, right: sample grown on earth with maximum size (by of courtesy of Fraunhofer IISB, Erlangen8).

hammer

oxygen

feedstock as powder

hydrogen burner flame

muffle

observation window

thin melt film

crystal

ceramic rod

Figure 18 Schematic principle of the Verneuil method.

Nowadays, the Verneuil technique is exploited for the economic mass production of sapphire and ruby crystals for jewelry and precision bearings (B700 t per year). Most high-melting oxide compounds can be grown using this technique.

3

Solution Growth

Growth of a crystal from a solution is an option for cases where melt growth is not possible for reasons like extremely high melting temperature, extremely high vapor pressure, non-congruent melting or destructive structure transformations during cooling of the

12

Methods for Bulk Growth of Inorganic Crystals: Crystal Growth

ti stirrer

immersion heater

crystals

temperature sensor

stirrer solution

fluid bath

Figure 19 Apparatus for crystal growth by the temperature changing method using a fluid bath. Source: K. Sangwal, Growth from Solutions in Encyclopedia of Materials: Science and Technology, Elsevier 2001.

crystal (e.g., quartz). In solution growth the elements or compounds of the material to be crystallized are dissolved in a suitable solvent. Crystal growth is achieved by supersaturation of the solution and deposition of the crystal material on a seed crystal.

3.1

Principle of Solution Growth

Firstly a solvent must be found which dissolves the crystal material and is not incorporated in the growing crystal. Then the solution has to be supersaturated which can be achieved by different means: 1. In the temperature changing technique, the solution is supersaturated by slow cooling as the solubility is usually decreasing with decreasing temperature (supersaturation is achieved by slow heating, if the solubility decreases with increased temperature) 2. In the evaporation technique, the supersaturation is obtained by controlled evaporating of the solvent at a constant temperature. 3. In the temperature gradient technique, two regions of different temperatures T1 and T2 (oT1) are established. In the region T1 the feed material is dissolved, while crystal growth takes place in the region with temperature T2. In solution growth the growth rate is mostly limited by the transport rate of the solute to the growth interface. Therefore, an active mixing for example, by stirring and crucible rotation techniques are in use (see Figure 19). An important issue in solution growth is the selection of a suitable solvent. At the growth temperature the solvent should generally have a sufficient solubility of the material to be grown and also a low vapor pressure and a low viscosity.

3.2

Low Temperature Solution Growth

Low temperature solution growth is one of the simplest methods to grow single crystals especially if aqueous solutions are used (Figure 19). Industrially important crystals grown by low temperature solution growth are potassium dihydrogen phosphate (KDP, Figure 20) ammonium dihydrogen phosphate (ADP), both for electro-optic applications.

3.3

High Temperature Solution Growth

Practically any material can be grown by high-temperature solution growth (often also called flux growth). Mainly low melting temperature solvents (fluxes), often molten salts and oxides, are used as solution. For a variety of technical important materials like Sr1-xBaxNb2O6 (SBN), Pb1-xBaxNb2O6 (PBN), LiNbO3, BaTiO3 (BTO), BaB2O4 (BBO), the so-called Top Seeded Solution Growth method (TSSG) is used (Figure 21). TSSG corresponds to the Cz method the only difference is that a solution is used instead of the melt.

Methods for Bulk Growth of Inorganic Crystals: Crystal Growth

13

Figure 20 Potassium dihydrogen phosphate (KDP) single crystal grown from aqueous solution (by of courtesy of CEA9).

seed heater crystal T2

solution

T1 >T2 nutrient

baffle

crucible

Figure 21 Typical set up for the temperature gradient solution growth technique with top seeding (TSSG).

A special industrial application case for solution growth is CdTe crystals for X-ray detectors which are grown by the so-called Traveling Heater Method (THM). THM is a variant of zone melting, but the ‘melt’ zone is replaced by a solution (e.g., Te in the case of CdTe). The movement of the ‘traveling’ heater causes the dissolution of the feed at the high temperature end of the zone and growth at the low temperature end. For other semiconductor crystals the use of solution growth is limited to research. In some laboratories the solution growth of ZnSe, ZnS, ZnTe as well as of SiC and GaN is investigated. The principle of solution growth is also used to grow thin layers epitaxially on substrates. This technique, which is called Liquid Phase Epitaxy (LPE) is used for example, for growth of HgCdTe layers on CdZnTe substrates for infrared applications. In summary one can state that crystals grown from solution have less structural defects than crystals grown from melts due to the lower growth temperature. However, the growth rates are small compared to melt growth techniques and sometimes the solvent contaminates the crystal.

3.4

Hydrothermal Growth

Hydrothermal growth is a special variant of solution growth from aqueous solution at temperatures above 100 1C. The increased temperatures (up to about 400 1C) are necessary to enhance the solubility of the solute in the aqueous solvent. But this causes

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Methods for Bulk Growth of Inorganic Crystals: Crystal Growth

Figure 22 Production of quartz crystals in the world’s largest autoclave. Source Bulk Crystal Growth (Editor P. Rudolph) in Handbook of Crystal Growth, 2nd Edition, Elsevier 2014.2

extremely high pressure (several kbars) of the supercritical aqueous solvent. The growth process includes typically heterogeneous reactions and is supported by mineralizers. A precisely controlled temperature gradient is used to achieve the crystallization at the seed location (principle c in Section 3.1). The most important application of the hydrothermal method is for industrial growth of synthetic quartz (SiO2) crystals (Figure 22). It is carried out in special vessels called autoclave or bomb under pressures of typically 170 MPa and temperatures around 400 1C. This technique is also used in industry to grow berlinite (AlPO4), calcite (CaCO3), gallium phosphate (GaPO4), triglycine sulfate (TGS) and zincite (ZnO) crystals. In addition, another variant of the hydrothermal method, called ‘ammono’thermal method, was developed which uses supercritical ammonia (instead of water). This ammono method is in industrial production for growing GaN crystals. Also other nitrides can be synthesized by this technique.

3.5

Growth under Extreme High Pressure

High pressure synthesis refers to the growth of diamond crystals. Diamond can be grown from graphite at a temperature around 1500 1C under a pressure of around 60 kbar. These conditions are provided by a set up with special anvils generating the high pressure in the heated growth cell. The growth cell contains graphite mixed with a metal or alloy that serves as a solvent/catalyst. At around 1500 1C diamond is the stable modification whereas graphite becomes meta-stable and is partially dissolved in the solvent. The solubility difference acts as driving force for the conversion of carbon from graphite to diamond crystals.

4

Vapor Growth

Vapor growth is like solution growth usually applied when melt growth is impractical. Due to the lower temperatures as compared to melt growth, many thermally activated processes like impurity incorporation, compositional uniformity, structural imperfection, are usually decelerated. Therefore, the crystal quality can be enhanced. However, in vapor growth the growth rates are usually also considerably decreased due to the small density of crystal material in the gas-phase, the low transport rate in the vapor to the growth interface and the decreased interface kinetics with decreasing temperature. It is therefore mainly used in cases where melt and solution growth are not applicable or has severe disadvantages.

Methods for Bulk Growth of Inorganic Crystals: Crystal Growth 4.1

15

Sublimation Technique (Physical Vapor Transport)

Crystal growth by Physical Vapor Transport (PVT) means that a source material is heated to a temperature (T1) where it sublimates, and its vapor is transported by diffusion and convection to a region with a seed crystal with a temperature T2 (oT1) where it is deposited i.e., crystallizes. The supersaturation at the crystallization front depends on the difference of the partial pressures of the sublimated material in the source and equilibrium pressure in the growth region. The transport rate (i.e., growth rate) depends also on the total vapor pressure in the system. The supersaturation cannot be too high in order to avoid homogeneous nucleation in the gas phase. The sublimation technique is industrially used to produce silicon carbide crystals with diameters of up to 150 mm (Figure 23) for electronic applications. The growth takes place in an inductively heated graphite crucible at elevated temperatures (2100 to 2400 1C). The transport of gaseous Si, Si2C, and SiC2 species is established from the SiC source to the growing SiC single crystal in an Argon atmosphere 10 to100 mbar. Also aluminum nitride is grown in industry by the PVT method at temperatures above 2000 1C. The PVT method is also used for II-VI compounds.

Figure 23 SiC crystal with 50 mm diameter grown by the PVT method (by of courtesy of Department for Material Science, Chair for Materials for Electronics and Energy Technology, University Erlangen-Nuremberg10).

Figure 24 1 mm thick GaN crystal with 2” diameter grown by the HVPE method on a sapphire seed wafer (by of courtesy of Fraunhofer IISB, Erlangen8).

16

Methods for Bulk Growth of Inorganic Crystals: Crystal Growth

4.2

Chemical Vapor Transport and Chemical Vapor Deposition

In the crystal growth methods by chemical vapor transport (CVT) or deposition (CVD), respectively, a reactive gas is used which reacts with the source material or gaseous chemical compounds (called precursor) containing the crystal components. This chemical species containing the source material are transported in the gas phase to the growth zone of a reactor. Here, the reactions take place by forming the crystal components which are then deposited at the growth interface. Both methods CVT/CVD have nearly no importance in bulk crystal growth. Only thick free-standing gallium nitride wafers, respectively gallium nitride boules (Figure 24) are presently grown by a hydride CVD. But CVD is the dominant industrial method for growing thin crystal films, called epitaxy, especially by using metal organic precursors, called metal organic vapor deposition MOCVD.

5

Conclusions

Bulk growth of inorganic crystals is an important industrial key technology. Most of the industrially used crystal materials are grown by solidification of their melt. Only a few materials are grown from solution or vapor phase. The growth techniques are well established and suitable to grow in principle any inorganic crystal material. Nevertheless continuous R&D efforts are still necessary for a further development of the crystal growth processes to fulfill the demands on special crystal properties coming from the various fields of applications. Among the most important R&D tasks are 1. avoiding of crystal defects which are deleterious for the performance of devices made from the crystals, 2. better control of crucial processing parameters in order to tailor the physico-chemical properties of the crystals according to the needs of their applications, 3. an increased uniformity of the relevant crystal properties in the micro- and macro-scale, and finally 4. up-scaling of the growth systems in order to grow crystals of larger dimension for higher productivity and yield.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

Nishinaga, T., 2015. Handbook of Crystal Growth Vol. I, second edn. Amsterdam: Elsevier/North-Holland. Rudolph, P., 2015. Handbook of Crystal Growth Vol. II, second edn. Amsterdam: Elsevier/North-Holland. Kuech, T.F., 2015. Handbook of Crystal Growth Vol. III, second edn. Amsterdam: Elsevier/North-Holland. http://www.siltronic.com http://www.freiberger.com http://www.hellma-materials.com http://www.wtm.uni-erlangen.de http://www.iisb.fraunhofer.de http://www-lmj.cea.fr/ http://crystals.tf.fau.de/