1

The Development of Crystal GrowthTechnology
HANS J. SCHEELSCHEEL CONSULTING, CH-8808 Pfaeffikon SZ, Switzerland

ABSTRACTThe industrial production of crystals started with A. Verneuil with his flamefusion growth method 1902. He can be regarded as the father of crystal growthtechnology as his principles of nucleation control and crystal-diameter control are adapted in most later growth methods from the melt, like Tammann,Stöber, Bridgman, Czochralski, Kyropoulos, Stockbarger, etc. The important
crystal pulling from melts named after Czochralski was effectively developed
by Teal, Little and Dash.
The multi-disciplinary nature of technology of crystal and epilayer fabrication,
the complex multi-parameter processes – where ten or more growth parameters have to be compromised and optimized, and also the scaling problem have
impeded the scientific development of this important area. Only recently has thenumerical simulation of Czochralski melts started to become useful for growthtechnologists, the deep understanding of the striation problem allowed the experimental conditions to grow striation-free crystals to be established, and the controlof epitaxial growth modes permitted the preparation of atomically flat surfaces
and interfaces of importance for the performance of opto-electronic and superconducting devices.
Despite the scale of the multi-billion dollar crystal and epilayer fabricationand crystal-machining industry and the annual need worldwide of at least 400
engineers, there is so far no formation of specialists for crystal production, epitaxy technology, crystal machining and surface preparation. A special curriculum
is required due to the multi-disciplinary character of crystal-growth technology(CGT) which does not fit into a single classical university discipline like chemical,
mechanical, materials, or electrical engineering, or crystallography, thermodynamics, solid-state physics, and surface physics. The education scheme for CGT
has to include all these disciplines and basic sciences to such an extent that thefinished engineers and scientists are capable of interacting and collaborating withspecialists from the various disciplines. It is up to the interested industries torequest CGT engineers from the technical universities and engineering schools.
Crystal Growth Technology, Edited by H. J. Scheel and T. Fukuda
 2003 John Wiley & Sons, Ltd. ISBN: 0-471-49059-8

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Crystal Growth Technology

Crystal-growth technology and epitaxy technology had developed along withthe technological development in the 20th century. On the other hand, therapid advances in microelectronics, in communication technologies, in medical
instrumentation, in energy and space technology were only possible after
the remarkable progress in fabrication of large, rather perfect crystals andof large-diameter epitaxial layers (epilayers). Further progress in CGT andeducation of CGT engineers is required for significant contributions to the energy
crisis. High-efficiency white light-emitting diodes for energy-saving illuminationand photovoltaic/thermo-photovoltaic devices for transforming solar and otherradiation energy into electric power with high yield depend on significant
advances in crystal growth and epitaxy technology. Also, the dream of laserfusion energy and other novel technologies can only be realized after appropriate
progress in the technology of crystal and epilayer fabrication.

1.1 HISTORICAL INTRODUCTION
Fundamental aspects of crystal growth had been derived from early crystallization experiments in the 18th and the 19th century (Elwell and Scheel 1975,
Scheel 1993). Theoretical understanding started with the development of thermodynamics in the late 19th century (Gibbs, Arrhenius, Van’t Hoff) and with thedevelopment of nucleation and crystal growth theories and the increasing understanding of the role of transport phenomena in the 20th century. The phenomenaof undercooling and supersaturation and the heat of crystallization were alreadyrecognized in the 18th century by Fahrenheit and by Lowitz. The corresponding
metastable region, the existence range of undercooled melts and solutions, wasmeasured and defined in 1893/1897 by Ostwald and in 1906 by Miers, whereas
the effect of friction on the width of this Ostwald–Miers region was describedin 1911 and 1913 by Young. Although the impact of stirring on this metastableregion is important in mass crystallization of salt, sugar and many chemicals, it
is not yet theoretically understood.
The rates of nucleation and crystallization in glasses were the foundation to
nucleation theories. The crystal surface with steps and kinks of Kossel in 1927
allowed Stranski and Kaishew in 1934 to define the work of separation of crystal
units as repeatable steps as the basis of the first crystal-growth theories. With theunderstanding of facet formation as a function of the entropy of fusion in 1958 byJackson, and depending on the density of bonds in the crystal structure 1955 by
Hartman and Perdok, the role of screw dislocations as continuous step sources in
the formation of growth hillocks (Frank 1949), and with the generalized crystal
growth theory of Burton, Cabrera and Frank 1951, many growth phenomena
could be explained.
In the growth of crystals from a fluid medium (melt, solution, gas phase) theheat and mass transport phenomena also play a significant role, as was observed
early by Rouelle 1745 and Frankenheim 1835. The diffusion boundary layer

Development of Crystal Growth Technology

5defined by Noyes and Whitney 1897 was used in the growth-rate equation of
Nernst 1904 and confirmed by interferometric measurements of concentration
profiles around growing crystals by Berg 1938 and by others. Forced convection was recognized to be beneficial for diffusion-limited growth by Wulff 1886,
Krüger and Finke 1910, and Johnsen 1915 for open systems with stirrers, whereassmooth stirring in sealed containers can be achieved with the accelerated crucible rotation technique ACRT of Scheel 1971/1972. The growth of inclusion-freecrystals from the melt can be accomplished by observing the principles of “diffusional undercooling” of Ivantsov 1951 and “constitutional supercooling” of Tiller
et al. 1953. Formation of inclusions, i.e. growth instability, can be prevented in
growth from solutions by sufficient flow against or along the crystal facets: Carlson 1958 developed an empirical theory which was utilized by Scheel and Elwell1972 to derive the maximum stable growth rate and optimized programming ofsupersaturation for obtaining large inclusion-free crystals.
Microscopic and macroscopic inhomogeneities in doped crystals and in solid
solutions are caused by segregation phenomena, which are related to mass andheat transfer. Based on the derivation of effective distribution coefficients formelt growth by Burton et al. (1953) and by van Erk (1982) for growth fromsolutions, the theoretical and experimental conditions for growth of striation-freecrystals could be established (Rytz and Scheel 1982, Scheel and Sommerauer
1983, Scheel and Swendsen 2001).
There have been remarkable developments with respect to size and perfectionof crystals, with silicon, sapphire, alkali and earth alkali halides reaching diameters up to 0.5 m and weights of nearly 500 kg. These advances in Czochralski,
Kyropoulos, heat-exchanger method, and Bridgman–Stockbarger growth wereaccompanied by numerical simulations which have become increasingly powerful to predict the optimized conditions. However, further advances in computermodelling and in the reliability of the used physico-chemical data are requiredin order to increase the efficiency and precision of computer simulations andto allow the prediction of the best crystal-growth technology including growthparameters for the growth of new large and relatively perfect crystals.

1.2 THE DEVELOPMENT OF CRYSTAL-GROWTH METHODS
Industrial crystal production started with Verneuil 1902 who with the flame-fusion
growth process named after him achieved for the first time control of nucleationand thus single crystals of ruby and sapphire with melting points above 2000 ◦ C.
By formation of the neck and the following enlargement of the crystal diameter,
not only was the seed selection achieved in a crucible-free process, but also thestructural perfection of the growing crystal could be controlled to some extent.
Figure 1.1 shows the stages of Verneuil growth of high-melting oxides, andFigure 1.2 the relatively simple apparatus where powder is fed into the hydrogen–oxygen flame and enters the molten cap of first the cone and neck, and6

Crystal Growth Technology
Verneuil
Hydrogen-Oxygen
Flame with Feed
PowderMoltenCap

SinterCone

Neck

Alumina
Rod
(a)

(b)

(c)

Figure 1.1 Stages of flame-fusion (Verneuil) growth of ruby, schematic: (a) formation ofsinter cone and central melt droplet onto alumina rod, (b) growth of the neck by adjustmentof powder supply and the hydrogen-oxygen flame, (c) Increase of the diameter without
overflow of the molten cap for the growth of the single-crystal boule. (Reprinted from
H. J. Scheel, J. Cryst. Growth 211(2000) 1–12, copyright (2000) with permission from
Elsevier Science.)then the molten surface layer of the growing crystal. The latter is slowly moved
downwards so that the surface-melting conditions remain within the observation
window. Historical details of Verneuil’s development were given by Nassau 1969
and Nassau and Nassau 1971. The industrialization for production of ruby watchstones and synthetic gemstones started with Verneuil’s laboratory in Paris in 1910
with 30 furnaces and with the Djevahirdjian factory at Monthey VS in Switzerland where the CIBA chemical company could provide hydrogen. The annual
ruby production reached 5 million carats in 1907, and 10 million carats ruby and6 million carats sapphire in 1913. Nowadays, the production capacity of sapphire for watch windows and other applications is 250 tons (1.25 × 109 carats),
the largest fraction from the Djevahirdjian factory with 2200 Verneuil furnaces,
and this with the practically unchanged process.
The principles of the Verneuil method with nucleation, growth rate and diameter control have been applied in most of the growth processes described in
the following years: Tammann 1914, Stöber 1925, Bridgman 1923, Stockbarger
1936, vertical-gradient-freeze growth of Ramsberger and Malvin 1927, and in
the Czochralski process 1918/1950, see Figure 1.3 and Wilke-Bohm 1988. Thus,
Verneuil can really be regarded as the founder of crystal-growth technology.
However, the most important method of fabrication of semiconductor andoxide crystals consists of pulling crystals from melts contained in crucibles, amethod named after Czochralski. The origin of this started with the topical area

Development of Crystal Growth Technology

7

Figure 1.2 The relatively simple apparatus of Verneuil for flame-fusion growth.
(Reprinted from D. T. J. Hurle (ed.) Handbook of Crystal Growth 1, Fundamentals,
Part A, Chapter 1, 1993, copyright (1993) with permission from Elsevier Science.)of physical chemistry for 50 years, the measurement or crystallization velocities,
which was initiated by Gernez 1882, who had found a high crystallization rate ofmolten phosphorus. After Tammann’s studies of glass crystallization, Czochralski presented in 1918 “a new method for measurement of the crystallization
velocity of metals” where he pulled fibers of low-melting metals from melts.
Soon after this, von Gomperz used floating dies to control the fiber diameter

8

Crystal Growth Technology
Molten cap
A. Verneuil(1890,1902 –1904)

Diameter control/
Growth rate control
Seed selection &nucleation control
Crucible-free economic
crystal-production process

Melt
v

v
Melt

Seed
selection orseed crystal

Horizontal Bridgman 1923
Tammann 1914 / Stöber 1925
Bridgman 1923 / Stockbarger 1936
Vertical Gradient Freeze VGF
Seed crystal

Ge:Teal & Little
1950

Melt

Melt
Czochralski 1918

Kyropoulos 1926

Figure 1.3 Modification of Verneuil’s principles of nucleation control and increasing
crystal diameters in other crystal-growth techniques. (Reprinted from H. J. Scheel, J.
Cryst. Growth 211(2000) 1–12, copyright (2000) with permission from Elsevier Science.)(an early Stepanov or EFG process) 1921, and in 1928 Kapitza measured crystallization rates and prepared single-crystalline bismuth in a vertical glass tube
(early zone melting).
Czochralski never considered pulling a crystal for research, although he could
have adapted the inverse Verneuil principle, which was well known at that time.
The metal crystals for research in Czochralski’s laboratory were prepared by the
Bridgman method. Therefore, it is no wonder that Zerfoss, Johnson and Egliin their review on growth methods at the International Crystal Growth Meeting in Bristol UK 1949 mentioned Verneuil, Bridgman–Stockbarger, Stöber andKyropoulos techniques, but not Czochralski.

Development of Crystal Growth Technology

9With the discovery of the transistor at Bell Laboratories there arose a need forsingle crystals of semiconductors, first of germanium. Teal and Little invented anddeveloped crystal pulling of relatively large Ge crystals in 1950. Then it was thebook of Buckley in 1951, who erroneously named pulling of large crystals after
Czochralski, and this name became widespread with the fast-growing importanceof crystal pulling. Possibly it is too late to change the name to Teal–Little–Dash,the latter due to his contribution to dislocation-free crystals by optimized seed
and neck formation. The development of Czochralski and liquid-encapsulated
Czochralski growth was discussed by Hurle 1987.
The majority of crystals is produced by pulling from the melt. Crystal dimensions have reached 10 cm for InP, LiTa03 , sapphire and other oxide crystals,
15 cm for LiNbO3 and GaAs, and 30 to 40 cm for silicon and for halide scintillator crystals with melt dimensions up to 1 m. Numerical modelling has assisted
these developments, for instance for the optimization of the crystal and crucible
rotation rates and of the temperature distribution and the design of heaters, afterheaters and heat shields. However, for the axial segregation and the striation
problems the final general solution has not yet been established. The following
approaches to reduce convection have been partially introduced in crystal production: double crucible for GaAs and stoichiometric LiNbO3 , magnetic fields invarious configurations for semiconductors, and shallow melts or small melt volumes with continuous feeding for silicon and for halide scintillator crystals: theseapproaches are discussed in this volume. An alternative approach to solve, or atleast minimize, the axial segregation and the striation problem is the co-rotating
ring Czochralski (CRCZ) method which induces a hydrodynamic double-crucible
effect: by optimized rotation rate of crystal and co-rotating ring (inserted into themelt) and counter-rotation rate of the crucible, a nearly convection-free melt
fraction is achieved below the growing crystal, whereas the larger fraction of themelt is homogenized by combined forced and natural convection (Scheel 1995).
Only a few Czochralski-grown elements like silicon, germanium, and copper
are produced dislocation-free by application of necking as developed by Dashin 1959. Most crystals have dislocations of typically 103 to 106 cm−2 due to thegenerally large temperature gradients at the growing interface and due to the postgrowth defect agglomeration behind the growth front (Völkl and Müller 1989).
Attempts to grow with low-temperature gradients at the Institute of Inorganic
Chemistry in Novosibirsk or to grow the crystal into the melt by the Kyropoulos
method (SOI-method in St. Petersburg) have yielded low dislocation crystals of,for example, sapphire up to 40 cm diameter, but have not yet been developed
for large-scale production. However, alternative technologies like Bridgman andvertical gradient freeze (VGF) have gained importance for production of semiconductor crystals of improved structural perfection like GaAs, InP, and CdTe
and its solid solutions. In vertical Bridgman and in VGF growth the forced convection by the accelerated-crucible rotation technique (ACRT) of Scheel 1972
has increased homogeneity, structural perfection, and stable growth rates in thecases of halides (Horowitz et al. 1983) and doped CdTe (Capper 1994, 2000).

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Crystal Growth Technology

In silicon, the octahedral void defects (see Falster and Itsumi in this volume)and their distribution as well as oxygen and dopant homogeneity require further
understanding (see Abe, Falster) and process optimization, otherwise an increasing fraction of silicon wafers have to be provided with epitaxial layers in orderto achieve the homogeneity of surface properties required for highly integrated
microelectronic circuit structures.
Examples of other remarkable developments in the growth of bulk crystals
are briefly given: Hydrothermal growth of large high-quality low-dislocationdensity quartz by VNIISIMS/Russia and Sawyer/USA, hydrothermal growth oflarge ZnO and emerald crystals, high-temperature solution growth of perovskites
and garnets by top-seeded solution growth TSSG by Linz et al. and by theaccelerated crucible rotation technique (ACRT) by Scheel and by Tolksdorf,
TSSG growth of KTP and CLBO (the latter by Sasaki and Mori), aqueoussolution growth of KDP (Sasaki, Zaitseva), Bridgman–Stockbarger growth ofalkali halides and of CaF2 , float-zone growth of TiO2 rutile by Shindo et al.,skull-melting growth of zirconia by Wenckus and Osiko, and Czochralski growthof numerous oxide compounds mainly in US, French, Japanese and recently
Korean and Taiwan companies.

1.3 CRYSTAL-GROWTH TECHNOLOGY NOW
The world crystal production is estimated at more than 20 000 tons per year, of
which the largest fraction of about 60 % are semiconductors silicon, GaAs, InP,
GaP, CdTe and its alloys. As can be seen in Figure 1.4 , optical crystals, scintillator crystals, and acousto-optic crystals have about equal shares of 10 %, whereas
laser and nonlinear-optic crystals and crystals for jewelry and watch industry haveshares of a few % only. This scale of crystal production and the fact that most
5%

3%

10%

SEMICONDUCTORSSCINTILLATION CRYSTALS

10%OPTICAL CRYSTALSACOUSTIC OPTICS CRYSTALS60 %LASER & NONLINEAR CRYSTALS12%JEWELRY & WATCH INDUSTRY

Figure 1.4 Estimated shares of world crystal production in 1999. (Reprinted from
H. J. Scheel, J. Cryst. Growth 211(2000) 1–12, copyright (2000) with permission from
Elsevier Science.)

Development of Crystal Growth Technology

11

crystals are produced in factories specialized in silicon or GaAs or LiNbO3 , etc.
has caused an increasing degree of specialization. Furthermore, crystal growth
has split into five major areas so that cross-fertilization and communication are
more and more reduced:
1.
2.
3.
4.
5.

Fundamental theoretical and experimental crystal growth studies.
Laboratory crystal growth for preparing research samples.
Industrial fabrication of single crystals and their machining and characterization.
Fabrication of metallic/dendritic crystals (e.g. turbine blades).
Mass crystallization (salt, sugar, chemicals).

Epitaxy has similarly separated from bulk crystal growth and has split into:
1.
2.
3.

Fundamental/theoretical and experimental epitaxy studies and surface phenomena.
Epitaxy growth methods (VPE, MOCVD/MOVPE, MBE, ALE, LPE).
Materials classes (GaAs, InP, GaN, II-VI compounds, high-Tc superconductors, low-dimensional/quantum-well/quantum-dot structures, etc.).

As a consequence of these separations and splits, numerous specialized conferences, workshops and schools are established and regularly organized, newspecialized journals appear, and the common language gets more and more lost,even to the extent that formerly well-defined terms are misused.
The situation is critical in research and development involving crystal growthand epitaxy. Due to a worldwide lack of education in these areas, frequently
growth methods are employed for problems for which they are not well-suitedor even not suited at all. It is postulated (Scheel 2002) that for a specific crystalof defined size and perfection there is only
one optimum and economic growth technology
considering thermodynamics, growth kinetics, and economic factors. This aspect
is schematically shown in Figure 1.5, where the crystal performance, the crystal
size, the efficiency of the growth method, and the price of crystal are shown. The
shape factor may play a role when, for instance, special shapes are needed in theapplication, for example square, rectangular or oval watch faces of sapphire. The
concept of one or maximum two optimum technologies should be considered forindustrial crystal fabrication, whereas in laboratories the existing equipment orexperience may determine a nonoptimum growth method. This postulate holds
obviously also for epitaxial methods: for example, high-Tc compounds have been
prepared by >1000 groups (after the discovery of high-temperature superconductivity, HTSC) by vapour-phase epitaxy, which cannot yield the atomically flat
surfaces required for reliable HTSC tunnel-device technology. Instead only liquid

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Crystal Growth Technology
Shape factor

Crystal Quality

Optimum
Performance

CrystalSize

Minimum

Efficiency of Crystal Growth Method
Price of Crystal& Energy consumption

Figure 1.5 Schematic diagram of the efficiencies of various crystal-growth methods
and the achieved crystal sizes and crystal quality. Note that there is normally only one
optimum economic technology for the required crystal performance and size.

phase epitaxy near thermodynamic equilibrium can give atomically flat surfaces,
as discussed elsewhere in this volume. Here, it could be added that the lack ofrecognition of the materials problems and specifically of the crystal growth andepitaxy problems have caused tremendous economic (and intellectual) losses not
only in industries but also in research. As a recent prominent example, HTSC asa whole could not be properly developed, neither its physical understanding nor
its many predicted applications because the chemical and structural complexityof the not very stable HTSC compounds were neglected. Most physical HTSC
measurements were nonreproducible because the used crystals and layers were
not sufficiently characterized for those structural and chemical defects that havean impact on these measurements or applications.
We realize that ‘education’ is needed for the important field of crystal-growthtechnology in order to achieve efficient developments in novel high technologies, in energy saving, in solar energy generation, in future laser-fusion energy
and with a better approach in HTSC: in all these areas where crystal-growthtechnology is the progress-determining factor (Scheel 2002). However, in viewof the multidisciplinary nature of crystal growth technology it will be not easy
to initiate a curriculum in a classical university with the rather strict separationof disciplines in faculties or departments. In addition, the specialized educationin CGT can start only after the multidisciplinary basic education and requires
additional courses of 1 to 2 years. Furthermore, there are no teachers for thisfield as most crystal growers are specialized and experts in specific materials or
specific methods only. In addition education in crystal growth technology andDevelopment of Crystal Growth Technology

13

Table 1.1 Complexity of crystal growth technology CGT
a. Multi-Disciplinaryž Chemistry (all fields) including Chemical Engineeringž Materials Science & Engineeringž Mechanical & Electrical Engineering (especially hydrodynamics, machine design,
process control)ž Theoretical Physics (especially thermodynamics, non-equilibrium thermodynamics,
statistical mechanics)
ž Applied Crystallography and Crystal Chemistryž Solid-State Physics
b. Complexity
ž Phase Transformation from Fluid (melt, solution, vapour) to Crystal
ž Scaling Problem: Control of surface on nm scale in growth system of ∼m size,
hampers numerical simulation
ž Complex Structure & Phenomena in Melts and Solutions
ž Multi-Parameter Processes: Optimize and compromise ∼10 parameters
There is only one Optimum and Economic Growth Technology for a Specific Size and Performanceof Crystal or Crystalline Layer!

epitaxy technology should be complemented by practical work and by periods
in industries: another obstacle where, however, interested industries will collaborate. This practical work is needed in order to understand and appreciate thecomplexity (see Table 1.1) of crystal-growth processes in view of the involved
phase transformation, of the many parameters that have to be compromised andoptimized, and in view of the scaling problem, which hampers realistic numerical simulation.

1.4 CONCLUSION
Crystal-growth technology (CGT) including epilayer fabrication technology isof greatest importance for energy saving, for renewable energy, and for novel
high technologies. Therefore the education of CGT engineers and CGT scientists
is needed as well as significant support and publicity. Excellent students withintuition and motivation can have a great impact in crystal-growth technology
and thus contribute to the energy problem and to high-technology developmentsof mankind.
REFERENCES
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Burton J. A., Prim R. C. and Slichter W. P. (1953), J. Chem. Phys. 21, 1987.
Capper P. (1994), Prog. Crystal Growth Charact. 28, 1.

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Crystal Growth Technology

Capper P. (2000), at second International School on Crystal Growth Technology ISCGT-2
Mount Zao, Japan August 24–29, 2000 (book in preparation).
Elwell D. and Scheel H. J. (1975), Crystal Growth from High-Temperature Solutions, Academic Press, London- New York, (reprint foreseen 2003).
Horowitz A., Gazit D., Makovsky J. and Ben-Dor L. (1983), J. Cryst. Growth 61, 323
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Scheel H. J. (1995), US Patent No. 5471943.
Scheel H. J. (2002), Plenary lecture at Second Asian Conference on Crystal Growth andCrystal Technology CGCT-2 August 28–31, 2002 in Seoul, Korea, (proceedings in
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Scheel H. J. and Elwell D. (1972), J. Cryst. Growth 12, 153.
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Tiller W. A., Jackson K. A., Rutter J. W. and Chalmers B. (1953), Acta Met. 1, 428.
van Erk W. (1982), J. Cryst. Growth 57, 71.
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