theories and methods of crystal growth: a general review · crystal growth: a general review 1.1...
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Chapter 1
Theories and Methods of Crystal Growth: A General Review
1.1 Introduction
The growth process of single crystals has been developed over the
years to meet the requirements of basic research and technological
applications. Crystals play a significant role in electronics industry,
photonics industry and fibre-optic communications. The semiconductor
based electronics industry, requires high quality semi-conducting,
ferroelectric, piezoelectric, single crystals. Integrated micro electronics
and opto-electronics have necessitated improvements in crystal growth
techniques. The developments in the theory and techniques of crystal
growth have made possible the commercial scale growth of large, defect-
free crystals of silicon and GaAs which find application in the fabrication of
electronic, photonic and microwave devices.
The crystal growth process is a complex one as it involves
optimization of various parameters such as temperature, pressure,
chemical potential, electrochemical potential etc. In a well-defined growth
process, just one of these parameters is held minimally away from its
equilibrium value to provide a driving force for growth. The process of
crystal growth involves phase transitions of the following types: solid-solid,
liquid-solid and gas-solid. The methods of crystal growth are classified
accordingly. A general review of the theories and techniques of crystal
growth is given in this chapter.
2 Chapter 1
1.2 Thermodynamics of crystal growth
Crystal growth being a non-equilibrium process, control of the
crystal growth environment and a consideration of growth kinetics, both at
the macroscopic and atomic levels are significant. The phase
transformation during the crystal growth process occurs due to lowering of
the free energy of the system. The free energy is related to the entropy
and internal energy of the system by the Gibbs [1] equation
G = H – TS (1.1)
where H is enthalpy, S is entropy and T is temperature.
The formation of a crystal can be considered as a controlled
change of phase to the solid state. The driving force of crystallization
comes from the lowering of the free energy of the system during this
phase transformation. This change of free energy is
∆G = ∆H – T. ∆S (1.2)
where ∆H = HL - HS
∆S = SL - SS
∆G = GL - GS
At equilibrium, ∆G = 0
∴ ∆H = Te. ∆S
where Te is the equilibrium temperature.
∴ ∆G = ∆H . ∆T/Te (1.3)
where ∆T = Te – T
When Te > T, ∆G is positive and it depends on the latent heat of transition.
The free energy change can also be represented as the product of
the entropy change and supercooling ∆T.
∆G = ∆S. ∆T
Theories and methods… 3
This representation is convenient for melt growth. In the case of
solution growth and vapour growth, what is taken into account is
concentration rather than supercooling. Hence this relation becomes
∆G ∼ R T ln (C/Co)
∆G ∼ R T ln (P/Po)
where C, P are solute concentration and vapour pressure and C0, P0 are
equilibrium solute concentration and saturation vapour pressure.
In general
∆G ∼ R T ln S (1.5)
where S is the supersaturation ratio.
The rate of growth of a crystal can be regarded as an increasing
function of ∆G, if the other parameters remain the same.
1.3 Nucleation
Nucleation is the precursor of crystal growth and the overall
crystallization process. In a supersaturated or supercooled system, when
a few atoms or molecules join together, a change in free energy takes
place in the process of formation of a cluster in the new phase. The
cluster consisting of such atoms or molecules is called a nucleus. If the
nucleus grows to a particular size known as ‘critical size’, then there is a
greater probability for the nucleus to grow. There are four stages involved
in the formation of a stable nucleus:
(1) the development of saturated stage, which may be due to a
chemical reaction, a change in pressure, temperature or any other
chemical or physical condition.
(2) the generation of nucleus.
(1.4)
4 Chapter 1
(3) the growth of nucleus from the unstable state to stable state or to
the critical size.
(4) the relaxation process where the texture of the new phase
changes.
Nucleation may occur spontaneously or it may be induced
artificially. Based on these, nucleation is classified as follows:
(1) Homogeneous nucleation and
(2) Heterogeneous nucleation.
The spontaneous formation of crystalline nuclei in the interior of the
parent phase is called homogeneous nucleation [2].
If the nuclei form heterogeneously around ions, impurity molecules
or dust particles, it is called heterogeneous nucleation [3].
Considering the total free energy for a group of atoms, a theory for
the formation of a nucleus was put forward by Volmer and Weber [4].
The free energy change associated with the formation of a nucleus
can be written as
∆G = ∆Gs + ∆Gv (1.6)
where ∆Gs is the surface excess free energy of the interface separating
the parent and the product phases, and ∆Gv is the volume excess free
energy change per unit volume, which is a negative quantity.
For a spherical nucleus
∆G = 4πr2σ –
4
3 πr3 ∆Gv (1.7)
Free energy of the system decreases by ∆Gv for each unit volume
of the solid created but increases by an amount equal to the interfacial
energy σ, for each unit area of the solid-liquid interface formed.
Theories and methods… 5
A graphical representation of equation (1.7) is given in Figure 1.1.
The surface energy term increases with r2 and the volume energy term
decreases with r3. The net free energy change increases with increase in
size, attains a maximum and decreases for further increase in the size of
the nucleus. The size of the nucleus corresponding to the maximum free
energy change is known as the critical nucleus. It is the smallest size of
the nucleus which can grow further. If the size of the nucleus is below the
critical dimensions, no further growth is possible and it redissociates into
the mother system.
Figure 1.1 Curve showing the dependence of the change in surface energy
(∆Gs), volume energy (∆Gv) and the net free energy (∆G) on radius (r) of the nucleus
By setting the condition d
dr∆G = 0, the radius r* of the critical
nucleus is obtained as
r* = v
2σ
∆G (1.8)
∆∆∆∆G*
r
∆∆∆∆G
r*
∆∆∆∆Gs
∆∆∆∆Gv
0
Fre
e en
ergy
ch
ange
∆∆ ∆∆G
6 Chapter 1
Substituting the value of r* in equation (1.7), the free energy change
associated with the critical nucleus is given by
∆G* = 3
2
v
16 σ
3∆G
π (1.9)
According to Gibbs - Thomson relation
ln 0
C
C
= ln S = 2 σ Ω
k T r (1.10)
where k is the Boltzmann’s constant and Ω is the molecular volume.
Using this relation equation (1.9) can be written as
∆G* = ( )
3 2
2
16πσ Ω
3 k TlnS (1.11)
The rate of nucleation J, ie., the number of nuclei formed per unit
volume per unit time can be expressed as
J=J0 exp -∆G*
kT
Substituting for *G in this equation,
J = J0 exp 3 2
3 3 2
-16πσ Ω
3k T (lnS)
(1.12)
where J0 is the pre-exponential factor.
This equation shows that the rate of nucleation is controlled by
temperature T, degree of supersaturation S and interfacial energy σ.
Rearranging equation (1.12) and arbitrarily choosing J = 1 so that
ln J = 0, the expression for critical supersaturation becomes
Scri = exp
1/ 23 2
3 3
0
16πσ Ω
3k T lnJ (1.13)
Theories and methods… 7
Using the values of various parameters, the critical supersaturation
required for spontaneous nucleation can be estimated.
For heterogeneous nucleation, the presence of a foreign substrate
induces nucleation at supersaturations lower than that required for
spontaneous nucleation. The free energy change associated with the
formation of critical nucleus under heterogeneous conditions ∆G*het must
be less than the corresponding free energy change ∆G*hom associated
with homogeneous nucleation.
∆G*het = φ ∆G*hom
where the factor φ is less than unity.
1.4 Theories of crystal growth
The process of crystal growth involves the following steps:
generation of reactants, transport of reactants to the growth surface,
adsorption at the growth surface, nucleation, growth, and removal of
unwanted reaction products from the growth surface. When a crystal
nucleus attains the critical size, it grows into crystal of macroscopic
dimension with well developed faces. To understand the kinetics and
mechanism involved in the process of crystal growth, many theories such
as surface energy theory, diffusion theory and surface adsorption theory
have been proposed.
Gibbs proposed a theory by considering the growth of a crystal
analogous to the growth of water droplet in mist. Later Kossel [5] and
others analysed the atomic inhomogeneity of a crystal surface and
explained the role of step and kink sites on the growth process. However,
this theory could not provide a complete explanation for the continuous
growth of a crystal surface. A complete explanation for continuous growth
at low supersaturation was given by Frank [6]. He showed that crystal
dislocations were capable of providing the sources of steps required for
8 Chapter 1
the continuous growth of a crystal. These theories have been extensively
described by many authors [7-9].
1.4.1 Surface energy theory
According to the surface energy theory proposed by Gibbs, a
growing crystal assumes the shape which has minimum energy. The
thermodynamical treatment suggested by Gibbs was later extended by a
number of researchers. Curie [10] calculated the shapes and end forms
of crystals in equilibrium with solution or vapour, consistent with Gibbs
criterion. He suggested that when the volume free energy per unit volume
is constant, the sum of the surface energies of all faces of the crystal will
be minimum.
Wulff [11] deduced the relation connecting the growth velocity
measured normal to any surface and the surface energy of that surface
and found that these two are proportional to each other. Marc and Ritzel
[12] further developed the concepts of Wulff, stating that different faces
have different solubilities. They suggested that when the difference in
solubility is small, growth is mainly governed by surface energy and the
change in surface of one form is necessarily at the expense of the other.
Berthoud [13] and Valeton [14] disputed the surface energy theory on the
basis of supersaturation. According to the theory, as the supersaturation
increases, growth becomes rapid in all directions and this results in the
spherical shape of the crystal. But, experimentally, it has been observed
that well defined faces are developed when the supersaturation is high.
1.4.2 Diffusion theory
The diffusion theories, proposed by Noyes and Whitney [15] and by
Nernst [16] are based on the following assumptions.
(a) There is a concentration gradient in the vicinity of a growing
surface.
(b) The growth is a reverse process of dissolution.
Theories and methods… 9
The amount of solute molecules that will get deposited over the
surface of a growing crystal in a supersaturated solution can be written as
0
dm D = A (C - C )
dt δ (1.14)
where dm is the mass of the solute deposited over the crystal surface of
area A during time dt, D is the diffusion coefficient of the solute, C and C0
are the actual and equilibrium concentration of the solute and δ is the
thickness of the stagnant layer adjacent to the crystal surface. The value
of δ depends on the relative motion between the crystal surface and the
solution. But this theory also fails due to the inconsistency with the
experimental results.
1.4.3 Adsorption layer theory by Kossel, Stranski and Volmer (KSV theory)
The role of surface discontinuities as nucleation sites was first
recognized by Kossel [17], Stranski [18] and Volmer [19]. According to
this theory the growth units approaching a crystal surface do not
incorporate immediately to the lattice, but become adsorbed and migrate
over the surface. The possible lattice sites for the attachment of adsorbed
atoms on the crystal surface are terrace, ledge and kink site as shown in
Figure 1.2.
Figure 1.2. Possible lattice sites for the attachment of adsorbed atom A-terrace site, B- ledge site, C-kink site
10 Chapter 1
The binding energy between an adatom and existing lattice
increases from terrace to ledge to kink site.
Volmer first suggested the concept of crystal growth mechanism
based on the existence of adsorbed layer of solute atoms or molecules on
a crystal face. Volmer’s theory (Gibbs-Volmer theory) is based on
thermodynamic reasoning. When units of the crystallizing substance
arrive at the crystal face, they are not immediately integrated into the
lattice, but merely lose one degree of freedom and are free to migrate
over the crystal face (surface diffusion). Hence there will be a loosely
adsorbed layer of integrating units at the interface, and dynamic
equilibrium is established between this layer and the bulk solution. The
adsorption layer plays an important role in crystal growth, secondary
nucleation and precipitation phenomena.
At the ‘active centres’ where the attractive forces are the greatest,
atoms, ions or molecules are linked into the lattice and under ideal
condition this stepwise build-up will continue until the whole plane face is
complete. Before the crystal face can continue to grow, a centre of
crystallization must come into existence on the plane surface and Gibbs -
Volmer theory suggests that a monolayer island nucleus, usually called a
two-dimensional nucleus, is created.
When an advancing step covers the whole surface completely,
further growth is possible only by the initiation of a two dimensional-
nucleus. According to Volmer, this is possible on account of thermal
fluctuations. The free energy change associated with the formation of
such a two-dimensional nucleus may be written as
∆G = a γ − V ∆Gv (1.15)
where a and V are the area and volume of the nucleus. Assuming a
circular disc-shaped nucleus of radius ‘r’ and height ‘h’ the equation
becomes
∆G = 2 π r h γ − π r2 h ∆Gv (1.16)
Theories and methods… 11
where γ is the edge free energy. The activation energy for two-
dimensional nucleation can be calculated as
∆G* = 2
π h γ Ω
k T Sln (1.17)
The rate of two - dimensional nucleation can be expressed as
J′ = C exp
-∆G*
kT
= C exp 2
2 2
-πh γ Ω
k T lnS
(1.18)
The expression for critical supersaturation is given by
Scri = exp 2
2 2
πh γ Ω
k T lnC
(1.19)
The rate of growth of a singular face is in principle controlled by the
rate of nucleation and rate of advance of a step and can be expressed as
R = 1/3 2/3h J V′ (1.20)
According to KSV theory, the growth of a crystal is controlled by
the probability of two-dimensional nucleation which is not appreciable until
the supersaturation reaches a considerable percentage order. However it
is observed that most of the real crystals grow at supersaturation down to
a value of 1% or even lower.
1.4.4 Screw dislocation theory by Burton, Cabrera and Frank (BCF theory)
One major drawback of KSV theory is that once the kinked ledge
has received sufficient ad atoms to move it to the edge of the crystal, it
could no longer function as a low energy nucleation site. But if points of
dislocations with screw components (screw dislocations) at crystal surface
are present they can provide a continuous source of steps which can
12 Chapter 1
propagate across the surface of the crystal. A theory of crystal growth
including the mechanism of step generation and transport into the steps
was given by Burton, Cabrera and Frank [20].
A screw dislocation emerging at a point on the crystal surface
provides a step on the surface with a height equal to ‘a’, the projection of
the Burgers vector of the dislocation. Since the step provided by the
screw dislocation is anchored at the emergence point of the dislocation,
and since the inner parts of the step move radially at a faster rate than the
outer parts, further growth takes place only by the rotation of step around
the dislocation point. This mechanism is illustrated in Figure 1.3. Under a
given condition of supersaturation these steps wind themselves up into a
spiral, centred on the dislocation.
Figure 1.3. Development of Spiral
A relation between the rate of growth R and the relative super
saturation was given by Burton, Cabrera and Frank. It is expressed as
R = C 2
1
S
S tanh 1
S
S (1.21)
where S1 = s
γ
k T X
19
2
Ω
Theories and methods… 13
and C = s se
2
s
D
X
β Ω∩
where γ - free energy,
S - relative supersaturation,
S1 - a constant for B C F model,
se∩ - equilibrium concentration of growth units on surface,
β - retardation factor,
Ω - volume of the growth unit.
The variation of the growth rate with supersaturation thus depends
on two parameters – C, which determines the absolute value of growth
rate and S1 which determines the actual growth rate.
The BCF theory predicts that the growth rate is proportional to the
square of the supersaturation for low supersaturation, changing to a linear
dependence at higher supersaturations. The calculated growth rate for
this mechanism is found in good agreement with observations. The spiral
growth patterns have been observed on a large number of crystals grown
by different methods.
1.5 Crystal growth methods
The process of crystal growth is a controlled phase change to solid
state from solid, liquid or vapour states. Depending on the phase
transition involved, crystal growth methods are classified into four main
categories [21-25].
(1) Solid growth (solid to solid)
(2) Melt growth (liquid to solid)
(3) Vapour growth (vapour to solid)
(4) Solution growth (liquid to solid)
14 Chapter 1
Growth of crystals from liquid phase is treated as two categories
due to the independent behaviour of melt growth and solution growth
techniques. A brief description of the various methods of growth is
presented in the following sections.
1.5.1 Growth from solid
In this method, single crystals are obtained by the preferential
growth of a polycrystalline mass. This can be achieved by straining the
material and subsequent annealing. Large crystals of several materials,
especially metals, have been grown by this method [26]. Recently certain
rare earth compounds are also grown using this method [27]. Solid state
growth is possible by atomic diffusion also [28]. At normal temperature
such diffusion is very slow except in the case of super ionic materials.
1.5.2 Growth from melt
Melt growth is the process of crystallization by fusion and
resolidification of the pure material, crystallization from a melt on cooling
the liquid below its freezing point. In principle, all materials can be grown
in single crystal form from the melt, provided they melt congruently, do not
decompose before melting and do not undergo a phase transition
between the melting point and room temperature. The rate of growth is
much higher than any other methods and for this reason this method is
used in the growth of crystals for commercial purposes.
The melt growth can be subdivided into the following techniques:
(1) Bridgeman technique
(2) Czochralski technique
(3) Flame fusion technique
(4) Zone melting method
(5) Float zone method
Theories and methods… 15
In the Bridgeman technique, solidification is obtained by the
withdrawal of a boat (crucible) containing molten material through a
temperature gradient [29,30]. There are two versions – Horizontal
Bridgeman method and Vertical Bridgeman method. This method is best
suited for low melting point materials. The Bridgeman technique is used
for the growth of metals, semiconductors and alkaline earth halides and
non-linear optical crystals [31-36]. Single crystals of rare earth
compounds are also grown by this method [37].
Czochralski method is basically a crystal pulling technique used for
producing high quality crystals [38 - 41]. An advantage of this method is
that growth from a free space accommodates the volume expansion
associated with solidification of many materials. This method is widely
used to grow refractory oxides such as sapphire and ruby and
semiconductors like silicon [42 - 46]. This method has been used in the
growth of certain laser crystals as well as rare earth compounds [47 – 52].
The crystal growth of certain ferroelectric and non-linear optical material is
also reported by this method [53, 54].
The flame fusion technique (Verneuil technique) [55, 56] is a
method of crystal growth in which no crucible is used. The charge is
taken in a finely powdered form and is carried through an oxy-hydrogen
flame. There it is melted and a single crystal grows around a seed crystal.
Crystals of ruby, sapphire and corundum can be grown by this method.
In zone melting, the melt is contained in a closed container that can
be mounted horizontally or vertically. A zone or part of the solid material
is melted and this molten zone travels together with the heating elements.
An advantage of zone melting is that multiple recrystallization is possible
which permits chemical purification of the substance [57].
Float zone method is essentially zone melting method in the
vertical configuration without container. The molten zone is sustained by
16 Chapter 1
surface tension forces. This method is most suitable for materials with
high surface tension and low density. This method is used for silicon
crystals with very low oxygen concentration and also for high purity
refractory metals like tungsten and tantalum. Recently, the growth of rare
earth compounds is reported using float zone method [58].
1.5.3 Growth from vapour
Numerous methods of crystallization from the vapour phase have
been developed in connection with the requirements of modern
technology, particularly in semiconductor electronics. These methods
are used for growing bulk crystals, epitaxial films, thin coatings and
platelet crystals. Growth from vapour phase may be broadly classified
into physical vapour transport and chemical vapour transport.
Physical vapour transport involves two techniques, sublimation –
condensation and sputtering. The first method involves the sublimation
of the charge at a high temperature end of the furnace followed by the
condensation at the colder end [59]. Sputtering techniques are mainly
used to prepare thin films rather than discrete crystals. The advantage
of this method is that film growth is possible at a temperature lower than
in sublimation-condensation growth. Various crystals and epitaxial films
have been produced by PVT method [60 – 62].
Chemical vapour transport involves a chemical reaction between
the source material to be crystallized and the transporting agent. The
material to be crystallized is converted into a gaseous product, which
either diffuses to the colder end or gets transported by the transporting
gas. At the colder end, the reaction is reversed so that the gaseous
product decomposes to deposit the parent material, liberating the
transporting agent which diffuses to the hotter end and again reacts with
the charge. The temperature of the hot zone and the crystallization zone
can be theoretically predicted [63, 64]. Commercially important materials
can be grown by this method [65-68].
Theories and methods… 17
1.5.4 Growth from solution
Growth of crystals from solutions is mainly a diffusion - controlled
process. The constituents of the material to be crystallized are dissolved
in a suitable solvent and crystallization occurs as the solution becomes
critically supersaturated. This may be achieved by lowering the
temperature of the solution by slow evaporation or by continuous supply
of the material to compensate for that which is precipitated out. The
solution growth method is a widely practiced method of crystal growth as
it requires lower temperature and can lead to lower density of lattice
defects.
The solution growth methods are classified according to the
temperature range and to the nature of the solvents used. The important
methods are
(a) High temperature solution growth
(b) Hydrothermal growth
(c) Low temperature solution growth
(d) Gel growth
Flux method is the most widely used high temperature solution
growth technique [69]. In this method, the components of the desired
material are dissolved in a solvent, the so called flux. Under controlled
condition, crystals are formed. Crystallization occurs at a temperature
much lower than the melting point of the crystallizing substance. Rare
earth compounds have been grown by this method [70, 71]. Another
technique in high temperature solution growth is Liquid phase epitaxy
[72] in which a thin layer of crystalline material is deposited from solution
onto a substrate of similar composition or surface structure.
In hydrothermal method, an aqueous solvent is used at elevated
temperatures and pressures to dissolve a solute which would ordinarily
18 Chapter 1
be insoluble at ambient conditions. This is an excellent method for the
growth of low temperature polymorphs of refractory materials. The
growth of rare earth based salts using this method has been reported
[73, 74].
Low temperature solution growth is sub-divided into (a) slow
cooling method and (b) slow evaporation method. In slow cooling
method, the supersaturation is achieved by a change in temperature, ie.,
the saturated solution of the material is prepared in a suitable solvent at
a high temperature and crystallization is initiated by slow cooling. In
slow evaporation method the solvent is allowed to evaporate slowly
which results in the supersaturation of the solution and crystallization of
the material.
A controlled growth of crystals with high perfection can be
achieved by gel technique [75, 76]. This technique is employed to grow
crystals with low solubility in water and high thermal instability. Metallic,
non-metallic and ferroelectric crystals are grown by this method [77, 78].
The gel method has been used for growing crystals of non-linear optical
materials [79, 80], cholesterol and urinary stones [81-83]. Most of the
oxalate crystals are insoluble in water and decompose before melting.
Hence gel method is best suited for the growth of these crystals. In the
present work, gel technique is employed for the growth of mixed rare
earth oxalate crystals.
1.6 Rare earth oxalate crystals and the present work
The electric, magnetic, luminescent and superconducting properties
of rare earth compounds make them technologically important materials
[84-86]. An alloy of Nd-Fe-B is found to have remarkable magnetic
properties [87, 88]. Some of the high temperature superconductors are
La2-x Mx CuO4 (M = Ba, Sr, Ca) and Ln Ba2 Cu3 O7-x (Ln = Y, Nd, Sm, Gd,
Dy, Ho, Er, Tm, Yb) with x = 0.2. Much of the current research, both
academic and technological, on rare earth-based material is centred on
intermetallic compounds [89].
Theories and methods… 19
The oxalate family of rare earths have widespread optical
applications [90, 91]. The rare earth oxalates are significant due to their
ionic conduction and because of their easy conversion into the
corresponding oxides [92, 93]. Superconducting compounds of rare
earths have been synthesized by the controlled co-precipitation of
oxalates followed by calcination [94].
The present work includes the growth and characterization of a
few mixed rare earth oxalate crystals namely gadolinium samarium
oxalate, gadolinium neodymium oxalate and gadolinium cerium oxalate.
Since these oxalate crystals decompose before melting, high
temperature methods cannot be adopted for their growth. The
insolubility of oxalates in water has been utilized for growing these
crystals by silica gel method. A detailed account of the method of
growth, characterization and study of the physical properties of the
grown crystals is given in the following chapters.
20 Chapter 1
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