07_chapter 1

1. INTRODUCTION
1.1 INTRODUCTION
Technological advances have been accompanied by rapid strides in crystal
growth. Single crystals are the fundamental building blocks for modern technology. A
single crystal is made up of a three-dimensional periodic array of atoms. The
properties of a material are extensively studied when the material is prepared in single
crystalline form. A variety of crystals are needed to meet some very important gaps in
conventional production engineering. The solid-state material is classified into single
crystals, poly crystals and amorphous materials depending upon the arrangement of
atoms or molecules or ions. Crystals are solids in their most ordered form. Crystals,
once valued only for their beauty, are now found in one form or another in most
electronic, optoelectronic and numerous optical devices. These devices, in turn, have
permeated almost every home and village throughout the world. In fact it is hard to
imagine what our electronics industry, much less our entire civilization, would have
been like, if crystal growth scientists and engineers were unable to produce the large,
defect free crystals required by device designers.
1.2 CRYSTALS AND THEIR IMPORTANCE
Crystals are at the root of much of today's advanced technologies. Over the
past few decades, the advancement in science and technology has made single crystals
indispensable for the development of new generation devices. Crystal growth is an
interdisciplinary subject covering physics, chemistry, electrical engineering,
metallurgy, crystallography, mineralogy, etc. In recent past, there has been a growing
interest in crystal growth process, particularly in view of the increasing demand for
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materials for technological applications [1-3]. The strong influence of single crystals in
the present day technology is evident from the recent advancements in
semiconductors, polarizers, transducers, infrared detectors, ultrasonic amplifiers,
ferrites, magnetic garnets, solid state lasers, nonlinear optic, piezoelectric, acoustooptic, photosensitive materials and crystalline thin films for microelectronics and
computer industries. It is a challenging task for a crystal grower to grow bulk size
crystals with high figure of merit and hence realize their applications for various fields.
The growth of single crystals and their characterization with an insight for device
fabrication has assumed great impetus due to their importance in both academic
research as well as applied research.
1.3 METHODS OF CRYSTAL GROWTH
The consistency in the characteristics of devices fabricated from the crystals
depends on the homogeneity and defect contents of the crystals. Hence, the process of
producing single crystals, which offers homogeneous media in the atomic level with
directional properties, attracts more attention than any other process. The methods of
growing crystals are very wide and mainly dictated by the characteristics of the
material and its size [4-5].
The methods of growing single crystals may be classified according to their
phase transformation as presented in Table 1.1.
The mode of selection of a particular technique of crystal growth depends on
the characteristic properties of the materials such as melting point, vapour pressure,
decomposition and solubility in solvents.
The growth methods are also equally
dependent on growth kinetics involved, crystal size, shape, purity and nature of
application of the crystal.
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The conversion of a polycrystalline piece of material into single crystal by
causing the grain boundaries to be swept through and pushed out of the crystal takes
place in the solid-growth of crystals [6]. The above methods have been discussed in
detail by several authors [7-9].
Table 1.1 Various phase transitions
Melt growth
Liquid to solid phase transition
Solid growth
Solid to solid phase transition
Solution growth
Liquid to solid phase transition
Vapour growth
Vapour to solid phase transition
The different techniques of each category are found in reviews and books by
Faktor and Garret on vapour growth[10], Brice on melt[11], Henisch on gel
growth[12], Buckley on solution growth[13] and Elwell and Scheel on high
temperature solution growth[14].
An efficient process is the one, which produces crystals adequate for their use
at minimum cost. The growth method is essential because it suggests the possible
impurity and other defect concentrations. Choosing the best method to grow a given
material depends on material characteristics.
1.3.1 Growth from Vapour Phase
The principle used in this method is that if the vapour pressure is greater than
the equilibrium vapour pressure around a seed crystal, there will be a net deposition
and the crystal will grow. Since the large number of variables involved in this process
tend to make it difficult, its use is generally limited to materials which cannot be
readily grown from the liquid phase, such as the II-VI group materials and silicon
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carbide. Its most important use is in the production of epitaxial layers both on similar
and foreign substrates, where precise control of thickness, surface topography and
impurity content are required. The process is of great commercial significance since it
forms the basis of many semiconductor, optoelectronic and acoustoelectric devices.
Zha et al have grown large (0.5 kg) inclusion free mercuric iodide crystals using the
vapour growth method [15]. The following are the important techniques followed in
this method:
(i) Physical vapour transport technique,
(ii) Chemical vapour transport technique.
1.3.2 Growth from Liquid Phase
The crystal growth from liquid can be classified into the following four
categories.
(i)
Melt growth
(ii)
High temperature solution growth (Flux growth)
(iii)
Hydrothermal growth and
(iv)
Low temperature solution growth
There are number of growth methods in each category. Among the various
methods of growing single crystals, solution growth at low temperature occupies a
prominent place owing to its versatility and simplicity. Growth from solution occurs
close to equilibrium conditions and hence crystals of liquid perfection can be grown.
Study of anisotropy of the properties of crystals requires specimens cut in different
orientations from the same single crystal. This can be easily done from crystals of
large size.
The advantages of solution growth techniques are the following:
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(i)
Simple growth apparatus
(ii)
Growth of strain and dislocation free crystals
(i)
Permits the growth of prismatic crystals by varying the growth
conditions
(ii)
Only method which can be used for substances that undergo
decomposition before melting
The following are the conditional limitations of solution growth techniques.
(i)
The growth substance should not react with solvent
(ii)
This method is applicable for substance fairly soluble in a
solvent
The following are the disadvantages of solution growth techniques.
(i)
The interstitial incorporation of solvent ions into the crystal
lattice causing the formation of cloudiness
(ii)
Impurities may be absorbed on the growth face of the crystal
whereby changing the crystal habit
(iii)
The slow growth rate can be improved only to a certain extent.
It is beyond our control
(iv)
Non-uniform doping
However, the selection of a proper solvent of high purity, regulation of growth
by control of the temperature and circulation of the solution by efficient stirring can
rectify the above disadvantages.
1.3.3 Growth from Melt
This method is very popular because the growth rate of crystals grown by this
method is quite high. It has been used for growing about 70% of the crystals. The
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preferential role of the electrochemical process responsible for the change in
composition of the crystals when they grow in melt in an applied field has been
studied [16]. Sirdeshmukh et al studied the systematic hardness variation
measurements in doped RbBrxI(1–x) and KxRb(1–x)I mixed crystals[17] .In this method,
the following popular techniques are used.
(i)
Bridgman technique
(ii)
Czochralski technique
(iii)
Zone melting technique
(iv)
Verneuil technique
1.3.4 High Temperature Growth (Flux growth)
Flux and hydrothermal growths form the category of high temperature solution
growth. In this method, a solid (molten salt/flux) is used as the solvent instead of
liquid and the growth takes place well below the melting point of the solute [18]. This
technique can be applied to incongruently melting materials. Mixed crystals of solid
solution can also be grown by the choice of optimum growth parameters. The crystals
grown from melt will have lower concentration of equilibrium defects and lower
dislocation density.
1.3.5 Hydrothermal Growth
A number of metals, metal oxides and other compounds, practically insoluble
in water up to its boiling point, show an appreciable solubility when the temperature
and pressure are increased
well
below 100oC and 1 atmosphere, respectively.
The requirements of high pressure cause practical difficulties and there are only a few
crystals of good quality and large size grown by this technique. Quartz is the crystal
grown industrially by this technique.
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1.3.6 Low Temperature Solution Growth
Low temperature solution growth is the most widely used method for the
growth of single crystals when the starting materials are unstable at high temperatures.
This method demands that the materials must crystallize from solution with prismatic
morphology. In general, this method involves seeded growth from a saturated solution.
The driving force i.e., the super-saturation is achieved either by temperature lowering
or by solvent evaporation. This method is widely used to grow bulk crystals, which
have high solubility and have variation in solubility with temperature [19]. After many
modifications and refinements, the process of solution growth now yields good quality
crystals for a variety of applications. Growth of crystals from solution at room
temperature has many advantages over other growth methods though the rate of
crystallization is slow. Since growth is normally carried out at room temperature or at
temperatures closer to it, the structural imperfections in solution grown crystals are
relatively low [11].
Low temperature solution growth can be subdivided into the following
methods.
(i)
Slow cooling technique
(ii)
Slow evaporation technique
(iii)
Temperature gradient technique
1.3.6a Slow cooling method
This is the most suitable method among various methods of solution growth.
However, the main disadvantage of slow cooling method is the need to use a range of
temperature. The possible range of temperature is usually narrow and hence much of
the solute remains in the solution at the end of the growth run. To compensate this
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effect, large volume of solution is required. The use of wide range of temperature may
not be desirable because the properties of the grown crystals may vary with
temperature. Temperature stability may be increased by keeping the solution in large
water bath or by using a vacuum jacket. Achieving the desired rate of cooling is a
major technological difficulty. This technique needs only a vessel for the solution in
which the crystals grow. The height, radius and volume of the vessel are so chosen to
facilitate the achievement of the required thermal stability. Even though this method
has technical difficulty of requiring a programmable temperature controller, it is
widely used with great success.
In
general,
the
crystals
produced
are
small
and
the
shapes
of the crystals are unpredictable. Koichi Watanabe has reported the growth of single
crystals of willemite (Zn2SiO4) containing small amount of Mn2+ and Sb3+
from
Li2MoO4 solvent by the slow cooling technique within the temperature range 1300 and
900 K [20]. Single crystals of cadmium mercury thiocyanate (CdHg (SCN)4) is grown
by using two different solvent mixtures of water-NaCl, water-KCl employing
temperature lowering method [21].
1.3.6b Slow evaporation technique
As far as apparatus is concerned, slow cooling and slow evaporation methods
are similar to each other. In this method, the saturated solution is kept at a particular
temperature and provision is made for evaporation. If the solvent is non-toxic like
water, it is permissible to allow evaporation into the open atmosphere. Typical growth
conditions involve a temperature stabilization of about 0.05oC and rate of evaporation
of a few mm3/h. The evaporation technique has an advantage viz. the crystals grow at a
fixed temperature. But inadequacies of the temperature control system still have a
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major effect on the growth rate. This method can effectively be used for materials
having very low temperature coefficient of solubility. But the crystals tend to be less
pure than the crystals produced by slow cooling technique, as the size of the crystal
increases more impurities find place in the crystal faces. Evaporation of solvent from
the surface of the solution produces high local super-saturation and unwanted nuclei
are formed. Small crystals also form on the walls of the vessel near the surface of the
liquid from the material left after evaporation. These fall into the solution and hinder
the growth of the crystal. Another disadvantage lies in controlling the rate of
evaporation. A variable rate of evaporation may affect the quality of the crystal. In
spite of all these disadvantages, this is simple and convenient technique of growing
single crystals of large size.
Crystals of HgCl2.2KCl.H2O was grown by Sastry et al [22] from aqueous
solutions by the slow evaporation technique. A large improvement in stability was
observed in NaCl-KCl mixed crystals grown from (NaCl)x (KCl)0.9−x (KBr)0.1 solution
than in crystals grown from NaxK1−xCl solution [23]. Mixed crystals of two nonlinear
optical materials, L-arginine hydrochloride monohydrate (LAHCl) and L-arginine
hydrobromide monohydrate (LAHBr) have been grown from aqueous solutions by
using slow solvent evaporation technique by Tanusri Pal et al [24].
L-histidine
tetrafluoroborate (L-HFB) single crystals have been successfully grown from solutions
of three different pH values by evaporation and temperature lowering techniques by
Rajendran et al [25] and the crystals grown at lower pH values are found to have
higher solubility. Ramajothi and Dhanuskodi [26] have reported the growth of Lhistidine tetrafluoroborate (L-HFB) from its aqueous solution kept at 26°C by
employing slow evaporation technique.
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1.3.7 Gel Growth
Henisch gave an excellent survey of the gel growth process [12]. Only small
crystals can be grown by this technique. Gels are two-phase systems comprising a
porous solid with liquid filled pores.
The pore dimensions depend on the
concentration of the gel material. The most frequently used gels are based on silica.
But gels based on gelatine, various soft soaps and pectin are also used. Seed crystals
should be used to reduce flaws in the center of the crystals. It is very important to
provide constant ambient temperature. The gel technique has been successfully
employed for growing mixed crystals also. Mixed crystals of oxalates [27], tartarates
[28], sulphates [29], etc of many elements have been reported to be grown by this
method. Lead bromide crystals of high optical perfection and of different habits with
interesting surface features have been grown in silica gel [30].
1.4 MIXED CRYSTALS
Mixed crystals are those of materials chemically similar in nature and having
same crystal structure. The mixed crystals are formed from a homogeneous growth of
one component in other occupying interstitial or substitutional positions. The mixed
crystals can be formed among substances which have similar crystalline structure.
Single crystals of barium mixed calcium tartrate tetrahydrate (CBT) have been grown
and reported [31]. The physico-chemical characterization of calcium strontium tartrate
crystals was reported [32]. Mixed crystals of strontium calcium tartrates have been
grown and reported [33]. A mixed crystal, in general has physical properties analogous
to those of the pure (end member) crystals. In some cases, the magnitude of the
physical properties for the mixed crystal even exceeds the values for the end members.
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In these cases, it is as if we have a new crystal in the family. In a few cases, mixed
crystals show exciting behaviour.
1.4.1 Alkali Halide and Semiconductor Mixed Crystals
Halide mixed crystals has ever been of keen interest to crystal scientist because
of their properties of luminescence and electrical conductivity. A large number of
investigators have been attracted to work on the alkali halide crystals for several
decades. The alkali halide crystals have always been at the centre state of solid-state
physics. They have been “model crystals” for testing many solid state theories. In
recent decades they have also proved useful in several applications ranging from X-ray
monochromators to tunable lasers. Because of this dual importance-both purely
scientific and technological- a vast amount of information has been generated on all
aspects of the alkali halides. The development of lasers revived the interest in alkali
halides as materials for optical components. This led to development of alkali halide
polycrystalline material for use as optical windows [34]. The use of pure simple alkali
halides is limited by the mechanical systems and hence there exist the need to
strengthen them. The mixed and impurity added (doped) crystals of alkali halides are
found to be harder than the end members and so they become more useful in these
applications. Alkali halide mixed crystals find their applications in optical, optoelectronic and electronic devices. For these reasons, it becomes necessary and useful to
prepare binary and ternary mixed crystals regardless of miscibility problem and
characterize them by measuring their physical properties [35]. Padma and Mahadevan
[36] have reported the growth and characterization of multiphased mixed crystals of
NaBr and KBr. Mahadevan and his co-workers [37] obtained larger and more stable
crystals from (NaCl)x(KCl)0.9-x(KBr)0.1 solutions than from NaxK1-xCl solutions. They
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grew the crystals from aqueous solutions only. Though the miscibility problem was
there, their study has evidenced that a KBr addition to NaCl-KCl system may yield a
new class of stable materials. Semiconducting mixed crystals have been extensively
studied because of their intrinsic physical properties and potential application in
optoelectronics[38–40] and spintronics [41,42].
1.4.2 Lead Halide Mixed Crystals
Among halide crystals lead halides forms a conspicuous group with these
properties prominent in them in addition to the marked exhibition of photo
conductivity. The growth and characterization of halides and mixed halides of lead
have been widely investigated. Figures 1.1 and 1.2 show the molecular and crystal
structures of PbBr2 and KBr /NaBr.
Lead is a member of Group 1V (IVA) of the Periodic Table because it has four
electrons in its outer, or valence, shell. However, the usual valence of lead is +2, rather
than +4. Lead (II) halides have become very important materials because of potential
applications in acousto-optical and room temperature X-ray and γ-ray detector devices.
The mechanical properties are also much better than some of the materials presently
being developed for nuclear detector application [43]. Lead halide crystals have been
well known as typical materials showing photolysis with ultraviolet light or X-ray
irradiation at room temperature [44]. In PbCl2 and PbBr2 crystals, two kinds of
intrinsic luminescence are observed under ultraviolet excitation at low temperature
[45, 46].
Among lead halides, lead bromide is a very important material [43,47, 48].
Because of its potential application in acousto optic (AO) and opto electronic devices,
single crystals of this material have highly unusual optical properties which are:
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spectral transmission range, photo elastic coefficient, acousto optic figure of merit,
acoustic velocity and acoustic attenuation. Depictive of the difficulties in growing
crystals of high optical qualities, the combination of these properties makes lead
bromide an interesting material in mid and far infrared wavelength region [49]. The
growth of crystals with low acoustic attenuation and good optical quality is still a big
challenge. In addition lead bromide possesses reasonably good mechanical properties.
These properties will be very helpful for AO devices used in optical signal properties.
Singh et al [50-52] have studied the growth and optical acoustic characteristics of
PbBr2. Lead bromide belongs to orthorhombic symmetric class with mmm space
group. In 1958, Medestrova and Sumarokova [53] reported a phase transformation in
lead bromide at 343oC. Very recently Singh et al [49] have presented a solid-solid
phase transformation observed in lead bromide at 365o C.
Figure 1.1: Molecular and crystal structure of PbBr2
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Figure 1.2: Molecular and crystal structure of KBr / NaBr
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Optical bromide crystals offer a unique combination of physio chemical
properties, in particular wide transparency range upto 30 mm, relative chemical
stability in air and appropriate mechanical and thermal properties [54–58].
Photoluminescence (PL) studies of RbBr1-xIx: Tl+ (0.1mol %) with x = 0.00, 0.05 and
0.10 mixed crystals grown in vacuum is reported at room temperature[59]. Easwaran
et al reported KBr1-xIx: Tl+ mixed crystals doped with Tl+ impurities and studied the
appearance of additional bands on the low energy [60]. They have also reported the
optical absorption (OA) studies of RbBr0.95 I0.05: Eu2+, Tl+ mixed crystal [61].Optical
properties in the surface of KClxBr1-x mixed crystals irradiated to gamma radiation was
studied by Bagheri and Malekfar [62]. To date several nonlinear optical and acoustooptical materials are found among complex bromides [54, 55, 58, 63-65]. Complex
lead-bromides containing heavy alkaline metals accept effective doping of rare earth
lasants and are promising for creation of new laser host mediums in the middle
infrared spectral range [57, 58, 66-68]. Only heavy atoms are present in the crystal
lattice of these materials and this is a key factor to obtain very low phonon frequency
and avoid luminescence quenching through multiphonon relaxation. Laser activity was
achieved for several lasant-doped bromide hosts in which single crystals were grown
with high enough quality and dimension. Also, it is a known fact that alloys are more
useful than the pure simple metals in device fabrications. Only little attention has been
paid to the crystallisation and characterization of ternary alkali lead bromides up to
now. The alkali-lead bromide crystals (RbPb2Br5 and CsPbBr3) were grown first in
1995 and the results of the study of crystal growth and luminescence properties were
reported [66]. In recent years, ternary lead halides were identified as new low phonon
energy laser host materials [69-77]. The large amount of experimental data allowed
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the investigation of the current interest of early theoretical studies [78-82]. Rare earthdoped potassium halides (KPb2Cl5, KPb2Br5) are of significant current interest for
infrared (IR) solid-state laser applications [66,69-72,83-88]. In contrast to many other
halides KPb2Cl5 (KPC) and KPb2Br5 (KPB) exhibit very small moisture sensitivity
which makes them attractive host for practical laser application. The lower maximum
phonon energy of KPb2Br5 compared to KPb2Cl5 leads to significant difference in the
RE emission properties as it was reported for Nd: KPb2Br5 [72, 85, 86] and Er:
KPb2Br5 [85-88]. KPb2Br5 has a maximum phonon energy of only 138 cm-1[87] and is
non hygroscopic which makes it an attractive candidate for solid state device
application. The rare-earth-doped potassium-lead bromide (KPb2Br5 or KPB) and
rubidium-lead bromide (RbPb2Br5 or RPB) crystals have properties similar to that of
KPC crystals, but lower phonon cut-off energy (~ 140 cm-1) [85]. There are only a
few reports in the literature on the crystal structure of KPb2Br5 [89-91]. According to
Beck et al [90] KPb2Br5 is biaxial and has monoclinic crystal structure (space group
P21/c) with an angle β very close to 90o. The melting point of KPb2Br5 is ~382oC and
a phase transition occurs at 240oC [89]. The refractive index is KPb2Br5 is ~ 2.1. The
moisture resistant property of these bromide crystals, the potential to incorporate rare
earth ions, and their low phonon frequency render them potentially useful laser crystals
[52]. The crystals of KPb2Br5 were prepared from an aqueous solution [92], from the
melt and by a solid state reaction [90-92]. There is a phase transition in this compound
at 242°C. The low-temperature modification should be orthorhombic [92] or tetragonal
[90] . The temperature, composition and crystallographic orientation dependence of
the ionic conductivity of the solid solutions PbCl2xBr2(1-x) have been reported
recently[93-94].CuPbBr3 single crystal has high conductivity of the order of
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2.3 x 10-4(Ω cm) -1at 22o C. The conductivity is mainly due to bromide ion vacancy
motion with activation energy of 0.27 eV [95].
Based on the above literature, in the present study, we have considered PbBr2,
KBr and NaBr for the growth of ternary alkali lead bromide crystals.
1.5 PRESENT WORK
Now we are in a position to understand the importance of lead halide mixed
crystals. They have several importance as acousto-optic devices, opto electronic
devices, low phonon energy laser host materials and mid-infrared and far ultraviolet
region devices. In view of this, it becomes necessary and useful to prepare ternary
mixed crystals of lead halides.
In the present study, mixed crystals of PbBr2 and KBr / NaBr have been grown,
for the first time, from slow evaporation of solution method and characterized.
The grown crystals were characterized chemically by determining the
composition using AAS analysis and EDAS data. They were characterized structurally
using powder X-ray diffraction (PXRD), and the lattice parameters were estimated
from single crystal XRD data. Thermal properties were studied from TGA / DTA
analysis. The optical properties of the grown samples were determined from UVVisible absorption spectra.
The morphology of all the grown samples were analyzed from SEM images.
The dielectric parameters like dielectric constant, dielectric loss and AC electrical
conductivity were measured by the parallel plate capacitor method for different
frequencies, viz. 100Hz, 1kHz, 10kHz, 100kHz and 1MHz at various temperatures
ranging from 40 -150o C. The DC electrical conductivity was measured by the two17
probe setup at various temperatures ranging from 40 -150o C. Activation energies were
also determined.
A detailed report of the present research work is provided in this thesis.
An overview of the various methods of crystal growth and, in particular, the
growth from solution and, an up-to-date level of achievements in the development of
mixed crystals have been provided in this Chapter.
The second Chapter itemizes the various techniques, instrumentation and
applications of spectroscopy to crystals. Apart from this, the other salient
instrumentations discussed are the single crystal X-ray diffraction, powder X-ray
diffraction, scanning electron microscopy (SEM), EDAS, atomic absorption
spectroscopy (AAS), thermal analysis such as TGA and DTA, dielectric and
conductivity measurements . These techniques have been employed for characterizing
the crystals grown in the present work.
The third Chapter is devoted to the growth and characterization of solution
grown potassium lead bromide single crystals, which can be used as a potential
candidate material for opto-acoustics. The growth conditions are optimized and the
results are discussed. The crystal data and morphology for the grown crystals are
determined by X-ray diffraction analysis. Chemical composition of the grown crystals
were analyzed by AAS and EDAS data. They were characterized thermally, optically
and electrically. The results of above mentioned studies are discussed in this Chapter.
The fourth Chapter presents the study of the opto- electronic material, sodium
lead bromide single crystals. Structural, optical, thermal and electrical studies are
undertaken on these crystals and the results are discussed.
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Fifth Chapter summarizes the results of the studies undertaken with respect to
the bromide mixed crystals dealt with in the present research work and goes a step
further by listing out the possible extensions of this study.
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