Experimentation for the growth of single crystals · 2019-01-01 · (Al 2O 3), GaAs, gadolinium...

Chapter – 2 Experimentation of Crystal Growth

~ 35 ~

Chapter – 2

Experimentation for the growth of

single crystals

Abstract

The detailed description about the Czochralski (CZ) and solution

growth methods has been given. The indigenously developed

Czochralski pullers have been employed for the growth of bul k crystals

of pure, Zn- and Fe- doped LiNbO3 and pure and liquid crystal induced

Benzophenone. For the growth of crack free and large-size LiNbO3

crystals a two zone furnace has been designed. In this furnace the main

heat source was a radio-frequency furnace. A coaxially coupled

resistive heater was installed over the RF furnace for fine control

temperature of the entire furnace zone particularly , during the post

growth slow rate cooling cycle . For Benzophenone crystal growth a

resistive furnace designed for low temperature crystal growth was

employed. The Eurotherm temperature controllers were used for the

precise controlling of the temperature. The slow evaporation solution

technique (SEST) has been adopted for the growth of pure, urea and

chromium doped tris(thiourea)zinc sulphate (ZTS) single crystals.


~ 36 ~

2.1 CZOCHRALSKI CRYSTAL GROWTH

As stated earlier the growth of bulk single crystals of nonlinear optical

materials is of great importance in view of their photonic applications. This section

gives the detailed description about the Czochralski method (CZ). CZ method is a

well-known and unique melt growth technique for the growth of bulk single crystals,

was invented by Prof. Jan Czochralski in ~1916 and became popular with his name

(Czochralski, 1918). This method is most suited for the growth of large size single

crystals of technologically important materials such as; silicon (Si), Ge, sapphire

(Al2O3), GaAs, gadolinium gallium garnet (GGG), bismuth geminate (BGO), LiNbO3,

Nd:YAG, Ti:sapphire, etc.

2.1.1 Salient features of Czochralski technique

The geometry for Czochralski growth is shown via schematic in Fig. 2.1(a).

The yellow line shows the temperature variation at various parts of the furnace and

crystallization zone. The material to be grown in to single crystal is melted in a

suitable non-reacting container under a controlled atmosphere by induction or

resistance heating. The temperature of melt is set slightly above the melting point and

the seed crystal (single crystal of same material) is lowered close to the melt surface

with continuous rotation. After thermodynamical equilibrium, the seed is contacted

with melt and the melt temperature raised to establish the desired growth interface

configuration. The pulling mechanism started to withdraw seed from melt at slow

rate. During this process the crystal diameter is gradually increased to the desired

value. Growth at constant diameter is achieved by maintaining the solidification

isotherm in a vertical position intersecting the meniscus at the point where the

isotherm becomes perpendicular to the melt surface. This condition is maintained by

the adjustment of following parameters: rate of pulling, rate of liquid level drop, heat

fluxes into and out of the system, and to a lesser extent by seed/crucible rotation.

The shape of the crystal is controlled by angle of contact ‘θ’ of meniscus with

the crystal (cylinder) as shown in the inset of Fig. 2.1(b). When θ < 0 diameter will

decrease, at θ = 0 diameter will remain constant and when θ > 0 the diameter will

increase and this can be controlled by controlling the power to melt.


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Ver

tica

l d

ista

nce

Temperature

Induction

Heater

coil

Seed

crystal

growing

crystal

Seed

holder

Melt

Insu

lati

on(a)

d‘ = 0

d‘ ˂ 0

d‘ ˃ 0

(i)

(iii)

(ii)

(b)

θ

Fig. 2.1: (a ) the schematic for the Czochralski crystal growth geometry

(http://www.answers.com/topic/crystal-growth) and (b) the schematic showing relation

between liquid-solid interface shapes and heat flow directions across interface (i) convex,

(ii) flat and (iii) concave and the inset shows contact angle

The contact angle decreases with increase in the power to melt and vice versa. A

standard necking process is employed to initiate the growth of high quality dislocation

free single crystals, by the precise controlling of power to melt.

The termination of crystal growth is also important. The termination of growth

by necking and slow cooling is required to reduce the amount of thermal shock

sustained by the crystal, generally ferroelectric crystals like LiNbO3, KNbO3, etc.

require very slow cooling rate after their growth cycle as these crystals undergo the

ferroelectric phase transition, during which a considerable volume change of unit cell

take place at their Curie temperature. Thermal gradients in the melt lead to change in

the composition of crystals and these thermal gradients are responsible for thermal

convection flows in which the temperature and velocity fields vary with time at any

point in space. The transient flow patterns cause transient segregation behavior at the

growth interface and often lead to uncontrolled inhomogeneous compositional

distributions in grown crystals.

The parameters which decide the control on growth of single crystals are

described here briefly. Good thermal conditions are crucial for the growth of high

quality crystals. In addition to heat, which is conducted up through the crystal and


~ 38 ~

seed rod, heat can be gained or lost from its surfaces by radiation or by convection

through gaseous convection. The control of gaseous and liquid convection is very

important for good quality of grown crystal. The rate of crystal pulling is an important

factor. If pulling rate is fp and the crystal of radius r is growing from crucible of radius

rc then as the material is conserved, the growth rate f can be obtained by equating the

mass of solid that has formed and the effective decrease in mass of the liquid. If dl and

ds are the densities of liquid and solid respectively, then,

pccs fdrfdrfdr 1

2

1

22 , which gives the value of f

22

1

2

1

rdrd

rdff

sc

cp

, if dl and ds are same, then

22

2

rr

rff

c

cp

Thus the average growth rate of a crystal is determined by the pulling rate and

radii of the crucible and the crystal. The radius of crystal is however controlled by the

melt temperature and temperature gradient at the growing interface.

Convection in liquid are complex and in case of high melting point materials

the inward flow from the periphery of the crystals are very severe, the shape of the

liquid/crystal interface is dominated by the flow and cone shaped interfaces and

pointed ends to the crystals become common (Carruthers, 1976; Hurle, 1976, 1983;

Normand et al., 1977). Rotation of crystal causes a centrifugal flow on the liquid

immediately adjacent to its surface and draws up liquid under its center. The high

temperature gradient at the solid/liquid interface causes the complex gaseous

convection at gaseous/liquid interface, which can be avoided by using low pressure in

chamber. The driving force for convection due to temperature gradient in horizontal

system is characterized by Grashof number ‘Gr’ given by, 24 / vTglGr H , here g

is the gravitational constant, α the thermal expansion, ΔTH horizontal temperature

gradient over length l and v is the kinematic viscosity of liquid. In case of vertical

convection the driving force is given by Rayleigh ‘Ra’ number, which is defined

as, vTgdRa v /4 , where, ΔTv is the vertical temperature gradient in the depth of

d, κ is the thermal diffusivity of melt (Normand et al., 1977). Increase in Gr or Ra

leads instability of plow pattern which lead to the turbulence. The non-stable

convective modes may result in periodic melt-back of growth interface and non-


~ 39 ~

uniform incorporation of alloy components or impurities to produce growth striations,

therefore, the low thermal gradients are advisable for good quality crystal growth. At

the industrial scale for very large crystal growth the crucible is made to rotate in

addition to seed rotation, to reduce the thermal asymmetry.

The shape of growth surface is dependent upon its crystallographic orientation

and the local temperature gradient. The possible shapes of the liquid-solid interfaces

are given in Fig. 2.1(b). For good quality, growth interface must be with ‘d’ = 0,

however it is difficult to maintain this condition. The slight thermal fluctuations

generate concave interface with ‘d’ < 0, due to excess input power to melt. In oxide

materials due to high surface tension, the excess input power raises liquid-solid

interface by several millimeters above the plane of melt surface (Santhanaraghavan &

Ramasamy, 2000). Under such conditions the radial heat loss at the periphery of

crystal become important and may lead to the formation of spines and ring facets, due

to rapid solidification. This condition may also lead to the entrapment of air bubbles

and propagation of elongated voids in grown crystals.

2.1.2 Czochralski growth of pure, Zn- and Fe-doped LiNbO3 single crystals

2.1.2.1 Phase diagram of LiNbO3

Lithium Niobate (LiNbO3) is thermally very stable above its melting point

(~1523 K), therefore it is suitable for the bulk crystal growth using Czochralski melt

technique. Matthias & Remeika, (1949), have first time obtained the single crystals of

LiNbO3 for ferroelectric properties. The major problem encountered during growth of

single crystals is assessing the composition of charge which leads to the proper

composition of grown crystals along their direction (Krol et al., 1980). The phase

diagram of LiNbO3 is given in Fig. 2.2(a) (Reisman & Holtzberg, 1958; Malovichko

et al., 2001). From diagram it is clear that LiNbO3 with congruent composition has

melting point ~1523 K, and varied with Li and Nb composition (Yih, 1989). The

stoichiometry of the crystals is defined by [Li]/[Nb] ratio and its value is different in

the crystals from the that in melt from which they have been grown. For the growth of

stoichiometric crystals ([Li]/[Nb] = 1) the initial composition of charge is taken Li

rich with [Li]/[Nb] > 1. The Li-rich and Li-poor phase boundaries have been located


~ 40 ~

by Holman, (1978) through the vapor equilibrium process. The L-rich phase boundary

was found to be a vertical line at 50 mol% of Li2O. The eutectic point on the solid-

liquid curve exists at ~48.85% of Li (O’Bryan et al., 1985). This composition is

known as congruent and has been preferred in the material history for most of the

research and technological applications. The crystals grown with starting this

composition exactly grow with same cationic ratio, and the liquid composition

remains unaltered along the process. The single crystals grown with composition

differ from this melt results in the compositional inhomogeneity along the pulling

direction (Rauber, 1978). The congruent lithium niobate crystals contain Li related

defects and efforts have been done to investigate the crystalline structure. Due to

deficiency of Li, niobium ‘Nb’ occupies Li sites and due to the charge neutrality

occurrence of other defects also takes place (Abrahams & Marsh, 1986).

This signifies that congruent lithium Niobate crystals are internally disordered,

which lead to the spectroscopic impurities. The large amount of foreign elemental

impurities can accommodate in such sponge-like structure depending on the electronic

structure of foreign element (Kuz’minov & Osiko, 1993). The temperature

corresponding to the melting of congruent composition is highest with respect to the

rest compositions, hence congruent crystals are easy to grow with homogeneity of

composition in the entire boule. For the stoichiometric composition it is very difficult

with only one composition of the solid solution. The stoichiometric crystals can be

grown with 50:50 ration of Li/Nb by taking initial composition Li rich with ~58% of

Li2O. This composition results in difficulties in growing the single crystals and

cooling process after growth. The special efforts have been made for the growth of

high quality stoichiometric LiNbO3 single crystals (Kitamura et al., 1992; Polgár et

al., 1997). The crystal in Czochralski is pulled vertically in cylindrical shape using

seed crystal cut is certain direction. LiNbO3 has rhombohedral lattice structure and

during the cooling cycle of grown crystal, it undergoes the structural phase transition

from paraelectric structure with space group R 3 c (D 6

3d ) and point symmetry 3m to

ferroelectric structure with space group R3c (C 6

3v ) (Abrahams, Reddy et al., 1966). In

ferroelectric phase the structure consists of planar sheets of oxygen atoms in a

distorted hexagonal closed pack configuration,


~ 41 ~

LiNbO3

+ Liquid (Liquid)

Congruent

point

LiNbO3

+ Li3NbO4

LiNbO3

+ Li3NbO4

(Li-Rich)

LiNbO3

+ LiNb3O8

(Li-Poor)

Li2O mol%

Tem

pera

ture

, oC

45 5550

1300

1200

1100

1000

600

900

800

700

Stoichiometric

LiNbO3

(a)

op

tic

axis

(b) +ve dipole end (c) neutral (nonpolar)

O

Li

Nb

Fig. 2.2: (a) Phase diagram of Li2O-N2O5 system (Arizmendi, 2004), (b) and (c) are the schematic

diagrams lithium niobate crystal lattice in ferroelectric and paraelectric phase respectively (Weis

& Gaylord, 1985)

the Li and Nb ions are situated in between these layers [Fig. 2.2(b)]. Above Curie

temperature the Li and Nb ions move into centrosymmetric position with Nb ion lying

in the plane of one of the oxygen layers and Li ion lying at half way between the

layers [Fig. 2.2(c)], which results in the paraelectric nonpolar state of lithium niobate.

The lattice parameters of the two phases are different and lead to the change is

volume during transition. Due to such structural change at high temperature it is very

important to control the cooling temperature of the grown crystal very precisely,

during phase transition.

2.1.2.2 Bulk crystal growth

For the growth of bulk single crystals of pure and Zn as well as Fe doped

LiNbO3 a CZ crystal puller designed, developed and fabricated at our laboratory,

National Physical Laboratory (NPL), New Delhi, was employed. A photograph of the

puller is shown in Fig. 2.3(a). The schematic of puller is shown in Fig. 2.3(b). Great

care has been taken to ensure that the pulling motion is smooth and uniform. Two

specially made stainless steel rods – one with a square cross section and surface

planarity within 5 μm over its entire length of 1 m and another with a circular cross

section – act as guides to the platform to which the seed is attached. The rate of

pulling can be varied stepwise in the range 2–20 mm h-1

. The total available length of

pulling is about 600 mm. A seed rotation assembly provides the desired rate


~ 42 ~

of rotation to the crystal during growth. Special efforts have been made to isolate the

furnace, the crucible, and the seed rotation and pulling assembly from vibrations

generated in the system and in the surroundings.

(1)

(4)

(3)

(a)

(2)

Fig. 2.3: (a) The photograph of high

temperature Czochralski puller, designed and

developed at National Physical Laboratory,

New Delhi, India: (1) pulling & rotational

assembly, (2) RF furnace, (3) computer

controlled RF generator, and (4) control panel

for seed rotation and pulling rates. The inset

shows display of Eurotherm 2404 controller,

at the bottom of it are the Process Variable

(PV) and Target Set point (tSP) controlled by

computer

(b) A line schematic of puller: (1) pulling rods,

(2) motor for translating pulling assembly, (3)

seed rotation assembly, (4) furnace, and (5)

concrete platform with shock absorbing

vibration free stage over it

(b) 1

5

3

4

2


~ 43 ~

150 mm

110 mm

60 mm

45 mm

45 mm

45 mm

75 mm

85 mm

7.5 mm

160 mm

20 mm

95 mm

150 mm 90 mm

40 mm

6

1

3

4

2

5

7

8

0 20 40 60 80 100 1201350

1400

1450

1500

1550

Tem

per

ature

(K

)

Distance from crucible bottom (mm)

Axis temperature

Fig. 2.4: (a) The line schematic of the designed furnace indicating two temperature zones of RFH

& RPGH, (b) axial temperature profile of the complete zone measured from the bottom of crucible

For this purpose, antivibration mountings of different types are used. The seed

rotation assembly is driven by a 12 W synchronous motor. Particular attention was

paid for ensuring that the wobble of seed rod with seed crystal around the rotation

axis is negligible. In the experiments reported here, the rate of rotation was kept at 30

1 Platinum crucible

2 Inner muffle

3 Middle muffle

4 Outer Muffle

5 Resistive heater

6 Ceramic blocks

7 RF coil

8 Hole for thermocouple

(b)

(a)


~ 44 ~

r min-1

. For the growth of single crystals at high temperature ~1523 a platinum

crucible of 50 mm diameter and 40 mm height was used.

A two zone furnace was designed to attain the required temperature for the

growth and cooling processes and the schematic of it is shown in Fig. 2.4(a). This

specifically designed furnace consists of a radio-frequency heating (RFH) as a main

zone in the lower portion and to lower the temperature gradient above this RFH zone

and also for a well-controlled post growth cooling, a coaxial resistive post growth

heater (RPGH) is placed just above the RFH zone. This RPGH acts as a second zone

of the furnace and power to it can be separately controlled to get the required

temperature gradient in the entire furnace zone. The lower RFH zone was powered

with a 30 kW radio-frequency (30 kHz) generator, power of this generator was

controlled by a programmable Eurotherm controller (2404) by proper interfacing and

tuning process. This Eurotherm controller was interfaced with a computer to achieve

the controlling of temperature of the lower RFH zone with a stability of ±0.05 K.

However, such minute variations in the lower zone were required to control the initial

growth and diameter of the growing crystal during the entire growth process could be

achieved with computer controlling.

Before starting the growth and also before switching on the RFH, RPGH was

kept at a constant temperature ~1073 K by giving a fixed voltage through a precise

variac. Then RFH switches on and the required temperature is attained through the

controller at the desired rate. As the RFH switches on, the temperature of the resistive

heating zone increased, and without changing the power of RPGH the temperature of

RFH was set ~1523 K. Under such condition the vertical temperature profile of the

furnace shown in Fig. 2.4(b) was recorded starting from the bottom of the crucible by

measuring the temperature at the axis of furnace.

As the temperature profile [Fig. 2.4(b)] indicates, due to the presence of

RPGH, a good gradient needed to grow lengthy crystals of the order of 100 to 125

mm could be obtained. For the growth process of all doped and pure crystals this

temperature gradient of ~14 K/cm was maintained just above the melt level of the

charge with the help of RPGH, to avoid the formation of extended structural defects


~ 45 ~

Fig. 2.5: The photographs of side and top views of the two zone furnace, the RFH coils and RPGH

like twins and grain boundaries which often occur at the early stage of crystallization

from seed (Cochet-Muchy, 1994). To maintain the temperature conditions very

precisely, the proper insulation of furnace was done. To avoid the dissipation of heat

from the lower side and maintain the height of crucible two ceramic blocks of

cylindrical shape were placed. The (Pt-Rh) thermocouple was taken out from

downward through a fine hole in these blocks. To avoid the radial dissipation of heat

from the furnace, proper insulation of ~4.3 cm thickness has been made.

The Fig. 2.5 shows the side and to view of the two zone furnace. From the

schematic [Fig. 2.4(a)] and top view of the furnace [Fig. 2.5] it can be seen that we

have used two coaxial ceramic (99%) muffles for insulation purpose. Outside of the

outer muffle the glass-wool was properly wrapped up and finally covered with a high

temperature resistive asbestos sheet. The RPGH of ~90 mm length was made from

kanthal wire by winding around the inner muffle, lower end of this heater was set at

the height of the topmost coil of RFH. The wounded coils of the heater were kept

separated from each other using ceramic beads over the entire length of wire used

(visible in top view of furnace). Voltage applied to this heater was controlled by using

a variac of high precision. During the growth process of crystals the furnace from top

was covered with a ceramic brick lid having two holes, one in center for insertion of

seed rod and other near the periphery inner muffle to get the slant visualization of

inner region of the furnace up to the bottom of crucible and seed crystal.

Resistive

heater RF furnace

Tip of thermocouple


~ 46 ~

(b) (c)(a)

10 mm

Fig. 2.6: (a) Platinum crucible, (b) Z-cut [001] crystal block of pure LiNbO3 which is used to

make seed crystal and (c) the seed holder attached with seed

The side hole helped the visual monitoring of the dipping of the lower end of seed in

to the melt of charge in crucible and the solid liquid interface during growth of

crystal. For clear visualization at high temperature a gold coated quartz/glass plate

was used to filter out the infrared radiations.

Figure 2.6 shows the photographs of a platinum crucible used for heating and

melting of charge, a Z-cut and lapped block of pure crystal used for the seed

preparation, here a upright blue arrow indicates the [001] direction of crystal, and the

platinum seed holder assembled with a seed crystal. The seed crystals used for the

growth bulk crystals were structurally perfect and free from the grain boundaries and

visible cracks. The Pt seed holder shown in Fig. 2.6(c) having seed crystal was

attached with a ceramic seed rod which was connected with the seed rotational

assembly of the puller. To perform the growth of bulk crystals of pure and doped

LiNbO3, the starting 4N purity (Sigma Aldrich) materials of LiNbO3, ZnO and Fe2O3

were used. For growth of doped crystals, Zn:LiNbO3 and Fe:LiNbO3, the required

amount of ZnO (1 mol%) and Fe2O3 (0.05 mol%) respectively, were properly mixed

with LiNbO3 and the charges were sintered at 1000 °C for 8 hours before melting. The

melted charges were left for ~4 hours for homogeneous melting and mixing of

dopants. The good quality 20 mm long crystals cut along [001] were used as seeds

one of such is shown in Fig. 2.6(b), to grow bulk crystals. The seed crystals were

prepared from the primary bulk crystal which was grown by using the platinum wire


~ 47 ~

as a seed. The crystal was cut in such a way so that we could get the [001] oriented

seed crystals. The orientation of the seed crystals was assessed by high resolution X-

ray diffraction.

At the growth temperature of melt, the lower end of the continuously rotating

seed was carefully dipped in to the melt charge and left for around one hour. In this

condition the temperature parameters were carefully monitored with the observations

weather the seed is melting or stable. A small portion of it got melted and at the

thermodynamic equilibrium nucleation started from the seed at this stage the pulling

process started. After the start of nucleation and growth of crystal from the seed, the

temperature of lower RFH zone was gradually reduced to increase the diameter of the

growing crystal. The pulling rate of crystal was precisely maintained at 2 mm/h with

constant rotation of 35 rpm over the complete growth process. After the complete

growth process at ~1528 K, the crystal was taken slightly lifted to break the contact

from melt and was left for 2 hours in the same condition at constant temperature for

post growth annealing of it. From this stage the grown crystal was cooled at very slow

rate of 12 K/h up to 1273 K to avoid the crack formation during the phase transition

(Lee, Kim et al., 1992) at ~1423 K. Later, the cooling rate was increased to 20 K/h till

the temperature reaches 1073 K and then further increased to 40 K/h till the

temperature reaches to ~ 600 K by controlling both RFH and RPGH heating

simultaneously. At this stage the RFH was switched off and cooling was performed

by controlling the temperature using RPGH alone, the temperature was reduced to

room temperature with previous cooling rate of 40 K/h. The grown crystal was

carefully taken out from the furnace, avoiding any mechanical jerks. Such sophisticate

growth conditions facilitated us to obtain the bulk single crystals of ~60 mm long and

~30 mm diameter. The above growth parameters were kept identical for growth of all

pure as well doped crystals. Such growth conditions facilitated us to obtain good

quality optically transparent and crack free bulk single crystals. For the

characterization purposes the crystals were cut into Z-cut wafers, and the specimens

of different dimensions suitable for the different characterization techniques were

prepared.


~ 48 ~

2.1.3 Czochralski growth pure and liquid crystal induced Benzophenone single

crystals

2.1.3.1 Materials properties and processing

Most of the organic materials decompose on or before their melting and hence

they are not suitable to grow into the bulk crystals by melt methods but can be

successfully grown by solution methods. However, Benzophenone with its melting

point ~ 322 K does not decompose above melting and stable up to ~ 417 K, therefore

suitable for the growth of bulk single crystals by melt technique (Bleay et al., 1978;

Scheften-Lauenroth at al., (1984); Katoh et al., (1985); Madhurambal et al., (2007);

Arivanandhan et al., (2004)). The SCE-13 liquid crystal falls in the category of

ferroelectric liquid crystals and above the room temperature it exhibits three phase

transitions: SmC*→SmA* at 61 °C, SmA*→N at 86 °C and N→I at 103 °C (O-Chou

et al., 1997), where Sm, N and I stand respectively for smectic, nematic and isotropic.

The LC molecules in smectic phase show a degree of translational order

(http://plc.cwru.edu/tutorial/enhanced/files/lc/phase/phase.htm). Due to ordering

nature of smectic phase at and below the growth temperature (50 °C), SCE-13 is

expected to induce the molecular ordering for BP molecules during the crystal growth

process.

2.1.3.1 Bulk crystal growth

The BP and SCE-13 liquid crystal were procured from CDH, India and BDH,

UK respectively. The Benzophenone has very low melting point ~322 K, therefore, it

is very essential to use the crystal growth system having a low temperature furnace

with perfect controlling of temperature. To solve this purpose we have employed our

in-house developed Czochralski crystal puller equipped with a resistive furnace, the

photograph of the puller is shown in Fig. 2.7(a). Except the furnace part this puller has

identical features as those of shown in Fig. 2.3. For melting the material a high quality

ceramic crucible, shown in Fig. 2.7(b), was used. The power supplied to this resistive

furnace was precisely controlled using Eurotherm (2404) temperature controller, as

melting point of benzophenone is less (~322 K) which is very near to room

temperature. To make the seed crystal primarily the pure Benzophenone crystal was

grown by making use of the capillary action process. Due to surface tension, the melt


~ 49 ~

Fig. 2.7: (a) The in-house developed Czochralski crystal puller with resistive furnace for growth

of low melting point crystals, (b) a ceramic crucible with remaining charge, and (c) the

photograph of in-situ growing crystal using a glass capillary

liquid rose inside the capillary and get solidified and the crystal outside the capillary

was grown. The photograph taken during the growth of Benzophenone crystal using

capillary is shown in Fig. 2.7(c).

The grown single crystal was cut carefully to get the seed crystals. For the

growth of pure crystal the seed was properly held with a metallic rod this rod was

attached with the seed rotation assembly. The charge was melted in the crucible and

seed was carefully dipped with continuous rotation at the speed of ~35 rpm. When

nucleation started from the seed, pulling of crystal started and throughout the

experiment pulling rate was kept at ~3 mm/h. For the growth of liquid crystal induced

Benzophenone crystal, 0.02 wt% of LC was properly mixed with powder of BP and

thereafter melted. The melted charge was kept at 65 °C for 4 hours for the proper

homogeneous mixing. The temperature of the charge was slowly lowered, and the

crystals were grown at 50 °C, just 1°C above its melting point, 49 °C (Vijayan et al.,

Capillary

Crystal

(a) (b)

(c)


~ 50 ~

2006) which was found to be the optimum for growth. Slowly the temperature of the

furnace decreased and set at the growth temperature of crystal and the crystal was

grown with the same parameters as those for the growth of pure crystal. For different

characterization purposes the grown crystals were cut in the required sizes and

crystallographic directions.

2.2 SOLUTION CRYSTAL GROWTH

2.2.1 Salient features of solution growth

Organic and even the semiorganic crystals also easily decompose and

therefore not suitable for the melt growth method. Most of the organic and

semiorganic NLO materials can be easily grown by slow evaporation solution

technique (SEST). In the solution growth method, the solution, solubility,

supersolubility, metastable zone width (MSZW), induction period and nucleation

kinetics are the important aspects which are described below in brief. The solution is a

homogenous mixture of a solute in a solvent. For the growth of good quality crystals

it is important to choose a suitable solvent. An ideal solvent should have the following

qualities: (i) high solubility of solvent, (ii) good temperature coefficient of solute

solubility, (iii) less viscosity, (iv) less volatility, (v) less vapour pressure, (vi) less

corrosion and non-toxicity, and (vii) low cost, etc. The General solvents are water

(both H2O and D2O), ethanol, methanol, acetone, carbon tetra chloride, hexane,

xylene etc. Almost 90% crystals are grown by taking water as solvent. This is due to

its high solvent action, which is related to its high dielectric constant, stability, low

viscosity, zero toxicity, availability and convenient mean for control of super

saturation exist by change of temperature or removal of solvent.

Solubility of a material in a solvent tells the amount of the material, which is

available for the growth and hence defines the total size limit. For less solubility,

growth rate will be small and hence size of crystal will also small. For high solubility,

it is easy to grow a good size single crystal. However, one should have a thorough

understating about the solubility and metastable zone width whose details are given

below.


~ 51 ~

Conce

ntr

atio

n

Temperature

Metastable

zone

ΔT, undercooling

ΔC, supersaturationStable

Unstable/liable

Solubility/saturation curve

Supersaturation curve

MSZW

Fig. 2.8: The saturation and supersaturation plots showing the metastable zone width

of a material for the growth of bulk single crystals (Kevin W. Smith, 2008)

2.2.2 Meta stable zone width and solubility curve

The curve of concentration vs. temperature is called the solubility curve of the

material. One can make a supersaturated solution at a particular temperature (T) by

adding some more solute as per the required supersaturation to the saturated solution

and increase the temperature slowly so that the added solute is just dissolved. Then

cool down the temperature back to the initial temperature T. Now the solution is said

to be supersaturated. There is a maximum value of solute that can be added to make a

supersaturated solution at a particular temperature T depending upon the solute and

solution. By finding such solute values at different temperatures one can draw a

supersaturated curve [Fig. 2.8]. As seen in Fig. 2.8, the concentration-temperature

field is divided into three regions with a metastable zone in between the saturation and

supersaturation curves. Growth is stable in the metastable zone. If the width of this

zone (MSZW) is higher, crystals grow nicely with big sizes as in the case of organic

crystals by slow cooling or by slow evaporation. In labile (unstable) region, due to

multi-nucleation, one cannot grow good size or good quality crystals, whereas in the

stable region, no nucleation of crystals takes place. By incorporating suitable dopants,

MSZW can be increased. [MSZW is the difference of temperature (∆T) between the

saturation condition and the supersaturation condition where one can observe the first

spec of crystal (fist nucleation) by slowly lowering the temperature from the

saturation point at a desired cooling rate like 4 K/h].


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2.2.3 Induction period and nucleation kinetics

The induction period, , is a measure of the ‘ability’ of the solution to remain

supersaturated. To measure at a selected supersaturation (S = C/C*; where C

* is the

saturated concentration), make the saturated solution at T, now add the solute to the

required supersaturation (S) and heat the solution slowly till the solute is just

dissolved, but completely. The solution is then cooled to the saturation temperature

(T) where the solution becomes supersaturated to the required degree of

supersaturation. Then observe the time at which the first spec of nucleation occurred.

The time of observation of the sparkling particle in the solution from the time at

which the solution reaches the saturation temperature (T) gives the induction period of

nucleation at T (Amedeo Lancia et al., 1999). The interfacial energy of the

interface between the growing crystal and the surrounding mother phase plays an

important role in the nucleation of crystals. The relationship between nucleation rate,

J, (representing the number of nuclei formed per unit time per volume) and induction

period is expressed as;

233

23*

)(ln3

16expexp

1

STkA

kT

GAJ

,

or 233

23

)(ln3

16)ln()ln(

STkA

(1.16)

here, A is a constant, v is molar crystal volume and the function, ln(A) weakly depends

on temperature, hence there is a linear dependence between ln(τ) and 1/(lnS)2, at

constant temperature. The plot of ln(τ) vs. 1/(lnS)2 is a straight line. The intercept of

the straight line on the y-axis gives the value of ln(A). The above equation suggests

that the slope of straight line fit for ln(τ) vs. 1/(lnS)2 is given by;

33

23

3

16

TR

Nm A

,

here V is the specific volume, R is the gas constant and NA is Avogadro’s number.

The interfacial energy σ of solid relative to solution is given by; ANV

mTR2

333

16

3

. The

energy of formation of critical nucleus can be expressed by the relation,

2

*

)(ln S

RTmG . From this relation it is clear that ∆G decreases as S increases from 1.


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Experimentally it was found that with additives like EDTA, the energy of formation

of a critical nucleus increases and hence indicates the growth promoting effects (more

the ∆G, lesser the multi-nucleation).

In the recent years, attention is paid to grow crystals from solution with faster

growth rates by adopting faster cooling rates. Possibility of faster cooling rate

depends upon stability of the solution which in terms depends upon metastable zone

width. Larger the zone width higher is the stability. Solution with narrow zone width

very often ends up with secondary nucleation which effects the growth of main

crystal. There are several reports in the literature on the effect of some specific

impurities on the crystal growth process. While most of the authors report the

secondary nucleation and reduction the growth rate by addition of impurities, few

other notify the enhancement of the growth rate by adding some suitable impurities so

that metastable zone width increases. The addition of some organic additives leads to

increase the metastable zone width (David et al., 1995). These additives make the

complexes with the active metal ion impurities present in the solution and enhance the

metastable zone width, and increase the growth rate leading to good quality single

crystals (Bhagavannarayana et al., 2006).

2.2.4 SEST growth of pure, urea and Cr3+

doped ZTS single crystals

2.2.4.1 Materials synthesis process

The tris(thiourea)zinc sulphate (ZTS) is as semiorganic material, it can be

synthesized in the aqueous medium and the single crystals have been grown by SEST.

The ZTS is soluble in water and was synthesized through the solution route from the

starting analytical grade materials thiourea and zinc sulphate heptahydrate. These

were taken in 1:3 molar ratio according to the following simple molar reaction

(Arunmozhi et al., 2004; Bhagavannarayana et al., 2006). At low temperature (333

K), the solutions of these have been made separately in double distilled water and are

mixed slowly in a separate beaker. The low temperature (~333 K) of the solution was

maintained to avoid decomposition of synthesized ZTS.

ZnSO4.7H2O + 3(CS(NH2)2 → Zn[CS(NH2)2]3SO4


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The continuous stirring of this mixture was performed for homogeneous mixing of

solutions after some time small crystallites started forming in the solution, slowly

water was added to dissolve these crystallites. The mixture of solutions was left for

continuous stirring overnight for ~12 hours. The saturated solution was obtained

through this process which was filtered and slowly cooled to the room temperature

and left for fast evaporation of the solvent (water). After the span of 5 days the

solvent was evaporated and we have obtained the ZTS in the crystalline form. This

synthesized ZTS was further purified by recrystallizing these crystals repeatedly three

times in doubled distilled water.

The recrystallized mass was characterized by powder X-ray diffraction and

Fourier transform infrared spectroscopy to confirm the structural phase and functional

groups of ZTS. After this process we confirmed that ZTS is free from the impurities

and has been used to grow the single crystals in pure form as well as with dopants (i.e.

urea and chromium).

2.2.4.1 Single crystal growth

For the growth of crystals the saturated solution of ZTS has been made in

beakers at ~ 300 K with slight acidic conditions (pH~5.9) (Bhagavannarayana et al.,

2008; Ushasree et al., 1999). For doping purpose the same solution has been divided

in different vessels and the required amount of dopants have been added separately. A

series of urea concentrations (0.1, 1.0, 2.5, 5.0, 7.5 and 12 mol%) for urea doped ZTS

crystals and 1.0 and 2.0 mol% of chromium chloride concentration for chromium

doped ZTS crystals, were used. After adding the required amount of dopants all the

solutions were left for the continuous stirring for ~12 hours, so that dopants could be

mixed homogeneously. To ensure the homogeneity of solution, it is heated to a few

degrees above the saturation temperature. After the proper mixing of dopants these

solutions were filtered using filter papers [Whatman, ashless, Grade 40, 8 μm, 150

mm Ø] in separate beakers. These beakers with solutions were covered from top to

avoid the uncontrolled evaporation of solvent and very fine holes were made in these

covers to provide the controlled evaporation of solvent at the desired rate for all the

beakers. All the beakers were housed in a constant temperature bath (CTB; METREX

– High Precision Water Bath) at 300 K. The temperature of water in CTB was


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precisely controlled by a Eurotherm temperature controller with the precision of

±0.01 K. All the growth parameters were carefully monitored and the experimental

conditions for all the solutions were kept constant. After a span of ~20 days, good

quality single crystals from all beakers have been harvested successfully. The grown

crystals possessed the natural facets with proper physical morphology and with good

visual transparency.

2.3 CONCLUSION

The details about the key parameters which decide the size and quality of the

crystal in CZ process have been discussed extensively. The two zone furnace of low

thermal gradient, consisting of RF and resistive heating sources, was successfully

developed for the growth of crack free LiNbO3 bulk single crystals. The resistive

heater sufficed the slow rate post growth cooling of the entire crystal boule. A precise

temperature controlled resistive furnace was developed for the growth of

Benzophenone single crystals. The ZTS was synthesized and its single crystals were

grown by slow evaporation solution technique.

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Experimentation for the growth of single crystals · 2019-01-01 · (Al 2O 3), GaAs, gadolinium...

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Transcript of Experimentation for the growth of single crystals · 2019-01-01 · (Al 2O 3), GaAs, gadolinium...