Growth of ethyl para-amino benzoate single crystal from vertical Bridgman technique

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Growth of ethyl 4-aminobenzoate (EPAB) single crystal

by vertical bridgeman technique

Kritika Sood

Shoolini University of Biotechnology and Management Sciences, Solan (H.P)

Guided by

Dr. N. Vijayan

CSIR – National Physical Laboratory, Dr. KS Krishnan Road, New Delhi 110012

Abstract

Literature review about single crystals and the methods employed for their growth. This field of crystal

growth has a major impact on modern society. Basics of crystal growth and theory of nonlinear optics was

attributed. Construction and working of vertical bridgeman furnace is elaborated. Ethyl 4-aminobenzoate

was used to grow single crystal through this technique. Previous research papers on ethyl 4-aminobenzoate

were also reviewed to optimize the necessary parameters for growing a crystal. All the trials for the

experiment were elucidated and also the temperature profiling was clearly explained through a graph

plotted. To verify the chemical structure, transmittance and lattice parameters of the entitled sample the

characterization techniques like Powder X-Ray diffraction (PXRD), Single crystal UV-Vis NIR and

Fourier Transform Infrared (FTIR) are carried out.

1. Introduction to single crystals

The crystal growth field, branch of material science, physics, chemistry and crystallography has a rich

historical background that goes back at least several millennia. It basically deals with understanding the

underlying mechanisms involved in the crystallization process and the technology to produce single

crystal from some medium in a controlled fashion.

The field of crystal growth encompasses a wide spectrum of scientific disciplines and includes:

(1) Experimental and theoretical studies of crystallization processes.

(2) The growth of crystal under controlled conditions for both scientific purposes and industrial

applications.

(3) Crystal characterization.

It also covers almost all the classes of materials i.e. inorganic and organic compounds, elemental materials

as well as biological macromolecules. Many methods have been developed over the years for producing

large single crystals, whose size ranges from nanometer to micro scale [1]. The processes for the

production of most single crystal are difficult which requires a technical skill in the synthesis of materials,

growth, processing and characterization [2].

This invention relates to a crystal growth method adapted for use in the formation of semiconductor light-

emitting devices and more particularly, to a crystal growth method wherein selective crystal growth of a

nitride semiconductor is carried out [3].

The opposite of a single crystal is an amorphous structure where the atomic position is limited to short

range order only. In between the two extreme exists polycrystalline which is made up of a number of

smaller crystals known as crystallites and polycrystalline phases. Crystals have an atomic ordering that

persists throughout their bulk and without the presence of grain boundaries .The two principal scientific

pillars upon which the field of crystal growth depends are thermodynamics and kinetics. Thermodynamics

properties of a system describe how solid, liquid and gaseous phases behave with respect to state variables

such as pressure, temperature, and composition. Kinetics factors on the other hand, influence our ability

to produce a crystal at a desired growth rate and a degree of perfection and uniformity suited to the

intended application.

It is a challenging attempt to grow a high quality single crystal. Satisfactory size of crystal(from fiber

crystals with diameter of tens of micrometers up to crystalline ingots of blocks with volume up to 1 m

)

and faultlessness (free from precipitates, inclusions, and twins with good uniformity and low concentration

of dislocations) are required for research and practical implementation on microelectronic circuits, electro-

optic switches and modulators, solid-state lasers, light emitting diodes, sensors, and many other devices

[4–8].

Over the past century, a sound theoretical foundation has been built up through the efforts of different

scientists and engineers working in materials related fields such as chemistry, physics and crystallography.

Although remarkable progress has been made, the complex nature of the field and its change emphasis on

newer materials and structures keep providing a constant source of challenges to the understanding of

crystallization process.

2. Crystal growth methods

Crystals can be prepared from various materials like elements, alloys and inorganic, organic and biological

compounds. From solution crystals have often been formed at relatively low temperatures by

crystallization, sometimes in the course of hundreds and thousands of years. Nowadays, crystals are

produced artificially to satisfy the needs of science and technology. It would be better to say that crystal

growth is an art than a science. Many attempts have been made for a long time to produce good crystals

of desired material. Presently, crystal growth specialists have moved from the periphery to the centre of

the materials-based technology.

The components can vary from binary mixtures to multicomponent systems having complex molecular

structures. Number of crystal growth methods available to the crystal grower is quite large. Crystal growth

needs the careful control of a phase change. Thus we may define three main categories of crystal growth

methods.

Growth from solid Processes involving solid-solid phase transitions

Growth from liquid Processes involving liquid-solid phase transitions

Growth from vapour Processes involving vapour-solid phase transitions

2.1. Growth from Solid

The solids are in general polycrystalline materials with very large number of crystallites. They can be

recrystallized by straining the material and subsequently annealing or by sintering. If a polycrystalline rod

or compressed powder of some materials is held at an elevated temperature below its melting point for

many hours some grains grow at the expense of other and it is called sintering or annealing. The

recrystallization is possible only in those materials, which are stable at high temperature where appreciable

diffusion can occur. This method is not suitable for growing large crystals [9].

2.2. Growth from Vapour

In vapour growth the vapour obtained from a solid phase at an appropriate temperature is subjected to

condense at lower temperature by utilizing the concept of chemical vapour transport reaction. Vapour

growth processes may be subdivided into three main types. They are sublimation, vapour transport and

gas phase reaction. In sublimation the solid is passed down a temperature gradient and crystals grow from

the vapour phase at the cold end of the tube. In vapour transport the solid material is passed down the tube

by a carrier gas. In gas phase reaction the crystals grow as a product precipitated from the vapour phase

as the direct result of chemical reaction between vapour species [10,11].

2.3. Growth from liquid

The crystal growth from liquid falls into four categories namely,

(i) Melt growth

(ii) Flux growth

(iii) Hydrothermal growth and

(iv) Low temperature solution growth.

There are a 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 can be grown

with high perfection. The present thesis deals with the growth of crystals by low temperature solution

growth. A brief outline of this important technique of crystal growth is described below.

2.4. Growth from solution

In this method, a saturated solution of a material is dissolved in suitable solvent and from this crystal a

start growing after the solution is supersaturated by lowering the solution temperature. Main advantage of

solution method is that, it is simple and inexpensive. Materials which melt incongruently decompose

before melting or undergo a phase transformation between the melting point and the room temperature,

are grown from solution. Materials which have high solubility and have variation in solubility with

temperature can be grown easily by this method. According to the type of the solvent used solution growth

method is classified into aqueous-solution, molten-salt (flux), and metallic solution and hydrothermal. The

growth of crystal from solution is possible in two different ways:

 Low temperature solution growth.

 High temperature solution growth.

Low temperature solution growth is most commonly used. In it the following methods are used to produce

the required super saturation:

 Slow evaporation solution technique (SEST)

 Slow cooling method

 Temperature gradient method

 Sankaranarayanan- Ramasamy (SR) method

2.4.1. Choice of solvent

In solution growth, it is very important to choose the correct solvent to grow the crystals. A good solvent

should have the following characters.

a) Good solubility for the given solute

b) Good temperature coefficient of solute solubility

c) Non corrosiveness

d) Non toxicity

e) Non volatility

f) Non flammability

g) Less viscosity

h) Maximum stability

i) Small vapour pressure

j) Cost advantage

Almost 90% of the crystals produced from low temperature solutions are grown by using water as a

solvent. Probably no other solvent is as generally useful for growing crystal as is water because of its

higher boiling point than most of the organic solvents commonly used for growth, it provides a reasonably

wide range for the selection of growth temperature. Moreover, it is chemically inert to a variety of glasses,

plastics and metals used in crystal growth equipment [10].

2.5. Gel growth

It is an alternative technique to solution growth with controlled diffusion and the growth process is free

from convection. Gel is a two component system of a semisolid rich in liquid and inert in nature. The

material, which decomposes before melting, can be grown in this medium by counter diffusing two

suitable reactants. Crystals with dimensions of several mm can be grown in a period of 3 to 4 weeks. The

crystals grown by this technique have high degree of perfection and fewer defects since the growth takes

place at room temperature.

2.6. Growth from melt

Melt growth is most popular method of growing large single crystal at high rates. More than half of the

technological crystals are currently obtained from this technique.

All materials can be grown in single crystal form from the melt provided they melt congruently without

decomposition at the melting point and do not undergo any phase transformation between the melting

point and room temperature. Depending on the thermal characteristics, the following techniques are

employed.

1. Bridgman technique

2. Czochralski technique

3. Kyropoulos technique

4. Zone melting technique

5. Verneuil technique

In Bridgman technique the material is melted in a vertical cylindrical container, tapered conically with

a point bottom. The container is lowered slowly from the hot zone of the furnace in to the cold zone. The

rates of movement for such processes range from about 1–50 mm/h. Crystallization begins at the tip and

continues usually by growth from the first formed nucleus. This technique cannot be used for materials,

which decompose before melting. This technique is best suited for materials with low melting point.

In Czochralski method, the material to be grown is melted by induction or resistance heating under a

controlled atmosphere in a suitable non-reacting container. By controlling the furnace temperature, the

material is melted. A seed crystal is lowered to touch the molten charge. When the temperature of the seed

is maintained very low compared to the temperature of the melt, by suitable water cooling arrangement,

the molten charge in contact with the seed will solidify on the seed. Then the seed is pulled with

simultaneous rotation of the seed rod and the crucible in order to grow perfect single crystals.

Liquid Encapsulated Czochralski abbreviated as LEC technique makes it possible to grow single crystals

of materials, which consists of components that produce high vapour pressure at the melting point. This

refined method of Czochralski technique is widely adopted to grow III-V compound semiconductors.

In Kyropoulos technique, the crystal is grown in a larger diameter. As in the Czochralski method, here

also the seed is brought into contact with the melt and is not raised much during the growth, i.e. part of

the seed is allowed to melt and a short narrow neck is grown. After this, the vertical motion of the seed is

stopped and growth proceeds by decreasing the power into the melt. The major use of this method is

growth of alkali halides to make optical components.

In the zone melting technique, the feed material is taken in the form of sintered rod and the seed is

attached to one end. A small molten zone is maintained by surface tension between the seed and the feed.

The zone is slowly moved towards the feed. Single crystal is obtained over the seed. This method is applied

to materials having large surface tension. The main reasons for the impact of zone refining process to

modern electronic industry are the simplicity of the process, the capability to produce a variety of organic

and inorganic materials of extreme high purity, and to produce dislocation free crystal with a low defect

density.

In the case of vertical normal freezing, the solid-melt interface is moved upwards from the cold bottom to

the hot top so as to get better quality crystals. The method is more applicable in growing single crystals

of materials with volatile constituents like GaAs.

In the Verneuil technique, a fine dry powder of size 1–20 microns of the material to be grown is shaken

through the wire mesh and allowed to fall through the oxy-hydrogen flame. The powder melts and a film

of liquid is formed on the top of the seed crystal. This freezes progressively as the seed crystal is slowly

lowered. The art of the method is to balance the rate of charge feed and the rate of lowering of the seed to

maintain a constant growth rate and diameter. By this method ruby crystals are grown up to 90 mm in

diameter for use in jeweled bearings and lasers. This technique is widely used for the growth of synthetic

gems and variety of high melting oxides.

3. Applications of single crystal

3.1. Semiconductor industry

In the fabrication of semiconductors silicon single crystal is used. On the quantum scale that

microprocessors operate on, the presence of grain boundaries have an impact on the field effect transistors

because it alters the electrical properties. Therefore microprocessor fabricators companies are investing in

producing the single crystals of silicon.

3.2. Optics

Monocrystals of sapphire and other materials are used for laser and nonlinear optics.

Monocrystals of fluorite are used in objective lenses of apochromatic refracting telescopes.

3.3.Materials engineering

Used in the production of high strength materials with low thermal creep such as turbine blades [12], in

the absence of grain boundaries there is decrease in the yield strength and decrease the amount of creep.

This decrease in the amount of creep is of great importance as there is high temperature.

 
3.4. Electrical conductors 
The  conductivity of  commercial conductors  is  often  expressed  relative  to  the  International Annealed 
Copper Standard, according to which the purest copper wire available in 1914 measured around 100%. 
The purest modern copper wire is a better conductor, measuring over 103% on this scale. The gains are 
from two sources. First, modern copper is more pure. However, this avenue for improvement seems at an 
end.  Making  the  copper  purer  still  makes  no  significant  improvement.  Second,  annealing  and  other 
processes have been improved. Annealing reduces the dislocations and other crystal defects which are 
sources of resistance. But the resulting wires are still polycrystalline. The grain boundaries and remaining 
crystal defects are responsible for some residual resistance. This can be quantified and better understood 
by examining single crystals. 
As anticipated, single-crystal copper did prove to have better conductivity than polycrystalline copper 
[13]. 
The single-crystal copper not only became a better conductor than high purity polycrystalline silver, but 
with  prescribed  heat  and  pressure  treatment  could  surpass  even  single-crystal  silver.  And  although 
impurities  are  usually  bad  for  conductivity,  a  silver  single-crystal  with  a  small  amount  of  copper 
substitutions was a better conductor than them all. 
As  of  2009,  no  single-crystal  copper  is  manufactured  on  a  large  scale  industrially,  but  methods  of 
producing very large individual crystal sizes for copper conductors are exploited for high performance 
electrical applications. These can be considered meta-single crystals with only a few crystals per meter of 
length. 
 
 
3.5. In research 
Single crystals are essential in research especially condensed-matter physics, materials science, surface 
science  etc.  The  detailed  study  of  the  crystal  structure  of  a  material  by  techniques  such  as  Bragg’s 
diffraction and helium atom scattering is much easier with monocrystals. Only in single crystals it is 
possible to study directional dependence of various properties. Furthermore, techniques such as scanning 
tunneling microscopy are only possible on surface of single crystals. 
 
 
                

4. Vertical bridgeman technique (VBT)

4.1. Working

In VBT the material that is to be grown is generally taken in a crucible container which has a pointed

bottom. The container is lowered gradually (1–5 mm/hr) between two different zones, hot and cold which

are formed in a cylindrical furnace. Crystallization begins at the tip and continues to grow from the formed

nucleus. Only those materials which do not decompose or undergo solid state phase transformation prior

melting can be used to grow single crystal using VBT.

4.2. Procedure

 Should know the melting point.

 Profile the furnace at different input voltages such that the dwell temperature is slightly higher

than the melting point.

 Fill the material in ampoule and vacuum seal it.

 Attach it with nano stepper drive and position the ampoule at 0 cm level.