Summer Research Fellowship Programme of India's Science Academies 2017
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
3
)
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
[48].
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 150 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 120 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 (15 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.