CRYSTAL
GROWTH
TECHNIQUES
1
Growth of crystal ranges from a small inexpensive technique to a complex
sophisticated expensive process and crystallization time ranges from minutes, hours, days and
to months. Single crystals may be produced by the transport of crystal constituents in the
solid, liquid or vapour phase. On the basis of this, crystal growth may be classified into three
categories as follows,
1. Growth from solution
2. Growth from melt
3. Growth from vapour
There are a number of methods of growing crystals of a particular material. Choice of
method of growth is made according to the physical properties of the material and to the size
of crystal required.
GROWTH FROM SOLUTION
Materials, which have high solubility and have variation in solubility with
temperature in a proper solvent, can be grown easily by solution method. The growth of
crystals from liquid and gaseous solution, pure liquids and pure gases can only occur, if some
degree of supersaturation or supercooling has first been achieved in the system.
The attainment of the supersaturated state is essential for any crystallization operation
and the degree of supersaturation, or deviation from the equilibrium saturated condition is the
prime factor for controlling the deposition process.
In general, growth of crystals can be considered to comprise three basic steps.
1) Achievement of supersaturation or supercooling
2) Formation of crystal nuclei of microscopic size and
3) Successive growth of crystals to yield distinct faces.
There are two methods in solution growth depending on the solvents and the solubility
of the solute. They are
1. High temperature solution growth and
2. Low temperature solution growth
The solvents commonly used include water, organic liquid or molten salts also called
flux.
Growth from aqueous solution:
Crystal Growth from aqueous solution at normal pressure is carried out at
temperatures few degrees above the ambient.
Gel growth:
2
When growth of single crystals posses problem by conventional technique due to
decomposition before melting or non availability of suitable flux, gel growth serves as an
excellent alternative method. This method is extremely simple and in expensive.
A gel is defined as a semi solid having high viscosity. It is a porous material
comprised of a semirigid network.
Hydrothermal growth:
Crystals of materials which don't have sufficient solubility in water or other solvents at
normal temperatures and pressures may be grown under hydrothermal conditions i.e, at high
temperatures and pressures.
Flux growth:
In the absence of a proper solvent for low temperature solution growth, crystal growth
may be carried out using high temperature solvent, usually molten salts also called flux.
MELT GROWTH
Growth from melt is the most widely used method for the preparation of large single
crystals. The materials which melt congruently and show the absence of any phase transition
below melting point can be grown as single crystal by melt method.
This is the case with alkali halides, alkaline earth fluorides, majority of oxides,
elemental semiconductors (Ge, Si) etc.
In this method, the impurities can be added deliberately to disperse the homogeneity
largely in a large percentage of the grown crystals.
Several factors may affect the solidification process. Hence, there identification is
essential for successful crystal growth.
VARIOUS MELT GROWTH TECHNIQUES
a) Normal Freezing:
In this technique the ignot is gradually frozen from one end.
b) Crystal Pulling:
In this technique, crystal grows on a seed withdrawn from the melt
(Czochralski technique)
c) Zone Melting:
In this technique a molten zone is passed through an ignot.
d) Flame Fusion:
In this technique crystal grows below a melt which is fed from above.
Methods of growing from melt is not suitable if at the vapour-liquid transformation
the required polymorphic form is not obtained or if the melting point is so high that
3
contaminant of the melt becomes impossible. Among all the crystal growth techniques
available the melt growth is the only technique which is used for the production of large size
crystals of semiconductor material for device application.
TECHNIQUES FOR MELT GROWTH
Among all the crystal techniques available the melt growth method is the only
technique, which is used for the production of large size single crystals of semiconductor
material for device applications.
Two configurations namely Bridgman and Czochralski technique are commonly used
for the melt growth. The suitability of a particular technique for a given material is decided
by the specific properties of the materials like, contraction or expansion on solidification,
melt viscosity, thermal conductivities pf melt and solid, requirements of size and quality. In
addition, the compositional requirements can also decide the choice of a particular technique.
BRIDGMAN TECHNIQUE (BT)
There are basically two different types of Bridgman technique (i) Horizontal Bridgman
technique and (ii) Vertical Bridgman technique.
Principle:
In both the techniques a boat with the molten charge is moved accross a temperature
gradient so as to allow the molten charge contained in the boat to solidify from an oriented
seed.
Vertical Bridgman Growth Technique - Experimental Setup:
It consist of a vertical cylindrical in which the crystal to be grown is kept as shown in
fig. The container is tappered conivally with a point bottom and is surrounded by two
furnaces as shown in figure 1.35
Fig 1.35
Furnace-1 maintains hot zone and Furnace-2 maintains cold zone. Using a pulley, the
4
container can be moved up and down during crystallization process so as to heat/cool the
melt.
Crytallization:
The material to be crystallized is taken in a boat and is kept inside the container. The
furnace-1 is switched ON and the material is heated to a very high temperature. Now when
the material attains molten state, it is slowly moved across a temperature gradient, i.e., it is
lowered from the hot zone to the cold zone and vice versa. The movement should be in the
range of 1 to 30 mm/hour. At this stage the crystallization begins in the tip and start growing
from the first formed nucleus (seed) by solidification. Similar such process will make the
crystal to grow further to a larger size.
Advantages:
i) Relatively cheaper when compared to other pulling techniques.
ii) Simpler technology
iii) Melt composition can be controlled during the growth
iv) The thermal gradients can be easily minimized with a consequent reduction of the
dislocation and in addition it gives cylindrical crystals with no need of sophisticated
diameter control devices.
Disadvantages:
i) Growth rate is very low.
ii) Since the material is in contact with the walls of the container for long period, it lead
to dislocation of the nucleus.
iii) Sometimes instead of single crystal, polycrystals may grow.
iv) This technique can't be used for materials which decompose before melting.
CZOCHRALSKI METHOD (CRYSTAL PULLING)
Czochralski method or pulling from the melt is the principle method for the
production of bulk single crystals of silicon. A schematic of the technique is shown in figure
1.36
The material to be grown as single crystal (i.e, a polycrystalline silicon) is taken in a
suitable crucible and is heated by means of a radio frequency heater so that clear melt is
obtained with a free liquid surface at the top.
A monocrystalline seed with the required orientation is attached to a pulling rod and is
mounted above the melt surface. This rod can be vertically withdrawn and rotated during
growth. Next, the current fed to the heater coil is carefully adjusted so that a certain portion
of the dipped seed is remelted and a melt meniscus is formed.
The pull rod is then rotated as it is slowly drawn upwards. As the melt freezes on the
5
crystal it does so in the same orientation as the seed and a single crystal is grown. The pulling
rate, the rotation rate and the power to the heater controls the desired shape of the crystal
especially the diameter.
Fig 1.36
This technique is essential for the growth of dislocation free silicon and called
"necking procedure".
As the atmosphere must be controlled during the growth process, the whole assembly
is contained in a vessel filled with gas (inert gas argon for semiconductors, oxygen or air for
oxides).
CRYSTAL GROWTH FROM VAPOUR PHASE
In this technique the material to be grown is supplied in the form of vapour. The
powders of the desired crystalline material are atomized by exposing it to an electric arc or
hot flame. A seed crystal is brought somewhere near the melting point so that the arriving
atoms or molecules will have sufficiently high mobility on the growing surface.
At very low temperature many crystals in a very high state of purity has been grown
from the vapour phase.
Synthetic rubies and sapphires are grown by this method only. The methods of growth of
crystal from vapour phase are broadly classified into two types.They are
(a) Physical Vapour Deposition
(b) Chemical Vapour Deposition
PHYSICAL VAPOUR DEPOSITION
In this method, the solid material is converted into gaseous phase by physical process
of heating. Here, the crystal growth takes place from its own vapour.
The experimental set – up consists of a closed evacuated vessel in which the source
solid is heated until it vapourizes.
6
Fig 1.37
Once the vapour has become supersaturated within the vessel, it will tend to rise and migrate
to the cooler upper portions of the vessel. There it will begin to condense and deposit in a
regular pattern eventually growing into a large crystal.
CHEMICAL VAPOUR DEPOSITION
Chemical Vapour Deposition to form crystalline substance involves the reaction of
vapour phase material followed by decomposition to form solid phase single crystal.
Vapour growth by chemical vapour deposition
Fig 1.38
In this method, both carrier gas and the reactive gas are taken in two cylinders as
shown in figure 1.38.
The carrier gas carries the reactive gas into the inlet port of the reactive chamber. The
reactive chamber is evacuated with the help of vacuum pump. The outlet port of the reactive
7
chamber is used to send out the unwanted gases. The heater is arranged near to inlet port of the
reactive chamber, while coolant is arranged near to the outlet port.
A substrate is placed inside the reactive chamber near to outlet port on which the required
crystal will be grown. The temperature and pressure inside the chamber are regulated strictly as
these parameters determine the deposition rate. Both reactive and carrier gases are allowed into the
reactive chamber and heated to a suitable temperature, so that reaction can take place at high
temperature. During the reaction, the atoms / molecules of the reactive gas will fall on the substrate
and crystal will be grown over the surface of the substrate. By product can be sent out through the
outlet of the reactive chamber.
Advantages:
i) It is used for the crystalline materials having high melting point, less solubility in solvent
and difficult to grow from solutions or melt.
ii) These materials can be growing into single crystal from their vapour phase. iii) This
method provides the crystal of high purity.
THIN FILM DEPOSITON TECHNIQUES
Thin films can be single or multicomponent , alloy/compound or multilayer coatings
on substrates of different shapes and sizes.
Various thin film deposition techniques are,
a) Vacuum evaporation b) Sputtering and c) Chemical deposition.
These methods can be employed for preparation of metal, semiconductor and dielectric films.
Epitaxy:
Epitaxial growth technique involves the growth of single crystal semiconductor layers on a single
crystal semiconductor substrate. When a crystal is grown epitaxially on a substrate of same material it
is called homoepitaxy. The use of epitaxial growth reduces the growth time, wafering cost and
eliminates the wastage caused during the growth, cutting, polishing etc.,
The major advantage of epitaxy is the uniformity in the composition, controlled growth parameter
and better understanding of the growth itself.
Different techniques for epitaxial growth:
The prominent techniques used for the growth of compound semiconductor and other
materials are,
Liquid phase epitaxy (LPE) Liquid Phase Electron Epitaxy (LPEE), vapour Phase Epitaxy
(VPE), Molecular Beam Epitaxy (MBE), Chemical Beam Epitaxy (CBE) etc..,
8