The Novice Writer: Crystal Growth Explained!

Crystal Growth

Czochralski Method:

It is the most common method used to grow single crystals.
Crystals are pulled from a crucible containing purest form of molten poly-crystalline silicon.
Once the crystals are pulled out by the puller the silicon wafers can be formed using them.
Such a method of growing crystals is called the Czochralski method of growing crystals.

The puller consists of four subsystems:

Furnace:

It includes the crucible that contains the melt – (molten poly-crystalline silicon).
The crucible should have high melting point, thermal stability, and durability.
The crucible should be made up of a material that is un-reactive with molten silicon.
Crucible should be inexpensive and reusable.
A susceptor like graphite is used to support the quartz crucible containing the molten silicon.
Graphite is the best susceptor due to its high-temperature properties.
The whole assembly of the susceptor and the crucible is resting on the pedestal shaft connected to a motor that provides rotation.
This arrangement, hence allows us to lower, lift, and rotate, the assembly in a controlled manner helping us keep the melt level equidistance from the reference point.

Crystal Pulling Mechanism:

The pull rate and the crystal rotation parameters of the growth process are controlled by the crystal pulling mechanism.
The maximum pull rate Vmax is given by:
Vmax = (Ks/LD)*(dT/dx);
This equation helps us deduce that the pulling rate is inversely proportional to the diameter.

Vmax is maximum pull rate;
Ks is thermal conductivity of the solid;
L is the latent heat;
D is the density of the solid silicon;
T is the temperature;

Ambient Control:

This process should be carried out in an environment surrounded by inter gases or should be performed in the vacuum.
We need to take this precaution because the hot parts of graphite may react with oxygen and erode the graphite completely.
The gases in the air might react with molten silicon which would result in a change in the composition of the silicon crystal.

Control System:

It is used to control parameters such as temperature, crystal diameter, pull rate, and rotation speeds.
The control may be closed or open loop.
This helps us automate the system thus, reducing the human error.

Czochralski Process:

Controlled amounts of impurities are added to the melt contained in the crucible.
Graphite surrounding the crucible is heated via radio frequency induction and the temperature is maintained few degrees above the melting point of silicon.
The dipped seed is removed from the felt in such a way that it remains perpendicular to the ground as it is fastidiously rotated around the axis.
The molten poly-crystalline silicon melts the tip of the seed and as it is withdrawn, refreezing occurs.
No sooner the melt freezes then it forms a single crystal from the seed.
This process is continued until the molten metal is consumed.
The seed withdrawal rate and the seed rotation rate helps us to determine the size of the ingot.
The diameter and thickness of the wafer so produce, varies from 75 mm to 230mm and 0.25mm to 1mm, respectively.

Bridgman Growth of GaAs:

The Bridgman technique is used for producing single crystal compound semiconductors like Gallium Arsenide.
The basic components required by this method are a quartz ampoule or tube or boat, gallium arsenide charge, gallium arsenide seed, multi-zone temperature furnace, boron nitride, and solid arsenide.
Unlike the Czochralski method, this method is processed completely inside the crucible during the crystal growth. This leads to a disadvantage as the crystal is pressurized by the walls of the crucible resulting in the surface deformation of crystal – Line/Surface Defect.
During the transformation from liquid to solid (better known as solidification) the semiconductor material tends to increase in size. Since the material rest on the crucible, its walls introduce stress on the crystal resulting in the crystal defects.
In this method, we use a quartz boat containing gallium arsenide seed and poly-crystalline gallium arsenide is kept inside the quartz ampoule which is evacuated and sealed.
The ampoule is surrounded by the furnace that maintains the temperature within the various temperature zones of the apparatus.
The typical range of temperatures that we choose in this method is from 600 degree Celsius to 1300 degree Celsius.
The crystallographic orientation of the larger single crystal formed is defined by the seed crystal placed on the boat which serves as the initial growth site.
The furnace is slowly moved away from the seed crystal at the sped of 15 to 20 millimeters per hour along the axial direction of the ampoule.
Molten gallium arsenide crystallizes as it enters the low-temperature zone and cools.
The reaction takes places on the surface of the melt and the seed crystal.

Float Zone Growth:

This technique uses poly-silicon rod called feed, single crystalline seed, and RF heater.
A small part of the crystal is kept molten by the RF heater and the position of the heater is not fixed.
The heater is moved such that the molten zone moves along the length of the rod.
This method produces crystal with much higher purity and resistivity.
This possible because the molten crystal zone which later solidifies into a single crystal is not in contact with anything but the ambient gases, thus doesn’t introduce any impurities by dissolving the crucible material as happens in CZ method.
This also avoids contamination of the crystal with oxygen as it happens in CZ method.