In this video, we will discuss Crystal Growth.
So, first, I want to introduce you to what are semiconductors, or
what atomic species make semiconductors.
So, this is a periodic table.
And if you look at the right side of the periodic table,
you have this non-metal species.
If we just focus on those,
there are elements that belong to group four,
and they are silicon and germanium.
So here is silicon and here is germanium.
And these both make semiconductors.
And silicon is obviously is the most widely used semiconductor.
And these are called the elemental semiconductor,
because they are made of only one atomic species.
You could have a compound semiconductor.
For example, if you have a gallium and
arsenic, and they form gallium arsenide.
And this is a compound made of group three element and
group five element, we called it three-five compound semiconductor.
Another example of three-five compound semiconductor is indium phosphide.
And gallium arsenide, indium phosphide, and
other related compounds make up a very large portion of
optoelectronic semiconductors that make optoelectronic devices.
You could also have a compound between two element and group six element.
And, for example, zinc selenide, Is an example of two-six compound.
Zinc is here and selenium is here.
Another example will be cadmium.
Cadmium telluride, and
that's another example of a two-six compound semiconductor.
So those are the chemical composition of semiconductors.
Now, how do we make them?
You can grow bulk crystal,
large crystal ingot using several techniques.
The most widely used technique is a technique called Czochralski technique.
Float zone technique is another widely used technique.
So to show how Czochralski growth works,
you first take silica, SiO2,
which is commonly found in sand.
In fact, silicon and oxygen are the two elements that are most commonly found,
most abundant, in earth's crust.
So, you take silica, and then there are some purification process.
There are some purification process that's shown here in this flow chart on the left.
And you make a melt.
So at this stage, you make a melt.
And that goes in here in this schematic shown here in the middle.
And this melt is then put in touch with a very small seed crystal.
And the seed crystal is putting contact with melt and slowly pulled up.
And as this melt cool down, they crystallize.
And they crystallize along according to the orientation of your seed crystal.
And that's how you grow the this big ingot of single crystal in
a semiconductor material.
For compounds semiconductor, you use something similar, but
you have to be more careful to exclude
any atomic species that may sit in from your ambiance.
So the technique is called the encapsulated Czochralski technique.
So here is a figure of the Czochralski Growth Tower.
There is a person here, it's a pretty big tower.
And here on the right is a photograph of the ingot of
piece of single crystal in wafer.
And you would basically chop once you make this ingot, then you would cut these.
Then you would cut these into small wafers, and
those are typically what we purchase from manufacturers.
Now, this is a bulk crystal growth, so you make a piece big piece of crystal.
And another very, very important technology is epitaxy.
Epitaxy means growth of crystalline film on a crystalline substrate.
And the film then conforms to the substrate, so it grows
along the direction of the crystal dictated by your substrate.
There are several different types of epitaxy techniques, molecular beam
epitaxy, metal-organic chemical vapor deposition, liquid phase epitaxy,
and atomic layer epitaxy, or more often it's called the atomic layer deposition.
All of them are currently in use.
Metal-organic chemical vapor depositions are very common, and atomic layer
deposition also are very commonly used in production of these electronic devices.
So here's a schematic of a typical metal organic chemical
vapor deposition system, MOCVD system.
So basically it's composed of the source.
So here, you have precursor materials,
and sent into a reaction chamber here in where you have a substrate,
typically, heated to a high temperature.
So that these precursor materials that are delivered
will come in contact with the substrate surface which is at high temperature.
Then the thermal energy decompose the precursor
materials and leave only the desired element on the surface, and
that results in crystal growth on the substrate.
So here are some photographs of a typical MOCVD system.
You can see that there are many, many wafers all together so
that it can grow thin films in large quantity.
And these are the various gas lines that are needed to control the flow
of precursor materials, and carrier gasses, and other things that are needed.
So here is an example of the film that was grown by MOCVD technique.
This is a gallium nitride and
other related nitride compounds, thin film grown on a substrate.
So you can see very high quality thin films, can be grown by this technique.
Another important technique is atomic layer deposition.
This technique is originally developed as an epitaxy technique.
But nowadays, it's more commonly used as just an atomic layer deposition.
In other words,
the film doesn't really conform to the crystals structure of the substrate.
The atomic layer,
that position is very similar to chemical vapor of that position.
However, it uses a particular chemistry that is self-limiting.
So here in this example,
the schematic on the left shows the growth of zinc sulfide film.
And it uses two different precursor species, zinc chloride and
hydrogen sulfide.
And first, the zinc chloride comes in.
When zinc chloride molecule is absorbed on the surface, it forms only one monolayer.
And the chemistry is such that that any of additional molecules that may come in
are not chemically bound on the surface.
So once they form one atomic layer, then the reaction stops.
And you can then purge any excess molecule that you may have,
and then you send in the next precursor which, in this case, hydrogen sulfide.
And the hydrogen sulfide then react with the zinc chloride that was
formed on the surface, and it leaves zinc and sulfur compound only.
And once again, the chemistry limits itself to one atomic layer, and
if this process continues to form a high-quality zinc sulfide thin film.
The advantage of atomic layer deposition is that
it can grow a precise thickness, a film with a precise thickness.
And also it can deposit film on a very complex geometry.
So, for example, here on the right,
it shows on the electron microgram showing a highly porous nano ports substrate.
And we have done a zinc sulfide growth, and actually titanium
oxide growth, in this case, on this very complex surface.
And you can see that despite this complexity,
the film is formed highly conformal fashion I shown here.
And this is the titania layer deposited by the ALD technique.
So ALD technique is now commonly used in the standard CMOS fabrication.