Semiconductor Optoelectronics

Prof. M. R. Shenoy
Department of Physics
Indian Institute of Technology, Delhi

Lecture - 16
Fabrication of Heterostructure Devices

So this is the last lecture of part one, there we were recalling or reviewing the basic
semiconductor device physics. The objective of this talk is primarily to indicate to you,
what are the various methods which are used in the fabrication of devices. It is
impossible to discuss the details of fabrication in one lecture, because there are several
intricacies. However, it is just to indicate what methods are used and those of you are
interested you can look at the references.

(Refer Slide Time: 01:13)

For beginners the good references will be, one Sze Physics and technology of
semiconductor devices for beginners, and also our reference Jaspreet Singh
Semiconductor of electronics by Jaspreet Singh and you can also see Semiconductor of
electronics by Pallab Bhattacharya. So, these are some of basic references. However,
there are plenty of specialist books and articles which are available on these growth
processes and growth technology.
We would simply introduce you to the various growth processes. So, if I take a typical
semiconductor optoelectronic device, it has a substrate, so, a substrate, a lower contact
electrode which is a metal and a upper contact electrode which is also a metal. What I am
showing is a longitudinal cross section, a cross section like this; longitudinal cross
section of a heterostructure device. So, if you see a typical heterostructure device then
the substrate would occupy most of the device because the thickness here is typically 60
to 100 60 to 100 micron is the substrate.

Over that you have layered structures 1 2 3 there may be many more layers and of
course, on top the contact electrodes one and contact electrode at the bottom. A typical
device, whose I have shown a longitudinal cross section. So, these devices could be for
example in a double heterostructure LED this could be aluminum gallium arsenide for
example, this could be gallium arsenide, this could be again aluminum gallium arsenide
and this is a p plus. So, I start with an n plus substrate. So, n plus substrate on which you
deposit a n aluminum gallium arsenide, a gallium arsenide, p aluminum gallium arsenide,
p plus gallium arsenide and just taking a simplest structure and on top again there is a
metal contact.

So, this is a simple double hetero structure LED (( )) and you know why we need n plus
and metal and n plus and p plus and metal and then we have contact between n and n
plus, we have contact between p and p plus because these are all only contacts. So, this
would be a typical structure. Therefore, what do we have? We have a substrate which is
a bulk crystal and followed by on top there are layers. These layers could be typically
anywhere the thickness could be anywhere from 0.1 to few microns may be 2 3 4
microns typical thickness of these layers.

So, the thickness of these layers is quite small when compared to the thickness of the
substrate. Therefore, the substrate is grown by different techniques compared to the top
layers here. They are grown by different techniques. So, these are grown by bulk crystal
growth techniques, so bulk crystal growth. And the top layers are grown by a epitaxial
techniques, epitaxial techniques, because the thickness of the layers which are grown
here are very small compared to the bulk crystal and these epitaxial techniques are
generally used when the thickness is very small anywhere from if tens of amstrongs to a
few microns, then these techniques to work otherwise, you have to go for bulk crystal
growth techniques. So, I will briefly discuss both bulk crystal growth techniques and
epitaxial growth techniques. Some of them just to give you an idea however as I
indicated that for details you may have to see several text or specific papers depending
on the details that you are looking for.

(Refer Slide Time: 07:21)

If you take bulk crystal growth here that is essentially from our point of view grow the
substrate here, there are two important techniques, which have used. One is called the
furnace method or Bridgman method there are several variations of this method.
Bridgeman technique or method or sometimes also called furnace and other one is very
well known Czochralski.

There are others and variation of these, but these two are basic process is to have bulk
crystal growth. Epitaxial techniques, to have epitaxial layers there are three important
techniques as I have (( )) liquid phase epitaxy, vapour phase epitaxy. So, this is LPE,
vapour phase epitaxy VPE and molecular beam epitaxy or MBE. Each one has various
advantages and features there, Molecular beam epitaxy. We will briefly discuss all these
three techniques.
(Refer Slide Time: 09:47)

Then we start first with Czochralski method here. Czochralski method, in this method
you have a chamber a lead nitrate crucible. There are different structures, different
variations. (( )) there could be because sometimes this process is done at atmospheric
pressure or sometimes could be done it high high pressure therefore we enclosed
volumes, sealed volumes at controlled pressure. What is done is this begins with a seal
crystal, this a molten melt, melt, molten starting solution here for example, if you have
growing a single crystal gallium arsenide. So, let us say this is you want to grow a single
crystal gallium arsenide or silicon or let us say silicon, then you start with the silicon
molten silicon here and then there is a single crystal, this molten silicon is, this melt is
the temperature of this melt is just above the melting temperature, just above the melting
molt.

The melt here is just above the melting point, this is a crucible a lead nitrate usually lead
nitrate crucible PBN and this is heated. What I am showing now from the sides is the
heating coils and there is a crystal is acting, this is a seal crystal as we draw all the parts,
everything in the setup will become clear and the seal crystal is slowly pulled up. This
has a temperature just above the melting point. Therefore, as you slowly pull up the
temperature here is much less than the melting point. Therefore, it crystallizes the melt
crystallizes on the seal and as you pull up the crystal keeps on growing on the seal.
In fact as you pull up depending on the pulling rate, depending on the pulling rate the
size of the crystal could be much larger. For example, let me show you here and you
could have crystal growing big crystals you started with a small seal, a seal crystal here
and this is being pulled up. So, this is the seal and the crystal is getting, that is the melt is
crystallized to form the single crystal. So, this is the single crystal, this was the seal
crystal. So, to begin with the seal is in the melt. The melt has a temperature just above
the melting point and you start the, starts very slowly pulling the seal crystal.

So, the melt which is in the contact to the seal when it comes up, it crystallizes and
because it is in contact with the crystal seal, it crystallizes on the seal with exactly same
lattice structure with the same directions and therefore, you have a single crystal and then
as you, if you control the pulling speed then this diameter can be varied. That is the size
of the single crystal could be varied. It is simple pulling through this, but the non (( ))
control, it is not in open. Sometimes, this chamber may have atmosphere anywhere from
1 to 20 atmosphere 1 to 20 atmosphere because there are several issues involved in terms
of the vapor pressure of the material that you start, the starting material.

So, if you, this is called Czochralski method to grow single crystals from a melt. You can
grow for example, silicon crystals have with diameter as large as 30 centimeters. So, d or
more to start with a small seal and grow and grow a crystal which is as large as 30
centimeters in diameter. So, this is one of the techniques to grow what a, what are called,
if you see those boule or ingots, they are, this is called boule or ingot, like a huge
cylinder and then you cut these into slices to make the vapors out of this. So, you can
grow large boules or ingots from this technique, using this techniques Czochrolski
technique of single crystal growth and this is diced or sliced and normally it is called
dicing to make the substrates, the vapors that you have the silicon vapors that you see
that are obtained by dicing this boules which are formed by the Czochrolski technique.

The advantage of the, let me now discuss the Bridgman technique and then you see the
advantage that using Czochrolski technique you can grow very thick and very large
boules or large single crystals; whereas the Bridgman technique or furnace technique is
used to grow smaller crystals.
(Refer Slide Time: 17:22)

As the name indicates it has a furnace. So, let me now discuss the Bridgman technique
we have to discuss five techniques, so about 5 minutes each, or 5 to 10 minutes
maximum. So, in this technique, this is the formulas usually these are graphite boats,
graphite boats and this is a quartz, quartz cylinder heated, this indicate the heating
elements or heating coil. What is done is at one end of the boat is single crystal is placed,
single crystal and it is filled with the melt. The melt comprises of poly for example, if it
is silicon it is poly crystalline silicon. So, poly crystalline silicon, how do we get poly
crystalline silicon? These the starting points, the starting points, which are like poly
crystalline silicon is obtained by chemical methods, chemical methods of.
(Refer Slide Time: 19:38)

Let me show you one equation here. How to get poly crystalline silicon, so you start with
the chemical reactions in a furnace silicon carbide and S i O 2 are the starting points and
in a furnace at high temperatures you can generate metallurgic grade of silicon and S i O
2 is the gas and C O is the gas which goes out and what you get is metallurgic grade of
silicon. So, silicon carbide plus silica, starting point of silicon is silica and silica is plenty
in sand. So, this is the first step of the crude process to get metallurgic grade of silicon.
Metallurgic grade is approximately 98 percent purity.

The metallurgic grade silicon then is, it interacts with the H C l here and you get
trichlorosilane. S i H C l 3 is trichlorosilane, a compound trichlorocilane and hydrogen
gas. This is reduced further in a hydrogen environment reduction in hydrogen
environment gives you silicon plus 3 H C l, H C l goes as a gas. This is the chemical
reaction of heating and reducing reduction. This is called reduction process. So, what
you get is high purity silicon which is electronic grade silicon. High purity, it is the
impurities are 1 part per billion or less. So, 10 power minus 9 minus 10 the numbers that
refer to the impurity concentration. So, this is electronic grade silicon.

So, you can get poly, poly crystalline silicon by this technique and that is the starting
point here, both here and Czochrolski technique, the melt comprises of chemically
obtained silicon and then you have to grow single crystal. Growing single crystal means
what? Single crystal does not mean 1 cubic lattice 1 cell, single crystal means all over
that structure, all over that material you have a periodic structure of silicon. It is
therefore, you call it as a single crystal. So, single crystal means everywhere there is a
crystalline structure, poly crystalline means you have crystalline structure, but there are
domains.

(Refer Slide Time: 22:32)

So, you may have a poly crystalline means you have a poly crystalline silicon, it means
there could be domains where there is crystal structures. Crystal structures is not
uniformly present everywhere, but in this a particular plane may be in this fashion and in
another it could be like this, in another it could be like this. So, this what I am showing is
crystal planes.

So, this is poly crystalline because crystalline structure is not over the entire silicon here,
over the entire structure. Therefore, it is poly crystalline. Single crystal means over the
entire material or entire structure or the entire piece you have 1 crystal set up, that is the
lattice structure is the same everywhere. So, that is obtained by one that is the
Czochrolski technique which I discussed and the second one is the furnace method. In
the furnace method this boat is pulled and only giving you an idea, the techniques are
quite involved to the boat is pulled through a temperature gradient towards cooling. So,
this end is cooled end and this here, it is hot end.

In fact everywhere this is maintained at a high temperature and then this is cooler
temperature, itself lower temperature cool does not mean it directly does not go to
atmospheric temperature. It is at lower temperature. So, the boat is pulled through this,
what is the idea? Idea is at as this passes to temperature below the melting point, it starts
crystallizing because this end first meets temperature which are lower than melting
points. So, it starts solidification.

So, because there is a single crystal present the solidification takes crystalline form
because there is a seal which is already present and therefore, the entire thing if you pull
this through the entire thing forms a single crystal and this is called the furnace method
or Bridgman method. The idea behind formation of single crystals using bulk single
crystals using the furnace method and the Bridgman, the Bridgman method and the
Czochrolski method. As I said for details please see references and specialist articles
from this. So, from these single crystals we start, we get the substrates. Substrates are
usually diced vapors of anywhere 100 micron thick, depends on the material also, some
materials are thicker, thicker substrates are used. In some materials the substrate is
slightly thinner, typically 60 to 100.

(Refer Slide Time: 25:50)

Once, you have single crystal substrate, so, this is the substrate. So, the two methods that
are described is to obtain the substrate. If you have to have doped substrate in the melt
before the single crystal is formed in the melt you include the dopants, required dopants
are included in the melt, the starting melt, so, that you get for example, I wrote n plus
substrate. How do we get n plus substrate? For example, you add arsenide in silicon melt
then you will have n plus substrate. So, the dopants have to be added in the melt before
the single crystal is formed, if you want a doped substrate. So, we have a substrate. Now,
we want to have these methods, epitaxial methods to grow on top. So, epitaxy apparently
this is a combination of epi plus taxis. This means upon or on top this is apparently
Greek word.

So, on top or upon and taxis it seems ordered arrangement, arrangement, ordered
arrangement. So, epitaxial growth means growth on top ordered arrangement, ordered
growth on top, ordered arrangement of happens to forms layers, epitaxial, epitaxial
techniques. So, I have indicated the three techniques liquid phase epitaxy, vapor phase
epitaxy, this is also called CVD chemical vapor deposition and if the chemicals involved
organo metalic compounds you. So, this is CVD. CVD is also vapor phase epitaxy and
MOCVD in fact most of the gallium arsenide and aluminum gallium arsenide are grown
by MOCVD. MOCVD is metal organic chemical vapor deposition.

Because if the chemicals which are being, if the deposit, if the chemicals involved in the
CVD process, if they have organo metallic compounds, organo metallic, organo metallic
compounds then it is also called as MOCVD metal organic vapor chemical vapor
deposition. Organo metallic, what do I mean by? I will give you an example. For
example, trimethylgallium TMG, trimethylgallium. So, methyl group C H 3 3 gallium.
Trimethylgallium, this is methyl group is a organic, gallium is metal. So, it is an organo
metallic compound trimethylgallium. This is one of the starting point for a gallium
arsenide. Organo metallic, MOCVD and the last one is M B E.
(Refer Slide Time: 29:40)

So, let me in the remaining part of this talk, let me discuss briefly the three techniques
which are widely used or epitaxial layers. LPE this is the first epi, liquid phase epitaxy.
As the name indicates the starting material is a liquid, liquid phase epitaxy. So, this is
done in a chamber you have, let me draw this, you will see nice figures in books and you
will also see in internet there are several articles and which show videos. So, this is I am
drawing it because as you see you will understand what it is. There is slot here for
example, a rectangular slot through graphite, this is graphite.

This is, it is a graphite container through which there is slot, there are also on top there
are like till box 1 2 3 this is, these are boxes, cylindrical boxes drawn, till box which you
can pour some liquid into this. So, these cylinders go down into this and on a slider, there
is a slider here, my belief is that if I do this drawing in front of you then you can exactly
understand what it is rather than directly show you a diagram which is already existing.
So, there is a slider, it is a slider, this is a this is a slider on which you place the substrate
let say gallium arsenide.

I did not describe a how to get gallium arsenide single crystal. I talked about the silicon
because that was the easiest. Gallium arsenide is more complicated, it actually has to be
done in a sealed close close furnace because there, because of the vapor pressure
considerations, but anyhow let me not go into that right now. In this you put the melt. So,
gallium arsenide and aluminum gallium arsenide melt. Now, what is this gallium
arsenide melt? Gallium arsenide melt melt, gallium arsenide melt is super saturated
solution, super saturated solution, solution of gallium arsenide in gallium.

Gallium is the solvent, gallium is the solvent gallium arsenide in gallium dissolving
gallium, it is a super saturated solution of gallium arsenide in gallium. Similarly, here it
is for aluminum gallium arsenide, it is aluminum and gallium here, aluminum and
arsenic or aluminum plus arsenic in gallium. So, gallium is the solvent, molten, this is
melt. Melt means it is a molten solid. In this case metal, aluminum arsenide gallium are
all metal, molten that is why melt. Super saturated and why super saturated? You can
imagine it is a basic crystallographic process you remember even in the schools we have
making sugar crystals from sugar solution. You have super saturated sugar solution and
you cool it and when you cool sugar crystals are formed because it was super saturated at
a slightly elevated temperature, you cool it then it cannot form anymore those sugars and
sugar crystallizers.

It is the same process which is used in this that you have super saturated melt and this is
the gallium arsenide substrate, arsenide and you slide it so that this comes exactly, when
it comes exactly under this till box here, it just sets there. It just holds on to that and then
all of these are at some temperature. Let us say some temperature of I do not know, I am
saying let us say it is at, generally this is around 800 degree centigrade, 800 degree
centigrade, approximately around 800 degree centigrade. The melting point of gallium
arsenide is actually very high. It is I think 1237 1 2 3 7 or 38 degree centigrade is the
melting point of gallium arsenide.

However, when it is in a solution, its, it is in the liquid form even at around 800 degree
centigrade. Because a solution has a lower melting point compared to the pure salt. Now,
this is approximately around 800 when this gallium arsenide is sitting under the till box,
the temperature of this, we see the whole thing is in a oven when the temperature is
reduced, what happens is gallium arsenide crystalline gallium arsenide deposits on this.
It is the same process of crystallization of sugar, but now there is a substrate gallium
arsenide on which there are layers which are deposited, gallium arsenide gets deposited.
The time for which the temperature has been lowered determines the thickness of the
layer that is grown on gallium arsenide under this.
Next, if I want to grow aluminum gallium arsenide, the slider is pushed for the, to the
next slot under this till box there is aluminum gallium arsenide melt and you again lower
the temperature and aluminum gallium arsenide will get deposited on the substrate. So,
you have gallium arsenide on which aluminum gallium arsenide deposited. So, you, if
you need again gallium arsenide to be deposited or vice-versa to make double
heterostructures you can have more chamber seal. So, there are, there can be more
chambers to deposit layers after layers. What is the simplicity that you see here? The
arrangement is very, very simple. Only a simple graphite container with a slider and 2 till
boxes, you put the melt and whole thing is in a furnace.

I will next describe you the other two, you will see the complications in these. This is
simplest and even today they are used, when you need to deposit layers, the thickness in
LPE the thickness of layers, if you need thickness greater than or of the order of 1000
Armstrongs that is 100 nanometer LPE can be used. Why greater than this because the
control on the thickness is not very good because you have to slide it to the next point.
So, the control is not very good, you cannot grow, have control of 10 Armstrong 5
Armstrong which you have in the other two techniques.

If you do not need abrupt junctions with precise control then L P E is the best technique.
It is the simplest, most cost effective and whenever you need to grow thicker layers LPE
is the best and every device does not require such abrupt precise junctions to make
quantum bell structures. You cannot make quantum bell structures with LPE for
example, because you do not have so much of control. But for many applications you do
not need, you need to use thicker layers and LPE is still a commercial technique which
used to grow optoelectronic devices. This is LPE.

So, let me go over to the next vapor phase epitaxy. I keep repeating please refer to
literature for more details. I have given you an idea what the technique is and details you
can always go through other material. Let me describe vapor phase epitaxy, this is an
interesting technique.
(Refer Slide Time: 39:49)

There are some let me give you some equations that describe deposition of gallium
arsenide, aluminum gallium arsenide layers using MOCVD reaction.

(Refer Slide Time: 40:03)

You have a reaction chamber, this is a reaction chamber in which there are substrate
holders. So, what I have shown is substrate holders, these are substrate holders means on
top of these there are substrate city 1 2. So, these are substrates like I showed you in the
LPE gallium arsenide substrate will sit first. So, these are substrate city. There is an
exhaust here, exhaust. Gas enters a mixture of gases, required gases, what are these
gases? I will show you in a minute, enter from, this a reaction chamber.

So, mixture of gas enters from here and reaction takes place on the surface of the
substrate inside the reaction chamber. This is chemical reaction which takes place on the
surface of the substrate and layers get deposited. The remaining gas is exhausted. Now,
there could be, there are different techniques used, there could be lamp heating. So, these
are heating lamps or there could be RF heating, there are different heating, this is lamp
array, it is not coil now, it is lamp array.

Heated lamps or there could be RF heating, heating of this, heating of the substrates.
Now, we have to see what is this mixture? This mixture typically comprises of if I want
to grow I will draw it then I will explain to you the reaction. This is actually hydrogen
gas is used H 2 gas which is bubbling through these and the vapors are carried. I could
have preferably drawn this and shown you directly the setup, but it is okay. So, we have,
so, let say this is, there are, everywhere there are mass flow controllers which are shown
as taps.

So, the taps which I am showing here are basically mass flow controller let us say T 1 T
2 T 3 T 4. So, hydrogen gas from here and arsine from here, arsine is a highly poisonous
gas and if you need to add some dopants, let me erase this dopants. So, this is trimethyl
gal gallium TMG trimethylaluminum TMA, trimethylaluminum and arsine here and
hydrogen is the carrier gas. So, this entire mixture is entering the vapor chamber, the
reaction chamber. So, see the reaction. Now, let us look at the reaction trimethylgallium
plus arsine here gives you gallium arsenide plus hydrogen here is the carrier, the reaction
is taking place inside the chamber; so gallium arsenide plus methane gas.
Trimethylgallium plus trimethylaluminum plus arsine gives you aluminum gallium
arsenide plus methane. This is the reaction, the chemical reaction.

Chemical reaction is taking place inside this chamber, the C V D process is taking place
here. It is a vapor phase epitaxy because the trimethylgallium and trimethylaluminum are
carried by hydrogen gas in the form of vapors, hydrogen is bubbling through this. So,
these are mass flow controllers, the rate at which the vapors have to go is controlled by
the mass flow controllers T 1 T 2 T 3. So, here is arsine coming and any dopant if you
want to add you also add the dopant gas. So, the reaction is taking place on the surface of
the substrate and the gallium arsenide which is formed here is directly deposited on the
surface.

If you do not want in the next stage if you do not want gallium arsenide to be deposited
you simply close this tap and only you are bubbling this one if you want to deposit only
aluminum arsenide, but if you want to deposit only gallium arsenide close this tap T 3.
So, you can controlled by this technique the layers very precisely, the control is very
good because the mass flow controllers can adjust the deposition rate on this is
determined by the inflow of the mixture, the reactant mixture and therefore, the control is
very precise both in MOVCD and the normal VPE. The reaction for CVD is also shown
here. It is called CVD because now there is no metal organic compound. So, silicon
tetrachloride plus hydrogen gas gives you silicon plus HCl, it is called CVD basically
they are all VPE. MOCVD is only when the reactants have metal organic chemical
vapors. Alright, I go to the last technique that is MBE.

(Refer Slide Time: 49:20)

What this MBE? Can I erase this? Anyhow please see some good material to know more
details. I come to the last technique MBE which is widely used to deposit quantum, to
make quantum bell structures, molecular beam epitaxy. The setup is quite involved but
the control is extremely precise, you can deposit mono layers of gallium arsenide or
aluminum gallium arsenide. Molecular beam epitaxy which means there must be atomic
beams which are coming to form molecules directly on top of the substrate. So, the
chamber involves something like this, let me draw it approximately. I purely like
drawing that is I want to draw this. You have effusion chambers, effusion furnaces or
effusion chambers.

Several effusion chambers, through which atomic gas to atomic beams come out. So,
there are shutters for each chamber. So, these are effusion chambers. I think it is single
effusion chamber or furnace, effusion furnace. So, atomic beams come from this, what I
have shown is shutters say S 1 S 2 S 3 S 4 shutters. There is a main shutter and in front
of this here is the substrate holder; there are 5 involved setups with gear wheels and all
those things, I am not showing all those arrangements here, but I have a chamber, there is
a RHEED gun here. The diagram is more or less ready, this is RHEED. Have you heard
of RHEED? Reflection high energy electron diffraction gun, RHEED gun. So, high
energy electrons or electron beam is incident on this here and it is diffracted here there is
a, this is a fluorescent screen, fluorescent fluorescent screen, this is for rotating
mechanism.

It is actually rotating in a different way. So, on the substrate, what I have shown is the
substrate. This is the substrate, this is loading chamber, the double barrier ultra high
vacuum lock. Now, let me explain and then everything will become clear. These are
sources say gallium, aluminum, arsenic and some dopant, if you want to put some
dopant, these are atomic sources. So, atomic sources, it is atom by atom which is comes.
So, atomic beam is incident on the substrate. The substrate is kept heated at a certain
temperature, the atoms which come there on the surface of the substrate.

So, atoms are arriving on the surface and because this is gallium arsenide, the atoms
arrange themselves so that it forms, it fits to the gallium arsenide legs. So, at a time it is
growing one layer by layer that is why you call it to mono layer. You are allowing
gallium, atomic gallium to come from here, aluminum, arsenic, if you do not want
aluminum, you want to grow only gallium or arsenic you put this shutter closed. This is a
main shutter, if we do not want any reaction to take place or you are doing some setting
when you close the main. So, this is the main shutter. Molecular beam epitaxy, please see
this.

This is at ultra high vacuum of 10 power minus 10 to 10 power minus 11 torr. It is ultra
high vacuum chamber in which we have a substrate that is mounted. This is the substrate
holder, you can rotate the substrate keeps on rotating, atomic beams come from here,
gallium, aluminum, arsenic all these. They are actually this atomic, the effusion chamber
has, each one has a crucible boron nitride crucible usually boron nitride crucibles in
which you have aluminum. You have placed pure metallic aluminum and it is heated and
the atomic beam is coming out of these effusion guns or this chambers, effusion furnace.

So, there is independent beams are coming here, the atoms rearranged themselves in the
lattice on the surface of the substrate. So, you can imagine the control that you have. You
are allowing the rate at which the atoms are incident, the rate at which atoms are incident
here is controlled by the temperature. So, in a controlled rate of gallium, aluminum,
arsenic you can change the ratio G a 1 minus x A l x arsenic, if you are going the ternary
compound aluminum gallium arsenide A l x G a 1 minus x arsenic.

You can control this x so that required composition of aluminum gallium arsenide can be
obtained and the deposition is atomic mono layers, mono layer by mono layer which
means the control that you have is 1 atomic layer, 1 atomic mono layer which means the
control is of the order of 3 to 5 Armstrong. The layer thicknesses can be controlled,
correct to 1 mono layer which means you make abrupt junctions. You do not want
gallium or aluminum arsenide anymore you just block aluminum, the next layer will be
gallium arsenide gallium arsenide pure.

So, molecular beam epitaxy as it indicates, as the name indicates there are beams, atomic
beams which drawn. 1 dozen call it as atomic beam epitaxy because it is the layers which
are grown or molecules. So, molecular beam, there are beams of atoms which come here
and the layer is deposited on top. So, this is the third technique, you can read more
details about these. It is a very interesting technique and most of the quantum bell
structure are grown by MBE technique. So, I will stop here. I hope I given you an idea
about the different techniques which are used, plenty of details are required to
understand each one of them further. So, we will stop here and go over to part two.