Molecular beam epitaxy (MBE) is a method for growing thin films one layer at a time under ultra-high vacuum conditions. It involves heating solid sources of material in effusion cells to create molecular beams that are deposited on a heated substrate. The absence of carrier gases and ultra-high vacuum environment result in films of the highest purity. MBE is widely used to manufacture semiconductor devices and is considered a fundamental tool for nanotechnology development due to its precise control over layer thickness down to a single atomic layer.

1. Molecular Beam Epitaxy
Abu Syed Md. Jannatul Islam
Lecturer, Dept. of EEE, KUET, BD
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Department of Electrical and Electronic Engineering
Khulna University of Engineering & Technology
Khulna-9203

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Molecular Beam Epitaxy (MBE)
 Molecular-beam epitaxy (MBE) process was developed in the late
1960s at Bell Telephone Laboratories by J. R. Arthur and Alfred Y. Cho.
 Growth of epitaxial films on a hot substrate from molecular beams
under ultra-high vaccum conditions(10−8–10−12 Torr).
 For III-V semiconductors (also used for IV, II-Vis, metals, oxides etc.)
 The absence of carrier gases, as well as the ultra-high vacuum
environment, result in the highest achievable purity of the grown
films.

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Molecular Beam Epitaxy (MBE)
 MBE is widely used in the manufacture of semiconductor devices,
including transistors, and it is considered one of the fundamental
tools for the development of nanotechnologies
 It is also used for the deposition of some types of organic
semiconductors. In this case, molecules, rather than atoms, are
evaporated and deposited onto the wafer.
 MBE systems can also be modified accordingly to the needs. Oxygen
sources, for examples, can be incorporated for depositing oxide
materials for advanced electronic, magnetic and optical applications,
as well as for fundamental research.

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Molecular Beam Epitaxy (MBE)

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Molecular Beam Epitaxy (MBE)

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Molecular Beam Epitaxy (MBE)
Here in MBE reactants are introduced by molecular beams.
Create beams by heating source of material to melting point in an
effusion (or Knudsen) cell.
Both solid and gas source can be used.
Pyro-lytic boron nitride (PBN) is chosen for crucibles which is
chemically stable up to 1400’C).
Molybdenum and Tantalum are widely used for shutters.
Ultrapure materials are used as source.
The solid source (sublimation) provides an angular distribution of
atoms or molecules in a beam.
The gaseous elements can crack/condense on the wafer where they
may react with each other
Several sources (several beams of different materials) aimed at
substrate

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Molecular Beam Epitaxy (MBE)
UHV gives source molecules a large mean free path, forming a
straight beam.
Beam impinges on a heated substrate (600’C).
The beams can be shuttered in a fraction of second. A computer
controls shutters in front of each furnace, allowing precise control of
the thickness of each layer, down to a single layer of atoms. Intricate
structures of layers of different materials may be fabricated this way.
Such control has allowed the development of structures where the
electrons can be confined in space, giving quantum wells or even
quantum dots.
Incident molecules diffuse around the surface to the proper crystal
sites and form crystalline layers.

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Molecular Beam Epitaxy (MBE)
 Atoms on a clean surface are free to move until finding correct
position in the crystal lattice to bond
Characterization tools allow growth to be monitored in-situ.
During operation, Reflection High-Energy Electron Diffraction
(RHEED) is often used for monitoring the growth of the crystal layers.
 Mass spectrometer for monitoring the residual gases and checking
source beams for leaking.
 A cryogenic screening around the substrate as a pump for residual
gases.
Such layers are now a critical part of many modern semiconductor
devices, including semiconductor lasers and light-emitting diodes.
The term "beam" means that evaporated atoms do not interact with
each other or vacuum-chamber gases until they reach the wafer, due
to the long mean free paths of the atoms.

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MBE Abilities

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Effusion Sources

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Effusion Cell Construction

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RHEED
 Reflection High Energy Electron
Diffraction) for monitoring the
growth of the layer
 Probe only few monolayers.
 Information about the state of the
layers(2D, 3D etc.)
 Information about the crystallinity.
 Measures the lattice parameter
 Growth rate can be obtained from
RHEED oscillation
A typical MBE system*
Molecular Beam Epitaxy

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In-situ Characterization

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MBE Applications

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Novel structures as quantum devices
Silicon/Insulator/Metal Sandwiches
Superlattices
Microelectronic Devices
TEM image of MBE Growth of Ultra-Thin InGaAs/AlAsSb Quantum Wells
MBE Applications

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MBE in Industry

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Advantages Disadvantages
 Clean surfaces, free of an oxide layer  Expensive (106 $ per MBE chamber)
 In-situ deposition of metal seeds,
semiconductor materials, and dopants
 ATG instability
 Low growth rate (1μm/h)  Very complicated system
 Precisely controllable thermal
evaporation
 Epitaxial growth under ultra-high
vacuum conditions
 Seperate evaporation of each
component
 Substrate temperature is not high
 Ultrasharp profiles
Benefits and Drawbacks of MBE

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MBE
Mainly useful for research lab experiments.
Not efficient for mass production!
MOCVD
Useful for lab experiments & For mass production!
MANY MILLIONS OF
$$$$ FOR BOTH!!!!!
MBE vs MOCVD

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Both of these techniques allow crystals to be deposited on a
substrate one monolayer at a time with great precision.
Both techniques can produce highly epitaxial films with
excellent abruptness, allowing thin layers to be formed.
These techniques are very useful for artificial crystal structures
such as “superlattices” and “quantum wells”.
MBE vs MOCVD

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MOCVD
Gases are let into the reactor at high pressure ~ 1 torr
MOCVD has a higher growth rate and less downtime.
It also has no issues regarding phosphor deposition.
MBE
Always done under UHV conditions, with
pressures below 10-8 torr
The UHV of MBE allows for better in situ diagnostic techniques
to be employed.
Substrate temperatures are lower in MBE.
MBE is relatively safer
MBE vs MOCVD

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 MBE: reactions occur only at the substrate
 MOCVD: parasitic reactions can occur before the reactant
species reach the substrate.
 MBE growth, unlike MOCVD growth, is not
thermodynamically favorable and is governed by Kinetics
MBE vs MOCVD

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 LPE involves the precipitation of a crystalline film from a
supersaturated melt on to a substrate.
 The temperature is increased until a phase transition occurs and
then reduced for precipitation.
 By controlling cooling rates the kinetics of layer growth can be
controlled.
 Once can have either continuous reduction with the substrate
(equilibrium cooling) or separate reduction in increments followed by
contact with the substrate (step cooling).
 It is a low cost method yielding films of controlled composition,
thickness and lower dislocation densities.
 Disadvantages are rough surfaces and poor thickness uniformity.
Liquid Phase Epitaxy (LPE)

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Liquid Phase Epitaxy (LPE)