Molecular Beam Epitaxy

Dr Samir Tilouche

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Laboratory for Analysis and Architecture of Systems

Micro Nano Bio Technologies Departement

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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.

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• Growth of epitaxial films on a hot substrate from molecular beams under ultra-high vaccum conditions(10⁻⁸–10⁻¹² Torr).

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• For III-V semiconductors (also used for IV, II-Vis, metals, oxides etc.)

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• 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

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• 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.

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• 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)

Epitaxy types

• Homoepitaxy

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The deposition substrate is the same material as we are depositing from the beam (e.g Si on Si)

• Heteroepitaxy

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Substrate and material are of different composition in order to fabricate integrated crystalline layers of different materials (e.g GaAs on Si)

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• Metamorphic epitaxy

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Is the commonly used term to describe the process of growing lattice-mismatched materials on a foreign substrate until they reach a state of complete relaxation.

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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

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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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Benefits and Drawbacks of MBE

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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.

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These techniques are very useful for artificial crystal structures such as “superlattices” and “quantum wells”.

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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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Unlocking the potential of MBE

• I studied the impact of a porous silicon structures as a compliant substrate for the epitaxial deposition of GaAs layer, along with an intercalation of a SiGe buffer layer.
• Various growth approaches were adopted to improve the quality of the GaAs layer: GaAs/SiGe/PSi and GaAs/Si1-xGex/PSi metamorphic heterostructures were grown using ultrahigh-vacuum molecular beam epitaxy (MBE).
• The SiGe and Ge buffer were deposited utilizing the same MBE Riber 32 reactor operating at a base pressure below 2x10-9 Torr.