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The Molecular-Beam Epitaxy (MBE) Process

Key Takeaways

  • Molecular-beam epitaxy (MBE) is a highly specialized technique used for creating thin films of single-crystal materials in an ultra-high vacuum.

  • MBE involves directing atomic or molecular beams onto a heated substrate to form epitaxial layers; crucial in semiconductor growth, quantum-well lasers, and nanostructure formation.

  • MBE is distinct from Chemical Beam Epitaxy (CBE) and Metal-Organic Vapor-Phase Epitaxy (MOVPE) in its approach to layer deposition and growth rate.

Molecular-Beam Epitaxy in an Ultra-High Vacuum Chamber

Molecular-Beam Epitaxy in an Ultra-High Vacuum Chamber

Molecular-beam epitaxy (MBE) represents a specialized technique for depositing thin films of single-crystal materials. This method is useful in semiconductor device production and is recognized as a key instrument for advancing nanotechnology developments.

MBE is one of the most complex and demanding methods because the growth process occurs within an ultra-high vacuum (UHV) setting. In the MBE technique, epitaxial layers are created by directing atomic or molecular beams onto a heated substrate within an ultra-high vacuum. The beams’ constituents adhere to the substrate, forming a lattice-matched layer. By separately regulating the beam intensities, adjustments can be made for the differing sticking coefficients of the various constituents within the epitaxial layers.

The Molecular-Beam Epitaxy (MBE) Process 

Substrate Selection

Start with selecting a base material, known as the substrate. This could be a familiar semiconductor material such as silicon, germanium, or gallium arsenide.

Heating the Substrate

Heat the substrate to a high temperature, typically to several hundred degrees, in preparation for the deposition process.

Effusion Cells Preparation

Set up effusion cells – essentially "guns" for projecting beams of atoms or molecules in gas form towards the substrate. Each cell has an open end and contains a tapering crucible (a heat-resistant cup) filled with the material it will project.

Cell Components and Heating

Equip each effusion cell with an electrically controlled heater and a thermocouple for measuring and regulating temperature. The cells, like the substrate, are heated to high temperatures, with a maximum of about 1500–1600°C (2700–2900°F).

Number of Effusion Cells

The number of effusion cells depends on the complexity of the crystal being produced, with a range typically between 8 to 14 cells.

Atom/Molecule Projection

Use each effusion cell to shoot a different molecule at the substrate. This depends on the nature of the crystal you're trying to create. The molecules in gas form are fired from the cells towards the substrate.

Crystal Growth Process

As the molecules land on the substrate's surface, they condense and build up very slowly and systematically in ultra-thin layers. This process allows the formation of the complex, single crystal you're after, growing one atomic layer at a time.

MBE in Detail

Molecular-beam epitaxy (MBE) operates in either high vacuum or ultra-high vacuum environments, with pressure ranges from 10-8 to 10-12 Torr. The key feature of MBE is its slow deposition rate, typically under 3,000 nm per hour, which is crucial for epitaxial growth of films. The lack of carrier gases and the ultra-high vacuum environment contribute to the exceptionally high purity of the films produced by MBE.

Reflection high-energy electron diffraction (RHEED) is often employed during MBE operations to monitor crystal layer growth. Computer-controlled shutters in front of each furnace allow precise layer thickness control down to a single atom layer. This precision enables the fabrication of complex structures with layers of different materials, crucial for modern semiconductor devices like lasers and LEDs. These structures can confine electrons in space, forming quantum wells or dots.

In some MBE systems where substrate cooling is necessary, an ultra-high vacuum environment is maintained by cryopumps and cryopanels, cooled to around 77 kelvins (−196°C) using liquid nitrogen or cold nitrogen gas. The cold surfaces act as impurity traps, requiring even higher vacuum levels for film deposition under these conditions. In other systems, wafers may be mounted on a heated rotating platter during operation.

The final composition and stoichiometry of the film are influenced by several factors, including the temperature and atomic structure of the substrate's surface, along with the flux ratios of the individual components reaching the substrate. To achieve more uniform growth, the substrate may be slowly rotated about 1-2 rotations per minute using a stepper motor linked to a magnetic manipulator.

Applications of MBE

Application Category

Specific Material/System

Application Description

Semiconductor Growth

Si, Ge, III–V, and II–VI semiconductors

MBE is utilized for precise layer control in specialized devices and research applications where abrupt junctions and thin layers are required. This is particularly useful where high wafer volume isn't a primary concern.

Gallium Arsenide (GaAs) Growth

GaAs MBE System

Used in electronic devices for creating high-mobility 2DEGs at the junction of materials with different band gaps (e.g., GaAs/AlxGa1-xAs). This system operates at temperatures between 600-700°C with a growth rate of ~1 μm/h. Higher growth temperatures reduce oxygen incorporation, improving optical device efficiency.

Field-Effect Transistors

GaAs/AlxGa1-xAs

MBE-grown transistors based on 2DEGs are applied in communication and microwave technologies.

Heterostructure and Quantum-Well Lasers

AlGaAs Materials

Extensive work has been reported in developing lasers, microwave devices, and superlattice structures using AlGaAs materials prepared by MBE.

Nanostructure Formation

Silver on Palladium Surface

Creation of one-atom-thick silver islands on a palladium surface, polished and vacuum annealed, for advanced nanostructural studies. The coverage is calibrated using scanning tunneling microscopy (STM) and photoemission spectroscopy (ARPES). Images are typically 250 nm by 250 nm.

Oxide Material Deposition

Oxide Materials

Incorporating oxygen sources in MBE systems allows for the deposition of oxide materials for advanced electronic, magnetic, and optical applications, as well as fundamental research. A molecular beam of an oxidant is used to achieve the desired oxidation state in multicomponent oxides.

MBE Advantages

Advantages of using Molecular Beam Epitaxy (MBE) include its capability to produce high-quality semiconductor crystals with low defects and high uniformity. This is particularly effective for crystals made from compound elements found in groups III(a)-V(a) of the periodic table or from various elements rather than just a single element. Additionally, MBE allows for the fabrication of extremely thin films with a high degree of precision and control.

Drawbacks

MBE does have some limitations. It is a relatively slow and meticulous process, with crystal growth rates typically only a few microns per hour. This slow pace makes MBE more suitable for scientific research settings rather than high-volume production environments. Moreover, the equipment required for MBE is complex and costly, primarily due to the challenges in achieving and maintaining clean, high-vacuum conditions.

Comparing MBE With CBE and MOVPE

In contrast to Molecular-Beam Epitaxy (MBE), where the epitaxial layer is formed by directing atomic or molecular beams in an ultra-high vacuum environment, Chemical Beam Epitaxy (CBE) and Metal-Organic Vapor-Phase Epitaxy (MOVPE) employ different mechanisms. 

  • CBE involves the direct impingement of metal alkyls at high substrate temperatures, which differs from the ultra-high vacuum and slower deposition rates characteristic of MBE. 
  •  MOVPE uses metal alkyls to form a stagnant boundary layer over the heated substrate, leading to a diffusion-limited growth rate. This is unlike MBE's process, which is not boundary layer-dependent and allows for more precise control over layer formation. 

Ready to elevate your semiconductor design process? Discover how Allegro X Advanced Package Designer can help in designing packages for your integrated circuits that use Molecular-Beam Epitaxy techniques in their manufacturing. 

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