Lesson 13.3Micro & nanoChapter 13 · Micro- and nanofabrication

Molecular beam epitaxy (MBE)

Grows a crystal one atomic layer at a time from beams in ultra-high vacuum

Molecular beam epitaxy (MBE) grows a crystal one layer of atoms at a time. In a vacuum emptier than the space where the International Space Station flies, small ovens fire beams of pure elements at a hot wafer, and the atoms settle into its crystal grid. It builds the ultra-thin layers inside many lasers and high-frequency chips.

Photo: QGravelier, CC BY-SA 4.0, via Wikimedia Commons

MBE growth chamber in cross-section · 6 stepsThe big circle is a cut through the vacuum chamber; the small circle at top right zooms in on the wafer surface. Circled numbers match the steps below.

How it works

Pump down. The wafer enters through a load lock (a small airlock), so the main chamber never opens to air. Pumps and cryopanels (surfaces chilled with liquid nitrogen that trap stray gas) hold an ultra-high vacuum, around 10⁻¹⁰ Torr: roughly a ten-trillionth of normal air pressure.
Heat and clean the wafer. The wafer sits face down on a heated, slowly turning holder. Heating drives off its thin oxide skin and leaves an atomically clean crystal surface.
Heat the cells. Effusion cells (small ovens, also called Knudsen cells) hold ultra-pure elements such as gallium, aluminum and arsenic. Heated precisely, each one gives off a steady stream of atoms.
Open the shutters. In such a vacuum, atoms fly in straight lines without hitting anything, like spray paint in empty space. Each cell has a shutter that opens or closes in a fraction of a second, switching materials on and off.
Build the crystal layer by layer. Atoms landing on the hot wafer move around briefly, then lock into place, copying the crystal grid below; this copying is what epitaxy means. Growth runs at about one atomic layer per second, roughly a micrometer per hour.
Count the layers live. RHEED (reflection high-energy electron diffraction) sends an electron beam grazing across the surface onto a glowing screen. The pattern's brightness pulses once per completed atomic layer, so operators can count layers as they grow.

Key facts

Materials: Compound semiconductors such as gallium arsenide (GaAs), aluminum gallium arsenide and indium phosphide; also silicon-germanium, oxides and new 2D materials
Typical parts: Wafers carrying stacks of crystal layers, some only a few atoms thick, for lasers and transistors
Part size: Wafers typically 50–150 mm across; layers from one atom (about 0.3 nm) to a few micrometers thick
Precision: Thickness controlled to a single atomic layer, with very sharp boundaries between materials
Surface finish: Mirror-smooth, atomically flat
Batch size it suits: Research and high-value production; production machines grow a handful of wafers per run
Tooling and machine cost: No part-specific tooling; a system typically costs from several hundred thousand to a few million dollars
Speed: About one atomic layer per second (roughly 1 µm per hour); a full device stack takes hours

Strengths and limits

Strengths

Atomic-layer control of thickness and composition
Extremely pure material with sharp interfaces
Live monitoring with RHEED
Makes quantum wells, quantum dots and brand-new materials

Limits

Very slow growth
Ultra-high vacuum means long pump-downs, bake-outs and upkeep
Expensive machines and ultra-pure source materials
Lower output than MOCVD (metal-organic chemical vapor deposition), the gas-based method used for volume

Where it's used

PhotonicsLaser diodes, VCSELs (tiny lasers for face recognition and data links), quantum-cascade lasers
Telecom and radioHigh-frequency transistors (HEMTs) for 5G, satellites and radar
OptoelectronicsInfrared detectors, photodiodes
Quantum technologiesQuantum dots, materials for qubits
ResearchNew materials, 2D crystals, oxide layers

How to recognize it

You can't see it. MBE layers are a few atoms to a few micrometers thick, buried inside chips smaller than a grain of rice. Only an electron microscope reveals the stacked rows of atoms.
Where it hides. The lasers in phone face-recognition sensors, fiber-optic transmitters and satellite radio amplifiers rely on epitaxial layer stacks, grown either by MBE or by its gas-based cousin, MOCVD. Nothing on the outside tells you which.
A finished wafer looks like a plain mirror; nothing shows the dozens of layers inside.
In lab photos, look for a shiny steel chamber bristling with ports, ovens pointing up at its center and a glowing RHEED screen.