Chapter 4: Principles of Molecular Beam Epitaxy

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Abstract

Molecular beam epitaxy (MBE) is an elegant material growth technique that is most simply described as a very refined form of vacuum evaporation or physical vapor deposition, with exquisite control over material purity, interface formation, alloy compositions, and doping concentrations. The major feature that enables these qualities is the extremely clean ultrahigh-vacuum (UHV) environment in the material growth system. UHV conditions typically feature very low (<10-14Torr) partial pressures of background impurity gases, leading to extremely low background impurity levels in the resultant materials. In MBE, noninteracting molecular beams of constituent materials are evaporated or sublimed and allowed to chemically interact on a heated substrate. The beams do not interact in the gas phase because they possess mean free paths much longer than the typical 20-30cm source-to-substrate distance due to the UHV conditions. The use of pure solid-source material (e.g., highly purified gallium metal instead of an engineered precursor molecule like trimethylgallium) allows for not only very pure epitaxial growth but also much simplified chemistry when the source material reaches the substrate. MBE is extremely good at producing very sharp interfaces. The effusion cells that produce the molecular beams of source materials are typically paired with a mechanical shutter that can quickly start or stop the beams in much less time than it takes to grow an atomic layer of material. The temperature of the source, which is very reproducible and stable, controls the flux generated from the effusion cells. This makes the generation of specific fluxes of constituent materials simple to control for accurate alloy compositions and doping concentrations. The UHV environment also lends itself to the incorporation of various in situ monitoring techniques that can be used during growth, including reflection high-energy electron diffraction (RHEED), low-energy electron diffraction (LEED), and mass spectrometry. When combined with non-UHV specific techniques like spectral ellipsometry, laser reflectance, pyrometry, and band edge thermometry, MBE becomes a very powerful tool for the study of basic science and the production of very high-quality materials and devices.

Original languageAmerican English
Title of host publicationHandbook of Crystal Growth: Thin Films and Epitaxy: Basic Techniques (Second Edition)
Subtitle of host publicationVolume III, Part A
EditorsT. F. Kuech
PublisherElsevier Inc.
Pages161-192
Number of pages32
Volume3
ISBN (Electronic)9780444633057
ISBN (Print)9780444633040
DOIs
StatePublished - 2015

Bibliographical note

Publisher Copyright:
© 2015, 1993 Elsevier B.V. All rights reserved.

NREL Publication Number

  • NREL/CH-5J00-63345

Keywords

  • A1. Reflection high energy electron diffraction
  • A1. Surface structure
  • A3. Molecular Beam Epitaxy
  • B2. Semiconducting III-V materials

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