Chapter 4: Principles of Molecular Beam Epitaxy

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.