7.1 Epitaxy
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materials may be incorporated layer-wise as quantum wells or differently arranged
heterostructures inside a device’s semiconductor structure, or even be deposited to
form nano-sized islands in a self-organised fashion.
Samples grown by epitaxy are widely used in science and industry for the production of various electronic, optical and optoelectronic devices. Epitaxy has become
indispensable in research activities on nanostructured systems and quantum-physical
devices. Naturally, different methods are applied to serve different purposes, which
are briefly summarised using two common examples. More information is naturally found in the expert literature on these subjects (in the field of semiconductor
technology) and it is the aim of this section to merely provide a basic background.
7.1.1 Molecular Beam Epitaxy
Molecular-beam epitaxy (MBE) provides a powerful tool for structure growth with
enourmous precisions down to the sub-monolayer limit. It serves as the best example
for well-controlled growth in an evacuated environment using a beam of molecules
in which particles impinge on the substrate surface one after the other, with the beam
being in the so-called Knudsen regime. This regime is characterised by the meanfree path of a particle in the beam before a scattering process occurs which must be
larger than the chamber dimensions. Thus, ultra-high vacuum is necessary to provide
predictable growth.
Materials are provided by Knudsen cells that serve as furnaces from which the
molecular beam originates as a consequence of a heating process. While the temperature of the furnace regulates the flux, a shutter in front of the exit aperture allows
to control exposure times. For radial symmetric growth, the substrate holder is rotating, while adequate heating of the substrate provides kinetic energy to the deposited
molecules to form homogeneous films and enable surface reconstructions by means
of diffusion and migration. With the help of reflected high-energy electrons diffraction (RHEED), monolayer growth can be monitored in situ.
MBE is widely known for its high-quality growth, lab-scale production and good
material-interface control. However, at the same time, it is costly, lacks mass production capabilities and is relatively slow with one monolayer-per-second growth rate.
A micrometer-scale structure could take hours to grow. Moreover, precise control
and knowledge of the various growth parameters is needed to produce high-quality
structures deterministically. Ideally, the apparatus is operated in a clean-room environment to facilitate high-quality growth.
7.1.2 Chemical Vapour Deposition
An alternative method for epitaxy is given by chemical vapour deposition (CVD).
The concept of CVD differs drastically from MBE. Its layered growth results from
211
materials may be incorporated layer-wise as quantum wells or differently arranged
heterostructures inside a device’s semiconductor structure, or even be deposited to
form nano-sized islands in a self-organised fashion.
Samples grown by epitaxy are widely used in science and industry for the production of various electronic, optical and optoelectronic devices. Epitaxy has become
indispensable in research activities on nanostructured systems and quantum-physical
devices. Naturally, different methods are applied to serve different purposes, which
are briefly summarised using two common examples. More information is naturally found in the expert literature on these subjects (in the field of semiconductor
technology) and it is the aim of this section to merely provide a basic background.
7.1.1 Molecular Beam Epitaxy
Molecular-beam epitaxy (MBE) provides a powerful tool for structure growth with
enourmous precisions down to the sub-monolayer limit. It serves as the best example
for well-controlled growth in an evacuated environment using a beam of molecules
in which particles impinge on the substrate surface one after the other, with the beam
being in the so-called Knudsen regime. This regime is characterised by the meanfree path of a particle in the beam before a scattering process occurs which must be
larger than the chamber dimensions. Thus, ultra-high vacuum is necessary to provide
predictable growth.
Materials are provided by Knudsen cells that serve as furnaces from which the
molecular beam originates as a consequence of a heating process. While the temperature of the furnace regulates the flux, a shutter in front of the exit aperture allows
to control exposure times. For radial symmetric growth, the substrate holder is rotating, while adequate heating of the substrate provides kinetic energy to the deposited
molecules to form homogeneous films and enable surface reconstructions by means
of diffusion and migration. With the help of reflected high-energy electrons diffraction (RHEED), monolayer growth can be monitored in situ.
MBE is widely known for its high-quality growth, lab-scale production and good
material-interface control. However, at the same time, it is costly, lacks mass production capabilities and is relatively slow with one monolayer-per-second growth rate.
A micrometer-scale structure could take hours to grow. Moreover, precise control
and knowledge of the various growth parameters is needed to produce high-quality
structures deterministically. Ideally, the apparatus is operated in a clean-room environment to facilitate high-quality growth.
7.1.2 Chemical Vapour Deposition
An alternative method for epitaxy is given by chemical vapour deposition (CVD).
The concept of CVD differs drastically from MBE. Its layered growth results from