Epitaxial Growth
A common and versatile growth technique that is used extensively in device and integrated
circuit fabrication is epitaxial growth. Epitaxial growth is a process whereby
a thin, single-crystal layer of material is grown on the surface of a single-crystal substrate.
In the epitaxial process, the single-crystal substrate acts as the seed, although
the process takes place far below the melting temperature. When an epitaxial layer is
grown on a substrate of the same material, the process is termed homoepitaxy. Growing
silicon on a silicon substrate is one example of a homoepitaxy process. At present,
a great deal of work is being done with heteroepitaxy. In a heteroepitaxy
process, although the substrate and epitaxial materials are not the same, the two crystal
structures should be very similar if single-crystal growth is to he obtained and if
a large number of defects are to be avoided at the epitaxial-substrate interface.
Growing epitaxial layers of the ternary alloy AlGaAs on a GaAs substrate is one example
of a heteroepitaxy process.
One epitaxial growth technique that has been used extensively is called chemical
vapor-phase deposition (CVD). Silicon epitaxial layers, for example, are grown
on silicon substrates by the controlled deposition of silicon atoms onto the surface
from a chemical vapor containing silicon. In one method, silicon tetrachloride reacts
with hydrogen at the surface of a heated substrate. The silicon atoms are released in
the reaction and can he deposited onto the substrate, while the other chemical reactant,
HCI, is in gaseous form and is swept out of the reactor. A sharp demarcation between
the impurity doping in the substrate and in the epitaxial layer can be achieved
using the CVD process. This technique allows great flexibility in the fabrication of
semiconductor devices.
Liquid-phase epitaxy is another epitaxial growth technique. A compound of the
semiconductor with another element may have a melting temperature lower than that
of the semiconductor itself. The semiconductor substrate is held in the liquid compound
and, since the temperature of the melt is lower than the melting temperature of
the substrate, the substrate does not melt. As the solution is slowly cooled, a single-crystal
semiconductor layer grows on the seed crystal. This technique, which occurs
at a lower temperature than the Czochralski method, is useful in growing group Ill-V
compound semiconductors.
A versatile technique for growing epitaxial layers is the molecular bean1 epitaxy
(MBE) process. A substrate is held in vacuum at a temperature normally in the range
of 400 to 800"C, a relatively low temperature compared with many semiconductor processing
steps. Semiconductor and dopant atoms are then evaporated onto the surface
of the substrate. In this technique, the doping can he precisely controlled resulting
in vety complex doping profiles. Complex ternary compounds, such as AIGaAs,
can be grown on substrates, such as GaAs, where abrupt changes in the crystal composition
are desired. Many layers of various types of epitaxial compositions can be
grown on a substrate in this manner. These structures are extremely beneficial in optical
devices such as laser diodes.
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circuit fabrication is epitaxial growth. Epitaxial growth is a process whereby
a thin, single-crystal layer of material is grown on the surface of a single-crystal substrate.
In the epitaxial process, the single-crystal substrate acts as the seed, although
the process takes place far below the melting temperature. When an epitaxial layer is
grown on a substrate of the same material, the process is termed homoepitaxy. Growing
silicon on a silicon substrate is one example of a homoepitaxy process. At present,
a great deal of work is being done with heteroepitaxy. In a heteroepitaxy
process, although the substrate and epitaxial materials are not the same, the two crystal
structures should be very similar if single-crystal growth is to he obtained and if
a large number of defects are to be avoided at the epitaxial-substrate interface.
Growing epitaxial layers of the ternary alloy AlGaAs on a GaAs substrate is one example
of a heteroepitaxy process.
One epitaxial growth technique that has been used extensively is called chemical
vapor-phase deposition (CVD). Silicon epitaxial layers, for example, are grown
on silicon substrates by the controlled deposition of silicon atoms onto the surface
from a chemical vapor containing silicon. In one method, silicon tetrachloride reacts
with hydrogen at the surface of a heated substrate. The silicon atoms are released in
the reaction and can he deposited onto the substrate, while the other chemical reactant,
HCI, is in gaseous form and is swept out of the reactor. A sharp demarcation between
the impurity doping in the substrate and in the epitaxial layer can be achieved
using the CVD process. This technique allows great flexibility in the fabrication of
semiconductor devices.
Liquid-phase epitaxy is another epitaxial growth technique. A compound of the
semiconductor with another element may have a melting temperature lower than that
of the semiconductor itself. The semiconductor substrate is held in the liquid compound
and, since the temperature of the melt is lower than the melting temperature of
the substrate, the substrate does not melt. As the solution is slowly cooled, a single-crystal
semiconductor layer grows on the seed crystal. This technique, which occurs
at a lower temperature than the Czochralski method, is useful in growing group Ill-V
compound semiconductors.
A versatile technique for growing epitaxial layers is the molecular bean1 epitaxy
(MBE) process. A substrate is held in vacuum at a temperature normally in the range
of 400 to 800"C, a relatively low temperature compared with many semiconductor processing
steps. Semiconductor and dopant atoms are then evaporated onto the surface
of the substrate. In this technique, the doping can he precisely controlled resulting
in vety complex doping profiles. Complex ternary compounds, such as AIGaAs,
can be grown on substrates, such as GaAs, where abrupt changes in the crystal composition
are desired. Many layers of various types of epitaxial compositions can be
grown on a substrate in this manner. These structures are extremely beneficial in optical
devices such as laser diodes.
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