352
12 Heterostructures
12.2 Heteroepitaxy
12.2.1 Growth Methods
Since the thickness of layers in the active part of heterostructures has to be controlled to monolayer
precision and the thickness of layers can go down to the single monolayer range, special growth methods
have been developed [1089–1091]. Among these molecular beam epitaxy (MBE [1092]), chemical
vapor deposition (CVD [1093, 1094]) and metalorganic vapor phase epitaxy (MOVPE [1095]) are the
most common for Si, Ge, III–V and II–VI semiconductors. Thin films for many materials can also be
fabricated by atomic layer deposition (ALD [1096, 1097]). In particular oxide semiconductors (and
superconductors) are also fabricated with pulsed laser deposition (PLD [1098]). Liquid phase epitaxy
(LPE [1099]) used to be very important for the fabrication of LEDs but has lost its role largely to
MOVPE.
MBE is performed in an ultrahigh vacuum (UHV) chamber, pumped by getter pumps and cryoshrouds. The source materials are evaporated from effusion cells and directed towards the heated
substrate. If the source materials are supplied as a gas stream, the method is called gas-source MBE
(GSMBE). If metalorganic compounds are used as precursors, the method is denoted as MOMBE.
The atoms impinge on the substrate with thermal energy and are first physisorbed. After diffusion on
the surface they either desorb or they are chemisorbed, i.e., incorporated into the crystal. In order to
obtain high spatial homogeneity of material properties such as composition, thickness and doping, the
substrate is rotated during deposition.
During CVD and MOVPE the heated substrate is in a gaseous environment. The transport gas is
typically H 2 , N 2 or O 2 . Precursor materials are hydrides such as silane, germane, arsine or phosphine
(SiH 4 , GeH 4 , AsH 3 , PH 3 ) and (for MOVPE) metalorganic compounds, such as, e.g., trimethylgallium
(TMG). Due to the toxicity of the hydrides, alternative, i.e., less-toxic and less-volatile compounds
are used, such as TBAs ((CH 3 ) 3 CAsH 2 ). The crystal growth occurs after pyrolysis and catalysis of the
compounds close to or on the substrate surface. All remaining C and H atoms (and whatever else that
is not incorporated into the crystal) leave the reactor and are neutralized and stopped in a scrubber.
In ALD alternating injections of molecular precursors and purge cycles build up the thin film atomic
layer by atomic layer; this method is particularly advantageous for achieving conformal coatings.
In-situ monitoring is of importance to obtain information about the growth process while it is underway. Using the information in a feedback loop it is possible to achieve in-situ control of the process,
e.g., for precise determination of growth rates or layer thickness. Techniques are RHEED (reflection high-energy electron diffraction) [1100] (only for UHV systems) and RAS (reflection anisotropy
spectroscopy) [1101, 1102].
12.2.2 Substrates
Thin-film epitaxy is mostly performed on wafers, i.e., thin circular slices of substrate material. The
most common substrate materials are Si (currently up to 400 mm diameter [1103, 1104]), Ge (also
up to 300 mm [1105]), GaAs (up to 6 inch), InP (up to 4 inch) and sapphire (up to 6 inch). Typical wafer
thickness is 300–500 µm. Also, very thin, flexible Si wafers (8–10 µm) have been developed [1106].
A wafer is cut from a large single cylindrical crystal that is fabricated with suitable growth techniques
such as Czochralski (CZ) growth [1107, 1108] modified by Teal and Little [1109–1111]. In CZ growth
the crystal, starting with a small seed crystal, is pulled out of a melt of previously polycrystalline, pure
or doped material. All dislocations stop in the narrow neck between the seed and the main body of
the cylinder. The diameter of the crystal is controlled by the pulling rate (matching the crystallization
12 Heterostructures
12.2 Heteroepitaxy
12.2.1 Growth Methods
Since the thickness of layers in the active part of heterostructures has to be controlled to monolayer
precision and the thickness of layers can go down to the single monolayer range, special growth methods
have been developed [1089–1091]. Among these molecular beam epitaxy (MBE [1092]), chemical
vapor deposition (CVD [1093, 1094]) and metalorganic vapor phase epitaxy (MOVPE [1095]) are the
most common for Si, Ge, III–V and II–VI semiconductors. Thin films for many materials can also be
fabricated by atomic layer deposition (ALD [1096, 1097]). In particular oxide semiconductors (and
superconductors) are also fabricated with pulsed laser deposition (PLD [1098]). Liquid phase epitaxy
(LPE [1099]) used to be very important for the fabrication of LEDs but has lost its role largely to
MOVPE.
MBE is performed in an ultrahigh vacuum (UHV) chamber, pumped by getter pumps and cryoshrouds. The source materials are evaporated from effusion cells and directed towards the heated
substrate. If the source materials are supplied as a gas stream, the method is called gas-source MBE
(GSMBE). If metalorganic compounds are used as precursors, the method is denoted as MOMBE.
The atoms impinge on the substrate with thermal energy and are first physisorbed. After diffusion on
the surface they either desorb or they are chemisorbed, i.e., incorporated into the crystal. In order to
obtain high spatial homogeneity of material properties such as composition, thickness and doping, the
substrate is rotated during deposition.
During CVD and MOVPE the heated substrate is in a gaseous environment. The transport gas is
typically H 2 , N 2 or O 2 . Precursor materials are hydrides such as silane, germane, arsine or phosphine
(SiH 4 , GeH 4 , AsH 3 , PH 3 ) and (for MOVPE) metalorganic compounds, such as, e.g., trimethylgallium
(TMG). Due to the toxicity of the hydrides, alternative, i.e., less-toxic and less-volatile compounds
are used, such as TBAs ((CH 3 ) 3 CAsH 2 ). The crystal growth occurs after pyrolysis and catalysis of the
compounds close to or on the substrate surface. All remaining C and H atoms (and whatever else that
is not incorporated into the crystal) leave the reactor and are neutralized and stopped in a scrubber.
In ALD alternating injections of molecular precursors and purge cycles build up the thin film atomic
layer by atomic layer; this method is particularly advantageous for achieving conformal coatings.
In-situ monitoring is of importance to obtain information about the growth process while it is underway. Using the information in a feedback loop it is possible to achieve in-situ control of the process,
e.g., for precise determination of growth rates or layer thickness. Techniques are RHEED (reflection high-energy electron diffraction) [1100] (only for UHV systems) and RAS (reflection anisotropy
spectroscopy) [1101, 1102].
12.2.2 Substrates
Thin-film epitaxy is mostly performed on wafers, i.e., thin circular slices of substrate material. The
most common substrate materials are Si (currently up to 400 mm diameter [1103, 1104]), Ge (also
up to 300 mm [1105]), GaAs (up to 6 inch), InP (up to 4 inch) and sapphire (up to 6 inch). Typical wafer
thickness is 300–500 µm. Also, very thin, flexible Si wafers (8–10 µm) have been developed [1106].
A wafer is cut from a large single cylindrical crystal that is fabricated with suitable growth techniques
such as Czochralski (CZ) growth [1107, 1108] modified by Teal and Little [1109–1111]. In CZ growth
the crystal, starting with a small seed crystal, is pulled out of a melt of previously polycrystalline, pure
or doped material. All dislocations stop in the narrow neck between the seed and the main body of
the cylinder. The diameter of the crystal is controlled by the pulling rate (matching the crystallization