356
12 Heterostructures
Fig. 12.6 Schematic of the
three different fundamental
epitaxial growth modes
epitaxial layers is step-flow growth where adatoms are incorporated mainly at step edges. A more
detailed discusion can be found in [1134]. Further details on crystal growth can be found in [1135].
The nucleation and the initial film growth are important and determine the film quality. Several
techniques have been developed to overcome common problems. A typical strategy is the growth of a
low-temperature nucleation layer.
12.2.4 Heterosubstrates
If homosubstrates are not available or very expensive, semiconductors are grown on dissimilar substrates, e.g., (Al,Ga)N and ZnO on sapphire (Al 2 O 3 ) or SiC.
3 In Fig. 12.7 the interface region of hexagonal AlN on 6H-SiC (cmp. Fig. 3.31b) is shown. The change of crystallographic phase is obvious; the
perfect atomic arrangement justifies the term ‘epitaxy’, literally meaning ‘order on top’.
In many cases, the integration of III–V- or II–VI-based semiconductors for optoelectronic applications on silicon for electronic devices is desirable, such as GaAs/Si, InP/Si, GaN/Si or the growth on
economic substrates is attractive such as GaN/Al 2 O 3 and ZnO/Al 2 O 3 . For all such combinations the
epitaxial relationship, i.e., the alignment of the crystallographic directions of both materials, which
can have different space groups, has to be considered. Some examples for epitaxial relationships are
given in Table 12.1. The epitaxial relation is determined by the energetically favorite formation of the
interface and the early stages of growth.
In Fig. 12.8, X-ray diffraction data are shown from a ZnO layer on c-oriented sapphire. The hexagonal ZnO lattice is rotated by 30
◦ with respect to the trigonal sapphire lattice. In the case of growth
of ZnO on Si(111) an amorphous SiO 2 layer can form at the interface such that the crystallographic
information of the substrate is lost. The ZnO grains exhibit random in-plane orientation (Fig. 12.9).
Fig. 12.7 HRTEM cross
section of the interface
region of MOVPE grown
hexagonal AlN on 6H-SiC
substrate. Adapted
from [1137]
2H-AlN
6H-SiC
1nm
3 For ZnO, homosubstrates have recently been produced with 3-inch diameter [1136].
12 Heterostructures
Fig. 12.6 Schematic of the
three different fundamental
epitaxial growth modes
epitaxial layers is step-flow growth where adatoms are incorporated mainly at step edges. A more
detailed discusion can be found in [1134]. Further details on crystal growth can be found in [1135].
The nucleation and the initial film growth are important and determine the film quality. Several
techniques have been developed to overcome common problems. A typical strategy is the growth of a
low-temperature nucleation layer.
12.2.4 Heterosubstrates
If homosubstrates are not available or very expensive, semiconductors are grown on dissimilar substrates, e.g., (Al,Ga)N and ZnO on sapphire (Al 2 O 3 ) or SiC.
3 In Fig. 12.7 the interface region of hexagonal AlN on 6H-SiC (cmp. Fig. 3.31b) is shown. The change of crystallographic phase is obvious; the
perfect atomic arrangement justifies the term ‘epitaxy’, literally meaning ‘order on top’.
In many cases, the integration of III–V- or II–VI-based semiconductors for optoelectronic applications on silicon for electronic devices is desirable, such as GaAs/Si, InP/Si, GaN/Si or the growth on
economic substrates is attractive such as GaN/Al 2 O 3 and ZnO/Al 2 O 3 . For all such combinations the
epitaxial relationship, i.e., the alignment of the crystallographic directions of both materials, which
can have different space groups, has to be considered. Some examples for epitaxial relationships are
given in Table 12.1. The epitaxial relation is determined by the energetically favorite formation of the
interface and the early stages of growth.
In Fig. 12.8, X-ray diffraction data are shown from a ZnO layer on c-oriented sapphire. The hexagonal ZnO lattice is rotated by 30
◦ with respect to the trigonal sapphire lattice. In the case of growth
of ZnO on Si(111) an amorphous SiO 2 layer can form at the interface such that the crystallographic
information of the substrate is lost. The ZnO grains exhibit random in-plane orientation (Fig. 12.9).
Fig. 12.7 HRTEM cross
section of the interface
region of MOVPE grown
hexagonal AlN on 6H-SiC
substrate. Adapted
from [1137]
2H-AlN
6H-SiC
1nm
3 For ZnO, homosubstrates have recently been produced with 3-inch diameter [1136].