6.3 Band Structures of Selected Semiconductors
147
X
K
L
12
10
8
6
4
2
0
-2
-4
-6
CdO
Fig. 6.13 Calculated indirect band structure of CdO. The top of the valence band is at E = 0. Adapted from [470]
6.3.7 MgO, ZnO, CdO
Cadmium oxide is a cubic semiconductor in the rocksalt structure. Due to symmetry considerations,
coupling (repulsion) of oxygen 2p- and cadmium 3d-orbitals does not occur at the zone center in the
rocksalt structure. Repulsion occurs though away from the -point and therefore the valence band
maximum is not located at the zone center (Fig. 6.13). Thus CdO is an indirect semiconductor. A
similar effect would occur in rs-ZnO due to zinc 3d orbitals; however, ZnO has wurtzite structure for
which p–d coupling at the -point is allowed; thus ZnO is direct. In MgO, Mg of course only possesses
populated s- and p-orbitals and no such repulsion is present; thus MgO even with its rocksalt structure
is also direct [469].
6.3.8 Chalcopyrites
The experimental band gaps of a number of chalcopyrite semiconductors are listed in Table 6.2. The
band structures of CuAlS 2 , CuAlSe 2 , and CuGaSe 2 are compared in Fig. 6.14.
In Fig. 6.15, the theoretical band structure of GaN and its closest related chalcopyrite ZnGeN 2 are
compared, both shown in the chalcopyrite (orthorhombic) Brillouin zone. The band gap of ZnGeN 2
is smaller than that of GaN and the difference of 0.4 eV is fairly well reproduced by the calculation
2
(giving 0.5 eV).
2 Due to the local density approximation (LDA) the absolute values of the band gaps are too small by about 1 eV.
147
X
K
L
12
10
8
6
4
2
0
-2
-4
-6
CdO
Fig. 6.13 Calculated indirect band structure of CdO. The top of the valence band is at E = 0. Adapted from [470]
6.3.7 MgO, ZnO, CdO
Cadmium oxide is a cubic semiconductor in the rocksalt structure. Due to symmetry considerations,
coupling (repulsion) of oxygen 2p- and cadmium 3d-orbitals does not occur at the zone center in the
rocksalt structure. Repulsion occurs though away from the -point and therefore the valence band
maximum is not located at the zone center (Fig. 6.13). Thus CdO is an indirect semiconductor. A
similar effect would occur in rs-ZnO due to zinc 3d orbitals; however, ZnO has wurtzite structure for
which p–d coupling at the -point is allowed; thus ZnO is direct. In MgO, Mg of course only possesses
populated s- and p-orbitals and no such repulsion is present; thus MgO even with its rocksalt structure
is also direct [469].
6.3.8 Chalcopyrites
The experimental band gaps of a number of chalcopyrite semiconductors are listed in Table 6.2. The
band structures of CuAlS 2 , CuAlSe 2 , and CuGaSe 2 are compared in Fig. 6.14.
In Fig. 6.15, the theoretical band structure of GaN and its closest related chalcopyrite ZnGeN 2 are
compared, both shown in the chalcopyrite (orthorhombic) Brillouin zone. The band gap of ZnGeN 2
is smaller than that of GaN and the difference of 0.4 eV is fairly well reproduced by the calculation
2
(giving 0.5 eV).
2 Due to the local density approximation (LDA) the absolute values of the band gaps are too small by about 1 eV.