3.2 Polarization Effect of Group III Nitride Materials
25
because of these effects of the c-plane polarization in light-emitting devices that
researchers are continually trying to improve and search for non-polarized surfaces
(a-plane, m-plane) or semi-polarized surface devices [10, 11], hoping to avoid or
reduce the polarization effects [6].
3.3 Doping of Group III-Nitride LED Materials
3.3.1 Doping of Nitride LED Materials
The doping of the Group III nitride semiconductor is mainly exemplified by wurtzite
GaN. Since undoped GaN has N vacancies, it exhibits a weak n-type [12, 13]. The
electron concentration is on the order of 10
16 /cm
3 . Table 3.3 shows the ionization
energies of the donor doping elements in GaN. It can be seen from the table that the
Si doping in the place of the Ga site is a shallow donor impurity. As the minimum
ionization energy is 0.012–0.02 eV, the activation efficiency is quite high. In the
actual process, silane (SiH 4 ) is usually used for Si doping of the nitride material to
form an n-type material, which can be simultaneously completed during the epitaxial
growth of the material. Many studies have shown that Si doping can reduce point
defects in GaN materials, improve interface free energy, change the atomic mobility
during material growth, and improve the crystal quality of GaN materials. In the
multi-quantum well structure, the doped Si atoms in the GaN material are activated
to generate interface charges, partially shield the polarized electric field and attenuate
the quantum-confined Stark effect.
The n-type doping of GaN is easy to implement. On the other hand, p-type doping
is difficult to achieve. This has been a challenge to realize GaN-based semiconductor
material LEDs. Some of the important acceptor doping materials in GaN materials are
listed in Table 3.4. It can be seen from the table that the Mg has the smallest ionization
energy. The absolute value is, however, still large that leads to the low activation
efficiency. It is difficult to obtain a p-type GaN material with high hole concentration.
At the same time, the Mg is passivated by the H impurities during the growth of nitride
and cannot be activated effectively, resulting in high resistance characteristics. Until
H. Amano et al. successfully realized the p-type doping of Mg in GaN by low energy
electron beam irradiation (LEEBI), the Mg-doped GaN with a Mg concentration
of 10
20 /cm
3 can only achieve a hole concentration of 2 × 10
16 /cm
3 . Although the
Table 3.3 Common donor
doping in wurtzite Ga [4]
Donor Substitutional site Impurity ionization energy (eV)
Si
Gasite
0.012–0.02
V N
Nvacancy
0.03–0.1
C
Ga site
0.11–0.14
O
N site
0.03
25
because of these effects of the c-plane polarization in light-emitting devices that
researchers are continually trying to improve and search for non-polarized surfaces
(a-plane, m-plane) or semi-polarized surface devices [10, 11], hoping to avoid or
reduce the polarization effects [6].
3.3 Doping of Group III-Nitride LED Materials
3.3.1 Doping of Nitride LED Materials
The doping of the Group III nitride semiconductor is mainly exemplified by wurtzite
GaN. Since undoped GaN has N vacancies, it exhibits a weak n-type [12, 13]. The
electron concentration is on the order of 10
16 /cm
3 . Table 3.3 shows the ionization
energies of the donor doping elements in GaN. It can be seen from the table that the
Si doping in the place of the Ga site is a shallow donor impurity. As the minimum
ionization energy is 0.012–0.02 eV, the activation efficiency is quite high. In the
actual process, silane (SiH 4 ) is usually used for Si doping of the nitride material to
form an n-type material, which can be simultaneously completed during the epitaxial
growth of the material. Many studies have shown that Si doping can reduce point
defects in GaN materials, improve interface free energy, change the atomic mobility
during material growth, and improve the crystal quality of GaN materials. In the
multi-quantum well structure, the doped Si atoms in the GaN material are activated
to generate interface charges, partially shield the polarized electric field and attenuate
the quantum-confined Stark effect.
The n-type doping of GaN is easy to implement. On the other hand, p-type doping
is difficult to achieve. This has been a challenge to realize GaN-based semiconductor
material LEDs. Some of the important acceptor doping materials in GaN materials are
listed in Table 3.4. It can be seen from the table that the Mg has the smallest ionization
energy. The absolute value is, however, still large that leads to the low activation
efficiency. It is difficult to obtain a p-type GaN material with high hole concentration.
At the same time, the Mg is passivated by the H impurities during the growth of nitride
and cannot be activated effectively, resulting in high resistance characteristics. Until
H. Amano et al. successfully realized the p-type doping of Mg in GaN by low energy
electron beam irradiation (LEEBI), the Mg-doped GaN with a Mg concentration
of 10
20 /cm
3 can only achieve a hole concentration of 2 × 10
16 /cm
3 . Although the
Table 3.3 Common donor
doping in wurtzite Ga [4]
Donor Substitutional site Impurity ionization energy (eV)
Si
Gasite
0.012–0.02
V N
Nvacancy
0.03–0.1
C
Ga site
0.11–0.14
O
N site
0.03