6.1 Introduction of AlGaN Material System
95
Fig. 6.2 The schematic diagram of a the unit cell and b wurtzite structure of group-III nitride
order is…AaBb-AaBb… in the [0001] direction. The sphalerite structure is a cubic
structure, and the two sets of face-centered cubic structures are formed by translating
1/4 diagonal lines in the [111] direction. The stacked order is… AaBbCc–AaBbCc….
Since the III-nitride with the wurtzite structure lacks the inversion symmetry
center in the [0001] direction, the III-nitride has two different polarity in the [0001]
direction, Group-III atoms (Al, Ga or In) polarity or [0001] polarity and N polarity
or [000–1] polarity. If the vertical bond is from group-III atom to N atom, or the
group-III atom is above the (0001) bilayer, this is Ga polarity; if the vertical bond is
from N atom to group-III atom, or the N atom is located above the (0001) bilayer,
this is N polarity [3]. The polarity of nitrides has an important effect on the doping
and piezoelectricity during epitaxial growth [4]. In general, Ga polar face and N
polar face depends on the substrate and growth process, especially the initial stage
of growth [5].
The growth method of the AlGaN-based multiple quantum well structure is
mainly MOCVD technology. As an example of III-nitride growth by MOCVD,
without considering the intermediate reaction and the product, the simplified reaction
equation can be written as:
M(g) + NH 3 (g) → MH(s) + 3RH(g)
(6.1)
where R is an organic compound group such as trimethyl or triethyl, M is a trivalent
element of the deposit, such as Al and Ga. Therefore, the reaction equations for
MOCVD growth of AlN, GaN, and Al x Ga 1−x N can be abbreviated as follows:
(CH 3 ) 3 Ca + NH 3 → GaN + 3CH 4
(6.2)
(CH 3 ) 3 Al + NH 3 → AlN + 3CH 4
(6.3)
95
Fig. 6.2 The schematic diagram of a the unit cell and b wurtzite structure of group-III nitride
order is…AaBb-AaBb… in the [0001] direction. The sphalerite structure is a cubic
structure, and the two sets of face-centered cubic structures are formed by translating
1/4 diagonal lines in the [111] direction. The stacked order is… AaBbCc–AaBbCc….
Since the III-nitride with the wurtzite structure lacks the inversion symmetry
center in the [0001] direction, the III-nitride has two different polarity in the [0001]
direction, Group-III atoms (Al, Ga or In) polarity or [0001] polarity and N polarity
or [000–1] polarity. If the vertical bond is from group-III atom to N atom, or the
group-III atom is above the (0001) bilayer, this is Ga polarity; if the vertical bond is
from N atom to group-III atom, or the N atom is located above the (0001) bilayer,
this is N polarity [3]. The polarity of nitrides has an important effect on the doping
and piezoelectricity during epitaxial growth [4]. In general, Ga polar face and N
polar face depends on the substrate and growth process, especially the initial stage
of growth [5].
The growth method of the AlGaN-based multiple quantum well structure is
mainly MOCVD technology. As an example of III-nitride growth by MOCVD,
without considering the intermediate reaction and the product, the simplified reaction
equation can be written as:
M(g) + NH 3 (g) → MH(s) + 3RH(g)
(6.1)
where R is an organic compound group such as trimethyl or triethyl, M is a trivalent
element of the deposit, such as Al and Ga. Therefore, the reaction equations for
MOCVD growth of AlN, GaN, and Al x Ga 1−x N can be abbreviated as follows:
(CH 3 ) 3 Ca + NH 3 → GaN + 3CH 4
(6.2)
(CH 3 ) 3 Al + NH 3 → AlN + 3CH 4
(6.3)