98
6 AlGaN-Based Multiple-Quantum-Well Materials and UV LEDs
6.3 Epitaxial Growth and Doping Techniques for AlGaN
Materials
For epitaxial growth of high-quality III-nitride thin films, the choice of substrate
is particularly important. Homogeneous substrate is theoretically the best substrate
choice for III- nitride growth. However, commercial production of high quality, large
size, low cost III-nitride homogenous substrates is still difficult [11], especially for
AlN single crystal substrates. Therefore, commercially available III-nitride devices
mainly use heterogeneous substrates such as (0001) sapphire, (111) Si, and 6H–
SiC substrates, where the (0001) sapphire is the most common substrate. For the
heteroepitaxial growth of III-nitrides, the lattice constant and thermal expansion
coefficient are two critical parameters. Table 6.1 lists the lattice constants and thermal
expansion coefficients and mismatch of AlN and different substrates. On one hand,
due to the existence of lattice mismatch, the strain accumulated in the epitaxial layer
relaxes by generating dislocations at the interface between substrate and epitaxial
layer, thus causes a large number of dislocations in the epitaxial layer. On the other
hand, due to the existence of thermal mismatch, cracking and dislocations of the
epitaxial layer are caused by the mismatch in lattice deformation between substrate
and epitaxial layer during heating or cooling process. To alleviate this problem, a
low-temperature AlN or GaN buffer layer is introduced as an initial nucleation layer
to reduce dislocation density and compensates for strain in the epitaxial layer caused
by thermal mismatch.
Figure 6.5 shows the lattice arrangement of (0001) AlN on the c-plane sapphire
substrate. The lattice point of AlN is rotated by 30° with respect to the lattice point
of sapphire [12]. Therefore, the actual lattice mismatch between AlN and sapphire
(13.3%) is calculated as follows:
Table 6.1 The lattice mismatch and thermal mismatch between AlN and different substrate
Lattice constant (Å)
Thermal
expansivity
(10 −6 /K)
Lattice mismatch
(%)
Thermal expansion
mismatch (%)
AlN
a = 3.112
c = 4.982
a/a = 4.2
c/c = 5.3
GaN
a = 3.189
c = 5.186
a/a = 5.59
c/c = 3.17
−0.24
−0.25
Sapphire
a = 4.758
c = 12.991
a eff. = 2.747
a/a = 7.5
c/c = 8.57
−34.6
13.3
−44
6H-SiC
a = 3.0817
c = 15.1123
a/a = 4.2
c/c = 4.68
0.98
0
Si (111)
a = 5.4301
a eff. = 3.843
a/a = 3.59 42.7
−19%
17
6 AlGaN-Based Multiple-Quantum-Well Materials and UV LEDs
6.3 Epitaxial Growth and Doping Techniques for AlGaN
Materials
For epitaxial growth of high-quality III-nitride thin films, the choice of substrate
is particularly important. Homogeneous substrate is theoretically the best substrate
choice for III- nitride growth. However, commercial production of high quality, large
size, low cost III-nitride homogenous substrates is still difficult [11], especially for
AlN single crystal substrates. Therefore, commercially available III-nitride devices
mainly use heterogeneous substrates such as (0001) sapphire, (111) Si, and 6H–
SiC substrates, where the (0001) sapphire is the most common substrate. For the
heteroepitaxial growth of III-nitrides, the lattice constant and thermal expansion
coefficient are two critical parameters. Table 6.1 lists the lattice constants and thermal
expansion coefficients and mismatch of AlN and different substrates. On one hand,
due to the existence of lattice mismatch, the strain accumulated in the epitaxial layer
relaxes by generating dislocations at the interface between substrate and epitaxial
layer, thus causes a large number of dislocations in the epitaxial layer. On the other
hand, due to the existence of thermal mismatch, cracking and dislocations of the
epitaxial layer are caused by the mismatch in lattice deformation between substrate
and epitaxial layer during heating or cooling process. To alleviate this problem, a
low-temperature AlN or GaN buffer layer is introduced as an initial nucleation layer
to reduce dislocation density and compensates for strain in the epitaxial layer caused
by thermal mismatch.
Figure 6.5 shows the lattice arrangement of (0001) AlN on the c-plane sapphire
substrate. The lattice point of AlN is rotated by 30° with respect to the lattice point
of sapphire [12]. Therefore, the actual lattice mismatch between AlN and sapphire
(13.3%) is calculated as follows:
Table 6.1 The lattice mismatch and thermal mismatch between AlN and different substrate
Lattice constant (Å)
Thermal
expansivity
(10 −6 /K)
Lattice mismatch
(%)
Thermal expansion
mismatch (%)
AlN
a = 3.112
c = 4.982
a/a = 4.2
c/c = 5.3
GaN
a = 3.189
c = 5.186
a/a = 5.59
c/c = 3.17
−0.24
−0.25
Sapphire
a = 4.758
c = 12.991
a eff. = 2.747
a/a = 7.5
c/c = 8.57
−34.6
13.3
−44
6H-SiC
a = 3.0817
c = 15.1123
a/a = 4.2
c/c = 4.68
0.98
0
Si (111)
a = 5.4301
a eff. = 3.843
a/a = 3.59 42.7
−19%
17