It is well known that AlN, GaN, InN and their crystal alloys crystallize are in the wurtzite structure with space group
, which means that there are strong spontaneous polarization and piezoelectric polarization in the InGaN and AlGaN materials [
25]. The highly polar nature yields a large amount of fixed charge at interfaces, which can induce a big electrostatic field in the LED. The total macroscopic polarization
P of InGaN (AlGaN) is defined as the sum of the spontaneous polarization
in the equilibrium lattice and the strain-induced piezoelectric polarization
. The spontaneous polarization
(
) can be obtained by numeric interpolations between the physical properties of GaN and InN (AlN) [
28]:
where
and
are the bowing parameters, defined as -0.038 C/m
2 for InGaN and -0.019 C/m
2 for AlGaN. Piezoelectric polarizations of InGaN and AlGaN are dependent on the strains in the materials, which can be expressed as [
29,
30]: