6.2 Optical and Electrical Properties of AlGaN Materials
97
6.2 Optical and Electrical Properties of AlGaN Materials
The forbidden bandwidths of AlN and GaN at room temperature are 6.03 and 3.42 eV,
respectively [8]. The forbidden bandwidth of the ternary alloy does not change
linearly with the change of the Al composition, but a bending coefficient b is required.
Thus the forbidden bandwidth of Al x Ga 1−x N is expressed as:
E g (Al x Ga 1−x N) = x · E g (AlN) + (1 − x) · E g (GaN) − b · x(1 − x)
(6.5)
The bending coefficient b is usually related to strain, buffer layer properties and
growth conditions [9]. For Al x Ga 1−x N, b is usually taken as 0.7. The lattice constant
can be calculated according to the Vegard theorem as follows:
a(Al x Ga 1−x N) = x · a(AlN) + (1 − x) · a(GaN)
(6.6)
The AlGaN material has a strong spontaneous polarization effect (its spontaneous
polarization direction is [0001]), which is toward the substrate. Its spontaneous polarization effect increases with increasing the Al composition. When the Al mole fraction of the well and the barrier of AlGaN/AlGaN MQWs are much different [10], a
highly polarized electric field (built-in electric field) is generated at the heterojunction interface or quantum well region along the c-axis, resulting in band-bending,
spatial separation of electron and hole wave functions, so-called Quantum Conned
Stark Effect (QCSE) as shown in Fig. 6.4. QCSE increases carrier lifetime, reduces
radiation recombination efficiency, and causes red shift of the emission wavelength.
Even without external bias, the spontaneous polarization and piezoelectric polarization electric field are also as high as MV/cm. The formation of two-dimensional
electron gas is advantageous for field effect devices such as HEMT. However, for
optoelectronic devices, the internal quantum efficiency will be largely reduced.
Fig. 6.4 The schematic
diagram of the effect of
QCSE on the energy band of
MQW
97
6.2 Optical and Electrical Properties of AlGaN Materials
The forbidden bandwidths of AlN and GaN at room temperature are 6.03 and 3.42 eV,
respectively [8]. The forbidden bandwidth of the ternary alloy does not change
linearly with the change of the Al composition, but a bending coefficient b is required.
Thus the forbidden bandwidth of Al x Ga 1−x N is expressed as:
E g (Al x Ga 1−x N) = x · E g (AlN) + (1 − x) · E g (GaN) − b · x(1 − x)
(6.5)
The bending coefficient b is usually related to strain, buffer layer properties and
growth conditions [9]. For Al x Ga 1−x N, b is usually taken as 0.7. The lattice constant
can be calculated according to the Vegard theorem as follows:
a(Al x Ga 1−x N) = x · a(AlN) + (1 − x) · a(GaN)
(6.6)
The AlGaN material has a strong spontaneous polarization effect (its spontaneous
polarization direction is [0001]), which is toward the substrate. Its spontaneous polarization effect increases with increasing the Al composition. When the Al mole fraction of the well and the barrier of AlGaN/AlGaN MQWs are much different [10], a
highly polarized electric field (built-in electric field) is generated at the heterojunction interface or quantum well region along the c-axis, resulting in band-bending,
spatial separation of electron and hole wave functions, so-called Quantum Conned
Stark Effect (QCSE) as shown in Fig. 6.4. QCSE increases carrier lifetime, reduces
radiation recombination efficiency, and causes red shift of the emission wavelength.
Even without external bias, the spontaneous polarization and piezoelectric polarization electric field are also as high as MV/cm. The formation of two-dimensional
electron gas is advantageous for field effect devices such as HEMT. However, for
optoelectronic devices, the internal quantum efficiency will be largely reduced.
Fig. 6.4 The schematic
diagram of the effect of
QCSE on the energy band of
MQW