6.3 Epitaxial Growth and Doping Techniques for AlGaN Materials
103
and fivefold enhancements, respectively. John Simon et al. proposed a polarization
doping method to induce Mg acceptor ionization. They achieved three-dimensional
mobile hole gas with high density of 2 × 10
18 cm
−3 using N-face AlGaN with
Al composition grading from 0 to 0.3 [65]. Lian Zhang et al. from our research
group also reported three-dimensional mobile hole gas induced by polarization in
metal-face Al-composition graded AlGaN [66].
To improve the current injection efficiency, Fujioka [67] and Sumiya [68] reported
that inserting a 1-nm thin AlN layer between AlGaN-MQWs and p-AlGaN electron
blocking layer could suppress carrier overflow and the 320-nm parasitic luminescence. The diffusion of Mg towards MQWs was also effectively prevented using this
method. Hideki Hirayama et al. proposed a multi-quantum-barrier electron blocking
layer to increase the effective barrier height to suppress electron leakage and increase
the carrier injection efficiency [69]. The EQE of 250-nm DUV LEDs was improved
by 2.7 times.
6.4 Structure Design and Fabrication of UV LEDs
High-efficiency DUV LEDs require high crystal quality AlGaN epitaxial layer, high
carrier-confinement multiple-quantum-well structure, and high carrier-injectioncapability electron and hole injection layers. Therefore, the improvement of the
photoelectric performance of DUV LEDs depends on the crystal quality of AlGaN
materials, the optimization of doping and quantum structure, and the improvement
of the device fabrication process. At present, AlGaN-based UV LEDs still encounter
significant obstacles such as low internal quantum efficiency, low current injection
efficiency, and low light extraction efficiency.
Low internal quantum efficiency: High-efficiency AlGaN-based DUV LEDs
require quantum structures with high carrier confinement capability. AlGaN material
has a strong spontaneous polarization effect along the [000–1] direction. The spontaneous polarization is pointing to the substrate direction, and its intensity enhances
as the Al composition increases. In addition, the significant Al composition difference between AlGaN quantum wells and barriers induces a strongly polarization
electrical field (built-in electrical field) along the c-axis. In consequence, the energy
bands bend and the electron and hole wave functions are separated spatially, which
brings in internal quantum efficiency issue.
Low current injection efficiency: Si and Mg are widely used n-type and p-type
doping elements in III-nitride materials. With increasing Al composition, it becomes
challenge to obtain n-type AlGaN with high conductivity and high carrier concentration. This can be ascribed to the fact that Si donor energy level becomes deeper.
The Si donor is compensated by acceptor-type defects (such as group III cation
vacancies and relevant complexes, impurities, dislocations) [52]. Unfortunately,
p-type doping of AlGaN is even more challenging. Activation energy of the acceptor
increases from 160 meV in GaN to 510–600 meV in AlN. Such phenomenon leads to
a fairly low hole concentration and poor conductivity of p-type AlGaN. It needs to be
103
and fivefold enhancements, respectively. John Simon et al. proposed a polarization
doping method to induce Mg acceptor ionization. They achieved three-dimensional
mobile hole gas with high density of 2 × 10
18 cm
−3 using N-face AlGaN with
Al composition grading from 0 to 0.3 [65]. Lian Zhang et al. from our research
group also reported three-dimensional mobile hole gas induced by polarization in
metal-face Al-composition graded AlGaN [66].
To improve the current injection efficiency, Fujioka [67] and Sumiya [68] reported
that inserting a 1-nm thin AlN layer between AlGaN-MQWs and p-AlGaN electron
blocking layer could suppress carrier overflow and the 320-nm parasitic luminescence. The diffusion of Mg towards MQWs was also effectively prevented using this
method. Hideki Hirayama et al. proposed a multi-quantum-barrier electron blocking
layer to increase the effective barrier height to suppress electron leakage and increase
the carrier injection efficiency [69]. The EQE of 250-nm DUV LEDs was improved
by 2.7 times.
6.4 Structure Design and Fabrication of UV LEDs
High-efficiency DUV LEDs require high crystal quality AlGaN epitaxial layer, high
carrier-confinement multiple-quantum-well structure, and high carrier-injectioncapability electron and hole injection layers. Therefore, the improvement of the
photoelectric performance of DUV LEDs depends on the crystal quality of AlGaN
materials, the optimization of doping and quantum structure, and the improvement
of the device fabrication process. At present, AlGaN-based UV LEDs still encounter
significant obstacles such as low internal quantum efficiency, low current injection
efficiency, and low light extraction efficiency.
Low internal quantum efficiency: High-efficiency AlGaN-based DUV LEDs
require quantum structures with high carrier confinement capability. AlGaN material
has a strong spontaneous polarization effect along the [000–1] direction. The spontaneous polarization is pointing to the substrate direction, and its intensity enhances
as the Al composition increases. In addition, the significant Al composition difference between AlGaN quantum wells and barriers induces a strongly polarization
electrical field (built-in electrical field) along the c-axis. In consequence, the energy
bands bend and the electron and hole wave functions are separated spatially, which
brings in internal quantum efficiency issue.
Low current injection efficiency: Si and Mg are widely used n-type and p-type
doping elements in III-nitride materials. With increasing Al composition, it becomes
challenge to obtain n-type AlGaN with high conductivity and high carrier concentration. This can be ascribed to the fact that Si donor energy level becomes deeper.
The Si donor is compensated by acceptor-type defects (such as group III cation
vacancies and relevant complexes, impurities, dislocations) [52]. Unfortunately,
p-type doping of AlGaN is even more challenging. Activation energy of the acceptor
increases from 160 meV in GaN to 510–600 meV in AlN. Such phenomenon leads to
a fairly low hole concentration and poor conductivity of p-type AlGaN. It needs to be