102
6 AlGaN-Based Multiple-Quantum-Well Materials and UV LEDs
AlGaN. It’s should be noted that the hole concentration is much lower than the electron concentration [53]. For AlGaN-based optoelectronic devices, the asymmetric
electron and hole concentration distribution can result in the electron injected in the
active region leaking into the p-type region. As a result, the radiative recombination
efficiency of quantum wells is reduced, accompanied by a long-wavelength parasitic luminescence in the p-type region. Besides, the bad current spreading capacity
in the n-type and p-type regions brings in the current crowding effect and the heat
accumulation, which can degrade the device luminous efficiency and reliability.
To improve the n-type doping efficiency, studies from Kansas State University,
University of California, Santa Barbara, University of South Carolina, and NTT in
Japan have shown that the n-type doping efficiency of AlGaN could be significantly
improved by suppressing the formation of self-compensation defects, and even n-type
conductive Si-doped AlN can be obtained [54–58]. Asif Khan’s research group used
the MEMOCVD technology to grow high-quality AlN and AlN/AlGaN SLs to reduce
the defects, the compensation and scattering centers in the AlGaN layer. P. Cantu
et al. found that the Si doping efficiency is strongly dependent on the Al composition
in AlGaN. The free carrier concentration decreases as the Al composition increases.
By utilizing Si-In co-doping, the formation of the deep acceptor-type centers could
be suppressed and the free carrier concentration in Al 0.65 Ga 0.35 N was increased by
two orders of magnitude. Yoshitaka Taniyasu et al. from NTT optimized the growth
process of Si-doped AlN, specifically suppressed the parasitic reaction of the source
gas and accurately controlled the Si doping level. The dislocation density was reduced
to 10
9 cm
−2 . The electron concentration and the carrier mobility reached 1.75 ×
10
15 cm
−3 and 125 cm
2 /(V s) at 300 K, respectively. H. X. Jiang’s research group
employed the δ-doping technique of Si and Mg in the n-AlGaN layer and the p-GaN
layer, respectively, to improve the material quality and reduce self-compensation
defects. The free carrier concentration and the conductivity of n-type and p-type
layers were both improved. The luminous efficiency of 340-nm UV LEDs was also
enhanced.
To improve the p-type doping efficiency, researchers from Boston University,
University of California at Santa Barbara, Carnegie Mellon University, and the US
Air Force Laboratory reported theoretically and experimentally that the Mg-doped
AlGaN/GaN superlattices, which were used to replace the conventional Mg-doped
AlGaN layer, could reduce the acceptor ionization energy and increase the Mg doping
efficiency, thereby increase the hole concentration and reduce the resistivity. The
optical absorption edge also moved to a short wavelength [59–63]. M. S. Shur et al.
and M. Shatalov et al. separately proposed to use p-GaN/p-AlGaN single heterojunction to obtain mass accumulation of holes near the heterojunction interface. Due to
the existence of only one barrier and the assistance of carrier transition by tunneling
and thermal emission, the vertical conductivity was improved. Currently, this method
has been widely used in DUV LEDs to improve hole injection efficiency and luminescence efficiency. H.X. Jiang’s research group adopted theδ-doping technique of Mg
dopant [64]. The dislocation density of p-GaN and p-AlGaN layers was reduced by
about one order of magnitude. Thanks to the improved material quality and reduced
self-compensation defects, the lateral and vertical conductivities presented twofold
6 AlGaN-Based Multiple-Quantum-Well Materials and UV LEDs
AlGaN. It’s should be noted that the hole concentration is much lower than the electron concentration [53]. For AlGaN-based optoelectronic devices, the asymmetric
electron and hole concentration distribution can result in the electron injected in the
active region leaking into the p-type region. As a result, the radiative recombination
efficiency of quantum wells is reduced, accompanied by a long-wavelength parasitic luminescence in the p-type region. Besides, the bad current spreading capacity
in the n-type and p-type regions brings in the current crowding effect and the heat
accumulation, which can degrade the device luminous efficiency and reliability.
To improve the n-type doping efficiency, studies from Kansas State University,
University of California, Santa Barbara, University of South Carolina, and NTT in
Japan have shown that the n-type doping efficiency of AlGaN could be significantly
improved by suppressing the formation of self-compensation defects, and even n-type
conductive Si-doped AlN can be obtained [54–58]. Asif Khan’s research group used
the MEMOCVD technology to grow high-quality AlN and AlN/AlGaN SLs to reduce
the defects, the compensation and scattering centers in the AlGaN layer. P. Cantu
et al. found that the Si doping efficiency is strongly dependent on the Al composition
in AlGaN. The free carrier concentration decreases as the Al composition increases.
By utilizing Si-In co-doping, the formation of the deep acceptor-type centers could
be suppressed and the free carrier concentration in Al 0.65 Ga 0.35 N was increased by
two orders of magnitude. Yoshitaka Taniyasu et al. from NTT optimized the growth
process of Si-doped AlN, specifically suppressed the parasitic reaction of the source
gas and accurately controlled the Si doping level. The dislocation density was reduced
to 10
9 cm
−2 . The electron concentration and the carrier mobility reached 1.75 ×
10
15 cm
−3 and 125 cm
2 /(V s) at 300 K, respectively. H. X. Jiang’s research group
employed the δ-doping technique of Si and Mg in the n-AlGaN layer and the p-GaN
layer, respectively, to improve the material quality and reduce self-compensation
defects. The free carrier concentration and the conductivity of n-type and p-type
layers were both improved. The luminous efficiency of 340-nm UV LEDs was also
enhanced.
To improve the p-type doping efficiency, researchers from Boston University,
University of California at Santa Barbara, Carnegie Mellon University, and the US
Air Force Laboratory reported theoretically and experimentally that the Mg-doped
AlGaN/GaN superlattices, which were used to replace the conventional Mg-doped
AlGaN layer, could reduce the acceptor ionization energy and increase the Mg doping
efficiency, thereby increase the hole concentration and reduce the resistivity. The
optical absorption edge also moved to a short wavelength [59–63]. M. S. Shur et al.
and M. Shatalov et al. separately proposed to use p-GaN/p-AlGaN single heterojunction to obtain mass accumulation of holes near the heterojunction interface. Due to
the existence of only one barrier and the assistance of carrier transition by tunneling
and thermal emission, the vertical conductivity was improved. Currently, this method
has been widely used in DUV LEDs to improve hole injection efficiency and luminescence efficiency. H.X. Jiang’s research group adopted theδ-doping technique of Mg
dopant [64]. The dislocation density of p-GaN and p-AlGaN layers was reduced by
about one order of magnitude. Thanks to the improved material quality and reduced
self-compensation defects, the lateral and vertical conductivities presented twofold