100
6 AlGaN-Based Multiple-Quantum-Well Materials and UV LEDs
UV-C light emission [21]. In 2002, Asif Khan’s research group used the AlN/AlGaN
superlattice structure grown by PALE technology as the stress release and dislocation
filter layer, which effectively reduced the stress and threading dislocation density in
the epitaxial layer and reduced the cracking probability of the epitaxial layer [22, 23].
Based on these, they reported sub-mW-level 250, 269, 285 and 324 nm AlGaN-based
DUV LEDs [24–26] and mW-level 278 and 280 nm AlGaN-based DUV LED [27].
Migration enhanced epitaxial technology: In 2005, the research groups of
SETI and Asif Khan in the United States developed migration enhanced MOCVD
(MEMOCVD) epitaxial technology to grow high-quality AlN templates and
AlN/AlGaN superlattices and improve the quality of AlGaN [28]. DUV LED was
achieved at the peak wavelength of 280, 295 nm under mW level, and 265, 270, and
275 nm under sub-mW level [29]. Since 2008, Ryan G. Banal et al. in Kyoto University have optimized the AlN nucleation process by using the Migration Enhanced
Epitaxy (MEE) and modified MEE (Modified-MEE) technology and achieved a
600 nm high-quality AlN on sapphire with the full width at half maximum (FWHM)
of the XRD rocking curves of (002) and (102) as 45 and 250 arcsec, respectively.
High-quality AlGaN/AlN MQWs were grown by using this technique as well [28,
29]. The quantum efficiency was as high as 36%.
Variable V/III ratio epitaxial growth: In 2006, Masataka Imura et al. in Meijo
University used the method of multiple V/III ratio modulation (sequential reduction) during high temperature to grow 9 μm AlN by MOCVD. At the same time
of preventing cracking, the dislocations are continuously annihilated in the transition process of growth mode. The dislocation density was reduced to 3 × 10
8 cm
−2
[30, 31].
NH 3 pulse multilayer growth and multilayer AlN buffer layer technology:
In 2007–2009, Hideki Hirayama in the Institute of Physical and Chemical Research
reported the high-quality AlN and AlGaN epitaxial layers with low defect density on
sapphire by NH 3 pulse multilayer growth and multilayer AlN buffer layer technology,
respectively. They also reported 222–273 nm AlGaN-based DUV LEDs and 282 nm
InAlGaN-based DUV LEDs [32–34].
Pulse lateral overgrowth (PLOG): Since 2006, the Asif Khan’s team in the
University of South Carolina has used the NH 3 pulsed lateral overgrowth to grow
AlN with flat surface on a micron-sized trench-type AlN/sapphire template (PLOGAlN), where the threading dislocation density is reduced by 10
2 –10
3 cm
−2 [35]. They
further achieved 214 nm laser emission of AlN by optical pump at room temperature
[36] and significantly improved the luminous efficiency, saturation characteristics
and heat dissipation performance of 290 nm UV LED [37]. The lifetime was over
5,000 h. The 276 nm vertical UV-C LED was achieved by wet stripping the sapphire
substrate [38]. In 2009, Hideki Hirayama et al. in the Institute of Physical and Chemical Research used the same method to grow AlN on the micron-sized grooved
AlN/sapphire template to reduce the threading dislocation density to 10
8 cm
−2 and
achieved 270 nm UV-C LEDs with a maximum light output of 2.7 mW [39].
Migration enhanced lateral overgrowth: In 2008, R. Jain in SETI Corporation
and M. S. Shur in Rensselaer Polytechnic Institute used MEMOCVD technology to
laterally overgrow 20 μm AlN on a micro-groove-type AlGaN template to obtain a
6 AlGaN-Based Multiple-Quantum-Well Materials and UV LEDs
UV-C light emission [21]. In 2002, Asif Khan’s research group used the AlN/AlGaN
superlattice structure grown by PALE technology as the stress release and dislocation
filter layer, which effectively reduced the stress and threading dislocation density in
the epitaxial layer and reduced the cracking probability of the epitaxial layer [22, 23].
Based on these, they reported sub-mW-level 250, 269, 285 and 324 nm AlGaN-based
DUV LEDs [24–26] and mW-level 278 and 280 nm AlGaN-based DUV LED [27].
Migration enhanced epitaxial technology: In 2005, the research groups of
SETI and Asif Khan in the United States developed migration enhanced MOCVD
(MEMOCVD) epitaxial technology to grow high-quality AlN templates and
AlN/AlGaN superlattices and improve the quality of AlGaN [28]. DUV LED was
achieved at the peak wavelength of 280, 295 nm under mW level, and 265, 270, and
275 nm under sub-mW level [29]. Since 2008, Ryan G. Banal et al. in Kyoto University have optimized the AlN nucleation process by using the Migration Enhanced
Epitaxy (MEE) and modified MEE (Modified-MEE) technology and achieved a
600 nm high-quality AlN on sapphire with the full width at half maximum (FWHM)
of the XRD rocking curves of (002) and (102) as 45 and 250 arcsec, respectively.
High-quality AlGaN/AlN MQWs were grown by using this technique as well [28,
29]. The quantum efficiency was as high as 36%.
Variable V/III ratio epitaxial growth: In 2006, Masataka Imura et al. in Meijo
University used the method of multiple V/III ratio modulation (sequential reduction) during high temperature to grow 9 μm AlN by MOCVD. At the same time
of preventing cracking, the dislocations are continuously annihilated in the transition process of growth mode. The dislocation density was reduced to 3 × 10
8 cm
−2
[30, 31].
NH 3 pulse multilayer growth and multilayer AlN buffer layer technology:
In 2007–2009, Hideki Hirayama in the Institute of Physical and Chemical Research
reported the high-quality AlN and AlGaN epitaxial layers with low defect density on
sapphire by NH 3 pulse multilayer growth and multilayer AlN buffer layer technology,
respectively. They also reported 222–273 nm AlGaN-based DUV LEDs and 282 nm
InAlGaN-based DUV LEDs [32–34].
Pulse lateral overgrowth (PLOG): Since 2006, the Asif Khan’s team in the
University of South Carolina has used the NH 3 pulsed lateral overgrowth to grow
AlN with flat surface on a micron-sized trench-type AlN/sapphire template (PLOGAlN), where the threading dislocation density is reduced by 10
2 –10
3 cm
−2 [35]. They
further achieved 214 nm laser emission of AlN by optical pump at room temperature
[36] and significantly improved the luminous efficiency, saturation characteristics
and heat dissipation performance of 290 nm UV LED [37]. The lifetime was over
5,000 h. The 276 nm vertical UV-C LED was achieved by wet stripping the sapphire
substrate [38]. In 2009, Hideki Hirayama et al. in the Institute of Physical and Chemical Research used the same method to grow AlN on the micron-sized grooved
AlN/sapphire template to reduce the threading dislocation density to 10
8 cm
−2 and
achieved 270 nm UV-C LEDs with a maximum light output of 2.7 mW [39].
Migration enhanced lateral overgrowth: In 2008, R. Jain in SETI Corporation
and M. S. Shur in Rensselaer Polytechnic Institute used MEMOCVD technology to
laterally overgrow 20 μm AlN on a micro-groove-type AlGaN template to obtain a