6.3 Epitaxial Growth and Doping Techniques for AlGaN Materials
101
fully merged smooth surface AlN with low dislocation density [40, 41]. The FWHM
of the XRD rocking curves of (002) and (102) are 157 and 291 arcsec, respectively.
The threading dislocation density was reduced to 2.8 × 10
8 cm
−2 .
High-temperature lateral epitaxy: Since 2006, the Isamu Akasaki’s team in
Meijo University has combined ELOG with high-temperature growth to grow AlN
and AlGaN on micro-scale trench-type AlN/sapphire template [42, 43]. The threading
dislocation density of AlN and AlGaN is reduced to 10
7 cm
−2 . Based on this method,
345 nm UV-A LEDs with EQE > 6.7% [44, 45] and 356 nm UV-A LDs with electric
pump [46] were achieved. The FWHM of (0002) and (10–10) XRD rocking curves
of ELO-AlN on the grooved sapphire substrate were only 148 and 385 arcsec respectively. The LOP of 266 nm LED reached 5.3mW at 60 mA, and the EQE reached
1.9%. The LOP of 278 nm LED reached 8.4 mW, and the EQE reached 3.4%. High
temperature growth can increase the surface mobility of Al atoms. However, it also
causes greater thermal mismatch due to the large difference in thermal expansion
coefficient between AlN and sapphire substrates. The pre-reaction of TMAl and
NH 3 in the gas phase is also introduced simultaneously.
AlN bulk substrate: The homogeneous AlN bulk substrate is a better alternative to
improve the crystal quality of Al(Ga)N materials and the performance of UV LEDs.
At present, Crystal IS (acquired by Asahi Kasei from Japan in 2011), HexaTech
and Nitride Solutions from USA, and CrystAL-N from Germany have made great
progress in the growth of AlN substrates and high-performance UV LEDs. Crystal IS
reported that the dislocation density of Al 0.5 Ga 0.5 N was reduced to 7.5 × 10
5 cm
−2
using PVT-AlN substrate [47]. In 2012, they achieved 260-nm UVC LEDs with IQE
as high as 70% and output power of 53 mW at a WPE of 3.6% at 200 mA in CW
operation [48]. In 2013, they presented 271 nm UVC LEDs with output power of
66.8 mW at a WPE of 2.5% at 300 mA by AlN substrate thinning and encapsulation
[49]. HexaTech collaborating with Japan’s HVPE developer Tokuyama successfully
reduced the dislocation density of MQWs to less than 10
6 cm
−2 on HVPE AlN
substrates, and the 268 nm UVC LEDs exhibited output power of 28 mW at an EQE
of 2.4% at 250 mA in 2012 [50]. The 261 nm DUV LEDs fabricated on HVPEAlN substrates exhibited LOP of 10.8 mW at 150 mA, and an estimated lifetime
over 5000 h at a high driven current of 150 mA in 2013 [51]. However, only 2”
AIN wafers are available on the market at present and AlN substrates cannot be
extensively used due to their much higher price than that of sapphire substrates.
For AlGaN and other III-nitrides, Si and Mg are the most common n-type and ptype doping elements, respectively. With the increase of Al composition, it’s difficult
to obtain n-type AlGaN with high conductivity and high carrier concentration. As the
Si donor energy level becomes deeper, the activation energy increases continuously.
The Si donor can be compensated by acceptor-type defects (such as group III cation
vacancies and relevant complexes, impurities, dislocations) [52]. The p-type doping
of AlGaN is even more difficult to realize. Due to the deep acceptor feature of Mg
in AlGaN, the activation energy increases almost linearly from 160 meV in GaN
to 510–600 meV in AlN as the Al composition increases. The acceptor activation
efficiency is low, leading to low hole concentration and poor conductivity of p-type
101
fully merged smooth surface AlN with low dislocation density [40, 41]. The FWHM
of the XRD rocking curves of (002) and (102) are 157 and 291 arcsec, respectively.
The threading dislocation density was reduced to 2.8 × 10
8 cm
−2 .
High-temperature lateral epitaxy: Since 2006, the Isamu Akasaki’s team in
Meijo University has combined ELOG with high-temperature growth to grow AlN
and AlGaN on micro-scale trench-type AlN/sapphire template [42, 43]. The threading
dislocation density of AlN and AlGaN is reduced to 10
7 cm
−2 . Based on this method,
345 nm UV-A LEDs with EQE > 6.7% [44, 45] and 356 nm UV-A LDs with electric
pump [46] were achieved. The FWHM of (0002) and (10–10) XRD rocking curves
of ELO-AlN on the grooved sapphire substrate were only 148 and 385 arcsec respectively. The LOP of 266 nm LED reached 5.3mW at 60 mA, and the EQE reached
1.9%. The LOP of 278 nm LED reached 8.4 mW, and the EQE reached 3.4%. High
temperature growth can increase the surface mobility of Al atoms. However, it also
causes greater thermal mismatch due to the large difference in thermal expansion
coefficient between AlN and sapphire substrates. The pre-reaction of TMAl and
NH 3 in the gas phase is also introduced simultaneously.
AlN bulk substrate: The homogeneous AlN bulk substrate is a better alternative to
improve the crystal quality of Al(Ga)N materials and the performance of UV LEDs.
At present, Crystal IS (acquired by Asahi Kasei from Japan in 2011), HexaTech
and Nitride Solutions from USA, and CrystAL-N from Germany have made great
progress in the growth of AlN substrates and high-performance UV LEDs. Crystal IS
reported that the dislocation density of Al 0.5 Ga 0.5 N was reduced to 7.5 × 10
5 cm
−2
using PVT-AlN substrate [47]. In 2012, they achieved 260-nm UVC LEDs with IQE
as high as 70% and output power of 53 mW at a WPE of 3.6% at 200 mA in CW
operation [48]. In 2013, they presented 271 nm UVC LEDs with output power of
66.8 mW at a WPE of 2.5% at 300 mA by AlN substrate thinning and encapsulation
[49]. HexaTech collaborating with Japan’s HVPE developer Tokuyama successfully
reduced the dislocation density of MQWs to less than 10
6 cm
−2 on HVPE AlN
substrates, and the 268 nm UVC LEDs exhibited output power of 28 mW at an EQE
of 2.4% at 250 mA in 2012 [50]. The 261 nm DUV LEDs fabricated on HVPEAlN substrates exhibited LOP of 10.8 mW at 150 mA, and an estimated lifetime
over 5000 h at a high driven current of 150 mA in 2013 [51]. However, only 2”
AIN wafers are available on the market at present and AlN substrates cannot be
extensively used due to their much higher price than that of sapphire substrates.
For AlGaN and other III-nitrides, Si and Mg are the most common n-type and ptype doping elements, respectively. With the increase of Al composition, it’s difficult
to obtain n-type AlGaN with high conductivity and high carrier concentration. As the
Si donor energy level becomes deeper, the activation energy increases continuously.
The Si donor can be compensated by acceptor-type defects (such as group III cation
vacancies and relevant complexes, impurities, dislocations) [52]. The p-type doping
of AlGaN is even more difficult to realize. Due to the deep acceptor feature of Mg
in AlGaN, the activation energy increases almost linearly from 160 meV in GaN
to 510–600 meV in AlN as the Al composition increases. The acceptor activation
efficiency is low, leading to low hole concentration and poor conductivity of p-type