8.7 Vertical Structure LEDs
177
into Ga and N 2 requires temperatures above 800 °C [34]. As can be seen from the
figure after a single pulse irradiation, the temperature at the GaN/sapphire interface
can reach the GaN decomposition temperature only when the pulse energy density
reaches 500 mJ/cm
2 . Considering that sapphire reflects and absorbs about 20% to
about 30% of the energy, the threshold energy density of the stripped substrate is
between about 625 and 715 mJ/cm
2 . In addition, it can be seen from the figure that the
high temperature region (>1000 °C) gradually increases as the laser energy density
increases. When the energy density is 500 mJ/cm
2 , the high temperature region is
less than 50 nm. The range of the high temperature region rapidly expands to nearly
800 nm when the energy density is increased to 800 mJ/cm
2 . Therefore, in order to
reduce the damage when the substrate is peeled off, a smaller energy density should
be used as much as possible.
When irradiated by a high-energy laser beam, GaN at the interface is decomposed
into Ga and N 2 gases. These gases can only be confined to a limited area of the spot
size. Therefore, there is usually a high gas pressure generated during the decomposition process. A pressure of 6 GPa is high enough to cause GaN to crack as calculated
by Karpinski et al. [35], Tavernier and Clarke [36]. Therefore, strategies to avoid
GaN cracking must be considered. To avoid GaN cracking, we can start with the
following aspects: firstly, we should choose the appropriate laser energy density to
minimize the gas pressure during laser lift-off. Secondly, we can make a channel that
so that the generated gas can be released instantaneously. In addition, it is necessary
to have a perfect interface without holes or voids between the GaN and the transfer
bonding of the substrate to ensure the integrity of GaN.
8.7.3.1 AC/High Voltage LEDs
AC (Alternating Current, AC) is used to deliver power to individual houses, business
or public electricity, mainly to avoid long-distance power transmission loss. Since
AC power is delivered to the end users, the appliance must be designed accordingly
to avoid short circuit due to voltage mismatch.
Conventional light-emitting diodes (Light-Emitting Diode, LED) are required to
use DC (Direct Current, the DC) as the drive. The use of an alternating current as
a power supply at the same time must be accompanied by the rectifier transformer
AC/DC converter to ensure LED normal operation. For applications, it is desirable
to save power of LED operation. However, in the AC/DC conversion process, power
loss can be as high as 15–30% of the electricity. It can revolutionize the lighting
applications if the LED can be driven by an AC source. Such a change will completely
get rid of the bottleneck of the driver power lifetime and the chip heat dissipation
problems. Furthermore, the product reliability will be improved, and the application
cost will be significantly reduced.
AC-LEDs are a class of LED products that integrate various processing technologies. They include a variety of devices or cores that can be directly driven by the AC
power of the AC grid without the need of additional transformers, rectifiers or drive
circuits. This allows LED products to be directly connected to the home and office
177
into Ga and N 2 requires temperatures above 800 °C [34]. As can be seen from the
figure after a single pulse irradiation, the temperature at the GaN/sapphire interface
can reach the GaN decomposition temperature only when the pulse energy density
reaches 500 mJ/cm
2 . Considering that sapphire reflects and absorbs about 20% to
about 30% of the energy, the threshold energy density of the stripped substrate is
between about 625 and 715 mJ/cm
2 . In addition, it can be seen from the figure that the
high temperature region (>1000 °C) gradually increases as the laser energy density
increases. When the energy density is 500 mJ/cm
2 , the high temperature region is
less than 50 nm. The range of the high temperature region rapidly expands to nearly
800 nm when the energy density is increased to 800 mJ/cm
2 . Therefore, in order to
reduce the damage when the substrate is peeled off, a smaller energy density should
be used as much as possible.
When irradiated by a high-energy laser beam, GaN at the interface is decomposed
into Ga and N 2 gases. These gases can only be confined to a limited area of the spot
size. Therefore, there is usually a high gas pressure generated during the decomposition process. A pressure of 6 GPa is high enough to cause GaN to crack as calculated
by Karpinski et al. [35], Tavernier and Clarke [36]. Therefore, strategies to avoid
GaN cracking must be considered. To avoid GaN cracking, we can start with the
following aspects: firstly, we should choose the appropriate laser energy density to
minimize the gas pressure during laser lift-off. Secondly, we can make a channel that
so that the generated gas can be released instantaneously. In addition, it is necessary
to have a perfect interface without holes or voids between the GaN and the transfer
bonding of the substrate to ensure the integrity of GaN.
8.7.3.1 AC/High Voltage LEDs
AC (Alternating Current, AC) is used to deliver power to individual houses, business
or public electricity, mainly to avoid long-distance power transmission loss. Since
AC power is delivered to the end users, the appliance must be designed accordingly
to avoid short circuit due to voltage mismatch.
Conventional light-emitting diodes (Light-Emitting Diode, LED) are required to
use DC (Direct Current, the DC) as the drive. The use of an alternating current as
a power supply at the same time must be accompanied by the rectifier transformer
AC/DC converter to ensure LED normal operation. For applications, it is desirable
to save power of LED operation. However, in the AC/DC conversion process, power
loss can be as high as 15–30% of the electricity. It can revolutionize the lighting
applications if the LED can be driven by an AC source. Such a change will completely
get rid of the bottleneck of the driver power lifetime and the chip heat dissipation
problems. Furthermore, the product reliability will be improved, and the application
cost will be significantly reduced.
AC-LEDs are a class of LED products that integrate various processing technologies. They include a variety of devices or cores that can be directly driven by the AC
power of the AC grid without the need of additional transformers, rectifiers or drive
circuits. This allows LED products to be directly connected to the home and office