210
10 Reliability Analysis of Group III Nitride LEDs Devices
the phonon energy and then ionized. The generated electron trap captures the activated state electrons to form a long afterglow illuminator. The thermal ionization
will deteriorate the thermal quenching characteristics of fluorescent agents [20].
Thermal quenching properties of rare-earth phosphor powder is influenced by
doping component and crystal structure. Take three different components of Lanbased dopant of Ce
3+ compound (La-SiON: Ce
3+ ) as an example, Fig. 10.4 shows
the PL spectra of the three samples at different temperatures [21]. The emission peak
wavelengths of the three samples did not move with increasing temperature, but the
FWHM of spectrum broadened. This indicates that the thermal stability of the color
coordinates of the nitrogen–oxygen fluorescent agent was good. In addition, in the
relationship of the peak of PL spectrum with temperature, the emission intensity of
La 5 Si 3 O 12 N: 2% Ce, La 3 Si 8 O 4 N 11 : 6% Ce phosphor agent change uniformly with
temperature, while LaSiO 2 N: 4% Ce decreased rapidly with increasing temperature.
Figure 10.5 is a normalized PL as a function of temperature. Among three samples,
LaSiO 2 N: 4% Ce was found with the worst thermal stability, La 3 Si 8 O 4 N .11 : 6%
Ce showed the highest thermal quenching temperature. The corresponding reason is
that La 3 Si .8 O .4 N 11 : 6% of Ce has a more compact crystal structure and the highest
N/O ratio [21] in comparison with other two materials.
Sublayer
When high-power LED device is working at high temperature thermal cycling environment for a long time, delamination may take place between the device and
the encapsulating layer, between an encapsulation layer and the lead frame of the
package, as well as between the LED die and the substrate. When delamination
occurs in the LED optical path of the device, the output light flux of the device will
be reduced, and the LED’s illuminating color may also change. The common place
can be between the die and the encapsulant layer, between the die and the phosphor layer, as well as between the lens of high-power LED and the encapsulation
layer. Zhou et al. used GaN-based high-power LED devices, aged at 350 mA forward
current for 256 h (25 °C, 54 is% RH), and subsequently discovered appearance of
division layer around the die since a large amount of light is restricted by reflection
in the sub-layer inside, and “rainbow belt” phenomenon appeared [22].
10.2 The LED Aging Test and an Aging Mechanism
Unlike other electrical elements, the LED device have a long service life. Except for
sudden failure, light attenuation to a certain extent beyond the requirement in most
cases is the reason for the undesired service life. In order to quickly understand the
reliability and life of LED, accelerated aging tests are applied generally. This section
will primarily describe the aging test and the mechanism where the temperature,
humidity, and electrical stress are used to conduct the measurement.
10 Reliability Analysis of Group III Nitride LEDs Devices
the phonon energy and then ionized. The generated electron trap captures the activated state electrons to form a long afterglow illuminator. The thermal ionization
will deteriorate the thermal quenching characteristics of fluorescent agents [20].
Thermal quenching properties of rare-earth phosphor powder is influenced by
doping component and crystal structure. Take three different components of Lanbased dopant of Ce
3+ compound (La-SiON: Ce
3+ ) as an example, Fig. 10.4 shows
the PL spectra of the three samples at different temperatures [21]. The emission peak
wavelengths of the three samples did not move with increasing temperature, but the
FWHM of spectrum broadened. This indicates that the thermal stability of the color
coordinates of the nitrogen–oxygen fluorescent agent was good. In addition, in the
relationship of the peak of PL spectrum with temperature, the emission intensity of
La 5 Si 3 O 12 N: 2% Ce, La 3 Si 8 O 4 N 11 : 6% Ce phosphor agent change uniformly with
temperature, while LaSiO 2 N: 4% Ce decreased rapidly with increasing temperature.
Figure 10.5 is a normalized PL as a function of temperature. Among three samples,
LaSiO 2 N: 4% Ce was found with the worst thermal stability, La 3 Si 8 O 4 N .11 : 6%
Ce showed the highest thermal quenching temperature. The corresponding reason is
that La 3 Si .8 O .4 N 11 : 6% of Ce has a more compact crystal structure and the highest
N/O ratio [21] in comparison with other two materials.
Sublayer
When high-power LED device is working at high temperature thermal cycling environment for a long time, delamination may take place between the device and
the encapsulating layer, between an encapsulation layer and the lead frame of the
package, as well as between the LED die and the substrate. When delamination
occurs in the LED optical path of the device, the output light flux of the device will
be reduced, and the LED’s illuminating color may also change. The common place
can be between the die and the encapsulant layer, between the die and the phosphor layer, as well as between the lens of high-power LED and the encapsulation
layer. Zhou et al. used GaN-based high-power LED devices, aged at 350 mA forward
current for 256 h (25 °C, 54 is% RH), and subsequently discovered appearance of
division layer around the die since a large amount of light is restricted by reflection
in the sub-layer inside, and “rainbow belt” phenomenon appeared [22].
10.2 The LED Aging Test and an Aging Mechanism
Unlike other electrical elements, the LED device have a long service life. Except for
sudden failure, light attenuation to a certain extent beyond the requirement in most
cases is the reason for the undesired service life. In order to quickly understand the
reliability and life of LED, accelerated aging tests are applied generally. This section
will primarily describe the aging test and the mechanism where the temperature,
humidity, and electrical stress are used to conduct the measurement.