204
10 Reliability Analysis of Group III Nitride LEDs Devices
as defects in the epitaxial material itself, ohmic contact degradation, etc. the other
one is the packaging process. The potential problems can come from the aging and
discoloration of the lens, the reduction of the excitation efficiency of the phosphor,
and the failure caused by improper solid-state bonding wires.
10.1.1 Light Decay
After GaN-based LEDs work for a period of time, the light output power decays as
the result of the degradation of the active area, the p-type GaN layer and the ohmic
contact.
(1) Degradation of the active region
Because there is no matching substrate material, epitaxially grown films often contain
a large number of defects. When the device is working, these defects, under the
influence of external temperature or current, can act as non-radiative recombination centers and carrier tunneling channels. This can lead to the formation of lowresistance ohmic channels, which cause LEDs light attenuation. As the injected
current becomes larger and the stress time becomes longer, the output optical power
degradation will become more obvious as shown in Fig. 10.1 [1]. The electrical characteristics of the LEDs after failure are mainly manifested by the same magnitude
increase in the leakage current at low voltages in the reverse and forward directions
[2].
Research and analysis show that there are three main sources of non-radiative
recombination centers in LEDs devices: ➀ Existing non-radiative recombination
defects (point defects, dislocations, etc.) in the device material have migrated or
expanded to the active region [3],➁ Mg impurities atoms diffused into the active
region [4] and acted as non-radiative recombination centers,➂ New N vacancies
were generated in the active region [5].
(2) Degeneration of ohmic contact
The degradation of ohmic contacts and the current crowding effect can lead to light
attenuation. Comparing the I-V characteristics before and after aging, the increased
parasitic series resistance of the degraded LEDs makes the forward current decrease
at the same voltage bias. It is generally believed that the semi-transparent ohmic
contact and the upper surface of the p-type GaN layer become deteriorated due to the
high current and high temperature, which leads to an increase in series resistance.
Thus, the current crowding effect will reduce the optical power [6].
In the Ni/Ge/Au ohmic contact, Ga in GaN diffuses outward through Ni/Ge to
reach the Au layer, which will form a non-stoichiometric region. At the same time,
Au diffuses inward to form a high-resistance alloy, which makes contact resistance
higher [7]. Failure modes caused by electrical contact metallurgical inter-diffusion
of LEDs devices include degradation of light output, increase of parasitic series
resistance and short circuits.
10 Reliability Analysis of Group III Nitride LEDs Devices
as defects in the epitaxial material itself, ohmic contact degradation, etc. the other
one is the packaging process. The potential problems can come from the aging and
discoloration of the lens, the reduction of the excitation efficiency of the phosphor,
and the failure caused by improper solid-state bonding wires.
10.1.1 Light Decay
After GaN-based LEDs work for a period of time, the light output power decays as
the result of the degradation of the active area, the p-type GaN layer and the ohmic
contact.
(1) Degradation of the active region
Because there is no matching substrate material, epitaxially grown films often contain
a large number of defects. When the device is working, these defects, under the
influence of external temperature or current, can act as non-radiative recombination centers and carrier tunneling channels. This can lead to the formation of lowresistance ohmic channels, which cause LEDs light attenuation. As the injected
current becomes larger and the stress time becomes longer, the output optical power
degradation will become more obvious as shown in Fig. 10.1 [1]. The electrical characteristics of the LEDs after failure are mainly manifested by the same magnitude
increase in the leakage current at low voltages in the reverse and forward directions
[2].
Research and analysis show that there are three main sources of non-radiative
recombination centers in LEDs devices: ➀ Existing non-radiative recombination
defects (point defects, dislocations, etc.) in the device material have migrated or
expanded to the active region [3],➁ Mg impurities atoms diffused into the active
region [4] and acted as non-radiative recombination centers,➂ New N vacancies
were generated in the active region [5].
(2) Degeneration of ohmic contact
The degradation of ohmic contacts and the current crowding effect can lead to light
attenuation. Comparing the I-V characteristics before and after aging, the increased
parasitic series resistance of the degraded LEDs makes the forward current decrease
at the same voltage bias. It is generally believed that the semi-transparent ohmic
contact and the upper surface of the p-type GaN layer become deteriorated due to the
high current and high temperature, which leads to an increase in series resistance.
Thus, the current crowding effect will reduce the optical power [6].
In the Ni/Ge/Au ohmic contact, Ga in GaN diffuses outward through Ni/Ge to
reach the Au layer, which will form a non-stoichiometric region. At the same time,
Au diffuses inward to form a high-resistance alloy, which makes contact resistance
higher [7]. Failure modes caused by electrical contact metallurgical inter-diffusion
of LEDs devices include degradation of light output, increase of parasitic series
resistance and short circuits.