21-07-2012, 10:41 AM
A Review on the Reliability of GaN-Based LEDs
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Abstract
We review the degradation mechanisms that limit
the reliability of GaN-based light-emitting diodes (LEDs). We
propose a set of specific experiments, which is aimed at separately
analyzing the degradation of the properties of the active layer,
of the ohmic contacts and of the package/phosphor system. In
particular, we show the following: 1) Low-current density stress
can determine the degradation of the active layer of the devices,
implying modifications of the charge/deep level distribution with
subsequent increase of the nonradiative recombination components;
2) high-temperature storage can significantly affect the
properties of the ohmic contacts and semiconductor layer at the
p-side of the devices, thus determining emission crowding and
subsequent optical power decrease; and 3) high-temperature stress
can significantly limit the optical properties of the package of
high-power LEDs for lighting applications.
INTRODUCTION
OVER THE past years, the technology of InGaN/GaN
light-emitting diodes (LEDs) has shown impressive improvements,
owing to the efforts of many research groups. As
a result, devices with an efficiency above 90 lm/W are currently
commercially available, and efficiencies above 120 lm/W
have already been demonstrated for research samples [1]. GaNbased
LEDs are considered excellent candidates for the replacement
of the widely used incandescent lamps, due to their high
efficiency, good robustness to shocks and atmospheric agents,
fast modulation speed, and for their linear behavior under
continuous current reduction and pulsedwidth modulation dimming.
LEDs also present advantages with respect to compact
fluorescent lamps (CFLs): In fact, CFLs contain mercury, which
has been classified as a hazardous material, and must be treated
in a specific way after the useful life of the lamps. On the other
hand, LED-based illumination devices are mercury free and,
therefore, do not present such issues.
EXPERIMENTAL DETAILS
The work has been carried out on commercially available
samples and research prototypes, grown ad hoc for the analysis
of specific degradation processes.
The analysis of the degradation of the properties of the
active region and of the ohmic contacts has been carried out
on bare chips grown on SiC, with 250-μm side, emitting at
460 nm. In order to have the possibility of characterizing the
doping/charge distribution in the active layer, we have used
multi-QW (MQW) LED structures: These devices have a wide
space charge region (SCR), whose properties can be explored in
detail by means of capacitance–voltage (C–V ) and deep-level
transient spectroscopy (DLTS) during stress. In order to reduce
the surface-leakage current and to avoid semiconductor contamination,
a silicon nitride passivation layer has been deposited
on the chip: After processing, the LEDs have been mounted
on TO18 metallic package for characterization and stress
tests.
DISCUSSION AND CONCLUSION
In this paper, we have presented a detailed analysis of the
reliability of GaN-based optoelectronic devices. The analysis
was aimed at identifying the physical mechanisms and the
failure modes related with the degradation of the different parts
of the devices’ structure. In particular, by means of specific
stress tests, we have separately analyzed the following: 1) the
degradation of the active layer properties; 2) the degradation
of the p-side contact and semiconductor layer; and 3) the
degradation of the package and phosphorous layer.