Reliability and Performance of Pseudomorphic Ultraviolet Light Emitting Diodes on Bulk
Aluminum Nitride Substrates
James R. Grandusky
1
, Yongjie Cui
1
, Mark C. Mendrick
1
, Shawn Gibb
1
, and Leo J. Schowalter
1
Crystal IS, 70 Cohoes Avenue, Green Island, NY 12183, U.S.A.
ABSTRACT
Reliability and performance of ultraviolet light emitting diodes have suffered due to the high
dislocation density of the AlN and high Al-content Al
x
Ga
1-x
N layers when grown on foreign
substrates such as sapphire. The development of pseudomorphic layers on low dislocation
density AlN substrates is leading to improvements in reliability and performance of devices
operating in the ultraviolet-C (UVC) range. One major improvement is the ability to operate
devices at much higher current densities and input powers than devices on sapphire substrates.
This is due to the better thermal properties and lower dislocation density of devices on AlN
substrates. Devices with active area of 0.001 cm
2
emitting at ~265 nm have been measured for
their reliability and change in power output over time at input currents of 20 mA (20 A/cm
2
), 100
mA (100A/cm
2
) and 150 mA (150 A/cm
2
). When operating at currents of 20 mA over 3500
hours of consecutive operation has been demonstrated with typical decay of ~27% over the 3500
hours. Extrapolating the decay with a linear fit gives a L50 (time to 50% of initial power) of
~5000 hrs. However it is desirable to be able to model the decay to better understand the
kinetics and better understand the mechanisms. In order to do this, the lifetime at 20 mA and
100 mA were modeled using an exponential decay function, square root transformation and a log
transformation to both be able to fit the experimental data and predict future performance.
INTRODUCTION
Ultraviolet disinfection is becoming very important as an efficient means of providing
disinfection to water, air and surfaces without the use of chemicals. This requires a light source
that is emitting in the ultraviolet-C (UVC) range (<300 nm) for efficient disinfection due to a
peak in DNA absorption at ~ 265 nm
1
. Traditionally, mercury lamps are used, however these
suffer the disadvantages of short lifetimes, slow start up, and the use of toxic mercury which can
be a hazard if the bulb is broken and leads to problems with disposal. Light emitting diodes
(LEDs) have the capability of high efficiencies, fast start up which can be synchronized with
water flow, long lifetimes, variable wavelengths, and no toxic materials.
However, currently devices are fabricated from Al
x
Ga
1-x
N layers on sapphire substrates
which lead to a high dislocation density and thus low efficiencies and short lifetimes
2
. Bulk AlN
substrates offer several advantages over growth on sapphire, including low lattice and thermal
mismatch between the substrate and the device layers. In addition, pseudomorphic growth of
Al
x
Ga
1-x
N with x>0.6 can be obtained resulting in device layers with low dislocation densities,
low resistivities, and atomically smooth surfaces
3
.
Mater. Res. Soc. Symp. Proc. Vol. 1195 © 2010 Materials Research Society 1195-B03-04