Reliability and Performance of Pseudomorphic Ultraviolet Light Emitting Diodes on Bulk Aluminum Nitride Substrates
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1195-B03-04
Reliability and Performance of Pseudomorphic Ultraviolet Light Emitting Diodes on Bulk Aluminum Nitride Substrates James R. Grandusky1, Yongjie Cui1, Mark C. Mendrick1, Shawn Gibb1, and Leo J. Schowalter1 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 AlxGa1-xN 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 cm2 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/cm2), 100 mA (100A/cm2) and 150 mA (150 A/cm2). 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 (0.6 can be obtained resulting in device layers with low dislocation densities, low resistivities, and atomically smooth surfaces3.
Figure 1. Performance of pseudomorphic UVC LEDs on bulk AlN substrates when driven to high currents for 4 different devices. The Y axis (output power) is normalized to 20 mA and shows linear increase in output power up to 150 mA drive current.
EXPERIMENT Epitaxial growth was carried out as discussed previously3. The device structure consisted of an n-type AlxGa1-xN layer, a 5 period multiple quantum well, an electron blocking layer, and a ptype GaN contact layer. Devices were processed using standard LED processing with a mesa diameter of 350 µm. The devices are flip chip mounted to an AlN submount and packaged in TO-8 and surface-mounted design (SMD) packages. For testing of the diodes, the packaged devices were mounted on a pin fin anodized Al heat sink (40 mm x 40 mm) with forced convection cooling which had an experimentally validated theoretical thermal resistance of 2.5 oC/W. With an applied forward
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