Reliability and Performance of Pseudomorphic Ultraviolet Light Emitting Diodes on Bulk Aluminum Nitride Substrates
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1202-I10-02
Reliability and Performance of Pseudomorphic Ultraviolet Light Emitting Diodes on Bulk Aluminum Nitride Substrates Shawn R. Gibb1 James R. Grandusky1, Yongjie Cui1, Mark C. Mendrick1, and Leo J. Schowalter1 1 Crystal IS Inc., 70 Cohoes Avenue, Green Island, NY 12183 U.S.A. ABSTRACT Low dislocation density epitaxial layers of AlxGa1-xN can be grown pseudomorphically on c-face AlN substrates prepared from high quality, bulk crystals. Here, we will report on initial characterization results from deep ultraviolet (UV) light emitting diodes (LEDs) which have been fabricated and packaged from these structures. As reported previously, pseudomorphic growth and atomically smooth surfaces can be achieved for a full LED device structure with an emission wavelength between 250 nm and 280 nm. A benefit of pseudomorphic growth is the ability to run the devices at high input powers and current densities. The high aluminum content AlxGa1-xN (x~0.7) epitaxial layer can be doped n-type to obtain sheet resistances < 200 Ohms/sq/µm due to the low dislocation density. Bulk crystal growth allows for the ability to fabricate substrates of both polar and non-polar orientations. Non-polar substrates are of particular interest for nitride growth because they eliminate electric field due to spontaneous polarization and piezoelectric effects which limit device performance. Initial studies of epitaxial growth on non-polar substrates will also be presented. INTRODUCTION Ultraviolet disinfection is becoming very important as an efficient means of providing disinfection to water, air and surfaces without the use of potentially harmful 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. EXPERIMENT Epitaxial growth was carried out via OMVPE in a Veeco D180 vertical rotating disc reactor on low dislocation density bulk AlN substrates of both polar (c-plane) and non-polar (mplane) orientations. The growths utilized trimethylgallium, trimethylaluminum, 100 ppm silane
and ammonia as precursors in hydrogen ambient. Prior to epitaxial growth the AlN substrates are etched to remove surface oxides and an in-situ high temperature annealing step is performed. For polar growth 500 nm of homoepitaxial AlN was grown post anneal and the remainder of the device structure consisted of an n-type Al0.7Ga0.3N layer, a 5 period multiple quantum well (MQW), an electron blocking layer, and a p-type GaN contact layer. X-ray diffraction was used to characterize the strain state of the layers using both the symmetric (0002) and asymmetric (1012) reflections. Aluminum content and strain were calculated as described previously3. Growth on non-polar substrates was performed in the same hardware utilizing the precursors described above. Devices on c-plane AlN substrates were processed using standard LED processing with a mesa diameter of 350 µm. The devices are flip chip mounted to an AlN submoun
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