Enhanced Emission from In x Ga 1-x N-based LED Structures Using III-Nitride based Distributed Bragg Reflector
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Enhanced Emission from InxGa1-xN-based LED Structures Using III-Nitride based Distributed Bragg Reflector K. Lee , A. Kadiyala, L. E. Rodak, R. Goswami, V. Kumbham, B. A. Bearce, J. Justice, J. Peacock, J. M. Dawson, L. A. Hornak, and D. Korakakis Lane Department of Computer Science and Electrical Engineering, West Virginia University, Morgantown, WV 26506, USA ABSTRACT InxGa1-xN-based LED structures were grown on digital AlxGa1-xN/GaN DBR substrate to enhance emission extraction. Same LED structure was grown on sapphire substrate as a comparison. LEDs grown on DBR substrate exhibited similar IV characteristics to that grown on sapphire substrate but emission-angle-dependent EL spectra were observed. Also, the resonant vertical cavity modes were observed in EL spectra of LEDs with DBR structure and compared to simulated results. Image processing analysis results show that light extraction of LEDs is enhanced with use of DBR substrate. INTRODUCTION III-Nitride based light emitting diodes (LEDs) have emerged as promising devices for solid state lighting due to the ability to emit from ultra violet to infrared wavelengths by controlling the alloy composition in the active region. In addition to efficiency, LEDs are also anticipated to bring new functionality to lighting technologies. Depending on the applications, including but not limited to, general lighting, displays, sensors, and optical communications, the emission profile may need to be more or less directional. As a result, there is significant interest in increasing the light extraction efficiency of nitride based LEDs and also controlling the emission profile. Various forms of mirrors, including metal coatings and reflective cups, have been used in LED fabrication in order to utilize light emission from the backside of the device and create a more directional emission [1,2,3]. A bottom epitaixally grown distributed Bragg reflector (DBR) provides a compact solution to enhanced light extraction and when combined with a top-side mirror offers an attractive approach to resonant cavity LEDs (RCLEDs) [4]. RCLEDs typically exhibit narrow spectral linewidth, better emission directionality, and enhanced light extraction efficiency when compared to conventional LEDs [5,6]. In this work, visible wavelength LEDs have been integrated with III-Nitride based DBRs grown via Metal Organic Vapor Phase Epitaxy (MOVPE) on sapphire substrates. High reflectivity, crack-free AlN/GaN DBRs have been used as the bottom mirror under InxGa1xN/GaN multi-quantum-well LED structures designed for 440 nm emission. Angle-resolved electroluminescence results show light emission is reflected from the DBR substrate used as bottom mirror. Additionally, enhanced optical emission has been measured using image processing technique from the top of the structure as compared to LEDs grown without DBRs. Key design parameters of the LED structure will be presented along with device characterization including EL, IV, and image processing analysis.
EXPERIMENT InxGa1-xN based LED structures were grown on c
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