Low-Temperature Cathodoluminescence Mapping of Green, Blue, and UV GaInN/GaN LED Dies
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Low-Temperature Cathodoluminescence Mapping of Green, Blue, and UV GaInN/GaN LED Dies Yong Xia1,2, Theeradetch Detchprohm1,2, Jayantha Senawiratne1,2, Yufeng Li1,2, Wei Zhao1,2, Mingwei Zhu1,2, and Christian Wetzel1,2 1 Future Chips Constellation, Rensselaser Polytechnic Institute, 110 8TH ST, Troy, NY, 12180 2 Department of Physics, Applied Physics, and Astronomy, Rensselaer Polytechnic Institute, 110 8TH ST, Troy, NY, 12180 ABSTRACT GaInN based light emitting diodes (LEDs) play an important role as energy efficient light sources in solid state lighting. A controversial discussion addresses the origin of lateral light emission variations and their correlation with either of the identified defects, e.g., threading dislocations and V-defects. In order to establish any possible correlation of defects and luminescence centers, we analyze three UV, blue and green LED dies by microscopic mapping of spectroscopic cathodoluminescence and secondary electrons at variable low temperature from 7 K to room temperature. Particular effort is being placed on a quantitative analysis of the luminescence signal. Image intensities are not being scaled and offset for highest contrast as otherwise typical for imaging mode. In standard configuration, we analyze image areas of (0.037 mm)2 with pixel resolution of 72 nm. Following regions of strong and weak emission we find that remain bright and dark respectively even at low temperature. Those variations increase with the mean emission wavelength of the LEDs and with temperature. The largest peak wavelength variation associated with the intensity contrast was observed in the green LEDs and amounts to 5 nm. Here the peak wavelength is higher in the dark spots than in the bright ones. This finding corresponds to the general trend when comparing the lower efficiency in longer wavelength green emitters to the blue ones.
INTRODUCTION GaN based semiconductors are promising materials to make green, blue and UV light emitting diodes (LEDs) and laser diodes (LDs) due to their suitable bandgap energies. The bandgap energies of InN, GaN and AlN are 0.7 eV, 3.4 eV and 6.0 eV, respectively.[1] By adjusting the alloy composition, the bandgap energy of their alloy nitride could be tuned from the near-infrared to the UV regime. The multi-quantum well (MQW) structure is usually used to confine the carriers in order to increase the light emission. Metal organic vapor phase epitaxy (MOVPE) and molecular beam epitaxy (MBE) are two main techniques to grow the MQW structure LEDs. Due to lacking of the ideal GaN substrate, threading dislocations (TDs) form during the hetero epitaxy growth. The typical density of TD could reach 109-1010 cm-2. TDs make the lateral light emission from the LED inhomogeneous. At the TD region the light emission decreases. However, there is a debate about the role of TDs for the lateral light emission variations.[2-3] Whether TDs are nonradiative recombination centers of carriers or carriers don’t recombine radiatively at the TD regions is still not clear. For this reas
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