Micro-Photoluminescence from V-shape inverted pyramid in HVPE Grown GaN Film
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Micro-Photoluminescence from V-shape inverted pyramid in HVPE Grown GaN Film Chao-Kuei Lee, Y-B Chen, Shu-Chen Chang, Ci-Ling Pan, and S. C. Wang Institute of Electro-Optical Engineering, National Chiao Tung University 1001 Ta Hsueh Road, Hsinchu, TAIWAN 300 ABSTRACT We report the room-temperature micro-luminescence images from V-shaped inverted pyramids in undoped GaN films grown on (0001) sapphire substrate by hybrid vapor phase epitaxy. As the excitation laser spot at 325 nm was translated from the surface toward the center of the inverted pyramid along its slope, the center wavelength of the PL peak shows a trend of monotonic red-shift of from 373.9 nm to 379.1 nm. This could be attributed to the 3-dimensional release of stress and associated decrease of build-in piezoelectric field in the V-defects. A distinct and strong emission at 386.7 nm was observed at the apex of the V-defect. This could be originated from the threading dislocation at the onset of the defect. INTRODUCTION GaN is a wide band-gap semiconductor of important technological applications, such as short wavelength light-emitting diodes1), semiconductor lasers2-3), and other optical device.4) However, due to the lack of GaN substrate, the GaN epilayers generally grow on foreign substrate such as sapphire. As a result, the GaN epitaxial layer grown on the foreign substrate exhibits high dislocation density in the order of 1010 com-2 due to stress induced by lattice mismatch and the large difference in thermal expansion coefficient between GaN films and sapphire. Recently, a type of hexagonal defect structure with pyramid shape also named v-shaped defect due to its shape in the surface of the GaN material and InGaN/GaN MQW structure was observed and reported5-10]. These include the observation of the stimulated emission and the micro-cavity gallery mode from the hexagonal pyramids under optical pumping 5,6] and the blue-shift in emission spectrum from V-defect under optical excitation7]. There are various possible mechanisms reported related to the formation of the v-shape defects. The Romano’s group analyzed and suggested that the change of free energy from the strain release and stacking force through the pyramid are the main reason of the formation of the defect8]. Other based on the experiment reported the formation of the v-defect is initiated from threading dislocation(TD) and was considered to be due to the stress release by motion of the surface roughening5,6,7].The Kim’s group observed the inverted pyramid in the InGaN/GaN MQW which are partially formed at K1.2.1 Downloaded from https://www.cambridge.org/core. Columbia University Libraries, on 15 Aug 2017 at 11:48:25, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1557/PROC-722-K1.2
stacking mismatch boundaries induced by stacking faults rather than threading dislocation9]. Although the hexagonal GaN pyramids are not desired for GaN-based light emitting devices such as lasers, it has attracted a lot of attention due to its
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