The Investigation of InGaN MQW Electroabsorption Modulator using the LED/Modulator/Detector Monolithically Integrated St
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The Investigation of InGaN MQW Electroabsorption Modulator using the LED/Modulator/Detector Monolithically Integrated Structure S.W. Chung, Y.S. Zhao, C.H. Lin, and H.P. Lee Department of Electrical and Computer Engineering, University of California, Irvine CA 92697 ABSTRACT The strong piezoelectric effect and quantum confined stark effect (QCSE) in the InGaN/GaN quantum well structures allow one to modify the free exciton absorption by the extrinsic field. The QCSE is investigated using a monolithically integrated three-section device comprising an LED, electroabsorption modulator and a detector section. The experimental results show that the LED output can be modulated as indicated by the detector signal. The strength of modulation decreases monotonically with increasing In composition in the InGaN/GaN MQW. The result can be explained on the basis of the Stokes’ shift between the emission and absorption spectra in the InGaN/GaN QW structure, and a blue shift of the absorption spectrum due to the QCSE as a result of the piezoelectric effect. INTRODUCTION Recent advances in the epitaxial growth of high quality GaN-based materials have led to an enormous activity in the study of these materials and commercialization of GaN-based devices including high brightness blue/green light emitting diodes (LEDs), lasers diodes and UV detectors. Since quantum wells (QW) structures lie at the heart of GaN–based optical devices, there is considerable interest in understanding and characterizing the physical properties of InGaN/GaN QW system, such as excitonic emission properties [1], the phase segregation during the crystal growth [2], the emission mechanism [3], the strain-induced piezoelectric field [4], the quantum confined stark effect (QCSE) [5], the electro-optic effect [6], and the Stokes shift [7]. QCSE is of particular interest for device application because it is the operating principle for electroabsorption modulators. The feasibility of the InGaN/GaN QW electro-optic modulator and electroabsorption modulator has been discussed theoretically in recent publication [6]. GaNbased modulators integratable with light sources such as lasers or LEDs are highly attractive for applications in high-speed high-density optical storage and environmental/chemical sensing. To our knowledge, however, there has been no report of such devices in the GaN-based material systems. In this article, we report on the investigation of the QCSE in InxGa1-xN/GaN MQW electroabsorption modulators monolithically integrated with a LED and a detector. Using this structure, we have demonstrated electroabsorption modulation of the LED light due to the blue shift of the QCSE. By increasing the In composition in the InxGa1-xN/GaN MQW, we observed a monotonic decrease of modulation efficiency. The results can be explained by the dependence of the Stokes’ shift on the peak emission energy of the InGaN/GaN MQW system. DEVICE STRUCTURE AND FABRICATION PROCESS The samples used in this work are commercial-graded InGaN/GaN MQW LED samples. The epi-materials are gr
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