Scanning Electron Microscopy Cathodoluminescence Studies of Piezoelectric Fields in an InGaN Multiple Quantum Well Light
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E11.41.1
Scanning Electron Microscopy Cathodoluminescence Studies of Piezoelectric Fields in an InGaN Multiple Quantum Well Light Emitting Diode
Kristin L. Bunker, Roberto Garcia, and Phillip E. Russell Analytical Instrumentation Facility and Materials Science and Engineering Department, North Carolina State University, Box 7531, Raleigh, NC 27695 USA ABSTRACT Scanning Electron Microscopy (SEM)-based Cathodoluminescence (CL) experiments were used to study the influence of piezoelectric fields on the optical and electrical properties of a commercial InGaN-based Multiple Quantum Well (MQW) Light Emitting Diode (LED). The existence and direction of a piezoelectric field in the InGaN-based LED was determined with voltage dependent SEM-CL experiments. The CL emission peak showed a blueshift followed by a redshift with increasing reverse bias due to the full compensation of the piezoelectric field. It was determined that the piezoelectric field points in the [000-1] direction and the magnitude was estimated to be approximately 1.0±0.2 MV/cm. SEM-CL carrier generation density variation and electroluminescence experiments were used to confirm the existence of a piezoelectric field in the InGaN-based MQW LED.
INTRODUCTION The existence of piezoelectric fields in InGaN/GaN quantum well optoelectronic devices has attracted a lot of interest due to the influence of these fields on the optical and electrical properties of the devices. The investigation of the existence, direction, and magnitude of piezoelectric fields in InGaN-based quantum-well (QW) devices is significant for further improvement of device design, engineering, and performance. The origin of polarization in the III-V nitride material system is due to both a piezoelectric and spontaneous component and depends upon the crystal growth direction [1, 2]. The piezoelectric polarization results from lattice mismatch strain which, in the GaN/InxGa1-xN system, can approach 10%. Theoretical calculations have shown that the III-nitrides have piezoelectric constants that are approximately ten times larger than conventional III-V and II-VI compounds and fields on the order of MV/cm can be generated in strained InxGa1-xN layers [3, 4]. Theory predicts that in a compressively strained InxGa1-xN layer grown in the [0001] direction, the piezoelectric polarization will point in the [0001] direction [3, 4]. Theoretical results have also indicated that III-nitrides have a large spontaneous polarization that exists in the absence of any external influence including strain or an externally applied electric field [3, 4]. The spontaneous polarization component is due to the difference in spontaneous polarization between AlN, GaN, and InN. Calculations have shown that the spontaneous polarization charge for GaN, AlN, and InN, is -0.029C/m2, -0.081C/m2, and -0.032C/m2, respectively, and that a structure grown in the [0001] direction will have a spontaneous polarization oriented in the [0001] direction [3, 4]. Since there is only a 9% difference between the spontaneous polarization in G
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