Characterization of indium gallium nitride led structures using electron microscopy

Robert Colby, Purdue University

Abstract

Lighting accounts for a significant portion of our energy consumption, but conversion to more efficient solid state lighting alternatives, namely light emitting diodes (LEDs), has been hampered by a number of technical and scientific hurdles. One of the foremost among these is the lack of an efficient green emitter, necessary for the development of color-mixing LEDs capable of producing warm white light efficiently, without the need for a phosphor down- conversion. (In,Ga)N, has been considered as a suitable candidate, having a range of direct bandgaps encompassing the visible spectrum for the various mixtures of InN and GaN. However, challenges confront the (In,Ga)N system, including lack of well-matched substrates for GaN, and the broad miscibility gap between GaN and InN. These concerns have been addressed by exploitation of nano-scale heteroepitaxy: specifically by growing GaN/(In,Ga)N-based LEDs through a nanoporous mask. With this approach, it is possible to grow individual nanorod LED structures by otherwise very conventional means, yet with negligibly low dislocation densities in the active region, and greater InN incorporation. Such systems were characterized through the use of transmission electron microscopy (TEM). Extensive TEM-based sampling and numerical modeling were employed to demonstrate that the nanorod geometry results in a better than two orders of magnitude reduction in the typically high density of threading dislocations of conventional GaN grown on sapphire. The structure of the pyramidal (In,Ga)N wells deposited on these essentially dislocation free nanorods were characterized as a function of attainable processing parameters, using a combination of TEM, scanning transmission electron microscopy (STEM) and energy dispersive spectroscopy (EDS). Optoelectronic measurements confirmed that the nanoscale geometry was effective at in incorporating higher than typical concentrations of InN, with preliminary devices emitting in the yellow- orange. Unexpected planar defects and phase transitions were observed and characterized as a function of processing conditions. TEM was used to demonstrate that new dislocations practically never nucleated within the pyramids—despite the encapsulated, strained (In,Ga)N layer and the frequent planar defects—confirming that dislocation-free nanoscale LED devices based on the nanorod design are a promising possibility.

Degree

Ph.D.

Advisors

Kvam, Purdue University.

Subject Area

Materials science

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