High Resolution Tomographic HyperSpectral Imaging of (InGa)N Nanopyramid LEDs

Achieving high efficiency light emitting diodes (LED's) is an important aspect of our goal of optimizing energy efficiency in lighting systems. One approach to this problem is the development of III-nitride materials systems, where quantum wells with high InN content create highly efficient LEDs within the green/yellow portion of the visible spectrum. One of the road-blocks to creating such a device is the immiscibility of InN and GaN under standard conditions. In this work, InGaN LED's are grown through a porous dielectric mask, creating an array of nanorods capped with hexagonal pyramids. It was predicted that strain relief at the facets and apex of these pyramids will allow for the incorporation of a higher concentration of InN creating conditions for green/yellow emission. To characterize these structures, site specific analysis of the InN concentration is necessary at the nanoscale. To this end, engineered nano - heterostructures containing Indium were grown selectively by organometallic vapor phase epitaxy on top of GaN films at Purdue University and then studied by high resolution electron microscopy and microanalysis at Purdue as well as the ANL EM Center.


Click on the above to view the Reconstruction
. Note: This Tomographic movie requires the FREE QuickTime MultiMedia Player

The active (light emitting) region of the LEDs which form in this material was identified to exist principly at the apex of nanopyramids ( < 100 nm in size). An example of these pyramids are shown in the Scanning Transmission Electron Microscope (STEM) image in the figure above. In order to understand the role and location of Indium in these devices, high resolution hyperspectral imaging was used to elucidate the morphology and also to measure the elemental distribution of Gallium, Indium, and Nitrogen (Ga, In, N) within these complex heterostructures. Tomographic reconstruction was used to analyze and subsquesntly visualize the three dimensional distribuion of these elements the results of which are illustrated in the movie.

In this reconstruction, one can clearly visualize the outline of the truncated nanopyramid caps which have grown upon the seed GaN nanorods, within these pyramids. Using x-ray spectroscopy we find that a (In,Ga)N structure exists preferentially at the apex. Detailed inspection of hyperspectral images shown in the reconstruction identifies this area to consist of a solid, coherent (In,Ga)N hexagonal nanopyramids having a heights of ~ 20 nm. These nanopyramids forming as a quantum structures within the larger GaN pyramidal features.

Robert Colby, 1,2, Eric A. Stach,1,2, Nestor J. Zaluzec,3
1School of Materials Engineering, Purdue University, West Lafayette, Indiana 47906, USA
2Birck Nanotechnology Center, Purdue University, West Lafayette, Indiana 47906, USA
3Electron Microscopy Center, Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, USA

More detailed description of the material can be found in Wildeson I.H. etal J. Appl. Phys. 108, 044303 (2010); doi:10.1063/1.3466998

Acknowledgements
This material is based on work supported by the Department of Energy under Award No. DE-FC26-06NT42862. Portions of the electron microscopy were accomplished under proposal number 081113-02A at the Electron Microscopy Center, Argonne National Laboratory, a U.S. Department of Energy Office Science Laboratory operated under Contract No. DE-AC02-06CH11357 by UChicago Argonne, LLC.

The Electron Microscopy Center (EMC) is an Office of Science User Facility operated for the U.S. Department of Energy Office of Science by Argonne National Laboratory.