Solid state conversion of aluminum thin films to single crystal sapphire
Jeff Biser, Sreya Dutta, Jason Perkins, Jon Regina, Hyoungjoon Park, Hongwei Li,
Richard P. Vinci, and Helen M. Chan
Department of Materials Science and Engineering, Lehigh University
Introduction
Sapphire (single crystal alumina) is used for a variety of optics applications, from windows for supermarket scanners to substrates for growth of solid state lasers. Preparation of the sapphire surface ordinarily requires extensive grinding and polishing. An alternative that we have explored is the direct conversion of a rough sapphire surface to a smooth surface. A smooth and defect free surface is a basic necessity for IR windows, and for devices such as LEDs and laser diodes. The process of generating a high quality surface consists of 3 stages, Aluminum deposition, Growth of Oxide, and Grain growth (AGOG), as follows:
1. Sputter deposition creates a thin coating of polycrystalline aluminum that conforms to the surface defects.
2. Annealing at low temperatures forms a polycrystalline aluminum oxide layer on top of the aluminum.
3. Annealing at high temperature causes Solid State Conversion, whereby the aluminum layer is fully oxidized, surface defects are smoothed out, and the single crystal substrate consumes the grains in the surface oxide layer to form a single crystal.
Our fundamental research work is aimed at understanding the underlying mechanisms for the AGOG process. In a related project, in collaboration with Prof. Nelson Tansu and his group in ECE, we are working to extend the process to create patterned sapphire surfaces for improved growth of wide bandgap compounds such as GaN. Improvements in GaN device cost and performance are needed for wide commercialization of LED solid state lighting.
Sample Images
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A GaN layer grown on sapphire is shown in this SEM image. The substrate
beneath the two blocks on the left side was patterned with sapphire nanodots
using the AGOG process. The substrate on the right side was left in the
normal flat condition. The GaN was grown using our Abbreviated Growth Mode
(AGM) technique that reduces processing time while increasing material
quality. It can be seen in the image that the AGM GaN on the patterned region
is excellent while growth on the planar side is poor. (From reference 8
below.) |
Goals
- Understand the fundamental physics underlying the AGOG process.
- Extend the AGOG process to nano-patterned sapphire surfaces for improved growth of GaN and other wide bandgap materials.
Findings
- The AGOG process has been successfully demonstrated on 2-inch sapphire substrates.
- Single crystal "mesa" structures 100 nm high x 100-400 nm square have been fabricated using the process.
- GaN growth on AGOG patterned sapphire can lead to significant improvements in LED efficiency.
Related Publications
- H. Park, H.M. Chan and R.P. Vinci, “Patterning of Sapphire Substrates Via a Solid State Conversion Process,” J. Mater. Res., 20, 2005, 417-23.
- J.M. Biser, J.T. Perkins, H. Li, H.M. Chan, R.P. Vinci, Fabrication and morphological stability of aluminum nanostructures en route to nanopatterned sapphire, Advances in Science and Technology, 45, 2006, 945-50.
- S. Dutta, H.M. Chan, and R.P. Vinci, Sub-surface Oxidation at the Aluminum-Sapphire Interface during Low Temperature Annealing, J Am Cer Soc 90(8), 2007, 2571-75.
- Y. K. Ee, R. A. Arif, N. Tansu, H. Li, H. M. Chan, R. P. Vinci, P. Capek, N. K. Jha, and V. Dierolf, Improved Photoluminescence of InGaN Quantum Wells Grown on Nano-Patterned AGOG Sapphire Substrate by Metalorganic Vapor Phase Epitaxy, in Proc. of the 20th IEEE Laser and Electro-Optics Society (LEOS) Annual Meeting 2007, Lake Buena Vista, FL, October 2007, pp. 902-903.
- H. Li, J. M. Biser, J. T. Perkins, S. Dutta, R. P. Vinci, and H. M. Chan, Thermal Stability of Cu Nanowires on a Sapphire Substrate, J Appl Phys 103(2), 2008, 024315-024323.
- D. A. Browne, H. Li, E. Giorgi, S. Dutta, J. M. Biser, R. P. Vinci, and H. M. Chan, Templated Epitaxial Coatings on Magnesium Aluminate Spinel using the Sol-Gel Method, J. Mater Sci, 44(5), 2009, 1180–1186.
- Y. K. Ee, J. Biser, W. Cao, H. M. Chan, R. P. Vinci, and N. Tansu, Growths of InGaN Quantum Wells Light-Emitting Diodes on Nano-Patterned AGOG Sapphire Substrate Using Abbreviated Growth Mode, in Proc. of the IEEE/OSA Conference on Lasers and Electro-Optics (CLEO) 2009, Baltimore, MD, May 2009.
- Y. K. Ee, J. Biser, W. Cao, H. M. Chan, R. P. Vinci, and N. Tansu, Metalorganic Vapor Phase Epitaxy of III-Nitride Light-Emitting Diodes on Nano-Patterned AGOG Sapphire Substrate by Abbreviated Growth Mode, IEEE J. Selected Topics in Quantum Electronics 15(4), 2009, 1066-1072.
Support for this work has been provided by:
- NSF DMR-0705299
- NSF DMR-0211078
- the Pennsylvania DCED (Department of Community and Economic Development)
- the U.S. Army Research Office and U.S. Army Research Laboratory, Cooperative Agreement Number DAAD19-02-2-0030.
The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressed or implied, of the Army Research Office, Army Research Laboratory, or the U.S. Government.
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Last updated: October 29, 2009