Structural Characterization of Laser Lift-off GaN
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STRUCTURAL CHARACTERIZATION OF LASER LIFT-OFF GaN ERIC A. STACH,* M. KELSCH,*# W.S. WONG,"'1 E.C. NELSON,* T. SANDS" AND N.W. CHEUNG§ * National Center for Electron Microscopy, Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720: email: [email protected] ; http://ncem.lbl.gov t Department of Materials Science and Engineering, University of California, Berkeley, CA 94720; § Department of Electrical Engineering and Computer Science, University of California, Berkeley 94720. f On leave from the Max Plank Institute ftir Metallforschung, Stuttgart, Germany. f Present address: Xerox Palo Alto Research Center, Palo Alto, CA 94304. ABSTRACT Laser lift-off and bonding has been demonstrated as a viable route for the integration of IIlnitride opto-electronics with mainstream device technology. A critical remaining question is the structural and chemical quality of the layers following lift-off. In this paper, we present detailed structural and chemical characterization of both the epitaxial layer and the substrate using standard transmission electron microscopy techniques. Conventional diffraction contrast and high resolution electron microscopy indicate that the structural alteration of the material is limited to approximately the first 50 nm. Energy dispersive electron spectroscopy line profiles show that intermixing is also confined to similar thicknesses. These results indicate that laser lift-off of even thin layers is likely to result in materials suitable for device integration. Additionally, because the damage to the sapphire substrate is minimal, it should be possible to polish and re-use these substrates for subsequent heteroepitaxial growths, resulting in significant economic benefits. INTRODUCTION IH-nitride semiconductor alloys are promising materials for opto-electronic devices in the ultraviolet to blue/green spectrum. This is because the Ill-nitrides form a continuous alloy system with direct band gaps over the range of 1.9 eV (InN) to 3.4 eV (GaN) to 6.2 eV (AIN). This has resulted in the successful creation of blue and green laser diodes, as well as the full color spectrum of light emitting diodes. 1,2 However, because of the low decomposition temperature of GaN (on the order of 900 'C), significant problems remain in the growth of materials of high crystal quality. This is because this low decomposition temperature makes bulk crystal growth difficult using standard methods. Additionally, the dissociation of nitrogen from typical carrier gases used in metallorganic chemical vapor deposition (MOCVD) requires high temperatures that are often incompatible with growth on conventional substrates. As a result, the majority of III-nitride devices are grown heteroepitaxially onto either sapphire (single crystal (0001) A120 3), or less frequently, SiC. These two materials provide a hexagonal template for the growth of wurtzite GaN, and can easily withstand high crystal growth temperatures. However, both sapphire and SiC have electrical and thermal conductivity constraints which may limi
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