X-Ray Photoemission Spectromicroscopy of Gan and AIGan
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Y. KIM #, Y. CHO #, C. KISIELOWSKI #, J. KRUEGER #, AND E. R. WEBER # *Center of X-ray Lithography, University of Wisconsin, Madison, WI "**Center of X-ray Optics, Lawrence Berkeley Laboratory, Berkeley, CA #Department of Material Science and Mineral Engineering, University of California, Berkeley, CA. ABSTRACT We investigate here for the first time GaN and AlGaN films by using x-ray photoemission spectromicroscopy. As compared to conventional x-ray photoemission spectroscopy (XPS), spectromicroscopy can provide spatially resolved information on the chemical composition of the sample surface. The experimental results where obtained by using MAXIMUM, a scanning photoemission microscope installed on 12.0 undulator beamline at the Advanced Light Source (ALS), Berkelely, allowing for a spatial resolution of 100 nm. We investigate here GaN and AlGaN thin films grown on sapphire substrate by metalorganic chemical vapor deposition (MOCVD). The results clearly indicate the great potential of spectromicroscopy in investigating chemical inhomogeneity, inpurities and localization in GaN and AlGaN thin films.
Figure 1: Secondary electron X-ray photoemission micrograph of AlGaN thin film (sample A). The field is 60 gim by 30 gtm. The kinetic energy is E.=5 eV. This image clearly shows the surface morphology of the sample, indicating a mean grain size of about 2 um, in agreement with other atomic force microscope results.
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Mat. Res. Soc. Symp. Proc. Vol. 512 ©1998 Materials Research Society
INTRODUCTION Gallium nitride and related wide band gap semiconductors are an important class of electronic materials because of potential use in optoelectronic devices operating in the blue range [1]. As a consequence, many efforts have been devoted to investigate the electronic structure of such nitrides in the last few years, also by means of x-ray photoemission spectroscopy (XPS) [2]. Among other results, these investigations indicated a substantial band bending due to Fermi level pinning at the sample surface, related to intrinsic localized surface states [3].
Figure 2: Close-up of the morphological structure of an AlGaN film. The field is 12 jim by 12 gim. The kinetic energy is E.=5 eV. The grains show a fine structure, related to their crystalline orientation (arrows). The inset shows a AFM image with sam field on the same sample.
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Although XPS has been widely used to study GaN and related compounds, no investigation using spectromicroscopy has been reported up to now, according to our knowledge. As compared to conventional XPS, spectromicroscopy [4] can provide spatially resolved information on the chemical composition of the sample surface, as well as standard morphological and chemical analysis. Hence, this technique provides a deeper insight in the puzzling microstructure of these compounds, characterized by lateral inhomogeneity and by dislocation densities several order of magnitude above those in other semiconductors. We investigate here for the first time GaN and AIGaN films by using x-ray photoemission spectromic
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