Chapter 4
Autocompensated Surface Structure of GaN Film on Sapphire 1
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Downloaded by NORTH CAROLINA STATE UNIV on October 4, 2012 | http://pubs.acs.org Publication Date: December 15, 1997 | doi: 10.1021/bk-1998-0681.ch004
M . M . Sung , J. Ahn , V. Bykov , D. D. Koleske , A. E. Wickenden , and J. W. Rabalais 1,3
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Department of Chemistry, University of Houston, Houston, TX 77204-5641 Naval Research Laboratory, Code 6861, 4555 Overlook Avenue, SW, Washington, DC 20375-5347 2
The surface composition and structure of a GaN film on sapphire has been determined through the use of time-of-flight scattering and recoiling spectrometry (TOF-SARS), classical ion trajectory simulations, low energy electron diffraction (LEED), and thermal decomposition mass spectrometry (MS). Elastic recoil detection (ERD) was used to determine the bulk hydrogen concentration. The totality of this data leads to the conclusions that the(1x1)surface is not reconstructed, that it is terminated in a Ν layer, and that Ga comprises the 2nd-layer. Hydrogen atoms are bound to 3/4 of the Ν atoms in the outerlayer and protrude outward from the surface, facilitating autocompensation of the otherwise unstable (1x1) structure. The group III nitrides (1,3), specifically GaN, have a large cohesive energy compared to group III phosphides and arsinides. This arises from the high electronegativity of nitrogen and the partial ionic character of the Ga-N bonds (4). The increased cohesive bond energy of the nitrides results in the wide band-gap emission which is observed in GaN based diodes (5,6) and lasers (7). The nitrides are also chemically stable, making them attractive for high power and high temperature device applications (1-3). To the best of our knowledge, there are no experimental surface structure measurements on any surface of GaN to date. Understanding the GaN surface structure will provide a starting point for determining how precursor molecules decompose leading to the growth of GaN. Also, the success of GaN electronic devices is dependent on the atomic level control of the elemental compositions and structures involved. One simple question is to what extent does the GaN {0001} surface reconstruct or relax, and i f so what are the possible surface structures. In this paper the surface and bulk composition, termination layer, and structure of a GaN{0001}-(lxl) film grown on a sapphire wafer are investigated. The techniques of time-of-flight scattering and recoiling spectrometry (TOF-SARS), 3
Corresponding author
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© 1998 American Chemical Society
In Synthesis and Characterization of Advanced Materials; Serio, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.
4. SUNG ET AL.
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Surface Structure of GaN Film on Sapphire
Downloaded by NORTH CAROLINA STATE UNIV on October 4, 2012 | http://pubs.acs.org Publication Date: December 15, 1997 | doi: 10.1021/bk-1998-0681.ch004
classical ion trajectory simulations, low energy electron diffraction (LEED), and thermal decomposition mass spectrometry (MS) were used in the surface analysis. The bulk hydrogen concentration was determined by elastic recoil detection (ERD). The combination of these techniques allows characterization of the elemental composition in the outermost two atomic layers, the element which constitutes the surface termination layer, the surface symmetry, and possible reconstructions or relaxations. In addition, TOF-SARS has high sensitivity to surface hydrogen and allows one to probe the involvement of hydrogen in the surface structure. Experimental Methods GaN Sample. GaN was grown on the c plane of polished sapphire using the N R L facilities. This consisted of a vertical, inductively heated, water-cooled quartz O M V P E reactor at reduced pressure (57 torr) using H as the carrier gas (
