Vacancy structures on the GaN(0001) surface
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Vacancy structures on the GaN(0001) surface William E. Packard and John D. Dow Department of Physics and Astronomy, Arizona State University, Tempe, Arizona 85287-1504
Kathleen Doverspikea) and Ray Kaplan Naval Research Laboratory, 4555 Overlook Avenue, S.W., Washington, DC 20375
Ruth Nicolaides Institute for Postdoctoral Studies, 1842 Estate Enighed, Cruz Bay, St. John, U.S. Virgin Islands 00830 (Received 27 December 1995; accepted 24 September 1996)
Scanning tunneling microscopy images are reported for the wurtzite GaN(0001) surface. Terraces are observed, with three kinds of defect structures that are assigned to ordered N-vacancies: (i) striations perpendicular to the step edges, (ii) row defects ˚ that intersect the steps at an angle of 30±, and (iii) “oval” defects spaced about 16 A that result from intersections of lines of vacancies (oriented at 60± with respect to step edges) with the row defects.
I. INTRODUCTION
Although GaN is becoming increasingly important for applications in short-wavelength optical light emitters, its surface structure is not well understood presently. In this paper, we report the results of scanning tunneling microscopy studies of the wurtzite GaN(0001) surface, which reveal terraces separated by single atomic-bi-layer steps. The terraces are not smooth, but exhibit considerable “disorder” with regular features, including (i) striations perpendicular to the terrace edges, (ii) closely spaced rows of missing atoms on the terraces, and (iii) “oval” structures along these rows. Here we report images of these features, together with models of them in terms of N-vacancy aggregates. To our knowledge, these are the first ultrahigh vacuum (UHV) scanning tunneling microscopy (STM) images of the GaN(0001) surface reported.
III. RESULTS
II. EXPERIMENTAL ELEMENTS
The GaN films used in these experiments were grown at 1000 ±C on (0001) sapphire substrates by low-pressure, metal-organic chemical vapor deposition (MOCVD), and were approximately 3 mm thick. Prior to deposition of the GaN, an AlN buffer layer, approxi˚ thick, was deposited at 450 ±C. Details of mately 200 A the growth scheme are presented elsewhere.1 Kahn et al.2 studied similar films, using low-energy electron diffraction (LEED) and Auger spectroscopy, and obtained spectra which clearly favor the conclusion that the GaN(0001) surface is terminated by N atoms; we assume this identification in our own analyses. a)
Present address: Cree Research, Inc., 2810 Meridian Parkway, Durham, North Carolina 27713.
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We imaged our films in ultrahigh vacuum with a scanning tunneling microscope. The original, as-received surfaces showed neither terraces nor order when imaged, and so we cleaned them in UHV by heating the sapphire substrate to ,900 ±C in 10 s increments, for aggregate periods up to several minutes—after which terraces, separated by single atomic-bi-layers, formed on the surface. An increase of surface temperature was achieved by ohmically heating
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