Abstract
(a) High Pressure Research Center, Polish Academy of Sciences, ul. Sokolowska 29,PL-01-141 Warsaw, Poland(b) Institute of Experimental Physics, University of Warsaw, Hoza 69,PL-00-681 Warsaw, Poland(c) Experimental Solid State Physics III, RIM, University of Nijmegen, Toernooiveld 1,NL-6525 ED Nijmegen, The Netherlands(Received July 4, 1999)Hexagonal hillocks are dominant surface defects in homoepitaxial GaN layers grown on N-polarGaN substrates. AFM observations of the flat-topped, hexagonal growth hillocks reveal a networkof concentric atomic steps of the height of one c-axis lattice constant (5.6 A). These steps areinterlaced and perfectly aligned perpendicular to h1––100idirections. Analysis of the step struc-ture in the center of hillocks reveals the nature of its core. Usually it consists of one or morescrew dislocations. Single screw and pairs of screw dislocations of opposite signs have been identi-fied.
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