AVS 45th International Symposium
    Electronic Materials and Processing Division Wednesday Sessions
       Session EM2-WeA

Paper EM2-WeA5
Structural and Morphological Studies of GaN Heteroepitaxy on SiC(0001)

Wednesday, November 4, 1998, 3:20 pm, Room 316

Session: Application of Scanning Probes to Electronic Materials
Presenter: V. Ramachandran, Carnegie Mellon University
Authors: V. Ramachandran, Carnegie Mellon University
A.R. Smith, Carnegie Mellon University
R.M. Feenstra, Carnegie Mellon University
D.W. Greve, Carnegie Mellon University
Correspondent: Click to Email
Heteroepitaxial growth of GaN on SiC has been studied using scanning tunneling microscopy (STM), atomic force microscopy (AFM) and high resolution x-ray diffraction. Growth was performed by molecular beam epitaxy (MBE) on hydrogen etched SiC(0001) substrates that show a @sr@3x@sr@3 R-30° reconstruction. Significant differences are seen in the growth morphology in different growth temperature regimes, both at low and high film thicknesses. At a coverage of a few monolayers, STM observation of GaN grown at temperatures around 550°C shows layer-by-layer growth. Terraces show sub-Å corrugation corresponding to the formation of misfit dislocations at the heterointerface, indicating that the film has relaxed. Films grown at around 650°C with a similar thickness show columnar, flat-topped islands of uneven heights with deep crevices between them. This difference in morphology may be understood in terms of the different strain relaxation mechanisms in the two temperature regimes. At higher temperature, films prefer to relieve strain by forming 3-D islands, which can be distorted to relieve strain. Alternatively, at lower temperature, the films remain pseudomorphic and grow in a layer-by-layer manner both before and after the formation of misfit dislocations. AFM measurements also show remarkable morphological differences between thicker (200 nm) films grown in the two temperature regimes mentioned above. Films grown at 550°C show a large number of spiral growth fronts while films grown at 650°C show a stepped layer-by-layer structure. Based on the growth model outlined above, it is apparent that in the low temperature 2-D growth mode, misfit dislocations at the interface are associated with screw dislocations which extend upto the surface and act as a source for the spiral growth. In 3-D growth, misfit dislocations can form at the edges of islands without the need for as many screw dislocations. X-ray rocking curve measurements on GaN films 200-400 nm thick show decreasing FWHM with increasing growth temperature. This would agree with the above model, where, as the growth temperature rises, the film requires fewer screw dislocations in order to create the necessary number of misfit dislocations at the interface.