Properties of GaN epilayers grown on misoriented sapphire substrates
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Internet Journal Nitride Semiconductor Research
Properties of GaN epilayers grown on misoriented sapphire substrates Carol Trager-Cowan 1, S. McArthur1, P. G. Middleton1, K. P. O'Donnell1, D. Zubia2 and S. D. Hersee2 1Department 2CHTM,
of Physics and Applied Physics, University of Strathclyde, University of New Mexico, Albuquerque,
(Received Friday, June 19, 1998; accepted Monday, October 12, 1998)
Three silicon-doped 3 µm thick GaN epilayers were grown simultaneously by metalorganic chemical vapour deposition on (0001) sapphire substrates misorientated by 0°, 4° and 10° toward the m-plane (1010). A comparative study of these epilayers was undertaken using photoluminescence (PL) spectroscopy, atomic force microscopy (AFM), scanning electron microscopy (SEM), cathodoluminescence (CL) imaging, CL spectroscopy and Hall effect measurements. Low temperature PL of the 0° and 4° epilayers shows donor bound exciton (BE) emission between 3.47 and 3.48 eV and a low level of yellow band emission. The peak intensities of both emission bands are a factor of 2 higher for the 4° layer. In the 10° epilayer, the BE band is 3x stronger than in the 0° epilayer but there is no discernible yellow band. However, a number of additional bands appear at 3.459, 3.417, 3.362, 3.345, 3.309, and 3.285 eV. These bands, some of which are acceptor related, may be attributed to the presence of structural defects in this epilayer, pointing to an abrupt degradation of its structural quality compared to the others. This degradation is confirmed by AFM studies. On a 20 µm x 20 µm image the 0° and 4° epilayers exhibit smooth surface morphologies, while the 10° epilayer shows a high density of hexagonal pits. Finally, SEM images reveal the surface of the 10° epilayer to be “streaked” and pitted. Low temperature CL images at 3.48 eV (bound exciton region) show random spotty emission, while those at 3.28 eV and 3.41 eV exhibit a streaky appearance similar to the SEM image. This suggests that these luminescence bands are indeed associated with structural defects.
1 Introduction Since the first GaN epilayers were grown on sapphire substrates in 1971 [1], various techniques have been applied to optimise the quality of the resulting films, such as nitridation [2] of the substrate, or the growth of a low temperature AlN [3] or GaN buffer layer [4]. In this paper we will describe comparative morphological and low temperature luminescence studies of silicon doped GaN epilayers grown on misoriented sapphire substrates. In a number of material systems, growth on misoriented substrates has been shown to improve epitaxial growth. The crystallinity of epilayers may be improved by relieving strain where the epilayer and substrate are mismatched [5], the growth rate may be increased [6] as can the dopant concentration [7]. Previous studies of GaN epilayers grown on misoriented substrates include those reported by Hiramatsu et al [8], Grudowski et al [9] and Ishibashi et al [10]. However,
to our knowledge, this is the first report of low temperature luminescence s
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