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RHEED images of nitrided sapphire and nucleation layers A and B. The N-rich layer, A, showed spots in the RHEED pattern, indicating a 3D growth mode. Growth of AlN using (layer B) gave a streaky pattern (two-dimensional growth) for layers thicker than 7 nm.
AFM images ( scale 5 nm) of AlN nucleation layers grown for 3 min with different flux ratios . Included in the figure is a line scan across each surface, showing typical height variations. Insets show magnification demonstrating the difference in nucleation layer morphologies.
Simulated (dotted) and measured (full) XRD response of the AlN(0002) reflection from the nucleation layers A and B. Using the peak positions and thickness fringes, the layer thickness and relaxation could be determined by simulations. The vertical line indicates the value for relaxed AlN.
AFM images of AlN buffer layers grown for 1 h on top of A and B nucleation layers. The flux ratios during nucleation layer and buffer layer growth are indicated below each image. Insets show magnification of interesting features of each surface. The surface in (a) ( scale 20 nm) has a uniform, column-like morphology. The surface of (b) ( scale 20 nm) is smooth with shallow pits (, pits excluded). The film in (c) ( scale 40 nm) is penetrated by deep holes, but is otherwise smooth (, holes excluded). Inset shows how a large hole has formed from merging of several smaller.
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