Control of threading dislocation density at the initial growth stage of AlN on c-sapphire in plasma-assisted MBE

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Crystalline properties of (1–2)-μm-thick AlN buffer layers grown by plasma-assisted molecular-beam epitaxy (PA MBE) on c-Al2O3 substrates with different AlN nucleation layers have been studied. The best quality layers are obtained on 50-nm-thick nucleation AlN layers grown by a migration enhanced epitaxy (MEE) at substrate temperature of 780°C. In this case the buffer layers possess the lowest FWHM values of the symmetric AlN(0002) and skew symmetric AlN(10–15) x-ray rocking curve peaks of 469 and 1025arcsec, respectively, which correspond to the screw and edge threading dislocation densities of 4.7×108cm−2 and 5.9×109cm−2. This improvement seems to be related with the larger diameter of the flat-top grains in the AlN nucleation layers grown in the MEE mode at high substrate temperatures.