ALN GAN SUPERLATTICES GROWN BY GAS SOURCE MOLECULAR-BEAM EPITAXY

AlN/GaN superlattices with layer thicknesses between 0.5 and 20 nm have been grown. The substrates were alpha(6H)-SiC(0001) and Al2O3(0001) (sapphire). The growth was performed using a modified gas source molecular beam epitaxy (MBE) technique. Standard effusion cells were used as sources of aluminum and gallium, and a small, MBE-compatible, electron cyclotron resonance plasma source was used to activate nitrogen gas prior to deposition. Auger, X-ray, and transmission electron microscopy studies confirmed the existence of well-defined layers. High resolution electron microscopy revealed pseudomorphic behavior between the two materials for layers thinner than 6 nm. By contrast, completely relaxed individual layers of GaN and AlN with respect to each other were present for bilayer periods above 20 nm. Cathodoluminescence showed a shift in the emission peak of up to 0.7 eV. The observed emission energy shifts were used to estimate the band discontinuities.