Epitaxial growth of AlN films via plasma-assisted atomic layer epitaxy

Thin AlN layers were grown at 200-650 degrees C by plasma assisted atomic layer epitaxy (PA-ALE) simultaneously on Si(111), sapphire (11 (2) over bar0), and GaN/sapphire substrates. The AlN growth on Si(111) is self-limited for trimethyaluminum (TMA) pulse of length > 0.04 s, using a 10 s purge. However, the AlN nucleation on GaN/sapphire is non-uniform and has a bimodal island size distribution for TMA pulse of <= 0.03 s. The growth rate (GR) remains almost constant for T-g between 300 and 400 degrees C indicating ALE mode at those temperatures. The GR is increased by 20% at T-g = 500 degrees C. Spectroscopic ellipsometry (SE) measurement shows that the ALE AlN layers grown at T-g <= 400 degrees C have no clear band edge related features, however, the theoretically estimated band gap of 6.2 eV was measured for AlN grown at T-g >= 500 degrees C. X-ray diffraction measurements on 37 nm thick AlN films grown at optimized growth conditions (T-g = 500 degrees C, 10 s purge, 0.06 s TMA pulse) reveal that the ALE AlN on GaN/sapphire is (0002) oriented with rocking curve full width at the half maximum (FWHM) of 670 arc sec. Epitaxial growth of crystalline AlN layers by PA-ALE at low temperatures broadens application of the material in the technologies that require large area conformal growth at low temperatures with thickness control at the atomic scale.