Hexagonal AlN thin films have been deposited by DC reactive magnetron
sputtering at room temperature. For a first set of samples, sputtered AlN
films were deposited on silicon Si (100) substrates. For a second set, AlN
films were deposited on 200 nm (002) oriented AlN epitaxial layer obtained
by Molecular Beam Epitaxy (MBE) on Si (111).
X-ray Diffraction (XRD) and High Resolution Transmission Electron Microscopy
(HRTEM) analysis of the synthesized films on Si (100) substrate have shown
an amorphous phase close to the interface followed by a nano-crystalline
layer exhibiting (100) and (002) orientations of the hexagonal AlN
crystalline phase. Finally a relatively well crystallised layer with a
single (002) orientation has been observed for the thickest films. This
improvement of crystalline quality with film thickness has been consistent
with a drastic decrease of the films stress from –1.2 GPa at 300 nm to no
stress around 800 nm and even 0.3 GPa tensile stress for 1.5 μm thick
film. This behaviour was different when epitaxial AlN was used as substrate.
In fact, we have observed thanks to HRTEM images and Selected Area Electron
Diffraction (SAED) patterns, that the AlN film deposited on such a substrate
exhibits the same crystalline quality and have the same orientation as the
AlN epitaxial layer during the first 500 nm of thickness. A further increase
of film thickness has caused a decrease on the crystalline quality. The
films became polycrystalline while preserving a (002) preferential
orientation.