ICMCTF2010 Session D1: Boron Nitride, Carbon Nitride-Based Multifunctional Materials and Group-III (Al, Ga, In) Nitride Materials

The ternary compounds of the B-C-N phase diagram have attracted much attention because it is potentially possible to combine unique properties of different compounds into one and, synthesize new ones. Hence, there is still much effort to synthesize crystalline ternary compounds of B-C-N. However, deposited films were mostly either amorphous or a mixture of h-BN and C/CN_x phases, even though there were numerous claims of hybrid films. In this study, in an effort to investigate chemical bonding and better document phase separation in B-C-N films, we have deposited h-BN, a-B₄C and CN_x thin films using reactive magnetron sputtering on Si (100) surfaces. Then, these films were subjected to ion-implantation by 40 keV C⁺, N₂⁺ and N⁺ ions. After implantation, chemical bonding in these films were investigated using X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FT-IR) spectroscopy. Local variations in structure for these films were also investigated by high-resolution transmission electron microscopy (HR-TEM ) and electron energy loss spectroscopy (EELS) was employed to investigate chemical environment of boron, carbon and nitrogen in these films. Similar data gathered by XPS, FT-IR and TEM studies on sputter deposited B-C-N films were comparatively examined, as well. Results from these studies indicated boron-nitrogen bond to be preferred over carbon-nitrogen or boron-carbon over a wide range of process parameters for B-C-N films. Hence, carbon tends to phase-segregate in to carbon clusters rather than displaying a homogeneous distribution in the both the implanted and deposited films.

During the last two decades, carbon-nitride (CN_x) coatings have enjoyed a growing interest in several disciplines. Amorphous and fullerene-like films exhibit very attractive properties, such as low friction and wear, high hardness, good chemical stability, and high resiliency to deformation, which make them suitable for replacing diamond-like carbon (DLC) films. The first successful industrial application of this material has been the use of very thin (~2 nm) amorphous CN_x films for the protection of hard disk drives. However, commercial use of thicker (1-5 µm) coatings on steel substrates has been difficult because of the development of high compressive intrinsic stresses during deposition, which causes delamination unless the coating/substrate adhesion can be improved.

Piezoelectric aluminum nitride (AlN) thin films were synthesized by reactive sputtering of Al metal target in different nitrogen-argon gas ratio atmosphere using a pulsed closed field unbalanced magnetron sputtering system on various substrates with thin Cr electrodes. The texture, orientation and the microstructure of AlN films were characterized by means of X-ray diffraction and scanning electron microscopy. The mechanical properties of the coatings were studied using the XP nanoindenter. Impedance analyzer was used to characterize the piezoelectric properties of these films. It was found that reactive gas ratio, the pulsing frequency and the working pressure significantly affected the (002) orientation in AlN films. Strong (002) orientation has been achieved in the AlN thin films under various pulsing conditions, which can be correlated to the high ion energies associated with the pulsed magnetron sputtering.

Growth of GaN by magnetron sputter epitaxy (MSE) has the potential to be used for high quality single crystalline epi-layer synthesis over large substrates at low temperatures. Development of growth stresses is a known issue in GaN epitaxy affect the material properties as well as devices performance and hence need to be controlled.

In-situ stress evolution during growth of GaN(0001) epilayers on (0001)-Al₂O₃ substrates has been studied during reactive MSE growth of 900 nm thick layers at different growth temperatures. A liquid Ga target was used and a working gas mixture of ultra pure Ar and N₂ held at 2.5 and 2 mTorr partial pressures, respectively. The in-situ stress measurements were made by measuring the curvature induced to the substrate by the stress exerted by the growing film. The equipment used was a multi beam optical stress sensor (MOS), based on the deflection of multiple laser beams. The crystalline quality of the films was studied by using x-ray diffraction techniques.

At 750°C , we observed a continuous compressive stress evolution during the film growth, reaching a total film stress of ~200 MPa at the completion of the growth. The FWHM of HR-XRD rocking curves for these films are ~720 arc sec. which is indicates a high crystalline quality. In contrast to this result, a tensile stress of ~150MPa was observed in a film growing at 500°C. This film also has a comparatively low crystalline quality with a rocking curve FWHM of 1200 arc sec. After cooling down to room temperature all films are under compression due to the differences in thermal expansion coefficients between GaN and sapphire.

Our results are in contrast to in-situ stress studies on MOCVD-grown GaN(0001)/Al₂O₃(0001) where a tensile stress evolution to 200-400 MPa was observed during growth at 1000 °C. We attribute the origin of the compressive stresses in our films grown at 750 °C to atomic peening by energetic particle bombardment and/or coherency stresses due to a compressive substrate lattice mismatch. At 500°C, we conclude that tensile stresses due to cohesion over grain boundaries and annihilation of defects (caused by low adatom mobility) dominate over the effect of atomic peening.