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The effects of reactive gas flow conditions on nonpolar AlN film growth on MnS/Si (100) substrates using reactive DC magnetron sputtering were investigated. During AlN deposition at a substrate temperature of 750 °C, the MnS surface can be unintentionally nitrided, resulting in a decrease in the crystallinity of the AlN. Low-temperature growth of the AlN layer at 300 °C prevents this nitridation and results in the crystallization of nonpolar AlN. A N2 flow equal to 30% of the Ar sputtering gas flow was found to improve the crystallinity of the nonpolar AlN and to reduce nitrogen defects, which play an important role in interfacial reactions. Nitrogen defects promote the formation of alloys such as AlMn and MnSi that degrade the interface and can significantly decompose the MnS. A higher proportion of N2 improves the nonpolar AlN crystallinity, reduces the concentration of defects and suppresses reactions at the AlN/MnS interface.

The effects of the N2 gas flow ratio and the substrate self-bias of reactive sputtering on the polarity of AlN film on various planar sapphire substrates were investigated to obtain a non-polar oriented AlN film. On a c-plane sapphire substrate, substrate self-bias enhanced c-plane AlN growth and Al termination at the surface, as confirmed by X-ray diffraction (XRD) and time-of-flight low-energy atom scattering spectroscopy. In addition, the AlN film deposited at an N2 gas:Ar sputtering gas ratio of 14% with substrate self-bias had high crystallinity with an XRD ω-rocking curve of 17 arcsec for the 0002 diffraction. In contrast, the AlN films on a- and r-plane sapphire substrates had semi-polar and non-polar plane orientations, respectively. However, on the a-plane sapphire substrate, the c-plane-orientated AlN phase was also observed, which was increased under substrate self-bias condition and an N2 flow rate below 30%. Preferable conditions for non-polar AlN growth by reactive sputtering were non-substrate self-bias and a high N2 flow rate of 30% or more.