Enhanced piezoelectric response of AlN via alloying of transitional metals, and influence of type and distribution of transition metals

Aluminum nitride (AlN) is an important piezoelectric material for a wide range of applications, but its low piezoelectric coefficient is the main problem it faces. Many efforts are devoted to improving its piezoelectric response by alloying transition metals (TMs). On the basis of the density functional theory, the influence of the type and distribution of TM on the piezoelectric response is discussed in this paper. TM0.0625Al0.9375N with twenty-eight different TMs are investigated, and most show higher values of piezoelectric strain modulus d33 than that of AlN. This is because the TM introduces weaker TM-N bonds and locates closer to the centre of three neighbouring N atoms. The location of TM is determined to be significantly correlated with its group number. Alloys of TMxAl1_xN (TM=Sc, Cr, Sr, Mo, Ru and Rh) with varying x are further studied. On the basis of the cost of the TMs and piezoelectric performances, the alloy with Mo is more effective in enhancing d33. A high d33 of 12.3 times that of pure AlN is realized in a metastable configuration of Mo0.167Al0.833N. The distribution of Mo plays a key role in the piezoelectric performance. A higher d33 is more likely to appear in MoxAl1_xN with more Al sublayers containing Mo atoms and with fewer dimers of Mo atoms along the z-axis.