Piezoelectricity, Phase Stability, and Surface Diffusion in Multicomponent Nitrides

The focus of the piezoelectricity research is on piezoelectric properties of both ordered and disordered nitrides. The exploration of disordered wurtzite nitrides revealed important aspects of the nitride alloying physics and the implications for their piezoelectric response, in addition to the discovery of interesting alloy candidates and their synthesis, e.g., YxIn1-xN. For the ordered nitrides, novel TMZnN2 (TM = Ti, Zr, Hf) structures with high piezoelectric responses have been predicted as stable.

The focus of the piezoelectricity research is on piezoelectric properties of both ordered and disordered nitrides. The exploration of disordered wurtzite nitrides revealed important aspects of the nitride alloying physics and the implications for their piezoelectric response, in addition to the discovery of interesting alloy candidates and their synthesis, e.g., YxIn1-xN. For the ordered nitrides, novel TMZnN2 (TM = Ti, Zr, Hf) structures with high piezoelectric responses have been predicted as stable.

The thermodynamic stability of novel alloys with interesting properties is investigated in order to determine if equilibrium or non-equilibrium synthesis is feasible. The studies consist of ternary phase diagrams of TM-Zn-N, mixing enthalpies for disordered YxAl1-xN and YxIn1-xN that can be used to predict possible synthesis routes and guide experiments. In addition, mixing enthalpies for strained ScxAl1-xN/InyAl1-yN superlattices show that the stability of certain phases and, therefore, the crystalline quality can be improved by modifying in-plane lattice parameters through higher indium content in the InAlN layers.

Surface diffusion is studied because it is an important factor during thin film growth with, for example, physical vapor deposition. It is the main atomic transport mechanism and, thus, governs the structure development of thin films. Specifically, the research is focused on diffusion on the surfaces of disordered alloys, and in particular Ti, Al, and N adatom diffusion on TiN and TiAlN surfaces. The investigations revealed that Ti adatom mobilities are dramatically reduced in the presence of Al in the surface layer on the TiN and Ti0.5Al0.5N(0 0 1) surfaces, while Al adatoms are largely unaffected. Furthermore, the reverse effect is found on the TiN(1 1 1) surface, Al adatom migration is reduced while Ti adatom migration is unaffected. In addition, it is shown that neglecting the magnetic spin polarization of Ti adatoms will locally underestimate the binding energies and the diffusion path, e.g., underestimating the stability of TiN(0 0 1) bulk sites.