Growth of wurtzite Hf <i>x</i> Al1 <b>−</b> <i>x</i> N up to x  <b>=</b>  0.39 by sputtering with bandgap shift and c/a-ratio anomaly

Abstract

Alloying hafnium nitride (HfN) into aluminum nitride (AlN) has been predicted to enhance the piezoelectric response beyond that achieved by scandium nitride (ScN) alloying into AlN, making hafnium aluminum nitride (HfxAl1−xN) a promising material system for acoustic devices. Understanding the effect of hafnium (Hf) incorporation in the wurtzite (wz) phase—specifically the structural modifications and their impact on optical and electrical properties—is therefore essential. In contrast to the extensively studied scandium aluminum nitride (ScxAl1−xN), HfxAl1−xN is expected to exhibit distinct behavior because Hf has one additional d-electron compared to scandium (Sc). Existing experimental work has predominantly focused on the rock salt (rs) phase or on the wz phase at low Hf concentrations. Here, it is demonstrated that HfxAl1−xN can be grown in the wz phase with promising structural quality by sputtering, up to a Hf content of x=0.39. Structural characterization by atomic force microscopy and x-ray diffraction confirms the growth of wz-HfxAl1−xN using DC magnetron sputtering. Up to x=0.15, the experimental observations are consistent with theoretical predictions. At higher Hf contents, however, a reversal in the trend of the c/a ratio is observed, driven by an unexpected increase in the c lattice parameter. No phase transition is detected up to this Hf concentration. Consistent with theoretical expectations, the bandgap decreases progressively with increasing Hf content. These results contradict the current theoretical description of HfxAl1−xN, which predicts a phase transition to a layered-hexagonal phase, while simultaneously confirming the theoretically predicted decrease in the bandgap.