Role of strain and composition on the piezoelectric and dielectric response of AlxGa1−xN: Implications for power electronics device reliability

Gallium nitride (GaN) and AlxGa1−xN, its solid solution with Al, play a vital role in a variety of high-power applications owing to their high breakdown voltage, drift velocity, and sheet charge density. Their piezoelectric nature is critical for both the operation and reliability of GaN-based devices, and this is compounded by the lack of lattice-matched substrates and the lattice mismatch between GaN and AlxGa1−xN, which invariably results in strained films. Thus, accurate models of performance and reliability require knowledge of how strain affects dielectric and piezoelectric response. We used density functional theory to calculate the piezoelectric and dielectric constants for different compositions of AlxGa1−xN as a function of biaxial strain and use Gaussian process regression to develop models, including uncertainties, from the ab initio results. We find that the dielectric constants decrease with compressive biaxial strain and increasing Al content due to an increase in phonon frequencies. Meanwhile, the piezoelectric constants increase with compressive biaxial strain and with Al doping. Our results show that the presence of strain can explain discrepancies in experimental measurements of dielectric constants but not piezoelectric ones. Interestingly, the piezoelectric constants e33 and e31 (which control the elastic energy induced by the application of gate voltage in GaN high electron mobility transistors, which have been related to their degradation) vary by almost 100% within a biaxial strain range of ∼3%. These results indicate that incorporating strain-dependent and composition-dependent piezoelectric response into current degradation models based on inverse piezoelectricity is crucial for accurate reliability predictions in GaN-based transistors.