Self-organised ordering of scandium into basal monolayers of aluminium nitride and its implication for the growth of well-crystallized (Al,Sc)N materials for electronic devices

Aluminium scandium nitride appears as a promising material for future wide band gap semiconductor devices, due to its large spontaneous polarisation effects. Extensive annealing experiments with regard to time and temperature of sputtered (Al,Sc)N thin films result in an ordering of such disordered and metastable materials of wurtzite structure into a so far unknown layered phase, once temperature allows sufficient cationic and anionic diffusion beyond ∼1200 °C. Detailed transmission electron microscopy investigations of this layered phase reveal the complete and self-organised ordering of scandium into octahedrally coordinated basal planes in between several AlN layers of wurtzite structure. The specific numbers of layers spread statistically around an average value depending on the chemical composition and formation temperature. Further investigations of the onset of such phase formation at 1400 °C demonstrate that partially disordered sequences of Sc-bearing basal planes can be thermodynamically interpreted as exsolution lamellae of monolayer thickness. These structural alterations may considerably affect the thermal stability and reliability of devices. The theoretically outstanding physical properties can be lost by the formation of octahedrally coordinated Sc in AlN during any appropriate high temperature deposition or annealing processes during device fabrication as well as due to migration effects during device operation.