Face-to-face anneal temperature controls lattice parameter in Ta(C,N) virtual substrates for AlGaN power electronics

Abstract: Tantalum carbide (TaC) thin film ''virtual'' substrates are a highly desirable material for $\text{Al}<em>{0.5}\text{Ga}</em>{0.5}\text{N}$ vertical power electronics devices due to lattice matching, thermal expansion matching, and metallic conductivity. However, the material has not been demonstrated to support $\text{Al}<em>{x}\text{Ga}</em>{1-x}\text{N}$ layers of variable composition $x$, limiting the range of device applications. We present a method to achieve tunable rock salt $\text{Ta}\text{C}<em>{x}\text{N}</em>{1-x}$ virtual substrates via a face-to-face annealing of TaC thin films in an $\text{N}<em>2$ atmosphere. The results suggest that annealing temperatures below $1600\,<sup>{\circ}\text{C}$ promote partial uptake of nitrogen onto carbon and anion vacancy sites to form rock salt $\text{Ta(C,N)}$ with intermediate anion compositions. At temperatures $\ge 1600\,<sup>{\circ}\text{C}$, nitrogen primarily occupies the anion sublattice and the crystalline quality and surface morphology simultaneously degrade coincident with the formation of secondary phases. This study demonstrates the growth and processing parameters necessary to make tunable lattice constant virtual substrates for $\text{Al}</em>{x}\text{Ga}_{1-x}\text{N}$ from $x = 0.5\text{-}1$, enabling vertically conducting power electronic devices with reduced defect density at high Al-content.