Beyond structural stabilization of highly-textured AlN thin film: the role of chemical effects

The crystalline quality and degree of c-axis orientation of hexagonal AlN thin films correlate directly with their functional properties. Therefore, achieving AlN thin films of high crystalline quality and texture is of extraordinary importance for many applications, but in particular in electronic devices. Here, we present a systematic study revealing that the growth of c-axis orientated AlN thin films can be governed by a chemical stabilization effect in addition to the conventionally known structural, i.e. strain-induced, stabilization mechanism. The promotion of in-plane growth of AlN grains with c-axis out-of-plane orientation is demonstrated on Y, W or Al seed layers with different thicknesses and crystallinity preliminary exposed to N2 at room temperature. We establish that the stabilization mechanism is chemical in nature: the formation of an N-rich surface layer on the metal seed layers upon exposure to N2 pre-determines the polarity of AlN islands at initial stages of thin film growth while the low energy barrier for the subsequent coalescence of islands of the same polarity contributes to grain growth. These results suggest that the growth of c-axis oriented AlN thin films can be optimized and controlled chemically thus opening more pathways for energy-efficient and controllable AlN thin-film growth processes.