Tuning polytype selection during molecular beam epitaxy of GaN

GaN polytype heterostructures, composed of sequential wurtzite (WZ) and zincblende (ZB) GaN layers, are a promising platform for next-generation power electronics. Although these polytypes differ only in atomic stacking sequence, their lattice-matched interfaces are expected to host a 2D electron gas with higher mobility than in AlGaN/GaN structures. Recently, we demonstrated metal-mediated molecular beam epitaxy of WZ-on-ZB nanowires (NW) and films.1 Although the initial nucleation of the ZB polytype was attributed to a stabilizing effect of the Si substrate,2 the origins of the subsequent transition to the WZ polytype remains unknown. Interestingly, the ZB to WZ polytype occurs after 1 hour of growth, at a thickness of ~20 nm.  Due to the consistency in the timing of the transformation, we hypothesize that the ZB-to-WZ transformation is not due to a random event but instead due to an extrinsic effect associated with the growth process. We discuss the relative roles of positive charging (from nitrogen plasma) and negative charging (from reflection high energy electron diffraction) on GaN polytype selection, identifying new insights for precise control of GaN polytype selection for future GaN-based power electronic devices.

1. APL 119, 031601 (2021).

2. APL 125, 042107 (2024).