Why ‘Zn diffusion’ is not always diffusion: Surface physics and a 40-year-old epitaxy problem

Zinc is the typical p-dopant in metal–organic-vapour-phase-epitaxy (MOVPE) for most III–V applications. Nevertheless, Zinc’s reportedly long and “uncontainable” diffusion compromises current integrated devices, largely limiting their design, and constraining implementations to simple p-on-n structures. Here, we report on surprising findings on Zn dopant behaviour, including previously unreported long-range “forward-diffusion like” phenomenology and a substrate miscut (back-)diffusion dependence. Tailored secondary ion mass spectrometry experiments (obtained comparing “traditional” and “novel” low voltage methodologies for Zn determination) show that Zn (or its precursors) can behave as surfactant, accumulating on the surface during intentional doping while gradually incorporating in the nominally undoped layers even after the Zn source is shut-off. Evidence can be straightforwardly modelled (in its time dependence) with good qualitative agreement. Moreover, we show that this phenomenology can be suppressed either by introducing growth interruption steps or by introducing a competing surfactant species (Sb or its precursors). As part of our results, we also observed that substrate misorientation seems to regulate back-diffusion: indeed it can be suppressed with specific substrate choice. Our results highlight the relevance of often overlooked multi-faceted surface processes during MOVPE and will help the development of robust solutions for novel device designs; crucially enabling next-generation integrated III–V applications.