Simultaneous Selective Area Growth of Wurtzite and Zincblende Self-Catalyzed GaAs Nanowires on Silicon

Selective area epitaxy constitutes a mainstream method to obtain
reproducible nanomaterials. As a counterpart, self-assembly allows their growth without
costly substrate preparation, with the drawback of uncontrolled positioning. We propose a
mixed approach in which self-assembly is limited to reduced regions on a patterned
silicon substrate. While nanowires grow with a wide distribution of diameters, we note a
mostly binary occurrence of crystal phases. Self-catalyzed GaAs nanowires form in either a
wurtzite or zincblende phase in the same growth run. Quite surprisingly, thicker
nanowires are wurtzite and thinner nanowires are zincblende, while the common view
predicts the reverse trend. We relate this phenomenon to the influx of Ga adatoms by
surface diffusion, which results in different contact angles of Ga droplets. We demonstrate
the wurtzite phase of thick GaAs NWs up to 200 nm in diameter in the Au-free approach,
which has not been achieved so far to our knowledge.