The growth of InAs is carried out by applying a fixed potential to the In foil for a fixed time lapse (90 min), after having removed the oxide layer with a cathodic sweep (see experimental section). SEM images of as-grown samples (Figure and Figure ) reveal the appearance of crystal-like protruding nanoislands for all applied potentials, that were not present on the bare substrate. The morphology is also confirmed by atomic microscopy (AFM, see Figure ). The size of the nano-islands is similar for all samples and varies from a few tens to a few hundred nanometers. EDX maps (Figure ) of the top surface reveal the presence of As, which predominates in the nanoislands. Figure shows the SEM image of a cross section across a nano-island for the sample grown at -1.175 V. We observe a clear Z-contrast that extends to ~40 nm in thickness. Again, we find similar island dimensions in samples grown under different potential conditions. Figure shows the EDX peak intensity for different chemical elements along the nano-island (dashed line in Figure ), and it reveals that As is mostly contained within the nano-island and it becomes negligible outside (see also supplementary information S6). We compare the EDX linescans for the samples grown with the least and most cathodic potentials (i.e., -1.175 V, solid curve, and -1.35 V, dashed curve) and they are virtually the same. From the integrated deposition current one can estimate an upper limit to the InAs thickness (see supplementary information Figure ), by assuming (i) 100% faradaic efficiency for the reaction shown in Eq 1, (ii) a homogenous deposition throughout a perfectly flat substrate and (iii) full conversion of As(0) into stoichiometric InAs given that As reacts with In at room temperature (see supplementary information S3). We obtain an estimated film thickness of ~ 50 nm regardless of the applied potential, which is much larger than the 40 nm thick nano-islands observed by SEM. Therefore, we suspect that significant contaminant current occurs during deposition due to hydrogen and/or arsine evolution.