Fig. 1

The topography images of the scratched trench and the zoomed area before (a, e) and after NHH etching for totally 2 (b, f), 7 (c, g), and 17 min (d, h), respectively. The height profiles along the marked line in (a–d) are shown in (i). The zoomed plot in the inset of (i) clearly shows the decline of the wetting layer by each etching process. The height profiles along the marked line in (e–h) are shown in (j) and the deduced composition profiles are plotted in (k)