Self-organizing nanodot structures on InP surfaces evolving under low-energy ion irradiation: analysis of morphology and composition
Nanoscale Research Letters 2014, 9:403 doi:10.1186/1556-276X-9-403Published: 19 August 2014
Surfaces of InP were bombarded by 1.9 keV Ar+ ions under normal incidence. The total accumulated ion fluence Phi the samples were exposed to was varied from 1 x 1017 cm-2 to 3 x 1018 cm-2, and ion flux f of (0.4 - 2) x 1014 cm-2 s-1 were used. The surface morphology resulting from these ion irradiations was examined by atomic force microscopy (AFM). Generally, nanodot structures are formed on the surface; their dimensions (diameter, height and separation), however, were found to depend critically on the specific bombardment conditions. As a function of ion fluence, the mean radius r, height h, and spacing l of the dots can be fitted by power-law dependences: r [proportional to] Phi0.40, h [proportional to] Phi0.48, and l [proportional to] Phi0.19. In terms of ion flux, there appears to exist a distinct threshold: below f ~ (1.3 +/- 0.2) x 1014 cm-2 s-1, no ordering of the dots exists and their size is comparatively small; above that value of f, the height and radius of the dots becomes substantially larger (h ~40 nm and r ~ 50 nm). This finding possibly indicates that surface diffusion processes could be important. In order to determine possible local compositional changes in these nanostructures induced by ion impact, selected samples were prepared for atom probe tomography (APT). The results indicate that APT can provide analytical information on the composition of individual InP nanodots. By means of 3D APT data, the surface region of such nanodots evolving under ion bombardment could be examined with atomic spatial resolution. At the InP surface, the values of the In/P concentration ratio are distinctly higher over a distance of approximately 1 nm and amount to 1.3 to 1.7.