Self-organizing nanodot structures on InP surfaces evolving under low-energy ion irradiation: analysis of morphology and composition

Abstract

Surfaces of InP were bombarded by 1.9 keV Ar+ ions under normal incidence. The total accumulated ion fluence Φ the samples were exposed to was varied from 1 × 1017 cm−2 to 3 × 1018 cm−2, and ion fluxes f of (0.4 − 2) × 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 ∝ Φ0.40, h ∝ Φ0.48, and l ∝ Φ0.19. In terms of ion flux, there appears to exist a distinct threshold: below f ~ (1.3 ± 0.2) × 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.

Nanocone formation on ion-bombarded InP surfaces

“…While that work illustrated a distinct dependence of the dot dimensions (radius, height, separation) on the ion fluence and the ion flux, the detailed morphology of these nanostructures could not be determined. The main objective of the present work is to possibly elucidate the formation mechanisms of such structures by examining, using scanning electron microscopy (SEM), ion fluence/flux regimes which could not be addressed in [24]. Before presenting these data it appears useful to recapitulate briefly the previous findings.…”

“…From the AFM data it was observed [24] that while for any given fluence the dots exhibit fairly homogeneous distributions in terms of size (radius r and height h) and mutual spacing l, these values are increasing monotonously with the ion fluences ˚ investigated (1 × 10 17 cm −2 to 3 × 10 18 cm −2 ). In this fluence range, the radius was found to increase from r ∼15 to ∼70 nm, whereas the dot height changes from h ∼ 5 to ∼60 nm.…”

“…In a recent study, the formation of regular nanodot structures on InP surfaces by 1.9-keV Ar + ion irradiation has been investigated by atomic force microscopy (AFM) [24]. While that work illustrated a distinct dependence of the dot dimensions (radius, height, separation) on the ion fluence and the ion flux, the detailed morphology of these nanostructures could not be determined.…”

“…Typically, P was found to be depleted at the surface due to preferential sputtering; the In/P surface concentration ratio as determined from Auger electron spectroscopy amounts to ∼1.7 for Ar + ion energies of 1−5 keV [30] and no dependence on the Ar ion energy between 0.5 and 5 keV was found [31]. A recent study using X-ray photoelectron spectroscopy (XPS) determined the surface composition of InP bombarded by 1.9 keV Ar + ions [24]; the overall In/P concentration ratio was found to be higher by a factor of 1.6 in the irradiated region as compared to the pristine InP surface.…”

“…Further theoretical approaches [20][21][22][23] experimental investigation is quite challenging. In previous studies [24,25], such possible compositional variations in individual nanodots caused by ion bombardment, were examined by means of atom probe tomography [26].…”