Optical characterization of the evolution of ion-induced anisotropic nanopatterns on Ag(001)

Reflection anisotropy spectroscopy is used as an in situ probe for the emergence and evolution of surface patterns on Ag(001) during oblique incidence ion bombardment. The information is extracted from plasmon resonances induced by the nanoscale patterns, utilizing the fact that smooth Ag(001) is optically isotropic. The Rayleigh-Rice perturbation approach delivers the temporal development of the average periodicity and amplitude of the surface patterns. For ion bombardment at a polar angle of incidence of ${70}^{\circ }$ along a $〈110〉$ azimuth, strongly anisotropic surface features develop, giving rise to a single plasmon resonance, which is described well with a one-dimensional power spectral density function. For a smaller polar angle of incidence of $61.5^{\circ }$ multiple plasmon resonances are observed, which demand a two-dimensional power spectral density function for a perfect description. These result compare well with high-resolution low-energy electron diffraction data, taken after ion bombardment at both angles of incidence. The optical data, obtained at $61.5^{\circ }$, show coarsening and seem to suggest scaling of the periodicity and roughness, with critical exponents $0.27$ and $0.40,$ respectively.

Figure 1

Relation between periodicity and resonance energy obtained from a simulation of a 1D grating ($\circ$) and from the plasmon dispersion relation (Ref. 12).

Figure 2

(Color online) RAS measurement as a function of sputter time. The incident 2-keV ${\text{Ar}}^{+}$ ion beam had an ion current of $5\mu \text{A}/{\text{cm}}^2$ at normal incidence. The images show three different temperature and polar angle of incidence conditions: (a) 350 K, 70${}^{\circ }$, (b) 300 K, 70${}^{\circ }$, and (c) 400 K 61.5${}^{\circ }$. The red dashed line is a guide to the eye for the negative maximum, while the black dashed line is a guide to the eye for the observation of a positive maximum. The displayed spectra are the last spectra measured in the three time series.

Figure 3

HR-LEED pattern measured after ion bombardment of the Ag(001) surface as displayed in Figs. 2a and 2(c). (a) Polar angle 70${}^{\circ }$ and temperature 350 K, measured at an electron energy of 210.7 eV ($S_{\left[001\right]}=4.806$) and (b) polar angle 61.5${}^{\circ }$ and 400 K, measured at an electron energy of 216.0 eV ($S_{\left[001\right]}=4.866$).

Figure 4

(Color online) Position of the facet spots measured as a function of the electron energy for the pattern as displayed in Fig. 3b. (a) Left-hand panel: The four facet peak positions as a function of the parallel scattering vector of the electron beam ${\vec{q}}_{\parallel }$. The value of ${\vec{q}}_{\parallel }$ in the $\left[1\overline{1}0\right]$ and $\left[110\right]$ azimuth is plotted for electron energies between 203.1 and 223.2 eV. Right-hand panel: The average parallel components of the scattering vector change for the various facets vs the vertical scattering phase $S_{\left[001\right]}$. The surface component of the ions is parallel to the $\left[1\overline{1}0\right]$. (b) Sketch of the average etch pit shape with the four facet angles indicated. The ion beam is incident from the left. I indicates the illuminated facet and S indicates the shadowed facet. R and L indicate both right and left sides.

Figure 5

(Color online) (a) Contour plot of the anisotropic PSDF obtained from a fit to the RAS spectrum recorded after 18 h sputtering at a polar angle of $61.5^{\circ }$. (b) Measured and fitted results at three stages of the ion sputtering at a polar angle of $61.5^{\circ }$.

Figure 6

(Color online) Fit parameters of the RAS spectra obtained during sputtering at a polar angle of incidence of ${\theta }_i=61.5^{\circ }$. (Bottom) Time evolution of the strength of the two Gaussian functions. (Top) Time evolution of the average length scale in the two perpendicular directions.