Morphological evolution of ion-sputtered Pd(001): Temperature effects

We have investigated the kinetic effects on the morphological evolution of an ${\mathrm{Ar}}^{+}$-ion sputtered Pd(001) by in situ, real-time x-ray reflectivity and grazing-incidence small angle x-ray scattering measurements at various substrate temperatures. We find that surface roughness $W$ and its associated growth exponent $\beta$ increase with $T$ up to a certain temperature $T_m$. Beyond $T_m$, however, they decrease with increasing $T$. For $T<T_m$, surface roughening and coarsening kinetics are mostly driven by the deposition and diffusion of sputter-induced adatoms as reported in molecular beam epitaxial growth. When $T$ is near $T_m$, however, vacancies rather than adatoms become the dominant surface species. Above $T_m$, the surface smoothing via adatom detachment and vacancy diffusion across step edges becomes effective.

Figure 1

Specular x-ray reflectivity (symbols) as a function of the out-of-plane momentum transfer $q_z$ for increasing sputtering time $t$ with $ϵ=0.5\mathrm{keV}$ and $f=0.5\times {10}^{13}\text{ions}∕{\mathrm{cm}}^2∕\mathrm{s}$ at $421\mathrm{K}$. The solid lines are the theoretical prediction from the Parratt’s formula.

Figure 2

The surface width or roughness $W$ as a function of sputtering time $t$ for different temperature $T$ (symbols) with $ϵ=0.5\mathrm{keV}$ and $f=0.5\times {10}^{13}\text{ions}∕{\mathrm{cm}}^2∕\mathrm{s}$. Before $t_c$, the solid lines represent fitting results by power law, $W\left(t\right)\sim t^{\beta }$ with large $\beta$ in the range $\beta \approx 0.2\sim 0.5$. After $t_c$, nice fit can be made by either (a) small $\beta \approx 0.1$, or (b) $W^2\left(t\right)\sim \log t$.

Figure 3

Surface roughness $W$ as a function of the sample temperature, $T$, for different ${\mathrm{Ar}}^{+}$-ion fluences.

Figure 4

(Color online) Evolution of grazing-incidence small-angle x-ray scattering (GISAXS) profile. Each scan is made along the [110] direction, $q_{\Vert }$ through specular rods at $q_z=0.1769{\mathrm{\AA }}^{-1}$. The profiles are measured during sputtering with $ϵ=0.5\mathrm{keV}$ and $f=0.5\times {10}^{13}\text{ions}∕{\mathrm{cm}}^2∕\mathrm{s}$ for different temperatures $T$ (a) $306\mathrm{K}$, (b) $340\mathrm{K}$, (c) $392\mathrm{K}$, and (d) $410\mathrm{K}$. For all profiles, the position of the satellite peak shifts to smaller $q_{\Vert }$ with increasing sputtering time $t$.

Figure 5

Transverse scans along [110] through specular rods at various $q_z’\mathrm{s}$. Ion fluence is $9.2\times {10}^{16}\text{ions}∕{\mathrm{cm}}^2$ at different $T’\mathrm{s}$ from $306\text{to}363\mathrm{K}$.

Figure 6

Lateral characteristic length $\xi$ as a function of sputtering time $t$ for different substrate temperatures (symbols) with $ϵ=0.5\mathrm{keV}$ and $f=0.5\times {10}^{13}\text{ions}∕{\mathrm{cm}}^2∕\mathrm{s}$. Solid lines represent the curves best fit by the power law $\xi \left(t\right)\sim t^{1∕z}$. Inset: FWHM of the satellite peaks as a function of sputtering time $t$ for different $T$’s.

Figure 7

The temperature dependence of the growth exponent $\beta$ before $t_c$ and coarsening exponent $1∕z$. The coarsening exponent $1∕z$ shows weak temperature dependence compared with $\beta$ before $t_c$.

Figure 8

Evolution of the local slope $m$ of the mounds or pits as a function of sputtering time $t$ with $ϵ=0.5\mathrm{keV}$ and $f=0.5\times {10}^{13}$.

Figure 9

Recovery of specular anti-Bragg intensity after sputtering a sample at $440\mathrm{K}$ with $ϵ=0.5\mathrm{keV}$ and $f=0.5\times {10}^{13}\text{ions}∕{\mathrm{cm}}^2∕\mathrm{s}$. “on” signifies the start of sputtering and “off” the end of sputtering.