Influence of morphology on adsorbate-induced changes in thin-film dynamic conductivity

For ultrathin copper films of various morphology we studied adsorbate-induced changes in broadband infrared transmission at normal incidence of light. Smooth Cu films on Si(111) and mesoscopically rough Cu films on KBr(001) were exposed to CO and to ${\mathrm{C}}_2{\mathrm{H}}_4$. We observed significant broadband changes for each of these gases and for both surfaces. Applying a Drude-type model we calculated the optical spectra in accord with the experiment. We find that the effects related to a change in the electronic relaxation rate are weakly influenced by the mesoscopic roughness of the film, while the effects related to charge transfer are strongly enhanced due to such roughness. This paper shows that the real adsorbate-induced changes can be determined for both the homogeneous films and the inhomogeneous films beyond percolation. The increased surface area owing to mesoscopic roughness is merely one contribution to larger adsorbate-induced effects of inhomogeneous films. The other more interesting contribution is due to depolarization in rough metal films that is responsible for strong enhancement of charge transfer effects.

Figure 1

Sketch of the two types of Cu films that are investigated in this work: mesoscopically smooth films on Si and mesoscopically rough films on KBr. Due to the same growth temperature the same adsorption properties are expected for the surfaces of these films.

Figure 2

Imaginary part of the dielectric function ${ϵ}_B$ for a two-dimensional inhomogeneous medium composed of 60% $(F=0.6)$ copper $\left({\omega }_{Pm}=66000{\mathrm{cm}}^{-1}\right)$ and 40% dielectric with ${ϵ}_d=1$ (solid lines) and with ${ϵ}_d=2$ (dashed lines). The values for the relaxation ${\omega }_{\tau m}$ are $20{\mathrm{cm}}^{-1}$ (lower curves) and $2000{\mathrm{cm}}^{-1}$ (upper curves). Below the high wave number limit ${\omega }_{\max }$ (vertical lines for the two values of ${ϵ}_d$) the effective medium is well described by a Drude-type dielectric function $F\bullet {ϵ}_{D\mathrm{eff}}$ (circles) with ${\omega }_{P\mathrm{eff}}^2=0.335\bullet {\omega }_{Pm}^2$ and ${\omega }_{\tau \mathrm{eff}}={\omega }_{\tau m}$.

Figure 3

Dependence of ${\omega }_{P\mathrm{eff}}^2∕{\omega }_{Pm}^2$ and the relative sensitivity $S=\left(\partial {\omega }_{P\mathrm{eff}}^2∕\partial F\right)∕\left({\omega }_{P\mathrm{eff}}^2∕F\right)$ on the metal filling factor $F$ for two-dimensional films.

Figure 4

The relative transmittance $T_{\text{film}}∕T_{\text{substrate}}$ of copper films on KBr and on Si[II] at $T_s=100\mathrm{K}$ normalized to the transmittance of the bare substrate is shown (a) versus film thickness for two different wave numbers and (b) versus wave number for a thickness $d=5\mathrm{nm}$. The arrows in (a) indicate an optical crossover for the selected wave numbers at a thickness $d_c$. They roughly mark the onset of metallic conductivity. In (b) also the spectral fits to the experimental data are shown.

Figure 5

AFM images of mesoscopically rough Cu films on KBr(001) (left) and of smooth Cu films on Si(111)[II] (right). Film thickness is 5.5 nm for both films. Imaging is done in air at room temperature, scan size is $200\mathrm{nm}\times 200\mathrm{nm}$. Both films are grown at 100 K.

Figure 6

Spectra of adsorbate-induced change in broadband transmission (ACBT): (a) 5 nm $\mathrm{Cu}∕\mathrm{KBr}\left(001\right)$ and 7 nm $\mathrm{Cu}∕\mathrm{Si}\left(111\right)$ exposed to CO; (b) 5 nm Cu on KBr(001) and 5 nm Cu on Si(111) exposed to ${\mathrm{C}}_2{\mathrm{H}}_4$. Solid lines, from experiment; dashed lines, calculated with the values given in Table . The exposures are indicated by the labels. For the films on KBr the spectra also show absorption lines from adsorbate vibrations. For ${\mathrm{C}}_2{\mathrm{H}}_4∕\mathrm{Cu}∕\mathrm{KBr}$ the structure at $\sim 2100{\mathrm{cm}}^{-1}$ is due to CO adsorption from the residual gas $\left(p_{\mathrm{CO}}\lesssim {10}^{-10}\mathrm{mbar}\right)$ prior to ${\mathrm{C}}_2{\mathrm{H}}_4$ exposure.