Neutralization and wake effects on the Coulomb explosion of swift ${{\mathrm{H}}_2}^{+}$ ions traversing thin films

The Coulomb explosion of small cluster beams can be used to measure the dwell time of fragments traversing amorphous films. Therefore, the thickness of thin films can be obtained with the so-called Coulomb depth profiling technique using relatively high cluster energies where the fragments are fully ionized after breakup. Here we demonstrate the applicability of Coulomb depth profiling technique at lower cluster energies where neutralization and wake effects come into play. To that end, we investigated 50–200 keV/u ${{\mathrm{H}}_2}^{+}$ molecular ions impinging on a 10 nm ${\mathrm{TiO}}_2$ film and measured the energy of the backscattered ${\mathrm{H}}^{+}$ fragments with high-energy resolution. The effect of the neutralization of the ${\mathrm{H}}^{+}$ fragments along the incoming trajectory before the backscattering collision is clearly observed at lower energies through the decrease of the energy broadening due to the Coulomb explosion. The reduced values of the Coulomb explosion combined with full Monte Carlo simulations provide compatible results with those obtained at higher cluster energies where neutralization is less important. The results are corroborated by electron microscopy measurements.

Figure 1

2D MEIS spectrum measured with 175 keV/u of ${{\mathrm{H}}_2}^{+}$ projectiles under normal incidence on the ${\mathrm{TiO}}_2$ layer grown over crystal Si. The following signals are observed from top to bottom: Ti from the ${\mathrm{TiO}}_2$ film; Si from the substrate; O from ${\mathrm{TiO}}_2$ and ${\mathrm{SiO}}_2$ films; and C stemming from contamination. Blocking lines in Si are also visible. See text for further information.

Figure 2

1D energy spectrum obtained by 150 keV/nucleon ${{\mathrm{H}}_2}^{+}$ molecules striking on ${\mathrm{TiO}}_2$. The inset shows the 2D spectrum featuring the Ti signal obtained with the ${\mathrm{H}}^{+}$ beam. The best fittings to the ${{\mathrm{H}}_2}^{+}$ data (red line) and to the ${\mathrm{H}}^{+}$ data (blue line) obtained for the Ti signal were obtained through simulations using the powermeis code. Note that the ${\mathrm{H}}^{+}$ data were omitted from the picture for the sake of clarity.

Figure 3

Simulated energy spectra for 150 keV/nucleon ${{\mathrm{H}}_2}^{+}$ molecules striking on ${\mathrm{TiO}}_2$ with different atomic densities and thicknesses but with a fixed number of atoms/${\mathrm{cm}}^2$ (see text). The geometry is depicted in the inset. For comparison, the energy spectrum for protons with the same speed is shown by a black solid line.

Figure 4

The top panel shows the measured REELS spectrum (dots) which shows separate elastic peaks for electrons scattered from Ti an O. After deconvolution of the O contribution (solid line) the spectrum can be treated in the same way as a low energy REELS spectrum. The bottom panel shows the obtained normalised loss function (dots), and a fit using a Drude-Lorentz dielectric function (solid line). The dashed line corresponds to the limit of $Im[-1/\epsilon (k,\omega )]$ for $k=0$.

Figure 5

Wake potential at $\rho =0$ as a function of $\tilde{z}=z-vt$ for 150 keV ${\mathrm{H}}^{+}$ ions in ${\mathrm{TiO}}_2$. The red curve is derived from the Mermin energy loss function with Fuentes data [19, 29]. The green short-dashed-dot is obtained with the Drude-Lorentz dielectric function with Fuentes [29] and the blue dashed line corresponds to this work.

Figure 6

The Coulomb broadening as a function of the depth traversed by the fragments of ${{\mathrm{H}}_2}^{+}$ molecules in ultrathin ${\mathrm{TiO}}_2$ film obtained at energies of 50, 120, and 150 keV/u. The curves represent the calculations of the Coulomb broadening assuming a Coulomb repulsive potential screened by the wake potential.

Figure 7

(a) High-resolution TEM image of the ${\mathrm{TiO}}_2$ film using phase contrast. (b) Scanning transmission electron microscope (STEM) image of the same sample. In (b) no difference could be observed between the contributions of ${\mathrm{TiO}}_2$ and ${\mathrm{SiO}}_2$, present as a native oxide.

Figure 8

The Coulomb broadening $\left({\sigma }_C\right)$ as a function of the ${{\mathrm{H}}_2}^{+}$ energy for ions traversing a 10 nm thick ${\mathrm{TiO}}_2$ film along a trajectory 60 degrees away from the surface normal. The lines stand for calculations of ${\sigma }_C$ in different scenarios: assuming a free Coulomb explosion (full black line); assuming the Coulomb repulsive potential screened by Yukawa potential (dot-dashed red line); assuming a Coulomb repulsive potential screened by wake potential (dashed blue line). All calculations were obtained assuming two ${\mathrm{H}}^{+}$ fragments. The purple curve includes the possibility of having ${\mathrm{H}}^0$ fragments.

Figure 9

On the right the Coulomb broadening ${\sigma }_C$ for three fixed initial ${{\mathrm{H}}_2}^{+}$ orientations $({\Theta }_{\mathrm{in}}=0^{\circ },{30}^{\circ }$, and ${80}^{\circ })$ at 200 keV/u. On the left the distribution of final cluster orientations after traversing a layer of 200 Å.