Imaging self-sputtering and backscattering from the substrate during pulsed laser deposition of gold

The expansion dynamics of the plasma generated during pulsed laser deposition of gold in vacuum has been investigated at the laser fluences of 2.5, 6.0, and $9.0\mathrm{J}{\mathrm{cm}}^{-2}$. A severe distortion of the expansion is observed in the presence of a substrate that is accompanied at $9.0\mathrm{J}{\mathrm{cm}}^{-2}$ by the appearance of a secondary plasma front expanding from the substrate surface. Langmuir probe analysis at $9.0\mathrm{J}{\mathrm{cm}}^{-2}$ shows that the substrate surface is bombarded by a high transient flux of energetic ${\mathrm{Au}}^{+}$ ions $(3.0\times {10}^{19}\text{ions}{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1})$ having very large kinetic energies $(>400\mathrm{eV})$. Analysis of the plasma dynamics shows that these observations are consistent with self-sputtering of Au neutrals from the substrate induced by incident Au ions while a fraction of them are backscattered. Self-sputtering is found to be 2 orders of magnitude larger than backscattering. The comparison with experimental data allows concluding that the apparent recoil of the plasma front is caused by collision with self-sputtered neutrals, while the secondary emission is originated by backscattered ions.

Figure 1

(Color) Two dimensional time-gated images of the Au plasma generated at $9.0\mathrm{J}{\mathrm{cm}}^{-2}$ that corresponds to the emission of excited Au neutrals at 479.3 and $481.2\mathrm{nm}$. The emission intensity is represented in false colors and the data have been normalized by the maximum intensity for each time delay. The time delays with respect to the laser pulse are indicated in each image. The relative position of (T) target, (S) substrate, and (H) substrate holder are schematically shown in each image. The origin of the plasma expansion $(z=0)$ is the target surface.

Figure 2

(Color online) Temporal evolution of the spatial distribution of ${\mathrm{Au}}^{*}$ emitting at 479.3 and $481.2\mathrm{nm}$ generated at $9.0\mathrm{J}{\mathrm{cm}}^{-2}$. The time delays with respect to the laser pulse are indicated in the figure. The data have been normalized by the maximum intensity for each time delay. The inset schematically shows the spatial positions of the intensity spatial distributions, where the primary and secondary emission reach half of their maximum value. The temporal evolution of these positions is considered to be representative of the movements of the primary and secondary fronts.

Figure 3

(Color online) Space-time evolution of the primary and secondary emission fronts. Results obtained at (open symbols) $2.6\mathrm{J}{\mathrm{cm}}^{-2}$ and (full symbols) $9.0\mathrm{J}{\mathrm{cm}}^{-2}$ are included. (◻, ∎) Primary fronts expanding from the target, (▵, ▴) primary fronts moving toward the target, and (●) secondary plasma front. No secondary emission is observed at $2.6\mathrm{J}{\mathrm{cm}}^{-2}$. The top horizontal axis corresponds to the substrate surface position $(d=39\mathrm{mm})$. Dashed lines are linear fits to the experimental data. Dotted lines correspond to the linear extrapolation toward the substrate surface of the primary front moving toward the target and the secondary plasma front. The full line represents the space-time evolution of Au species expanding from the target with a velocity of $27.3\mathrm{km}{\mathrm{s}}^{-1}$. The numbers in the figure correspond to the absolute velocities determined from the slopes of the linear fits of the experimental data.

Figure 4

(Color online) Normalized velocity distributions determined for ${\mathrm{Au}}^{*}$ neutrals and ${\mathrm{Au}}^{+}$ ions at (solid line) $2.6\mathrm{J}{\mathrm{cm}}^{-2}$ and (dashed line) $9.0\mathrm{J}{\mathrm{cm}}^{-2}$.

Figure 5

(Color online) Calculated density distributions of (solid line) self-sputtered atoms, ${\mathrm{Au}}_{\mathrm{SS}}$, and (dashed line) backscattered ions, $\mathrm{Au}^{+}{}_{\mathrm{BS}}$, as a function of the velocity of ${\mathrm{Au}}^{+}$ ions arriving to the substrate for a laser fluence of $9.0\mathrm{J}{\mathrm{cm}}^{-2}$. Data corresponding to backscattered ions have been multiplied by a factor of 100 to ease the comparison.

Figure 6

(Color online) Calculated velocity distributions of (solid line) self-sputtered atoms, ${\mathrm{Au}}_{\mathrm{SS}}$, and (dashed line) backscattered ions, $\mathrm{Au}^{+}{}_{\mathrm{BS}}$, for a fluence of $9.0\mathrm{J}{\mathrm{cm}}^{-2}$. Data corresponding to backscattered ions have been multiplied by a factor of 100 to ease the comparison.

Figure 7

(Color online) Geometry used for the self-sputtering and backscattering simulations. (Dotted arrow) Trajectory of incident ${\mathrm{Au}}^{+}$ beam, (continuous arrow) trajectory of self-sputtered and/or backscattered species, $\left(\theta \right)$ its angle with respect to the target surface normal ($z$ axis), and (dashed arrow) velocity component of self-sputtered and/or backscattered species along the $z$ axis, $v_z^{\prime }$.

Figure 8

Simulated (∎) self-sputtering yield of neutrals, $Y_{SS}\left(v\right)$, and (엯) backscattering yield of ions, $Y_{BS}\left(v\right)$. $Y_{BS}\left(v\right)$ has been multiplied by a factor of 100 to ease the comparison. The data are presented as a function of the velocity of the ${\mathrm{Au}}^{+}$ ions arriving to the substrate. The dashed lines are best fits to the calculated data.

Figure 9

Calculated elemental velocity distributions of atoms self-sputtered by incident ${\mathrm{Au}}^{+}$ ions having velocities of $7\mathrm{km}{\mathrm{s}}^{-1}$, $22\mathrm{km}{\mathrm{s}}^{-1}$, and $31\mathrm{km}{\mathrm{s}}^{-1}$. Data correspond to a laser fluence of $9.0\mathrm{J}{\mathrm{cm}}^{-2}$.