Relevance and formation mechanisms of negative ions upon ablation of Al₂O₃

Abstract

The aim of this work is to study the significance of negative ions in the plasma produced by ablation of a simple oxide ceramic target (Al 2 O 3 ) at distances and fluences typically used in pulsed-laser deposition processes. The results show that negative ions are indeed produced, the majority of which (>82%) being O − that are predominantly produced by neutralization of O + followed by electron attachment. They represent one third of the O + population at low fluences for a distance of 4 cm from the target at which most deposition experiments are performed. AlO − represents up to 15% of the negative ions and their amount increases at the expense of O − as fluence is increased. The most abundant as well as the fastest species in the plasma is by far Al + that represent >80% of ions having kinetic energy <100 eV at low fluences. This result is consistent with earlier discussion on the possible existence of direct photoionization processes due to the high (6.4 eV) photon energy. Saturation effects, the formation mechanism for AlO − and Al 2+ , and expansion dynamics for negative ions are finally discussed.

Laser ablation of ceramic Al2O3 at 193 nm and 248 nm: The importance of single-photon ionization processes

“…Finally, it is worth to mention that upon ablation at 248 nm, no Al þþ was detected whereas for 193 nm ablation, the relative amount increases with fluence and shows no evidences of saturation in the studied fluence range as reported earlier. 16 The amount of O þ ( Fig. 2(c)) follows the expected trend for both wavelengths, i.e., an increase with increasing fluence, and the O þ produced at 193 nm nearly reaches saturation for fluences which are close to the threshold for observing O þ at 248 nm.…”

“…8,36 The KE distribution of Al þþ in Fig. 3(a) that is shifted to higher values and the fact that the amount of these double charged ions does not saturate 16 provides further support to this reasoning. The repulsive forces that arise between the hot and cold populations can explain the need for a stream velocity necessary to fit the experimental KE distributions in Fig.…”

“…In addition, the absolute amount of Al þ is higher than that of O þ (not shown) as reported earlier. 16 The KE distributions obtained from optical emission spectroscopy (Figs. 4(a) and 4(b)) show that there are ions of both types in the excited state with KE much higher than 100 eV and that the amount of excited Al þ and O þ with KE higher than 200 eV are, respectively, 12% and 8%.…”

“…The system allows measuring KE up to 100 eV and acquisitions were performed using a delay of 20 ls with respect to the signal of a photodiode collecting the laser output and an acquisition time of 400 ls. Since in an earlier work using the same experimental approach it was shown that the more relevant and faster species were monoatomic species, 16 we focus the present work to these species. The second diagnostic tool was imaging emission spectroscopy.…”

“…The works on laser ablation of ceramic Al 2 O 3 targets also relate to single wavelengths of 248 nm, 3,14 355 nm, 15 532 nm, 11 or 1064 nm. 4 Only recently, the ion yield and KE distributions produced by ablation of a ceramic Al 2 O 3 target at 193 nm have been reported showing that Al þ represent >80% of the ions 16 and 20% of the ions had KE > 200 eV around the threshold for plasma formation. 17 These results were related to the existence of direct photoionization processes in the plasma.…”