Studies of the plume accompanying pulsed ultraviolet laser ablation of zinc oxide

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The plume of ejected material accompanying pulsed laser ablation of a ZnO target at 193 nm in vacuum has been investigated using wavelength and spatially resolved optical emission spectroscopy and Langmuir probes. All lines in the observed optical emission spectra are assignable to electronically excited Zn+* cations, and Zn* and O* neutrals, all of which emitting species we attribute to the result of electron–ion recombination processes in the gas phase following material ejection, laser–plume interactions, ionization, and thus, plasma formation. Various contributory components can be identified within the plume. Included among these are: a fast distribution of Zn2+ ions (observed via emission from highly excited states of Zn+*) together with an accompanying subset of fast electrons—the relative importance of which increases with increasing incident fluence on the target; a more abundant slower component involving both Zn+ and O+ ions, which expand in association with the main body of the electron distribution; and a slow moving component of Zn* emitters, which we suggest should be associated with material that has been backscattered from the expanding plasma ball towards the target surface and then rebounded or desorbed into the gas phase. The observation that the postablated target surface is substantially enriched in Zn provides additional support for the importance of material backscattering from within the dense plasma ball, accommodation, and in this case, recondensation on the target. The deduction that the target surface in the vicinity of the irradiated area is Zn rich after just a few laser shots provides an explanation for the oft-reported observation that ZnO films deposited by pulsed laser ablation of ZnO in vacuum are nonstoichiometric, with a Zn:O ratio greater than unity. Such backscattering from the plasma volume and selective recondensation of the less volatile component or components within the plume prior to the next ablation pulse being incident on the target surface appear to account for virtually all reported instances of nonstoichiometric film growth by pulsed laser deposition (PLD) in vacuum. Indeed, given the deduced area of the target surface affected by such redeposition and the target translation speeds typically employed in PLD studies, it would appear that nonstoichiometric film growth is likely to be the norm whenever PLD is carried out in vacuum and at wavelengths and fluences that lead to formation of a sufficiently dense plasma to cause material redeposition on the target.