Diversity in plasma arrival sequence correlated to oxygen deficiencies of oxide thin films grown by PLD

Abstract

Oxide thin films grown in vacuum by pulsed laser deposition (PLD) are usually oxygen deficient, due to the well-known reasons such as preferential plasma scatterings or resputtering. The present work demonstrates another possibility that results in oxygen deficiencies associated to the much earlier arrival of O + as compared to other cation ions. The earlier arrived O + cannot be independently deposited as solid without other cation constituents. In particular, this effect is pronounced when performing PLD using a 248 nm laser, with a large proportion of O + generated via a 3-photon ionization process during the initial laser solid interactions.Pulsed laser deposition (PLD) is one of the most commonly applied deposition techniques, that is possible to realize a congruent growth of oxide thin films with complex compositions [1−10]. PLD transfers the target composition as thin films via laser-induced plasma plume in vacuum or under background partial pressures [1,4−6]. In most cases, the growth of oxide thin films is performed under oxygen containing circumstance in order to avoid the deficiency of oxygen in the oxide thin films, since the thin film grown in vacuum or quasi-vacuum (≤10 −3 mbar) is usually oxygen deficient. In vacuum, the plasma species possesses a large kinetic energy (E K ) up to several hundreds of eV [4−6], which is sufficient to re-sputter the film elements with a preference of lighter composition [7,8]. Meanwhile, the plasma elements can be recoiled or backscattered by the surface of the as-grown film, which is also in preferential for the lighter plasma constituents [9]. These well explain the oxygen deficiency in the oxide thin films grown in vacuum, since oxygen possesses a smaller atomic weight than most of other cation elements [5,6]. Nevertheless, there are also some cases that the desired oxide thin film properties cannot be achieved when using high background partial pressures.In the present work, we show that apart from the above already known causes, the arrival time and sequence of different plasma species can also result in oxygen deficiency in the oxide thin films prepared by PLD in vacuum. Tak [4,5] thin films grown in vacuum or quasi-vacuum by PLD. It can be obviously seen that the deficient oxygen compositions in Ca 3 Co 4 O 9 and La 0.6 Sr 0.4 MnO 3 thin films grown using a KrF excimer laser (λ = 248 nm), as well as La 0.6 Sr 0.4 MnO 3 thin film grown using an ArF excimer laser (λ = 193 nm). In contrast, the La 0.6 Sr 0.4 MnO 3 thin films grown using the same laser ablation fluence but a XeCl excimer laser (λ = 308 nm) shows an oxygen composition almost reaching the stoichiometric number. The diversity in composition of the as-grown La 0.6 Sr 0.4 MnO 3 thin films using various laser wavelengths further influences the crystal structures. For example, a strong diffraction peak from an epitaxial grown and crystallized LSMO film is observed for the deposition using λ = 308 nm, which is not achievable for the deposition using λ = 248 nm or 193 nm [4].The oxygen deficiency was attr...