Negative ions: The overlooked species in thin film growth by pulsed laser deposition

Abstract

Plasma plume species from a ceramic La 0.4 Ca 0.6 MnO 3 target were studied by plasma mass spectrometry as a function of laser fluence, background gas, and deposition pressure to understand the interplay between plasma composition and oxide thin film growth by pulsed laser deposition. The plume composition reveals a significant contribution of up to 24% of negative ions, most notably using a N 2 O background. The significance of negative ions for thin film growth is shown for La 0.4 Ca 0.6 MnO 3 films grown in different background conditions where the best structural properties coincide with the largest amount of negative plasma species.One of the most versatile deposition techniques in solid state physics and analytical chemistry is the vaporization of condensed matter using photons. For oxide thin film growth, pulsed laser deposition (PLD) has evolved into a powerful deposition technique with a high control over crystalline properties. 1,2 Material removed from the target is directed towards a substrate where it re-condenses to form a film. The film's growth kinetics will depend on the material flux, plume composition, laser repetition rate, growth temperature, substrate, pressure, and background gas (vacuum, reactive). In addition, wavelength and fluence will determine if thermal or non-thermal evaporation processes are dominant as well as control the ratio between neutral and ionized species in a plume. 3 An important characteristic of PLD is the ability to realize, in principle, a stoichiometric transfer of ablated material from multi-elemental targets. This is an interesting fact, often taken for granted 2-5 but not necessarily correct in general. 6 However, the question is which plasma conditions have to be realized in order to grow a stoichiometric thin film with good properties, e.g., crystallinity. The ablation and deposition processes are connected by the transfer of material via the created plasma plume, and the thin film properties are closely related to the dynamics and composition of the respective plasma plume and growth properties on a substrate. To enhance the oxygen content in oxide thin films, a background gas is introduced, which also helps to moderate the kinetic energy (KE) of plume species. 1 Which plasma species are important for thin film growth? For sputtering, plasma species have been studied in detail, including the influence of negative ions. A densification of a film in the initial stages by ion bombardment has been linked to the presence of negative ions. 7 Likewise, negative ions are involved in an increase of the adatom's mobility which is a function of adatom to ion ratio. This is of particular importance for the kinetic energies of plasma species as low as 2-18 eV (Ref. 8) and typical when a background pressure is applied during film growth. The result is an increased nucleation density of up to 10 9 cm À2 . It is noteworthy to point out that negative oxygen ions are the more active oxidizing species compared to O þ as shown for silicon oxidation. 9 The example of the sputter p...

Relevance and formation mechanisms of negative ions upon ablation of Al2O3

“…More recently, an improvement of film quality has been reported when ablating La 0.6 Ca 0.4 MnO 3 in the presence of N 2 O gas that coincides with the largest amount of negatively charged oxygen species [12,13]. However, these works show on the one hand that the amount of negatively charged species is less important and almost non-relevant in O 2 and vacuum, respectively.…”

Plasma interactions determine the composition in pulsed laser deposited thin films

“…18,19 Moderation of the kinetic energy of plasma species with increasing pressure, and the formation of negative species may improve film growth and crystallinity. 20 Analyzing these correlations using mass spectrometry, 20-24 emission/ absorption spectroscopy, [25][26][27] and plasma imaging 13,28 have led to an improved understanding of the physical properties of the laser induced plasma. A more quantitative approach to capture the simultaneously ongoing chemical aspects, in particular the reaction of plasma species with a background gas and to reliably determine the film composition, is still missing.…”

Structural, magnetic, and superconducting properties of pulsed-laser-deposition-grownLa1.85Sr0.15CuO4Das1, Sen2, Marozau3 et al. 2014Phys. Rev. BSelf Cite111180View full textAdd to dashboardCite

“…To the best of our knowledge it has not yet been used for the PLD growth of LSCO thin films. Nevertheless, it has recently been demonstrated that thin LCMO films grown with PLD in N 2 O atmosphere have a higher oxygen content and a superior crystalline quality than those grown in O 2 environment 18 . In this manuscript we describe the N 2 O assisted PLD growth of La superlattices with a repetition number of X = 1, 3, 5, 7 and 9 bilayers (in the following we refer to them as X BL samples) have been grown in a PLD chamber (SURFACE-TEC GmbH), equipped with an excimer KrF laser source (λ = 248 nm, t s = 25 ns) and an infrared laser (JENOPTIK, JOLD-140-CAXF-6A) for heating the substrate.…”