Plasma interactions with the N2O background gas: Enhancing the oxidization of alkaline-earth species for pulsed laser deposition

Abstract

Using N2O as an alternative background gas to O2 when growing oxide thin films by pulsed laser deposition (PLD) was previously expected to result in larger oxygen contents of as-grown thin films. In this work, we investigate the composition and kinetic energy of the pulsed laser induced plasmas that propagated in N2O and O2 by using mass spectrometry and plasma imaging. Two distinguished features were observed when using N2O instead of O2. (1) In N2O background gas as compared to O2, a larger proportion of negative oxygen ions was detected. (2) The alkaline-earth elements were fully oxidized in the N2O background, which is not achievable in O2. These observations are attributed to the smaller dissociation energy of the N2O molecules as compared to O2. The smaller dissociation energy of the background gas molecule is expected to reduce the interaction strength during their central collisions with the plasma species. As a result, the dissociable or electron detachable plasma species can be formed in larger amounts in N2O background, compared to O2. Comparing the composition of the deposited thin films indicates a larger oxygen content of the film grown in N2O background gas, as compared to O2, at pressures of ∼10−1 mbar, which is the most commonly used deposition pressure in PLD. Nevertheless, this was not achieved when performing PLD at ∼10−2 mbar, since the pressure was not high enough to trigger the formation of the shockwave front during the plasma expansion and thereby the lightest oxygen plasma species were preferentially scattered.