Oxide thin film depth profiling: interest and limitations of argon sputtering for photoemission spectroscopy
Résumé
Active research is currently conducted to find alternatives to the silicon-based CMOS semiconductor technologies.
Among the materials classes considered, thin films of oxides are promising candidates for emerging electronic
applications. In this context, perovskite oxide heterostructures are valuable since their interfaces are not only
conducting but can also present functional properties: ferroelectricity, superconductivity or thermoelectricity. For
characterization purpose, argon ion beam is traditionally employed for thin film depth-profiling analysis. This
gives trends on the reactions involved in the initial and final steps of the deposition processes. In this presentation,
we will focus on different aspects of this ion interaction with oxide surfaces grown by Pulse Laser Deposition
(PLD) and Atomic Layer Deposition (ALD): from cleaning with large Gas Cluster Ion Beam (GCIB) at low
energies[1] to etching with energetic monoatomic ions (Ar+
). The access to unmodified buried interfaces in
heterostructures will be discussed.
One of the most promising materials for emerging electronic applications is SrVO3 (SVO). This versatile
transparent conductive perovskite oxide can be considered as a metal, i.e. presenting an excellent electronic
conductivity with low work function. However, its high sensitivity to external stimuli remains a burning issue.
For instance, reconstructed thin layers are generally observed due to Sr migration under soft stress as oxygen
environment or temperature exposure. We showed using XPS depth profiling that the upper layer strongly differs
from the rest of the film. Indeed, Sr-enrichment at the extreme surface was measured whereas stoichiometric SVO
oxide was evidenced through the bulk[2]. However, information about the different chemical environments
involved was no longer accessible. Sputtering sources are known to induce disorder (atomic implantation inside
the matrix, amorphization, elemental dissociation, and/or chemical reduction). A clear modification of the Sr3d,
and O1s-V2p spectral regions induced by Ar+
ion bombardment was observed during the depth profiling making
the data not suitable for fine determination of the constituting element chemical environments. Experiments at
TEMPO beamline (SOLEIL synchrotron) were conducted to overcome this issue. Indeed, tuning the excitation
energy (probed depth) as well as gas exposure during photoemission experiments (NAP-XPS) successfully
revealed the “effective” SVO spectral signature.
Otherwise, the very sensitive surface reactivity of SVO has also been studied. We particularly obtained valuable
results after treatments in liquid water which remove the Sr-rich layer on top of SVO thin films and provide new
surface termination for the uppermost vanadium atoms[2]. Complementary angle resolved XPS and Low Energy
Ion Scattering (LEIS) experiments confirmed the vertical arrangement and the formation of V-rich surface after
water exposure.
Finally, we have monitored by XPS the influence of ion beam irradiations on the famous LaAlO3/SrTiO3
(LAO/STO) heterostructure. We demonstrated that a short-time cluster ion irradiation of the LAO surface induces
significant modifications in the chemical and electronic properties of the buried STO substrate[3]: i) a lowering of
Ti atoms oxidation states (from Ti4+ to Ti3+ and Ti2+) correlated to the formation of oxygen vacancies at the LAO
surface and ii) creation of new surface states for Sr atoms after atom migration through the covering layer.