Correlated oxide semiconductors offer a unique relation between local structure and electrical properties because small amount of impurity atoms allow for significant changes in the band structure^1,2. Electron doping of strongly correlated nickelates such as SmNiO₃ (SNO) by small ion incorporation has been observed to change resistivity up to ten orders of magnitude – manifesting a quantum mechanical driven phase transition via ~3eV band gap opening³. During this process, cations with small atomic radii intercalate, reducing the Ni valence in of the SNO. Nevertheless, it is difficult to directly characterize the small amounts of such light elements within the structure. This study focuses on the band gap opening of the correlated oxide SmNiO₃ upon charge doping as well as structural changes introduced by the dopants during this phase transition. The correlation among doping level – structural changes – electrical properties of SNO will be established.

Experimental

SNO thin films are grown by physical vapor deposition (PVD) from Sm and Ni targets in an Ar/O₂ atmosphere and subsequent high pressure O₂ annealing. LaAlO₃ and Si/SiO₂ substrates were used to create epitaxial and polycrystalline samples respectively. Films were then subjected to electrochemical treatment in a liquid environment to dope light elements into the lattice. Characterization of the changes in film resistance was used in conjunction to determine dopant distribution overall effect on SNO thin film properties. Synchrotron x-ray and neutron scattering will be used to investigate the structural evolution with doping of SNO.

Results and Discussion

Figure 1 shows resistivity changes as a function of temperature as the sample is doped with lightweight elements. It can be readily observed that this non-volatile doping process significantly changes not only the resistivity of the film, but also the characteristic metal-insulator transition observed in a number of pristine nickelates. This doping process is non-volatile due to resistivity remaining stable at ambient conditions. Notably, there is no change to the film resistance without further bias application.

To better understand the structural implications of the doping process, X-ray diffraction was used to explore the effects on the SNO lattice with the incorporation of these impurity ions. Epitaxial SNO on LAO substrates before and after light element doping show a clear shift of the diffraction peaks of SNO to lower angle. This observation is consistent in that the lattice expands to accommodate the dopant ions. Additional investigations on the electrochemical doping process are also explored through cyclic voltammetry and will be discussed during the presentation.

Summary

SNO thin films show a large change in resistivity when doped with light elements via charge transfer to the Ni orbitals. To better understand the phase transition as it relates to this doping process, we highlight the association between the changing structure and properties as the light elements are intercalated into the lattice. A better understanding of the carrier density modulation and distribution are correlated with structural changes imparted with the electrochemical treatment. Ultimately, new insights into this highly sensitive wide gap semiconductor may be useful for future sensing applications.

References

1. Rondinelli, J.M., May, S.J. and Freeland, J.W., 2012. Control of octahedral connectivity in perovskite oxide heterostructures: An emerging route to multifunctional materials discovery.MRS bulletin, 37(03), pp.261-270.
2. Middey, S., Chakhalian, J., Mahadevan, P., Freeland, J.W., Millis, A.J. and Sarma, D.D., 2016. Physics of Ultrathin Films and Heterostructures of Rare-Earth Nickelates.Annual Review of Materials Research, 46, pp.305-334.
3. Shi, J., Zhou, Y. and Ramanathan, S., 2014. Colossal resistance switching and band gap modulation in a perovskite nickelate by electron doping. Nature communications, 5, 4860.

Figure 1. Doping of SmNiO₃ changing the resistance as a function of temperature and derivative.