Interface effects on the electrical conductivity of oxide thin films

M. C. Göbel, E. Gilardi, G. Gregori

Not only the junctions between the grains of the same material but also the interfaces between different materials are known to have a strong influence on the conduction properties. In the context of our program to understand these interfacial ionic effects we investigate thin films of CeO₂ with different doping contents prepared with pulsed laser deposition (PLD) on a variety of substrates (such as SiO₂, Al₂O₃ or MgO). The electrical conductivity of the samples is measured as a function of thickness, temperature and oxygen partial pressure (pO₂) using impedance spectroscopy. Notably, in some cases both conductivity and activation energy vary as a function of film thickness suggesting the presence of interface effects between thin film and substrate. Conductance vs. film thickness diagrams allow us to identify and separate the bulk contribution from the substrate-film interface contribution (Fig. 1) .

Fig. 1 Conductance vs. thickness diagrams for (a) Ce0.9Gd0.1O1.95 and (b) nominally pure CeO2 thin films. The films grown on SiO2 are nanocrystalline with an average grain size which varies as a function of the thickness of the film [1]. Reproduced with permission of the PCCP Owner Societies. — Fig. 1 Conductance vs. thickness diagrams for (a) Ce_0.9Gd_0.1O_1.95 and (b) nominally pure CeO₂ thin films. The films grown on SiO₂ are nanocrystalline with an average grain size which varies as a function of the thickness of the film [1]. Reproduced with permission of the PCCP Owner Societies.

Fig. 1 Conductance vs. thickness diagrams for (a) Ce_0.9Gd_0.1O_1.95 and (b) nominally pure CeO₂ thin films. The films grown on SiO₂ are nanocrystalline with an average grain size which varies as a function of the thickness of the film [1]. Reproduced with permission of the PCCP Owner Societies.

It must be noticed however, that in polycrystalline films the average grain size varies with the film thickness and this can result in misleading interpretations of the intercept value on the y axis (conductance) of the diagrams shown in Fig. 1. As a matter of fact only the intercept of the pure films grown on Al₂O₃ correspond to a depletion of the charge carrier (electrons) at the film/substrate interface [1].

Grain boundary (GB) effects are able to strongly influence the conduction properties in many oxides. Here CeO₂ thin films of different doping content (pure, acceptor doped, donor doped) are investigated. After microstructure characterization (with XRD, FIB, TEM, etc.) the conductivity of the samples is investigated with electrochemical impedance spectroscopy (EIS) as a function of temperature and pO₂. The films with the high GB density (and therefore with the strongest GB effects) show a drastic change in the conductivity for several orders of magnitude (Fig. 2) and other unusual phenomena, such as a partially electronic conductivity of heavily doped samples at high pO₂ and low temperatures (Fig. 3) [2,4]. Such results can be quantitatively explained within the framework of the space charge model.

Fig. 2 Arrhenius plot of the Ce0.9Gd0.1O1.95 thin films with different microstructure: epitaxial, nanocrystalline with average grain size 40 nm (ht) and nanocrystalline with average grain size 10 nm (rt) [2,4]. Reproduced with permission of the PCCP Owner Societies. — Fig. 2 Arrhenius plot of the Ce_0.9Gd_0.1O1.95 thin films with different microstructure: epitaxial, nanocrystalline with average grain size 40 nm (ht) and nanocrystalline with average grain size 10 nm (rt) [2,4]. Reproduced with permission of the PCCP Owner Societies.

Fig. 2 Arrhenius plot of the Ce_0.9Gd_0.1O1.95 thin films with different microstructure: epitaxial, nanocrystalline with average grain size 40 nm (ht) and nanocrystalline with average grain size 10 nm (rt) [2,4]. Reproduced with permission of the PCCP Owner Societies.

Fig. 3 Oxygen partial pressure dependence of nanocrystalline thin films with an average grain size of 40 nm (black symbols) and 10 nm (red symbols), respectively. Remarkably at pO2 = 10-5 bar, 25% of the total conductivity is electronic [2,4]. Reproduced with permission of the PCCP Owner Societies. — Fig. 3 Oxygen partial pressure dependence of nanocrystalline thin films with an average grain size of 40 nm (black symbols) and 10 nm (red symbols), respectively. Remarkably at pO₂ = 10^-5 bar, 25% of the total conductivity is electronic [2,4]. Reproduced with permission of the PCCP Owner Societies.

Fig. 3 Oxygen partial pressure dependence of nanocrystalline thin films with an average grain size of 40 nm (black symbols) and 10 nm (red symbols), respectively. Remarkably at pO₂ = 10^-5 bar, 25% of the total conductivity is electronic [2,4]. Reproduced with permission of the PCCP Owner Societies.

Publications:

M.C. Göbel, G. Gregori, X.X. Guo and J. Maier, “Boundary effects on the electrical conductivity of pure and doped cerium oxide thin films”, Phys. Chem. Chem. Phys. 12 (42), 14351-14361 (2010). DOI: 10.1039/c0cp00385a
M.C. Göbel, G. Gregori, and J. Maier, “Mixed conductivity in heavily acceptor-doped CeO₂ thin films under oxidizing conditions”, Phys. Chem. Chem. Phys. 13, 10940-10945 (2011). DOI: 10.1039/c1cp20248k
M.C. Göbel, G. Gregori, and J. Maier, “Electronically blocking grain boundaries in donor doped cerium dioxide”, Solid State Ionics 251, 45-51 (2012). DOI: 10.1016/j.ssi.2012.03.036
G. Gregori, M.C. Göbel, and J. Maier, “Electronic conductivity in nanocrystalline Ce_0.9Gd_0.1O_1.95 thin films at high oxygen partial pressures”, ECS Transactions 45 [1], 19-24 (2012). DOI: 10.1149/1.3701307
M.C. Göbel, G. Gregori, and J. Maier, “Size effects on the electrical conductivity of ceria: Achieving low space charge potentials in nanocrystalline thin films”, J. Phys. Chem. C 117 [44], 22560-22568 (2013). DOI: 10.1021/jp407585w
K. Song, H. Schmid, V. Srot, E. Gilardi, G. Gregori, K. Du, J. Maier, P.A. van Aken, “Cerium reduction at the interface between CeO2 and Y2O3-stabilised ZrO2 and implications for interfacial oxygen non-stoichiometry”, APL Materials 2, 032104 (2014).
M.C. Göbel, G. Gregori and J. Maier, “Numerical calculations of space charge layers effects in nanocrystalline cerium oxide. Part 2”, Phys. Chem. Chem. Phys. 16 [21], 10175-10186 (2014).
M.C. Göbel, G. Gregori and J. Maier, “Numerical calculations of space charge layers effects in nanocrystalline cerium oxide. Part 1”, Phys. Chem. Chem. Phys. 16 [21], 10214-10231 (2014).