From zirconia to yttria: sampling the crystallographic and electronic phase diagram using sputter‐deposited
YSZ
thin films
Authors: Götsch Thomas , Stöger‐Pollach Michael , Penner Simon 
Published: December 2016

Yttria‐stabilized zirconia (YSZ) is one of the most extensively investigated materials with a large field of applications due do its chemical inertness and mechanical as well as thermal stability. Moreover, it features a high ionic conductivity at elevated temperatures; it is consequently an ideal candidate as electrolyte in chemical sensors and solid oxide fuel cells. While there is a large number of works on this topic, most only deal with a single composition and thicker films. Here, we present a systematic study regarding the influence of the amount of yttria on the crystallographic and electronic properties of YSZ thin films to elucidate the thin film phase diagram. These epitaxially‐grown thin films with a nominal thickness of 25 nm and compositions of 3 mol%, 8 mol%, 20 mol% and 40 mol% Y
2
O
3
(from now on called 3YSZ, 8YSZ, 20YSZ and 40YSZ, respectively) were prepared on NaCl single crystals using our home‐built direct‐current ion beam sputter source.[1] The films, which could be floated off the sodium chloride to yield unsupported thin films for TEM studies, were shown to be homogeneous and impurity‐free.

Although the distinction between the tetragonal and cubic phase of zirconium oxides is impossible with only selected area electron diffraction, due to both featuring planes with the same lattice spacings (see Figure 1a‐d), it is possible to determine that 3YSZ and 8YSZ are tetragonal and 20YSZ as well as 40YSZ are cubic. For this, an analysis of the lattice parameter
c
, which is calculated from multiple diffraction rings for both, the tetragonal and cubic polymorphs, was performed, showing a stagnation in this parameter between 8 to 20 mol% yttria (see Figure 1e). Because a higher concentration of the larger atom (Y) is present, the cell volume still has to increase, which can only be explained if the total unit cell volume is still increasing, which means that the cell is expanding in the other two directions (
a
and
b
) instead, i.e. a phase transformation to the cubic cell is occurring.[1]

In order to obtain the direct band gaps, valence EELS experiments have been conducted (using only the direct beam) at an acceleration voltage of 60 kV to avoid Čerenkov losses.[2]. If the resulting band gaps (after Richardson‐Lucy convolution of the spectra), which agree nicely with UV‐photoelectron data, are plotted as a function of the amount of yttria present in the sample, a similar trend to that of the lattice parameter (see above) can be observed, as seen in Figure 2. Going from 3YSZ to 8YSZ, the band gap decreases from 5.6 eV to 5.3 eV, respectively, only to then increase again to almost 5.8 eV in the case of 20YSZ, coinciding with the tetragonal‐cubic phase transition. Finally, 40YSZ features a much lower band gap of about 5.2 eV. In additional EELS experiments, the crystal field splitting between the e
g
and t
2g
bands (investigated using the oxygen K‐ELNES) is also shown to feature a dependency of the dopant level.

Furthermore, a Gatan Vulcan Cathodoluminescence system is used to spectroscopically measure the Čerenkov radiation emitted from the sample when exposed to a 200 kV beam. Determining the absorption edge seems to yield the distance from the valence band maximum to the gap states (present due to the oxygen vacancies), as determined by comparison with DFT band structures.