Quantitative local environment characterization in amorphous oxides

We perform density-functional-theory calculations of electronic core levels to obtain the tellurium x-ray photoelectron spectra in the amorphous solar-energy materials ${\text{CdTeO}}_x$ ($x=0.2$, 1, 2, and 3). We quantify the distribution of local tellurium environments that sum up to the total two-peak structure in the experimental spectrum. The general trend is that the more oxygen neighbors tellurium has the bigger the shift of its core-level energy. However, due to the structural complexity, the relation between the core-level shift and the number of oxygen neighbors does not obey simple rules. Hence, we show the importance of computer simulations when interpreting x-ray photoelectron spectra in this system, in particular, and amorphous oxides in general.

Figure 1

Correlation between the initial-state approximation and $Z+1$ excitation energy for CdTeO. The other compounds studied here also exhibit the same behavior.

Figure 2

(Right $y$ axis) Comparison between the CLS calculated by VASP and Q-E in the $Z+1$ approximation. (Left $y$ axis) Comparison of CLS calculated with Q-E within the excited-core approximation and the $Z+1$ approximation.

Figure 3

The calculated tellurium XPS spectra are shown for ${\text{CdTeO}}_{0.2}$, CdTeO, ${\text{CdTeO}}_2$, and ${\text{CdTeO}}_3$. The bars indicate the tellurium energy shifts. The height of the bars and the numbers in parenthesis denote the number of oxygen neighbors, which was calculated within a cut-off radius of $2.32\text{Å}$.

Figure 4

In the left panel we show the experimental XPS data for the $\text{Te}3d_{3/2}$ peak reproduced from Ref. 3. The labels on the vertical axis correspond to relative ${\text{NH}}_3$ pressures for each experiment expressed in units of $\times {10}^{-4}\text{Pa}$. In the right panel, we present the calculated spectra as obtained in the $\left(Z+1\right)$ approximation. The dashed vertical lines indicate core-level shifts for calculated $\alpha {\text{-TeO}}_2$, $c{\text{-CdTeO}}_3$, and CdTe crystal structures. The binding-energy scale of the theoretical results is not absolute and only energy differences may be compared to experiments.

Figure 5

The ratio between the Gaussian areas of each peak in the calculated tellurium XPS spectra is plotted in the upper graph as a function of the oxygen content $x$. The lower graph contains the same ratios as obtained from experiments (Ref. 3) but plotted as a function of ${\text{NH}}_3$ partial pressure. The dashed lines show the fits to the different data sets.

Figure 6

The $x\left(P\right)$ expression obtained in Eq. (5) is displayed. The dashed curve is the corresponding refitted function with the experimental saturation limit. The open pentagons show the experimentally determined oxygen content from Ref. 3.