Composition and crystallography dependence of the work function: Experiment and calculations of Pt-Al alloys

The work function (WF) of several phases of Pt-Al alloys is investigated. A first-principles study is performed to determine the dependence of the vacuum WF (VWF) on the phase, crystallographic orientation, and atomic termination. In parallel, the effective WF (EWF) of these alloys is experimentally measured using metal-oxide semiconductor devices. A detailed microstructural characterization based on x-ray diffraction and transmission electron microscopy allows the comparison between experiment and calculations. It is found that despite the formation of a complex microstructure, the calculated VWF values are in good agreement with the experimental EWF values. The possible VWF range for each phase has a relatively small (∼0.5 eV) span for different orientations and atomic terminations, and it is found that Pt atoms in the terminating plane are more dominant than Al atoms. The results demonstrate that first-principles calculations of the WF of alloys can be used to successfully predict the experimental WF and that knowledge of the microstructure is important for the accuracy of these calculations.

Figure 1

Representative slab models constructed for (a) and (b) (100)-oriented tetragonal Pt${}_3$Al phase and (c) and (d) (001)-oriented rhombohedral PtAl phase. The large (green) and the small (red) points represent Pt and Al atoms, respectively. The figure demonstrates (a) a mixed atomic termination vs (b) Pt atomic termination and (c) Al-terminated vs (d) Pt-terminated planes.

Figure 2

Schematic cross sections of (a) the beveled oxide sample, (b) uniform SiO${}_2$/Si thickness sample, and (c) the different metal electrodes.

Figure 3

Glancing mode XRD spectra of all samples, each normalized with respect to its largest peak. Vertical lines are positioned in accordance with the appropriate JCPDS reference for the phases specified in the legend and in Table .

Figure 4

(a) A cross-section HRTEM micrograph of a 1:1 Pt:Al sample showing the full thickness of the layer, (b) an FFT analysis of the larger image, and (c) a 3$\times$ enlarged segment of the larger image presented at a different contrast.

Figure 5

A dark-field (DF) cross-section TEM micrograph of a 1:1 Pt:Al sample showing a bright diffraction contrast from several grains highlighted with arrows.

Figure 6

(a) Flatband voltage ($V_{FB}$) vs EOT curves for Pt, Al, and their alloys. (b) A summary of the experimental EWF (exp.) obtained from the intercepts of Fig. 6a and prior knowledge of $\Delta$ vs the atomic Pt concentration of the samples. The DFT-calculated VWF of the different phases (Table ) are superimposed on the experimental results in the appropriate composition. The average VWF value for each phase is marked with a dashed line for clarity. In addition, the literature (Ref. 31) VWFs (lit.) for Pt and Al are included. The shaded region represents the possible range of values based on the results.

Figure 7

Representation of the Fig. 6b data vs the studied samples. Here, the EWF data points are enlarged for clarity to be at the size of their vertical error. The average VWF value and the likely VWF values based on a preferred orientation analysis are shown in lines for clarity.