Aluminum

Aluminum oxides and hydroxides (Al2O3, Al(OH)3 and AlOOH) are extremely difficult to differentiate by XPS. A compilation of Al 2p3/2 values and modified Auger parameter values (Al 2p - KL23L23(1D)) are presented below. Unfortunately, statistical speaking, the values for the varying oxides and hydroxides overlap each other.
Table 1. Al 2p3/2 binding energy values [1].

Table 2. Al 2p-KL23L23(1D) modified Auger Parameter values [1].

Reference:
[1] C.D. Wagner, A.V. Naumkin, A. Kraut-Vass, J.W. Allison, C.J. Powell, J.R.Jr. Rumble, NIST Standard Reference Database 20, Version 3.4 (web version) (http:/srdata.nist.gov/xps/) 2003.

Aluminum Oxide Thickness Measurement

If the metal:oxide ratio can be determined for a thin film oxide sample (~0-9 nm) and if the inelastic mean free path (IMFP, λ) of the metal (λm) and oxide (λox) is known (or can be calculated), the oxide film thickness can be calculated using the calculations of the type developed by Strohmeier [1] and Carlson [2] defined as follows:

d=λoxsinθ ln(((NmλmIox)/(NoxλoxIm))+1) (Eq. 1)

where θ is the photoelectron take-off angle, Iox and Im are the area percentages of the oxide and metal peaks from the high-resolution spectrum, and Nm and Nox are the volume densities of the metal atoms in the metal and oxide, respectively.

For an aluminum oxide film on an aluminum substrate, the Al 2p spectrum has well separated oxide and metal peaks (example shown in Figure 1) and Io and Im values can be readily ascertained. Using the λ and N values proposed by Strohmeier [1], an Excel based aluminum oxide thickness calculator is presented. Just input the percentage of metal from the high-resolution spectrum to get a film thickness in Angstroms. (Note: you must download the file to Excel to use it - it is locked in Google Docs).

Figure 1. Al 2p XPS spectrum of a thin film Al oxide on Al metal with a calculated oxide thickness of 3.7 nm.

References:
1. B.R. Strohmeier, Surf. Interface Anal. 15 (1990) 51.
2. T.A. Carlson, G.E. McGuire, J. Electron Spectrosc. Relat. Phenom, 1 (1972/73) 161.

Plasmon Loss

For some materials, plasmon loss peaks may occur. These involve an enhanced probability for loss of a specific amount of energy due to the interaction between the photoelectron and other electrons. For conductive metals, the energy loss (plasmon) to the conduction electrons occurs in well-defined quanta arising from group oscillations of the conduction electrons[1]. Plasmons attributed to the bulk of the material and its surface can sometimes be separately identified. An example of plasmon loss peaks in a spectrum of aluminum metal is presented in Figure 1. In some cases, such as with Al, the plasmon loss structure can interfere with the assignment and quantification of other spectral peaks such as Si 2p, Si 2s and P 2p.

Figure 1. Plasmon loss structure in a spectrum of aluminum metal.

Table 1. Plasmon loss structure peak positions, FWHM and percent peak area (% concentration) from a survey spectrum (160 eV pass energy) of sputter cleaned aluminum metal (Figure 1.)

Reference:
[1] J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer Corp, Eden Prairie, MN, 1992.