Room-temperature oxidation of ultrathin TiB 2 films
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J.F. Sullivan Department of Chemical Engineering, The University of Alabama, Tuscaloosa, Alabama 35487-0203
J.A. Barnardb) and M.L. Weaverc) Department of Metallurgical and Materials Engineering, The University of Alabama, Tuscaloosa, Alabama, 35487-0202, and Center for Materials for Information Technology, The University of Alabama, Tuscaloosa, Alabama 35487-0209 (Received 16 July 2001; accepted 23 January 2002)
Titanium diboride has been claimed as a very promising candidate material for protective applications in the magnetic recording. Its oxidation resistance at room temperature is a critical criterion in assessing this application potential. In this paper, the oxidation characteristics of ultrathin TiB2 thin films, such as overcoat erosion and oxide thickness, are investigated via a combination of x-ray reflectivity, x-ray photoelectron spectroscopy (XPS), and atomic force microscopy. It was found that a 96 h.
data listed in Table I. Since postdeposition oxidation was inevitable even during film cooling in high vacuum, t0 actually cannot be measured directly. However, it (preoxidation thickness) can be reasonably assumed to be a linear function of the nominal thickness tn (calculated thickness based on deposition rate). Thus, a plot of t0 versus tn should yield a straight line crossing the origin, the slope of which indicates the ratio of the as deposited thickness to the nominal one. Because ⌬t is independent of t0 and t1 + t2 ⳱ t0 + ⌬t (Fig. 1), a t1 + t2 versus tn plot should also yield a straight line whose intercept with the t1 + t2 axis is ⌬t and can be seen as having translated the t0 versus tn line upward by ⌬t. Such a t1 + t2 versus tn plot was shown in Fig. 5. As expected, the points really lie on a straight line with positive intercept with t1 + t2 axis. A least-squares analysis of the points yields t1 + t2 ⳱ tn + 5.9 Å with perfect linearity, which confirms that t0 is indeed linearly proportional to tn and t0 ⳱ tn on the average. Therefore, for these TiB2 films the thickness gain due to oxidation, ⌬t, is thus approximately 6 Å. Accordingly, the average erosion of these films te ⳱ t1 − ⌬t (Fig. 1) is approximately 9 Å. The oxide thickness t1 is significantly larger than the overcoat erosion te (t1/te ≈ 1.6), indicating considerable swelling induced by oxidation.
XPS was used to determine the chemical states of the Ti, B, and O at different depths throughout the film to further characterize the studied films. An XPS survey spectrum obtained from the as-prepared 10-nm TiB2 is shown in Fig. 6. Features originating from Ti, B, O, and C are evident. The presence of oxygen is not surprising since the sample had been exposed to ambient conditions, resulting in an oxide layer at the surface. Moreover, features corresponding to carbon disappeared completely prior to the collection of highresolution spectra, indicating either a volatile substance that rapidly evaporated or an area of local rather than uniform contamination. The composition of this
FIG. 5. Total thickness (t1 + t2) versus nominal thickne
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