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Langmuir 1986, 2, 30-36
Oxides Formed on Polycrystalline Titanium Thin-Film Surfaces: Rates of Formation and Composition of Oxides Formed at Low and High O2 Partial Pressures Michael C. Burrellt and Neal R. Armstrong* Department of Chemistry, University of Arizona, Tucson, Arizona 85721 Received January 4, 1985 The room temperature surface oxidation of evaporated titanium thin films was investigated by a quartz crystal microbalance and Auger electron spectroscopy. The initial oxidation reaction was characterized by a linear oxygen adsorption rate and the formation of a Ti02/Ti203layer and ends when ca. three monolayers of oxygen have been adsorbed. Continued oxidation occurs at a logarithmic rate, and predominantly Ti02is formed. The rate of oxygen adsorption and the limiting oxide thickness were dependent on the constant field growth mechanism proposed by Fehlner and Mott (1970). The results of these studies are used as controls in the investigation of the electrochemicaloxidation of titanium-the subject of the following paper in this issue.
Introduction The growth of oxide layers on active metal surfaces, and particularly titanium, has been of interest for a number of years.'-I We have been interested in the nature and extent of oxide growth on titanium thin films upon O2 exposure and subsequent oxidation in an electrochemical en~ironment.'-~The prehistory of the titanium surface dictates, in large measure, the reactivity upon contact with the electrolyte. This was of particular importance to us recently in the case where the titanium film had been fully hydrided (to TiH2)before contacting the electrochemical environment.2 The type and thickness of the oxide layer on the surface controlled the amount of electrochemical energy that had to be applied to the system to remove the hydrogen from that solid. It was clear, however, that a further understanding of the process awaited the use of more fully characterized titanium film surfaces. The hydrogen uptake (from the gas phase) by titanium and its alloys also continues to be of interest.&l' As predicted, the extent of prior oxide growth on those surfaces controls the energy input necessary to insert and later withdraw hydrogen. Once the oxide on the titanium surface has been fully characterized it is important to compare oxide formation on titanium surfaces that have been hydrided to various extents. It is also this oxide that is of major interest in subsequent electrochemical experiments. Surface characterization techniques such as Auger electron spectroscopy (AES) and quartz crystal microgravimetry (QCM) can be used in a quantitative fashion to monitor the progress of surface oxidation, hydriding, and even electrochemical processes. In previous publications1J2J3we have demonstrated that correct treatment of AES data (removal of electron-scattering contributions to the secondary electron emission) can lead to accurate compositional information about a titanium surface, within the sampling depth (ca. 20 A) of the predominate Ti and 0 AES transitions. The QCM can monitor the progress of reactions which proceed well beyond that distance, thus complementing the surface sensitive AES technique. In this first paper of this series, we document our recent characterization of the growth of oxide layers on titanium surfaces, during and beyond the initial oxidation regime which was reported ear1ier.l During initial oxidation (75 A equivalent thickness). First, the rate and extent of oxide formation (at constant pressure) of the thicker films was independent of initial Ti film thickness, showing that an identical passivating oxide layer was formed on each surface. Second, after the initial linear uptake of O2with time, the adsorption reaction continued at a slower rate which depended on the O2pressure. Figure l, for example, shows the QCM mass uptake curves for a series of 1000 A thick Ti films exposed to O2at pressures ranging from 5 X lo4 to 1.5 X lo4 torr. This pressure-dependent oxide growth on continuous titanium film surfaces will be considered in more detail below. The additional oxygen adsorption beyond initial oxidation was observed only in the case of the continuous Ti films greater than 40 A equivalent thickness. Oxidation Depth and Surface Roughness of Continuous Ti Films. In order to relate the observed oxygen mass uptake to the actual thickness of the oxide formed, (15)Sauerbrey, G., 2. Physik 1957,155, 206. (16)Kasemo, B. Proc. Int. Vac. Congr., 7th 1977,2,1197.
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lshase
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900 1800 EXPOSURE T I M E , sec
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Figure 1. Oxygen u take (frequency shift) as a function of time for a series of 1000-1Ti films at O2 partial pressures of (a) 1 X lo-' torr, (b) 2 X lo-', (c) 5 X lo-', and (d) 1.5 x lo4 torr. 0
K E ,cV
TI
Energy Loss,eV
Figure 2. AES and ELS spectra of Ti film surfaces oxidized to differing extents. (a) AES and (b) ELS spectra of a film with 670 ng/cm2 of oxygen (19 equivalent monolayers of oxide); (c) AES and (d) ELS spectra of a film with 2600 ng/cm2 of oxygen (330 equivalent monolayers of oxide).
the ratio of the true surface area to the projected geometric area of the adsorbing surface must be known (e.g., the surface roughness factor, R). Values of R typically range from 2 to 8 for evaporated films depending upon the metal and the film preparation procedure.l'J* The surface roughness in this case was estimated by measuring the total oxygen uptake for films exposed to atmospheric pressure 02,which forms a passive oxide (predominantly TiOJ whose thickness (40-50 A) has been previously determined by XPS measurement^.'^ The value of the surface roughness factor R was determined by: R = apparent oxide thickness (QCM data)/actual thickness (40 A) The total amount of adsorbed oxygen in the case where these same Ti films were exposed to atmospheric pressure O2 (see below), determined from the QCM frequency shift of 500 Hz,corresponded to an apparent Ti02thickness of 125 A. The value of R = 125 AI40 A = 3.1 was determined and is used in subsequent calculation of actual oxide thicknesses for all ranges of O2exposure. (17) Chopra, K. I. "Thin Film Phenomena"; McGraw-Hill New York, 1969. (18)Murt, E. M., Guldner, W. G., Eds. "Physical Measurement and Analysis of Thin Films"; Plenum Press: New York, 1969. (19)Plates, A.; Johansson, L. I.; Hagstram, A. L.; Karlsson, S. E.; Hagstram, S. B. M. Surf. Sci. 1977, 63,153.
32 Langmuir, Vol. 2, No. 1, 1986
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e
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Burrell and Armstrong
Fe on T
600
Figure 3. (a) AES spectrum of a fresh Ti film with 0.7 equivalent monolayers of Fe deposited on its surface and (b) AES spectrum of the same film after 3000 s or more exposure to 2 X torr O2to form an oxide of 25-30 A thickness.
Spectroscopic Characterization of Surface Oxides. Figure 2a shows the AES spectrum observed for a film which had adsorbed 670 ng/cm2 of O2 (17 equivalent monolayers) before oxidation was halted and is representative of spectra used to calculate stoichiometries of oxide for films during the initial stages of oxidation, yielding an average O/Ti ratio of 1.5. Since the corrected thickness (- 13-15 A) of the oxide is thinner than the Auger electron sampling depth, the underlying metal must contribute to the overall Ti Auger signal, meaning that part of the oxide is of higher stoichiometry than Ti203(e.g., Ti02), in order to account for the observed AES lollTi ratio. The features at ca.2 eV in the ELS spectrum of this initial surface oxide indicate that some of lower Ti oxidation states (Ti3+)are present as well which contribute ad d transition (Figure 3b).1220 After the initial oxidation stage has been passed ( x 3 20 A), the oxide film becomes characteristic of Ti02. Figure 2c shows an AES spectrum obtained from a film which had adsorbed 2600 ng/cm2 of O2a t a pressure of ca. 1torr (65 equivalent monolayers); the oxide at the surface in this case is stoichiometric Ti02. As expected the AES O/Ti ratio was identical with that measured from a Ti02 standard and the d d transition is absent from the ELS spectrum (Figure 2d). These results show that the predominant oxide formed in the near-surface layer is Ti02, except during the very early stages of reaction, when lower oxidation states are present as well. The rapid reaction of the lower oxide(s) to form Ti02 during continued oxygen exposure is consistent with the known reactivity of the lower oxide surfaces with 02.20,21 It is probable that a suboxide or oxidation state gradient exists below the Ti02 surface layer although it cannot be directly probed by the surface-sensitive techniques utilized in this study. However, other workers have demonstrated that the oxide film consists primarily of Ti02 (even for oxide thicknesses
