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Oxidation of thin scandium films
Applied Surface Science 211 (2003) 136–145
Oxidation of thin scandium films A. Shih*, J.E. Yater, C. Hor, R. Abrams Naval Research Laboratory, Code 6844, Washington, DC 20375, USA Received 31 October 2002; accepted 4 February 2003
Abstract Thin Sc films (namely, 20 layers or thinner) are studied as a function of oxygen exposure using Auger Electron Spectroscopy (AES) and Temperature Programmed Desorption (TPD) measurements. While AES data indicate that surface Sc layers oxidize rapidly, TPD data indicate that at room temperature the oxidation process is confined to the top layers. The surface oxide layer, about 5 layers thick, protects the Sc underneath from oxidation. However, thin Sc films can be completely oxidized by heating the W substrate to a temperature between 220 and 420 8C during oxygen exposure. The enhanced oxidation is the consequence of the coalescence of surface oxide, which breaks the protective surface oxide layer, exposing the Sc underneath to oxygen. AES measurements reveal that Sc oxide in the surface layers of an oxidized Sc film is Sc2O3. However, Sc and ScO are the only Sc-containing species found among the desorption products, and their relative amounts depend on the extent of oxidation of the film. Between 1400 and 1600 K during a TPD process, some of the Sc2O3 interact with W and Sc and dissociate into Sc2O2 while W and Sc are oxidized. Those Sc atoms which are not consumed during the oxidation–reduction interaction adsorb on Sc2O2. The large amount of Sc that desorbs from Sc2O2 between 1500 and 1700 K reveals that the surface area of Sc2O2 is 4.2 times the surface area of the W substrate, indicating that Sc2O2 coalesces into clusters. Sc2O2 then dissociates into ScO, which sublimates above 1650 K. In most oxidized Sc films, copious Sc2O3 could remain after depletion of the supply of reducing agents (W and Sc). The remaining layers of Sc2O3 dissociate into ScO and ScO2 above 1850 K when ScO sublimates, leaving behind ScO2. Published by Elsevier Science B.V. PACS: 82.65 My; 79.40þz Keywords: Thermionic emission; Dispenser cathode; Thermal desorption; Scandate cathode; Scandium; Scandium oxide
1. Introduction Dispenser cathodes are widely employed thermionic cathodes, particularly in applications where an emission density >1 A/cm2 is demanded. A standard dispenser cathode consists of a porous W disk impregnated with a Ba compound. During operation Ba is *
Corresponding author. Tel.: þ1-202-767-2260; fax: þ1-202-767-1280. E-mail address: [email protected] (A. Shih). 0169-4332/03/$ – see front matter. Published by Elsevier Science B.V. doi:10.1016/S0169-4332(03)00250-2
continuously dispensed onto the emitting surface, replenishing the surface Ba lost through evaporation. The emission density can be greatly enhanced by coating the W surface with a layers of Sc oxide and W mixture. The resulting dispenser cathode is known as a scandate cathode [1–4]. Given the same operating temperature, the emission from a scandate cathode can be as much as 200 times the emission from a standard dispenser cathode. The scandate cathode is the most promising high-emission-density thermionic cathode, and an emission density of 400 A/cm2 has
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been measured at 1030 8C [4]. However, such a high emission density has not been easily reproduced. One of the challenges in the fabrication process is to maintain the correct proportion and oxidation state of various components (i.e. W and Sc) in the scandate cathode coating [5]. Aside from fabrication challenges, scandate cathodes may present problems during actual operation. For example, the emission characteristics have revealed that the emission density may be non-uniform. In addition, the difficulty in emission recovery after ion sputtering anticipates a problem with robustness when the cathode is operated in a tube environment. To circumvent these problems, it was suggested that Sc rather than Sc oxide be used in the coating preparation (for examples, see [6]). The higher mobility of Sc compared to Sc oxide could lead to better coating uniformity and faster Sc replenishment after ion sputtering damage. However, our recent study determined the evaporation rates of Sc and Sc oxide as a function of temperature, and suggested that Sc in the coating would be depleted by sublimation during activation of the scandate cathode [7]. Consequently, Sc in the coating needs to be oxidized, while metallic Sc will have to be supplied from the chemical interaction of Sc oxide. Methods are being tried, e.g. addition of Re in the coating, in order to ensure adequate supply of Sc. In this study, we investigate the oxidation process of Sc as well the interaction of Sc oxide with W in absence of Re. This effort is part of our larger goal [7,8] to understand the interactions among Ba, Sc, W, Re, and O, which are present on the scandate cathode surface.
2. Experimental approach Thin Sc films (less than 20 layers) are deposited by evaporation of metallic Sc onto a W polycrystalline ribbon, and the films are then oxidized by exposure to oxygen. The oxidized films are analyzed by Auger Electron Spectroscopy (AES) and Temperature Programmed Desorption (TPD). AES data determine the thickness of the Sc films [7] and measure the extent of oxidation after each oxygen exposure. TPD data provide information on the species in the reaction products, the amount of each species, and the respective binding energies. Fig. 1 describes the experimental
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Fig. 1. Schematic of the experimental setup.
setup. Sc metallic chips are placed inside an evaporation crucible which is made of W wires. A variable leak valve is employed to control the oxygen partial pressure during oxygen exposure, and AES spectra are taken before and after each oxygen exposure. TPD spectra are taken with the sample surface normal pointing along the line of sight of a UTI Quadruple Mass Analyzer (QMA). During a TPD measurement, the desorption of a specific species is measured while the sample temperature (T) is raised linearly as a function of time (t), i.e. T ¼ T0 þ bt
(1)
where T0 is the initial temperature and b is the rate of the temperature rise. For a linear temperature rise, the desorption rate, N(T), of a specific species can be expressed [9] by g E NðTÞ ¼ n sn exp (2) kT b where E is the desorption energy, k is Boltzman constant, gn is rate constant, s is surface coverage, and n is the order of the desorption kinetics. The identification of the desorption kinetics will provide an essential clue in our understanding of the oxidation process (Sections 4 and 5). Zeroth-order desorption kinetics indicate that the nature of the adsorption state is a multiple-layer state. Because only the topmost layer contributes to desorption, the rate of desorption from a multiple layer is independent of s,
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i.e. n ¼ 0. First-order desorption kinetics can be associated with a monolayer or submonolayer film for which the desorption rate is proportional to s, i.e. n ¼ 1. Based on Eq. (2), the desorption characteristics of zeroth-order kinetics (n ¼ 0) are as follows. The rate of desorption rises exponentially as a function of sample temperature until depletion of the species. While desorption curves from films of various thickness share the same rapidly rising slope, the spectrum from a film of thicker coverage does not begin to fall off until a higher temperature. Consequently, the peak position increases with the film thickness [8]. In contrast, first-order desorption kinetics (n ¼ 1) lead to a constant peak position for films of different thickness [9]. QMA measurements enable us to study the desorpion of each individual species. We found that Sc and ScO are the major species desorbed from oxidized Sc films. We chose to record ScOþ spectra instead of ScOþþ spectra because the mass-to-charge ratio of ScOþþ is close to that of the isotopes of COþ, a major residual gas in our vacuum system. Scþ and Scþþ spectra are quite similar, but we chose to study Scþþ spectra since they were studied in our earlier investigation [7].
3. Oxidation of Sc films AES spectra record the oxygen intake by the Sc films after each oxygen exposure. Specifically, the ratio of the O-to-Sc Auger intensities is plotted using the O KLL transition 503 eV and Sc LMM transition at 340 eV. Fig. 2 shows a plot of the ratio taken on a 13-layer Sc film as a function of the oxygen exposure in Langmuirs (L) (1 L ¼ 106 Torr s; if every impinging molecule sticks on the surface, 1 L exposure results in one full layer of coverage on the surface). On the 13-layer Sc film, the O-to-Sc Auger ratio reaches saturation around 150 L. The inset, which expands the low range of the exposure scale, emphasizes the rapid rise of the O signal in the first 30 L of exposure. On a 3-layer Sc film, the O signal rises slightly faster than on the thicker film, increasing linearly for the first 15 L and reaching saturation at about 50 L. Because of the limited probing depth of Auger electrons, we learn from AES spectra about the che-
Fig. 2. O-to-Sc Auger peak height ratio is shown as a function of oxygen exposure. The AES data were taken on a 13-layer Sc film. The rise in the ratio is rapid over the first 50 L of exposure, and the inset shows the first 200 L of exposure.
mical changes in the surface layers only. At 503 and 340 eV, the O KLL and Sc LMM Auger lines have an escape depth of about 4.5 layers of Sc [10]. In other words, the Auger data reveal only the chemical composition within the top 4.5 layers and provide no information on the layers below. In order to ensure full oxidation, we expose each film to oxygen beyond saturation of the O AES signal. For example, 3-layer Sc films were exposed to 240 L of oxygen while 13-layer Sc films were exposed to 1920 L of oxygen. A ScOþ TPD spectrum, shown in Fig. 3, was taken from a 2-layer Sc film after 160 L of oxygen exposure.
Fig. 3. ScOþ TPD spectrum from a 2-layer Sc film after 160 L of oxygen exposure. Shown together for comparison is the Scþþ TPD spectrum from a freshly deposited 2-layer Sc film.
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Fig. 4. The amount of desorbed ScO is shown as a function of the Sc film thickness. The Sc films were oxidized by heavy oxygen exposure. The total area in each of the ScOþ TPD spectra provided the amount of the desorbed ScO.
We designate the ScO states by the lower case letters a, b, and c in order to distinguish them from the Sc states on W, which we have designated by the upper case letters A, B, and C [7]. In order to compare the peak positions of both types of states, a Scþþ TPD spectrum from a fresh 2-layer Sc film is also shown in Fig. 3. Clearly, ScO states a, b, and c are tighter binding states than Sc states A, B, and C [7], as manifested by the higher desorption temperatures. We will discuss the differences among states a, b, and c latter in Section 5, but for now only the sum of the areas under peaks a, b, and c is needed to provide a measure of the total amount of ScO present in the film. Fig. 4 shows the area sum as a function of the initial Sc film thickness on the W substrate for a series of Sc films. If the Sc films were thoroughly oxidized, we would expect a linear relation between the amount of Sc oxide and the initial Sc film thickness. The deviation from linearity can be interpreted as an incomplete oxidation of the Sc films. The larger the amount of Sc that remains metallic, the larger the deviation will be. This interpretation is supported by Scþþ TPD spectra taken from oxidized Sc films (see Section 4). Fig. 5 compares the peak positions in a Scþþ spectrum and a ScOþ spectrum (shown in Fig. 3) that were both taken from a 2-layer Sc film after 160 L of oxygen exposure. The positions of the three highest temperature peaks in the Sc spectrum coincide with
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Fig. 5. Scþþ and ScOþ TPD spectra from 2-layer Sc films after 160 L of oxygen exposure. The positions of peaks a, b, and c in the Scþþ TPD spectrum coincide with the respective peak positions in the ScOþ TPD spectrum; these states are the oxidized Sc states. The state a is an unoxidized Sc state.
states a, b, and c from the ScOþ spectrum, and the relative peak heights within each set of the triplets are nearly identical for the two spectra. They must have the same origin and consequently states a, b, and c are assigned to them. We believe that these three peaks in the Scþþ spectrum represent Scþþ ion fragments from ScO created by the ionization current of the QMA. The weakest binding state, designated as state a, is a Sc state and it does not appear in the ScOþ spectrum. The area under peak a gives a measure of the amount of unoxidized (or metallic) Sc. In the 2-layer Sc film after 160 L of oxygen exposure, the amount of metallic Sc contained in state a is equivalent to 0.15 layers of Sc. In a thicker Sc film, the amount of metallic Sc remaining is much greater as seen in Fig. 6. For example, in a 13-layer Sc film after 1920 L of oxygen exposure, 7 layers of Sc remained unoxidized while only 5 layers of Sc were oxidized! A possible way to increase the oxidation efficiency is to heat the substrate during oxygen exposure. We studied the oxidation of 13-layer Sc films while heating the W substrate at 220 and 420 8C. With the sample at 220 8C, 2.9 and 0.7 layers of Sc remained metallic after 48 and 765 L of oxygen exposure, respectively. With the sample at 420 8C, 1.47 layers of Sc remained metallic after only 48 L of oxygen exposure. The results are summarized in Fig. 7, which shows that the heating was effective in assisting the
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revealed by the TPD spectra must be in the Sc underneath the surface layers. This point becomes clearer by examining the Scþþ TPD spectra. 4. Scþþ TPD spectra
Fig. 6. Examples of Scþþ TPD spectra with various amounts of metallic Sc remaining in the film. Spectra (a): 0.6 layers of metallic Sc remaining in a 3-layer Sc film after 240 L of oxygen exposure at 20 8C. Spectra (b): 1.5 layers of metallic Sc film remaining in a 13layer Sc film after 48 L of oxygen exposure at 420 8C. Spectra (c): 7 layers of metallic Sc remaining in a 13-layer Sc film after 1920 L of oxygen exposure at 20 8C.
progress of the oxidation process. However, plots of the O-to-Sc Auger ratio versus oxygen exposure showed no significant difference with heating, indicting that moderate heating had no effect on oxygen diffusion through the Sc films. Since the Auger probing depth is 4.5 Sc layers, the increased oxidation
Fig. 7. Effect of substrate heating on the oxidation of 13-layer Sc films. While a large amount of Sc remains metallic in the film after oxidation at room temperature (20 8C), significantly less metallic Sc remains in the film after oxidation at elevated temperatures. For clarity, straight lines were used to connect the data points (although there are insufficient data points to warrant the linear fit).
Fig. 6 shows three Scþþ TPD spectra with various Sc populations in state a, and the figure caption describes how each curve was obtained. As designated in Section 3, states a and b are ScO states, while states a, A, and B are Sc states. State a contains 0.6, 1.5, and 4.2 layers of metallic Sc in spectra (a), (b), and (c), respectively. In addition, spectrum (c) contains 2.8 layers of metallic Sc in the multiple-layer states A and B [7]. State a has a higher binding energy than state C, the first layer state of Sc adsorbed on W, which was studied and reported in our previous study [7]. The position of peak a is 1630 25 K, which is 60 K higher than the position of peak C at 1575 22 K. We attribute state a to the first-layer Sc on Sc oxide because the presence of this state is found to require the establishment of Sc oxide first. Peak a is absent for oxidized Sc films with initial Sc coverage <1 layer while it is present when the initial Sc film is >1 layer as shown in the Scþþ TPD spectra of Fig. 8. During a TPD heating process, the Sc underneath the Sc oxide diffuses through the bulk to the top of the Sc oxide film before desorption.
Fig. 8. (a) Scþþ TPD spectrum taken from a 1-layer Sc film after 90 L of oxygen exposure. The a state is absent in this spectrum. (b) Scþþ TPD spectrum taken on a 3-layer Sc film after 240 L of oxygen exposure. State a contains about 0.6 layers of Sc.
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The vertical line in Fig. 6 demonstrates that the position of peak a remains constant while the population varies from 0.6 to 4.2 layers. This desorption characteristic is a characteristic of first-order kinetics [9], which suggests that state a is a monolayer or submonolayer adsorption state. Since multiple-layer states A and B appear after state a exceeds 4.2 layers, it follows that the surface area of the Sc oxide, on which state a is adsorbed, must be 4.2 times the surface area of the W substrate. The large surface area of the Sc oxide overlayer can be readily accounted for by postulating that the Sc oxide agglomerates into clusters during the TPD heating process. A surface area gain of 3.14 would result if the clusters are assumed to have a spherical shape, which minimizes the clusters’ surface area. The remaining gain could arise from several possibilities, e.g. imperfection in the spherical shape, the presence of clusters that pile on top of the first layer of clusters, or the presence of a Sc oxide monolayer on W. We believe that the last possibility is the most probable since the Sc oxide that sits directly on W binds more tightly to W and does not participate in the cluster formation. Furthermore, if there was exposed W (i.e. not covered by Sc oxide), Scþþ TPD spectra would contain desorption peaks of Sc on W. None of the Scþþ TPD spectra showed the presence of state C, the first Sc layer on W. 5. ScOþ TPD spectra Sc oxide cluster formation is also consistent with the characteristics of ScOþ TPD spectra. Fig. 9 shows ScOþ TPD spectra of oxidized Sc films with an initial Sc thickness between 0.5 and 6.5 layers. All oxygen exposures were carried out with the W substrate at ambient temperature, i.e. 20 8C. The ScOþ TPD spectra from Sc films oxidized at 220 or 420 8C are similar, but they have larger peak heights and peak areas than the spectra in Fig. 9 (given the same initial film thickness). Heating the substrate during oxygen exposure helps to oxidize the Sc in the deeper layers. As mentioned before, there are three adsorption states of ScO on W, states a, b, and c. Our earlier study of Sc desorption from fresh Sc films indicated a layer-by-layer growth of Sc on W [7]. The highest binding energy state (first layer state C) was nearly two-third occupied before the upper layer states
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Fig. 9. ScOþ TPD spectra from Sc films of different thickness. Spectra (a): 0.5-layer Sc film after 60 L oxygen exposure. Spectra (b): 0.9-layer Sc film after 120 L oxygen exposure. Spectra (c): 2.3layer Sc film after 360 L oxygen exposure. Spectra (d): 2.9-layer Sc film after 420 L oxygen exposure. Spectra (e): 6.5-layer Sc film after 960 L oxygen exposure.
(A and B) began to form. In contrast, all three states of ScO are present starting from the thinnest film studied, i.e. spectra (a) in Fig. 9, which was taken from an oxidized 0.5-layer Sc film. In the spectra of thicker films, only peaks a and b show significant growth in size. Above 3 layers, peak b becomes much larger than peak c that peak c is no longer resolvable from peak b. As we will show below, the observation can be explained by the nucleation of ScO into threedimensional islands, e.g. clusters, where states a and b are cluster states and state c is the first-layer ScO on W. Apparently, substrate heating during the TPD measurement causes ScO to agglomerate into clusters even if the film is thin. Desorption of multiple-layer films obeys zerothorder kinetics, as discussed in Section 2. Since peaks b and c in the ScOþ TPD spectra shown in Fig. 9 overlap with each other, they need to be resolved by a peak fitting program [11]. Fig. 10 shows the component peaks, which were resolved from three ScOþ TPD spectra. Among the functions provided by the program we found that a Voight function or a Gaussian Lorentian function yielded the best fit. The resolved peaks in Fig. 10 give clear evidence that states a and b are multiple-layer states: the peak positions shift to higher temperature for higher coverage, and the peaks share the rising part of the curve, although a small offset is observed in peak b. However,
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Fig. 10. ScOþ TPD spectra are resolved into respective component peaks in order to determine the kinetics of desorption of the component states. Both states a and b exhibit the characteristics of zeroth order kinetics. Spectra (a) taken from a 2.9-layer Sc film after 420 L of oxygen exposure at 20 8C, (b) 6.5-layer Sc film after 960 L of oxygen exposure at 20 8C, and (c) 7.7-layer Sc film after 780 L of oxygen exposure at 220 8C. The 7.7-layer film was oxidized while the substrate was heated in order to have thorough oxidation of the thick Sc film.
the offset is within the standard deviation in the peak positions, 20 K. Peak c is positioned at a slightly higher temperature than peak b. Unlike peaks a and b, the position and height of peak c do not change significantly with an increase in film thickness. For films with an initial Sc thickness over 2.3 layers, peak c is no longer distinct from peak b because of the close proximity of the peaks and the much larger height of peak b. For these films, the position and height of peak c were assumed using the values that were learned from the thinner films. AES measurements reveal that state c contains less than a monolayer of ScO. Since state c is the first layer of ScO on W, the peak position should be independent of the film thickness. We need to investigate why there are two multiplelayer states which grow simultaneously with increasing film thickness. The discussion in the next section provides a satisfying explanation.
6. Oxidation state in scandium oxide films We learn from chemistry that Sc2O3 is the commonly known Sc oxide [12], and from chemical cat-
alogues that Sc2O3 is the Sc oxide that is commercially available. It is perplexing that ScO is the only oxide species that sublimates from the thin Sc oxide films in this study. We need to investigate if the Sc oxide in the oxidized films is really ScO or Sc2O3. In fact, we do not know if Sc and O react to form ScO at all. A theoretical study [13] of Sc oxide molecules and clusters found that the stable gas phase species are Sc2O2, Sc2O3, Sc2O4 dimers and Sc2O4 clusters. Specifically, the result favored Sc2O2 cluster formation instead of two neutral ScO molecules. A study by gas phase photoelectron spectroscopy [14] detected clusters of stoichiometric Sc2O2, Sc2O3, and Sc2O4. Consequently, we searched specifically for Sc2O2, Sc2O3, and Sc2O4 among the desorbed species during the TPD studies, but found no significant presence of these species among the evaporants. For the study we prepared a series of Sc films with an initial Sc thickness of about 13 layers, and we then exposed the films to large oxygen exposures while the substrate was at 20, 220 or 420 8C. In all cases, Sc and ScO were the only desorption species that contained Sc, and the relative amounts reveal the extent to which the oxidation in the film had proceeded. One should not conclude that the stable molecular species of Johnson and Panas [13] did not form on the oxidized Sc films just because they were absent from the TPD spectra. They could have dissociated before desorption. Specifically, the thermal energy required for Sc2O2 dissociation into two ScO is 3.97 eV (383 kJ/mol [13]), which is lower than 5.06 eV (488 kJ/mol ) [7], the lowest desorption energy of ScO from W. It appears that any (ScO)n cluster present on the W surface would dissociate to ScO monomers at the temperature of ScO desorption. Furthermore, a large molecule, even if it sublimated, could be broken to fragments by the impact of the energetic electrons (with average energy about 70 eV) that the UTI QMA uses to ionize gaseous molecules. Consequently, it is often not straightforward to determine the actual compound that forms based on TPD measurement alone when only molecular fragments of the compound are detected. A surface analysis study of Sc oxide films by Gorodetsky and Martyuk [15] suggested the existence of two phases of scandium–oxygen interaction: a thin Sc2O2 layer on top of a thick Sc2O3 layer. The study found also that heating the oxide film to 1100–1400 K leads to Sc2O3 dissociation and to an increase in the Sc
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surface concentration. The findings motivated our study described below, which was designed to investigate the possibility of thermal decomposition. That study reveals that reduction of Sc oxide indeed occurs but not to Sc. The increase in the Sc surface concentration reported by Gorodetsky et al. could have emerged from incomplete Sc oxidation because Sc atoms in deep layers remain unoxidized during roomtemperature oxygen exposure. Upon heating, they diffuse to the top and increase the surface concentration of Sc. In our investigation of the possibility of thermal decomposition, we utilize AES to monitor the surface compositional change during TPD. Because the presence of unoxidized Sc could lead to an increase in the Sc surface concentration upon heating, we need to oxidize the Sc films thoroughly. Specifically, a 7-layer Sc film was exposed to 1080 L of oxygen while the W substrate was at 420 8C. As discussed in Section 3, such processing conditions should ensure complete oxidation of the Sc films. After cooling to ambient temperature, the substrate temperature was then raised linearly as a function of time, but the heating was terminated abruptly at a predetermined temperature, Tmax, to allow surface compositional analysis (i.e. AES measurements). The process was then repeated to successively higher Tmax, and after each heating, AES spectra were taken at three different locations on the sample. Fig. 11(a) shows the O-to-Sc Auger ratio as a function of Tmax (with the solid circles showing data taken from the 7-layer film.) The average of the three O-to-Sc Auger ratios and the calculated standard deviation are used for the data point and the error bar, respectively. It should be noted that while the O-to-Sc Auger ratio does not directly measure the O-to-Sc atomic ratio on the surface, the changes in the ratio do provide a measure of the changes in the surface composition. Specifically, the initial O-to-Sc Auger ratio is about 1.1, but it decreases to about 0.7 or 0.8 between 1600 and 1825 K, i.e. about a 40% reduction. We know that above 1600 K the surface layer consists of a 1:1 O-toSc stoichiometry because ScO is the only species found in the evaporant. Therefore, it follows from the change in the O-to-Sc Auger ratio that the initial film consists of a 3:2 O-to-Sc stoichiometry, i.e. Sc2O3. A similar reduction in the O-to-Sc Auger ratio was observed on the surfaces of fully oxidized 1.7- and 4-layer films. However, no clear correlation of the
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Fig. 11. (a) O-to-Sc Auger ratios, and (b) W-to-Sc Auger ratios are shown as a function of the maximum ramped temperature, Tmax, of two fully oxidized Sc films. The solid circles data were taken on the film started with a 7-layer Sc film, while the open circles data started with a 13-layer Sc film. After each temperature ramp, AES spectra were taken at three different locations on the sample. Each data point represents the average value, and the error bar represents the standard deviation in the three ratios.
O-to-Sc Auger ratio with Tmax was observed on the surface of a fully oxidized 13-layer Sc film (as seen by the open circles data in Fig. 11). The apparent inconsistency can be explained by the assumption that W, a reducing agent, is responsible for the reduction of Sc2O3. Since AES probes only the top 4.5 layers of Sc oxide films, these layers on the 13-layer film were not in contact with the W substrate. Consequently, we did not observe a stoichiometric change. In thinner films, the probed region includes the oxide-W interface, and the reduction in the Auger ratio is apparent. Above 1400 K the oxide coverage becomes thinner due to ScO desorption. The thinner the oxide coverage, the weaker the W signal attenuation, and the higher the contribution to the Auger spectra from the interface. Indeed, the reduction in the O-to-Sc Auger ratio coincides with a significant appearance of W in the AES spectra (as seen by the solid circles data in Fig. 11).
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This interpretation is supported by the calculated values of heats of formation by Johnson and Panas [13]. The lowest heat of formation for Sc2O3 is ScO þ ScO2 ! Sc2 O3
(3)
DH ¼ 617 kJ=mol ðor 6:40 eVÞ Since the heat of formation is greater than the desorption energy of state a of ScO (Ed ¼ 5:06 eV [7]), Sc2O3 dissociation at the desorption temperature of state a requires the help of a reducing agent such as W or Sc. The interaction of Sc2O3 with W leads to the formation of Sc2O2 and W oxide. Since the reaction ScO þ ScO ! Sc2 O2
(4)
DH ¼ 383 kJ=mol ðor 3:97 eVÞ is exothermic, the formation of Sc2O2 is favored thermodynamically over the formation of two ScO at low temperatures. The dissociation energy of Sc2O2 (DH ¼ 3:97 eV) is lower than the desorption energy of state a of ScO (Ed ¼ 5:06 eV). Therefore, the dissociation is expected to occur before the desorption of state a. Because the O-to-Sc atomic ratios are the same for Sc2O2 and ScO, AES measurements do not differentiate between the two species. When the reducing agents are exhausted through oxidation or desorption, Sc2O3 dissociation ceases and the ScO supply drops. By itself, Sc2O3 is quite stable, with the heat of formation being 6.40 eV or higher. This Sc2O3 is expected to decompose at a temperature above the state a desorption temperature. Indeed, we observed state b of ScO above 1850 K. At these high temperature, Sc2O2 is unstable and Sc2O3 dissociates directly to ScO. This interpretation explains satisfactorily the existence of two zeroth-order desorption states in ScOþ TPD spectra. In conclusion, an oxidized Sc film consists mainly of Sc2O3. Between 1400 and 1600 K, Sc2O3 dissociates to Sc2O2 assisted by W and unoxidized Sc. Sc2O2 dissociates to ScO which then desorbs above 1650 K, giving rise to peak a in the ScOþ TPD spectra. The top layers of Sc2O3 dissociate above 1850 K without the help of a reducing agent. The dissociation leads directly to ScO and ScO2. ScO desorbs and gives rise to peak b in the ScOþ TPD spectra, leaving behind ScO2. Consequently, the O-to-Sc Auger ratio rises again (the solid circles in Fig. 11a). The nature of
the highest binding energy state c is not yet clear. At high temperature, ScO2 and W may have formed some Sc–O–W surface complex, which decomposes to give rise to peak c in the ScOþ spectra. At 2050 K when the TPD process terminates, the desorption from neither state b nor state c is complete. AES measurements determined that the remaining film contained less than a Sc monolayer. We did not heat the sample to a higher temperature because we found that by doing so the sample life would be shortened drastically.
7. Summary AES measurements indicated rapid oxidation of Sc films: the O Auger intensity reaches saturation after about 50 L of oxygen exposure. However, at ambient temperature (20 8C) the rapid oxidation is confined to the top layers of Sc only. TPD measurements indicated that for a 13-layer Sc film, 7 layers of Sc remained unoxidized even after a very large oxygen exposure (1920 L). Once the top 5 layers of oxide form, they protect the metallic Sc underneath from oxidation. The conclusion is consistent with our experience with Sc evaporators. We use Sc metallic chips as a Sc evaporation source and the Sc chips are reusable even after long exposures to atmosphere. Heating of the substrate during oxygen exposure helps to bring the oxidation to near completion. A fully oxidized Sc film consists of Sc2O3, which decomposes upon heating. Between 1400 and 1600 K, Sc2O3 interacts with W and reduces to Sc2O2 while W oxidizes. Sc2O2 then breaks into ScO, which desorbs above 1650 K. Since AES does not differentiate Sc2O2 from ScO, we do not know the precise temperature at which the dissociation occurs. The upper layers of Sc2O3, which are remote from the W substrate, remain stable until 1850 K when Sc2O3 decomposes directly to ScO. Sc2O2 is not the intermediate product here because the temperature is too high for Sc2O2 to be stable. ScO sublimates, leaving behind ScO2. Scþþ TPD spectra taken on a partially oxidized Sc film have more features than the spectra taken on a fully oxidized Sc film. The upper layers of a Sc film oxidize first to Sc2O3 while Sc layers below remain metallic. During a TPD process, a Sc atom that is in contact only with other Sc atoms desorbs first and gives rise to the multiple-layer features in Scþþ TPD
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spectra (Fig. 6). The remaining Sc atoms, which adsorb on Sc oxide, desorb between 1500 and 1700 K. Note that the reduction of Sc2O3 films begins about 1400 K before desorption of the first Sc layer commences. The Sc oxide from which the Sc atoms desorb is, therefore, Sc2O2. Furthermore, as reducing agents, the Sc atoms may participate with W in reducing Sc2O3 while they are oxidized to ScO themselves. Above 1700 K, Sc desorption is complete while ScO desorption has already commenced, and the TPD proceeds in the same manner as from a fully oxidized Sc film from here on. Identification of the order of desorption kinetics from Sc and ScO desorption states is instrumental in formulating our model above. First-order kinetics from state a suggests that the Sc atoms are first layer adsorbates. As much as 4.2 layers of Sc can be present in that film, indicating the surface area of the Sc2O2 substrate is 4.2 times the surface area of the W substrate. Cluster formation is proposed to account for the large surface area. The order of desorption kinetics also sets a constraint on our model. For example, our model has to be able to explain why there are two zeroth-order (or multiple-layer) states, states a and b, in ScOþ TPD spectra. Interaction of Sc2O3 with W and Sc generates multiple layers of Sc2O2. Since each Sc2O2 dissociates into two ScO, the resulted ScO film has twice the layers of the initial Sc2O2 film. Consequently, the desorption rate of state a is coverage independent, i.e. a zeroth-order state. State b arises from the dissociation of upper layers of Sc2O3 which are remote from the source of reducing agent, i.e. the W substrate. Consequently, the rate of ScO desorption is again coverage independent. Moderate heating of the substrate (between 220 and 420 8C) during oxygen exposure brings the oxidation of a 13-layer Sc film to near completion. The enhanced oxidation does not occur through increased oxygen diffusion into the Sc film, but rather it occurs through the coalescence of the surface Sc oxide layer into clusters. The cluster formation breaks the protective oxide layers and allows the oxidation process to proceed. It should be pointed out that the large amount of Sc present in state a suggests the substrate on which the Sc adsorbs forms clusters. At the desorption temperature (1500–1700 K), Sc2O3 has already dissociated. Consequently, we do not have direct evidence that clusters have formed between 500 and
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700 K (i.e. 200–420 8C) when Sc oxide remains as Sc2O3. The resolution of the scanning electron beam employed by the AES (with a nominal resolution of 10 mm) was insufficient to determine the cluster formation. An in situ study with a high-resolution SEM, for example, could resolve the issue. Nevertheless, we believe that Sc2O3 coalescence occurs at the moderate temperatures because it explains why such moderate heating could be so effective in bringing the oxidation to completion. Regardless of the validity of our oxidation model, it is clear that modest heating between 220 and 420 8C is very effective at enhancing the oxidation process. In fact, the oxidation of a 29-layer-thick Sc film was near completion (<1 layer of metallic Sc remained) after 1800 L of oxygen exposure with the sample at 420 8C. Therefore, we recommend that the cathode body be kept above 220 8C during scandate coating by pulsed laser deposition or sputtering deposition in order to have the Sc thoroughly oxidized. When heating is not practical during the coating process, the coated cathode should be heated above 220 8C in an oxidizing environment before activation.
References [1] J. Hasker, J. van Esdonk, J.E. Crommneen, Appl. Surf. Sci. 26 (1986) 173. [2] S. Yamamoto, S. Sasaki, S. Taguchi, I. Watanabe, N. Koganezawa, Appl. Surf. Sci. 33/34 (1988) 1200. [3] S. Yamamoto, I. Watanabe, S. Taguchi, S. Sasaki, S. Yaguchi, Jpn. J. Appl. Phys. 28 (1989) 490. [4] G. Gartner, P. Geittner, H. Lydton, A. Ritz, Appl. Surf. Sci. 111 (1997) 11. [5] J.W. Gibson, G.A. Haas, R.E. Thomas, IEEE Trans. Electron Devices 36 (1989) 209. [6] G. Gartner, J. Hasker, US Patent No. 5,936,334 (1999). [7] A. Shih, J.E. Yater, C. Hor, R. Abrams, Appl. Surf. Sci. 191 (2002) 44. [8] A. Shih, J.E. Yater, R. Abrams, Appl. Surf. Sci. 146 (1999) 1 (in which Fig. 3 is a good example of TPD spectra of multiple-layer films). [9] P.A. Redhead, Vacuum 12 (1962) 203. [10] M.P. Seah, W.A. Dench, Surf. Interface Anal. 1 (1979) 2. [11] PeakFit program by SPSS Science. [12] S.S. Zumdahl, Chemical Principles. D.C. Heath and Company, Lexington, MA, Toronto, 1992, p. 850. [13] J.R.T. Johnson, I. Panas, Chem. Phys. 248 (1999) 161. [14] H. Wu, W.S. Wang, J. Phys. Chem. A 102 (1998) 9129. [15] D.A. Gorodetsky, A.V. Martyuk, Ukrayins’kyi-FizychnyiZhurnal 31 (1986) 1054.
Oxidation of thin scandium films A. Shih*, J.E. Yater, C. Hor, R. Abrams Naval Research Laboratory, Code 6844, Washington, DC 20375, USA Received 31 October 2002; accepted 4 February 2003
Abstract Thin Sc films (namely, 20 layers or thinner) are studied as a function of oxygen exposure using Auger Electron Spectroscopy (AES) and Temperature Programmed Desorption (TPD) measurements. While AES data indicate that surface Sc layers oxidize rapidly, TPD data indicate that at room temperature the oxidation process is confined to the top layers. The surface oxide layer, about 5 layers thick, protects the Sc underneath from oxidation. However, thin Sc films can be completely oxidized by heating the W substrate to a temperature between 220 and 420 8C during oxygen exposure. The enhanced oxidation is the consequence of the coalescence of surface oxide, which breaks the protective surface oxide layer, exposing the Sc underneath to oxygen. AES measurements reveal that Sc oxide in the surface layers of an oxidized Sc film is Sc2O3. However, Sc and ScO are the only Sc-containing species found among the desorption products, and their relative amounts depend on the extent of oxidation of the film. Between 1400 and 1600 K during a TPD process, some of the Sc2O3 interact with W and Sc and dissociate into Sc2O2 while W and Sc are oxidized. Those Sc atoms which are not consumed during the oxidation–reduction interaction adsorb on Sc2O2. The large amount of Sc that desorbs from Sc2O2 between 1500 and 1700 K reveals that the surface area of Sc2O2 is 4.2 times the surface area of the W substrate, indicating that Sc2O2 coalesces into clusters. Sc2O2 then dissociates into ScO, which sublimates above 1650 K. In most oxidized Sc films, copious Sc2O3 could remain after depletion of the supply of reducing agents (W and Sc). The remaining layers of Sc2O3 dissociate into ScO and ScO2 above 1850 K when ScO sublimates, leaving behind ScO2. Published by Elsevier Science B.V. PACS: 82.65 My; 79.40þz Keywords: Thermionic emission; Dispenser cathode; Thermal desorption; Scandate cathode; Scandium; Scandium oxide
1. Introduction Dispenser cathodes are widely employed thermionic cathodes, particularly in applications where an emission density >1 A/cm2 is demanded. A standard dispenser cathode consists of a porous W disk impregnated with a Ba compound. During operation Ba is *
Corresponding author. Tel.: þ1-202-767-2260; fax: þ1-202-767-1280. E-mail address: [email protected] (A. Shih). 0169-4332/03/$ – see front matter. Published by Elsevier Science B.V. doi:10.1016/S0169-4332(03)00250-2
continuously dispensed onto the emitting surface, replenishing the surface Ba lost through evaporation. The emission density can be greatly enhanced by coating the W surface with a layers of Sc oxide and W mixture. The resulting dispenser cathode is known as a scandate cathode [1–4]. Given the same operating temperature, the emission from a scandate cathode can be as much as 200 times the emission from a standard dispenser cathode. The scandate cathode is the most promising high-emission-density thermionic cathode, and an emission density of 400 A/cm2 has
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been measured at 1030 8C [4]. However, such a high emission density has not been easily reproduced. One of the challenges in the fabrication process is to maintain the correct proportion and oxidation state of various components (i.e. W and Sc) in the scandate cathode coating [5]. Aside from fabrication challenges, scandate cathodes may present problems during actual operation. For example, the emission characteristics have revealed that the emission density may be non-uniform. In addition, the difficulty in emission recovery after ion sputtering anticipates a problem with robustness when the cathode is operated in a tube environment. To circumvent these problems, it was suggested that Sc rather than Sc oxide be used in the coating preparation (for examples, see [6]). The higher mobility of Sc compared to Sc oxide could lead to better coating uniformity and faster Sc replenishment after ion sputtering damage. However, our recent study determined the evaporation rates of Sc and Sc oxide as a function of temperature, and suggested that Sc in the coating would be depleted by sublimation during activation of the scandate cathode [7]. Consequently, Sc in the coating needs to be oxidized, while metallic Sc will have to be supplied from the chemical interaction of Sc oxide. Methods are being tried, e.g. addition of Re in the coating, in order to ensure adequate supply of Sc. In this study, we investigate the oxidation process of Sc as well the interaction of Sc oxide with W in absence of Re. This effort is part of our larger goal [7,8] to understand the interactions among Ba, Sc, W, Re, and O, which are present on the scandate cathode surface.
2. Experimental approach Thin Sc films (less than 20 layers) are deposited by evaporation of metallic Sc onto a W polycrystalline ribbon, and the films are then oxidized by exposure to oxygen. The oxidized films are analyzed by Auger Electron Spectroscopy (AES) and Temperature Programmed Desorption (TPD). AES data determine the thickness of the Sc films [7] and measure the extent of oxidation after each oxygen exposure. TPD data provide information on the species in the reaction products, the amount of each species, and the respective binding energies. Fig. 1 describes the experimental
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Fig. 1. Schematic of the experimental setup.
setup. Sc metallic chips are placed inside an evaporation crucible which is made of W wires. A variable leak valve is employed to control the oxygen partial pressure during oxygen exposure, and AES spectra are taken before and after each oxygen exposure. TPD spectra are taken with the sample surface normal pointing along the line of sight of a UTI Quadruple Mass Analyzer (QMA). During a TPD measurement, the desorption of a specific species is measured while the sample temperature (T) is raised linearly as a function of time (t), i.e. T ¼ T0 þ bt
(1)
where T0 is the initial temperature and b is the rate of the temperature rise. For a linear temperature rise, the desorption rate, N(T), of a specific species can be expressed [9] by g E NðTÞ ¼ n sn exp (2) kT b where E is the desorption energy, k is Boltzman constant, gn is rate constant, s is surface coverage, and n is the order of the desorption kinetics. The identification of the desorption kinetics will provide an essential clue in our understanding of the oxidation process (Sections 4 and 5). Zeroth-order desorption kinetics indicate that the nature of the adsorption state is a multiple-layer state. Because only the topmost layer contributes to desorption, the rate of desorption from a multiple layer is independent of s,
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i.e. n ¼ 0. First-order desorption kinetics can be associated with a monolayer or submonolayer film for which the desorption rate is proportional to s, i.e. n ¼ 1. Based on Eq. (2), the desorption characteristics of zeroth-order kinetics (n ¼ 0) are as follows. The rate of desorption rises exponentially as a function of sample temperature until depletion of the species. While desorption curves from films of various thickness share the same rapidly rising slope, the spectrum from a film of thicker coverage does not begin to fall off until a higher temperature. Consequently, the peak position increases with the film thickness [8]. In contrast, first-order desorption kinetics (n ¼ 1) lead to a constant peak position for films of different thickness [9]. QMA measurements enable us to study the desorpion of each individual species. We found that Sc and ScO are the major species desorbed from oxidized Sc films. We chose to record ScOþ spectra instead of ScOþþ spectra because the mass-to-charge ratio of ScOþþ is close to that of the isotopes of COþ, a major residual gas in our vacuum system. Scþ and Scþþ spectra are quite similar, but we chose to study Scþþ spectra since they were studied in our earlier investigation [7].
3. Oxidation of Sc films AES spectra record the oxygen intake by the Sc films after each oxygen exposure. Specifically, the ratio of the O-to-Sc Auger intensities is plotted using the O KLL transition 503 eV and Sc LMM transition at 340 eV. Fig. 2 shows a plot of the ratio taken on a 13-layer Sc film as a function of the oxygen exposure in Langmuirs (L) (1 L ¼ 106 Torr s; if every impinging molecule sticks on the surface, 1 L exposure results in one full layer of coverage on the surface). On the 13-layer Sc film, the O-to-Sc Auger ratio reaches saturation around 150 L. The inset, which expands the low range of the exposure scale, emphasizes the rapid rise of the O signal in the first 30 L of exposure. On a 3-layer Sc film, the O signal rises slightly faster than on the thicker film, increasing linearly for the first 15 L and reaching saturation at about 50 L. Because of the limited probing depth of Auger electrons, we learn from AES spectra about the che-
Fig. 2. O-to-Sc Auger peak height ratio is shown as a function of oxygen exposure. The AES data were taken on a 13-layer Sc film. The rise in the ratio is rapid over the first 50 L of exposure, and the inset shows the first 200 L of exposure.
mical changes in the surface layers only. At 503 and 340 eV, the O KLL and Sc LMM Auger lines have an escape depth of about 4.5 layers of Sc [10]. In other words, the Auger data reveal only the chemical composition within the top 4.5 layers and provide no information on the layers below. In order to ensure full oxidation, we expose each film to oxygen beyond saturation of the O AES signal. For example, 3-layer Sc films were exposed to 240 L of oxygen while 13-layer Sc films were exposed to 1920 L of oxygen. A ScOþ TPD spectrum, shown in Fig. 3, was taken from a 2-layer Sc film after 160 L of oxygen exposure.
Fig. 3. ScOþ TPD spectrum from a 2-layer Sc film after 160 L of oxygen exposure. Shown together for comparison is the Scþþ TPD spectrum from a freshly deposited 2-layer Sc film.
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Fig. 4. The amount of desorbed ScO is shown as a function of the Sc film thickness. The Sc films were oxidized by heavy oxygen exposure. The total area in each of the ScOþ TPD spectra provided the amount of the desorbed ScO.
We designate the ScO states by the lower case letters a, b, and c in order to distinguish them from the Sc states on W, which we have designated by the upper case letters A, B, and C [7]. In order to compare the peak positions of both types of states, a Scþþ TPD spectrum from a fresh 2-layer Sc film is also shown in Fig. 3. Clearly, ScO states a, b, and c are tighter binding states than Sc states A, B, and C [7], as manifested by the higher desorption temperatures. We will discuss the differences among states a, b, and c latter in Section 5, but for now only the sum of the areas under peaks a, b, and c is needed to provide a measure of the total amount of ScO present in the film. Fig. 4 shows the area sum as a function of the initial Sc film thickness on the W substrate for a series of Sc films. If the Sc films were thoroughly oxidized, we would expect a linear relation between the amount of Sc oxide and the initial Sc film thickness. The deviation from linearity can be interpreted as an incomplete oxidation of the Sc films. The larger the amount of Sc that remains metallic, the larger the deviation will be. This interpretation is supported by Scþþ TPD spectra taken from oxidized Sc films (see Section 4). Fig. 5 compares the peak positions in a Scþþ spectrum and a ScOþ spectrum (shown in Fig. 3) that were both taken from a 2-layer Sc film after 160 L of oxygen exposure. The positions of the three highest temperature peaks in the Sc spectrum coincide with
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Fig. 5. Scþþ and ScOþ TPD spectra from 2-layer Sc films after 160 L of oxygen exposure. The positions of peaks a, b, and c in the Scþþ TPD spectrum coincide with the respective peak positions in the ScOþ TPD spectrum; these states are the oxidized Sc states. The state a is an unoxidized Sc state.
states a, b, and c from the ScOþ spectrum, and the relative peak heights within each set of the triplets are nearly identical for the two spectra. They must have the same origin and consequently states a, b, and c are assigned to them. We believe that these three peaks in the Scþþ spectrum represent Scþþ ion fragments from ScO created by the ionization current of the QMA. The weakest binding state, designated as state a, is a Sc state and it does not appear in the ScOþ spectrum. The area under peak a gives a measure of the amount of unoxidized (or metallic) Sc. In the 2-layer Sc film after 160 L of oxygen exposure, the amount of metallic Sc contained in state a is equivalent to 0.15 layers of Sc. In a thicker Sc film, the amount of metallic Sc remaining is much greater as seen in Fig. 6. For example, in a 13-layer Sc film after 1920 L of oxygen exposure, 7 layers of Sc remained unoxidized while only 5 layers of Sc were oxidized! A possible way to increase the oxidation efficiency is to heat the substrate during oxygen exposure. We studied the oxidation of 13-layer Sc films while heating the W substrate at 220 and 420 8C. With the sample at 220 8C, 2.9 and 0.7 layers of Sc remained metallic after 48 and 765 L of oxygen exposure, respectively. With the sample at 420 8C, 1.47 layers of Sc remained metallic after only 48 L of oxygen exposure. The results are summarized in Fig. 7, which shows that the heating was effective in assisting the
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revealed by the TPD spectra must be in the Sc underneath the surface layers. This point becomes clearer by examining the Scþþ TPD spectra. 4. Scþþ TPD spectra
Fig. 6. Examples of Scþþ TPD spectra with various amounts of metallic Sc remaining in the film. Spectra (a): 0.6 layers of metallic Sc remaining in a 3-layer Sc film after 240 L of oxygen exposure at 20 8C. Spectra (b): 1.5 layers of metallic Sc film remaining in a 13layer Sc film after 48 L of oxygen exposure at 420 8C. Spectra (c): 7 layers of metallic Sc remaining in a 13-layer Sc film after 1920 L of oxygen exposure at 20 8C.
progress of the oxidation process. However, plots of the O-to-Sc Auger ratio versus oxygen exposure showed no significant difference with heating, indicting that moderate heating had no effect on oxygen diffusion through the Sc films. Since the Auger probing depth is 4.5 Sc layers, the increased oxidation
Fig. 7. Effect of substrate heating on the oxidation of 13-layer Sc films. While a large amount of Sc remains metallic in the film after oxidation at room temperature (20 8C), significantly less metallic Sc remains in the film after oxidation at elevated temperatures. For clarity, straight lines were used to connect the data points (although there are insufficient data points to warrant the linear fit).
Fig. 6 shows three Scþþ TPD spectra with various Sc populations in state a, and the figure caption describes how each curve was obtained. As designated in Section 3, states a and b are ScO states, while states a, A, and B are Sc states. State a contains 0.6, 1.5, and 4.2 layers of metallic Sc in spectra (a), (b), and (c), respectively. In addition, spectrum (c) contains 2.8 layers of metallic Sc in the multiple-layer states A and B [7]. State a has a higher binding energy than state C, the first layer state of Sc adsorbed on W, which was studied and reported in our previous study [7]. The position of peak a is 1630 25 K, which is 60 K higher than the position of peak C at 1575 22 K. We attribute state a to the first-layer Sc on Sc oxide because the presence of this state is found to require the establishment of Sc oxide first. Peak a is absent for oxidized Sc films with initial Sc coverage <1 layer while it is present when the initial Sc film is >1 layer as shown in the Scþþ TPD spectra of Fig. 8. During a TPD heating process, the Sc underneath the Sc oxide diffuses through the bulk to the top of the Sc oxide film before desorption.
Fig. 8. (a) Scþþ TPD spectrum taken from a 1-layer Sc film after 90 L of oxygen exposure. The a state is absent in this spectrum. (b) Scþþ TPD spectrum taken on a 3-layer Sc film after 240 L of oxygen exposure. State a contains about 0.6 layers of Sc.
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The vertical line in Fig. 6 demonstrates that the position of peak a remains constant while the population varies from 0.6 to 4.2 layers. This desorption characteristic is a characteristic of first-order kinetics [9], which suggests that state a is a monolayer or submonolayer adsorption state. Since multiple-layer states A and B appear after state a exceeds 4.2 layers, it follows that the surface area of the Sc oxide, on which state a is adsorbed, must be 4.2 times the surface area of the W substrate. The large surface area of the Sc oxide overlayer can be readily accounted for by postulating that the Sc oxide agglomerates into clusters during the TPD heating process. A surface area gain of 3.14 would result if the clusters are assumed to have a spherical shape, which minimizes the clusters’ surface area. The remaining gain could arise from several possibilities, e.g. imperfection in the spherical shape, the presence of clusters that pile on top of the first layer of clusters, or the presence of a Sc oxide monolayer on W. We believe that the last possibility is the most probable since the Sc oxide that sits directly on W binds more tightly to W and does not participate in the cluster formation. Furthermore, if there was exposed W (i.e. not covered by Sc oxide), Scþþ TPD spectra would contain desorption peaks of Sc on W. None of the Scþþ TPD spectra showed the presence of state C, the first Sc layer on W. 5. ScOþ TPD spectra Sc oxide cluster formation is also consistent with the characteristics of ScOþ TPD spectra. Fig. 9 shows ScOþ TPD spectra of oxidized Sc films with an initial Sc thickness between 0.5 and 6.5 layers. All oxygen exposures were carried out with the W substrate at ambient temperature, i.e. 20 8C. The ScOþ TPD spectra from Sc films oxidized at 220 or 420 8C are similar, but they have larger peak heights and peak areas than the spectra in Fig. 9 (given the same initial film thickness). Heating the substrate during oxygen exposure helps to oxidize the Sc in the deeper layers. As mentioned before, there are three adsorption states of ScO on W, states a, b, and c. Our earlier study of Sc desorption from fresh Sc films indicated a layer-by-layer growth of Sc on W [7]. The highest binding energy state (first layer state C) was nearly two-third occupied before the upper layer states
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Fig. 9. ScOþ TPD spectra from Sc films of different thickness. Spectra (a): 0.5-layer Sc film after 60 L oxygen exposure. Spectra (b): 0.9-layer Sc film after 120 L oxygen exposure. Spectra (c): 2.3layer Sc film after 360 L oxygen exposure. Spectra (d): 2.9-layer Sc film after 420 L oxygen exposure. Spectra (e): 6.5-layer Sc film after 960 L oxygen exposure.
(A and B) began to form. In contrast, all three states of ScO are present starting from the thinnest film studied, i.e. spectra (a) in Fig. 9, which was taken from an oxidized 0.5-layer Sc film. In the spectra of thicker films, only peaks a and b show significant growth in size. Above 3 layers, peak b becomes much larger than peak c that peak c is no longer resolvable from peak b. As we will show below, the observation can be explained by the nucleation of ScO into threedimensional islands, e.g. clusters, where states a and b are cluster states and state c is the first-layer ScO on W. Apparently, substrate heating during the TPD measurement causes ScO to agglomerate into clusters even if the film is thin. Desorption of multiple-layer films obeys zerothorder kinetics, as discussed in Section 2. Since peaks b and c in the ScOþ TPD spectra shown in Fig. 9 overlap with each other, they need to be resolved by a peak fitting program [11]. Fig. 10 shows the component peaks, which were resolved from three ScOþ TPD spectra. Among the functions provided by the program we found that a Voight function or a Gaussian Lorentian function yielded the best fit. The resolved peaks in Fig. 10 give clear evidence that states a and b are multiple-layer states: the peak positions shift to higher temperature for higher coverage, and the peaks share the rising part of the curve, although a small offset is observed in peak b. However,
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Fig. 10. ScOþ TPD spectra are resolved into respective component peaks in order to determine the kinetics of desorption of the component states. Both states a and b exhibit the characteristics of zeroth order kinetics. Spectra (a) taken from a 2.9-layer Sc film after 420 L of oxygen exposure at 20 8C, (b) 6.5-layer Sc film after 960 L of oxygen exposure at 20 8C, and (c) 7.7-layer Sc film after 780 L of oxygen exposure at 220 8C. The 7.7-layer film was oxidized while the substrate was heated in order to have thorough oxidation of the thick Sc film.
the offset is within the standard deviation in the peak positions, 20 K. Peak c is positioned at a slightly higher temperature than peak b. Unlike peaks a and b, the position and height of peak c do not change significantly with an increase in film thickness. For films with an initial Sc thickness over 2.3 layers, peak c is no longer distinct from peak b because of the close proximity of the peaks and the much larger height of peak b. For these films, the position and height of peak c were assumed using the values that were learned from the thinner films. AES measurements reveal that state c contains less than a monolayer of ScO. Since state c is the first layer of ScO on W, the peak position should be independent of the film thickness. We need to investigate why there are two multiplelayer states which grow simultaneously with increasing film thickness. The discussion in the next section provides a satisfying explanation.
6. Oxidation state in scandium oxide films We learn from chemistry that Sc2O3 is the commonly known Sc oxide [12], and from chemical cat-
alogues that Sc2O3 is the Sc oxide that is commercially available. It is perplexing that ScO is the only oxide species that sublimates from the thin Sc oxide films in this study. We need to investigate if the Sc oxide in the oxidized films is really ScO or Sc2O3. In fact, we do not know if Sc and O react to form ScO at all. A theoretical study [13] of Sc oxide molecules and clusters found that the stable gas phase species are Sc2O2, Sc2O3, Sc2O4 dimers and Sc2O4 clusters. Specifically, the result favored Sc2O2 cluster formation instead of two neutral ScO molecules. A study by gas phase photoelectron spectroscopy [14] detected clusters of stoichiometric Sc2O2, Sc2O3, and Sc2O4. Consequently, we searched specifically for Sc2O2, Sc2O3, and Sc2O4 among the desorbed species during the TPD studies, but found no significant presence of these species among the evaporants. For the study we prepared a series of Sc films with an initial Sc thickness of about 13 layers, and we then exposed the films to large oxygen exposures while the substrate was at 20, 220 or 420 8C. In all cases, Sc and ScO were the only desorption species that contained Sc, and the relative amounts reveal the extent to which the oxidation in the film had proceeded. One should not conclude that the stable molecular species of Johnson and Panas [13] did not form on the oxidized Sc films just because they were absent from the TPD spectra. They could have dissociated before desorption. Specifically, the thermal energy required for Sc2O2 dissociation into two ScO is 3.97 eV (383 kJ/mol [13]), which is lower than 5.06 eV (488 kJ/mol ) [7], the lowest desorption energy of ScO from W. It appears that any (ScO)n cluster present on the W surface would dissociate to ScO monomers at the temperature of ScO desorption. Furthermore, a large molecule, even if it sublimated, could be broken to fragments by the impact of the energetic electrons (with average energy about 70 eV) that the UTI QMA uses to ionize gaseous molecules. Consequently, it is often not straightforward to determine the actual compound that forms based on TPD measurement alone when only molecular fragments of the compound are detected. A surface analysis study of Sc oxide films by Gorodetsky and Martyuk [15] suggested the existence of two phases of scandium–oxygen interaction: a thin Sc2O2 layer on top of a thick Sc2O3 layer. The study found also that heating the oxide film to 1100–1400 K leads to Sc2O3 dissociation and to an increase in the Sc
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surface concentration. The findings motivated our study described below, which was designed to investigate the possibility of thermal decomposition. That study reveals that reduction of Sc oxide indeed occurs but not to Sc. The increase in the Sc surface concentration reported by Gorodetsky et al. could have emerged from incomplete Sc oxidation because Sc atoms in deep layers remain unoxidized during roomtemperature oxygen exposure. Upon heating, they diffuse to the top and increase the surface concentration of Sc. In our investigation of the possibility of thermal decomposition, we utilize AES to monitor the surface compositional change during TPD. Because the presence of unoxidized Sc could lead to an increase in the Sc surface concentration upon heating, we need to oxidize the Sc films thoroughly. Specifically, a 7-layer Sc film was exposed to 1080 L of oxygen while the W substrate was at 420 8C. As discussed in Section 3, such processing conditions should ensure complete oxidation of the Sc films. After cooling to ambient temperature, the substrate temperature was then raised linearly as a function of time, but the heating was terminated abruptly at a predetermined temperature, Tmax, to allow surface compositional analysis (i.e. AES measurements). The process was then repeated to successively higher Tmax, and after each heating, AES spectra were taken at three different locations on the sample. Fig. 11(a) shows the O-to-Sc Auger ratio as a function of Tmax (with the solid circles showing data taken from the 7-layer film.) The average of the three O-to-Sc Auger ratios and the calculated standard deviation are used for the data point and the error bar, respectively. It should be noted that while the O-to-Sc Auger ratio does not directly measure the O-to-Sc atomic ratio on the surface, the changes in the ratio do provide a measure of the changes in the surface composition. Specifically, the initial O-to-Sc Auger ratio is about 1.1, but it decreases to about 0.7 or 0.8 between 1600 and 1825 K, i.e. about a 40% reduction. We know that above 1600 K the surface layer consists of a 1:1 O-toSc stoichiometry because ScO is the only species found in the evaporant. Therefore, it follows from the change in the O-to-Sc Auger ratio that the initial film consists of a 3:2 O-to-Sc stoichiometry, i.e. Sc2O3. A similar reduction in the O-to-Sc Auger ratio was observed on the surfaces of fully oxidized 1.7- and 4-layer films. However, no clear correlation of the
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Fig. 11. (a) O-to-Sc Auger ratios, and (b) W-to-Sc Auger ratios are shown as a function of the maximum ramped temperature, Tmax, of two fully oxidized Sc films. The solid circles data were taken on the film started with a 7-layer Sc film, while the open circles data started with a 13-layer Sc film. After each temperature ramp, AES spectra were taken at three different locations on the sample. Each data point represents the average value, and the error bar represents the standard deviation in the three ratios.
O-to-Sc Auger ratio with Tmax was observed on the surface of a fully oxidized 13-layer Sc film (as seen by the open circles data in Fig. 11). The apparent inconsistency can be explained by the assumption that W, a reducing agent, is responsible for the reduction of Sc2O3. Since AES probes only the top 4.5 layers of Sc oxide films, these layers on the 13-layer film were not in contact with the W substrate. Consequently, we did not observe a stoichiometric change. In thinner films, the probed region includes the oxide-W interface, and the reduction in the Auger ratio is apparent. Above 1400 K the oxide coverage becomes thinner due to ScO desorption. The thinner the oxide coverage, the weaker the W signal attenuation, and the higher the contribution to the Auger spectra from the interface. Indeed, the reduction in the O-to-Sc Auger ratio coincides with a significant appearance of W in the AES spectra (as seen by the solid circles data in Fig. 11).
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This interpretation is supported by the calculated values of heats of formation by Johnson and Panas [13]. The lowest heat of formation for Sc2O3 is ScO þ ScO2 ! Sc2 O3
(3)
DH ¼ 617 kJ=mol ðor 6:40 eVÞ Since the heat of formation is greater than the desorption energy of state a of ScO (Ed ¼ 5:06 eV [7]), Sc2O3 dissociation at the desorption temperature of state a requires the help of a reducing agent such as W or Sc. The interaction of Sc2O3 with W leads to the formation of Sc2O2 and W oxide. Since the reaction ScO þ ScO ! Sc2 O2
(4)
DH ¼ 383 kJ=mol ðor 3:97 eVÞ is exothermic, the formation of Sc2O2 is favored thermodynamically over the formation of two ScO at low temperatures. The dissociation energy of Sc2O2 (DH ¼ 3:97 eV) is lower than the desorption energy of state a of ScO (Ed ¼ 5:06 eV). Therefore, the dissociation is expected to occur before the desorption of state a. Because the O-to-Sc atomic ratios are the same for Sc2O2 and ScO, AES measurements do not differentiate between the two species. When the reducing agents are exhausted through oxidation or desorption, Sc2O3 dissociation ceases and the ScO supply drops. By itself, Sc2O3 is quite stable, with the heat of formation being 6.40 eV or higher. This Sc2O3 is expected to decompose at a temperature above the state a desorption temperature. Indeed, we observed state b of ScO above 1850 K. At these high temperature, Sc2O2 is unstable and Sc2O3 dissociates directly to ScO. This interpretation explains satisfactorily the existence of two zeroth-order desorption states in ScOþ TPD spectra. In conclusion, an oxidized Sc film consists mainly of Sc2O3. Between 1400 and 1600 K, Sc2O3 dissociates to Sc2O2 assisted by W and unoxidized Sc. Sc2O2 dissociates to ScO which then desorbs above 1650 K, giving rise to peak a in the ScOþ TPD spectra. The top layers of Sc2O3 dissociate above 1850 K without the help of a reducing agent. The dissociation leads directly to ScO and ScO2. ScO desorbs and gives rise to peak b in the ScOþ TPD spectra, leaving behind ScO2. Consequently, the O-to-Sc Auger ratio rises again (the solid circles in Fig. 11a). The nature of
the highest binding energy state c is not yet clear. At high temperature, ScO2 and W may have formed some Sc–O–W surface complex, which decomposes to give rise to peak c in the ScOþ spectra. At 2050 K when the TPD process terminates, the desorption from neither state b nor state c is complete. AES measurements determined that the remaining film contained less than a Sc monolayer. We did not heat the sample to a higher temperature because we found that by doing so the sample life would be shortened drastically.
7. Summary AES measurements indicated rapid oxidation of Sc films: the O Auger intensity reaches saturation after about 50 L of oxygen exposure. However, at ambient temperature (20 8C) the rapid oxidation is confined to the top layers of Sc only. TPD measurements indicated that for a 13-layer Sc film, 7 layers of Sc remained unoxidized even after a very large oxygen exposure (1920 L). Once the top 5 layers of oxide form, they protect the metallic Sc underneath from oxidation. The conclusion is consistent with our experience with Sc evaporators. We use Sc metallic chips as a Sc evaporation source and the Sc chips are reusable even after long exposures to atmosphere. Heating of the substrate during oxygen exposure helps to bring the oxidation to near completion. A fully oxidized Sc film consists of Sc2O3, which decomposes upon heating. Between 1400 and 1600 K, Sc2O3 interacts with W and reduces to Sc2O2 while W oxidizes. Sc2O2 then breaks into ScO, which desorbs above 1650 K. Since AES does not differentiate Sc2O2 from ScO, we do not know the precise temperature at which the dissociation occurs. The upper layers of Sc2O3, which are remote from the W substrate, remain stable until 1850 K when Sc2O3 decomposes directly to ScO. Sc2O2 is not the intermediate product here because the temperature is too high for Sc2O2 to be stable. ScO sublimates, leaving behind ScO2. Scþþ TPD spectra taken on a partially oxidized Sc film have more features than the spectra taken on a fully oxidized Sc film. The upper layers of a Sc film oxidize first to Sc2O3 while Sc layers below remain metallic. During a TPD process, a Sc atom that is in contact only with other Sc atoms desorbs first and gives rise to the multiple-layer features in Scþþ TPD
A. Shih et al. / Applied Surface Science 211 (2003) 136–145
spectra (Fig. 6). The remaining Sc atoms, which adsorb on Sc oxide, desorb between 1500 and 1700 K. Note that the reduction of Sc2O3 films begins about 1400 K before desorption of the first Sc layer commences. The Sc oxide from which the Sc atoms desorb is, therefore, Sc2O2. Furthermore, as reducing agents, the Sc atoms may participate with W in reducing Sc2O3 while they are oxidized to ScO themselves. Above 1700 K, Sc desorption is complete while ScO desorption has already commenced, and the TPD proceeds in the same manner as from a fully oxidized Sc film from here on. Identification of the order of desorption kinetics from Sc and ScO desorption states is instrumental in formulating our model above. First-order kinetics from state a suggests that the Sc atoms are first layer adsorbates. As much as 4.2 layers of Sc can be present in that film, indicating the surface area of the Sc2O2 substrate is 4.2 times the surface area of the W substrate. Cluster formation is proposed to account for the large surface area. The order of desorption kinetics also sets a constraint on our model. For example, our model has to be able to explain why there are two zeroth-order (or multiple-layer) states, states a and b, in ScOþ TPD spectra. Interaction of Sc2O3 with W and Sc generates multiple layers of Sc2O2. Since each Sc2O2 dissociates into two ScO, the resulted ScO film has twice the layers of the initial Sc2O2 film. Consequently, the desorption rate of state a is coverage independent, i.e. a zeroth-order state. State b arises from the dissociation of upper layers of Sc2O3 which are remote from the source of reducing agent, i.e. the W substrate. Consequently, the rate of ScO desorption is again coverage independent. Moderate heating of the substrate (between 220 and 420 8C) during oxygen exposure brings the oxidation of a 13-layer Sc film to near completion. The enhanced oxidation does not occur through increased oxygen diffusion into the Sc film, but rather it occurs through the coalescence of the surface Sc oxide layer into clusters. The cluster formation breaks the protective oxide layers and allows the oxidation process to proceed. It should be pointed out that the large amount of Sc present in state a suggests the substrate on which the Sc adsorbs forms clusters. At the desorption temperature (1500–1700 K), Sc2O3 has already dissociated. Consequently, we do not have direct evidence that clusters have formed between 500 and
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700 K (i.e. 200–420 8C) when Sc oxide remains as Sc2O3. The resolution of the scanning electron beam employed by the AES (with a nominal resolution of 10 mm) was insufficient to determine the cluster formation. An in situ study with a high-resolution SEM, for example, could resolve the issue. Nevertheless, we believe that Sc2O3 coalescence occurs at the moderate temperatures because it explains why such moderate heating could be so effective in bringing the oxidation to completion. Regardless of the validity of our oxidation model, it is clear that modest heating between 220 and 420 8C is very effective at enhancing the oxidation process. In fact, the oxidation of a 29-layer-thick Sc film was near completion (<1 layer of metallic Sc remained) after 1800 L of oxygen exposure with the sample at 420 8C. Therefore, we recommend that the cathode body be kept above 220 8C during scandate coating by pulsed laser deposition or sputtering deposition in order to have the Sc thoroughly oxidized. When heating is not practical during the coating process, the coated cathode should be heated above 220 8C in an oxidizing environment before activation.
References [1] J. Hasker, J. van Esdonk, J.E. Crommneen, Appl. Surf. Sci. 26 (1986) 173. [2] S. Yamamoto, S. Sasaki, S. Taguchi, I. Watanabe, N. Koganezawa, Appl. Surf. Sci. 33/34 (1988) 1200. [3] S. Yamamoto, I. Watanabe, S. Taguchi, S. Sasaki, S. Yaguchi, Jpn. J. Appl. Phys. 28 (1989) 490. [4] G. Gartner, P. Geittner, H. Lydton, A. Ritz, Appl. Surf. Sci. 111 (1997) 11. [5] J.W. Gibson, G.A. Haas, R.E. Thomas, IEEE Trans. Electron Devices 36 (1989) 209. [6] G. Gartner, J. Hasker, US Patent No. 5,936,334 (1999). [7] A. Shih, J.E. Yater, C. Hor, R. Abrams, Appl. Surf. Sci. 191 (2002) 44. [8] A. Shih, J.E. Yater, R. Abrams, Appl. Surf. Sci. 146 (1999) 1 (in which Fig. 3 is a good example of TPD spectra of multiple-layer films). [9] P.A. Redhead, Vacuum 12 (1962) 203. [10] M.P. Seah, W.A. Dench, Surf. Interface Anal. 1 (1979) 2. [11] PeakFit program by SPSS Science. [12] S.S. Zumdahl, Chemical Principles. D.C. Heath and Company, Lexington, MA, Toronto, 1992, p. 850. [13] J.R.T. Johnson, I. Panas, Chem. Phys. 248 (1999) 161. [14] H. Wu, W.S. Wang, J. Phys. Chem. A 102 (1998) 9129. [15] D.A. Gorodetsky, A.V. Martyuk, Ukrayins’kyi-FizychnyiZhurnal 31 (1986) 1054.