Ba and BaO on W and on Sc2O3 coated W

Applied Surface Science 242 (2005) 35–54 www.elsevier.com/locate/apsusc

Ba and BaO on W and on Sc2O3 coated W A. Shih1,*, J.E. Yater, C. Hor1 Naval Research Laboratory, Washington, DC 20375, USA Received 24 March 2004; accepted 26 July 2004

Abstract Temperature-programmed desorption (TPD) and Auger electron spectroscopy (AES) are used to characterize the surface layers that form under an evaporating flux of a dispenser cathode (which is a Ba and BaO source) on a W substrate and Sc2O3coated W substrate to simulate the surface layer of a conventional dispenser cathode and scandate cathode, respectively. The surface layers were prepared while the substrate was either at 940 8Cb (1272 K), a typical operating temperature, or at 1125 8Cb (1477 K), a typical activation temperature. Our investigation found that a partial layer of BaO formed on W, similar to the surface layer that forms on a dispenser cathode. Heating to the activation temperature causes the BaO to form a stronger bond with W. For the Sc2O3-coated W substrate, heating to the activation temperature is necessary for the inter-diffusion between the Sc2O3 and W to occur. BaO layers form a stronger bond to the inter-diffused layer than to pure W. However, the most important finding is that a stable BaO-containing compound forms and continues to accumulate under the impinging flux on the Sc2O3 and W covered substrate at 940 8Cb. Surface emission models describe successfully all other dispenser cathodes, but fail to explain the emission characteristics of scandate cathodes. Raju and Maloney proposed an alternate model, which requires the presence of a thick layer of semi-conducting material. Our finding suggests that it is possible to form a thick layer from simultaneous presence of BaO, Sc2O3 and W. However, further investigation is necessary to determine if the Raju and Maloney type layer is indeed present on top of scandate cathodes. # 2004 Published by Elsevier B.V. PACS: 82.65My; 79.40+z Keywords: Thermionic emission; Dispenser cathode; Thermal desorption; Scandate cathode; Barium oxide; Scandium oxide

1. Introduction Scandate cathodes are the most promising highemission-density cathodes, and an emission density of * Corresponding author. Present address: 9121 Bells Mill Road, Potomac, MD 2085, USA. E-mail address: [email protected] (A. Shih). 1 Retired. 0169-4332/$ – see front matter # 2004 Published by Elsevier B.V. doi:10.1016/j.apsusc.2004.07.067

400 A/cm2 at 1030 8C (1303 K) has been demonstrated [1]. However, there are obstacles to overcome before scandate cathodes are widely utilized. The problems, which are particularly of concern, are a possible non-uniformity in emission and the susceptibility to ion sputtering damage. Uniformity in emission is essential when high brightness is important, while emission recovery from ion damage

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is essential to cathode robustness and cathode life. The anomalous emission characteristics, which were commonly observed on scandate cathodes, leads to the speculation of a patchy work function and nonuniform emission [2–4]. An increase in the applied field breaks patchy fields, resulting in a continuous rise in emission current. Some of the early scandate cathodes were made of mixed powders of Sc2O3 and W, pressed to form the cathode bodies (mixed matrix scandate cathodes) or to form cathode coatings (toplayer scandate cathodes) [1]. Large particle sizes of the Sc2O3 and W powders were thought to be responsible for the patchy surfaces. Fabrication techniques such as sputter deposition [5], chemical vapor deposition [6] and pulsed laser deposition [1], were developed to achieve atomic mixing of Sc2O3 and W. Addition of ScH2 [7] or inter-metallic compounds of Sc–Re [8] in the matrix or coating were attempted to provide a source of mobile Sc, which serves to replenish the loss of Sc from the ion sputtering. New fabrication techniques do lead to higher emission densities, culminating in the 400 A/cm2 at 1030 8C [1]. However, the non-saturation emission characteristics still remain with these cathodes, and a method that dispenses Sc similar to Ba dispensers has not been found. It is essential to understand some of the fundamental issues in order to aid further development in the fabrication techniques. For example, what is the emission mechanism in scandate cathodes, and what is the Sc replenishment mechanism in scandate cathodes? The prevailing emission models are based on surface models [9–13], which require a patch-field effect to explain the emission non-saturation phenomenon. Raju and Maloney [14] proposed a semiconductor model to explain the emission characteristics. Because of the low concentration of free electrons in the semiconductor, the applied external field penetrates into the surface, bringing down the work function. In order for the semiconductor model to be applicable, the active material must have a substantial thickness. The absence or presence of such a layer must be investigated. The addition of metallic Sc in the cathode body or in the impregnants is not a viable approach. Our measurement on the Sc evaporation rate informed us that cathode activation processes would exhaust the metallic Sc [15], and Sc must be in a compound form,

e.g. Sc2O3. However, the volatility of Sc2O3 by itself is too low to be a practical Sc source [15,17]. Above 1600 K, Sc2O3 interacts with W leading to Sc2O2 formation, which sublimates as ScO [17]. The required temperature is too high to be a practical Sc replenishing method. Other chemical interactions that lead to volatile Sc or Sc compound at lower temperatures must be sought and be incorporated in the scandate cathode fabrication if discovered. It is essential to both endeavors to understand the chemical interactions on the surface of a scandate cathode. The surface chemistry on top of a scandate cathode is very complex, consisting of interactions among Ba, Sc, O, W, and maybe Re. Our approach is to investigate first the interactions among the subsets, such as Ba and O on W [16] and Sc and O on W [15,17]. The knowledge gained facilitates the study of the complete set. Investigations of Ba and BaO interactions with Sc2O3-coated W require oxidation of the Sc layer that is deposited on W. We know now that at room temperature the oxidation of Sc is limited to the top few layers. However, a moderate heating (200– 400 8C) is sufficient for the oxidation to proceed to completion. The oxide formed is Sc2O3, which is stable at the operating temperatures and activation temperatures [17]. A study of Ba on W was also reported [16]. However, the sample-heating design in the preliminary study constrained the W substrate temperature to below 1700 K, which was too low to clean all oxygen from the W surface. We redesigned the sample heating since then and will describe the design in Section 2, as a part of the discussion of the experimental setup. In Section 3 we will present our study of Ba and BaO adsorption on W substrate at room temperature. The results are essential in the analysis and interpretation of data in the following sections. In Section 4 we report on the results of Ba and BaO adsorption on W at elevated temperatures. Specifically, the study was focused on temperatures of 940 8Cb (1272 K) and 1125 8Cb (1477 K), where 940 8Cb is a typical cathode operating temperature [18,19], while 1125 8Cb is a typical cathode activation temperature [20,21]. The study is to simulate the surface layers on a standard dispenser cathode. In Section 5 we report on the study of Ba and BaO adsorption on Sc2O3-coated W substrate at 940 and 1125 8Cb. The study is to simulate the surface layers on a scandate cathode. Section 6 presents the

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summary. In addition, we discuss on the further studies that are required and on the suggested improvements of the experimental method.

2. Experimental approach and setup A dispenser cathode in general consists of a porous W button, which is impregnated with Ba and O containing compounds in the pores. During operation of a dispenser cathode, Ba and BaO are continuously dispensed onto the W surface via the pore openings. A typical top-layer scandate cathode has a mixture of a Sc oxide and W coating on top of the porous W button. We use a W ribbon to simulate a standard dispenser cathode surface, and an external source, such as a dispenser cathode, is used to supply the Ba and BaO flux. For synthesis of a scandate cathode surface, we deposit a layer of Sc on the W ribbon, and then oxidize the Sc layer to form Sc2O3. After the oxidation, we heat the W ribbon to allow W and Sc2O3 inter-diffusion. Fig. 1 shows a schematic of the experimental setup. The circle in the middle represents a sample carrousel mounted with the W ribbon sample. The carrousel allows the sample to be rotated in front of a Sc evaporator or the dispenser cathode for sample preparation, and in front of the Auger electron spectroscopy (AES) analyzer or the UTI Quadruple mass analyzer (QMA) for sample characterization.

Fig. 1. Schematic of the experimental setup.

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The Sc evaporator consists of a W-wire boat with Sc pellets placed inside. A shutter is used to control the deposition time. A variable leak valve attached with an oxygen bottle of 99.997% purity is used for a controlled oxidation of the deposited films. The evaporants from a Spectramat 411 type dispenser cathode are used to supply the Ba and BaO to the surface to simulate the active material supply from the pores. Hasker et al. found a much faster activation time of the scandate cathode using a 411 rather than 532 impregnant ratio [9]. The molar ratio of Ba:BaO evaporated from a 411 type dispenser cathode was measured to be 13.3:1 [22]. For convenience the Ba/ BaO source is placed at a distance of 3/8 in. (0.95 cm) from the sample during Ba/BaO depositions. Since the distance is very large compared to the dispenser size, which is 0.134 in. (0.34 cm) in diameter, long deposition times were often needed throughout the experiment. However, this afforded us good control of the Ba/BaO deposition. A VG Scientific CLAM 100 and a VG Scientific electron gun are used to take AES spectra, which provide surface chemical composition analyses and which are also instrumental in the determination of the evaporation rates from the Sc source and from the Ba/ BaO source. After the AES characterization, the sample is rotated in front of the UTI QMA for a temperature-programmed desorption (TPD) study. A TPD spectrum records the desoprtion of a specific species of interest as a function of the sample temperature. The desorption peak position Tp gives a measure of the binding energy of the species on the sample surface, while the area enclosed in a desorption spectrum provides a measure of the amount of the species that is present on the sample surface [23]. One of the challenges in this study is to meet the stringent requirements of the sample heating. In order to clean the W sample, it must be heated above 2000 K. The temperature must be uniform over the sample surface in order for the TPD data to be meaningful. Fig. 2 shows a rear view of our present design for the sample heating. A 0.4 cm  0.4 cm W ribbon of 0.0025 cm thickness is heated by passing current through two W wires of 0.025 cm diameter. Pt foils which are needed for spot-welding the W ribbon to the W wires are shown in Fig. 2. Since the Pt foils are in the rear of the sample, they have no contribution to the AES or TPD spectra. The mounting post made

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Fig. 3. Example of a temperature vs. time plot. Fig. 2. Rear view of the sample and its mounting.

of two 0.23 cm Ni rods keeps the heating wires taut and the W ribbon surface flat. A smaller sample is preferred to allow a higher maximum temperature. However, the smaller surface area provides a smaller flux of the desorbing species. Since the signal-to-noise ratio in TPD spectra becomes important, we chose the ribbon size as a compromise between the two factors. To attain an acceptable level of the signal-to-noise in TPD spectra, we set the mass resolution of the QMA to its maximum sensitivity (or minimum resolution). The typical QMA signals were between 0.1 and 5 nA, after electron-multiplier amplification. For sample temperature measurements, we rely on a W–3% Re and W–25% Re thermocouple and an infrared thermometer, since both methods are suitable for computer-controlled data acquisition. Specifically, we use an Ircon Modline 3 Infrared Thermometer series 200, which responds to infrared radiation in the band of 0.70–1.00 mm wavelengths, and which has a temperature range between 1023 and 2873 K. Below 1023 K, we have to rely on the thermocouple alone. An optical pyrometer is employed to calibrate both the thermocouple and the infrared thermometer. We know that the spectral emissivity of W at 0.65 mm (for optical pyrometer) is 0.43 [24]. In order to have a consistent reading between the optical pyrometer and the infrared thermometer, we set the emissivity of the latter at 0.350. A personal computer is used for data acquisition. At each 0.2 s interval, the computer records the outputs from the thermocouple, infrared thermometer, and QMA, respectively, and then increases the heater voltage. The advance in the

heating voltage is such that the temperature rise in the W ribbon sample is approximately linear since a linear temperature-programmed desorption is particularly conducive to data analysis [23]. Fig. 3 gives a typical plot of the temperature versus time measurement (9 8C/s temperature rise). The short life of the sample remains to be a major problem. We can perform between 30 and 60 TPD measurements before one of the heating legs burns. Since the sample is a polycrystalline W surface, the replacement is unlikely to have exactly the same distribution of crystal faces. Section 4 compares Ba and BaO desorption from two different samples and found good reproducibility. However, for a critical examination of the effect on the substrate heating or the surface coating, the measurements must be on the same sample. Although this paper is based on data collected on five different samples, the earlier samples were used as preliminary studies for planning the needed sequences of the study. Unless specified, all data presented in Sections 4 and 5 were taken on one single sample.

3. Ba and BaO adsorption on W at room temperature Since the evaporants from a 411 type dispenser cathode are predominantly Ba (Ba:BaO molar ratio is 13.3:1 [22]), we use the dispenser cathode as our Ba source. Two Ba2+ TPD spectra are shown in Fig. 4 taken after extended depositions of evaporants from the dispenser cathode while the W substrate was at the

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Fig. 4. Ba2+ TPD spectra. Spectrum (a) was taken from multi-layers of Ba on clean W. Spectrum (b) was taken from a Ba layer on W with O that remained after a 1700 K heating.

ambient temperature. The choice of Ba2+ TPD spectra over Ba+ TPD spectra was explained in an earlier paper [16]. Spectrum (a) was taken from the film deposited on a clean W substrate. An AES measurement indicated that the coverage was about 2.2 layers. Based on the systematic study of Ba adsorption on W [16], we know peak B in spectrum (a) is the desorption peak from the first layer Ba adsorbed on W, while peak A is from the upper layers. The desorption peak Tp shown is 1094 K, which is significantly lower than 1250 K, Tp reported in the preliminary study [16]. The discrepancy points out the sensitivity of the Ba binding energy to the substrate cleanliness. In that study, the maximum heating temperature was 1700 K, which was insufficient to clean all of the oxygen from the W surface. To reproduce the surface condition, we heat the present W sample also to only 1700 K before depositing a thin Ba layer (about 1 layer). Spectrum (b) shows the Ba2+ TPD from this layer, and Tp is 1220 K. Higher oxygen content would raise Tp further. Fig. 5 shows Ba2+ TPD spectra from three Ba films at various stages of oxidation. Spectrum (a) is the same as-deposited spectrum shown in Fig. 4. Spectrum (b) was taken from a six-layer Ba film after being exposed to 1800 L (Langmuir) of oxygen while the substrate was at room temperature. About 1.6 Ba layers remained un-oxidized, which adsorbed on the surface of BaO clusters. The oxidized Ba agglomerated to the clusters during the TPD process, and similar agglomeration was observed on oxidized Sc films

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Fig. 5. Ba2+ TPD spectra at different stages of oxidation. Spectrum (a), which is the same as spectrum (a) of Fig. 4 was taken from an asdeposited Ba layers. Spectrum (b) was taken from a six-layer Ba film, which was oxidized at room temperature. Spectrum (c) was taken from another six-layer Ba film, which was oxidized at 700 K.

[17]. Spectrum (b) shows that the desorption peak of Ba from the BaO surface is at 1287 K, a much higher temperature than the desorption peak of Ba from W. Complete oxidation of Ba layers requires substrate heating during oxidation similar to the oxidation of Sc films [17]. Spectrum (c) was taken from also a sixlayer Ba film after being exposed to 600 L of oxygen while the W substrate was at 700 K. The high desorption temperatures are the consequence of the high binding energies of BaO to W. Some of the BaO decomposed to Ba during desorption. Fig. 6 shows BaO+ TPD spectra from the Ba films that were oxidized with the substrate at room temperature. The initial Ba thicknesses were 0.59, 1.2, 2.4 and 4.8 layers for spectra (a)–(d), respectively. The oxygen exposure was 12 L for spectra (a)–(c), and 36 Langmuirs for spectra (d). Desorption peaks above 1600 K were from the first layer of BaO, while the peak near 1500 K was from the upper layers of BaO. The multi-layer feature appears long before the completion of the first layer. This observation suggests that BaO agglomerates to form BaO clusters upon heating; the cluster formations were also evoked in the interpretation of spectrum (b) in Fig. 5. The close proximity of the two peaks shifts the apparent peak positions of the peaks toward each other. With the use of a peak fit program [25], the peak position of the upper layers peak was found to be at 1510 K for the thinner films of spectra (b) and (c), and at 1522 K for

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Fig. 6. BaO+ TPD spectra of oxidized Ba layers. The initial thickness of the Ba layers was 0.59, 1.19, 2.4 and 4.8 layers for spectra (a)–(d), respectively. All oxidation was taken at room temperature.

the thicker film of spectrum (d). The peak position for a multi-layer film is known to shift to a higher value for a thicker film [15]. The peak position of the multiple BaO layers will be an important reference in Section 5. The areas enclosed by the spectra of Fig. 6 give a measure of the amount of BaO desorbed. Fig. 7 plots the amount as a function of the initial Ba film thickness. Fig. 7 includes also the BaO amount desorbed from a 15.9 Ba layer after exposing to 48 L of oxygen, in addition to the data shown in Fig. 6. If all of the Ba in the films were oxidized from the oxygen exposures, Fig. 7 would be a linear plot. However, above an initial thickness of 2.4 layers, the amount of BaO formed is significantly less than expected for full oxidation. Based

Fig. 7. Amount of BaO desorbed as a function of the initial thickness of the Ba films.

on the Ba2+ TPD spectra, 1.6 layers of Ba remained unoxidized after a 6.2-layer Ba film was exposed to 40 Langmuirs of oxygen. All of the above oxidations occurred with the substrate at room temperature. For full oxidation we only need to heat the Ba films to above 700 K during oxidation (see Fig. 5). The areas in the TPD spectra need to be translated into the number of layers of coverage. To determine the area-to-BaO coverage conversion, we divide the total area in a BaO+ TPD spectrum by its known coverage, the determination of which we relied on AES spectra. For Ba coverage on W, we use the AES peak–height ratio of Ba 584 eV to W 169 eV Auger lines [26]. (To improve the accuracy, we determined first the rate of Ba deposition by determining the Ba coverage as a function of deposition time. Each Ba film thickness is then the product of the Ba deposition rate times the deposition time.) The Ba films were then fully oxidized, and for Ba films above 2.54 layers, the substrate was heated to 700 K during oxygen exposure. Each Ba layer was assumed to become one BaO layer after oxidation. In this manner, we determined that each BaO layer gives rise to an area of 73  8 nA K in a BaO+ TPD spectrum. The average and the standard deviation were determined from five BaO films of various thicknesses. The determined number is only meaningful to this experiment. The vertical coordinate of Fig. 7 can now be translated into the number of BaO layers by dividing by 73. In order for the conversion factor to be useful to the entire study, we need to keep the experimental parameters constant throughout the study, e.g. the size of the sample, the distance from the sample to the QMA, the rate of temperature rise during TPD measurements, the QMA mass resolution, and the emission level of the ionization current. The conversion from the area in a Ba2+ TPD spectrum to the Ba coverage cannot be determined in a similar manner, because the Ba films prepared contained a significant fraction of BaO, which came partly from the BaO in the evaporants and partly from oxidation by the residual gases during the deposition and during the AES measurement. Another complexity is in the definition of a Ba monolayer. We will follow Moore and Allison’s definition of a Ba monolayer, which occupies only half of the available sites [27]. Following this definition one Ba layer will evolve into one BaO layer. Consider a W(1 0 0)

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surface, for instance: the Ba monolayer corresponds to the C(2  2) structure, namely half of the W sites are occupied by Ba and the other half are empty. After oxidation, the empty sites are occupied by oxygen. It is interesting to note that Ba coverage determined by the Auger peak–height ratio of Ba 584 eV to W 169 eV lines is consistent with this definition of a Ba monolayer [28] using the relative sensitivity factors determined from bulk BaO [29]. The area contained in peak B of Fig. 4 is 300. This is the area from a Ba layer occupying every site. The monolayer area should be 150 nA K based on our definition, since the area of peak B in spectrum (a) of Fig. 4 is 300 nA K. The total area contained in the spectrum (a) is 645, which translates to 4.3 Ba layers. The accompanying AES measurements indicated 4.6 layers of coverage, a reasonable agreement allowing some BaO in the film. Although the BaO fraction is low in the evaporant, the O presence can be significant in the sub-monolayer films deposited. Fig. 8 shows the Ba2+ and BaO+ TPD spectra from a sub-monolayer film. The accompanied AES spectrum, (a) in Fig. 9, indicates that the total coverage is about 0.65 layers based on the Ba 590 eV to W 179 eV Auger peak–height ratio. The AES spectrum shows also that the O 510 eV to Ba 584 eV Auger peak–height ratio is 0.7, indicating a significant O presence even though it is a Ba-rich layer, because the ratio is 2 for a stoichiometric BaO, i.e. 1-to-1 Bato-O composition [29]. However, it has a much higher oxygen content than expected for the small BaO in the flux. The areas from the TPD spectra indicated 0.38 Ba layers and 0.16 BaO layers. The ratio of 2.4:1 is much

Fig. 8. Spectra (a) and (b) are the Ba2+ and BaO+ TPD spectra, respectively, taken from 0.54-layer Ba films.

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Fig. 9. AES spectrum (a) taken on the film used for illustration in Fig. 8. AES spectrum (b) taken on the oxidized film used for illustration in Fig. 10.

smaller than the Ba-to-BaO ratio in the flux. The extra oxygen was mostly picked up during the AES measurement To be consistent, we took an AES spectrum for each TPD spectrum throughout the study. The presence of the low level of O can have a strong effect on the binding energy of Ba in sub-monolayer films. The sub-monolayer of Fig. 8 is a Ba rich layer. Why were there not two distinct peaks, one from Ba on W and one from BaO on W, in the Ba2+ TPD spectrum? Instead, the lowest temperature peak in the Ba2+ TPD spectrum of Fig. 8 is at 1480 K, which is too high to be a Ba state and too low to be a BaO state: we know that Tp for Ba on clean W is less than 1100 K, and Tp for Ba on BaO is 1287 K; spectrum (b) in Fig. 8 shows that Tp is about 1710 K for the BaO film. To interpret the spectra in Fig. 8, we must think of Ba and O on W collectively and not imagine them as individual Ba or BaO. Imagine that on a square lattice, such as W(1 0 0), each Ba has four oxygen neighbors for a stoichiometric BaO film. In an Odeficient film, each Ba has less than four oxygen neighbors, and consequently the binding energy is lower than for BaO on W. In other words, we think of the adsorption state of Fig. 8 as O-assisted Ba adsorption on W, and consequently the binding energy is stronger than that of Ba on W. However, the vacancy in O neighbors weakens the binding energy. We don not have any TPD spectrum for films with an O/Ba AES ratio less than 0.7. However, it is easy to prepare a film with O/Ba AES ratio larger than 0.7, e.g. simply by exposing a film to oxygen. Fig. 10 shows the pair of

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Ba2+ rate may be a factor of 10 smaller, we do not expect the double ionization contribution to Ba2+ TPD spectra to be substantial. This is further supported by the fact that the relative intensity between Ba2+ and BaO+ TPD spectra pairs varies significantly from film to film.

4. Ba and BaO adsorption on W at elevated temperature

Fig. 10. Spectra (a) and (b) are the Ba2+ and BaO+ TPD spectra, respectively, taken from 0.60-layer oxidized Ba films. The Ba films were exposed to 12 Langmuirs of oxygen at room temperature.

TPD spectra for a sub-monolayer film after exposure to 12 Langmuirs of oxygen. Spectrum (b) of Fig. 9 shows a 3.5 O/Ba AES ratio for the film. O vacancies must have been filled. The peaks in the Ba2+ TPD spectrum shift to higher temperatures and coincide with the peaks in the BaO+ TPD spectrum. TPD spectra from films with O/Ba AES ratios close to 1 will be presented in the next section. The pairs of TPD spectra (i.e. Ba2+ to BaO+) manifest two desorption branches from the same surface layer. The relative areas of Ba2+ to BaO+ desorption in Figs. 8 and 10 suggest that an increase in the O content suppresses the Ba desorption branch and enhances the BaO desorption branch. From the Odeficient film of Fig. 8, the branches were 0.38 layers desorbing to Ba and 0.16 layers desorbing to BaO. From the O-rich film, the branches were 0.1 layers desorbing to Ba and 0.5 layers desorbing to BaO. AES measures the total Ba coverage. For the O-deficient film, the AES spectrum (Fig. 9(a)) measures 0.65 layers, compared to 0.54 layers (0.38 + 0.16 layers) determined by the TPD spectra. For the O-rich film, the AES spectrum (Fig. 9(b)) measures 0.57 layers, compared to 0.60 layers (0.1 + 0.5 layers) determined by the TPD spectra. Both are in reasonable agreement considering the poor signal to noise ratio in both AES spectra of Fig. 9. Ba2+ could also result from dissociation and double ionization by the electron beam in the ionization chamber of the QMA. Koitabashi [22] measured the Ba+-to-BaO+ ratio to be 0.6 in a BaO stream. Since the

The last section described Ba and BaO adsorption on W at room temperature. The study paved the groundwork for the next two sections. We know now the nature of various desorption peaks and how to convert the desorption areas to the coverage. Although the interpretations of Ba2+ and BaO+ TPD spectra were made based on all of the data, we discussed them at the end of the last section to facilitate the presentation of the results for the remaining of the paper. Pertinent to the dispenser cathode performance is the interaction of Ba and BaO with the substrate at elevated temperatures rather than at room temperature. During cathode operation or activation, the active material (mainly Ba and BaO) is evaporated from the impregnants in the pores and diffuses onto the hot substrate. In order to simulate the surface layer that is on a dispenser cathode surface, a hot dispenser cathode is placed in front of a hot W substrate. We choose a 411 type dispenser cathode for this purpose, because Hasker et al. [9] found that a shorter activation time using the 411 impregnant ratios than using a 532 impregnant ratio. The W substrate was either at 940 8Cb (1272 K), a typical cathode operating temperature [18,19], or at 1125 8Cb (1477 K), a typical activation temperature [20,21]. We will use the brightness temperature (8Cb) for the remaining of the paper because it is the most commonly used unit in the thermionic cathode community. All data presented in this section and the next section, unless specified, were taken using the same W substrate. We exposed the W substrate to the same Ba and BaO flux for all of the study below, for which the dispenser cathode source was kept at 1125 8Cb. A source temperature of 940 8Cb was also tried, but the evaporation rate was so low that the long deposition times made it impractical. Even at 1125 8Cb it took

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Fig. 11. AES spectrum (a) taken after 60 min of deposition on a W substrate at 940 8Cb, and AES spectrum (b) taken after 60 min of deposition on a W substrate at 1125 8Cb. From here on all deposition will be from the same dispenser cathode at 1125 8Cb with the exception of Fig. 32 where the cathode temperature will be higher.

over an hour to reach a steady state. Fig. 11 shows two AES spectra taken on the W substrate after 60 min of deposition. With the substrate at 940 8Cb during deposition, spectrum (a) shows that 0.68 layers of BaO covered the surface. The O 510 eV to Ba 584 eV Auger peak–height ratio is 1.92, close to the stoichiometric BaO ratio of 2 [29]. With the substrate at 1125 8Cb during deposition, spectrum (b) shows that 0.45 layers of BaO covered the surface. The O 510 eV to Ba 590 eV Auger peak–height ratio is 2.02. The BaO coverage was similar to what was commonly observed on a dispenser cathode [30]. Fig. 12 shows Ba2+ and BaO+ TPD spectra taken from the film of 60 min deposition with the substrate at 940 8Cb. According to the desorption areas, 0.35 layers of the film desorbed as Ba while 0.36 layers desorbed as BaO. A sum of 0.71 layers is very close to the 0.68 layers determined by the AES spectrum of Fig. 11. Fig. 13 shows Ba2+ and BaO+ TPD spectra taken from the film of 60 min deposition with the substrate at 1125 8Cb. 0.20 layers and 0.39 layers of the film desorbed as Ba and BaO, respectively, for a total of 0.59 layers, which is a little higher than the 0.45 layers determined by AES. But the discrepancy is not significant because of the large uncertainties in both measurements. In particular, the coverage determination by AES, which requires the quotient of the two noisy Auger peak heights, could lead to an error over 10%. In addition, there may be variation in

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Fig. 12. Ba2+ TPD spectrum (a) and BaO+ TPD spectrum (b) taken after 60 min of deposition on a W substrate at 940 8Cb.

coverage over the surface, and AES was taken on only one spot on the surface. The effect of the substrate temperature during deposition is apparent in the relative peak positions and relative peak areas in the Ba2+ and BaO+ TPD spectra. From the 940 8Cb sample, Tp is 1600 K in the Ba2+ spectrum, which is lower than the Tp of 1658 K in the BaO+ spectrum. As pointed out in Section 3, the presence of the low temperature peak in the Ba2+ spectrum indicates an O deficiency in the film. The O/ Ba AES ratio in this film is 1.92 and Tp is 1600 K. Recall that the O/Ba AES ratio in the film of Fig. 8 was 0.7 and Tp was 1480 K, while the O/Ba AES ratio in the film of Fig. 10 was 3.5 and Tp was 1646 K. The correlation supports the interpretation that an O deficiency weakens the O-assisted Ba bonding to

Fig. 13. Ba2+ TPD spectrum (a) and BaO+ TPD spectrum (b) taken after 60 min of deposition on a W substrate at 1125 8Cb.

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W; the higher the O deficiency present in the film, the weaker the bond is. From the 1125 8Cb sample, the onset of the Ba2+ spectrum coincides with the BaO+ spectrum, indicating the absence of O deficiency, in agreement with the O/Ba AES ratio of 2.02 measured from the film (Fig. 11, spectrum (b)). The Ba and BaO branching ratios found in the TPD spectra change monotonically with the O content. From the 940 8Cb sample, 0.35 layers desorb as Ba and 0.36 layers desorb as BaO. Consequently, the Ba and BaO branching ratio is 0.97 (0.35/0.36) for the film with an O/Ba AES ratio of 1.92. From the 1125 8Cb sample, the Ba and BaO branching ratio is 0.51 (0.20/0.39) for the film with an O/Ba ratio of 2.02. From the film deposited at room temperature (Fig. 8), the Ba to BaO branching ratio was 2.4 (0.38/0.16) for an O/Ba AES ratio of 0.7. From the fully oxidized film (Fig. 10), the Ba to BaO branching ratio was 0.2 (0.10/0.50) for O/ Ba AES ratio of 3.5. The major effect of the substrate temperature is in the binding of BaO to W. Fig. 14 compares the Ba2+ spectrum of 940 8Cb (spectrum (a)) to the Ba2+ spectrum of 1125 8Cb (spectrum (b)). For the comparison, spectrum (b) of Fig. 12 and spectrum (b) of Fig. 13 are reproduced as spectra (a) and (b), respectively. It is clear that the 1125 8Cb spectrum is the same as the 940 8Cb spectrum with the removal of the low temperature peak. The 1125 8Cb heating desorbed the Ba from the O deficient area; the remaining film had a balanced Ba-to-O stoichiometry. The BaO+ spectra revealed that a 1125 8Cb heating resulted in a tighter BaO-to-W bond with a narrower

energy spread than for a 940 8Cb heating. Tp in the 1125 8Cb spectrum (spectrum (a) in Fig. 13) is 1702 K, while Tp in the 940 8Cb spectrum (spectrum (a) in Fig. 12) is 1658 K. The full-width-at-half-maximum (FWHM) in the 1125 8Cb spectrum is 233 K, while the FWHM in the 940 8Cb spectrum is 327 K. We studied the film coverage change as a function of deposition time. The Ba2+ spectra showed little change with increasing deposition time. Fig. 15 compares a Ba2+ spectrum of 30 min deposition time with the Ba2+ spectrum of 60 min deposition time. The substrate temperature was 940 8Cb during deposition for both spectra. The difference is very small. In fact, the areas from the spectra indicate a mere increase from 0.30 to 0.35 layers. For a 1125 8Cb substrate temperature, the coverage increased from 0.18 layers to 0.20 layers for the same deposition time durations. In contrast the BaO+ TPD spectra indicated a significant coverage growth even beyond 60 min deposition time. Fig. 16 shows 940 8Cb BaO+ TPD spectra (a)–(d) for 30, 60, 120 and 240 min of deposition, respectively. The desorption areas indicate a BaO coverage of 0.25, 0.37, 0.62 and 0.83 layers in order of increasing deposition time. Tp for the spectra of the 30 and 60 min films was 1682 K, decreasing to 1648 K for the 120 and 240 min films. The peaks appear to be rather broad with a FWHM of 318  24 K. The substrate is a polycrystalline W ribbon, and the surface is composed of a collection of various crystal orientations. BaO is known to bind differently on different single crystal faces [31]. The binding energy spread over various faces on the

Fig. 14. Ba2+ TPD spectra taken after a 60 min deposition on a W substrate at 940 8Cb (a) and at 1125 8Cb (b).

Fig. 15. Ba2+ TPD spectra taken on a W substrate at 940 8Cb after 30 min of deposition (a) and after 60 min of deposition (b).

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Fig. 16. BaO+ TPD spectra taken on a W substrate at 940 8Cb after different lengths of deposition time: (a) 30 min; (b) 60 min; (c) 120 min; (d) 240 min.

Fig. 17. BaO+ TPD spectra taken on a W substrate at 1125 8Cb after different lengths of deposition time: (a) 30 min; (b) 60 min; (c) 120 min; (d) 240 min.

polycrystalline W may account for the broad desorption peaks. BaO occupied the tighter binding faces first before occupying the weaker binding faces, resulting in the downward shift of Tp with higher coverage. We attempted to resolve the TPD spectra into two or more component peaks but found little consistency in the results for the spectra in Fig. 16. For a satisfactory peak fit, we need to know the crystalline composition of the substrate and the correct functional form of the desorption peaks. The functions available in the peak fit program [25] fitted well the Sc2+ and ScO+ desorption peaks in the TPD spectra [15,17]. None of the functions seems to fit the Ba2+ and BaO+ desorption peaks as satisfactorily. Fig. 17 shows BaO+ TPD spectra (a)–(d) for 30, 60, 120 and 240 min of deposition, respectively, at 1125 8Cb substrate temperature. The desorption areas indicate a BaO coverage of 0.29, 0.40, 0.46 and 0.62 layers in order of increasing deposition time. Tp was 1711  3 K for the spectra of the 30, 60 and 120 min films, and was 1690 K for the 240 min film. The FWHM of the peaks was 236  9 K, much narrower than those in the 940 8Cb spectra. We recall that the higher heating temperature removed the lower binding energy peak from the 940 8Cb Ba2+ spectrum (Fig. 14). The effect on the BaO+ TPD spectra was entirely different; the higher heating temperature shifted Tp to a higher temperature (from 1682 to 1711 K) and sharpened the peak width (from 318 to 236 K in FWHM) but with no loss in coverage. Therefore, the effect of the activation temperature is to form stronger

BaO-to-W bonds with a smaller spread in the binding energy. The areas in the TPD spectra give a measure of the coverage. Fig. 18 shows the coverage as a function of deposition time at 940 8Cb substrate temperature. The solid circles were derived from the BaO+ TPD spectra, and were the coverage that lead to the BaO desorption branch. The crosses were derived from the Ba2+ TPD spectra, and were the coverage that lead to the Ba desorption branch. The sum of the two is the total BaO coverage, which agrees well with the coverage derived from the AES spectra (open squares in Fig. 18). For

Fig. 18. Coverage is plotted as a function of deposition time on a W substrate, which was at 940 8Cb during deposition. The solid circles mark the coverage determined from the BaO+ TPD spectra (Fig. 16). The crosses mark the coverage determined from the Ba2+ TPD spectra (Fig. 15). The open squares mark the coverage determined by AES spectra.

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example, from the 30 min deposition data, the 0.58layer coverage was determined by AES, while the sum of 0.59-layer coverage were determined from TPD spectra, i.e. 0.30 and 0.29 layers from the Ba2+ and BaO+ TPD spectra, respectively. The good agreement on the 60 min data was discussed earlier. At low coverage, a significant fraction of BaO desorbs as Ba, indicating a significant presence of O deficient areas. As the coverage increases, the desorption to BaO becomes dominant, indicating a decrease in the O deficient areas and an increase in the stoichiometric BaO areas. The AES spectra were not sufficiently accurate to provide useful information on the subtle stoichiometry change. The O/Ba AES peak height ratio was 1.95  0.15, averaged over all AES data. It is not surprising that the surface layer should be O deficient. The deposition flux is overwhelmingly Ba rich, namely 13.3:1 Ba-to-BaO atomic ratio [22]. At 940 8Cb substrate temperature, the sticking coefficient of Ba must be very low compared to the sticking coefficient of BaO. The coverage determined from the BaO+ TPD spectra showed a continued increase even near 240 min deposition time. However, the decrease in the O deficient areas compensated the increase, and the total coverage reached saturation at about 120 min. The long time to reach the saturation coverage indicates that the Ba and BaO flux employed in this study was much lower than the rate of dispensing on a dispenser cathode. Marrian and Thomas used a close-space diode configuration to simulate the dispensing rate of dispenser cathodes, and found it required about 20 min to reach a steady state [31]. The reason for the low flux rate here was the large source to sample distance, 0.95 cm, which was large compared to the source diameter, 0.34 cm. Fig. 19 shows the coverage change as a function of deposition time on the substrate at 1125 8Cb. The markers are the same as those in Fig. 18. Similar to the 940 8Cb data, each of the two desorption branches yields a partial coverage, and the sum agrees with the total coverage determination of AES. For example, at 30 min deposition AES indicated 0.39 layers of coverage, while the partial coverages from Ba and BaO desorption branches were 0.18 layers and 0.25 layers respectively, for a total of 0.43 layers. The results from 60 min deposition have been discussed above. In contrast with the 940 8Cb data, the 1125 8Cb data indicate that the surface BaO layer has by and

Fig. 19. Coverage is plotted as a function of deposition time on a W substrate, which was at 1125 8Cb during deposition. The solid circles mark the coverage determined from the BaO+ TPD spectra (Fig. 17). The crosses mark the coverage determined from Ba2+ TPD spectra. The open squares mark the coverage determined by AES spectra.

large a balance in Ba and O. The O/Ba AES peak height ratio is 2.04  0.18, indicating a 1:1 Ba-to-O stoichiometry. From these films we observed small but significant Ba desorption, but the peak positions indicate the absence of O deficiency (see Fig. 13). In comparison with the 940 8Cb data, the coverage on the 1125 8Cb substrate is lower and shows no indication of saturation even after 240 min of deposition. However, the main goal of the study is to show that surface layers below a mono-layer of BaO are formed on W from the flux of a dispenser cathode, in agreement with surface layer models, which are well established for standard dispenser cathode surfaces [30,32]. A similar study on Sc2O3-coated W presents a quite different finding, and will be discussed in Section 5. BaO forms stronger bonds with a smaller spread in binding energies on a W substrate at 1125 8Cb than at 940 8Cb. To find the temperature at which this transition occurs, BaO+ TPD spectra were also taken on the films deposited at substrate temperatures of 20 8C and 1025 8Cb. Fig. 20 shows the BaO+ TPD spectra together with the 940 and 1125 8Cb spectra, and the deposition time was 60 min for every spectrum. (There is a small peak near 1200 K at spectrum (a), and the peak temperature is too low to be from a BaO adsorption state. This peak coincides with the peak in the Ba2+ TPD spectrum of a Ba monolayer desorbing from clean W. A small fraction of the copious amount of Ba may have picked up O during

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Fig. 20. BaO+ TPD spectra after 60 min of deposition on a W substrate, which was at (a) 20 8C, (b) 940 8Cb, (c) 1025 8Cb, and (d) 1125 8Cb.

desorption and shows up in the BaO+ TPD spectrum.) In Fig. 20 the vertical lines help to show that the peak position Tp moves toward a higher temperature for a higher substrate temperature. It is less apparent in the figure that the peak width decreases with the higher substrate temperature. For clarity, Fig. 21 presents Tp and FWHM as functions of the substrate temperature. The desorption peak is the sharpest at the highest substrate temperature, but there is still contribution from the polycrystalline nature of the substrate to the sharpest peak. Fig. 22 shows two 1125 8Cb BaO+ TPD spectra, both of which were taken after 60 min deposition but on two substrates. The spectrum with the broader peak with FWHM of 226 K was taken on the same sample (sample #13) as most of the data of Sections 4 and 5. The spectrum with the sharper peak, which has FWHM of 164 K, was taken on a different sample (sample #12). The values of Tp for the two spectra were very close to each other, illustrating the reproducibility among samples. There were only limited data taken on sample #12. In order to compare the results from the two samples, we included all data regardless of the deposition time. For the 1125 8Cb data from sample #13, the Tp was 1704  12 K and the FWHM was 236  9 K. From sample #12, the Tp was 1711  13 K and the FWHM was 161  9 K. The 940 8Cb data did not show such good reproducibility. From sample #13, the Tp was 1660  18 K and the FWHM is 319  24 K. From sample #12, the Tp is 1630  3 K and the FWHM was 276  18 K. The large discrepancy can be explained by the polycrystal-

Fig. 21. Desorption peak temperature Tp and full-width-at-halfmaximum (FWHM) are shown in part (A) and part (B), respectively, as a function of the substrate temperature during deposition.

line effect. A difference in the crystal face distribution between the two samples results in a different distribution in binding states, giving rise to the difference in the apparent peak position. This

Fig. 22. Comparison of BaO+ TPD spectra on two different W substrates.

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observation underscores the importance of comparing data from the same sample. The data to be presented in Section 5 is going to be compared with the data presented in this section. In Section 5, the W sample was coated with Sc2O3, while in this section the W sample was not coated. This comparison has to be made on the same W substrate, and indeed both studies were carried out on sample #13.

5. Ba and BaO adsorption on Sc2O3 coated W The simplest and most successful approach in the fabrication of scandate cathodes is to coat a Sc2O3 and W mixture onto the surface of a standard dispenser cathode. For good control of the coating composition, alternate layers of Sc2O3 and W of the appropriate relative thickness are often used. Intermixing of the two components relies on thermal inter-diffusion [6]. To simulate the surface of the scandate cathode, a layer of Sc was deposited on the W substrate, and then the Sc layer was oxidized. After the oxidation the substrate was heated to induce the mixing. The 411 type dispenser cathode was also used to introduce Ba and BaO onto the coated W substrate at the elevated temperatures, i.e. at 940 8Cb or at 1125 8Cb. A typical coating preparation involved placing the W substrate in front of the Sc evaporator (see Fig. 1) for 30 min deposition. The thickness of the Sc coating was about 13 layers as determined by the area in the Sc2+ TPD spectrum [15]. During the oxidation process, the substrate was heated to 700 K while oxygen was leaked in via a variable leak valve. The pressure was raised to 6  10 6 Torr for 10 min, which is equivalent to 3600 Langmuirs of oxygen exposure. The substrate heating is necessary in order to oxidize the Sc layer completely [15]. Fig. 23 shows three AES spectra taken on Sc films at the different stages of preparation. Spectrum (a) was taken on the asdeposited film, which had only a small amount of O present. Spectrum (b) was taken after oxidation, and the O 510 eV-to-Sc 340 eV peak–height ratio of 1.1 indicates that the surface layer was Sc2O3 [17]. Both spectra show that W was absent from the surface layer. It is known that heating causes rapid inter-diffusion ˚ -thick Sc2O3 between W and Sc2O3 [6]. For the 50 A covered W, the 1125 8Cb heating induced a significant inter-diffusion. Spectrum (c) was taken after the

Fig. 23. AES spectra taken on Sc films at different stages of preparation: (a) as deposited; (b) oxidized at 700 K from exposure to 3600 L of oxygen; (c) heated to 1125 8Cb for 65 min following the oxidation.

substrate was heated to 1125 8Cb for 65 min. The W 169 eV-to-Sc 340 eV peak–height ratio indicates that the film consisted of 37% W and 63% Sc. In this analysis, we assumed a homogenous mixing in the surface layer that contained only two elements, W and Sc. O was not included in the analysis because the O AES peak–height is affected strongly by its environment (known as the matrix effect). In all heated films independent of the relative peak heights between Sc and W, the peak height of O 510 eV was correlated strongly with that of Sc 340 eV but not with W, and the O 510 eV-to-Sc 340 eV peak height ratios were all around 1.1. The heating appears to cause no change in the oxidation states of Sc2O3 and W, but only the mixing of the two. We are interested in the surface layers formed on scandate cathodes during operation and during activation. Substrate temperatures of 940 and 1125 8Cb were chosen during deposition for the respective states. A 940 8Cb heating was not sufficiently high to cause a significant mixing of Sc2O3 and W. Therefore, before each deposition at 940 8Cb the coated substrate was heated to 1125 8Cb for 60 min. In this manner most of the films studied were on coatings that consisted of 35% to 40% W based on the twoelement compositional analysis. Fig. 24 shows TPD spectra taken after 60 min deposition on the coated substrate at 940 8Cb. The Ba2+ spectrum is significant in intensity, indicating that there were Ba-rich areas in spite of the large O

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Fig. 24. Ba2+ TPD spectrum (a) and BaO+ TPD spectrum (b) taken after 60 min of deposition on the coated W substrate at 940 8Cb.

Fig. 26. Ba2+ TPD spectra after 60 min deposition on the coated substrate at 940 8Cb (a) and at 1125 8Cb (b).

presence of the coated surface. The Sc2O3 coating had little effect on the peak positions in the Ba2+ and the BaO+ spectra but had a significant effect on the peak areas. Fig. 25 compares the Ba2+ spectra on coated and uncoated substrates. While the peak positions remain unchanged, the peak area of the first peak reduces from 0.21 layers in the spectrum on the uncoated substrate to 0.10 layers in the spectrum on the coated substrate. The second peak area remains of 0.14 layers. However, the decrease in the coverage that desorbed as Ba from 0.35 to 0.24 layers was more than compensated by the increase in the coverage that desorbed as BaO from 0.36 to 0.69 layers. Therefore, the total coverage after 60 min of deposition was higher on the coated substrate (0.93 layers) than on the bare W substrate (0.71 layers), indicating a higher

sticking coefficient. The presence of Sc2O3 on the surface may enhance the sticking coefficient of Ba, which is bountiful in the source flux (13.3:1 in the Ba:BaO ratio) and which does not stick well on the hot W substrate. The Ba2+ desorption intensities were even smaller from the layers deposited at 1125 8Cb than at 940 8Cb. Fig. 26 compares two Ba2+ spectra, (a) and (b), after 60 min deposition on the coated substrate at 940 and 1125 8Cb, respectively. The 1125 8Cb spectrum is what remains after removing the low temperature peak from the 940 8Cb spectrum. A similar relation between the Ba2+ spectra at the two temperatures was observed on the bare W substrate (see Fig. 14). The absence of the low temperature peak in Ba2+ TPD spectra indicates the absence of Ba rich areas in the 1125 8Cb films. Fig. 27 shows Ba2+ and BaO+ TPD spectra after 60 min deposition at 1125 8Cb. The coverage that desorbed as Ba was 0.15 layers, compared to 0.20 layers on the bare W substrate for the same deposition time. The coverage that desorbed as BaO was 0.56 layers, compared to 0.39 layers on the bare W substrate for the same deposition time. After 60 min of deposition, the coverage was 0.70 and 0.59 layers on the coated and bare W substrates, respectively. Therefore, on the 1125 8Cb substrate just like on the 940 8Cb substrate, the sticking coefficient was higher on the coated substrate than on the bare W substrate. Fig. 28 illustrates the point further. The figure shows the coverage as a function of deposition time on the substrate at 1125 8Cb. The solid circles mark the coverage determined from the BaO+ TPD

Fig. 25. Ba2+ TPD spectrum (a) on a W substrate is compared to Ba2+ TPD spectrum (b) on the coated W substrate. For both spectra, the deposition time was 60 min and the substrate was at 940 8Cb.

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Fig. 27. Ba2+ TPD spectrum (a) and BaO+ TPD spectrum (b) taken after 60 min of deposition on the coated W substrate at 1125 8Cb.

Fig. 29. BaO+ TPD spectra taken on the coated W substrate at 1125 8Cb after different lengths of deposition time: (a) 60 min; (b) 120 min; (c) 240 min.

spectra while the crosses mark the coverage from the Ba2+ TPD spectrum. No AES-determined coverage is shown in Fig. 28, partly because the presence of multiple elements in the substrate complicates the coverage determination. More importantly the Ba 584 eV peaks in the spectra were mostly small and noisy. We will discuss this in more detail in the discussion on the 940 8Cb data. We noted from the 60 min data that the total coverage on the coated surface was higher than on the bare W surface. Fig. 28 informs us that the saturation in coverage was reached near 60 min. In contrast, Fig. 19 showed a continuous rise in coverage on the bare W substrate beyond a

120 min deposition time, although the total coverage was comparable on the two substrates. Specifically, after 240 min of deposition, the total coverage on the coated substrate was 0.75 layers (0.14 + 0.61 layers), while the total coverage on the bare W surface was 0.79 layers (0.17 + 0.62 layers). The major effect of the coating is found in the binding energy of the BaO layer. Fig. 29 shows BaO+ TPD spectra (a)–(c) taken after 60, 120 and 240 min depositions, respectively, on the coated substrate at 1125 8Cb. The peak position Tp increases monotonically with increasing deposition time and is 1713, 1761 and 1791 K for the 60, 120 and 240 min of deposition, respectively. The increase in the binding energy (i.e. increasing Tp) must be the result of the increasing reaction time, because there was no significant change found either in the BaO coverage or in the coating composition. The accompanied AES spectra revealed that the coating consisted of 35%, 35% and 41% W in the increasing order of deposition time. A noticeable broadening in the peak width was also observed: the FWHM is 237, 281 and 282 K, respectively, in the increasing order of deposition time. The later two are significantly broader than 236  9 K, the FWHM for the BaO+ TPD spectra on the bare W surface. The most significant observation is the continuous coverage growth on the coated substrate at 940 8Cb. Fig. 30 shows BaO+ TPD spectra (a)–(d) with 30, 60, 120 and 240 min of deposition, respectively, on the coated substrate at 940 8Cb. The peak position of spectra (a)–(c) are 1657  K, which is close to

Fig. 28. Coverage is plotted as a function of deposition time on the coated W substrate, which was at 1125 8Cb during deposition. The solid circles mark the coverage determined from the BaO+ TPD spectra (Fig. 29). The crosses mark the coverage determined from Ba2+ TPD spectra.

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Fig. 30. BaO+ TPD spectra taken on the coated W substrate at 940 8Cb after different lengths of deposition time: (a) 30 min; (b) 60 min; (c) 120 min; (d) 240 min.

1660  13 K, the peak position of the Ba2+ TPD spectra on bare W at 940 8Cb. The peak widths are comparable too, and the FWHM is 328  31 and 318  24 K on the coated and bare W respectively. The peak position of spectrum (d) is slightly higher at 1678 K. However, the coverage derived from the area of the desorption peaks is different from the bare W substrate. Fig. 31 shows the coverage as a function of the deposition time. The solid circles are the coverage derived from the areas of the BaO+ TPD spectra shown in Fig. 30. The crosses are the coverage derived from two companion Ba2+ TPD spectra. The coverage that gives rise to Ba desorption is very small compared to

Fig. 31. Coverage is plotted as a function of deposition time on the coated W substrate, which was at 940 8Cb during deposition. The solid circles mark the coverage determined from the BaO+ TPD spectra (Fig. 30). The crosses mark the coverage determined from Ba2+ TPD spectra.

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the coverage that gives rise to BaO desorption. For example, from the 60 min film, 0.25 layers desorbed as Ba and 0.69 layers desorbed as BaO. From the 240 min film, 0.40 layers desorbed as Ba and 1.9 layers desorbed as BaO. It is well established that a partial layer of BaO covers the surface of a standard dispenser cathode [30,32]. With the W substrate held at 940 8Cb while exposed to the flux from a dispenser cathode, we found a steady state of about 0.8 layers of BaO covering the W substrate. The same technique is used on the coated substrate; we found instead a continuous rise in coverage, which exceeded substantially a full monolayer. Will the coverage increase further with more deposition? Deposition times of 240 min were already very long. To accelerate the evaporation rate, we increased the dispenser cathode temperature from 1125 to 1225 8Cb. Spectra (e) and (f) in Fig. 32 are BaO+ spectra taken after 60 and 210 min, respectively, in the enhanced flux, and the desorbed amount is 2.0 and 5.1 layers of BaO, respectively. Undoubtedly a bulk-type layer has formed instead of a surface layer. We postulate that a BaO-containing compound has formed through the interaction of Ba and BaO with the Sc2O3 and W mixture. AES spectra that accompanied the TPD spectra revealed that Ba atoms have been absorbed into the interior probably

Fig. 32. BaO+ TPD spectra (e) and (f) were taken on the coated W substrate after 60 and 210 min depositions, respectively, but the deposition rate was increased by raising the temperature of the dispenser cathode source from 1125 to 1225 8Cb during deposition. Spectrum (c), the same as spectrum (c) in Fig. 30 was taken with the source at 1125 8Cb. Spectrum (g) was taken from 4.9 layers of BaO on W, and is shown here as a reference for the peak positions from a thick BaO film.

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through compound formation; the Ba 584 eV peak heights were much too small for the amount of BaO desorbed. Spectrum (a) of Fig. 33 shows the spectrum taken on the 120 min film. The Ba AES signal is comparable to the noise even though the companion BaO+ TPD spectrum (spectrum (c) in Fig. 30) suggests that 1.14 layers of BaO were present in the film. If it were a surface BaO layer, a much large Ba 584 eV AES signal would be expected. Spectrum (b) of Fig. 33 shows the Ba 584 eV peak for a 0.71 layers of BaO on W, which was taken on the 60 min film deposited on W at 940 8Cb. Even on the layer that desorbed 5.1 layers of BaO, the Ba 584 eV signal was modest in size, as shown by spectrum (c) of Fig. 33. In contrast, spectrum (d) shows a spectrum of 2.3 layers of BaO. The film was formed after 4.5 min of deposition on W at room temperature, followed by 12 Langmuirs of oxygen exposure to oxidize the film.

Fig. 33. AES spectra are shown here to compare the Ba 590 eV peak heights. They were taken after (a) 120 min deposition on the coated W at 940 8Cb, (b) 30 min deposition on W at 940 8Cb, (c) 210 min deposition with the accelerated deposition rate on coated W at 940 8Cb, and (d) 2.3 layers of BaO film.

The BaO-containing compound is not simply bulk BaO because they differ in the desorption temperature. Data from films of comparable thickness must be compared, because the peak temperature increases with increasing film thickness for a multi-layer film. Spectrum (g) of Fig. 32 shows BaO+ TPD spectrum from 4.9 layers of BaO film. The peak position Tp is at 1543 K, which is much lower than 1720 K, the Tp in spectrum (f), which contains 5.1 desorbed BaO layers. In fact, the Tp 1543 K is much lower than the peak temperatures in spectra (a) through (d) of Fig. 30 and in spectra (c) through (f) of Fig. 32. The difference in the desorption temperature made the postulation of a BaO-containing compound formation necessary. The compound has to decompose in order to release BaO before its sublimation. The higher desorption temperature of spectra (a)–(f) indicates that the decomposition temperature must be higher than the desorption temperature of the bulk BaO. The precise composition of the BaO-containing compound remains to be determined. One approach is to determine the maximum amount of BaO that is contained in a given thickness of Sc2O3 and W coating mixture. Through the variation in the thickness and coating mixture, the stoichiometry of the compound can be deduced. We found that the presence of W in the Sc2O3 coating is important. Fig. 34 compares two BaO+ spectra taken after 30 min of deposition on a coated surface. For spectrum (a), the coated substrate was heated to 1125 8Cb for 60 min before the 30 min deposition at a 940 8Cb substrate temperature, while for

Fig. 34. BaO+ TPD were taken after 30 min of deposition on a coated substrate at 940 8Cb. (a) The coated substrate was heated to 1125 8Cb for 65 min before the deposition. (b) The coated substrate was not heated before the deposition.

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spectrum (b) the 30 min deposition was done without preheating. The two-element analysis gave 40% W in the coating for the former case and 0% W for the latter case. The compound formed on the coating with 40% W released 0.42 layers of BaO upon heating. In contrast, only 0.06 layers of BaO were released from the film formed on the substrate without W.

6. Summary and discussion The presence of O on W has a strong effect on the binding energy of Ba on W. Heating to 1700 K is not sufficient to remove all O. The desorption peak position Tp for a full Ba layer deposited on W with residual O is between 1220 and 1250 K, and on thoroughly cleaned W, the Tp decreases to below 1100 K. Since a small amount of BaO is present in the Ba source (a dispenser cathode), the deposited film is never O-free. Only the upper limit of Tp has been determined. Each O atom is not tied to a specific Ba. Instead it interacts with several neighboring Ba atoms, and strengthens their binding to W. From a partial monolayer on W with less O than Ba, we did not observe two distinct binding states that can be associated with Ba binding to W and BaO binding to W. Instead we found two binding states that are associated with an O-deficient region and an O and Ba balanced region. Ba and BaO were observed to be the desorbing species from both regions. The binding energy in the O-deficient region increases with increasing O content until reaching a balance between Ba and O, while the relative amount of Ba desorption to BaO desorption decreases with increasing O content. On a dispenser cathode surface, the Ba and BaO supply depends on the diffusion of Ba and BaO supplied by the impregnants in the pores. On the simulated surface in this study, the Ba and BaO supply comes instead from the impinging Ba and BaO flux evaporated from a dispenser cathode facing the sample. It is well known that a partial layer of BaO covers the surface of a standard dispenser cathode at operating temperature [30,32]. The simulation does produce, at steady state, 0.8 layers of BaO on a W substrate at a typical operating temperature of 940 8Cb. We found further that this partial layer is slightly O deficient, because the evaporants from a dispenser cathode are predominantly

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Ba. This Ba and O imbalance is rather favorable to the cathode operation. We found only Ba and BaO but no O or O2 in the desorption products. Ba desorption, which leaves behind O, could lead to excessive O build-up, which has been indeed observed on aging dispenser cathodes [30]. However, we found that an increase in the O/Ba ratio increases the BaO desorption branch and decreases the Ba desorption branch. The excess Ba in the supply together with the desorption dynamics help to keep the surface Ba and O in balance [32]. On Sc2O3 coated W at 940 8Cb, we no longer have partial surface layers of BaO coverage. Instead we found that a BaO-containing compound forms, which could accumulate to form a very thick layer. It should be noted that on an actual scandate cathode the accumulation of this layer can be much more rapid than our simulated situation with the dispenser source far away from the substrate. With the experimental geometry, it took 2–4 h to form a steady state on a W substrate. On a standard dispenser cathode surface, 20 min are sufficient [31]. Furthermore, activation of scandate cathodes often involves drawing emission from the scandate cathode to an anode at a closespaced geometry. This process could speed up the compound accumulation, because of the increased Ba and BaO supply from electron stimulated desorption from the anode as well as from atomic and molecular backscattering off the anode. The effect of 1125 8Cb heating may be several. The high temperature is necessary for the inter-diffusion to occur among the interlacing W and Sc2O3 layers, which are present on many as-deposited scandate cathodes. We found also that the high heating temperature causes a tight binding BaO partial layer to form on the coated surface. The layer may serve to reduce the electron affinity of the compound formed underneath, if the compound formation causes the scandate cathodes to behave like an oxide cathode [33]. Raju and Maloney proposed a semiconductor model for scandate cathodes by an analogue to oxide cathodes [14]. This model explains well the anomalous emission characteristics commonly observed on the emission versus voltage measurements. The external electric field is not screened by the semiconductor surface; rather the field penetrates into the surface and lowers the work function. However, in order for the semiconductor model to be applicable, the compound formed must be thick and the doping

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density must be low. The depth and doping density together affect the calculated field enhancement. It remains to be determined how thick the BaOcontaining compound is, whether it is a semiconductor, and what doping level it has. First of all, we need to determine what the BaO-containing compound is, and a possible approach was suggested in Section 5. Unfortunately, we cannot pursue this subject further because of the termination of our research program. However, we hope others will continue the pursuit. We describe below some suggestions on needed improvements in the experimental technique. A redesign of the sample-heating setup is needed. In our current design, the temperature must be kept below 2100 K to avoid burning the heater wires. This temperature is not sufficiently high to clean the W substrate after Sc2O3 deposition. Even using the precaution, the sample was usable for only 30–60 TPD spectra. More in situ analytical techniques are needed to correlate with TPD and AES studies. In particular, XPS and a work function measurement probe are desirable. Wang and Pan [34] applied XPS and investigated the surface of Sc2O3-coated cathodes by pulsed laser deposition, and demonstrated that XPS can provide some much-needed chemical information. Furthermore, work function measurements are needed to determine if the synthesized layers correspond to the same low work function layers as observed on scandate cathodes. Improvements are needed in the data analysis of TPD spectra. We were unable to resolve the desorption peaks of Ba2+ and BaO+ TPD spectra with the peak fit program, which successfully resolved Sc2+ and ScO+ spectra. Finding the correct functional form and use of single crystal surfaces may be the required solution.

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