Investigation by Auger spectroscopy of the composition and surface oxidation characteristics of oxygen saturated zirconium

302

Applied

Surface

Science 35 (1988-89) 302-316 North-Holland, Amsterdam

INVESTIGATION BY AUGER SPECTROSCOPY OF THE COMPOSITION AND SURFACE OXIDATION CHARACTERISTICS OF OXYGEN SATURATED ZIRCONIUM Max C. DEIBERT, Department

Received

Brian Paul THIESEN

of Chemical Engineering,

25 July 1988; accepted

Montana

for publication

* and Ramazan

State University, Bozeman,

13 September

KAHRAMAN MT 59717, USA

1988

The surface oxidation characteristics of oxygen saturated zirconium (Zr: O,,) have been investigated between room temperature and 1190 K using Auger spectroscopy and sputter-depthprofiling techniques. Zr : O,, substrates were prepared by absorbing oxygen from surface oxide layers into Zr foils at high temperature. Quantitative Auger analysis confirms that the composition of Zr : O,, is approximately Zr00,44. Zr : O,, oxidizes slightly faster at room temperature than pure Zr during initial 0, exposure. The high temperature surface oxidation of Zr : O,, was studied in the absence of interference from the simultaneous bulk absorption of oxygen, which occurs with pure Zr substrates. The thickness of the ZrO, surface layer formed on Zr : O,, increases with the extent of 0, exposure, exposure time and oxidation temperature up to 900 K. Above 900 K the surface oxidation is not complete to Zr02.

1. Introduction The room temperature surface oxidation of pure zirconium (Zr) metal has been the subject of a large number of surface analytical studies [l-4]. The simultaneous surface oxidation and diffusion of oxygen into the underlying bulk Zr at high temperatures has also been investigated [1,4,10]. The hcp crystal lattice of a-Zr absorbs this oxygen as an interstitial solute with little change in its lattice dimensions [15]. Zr : OS, has a composition of about ZrOe,+, [16,17]. The rate of diffusion of oxygen into pure Zr becomes significant compared to the rate of surface oxidation at temperatures above about 500 K. It therefore becomes very difficult to study the surface oxidizing characteristics of Zr at elevated temperatures in the absence of complications arising from simultaneous bulk oxygen absorption. This complication has been avoided in the present study by preparing samples of Zr : OS, on which a surface oxide layer is stable at elevated temperatures. The surface composition of sputter* Present

address:

EG&G

Idaho,

Inc., Idaho Falls, ID 83415, USA

0169-4332/89/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)

B.V.

M. C. Deibert et al. / Composition and oxidation characteristics

of Zr: O,,

303

cleaned surfaces of these Zr : O,, substrates have been evaluated by quantitative Auger analysis. In addition, the surface oxidation characteristics and stability of surface oxide layers on Zr : O,, have been investigated at temperatures up to 1190 K.

2. Experimental 2. I. Sample preparation Each sample of Zr : O,, was prepared by absorbing a pre-formed surface oxide layer into the underlying Zr foil at high temperature in an inert environment. This method is an adaptation of a technique demonstrated by Pemsler [18] in which pre-oxidized tapered disks of Zr were annealed in vacuum. The surface oxide on the thick portion of the disks dissolved into the underlying Zr during the high temperature anneal, while some oxide was retained on the surface of the thin edges. The underlying Zr in these thin regions of the disks had become saturated with oxygen and were in equilibrium with the retained surface oxide. The Zr utilized in this study was 0.25 mm thick foil with a purity of 99.99%. Samples of this foil were suspended in a vertical quartz tube furnace by a fine nickel-chromium alloy wire. This wire was attached to a Cahn/Ventron R-100 null beam balance which monitored the mass gain of the sample during oxidation. The oxidation of the Zr foil was conducted at 1173 K in 200 Torr of ultra-pure 0,. The quartz tube was evacuated to remove the 0, when the mass gain of the sample reached 7.5% (Zr : O,, corresponds to 7.01% oxygen by mass) and was then backfilled with argon. The system temperature was then held at 1243 K for 30 h or longer. During this time most of the oxygen in the surface oxide layer, which was formed during the 0, exposure, diffused into the underlying Zr. The surface oxide layer served to maintain oxygen saturation on the surface of the underlying Zr and to supply the amount of oxygen necessary to oxygen saturate all of the Zr in the foil. It has been demonstrated that the interface of metallic Zr, which is in contact with a surface ZrO, layer, is saturated with oxygen [17,19]. The diffusion of oxygen in Zr has been well characterized [20]. The duration of the high temperature anneal in argon was calculated to insure that, at the diffusion constant of 6 X lop9 cm*/s for oxygen in Zr at 1243 K, the midplane of the foil would reach at least 99.99% saturation. At temperatures of about 773 K and lower the diffusion of oxygen through polycrystalline Zr is primarily along grain boundaries, while at the oxygen saturation temperature used in this study, 1243 K, bulk diffusion predominates [20]. For this reason, the grain size in the Zr foil used in this work should not significantly influence the rate or distribution of oxygen penetration into the foil. The determination

304

M.C. Deibert et al. / Composition and oxidation characteristics of Zr: OS,

of the time necessary to achieve oxygen saturation in the Zr utilized the transient transport analysis for a two-dimensional solid developed by Heisler

WI. The Zr : OS, samples were lightly polished with very fine grit silica-carbide paper to remove excess surface oxide and were then etched for 10 s in 60% HNO,, 35% H,O, 5% HF and rinsed prior to their insertion into the surface analysis system. 2.2. Surface analysis system The surface analysis and oxidation studies of the Zr : O,, samples utilized the PHI 595 scanning Auger microprobe with Ar+ sputter profiling capability located in the Center for Research in Surface Science and Submicron Analysis (CRISS) at Montana State University. This system, which has a base pressure of lo-” Torr, is completely computer controlled and is set up for digital data recording and computer assisted data analysis. The 3.0 keV primary electron beam was operated at a current of 200 + 5 nA, which corresponds to an electron beam diameter of approximately 800 nm. The electron beam was rastered over a 0.02 x 0.02 mm2 area. This area was central to a 2 x 2 mm2 area impacted by a rastered 3.0 keV Ar+ beam during ion milling. The Ar+ beam current was approximately 3.0 PA or about 75 PA/cm2 over the area of the rastered beam. The normal to the samples was oriented at 30” to the primary electron beam and 60 o to the Arf beam. All ion milling and spectral measurements were conducted at room temperature. The Zr : O,, samples were mounted on a 0.4 cm wide strip of 0.075 cm thick Ta foil in the surface analysis system. This foil was resistively heated by direct current to achieve high sample temperatures. A W-5’%Re/W-26%Re thermocouple was spot-welded to the Ta foil for sample temperature measurements. The primary AES scans focused on the principal Zr, carbon and oxygen Auger features by multiplexing three selected energy regions in the undifferentiated E x N(E) mode: 78 to 178 eV for the primary Zr Auger peaks; 240 to 280 eV to detect carbon; 495 to 515 eV for oxygen detection. In addition, survey AES scans from 30 to 1030 eV were conducted to detect the presence of impurities with Auger peaks in this energy region. Only a trace of the primary argon Auger peak at 211 eV was detected indicating that little of this gas absorbed in the samples during the high temperature saturation period. The spectra also showed only low intensity Auger signals for carbon at 263 eV after the initial sputter-cleaning of the Zr : O,, samples. The E x N(E) versus E data collected during the AES scans were stored on magnetic disks for later display, manipulation and analysis. PHI software was utilized to process the E x N(E) versus E data as required. Quantitative

M. C. Deibert et al. / Composition and oxidation characteristics

of Zr: Q,,

305

Auger peak intensities were established either from the peak heights above the background spectral intensity or, in the automated PHI software utilized in depth profiling, the peak-to-peak height of the differentiated E X N(E) spectra. In both cases, the core level Zr(MNN) peak at about 88 eV and the O(KVV) peak at about 508 eV were used for Zr and oxygen Auger peak intensity quantification, respectively. No significant changes in the intensity of the Auger peaks for Zr or oxygen were observed when sequential spectral runs were conducted on the same sputter-cleaned or oxidized Zr : OS, surface. This indicates that no significant electron stimulated desorption of oxygen occurred during the Auger tests. Electron stimulated desorption of oxygen from oxidized Zr surfaces has been shown to be significant under some experimental conditions [3].

3. Results 3.1. Auger spectral characteristics Examples of the principal Auger transitions occurring between 78 and 178 eV for sputter-cleaned surfaces of pure Zr foil and Zr : OS, are presented in fig. 1. Also shown in fig. 1 is the spectra for sputter-cleaned Zr : OS, after 300 L of 0, exposure at 900 K. This latter surface is shown below to have a ZrO, surface layer about 42 nm thick. The spectrum for sputter-cleaned pure Zr foil exhibits peaks near 88, 114, 123, 114 and 172 eV. The ZrO, surface on high temperature oxidized Zr : OS, exhibits peaks near 86, 111, 121, 139, 144 and 170 eV. The Zr(MNV) peak at 144 eV is diminished by the surface oxidation of the Zr while a new interatomic Zr(MN)O(V) peak develops at 139 eV [6]. The Zr(MVV) peaks at 173 and 123 eV are strongly diminished. These changes, together with the shifting to lower energies of the Zr(MNN) peaks at 88 and 114 eV, provide convenient indications of the oxidation state of Zr from its Auger spectra and are consistent with the observations of Axelsson et al. [3]. The spectrum for the sputter-cleaned Zr : OS, surface in fig. 1 is essentially the same as that for sputter-cleaned Zr except for a small shoulder near 139 eV on the 144 eV peak. This shoulder most probably represents a small contribution from the interatomic Zr(MN)O(V) transition. There are no other significant indications that the Zr near the surface of this sputter-cleaned Zr : OS, is oxidized. The argon ion sputtering process might be expected to promote preferential oxygen removal from the surface of the Zr : OS,, producing an Auger spectra characteristic of a more reduced Zr than one that would be measured on a UHV-fracture exposed Zr : OS, surface. This effect has been noted for sputtercleaned ZrO, surfaces by Tanabe et al. [15]. However, they also observed that

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M. C. Deibert et al. / Composition and oxidation characteristics

L

80

I

I

I

100

Electron

I

120

I

140

Energy

I

of Zr : OS,

I

160

(eV)

Fig. 1. 78 to 178 eV region of E X N(E) Auger spectrum of (a) sputter-cleaned Zr; (b) sputter-cleaned Zr : OS,; and (c) sputter-cleaned Zr : O,, after 300 L of O2 exposure at 900 K (ZrO, surface).

no significant surface depletion of oxygen occurred during the sputter-cleaning of sintered pellets of mixed Zr and ZrO, powders which contained between 5 and 40 at% oxygen. Our Zr : OS, samples contained about 30 at% oxygen. There is additional evidence that preferential sputter removal of oxygen is small in our study since an anneal of the sputter-cleaned Zr : OS, at 900 K for 10 min in UHV did not significantly change the relative intensities of the Zr and oxygen Auger spectra. A small surface accumulation of carbon during this anneal was evidenced by an increased intensity of the C(KLL) Auger peak intensity at 263 eV. 3.2. Oxygen concentration

in Zr: OS,

The relative AES peak height intensities of E X N(E) ZrO, and sputter-cleaned Zr : OS, surfaces were used

spectra measured on to estimate the con-

M.C. Deibert et al. / Compositi& and oxidation characterisrics of Zr: OS,

301

centration ratio of oxygen to Zr on the surface of sputter-cleaned Zr : O,, using the quantitative Auger analysis method of Seah [22]. In Seah’s method of analysis of a binary mixture, the atomic ratio of the elements in the surface region which contribute to the measured Auger signal intensities can be related to their Auger intensities by: (X*/XB)s

= C(L/&3)?

(1)

where X, is the surface region mole fraction of component A; X, the surface region mole fraction of component B, Xa = 1 - X,; IA the the Auger signal intensity of element A; Zs the the Auger signal intensity of element B; and C a proportionality constant. Auger spectra of four sputter-cleaned Zr: O,, surfaces which had been oxidized to ZrO, at 900 K were measured. The average and standard deviation of the Auger peak height intensities, Zo/Zz,, for these four ZrO, surfaces was 7.72 and 0.25, respectively. The ratio of these Auger intensities measured on thirteen separate sputter-cleaned but not re-oxidized Zr : O,, surfaces averaged 1.71 with a standard deviation of 0.08. There were no significant differences among the sequential Zo/Zz, ratios measured during interruptions of extended sputterings of the Zr : O,, surface and those measured after each of several oxidizing and sputter-cleaning cycles. This consistency of the IO/Z,, Auger signal intensity ratio for sequentially exposed surfaces of Zr : O,, indicates that the oxygen concentration was essentially constant with depth. This consistency would be expected from the long, high temperature, saturation period used in the preparation of the Zr : O,, substrates. Although the sputter-cleaned surfaces of Zr : O,, were undoubtedly covered by microcracks, especially in the vicinity of grain boundaries, any more heavily oxidized Zr in these regions will be preferentially removed during Ar+ sputtering, leaving a surface which is primarily sputter-cleaned Zr : O,,. The consistency of Zo/Zzr values measured on a large set of sputter-cleaned Zr : O,, surfaces supports this assumption. The proportionality constant C in eq. (1) was established from the Auger signal intensity ratio measured on the heavily oxidized Zr : O,, surfaces assuming that they achieve a ZrO, composition ((X,/X,,), = 2.0). The validity of this assumption is supported by Tapping’s observation from XPS measurements of oxidized Zr that the surface oxide has ZrO, stoichiometry [ll]. The ratio (X,/X,,), for the thirteen sputter-cleaned surfaces were then estimated from their (Zo/Zz,) Auger intensity ratios. By this method, the surface concentration ratio was established to be in the range of 0.44 + 0.02 (equivalent to about ZrO,,,). This estimate of the concentration of oxygen in Zr : O,, is equivalent to the value of 0.44 reported by Gebhardt et al. [16] from electrical resistivity measurements. It is also close to the value of between 0.42 and 0.43 reported by David et al. [17] from nuclear microanalysis measurements of the oxygen saturated Zr at the interface between a ZrO, layer and the underlying metal after oxidation of the Zr at high temperature. The

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M. C. Deibert et al. / Composition and oxidation characteristics

of Zr : OS,

validity of the application of Seah’s quantitative Auger analysis technique to the Zr-oxygen system is therefore demonstrated. The use of eq. (1) to predict the (X,/X,,), ratio of incompletely oxidized surfaces is based on the assumption of the consistency of the proportionality constant, C, over a wide range of surface oxygen concentration up to that of ZrO,. This assumption is supported by the demonstration by Hall and Morabito [23] that these constants change very little for binary pairs of elements throughout the range of concentrations between the pure components and also by the correspondence of the surface composition of Zr : OS, (ZrO,,.,) predicted by this method with those measured by other methods in independent studies [16,17]. 3.3. Room temperature oxidation The room temperature oxidation of sputter-cleaned pure Zr foil and of sputter-cleaned Zr : OS, were investigated to establish if the interstitially dissolved oxygen in the Zr : OS, significantly alters the initial reaction with 0,. After a multiplexed AES spectra of the sputter-cleaned Zr or Zr : OS, was recorded, an oxygen flow was established through the vacuum chamber from a leak valve to achieve the series of time-pressure conditions listed in table 1. Multiplexed AES spectra were recorded after each of the seven sequential 0, exposures between 0.3 to 300 L. The Io/I,, Auger intensity ratios established from these spectra were converted to (X,/X,,), ratios using eq. (1) and the proportionality constant, C, determined from Auger intensities measured using the high temperature oxidized Zr : OS, surfaces. The variation of (X,/X,,), with 0, exposure is shown in fig. 2. The Zr : OS, appears to oxidize more rapidly than Zr during initial 0, exposure. As the result of the first 0.3 L of 0, exposure, the (X,/X,,), ratio for the Zr : OS, substrate increased 0.62 (from 0.44 to 1.06), while that for pure Zr increased 0.47 (from 0.00 to 0.47). Both surfaces approach the saturation (X,/X,,), value of 2.0 after 300 L of 0, exposure, although the surface of Zr: OS, Table 1 Sequential

0, exposure

pressures

and times utilized in room temperature

oxidation

studies

Exposure

0, pressure

number

(Torr)

Exposure time (s)

Added 0, exposure (L)

Total 0, exposure (L)

1 2 3 4 5 6 7

1x10-s 1x10-s 2x10-s 7x10-8 2x10-7 7x10-7 1x1o-6

30 70 100 100 100 100 200

0.3 0.7 2 7 20 70 200

0.3 1 3 10 30 100 300

M.C. Deibert et al. / Composition and oxidation characteristics

OL, Clean

.z aB

z

5 IX

OS0

1

I

I.0

2.0

of Zr: OS,

309

I

3.0

Log,, [Oxygen Exposure (Longmuirs)] O2 exposure on the molar ratio of oxygen to Zr for Fig. 2. Influence of room temperature sputter-cleaned surfaces of (a) Zr : O,,; and (b) pure Zr.

maintains a higher total surface oxygen concentration than does pure Zr throughout the 0, exposure range investigated. Osthagen and Kofstad [24] have observed that the rate of macroscopic mass gain of Zr samples increases as the concentration of oxygen dissolved in the Zr increases. The present results indicate that this acceleration of the Zr-0, surface reaction may also be active during its very initial stages. 3.4. High temperature oxidation Sputter-cleaned Zr : OS, surfaces were oxidized at temperatures between 500 and 1090 K and 0, exposures between 10 and 600 L. Examples of sputter-depth -profiles taken after 0, exposure under three separate conditions are shown in fig. 3. In conducting these sputter-depth-profiles by the automated PHI procedure, two sets of AES spectra for the Zr(MNN) peak near 88 eV and for the O(KVV) peak near 508 eV were run before the first 30 s sputter cycle and one set after each successive 30 s sputter cycle. The PHI software which controls the sputter-depth profile procedure utilizes the peak-to-peak height of the differentiated Auger peaks as a measure of their intensity. The oxygen and Zr atomic concentration profiles shown in fig. 3 were calculated using standard sensitivity factors for these peaks [25]. The changes in the ratio of the oxygen to Zr Auger intensities, (1,/I,,), between sputter-cleaned and oxidized Zr : OS, which were measured by the automated PHI sputter-depth-profile procedure are significantly different from those determined from either the peak heights (or peak areas) of the undifferentiated Auger spectra. The undifferentiated spectral peak height data

310

h4.C. Deibert et al. / Composition and oxidation characteristics of Zr: OS,,

$--=J( 0

3

6

9

Sputter

12

15

Time,

“,

16

,

21

24

,I 27

30

Min,

Fig. 3. Relative oxygen and Zr atomic concentration profiles calculated from sputter-depth-profile data for sputter-cleaned Zr : O,, surfaces after (a) 600 L 0, exposure at 900 K; (b) 300 L 0, exposure at 1190 K; and (c) 300 L 0, exposure at 900 K followed by an anneal in UHV for 5 min at 1100 K. (The relative concentrations determined from the two AES scans conducted prior to the first 30 s sputter cycle are shown before and at zero sputter time.)

presented in section 3.2 indicate that this ratio changes by a factor of 0.22 (1.71/7.72) as the result of sputter-cleaning an oxidized surface of Zr : OS,. (Peak area measurements of the undifferentiated Zr(MNN) peak near 88 eV and of the O(IVV) peak near 508 eV provide essentially the same value for this factor.) The ratios, (lo/lz,), determined by the automated PHI procedure for 31 sputter-cleaned Zr : OS, surfaces averaged 1.45 while those for 19 heavily oxidized Zr: OS, surfaces averaged 4.70. This intensity ratio is therefore indicated to change by a factor of 0.31 (1.45/4.70) by peak-to-peak height intensity measurements of differentiated Auger spectral peaks. The factor by which this intensity ratio changes, measured to be 0.22 from undifferentiated Auger spectral peaks, agrees with other independent measurements [16,17] of the relative oxygen and Zr concentrations in Zr : OS, (ZrO,,,) and ZrO,. Measurements of Auger intensities from the peak heights or areas of undifferentiated spectra, therefore, appear to be more valid for this system than

M. C. Deiberr et al. / Composition and oxidation characteristics

of Zr: OS,

311

those measured in the more traditional method using differentiated Auger spectra which depend on the slopes of the respective Auger peaks. The sputter-depth profiles of oxygen and Zr concentrations presented in fig. 3 should therefore be considered to represent only trends in the relative concentrations of these elements and not accurate estimates of their actual atomic proportions. The sputter-depth-profile in fig. 3 for the Zr : O,, sample which had been oxidized at 900 K with 600 L of 0, indicates that during the first 9 n-tin of sputtering the sputter exposed surfaces are heavily oxidized with a relatively consistent composition close to that of ZrO,. There is some indication of a slight reduction in the oxygen concentration (and corresponding increase in the Zr concentration) during the initial 9 min of sputtering. Some decrease in oxygen concentration through the ZrO, surface layer can be expected in order to provide the concentration gradient needed to promote the diffusion of oxygen to the Zr: O,, substrate. The diffusion of oxygen through the oxide layer to the oxide-metal interface has been shown to be the primary transport process in ZrO, surface layer growth, rather than the diffusion of Zr to the oxide-gas interface [26]. Between 9 and 18 min of sputtering there is a transition in composition to near that of the substrate Zr : O,,. The depth of the surface ZrO, layer can be estimated from the time required for its removal by sputtering. The sputter rate of SiO, for the experimental system was measured to be 8.1 nm/min. The ratio of sputter rates of SiO, and ZrO, has been established to be 0.14/0.12 using 4.0 keV argon ions [27]. Although 3.0 keV argon ions were used in this study, this difference in ion energy should not significantly change the ratio of these sputter rates. The ZrO, sputter rate should therefore have been approximately 6.9 nm/min. The 9 min sputter time during which the surface concentration remained nearly consistent indicates that the Zrq surface layer was about 62 nm thick. The rate of sputter removal of material decreases as the surface becomes less oxidized. There is no readily available means of establishing the thickness of the transition region penetrated between about 9 and 18 min of sputtering from the Auger sputter-depth-profile data. There are several possible measures of the thickness of the surface oxide layer formed on Zr: O,, during a series of high temperature exposures of sputter-cleaned Zr : O,, to 0,. The sputter-time at which the concentrations of oxygen and Zr begin to change from values close to those in ZrO, is difficult to accurately establish for many of the sputter-depth profiles. Therefore, the measurement utilized in this analysis is the sputter time required to achieve equivalence in the oxygen and Zr surface concentrations. For the profile presented in fig. 3 (curve a) for the Zr : O,, surface oxidized at 900 K with 600 L of O,, 13.8 mm of sputtering were required for this equivalence to be achieved. (Actually the oxygen/Zr surface concentration ratio was greater than 1.0 after the 30 s sputter ending at 13.5 total min of sputter time and less

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M.C. Deibert et al. / Composition and oxidation characteristics

Table 2 Surface oxide layer thicknesses

formed

on Zr : OS, by high temperature

of Zr: Q,,

exposure

to 0,

Oxidation temp.

0, exposure

Exposure time a)

(K)

(L)

(s)

Oxide layer sputter-time (min)

500 700 900 1100 1190 900 900 900 900 900 900 900

300 300 300 300 300 10 30 30 100 100 600 600

300 300 300 300 300 100 30 300 100 300 300 600

0.7 2.2 11.0 b’ 12.8 9.8 0.0 1.1 1.1 2.6 4.4 13.8 16.2

‘) The 0, exposure pressure in Torr = L/s X 10K6. b, Average of 4 runs with sputter times of 11.5, 11.2, 9.8 and 11.6 mitt, respectively.

than 1.0 after the next 30 s sputter cycle.) Using this measure for the depth of oxygen penetration into the Zr : O,,, table 2 presents the oxide layer thicknesses achieved by oxidizing sputter-cleaned Zr : O,, at several temperatures and 0, exposure conditions. The first five surface oxidation test results listed in table 2 involved 300 L of 0, exposure (10e6 Torr for 300 s) at temperatures between 500 and 1190 K. In these tests the oxide layer sputter time increased with oxidation temperature up to 1100 K and then decreased at 1190 K. The sputter-depth-profile of the Zr: O,, surfaces oxidized at 500, 700 and 900 K indicated that the composition of a significant portion of the surface oxide layer was close to that of ZrO,. The Zr : O,, surfaces oxidized at 1100 and 1190 K did not reach high an oxygen concentration. This is shown in the sputter-depth-profile in fig. 3 (curve b) for the sputter-cleaned Zr : O,, surface which had been exposed to 300 L of 0, at 1190 K. The maximum relative atomic oxygen concentration established by the automated PHI procedure using differentiated AES peak heights is about 56% for this surface. The maximum relative atomic oxygen concentration is about 61% for the surface oxidized at 1100 K, while those oxidized at lower temperatures exhibited values closer to 64% by this method. The oxidation of Zr: O,, at the higher temperatures investigated appears, therefore, to result in incomplete oxidation. This phenomenon has been observed by Krishnan et al. for pure Zr substrates [4]. This is the subject of ongoing investigations. The influences of the extent of 0, exposure at 900 K and the pressure-time conditions utilized to achieve some of the exposure levels are also shown in

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313

table 2. As expected, the sputter time required to achieve equivalent oxygen and Zr concentrations increases as the extent of 0, exposure increases. At 0, exposures of both 100 and 600 L, the depth of oxygen penetration increases when longer 0, exposure times (lower 0, pressures) were used. The reduced surface access rate of 0, to the oxidizing surface at low 0, pressures is, apparently, more than offset by the additional time available for the oxygen to diffuse through the developing surface oxide layer. At 30 L of 0, exposure these two effects appear to balance out to produce essentially equivalent extents of surface oxidation even with offsetting order-of-magnitude changes in the exposure time and 0, pressure. 3.5. Surface oxide layer stability The stability of the surface oxide layers on Zr: O,, were evaluated by vacuum annealing each of six surfaces which were pre-oxidized at 900 K by 300 L of 0, exposure. Oxidized Zr : O,, should exhibit no significant diffusion of oxygen from the surface oxide layer into the substrate during high temperature annealing. The data in table 3 indicates that the six pre-oxidized surfaces annealed at temperatures between 700 and 1100 K exhibited no significant change in the oxide layer sputter time (thickness) compared to similar oxidized surfaces which were not annealed. The average sputter time for these six annealed surfaces is 10.8 min. This is very close to the average of 11.0 min for the four surfaces listed in table 2 which were oxidized under the same conditions but were not annealed after they were cooled to room temperature immediately after oxidation. The sputter-depth-profile of an oxidized (300 L 0, at 900 K) Zr: OS, surface which was subsequently annealed at 1100 K in UHV for 5 min is shown in fig. 3 (curve c). There is no significant change in the form of the oxygen-Zr concentration versus sputter time curves from those observed on similarly oxidized surfaces which were not annealed, except for an apparent reduction in the oxygen concentration on the outer surface. This oxygen Table 3 Effects of high temperature Anneal temp. (K)

Anneal time (min)

700 900 900 900 1100 1100 a) All surfaces

UHV annealing

5 5 10 60 5 5 were initially

oxidized

on the thickness

of surface

Oxide layer sputter-time

oxide layer on Zr : O,, ‘)

(min)

12.2 10.8 9.8 10.8 10.5 10.6 at 900 K with 300 L of 0, exposure

(10m6 Torr for 300 s).

314

M.C. Deibert et al. / Composition and oxidation characteristics

of Zr: OS,

depleted outer surface layer was removed during the first 30 s sputter cycle. The oxygen concentration remains approximately constant for the next six min of sputtering, indicating that the surface ZrO, layer was about 42 nm thick. The sputter-depth-profiles of the other five high-temperature annealed surfaces were also essentially identical to those of the oxidized surfaces which were not annealed, except for various degrees of reduction of the outer surface. The surface oxide layers on elemental Zr are absorbed at significant rates into the bulk metal at lower temperatures and over shorter times than those indicated in table 3. Krishnan et al. [4] noted a significant loss of surface oxygen from pre-oxidized pure Zr foil at increasing rates at higher temperatures. The surface oxide layers were essentially depleted after about 2 min of UHV treatment at 1008 K; the highest temperature they investigated. Foord et al. [l] similarly observed that thin surface oxide layers on pure Zr dissociate into the underlying bulk at faster rates with increasing temperature, with times on the order of 0.2 min required for an 80% depletion of surface oxygen at 614 K. Since the thicknesses of the surface oxide layers, formed at 900 K during 300 L of 0, exposure in this work, are essentially unaffected by subsequent high temperature annealing in UHV at temperatures of up to 1190 K, the system behavior is that expected of Zr : O,, and is a further demonstration that the substrates formed in this study were oxygen saturated. The reduction of the oxygen concentration on the outer surface of the oxidized Zr : O,, during high temperature treatment in UHV is the subject of ongoing studies. Axelsson et al. [3] noted that atomic hydrogen caused the reduction of ZrO, surfaces at 1020 K and that surface reduction can result from electron beam stimulated desorption of oxygen. The apparent high temperature reduction of the ZrO, surfaces in UHV observed in this work occurred in the absence of either of these influences.

4. Summary and conclusions Samples of oxygen saturated Zr (Zr : O,,) were produced by absorbing the oxygen from a pre-formed surface oxide layer into the bulk of a Zr foil substrate at high temperature (1243 K) in an argon gas environment. The oxygen concentration in this Zr : O,, determined by quantitative Auger specThis oxygen concentration level in Zr : O,, is troscopy is equivalent to ZrO,.,. essentially the same as that determined in separately reported studies using other analytical techniques. This demonstrates the validity of the application of Seah’s quantitative Auger method to the surface analysis of this metal-oxygen system using a stoichiometric surface oxide for calibration. The Auger spectrum of sputter-cleaned Zr : O,, is very similar to that of elemental Zr, showing only a trace indication of Zr oxidation. This indicates

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of Zr: OS,

315

that even though the oxygen concentration in Zr : OS, is 22% of that in ZrO,, the Zr apparently does not strongly interact chemically with the interstitially dissolved oxygen. The rate of the room temperature oxidation of sputter-cleaned Zr: OS, during an initial 0.3 L exposure to 0, appears to be slightly faster than that of sputter-cleaned pure Zr. If the oxidation is conducted at temperatures up to 900 K, the surface oxide layer has a stoichiometry close to that of ZrO,. The surface oxide layer developed during 0, exposure at 900 K does not dissolve into the bulk Zr during high temperature annealing at temperatures up to 1100 K. Surface analytical investigations, such as Auger spectroscopy, can be conducted on ZrO, surface layers formed on Zr : OS, substrates with little interference from surface charging. The electrically conductive Zr : OS, substrate provides for the dissipation of surface charge. The Auger peak energies obseved on these ZrO, surfaces were essentially identical to those previously reported on bulk ZrO, [3]. A shift in these energies would be expected if surface charge interference was present. There is some indication that the oxidation of sputter-cleaned Zr : OS, at 1100 and 1190 K produces a surface oxide with less oxygen content than ZrO,, with the deficiency of oxygen being higher at the higher oxidation temperature. The surface oxygen concentration on ZrO, also appears to become slightly reduced during high temperature annealing in UHV.

Acknowledgments We are grateful to the staff members of CRISS (supported by the NSF, Grant No. DMR-83-09460) for their excellent technical support. Portions of this work were supported by NSF(EPSCoR) Grant No. ISP-8011449, with matching funds from the State of Montana.

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