A study of oxygen interaction with a LaB6(100) single crystal surface

Applications of Surface Science 8 (1981) 197—205 North-Holland Publishing Company

A STUDY OF OXYGEN INTERACTION WITH A LaB6(100) SINGLE CRYSTAL SURFACE

*

Paul R. DAVIS Department ofApplied Physics, Oregon Graduate Center, Beaverton, OR 97006, USA

and Scott A. CHAMBERS Division ofNatural Science, George Fox College, Newberg, OR 97132, USA Received 19 May 1980 Revised manuscript received 22 July 1980

The desorption of boron and lanthanum oxides from a LaB6 (100) single crystal surface has been studied by standard surface analysis techniques. Measurements were made both on preadsorbed oxygen layers in vacuum and under steady-state conditions in fixed oxygen pressures. The observed oxide desorption products were BO, B202, B203 and LaO. Temperature and pressure dependencies of the boron oxide species have been studied in detail. The findings are discussed from the point of view of cathode applications.

An important consideration in thermionic cathode design and use is the effect of system background gases upon the surface properties of the cathode material. In particular the emission characteristics and useful lifetime of the cathode may be highly sensitive to the presence of traces of active gases such as oxygen. We have studied the evaporation of oxide species from a LaB6(lOO) surface using two different approaches. First, thermal desorption studies yielded information on the nature of surface oxide species and their thermal removal characteristics under negligible oxygen partial pressure (P02 < 1 X 1010 Torr) conditions. These experiments allowed highsensitivity mass spectrometric measurements to be made on desorbing species. Second, steady-state desorption and Auger measurements in various fixed pressures of oxygen provided more detailed information on the kinetics of formation and removal of the various oxide species under conditions similar to those of actual cath*

This work was performed for the Jet Propulsion Laboratory, California Institute of Technology, sponsored by the National Aeronautics and Space Administration under contract NAS7100.

0 378-5963/81/0000—0000/s 02.75 © 1981

North-Holland

P.R. Davis, S.A. Chambers / 02 interaction with LaB

198

6(100,) surface

Table 1 Thermal desorption of oxides from LaB6 (100): species monitored Mass (species)

Conditions of detection

Comments

139 (La~)

T ~ 1470 K, clean or oxygenated sample Same as La~

Shows fragmentation peak from LaO when surface is oxygenated

2~) 69.5 (La 326 (La 2O~)

Not observed

294 (La2O~)

Not observed

171 (LaO~)

Not observed

155 (LaO~)

Observed whenever the LaB6 surface has any oxygen present, regardless of adsorption temperature Same as LaO~

2~) 77.5 (LaO 70 (B 20)

Observed after adsorption at T ~ 1000 K only with exposure 10 L. Not observed after room temperature oxygen adsorption

Broad peak, may be second order, but coverage dependence has not been studied in detail

54 (B20)

Observed after adsorption at 300 K or 1000 K at any oxygen exposure

Appears to be the dominant B oxide species desorbed

43 (BO~)

Not observed above background

27 (BO~)

Observed after any oxygen cxposure at 300 K or 1000 K

Single peak observed after 2 L 02 adsorption, multiple peaks after ~ 10 L 02 dose at 1000 K

11 (B~)

T ~ 1630 K, clean or oxygenated sample

When oxygen is present on the surface, lower temperature peaks from fragmentation of boron oxides are observed

ode operation. All the measurements wcre made on a polished, (100) oriented, Larich LaB6 sample (precise stoichiometry IaB5 74+005) under controlled UHV conditions. The sample was prepared and analyzed as described elsewhere Fl I. A summary of the species monitored and observed in the thermal desorption studies is given in table 1. Fig. 1 shows desorption spectra obtained after °2 adsorption of 4 X l0~ Torr sat 1000 K. Only LaO and the high energy BO and B202 peaks were observed after exposures less than 1 X 10~Torr s, while doses greater than 1 X 10~ Torr s at 1000 K caused additional BO and B202 binding states to form, with small quantities of B203 then observable by thermal desorption spec-

P.R. Davis, S.A. Chambers / 02 interaction with LaB

6 (100) surface

199

THERMAL DESORPTION: O2/LaB~74(100) INITIAL 02 DOSE 4xIO~Torr-sec ADSORBED AT 1000K +

a. b: C: d:

B202 LaO~ t BO B 2O3~

A

b

800

1000

1200

400

600

1800

T(K) 75 90 126 50 t(sec) Fig. 1. Thermal desorption of oxide species from a LaB6 (100) surface after oxygen adsorption of 4 x 10~ Torr s at 1000 K. Curve a: B2O2 curve b: LaO; curve C: BO; curved: B2O3. The relative overall sensitivity for each curve is indicated. 55

63

troscopy. Adsorption to saturation at room temperature yielded spectra identical to those observed for 1 X l0~ Torr s exposure at 1000 K. The desorption energies of the observed oxide species were determined by the Redhead technique [2] with the assumption of first-order desorption in all cases. The results of these desorption energy calculations are shown in table 2. The LaO and BO binding energies determined in this study are very close to those measured by Bas et al. [3] even though the samples have different surface geometry and presumably different bulk stoichiometries. Fig. 1 clearly shows that significant desorption of boron oxides occurs at temperatures well below the onset of La or LaO desorption. The onset of B202 desorption is T < 1100 K. For the sample studied here, the evaporation energy of elemental La is 4.3 ±0.1 eV, while that for elemental B is 7.2 ±0.2 eV. The oxide evaporation studies in steady 02 pressures utilized both mass spectrometric analysis and Auger electron spectroscopy. Thus, it was possible to charac-

200

P.R. Davis, S.A. Chambers/0

2 interaction with LaB6(100) surface

Table 2 Thermal desorption of oxides from LaB6: Binding energies Binding energy (eV) a) Species

LaO BO

-

LaB6 (100) b,c) (this work)

LaB6 (100) d) (ref. [6J)

LaB6 (110) e) (ref. 131)

4.3±0.20

5.5

4.2

3.6±o.lg) 4.3

—

±0.1

B202

3.3 ±o.i h) 3.5 ±0.1

B203

3.4

±0.1

-~

-

3.4 3.9 4.1 —

—

—

a) b) c) d)

Bindmg energies of oxide species determined by Redhead method. After 02 adsorption at 1000 K. Nominal composition. Precise bulk stoichiometry LaB574 ±~ Apparently measured in continuous 02 background pressure — clean sample claimed. LaO binding energy determined from Arrhenius plot. e) After 02 adsorption at 300 K. O Peak broadens toward low temperature at highest 0~doses. g) Low energy peak not present for 02 dose < 1 X 111 Torr s. h) Exposures > 20 L cause a lower energy state to form, After 200 L at 1000 K, for example, desorption from this third state is observed forT> 1000 K.

terize desorbing species and to study the nature of the sample surface itself. The combination of these two techniques yielded more complete information on the oxidation process than either technique alone could provide. Some limitations on the mass spectrometric measurements were imposed by the presence of a high 02 pressure. Several possible evaporating species could not be monitored because of interference from identical or nearby species present in the background gases. For example, 0 (mass 16) and 02 (mass32) couldnot be detected as desorbing from the sample, because the background level of these species was orders of magnitude larger than the expected desorbing yield. Similarly, the BO (mass 27) peak could not be separated from overlap of the much larger background CO (mass 28) peak (about 3% of the total oxygen pressure) because of limited mass resolution. On the other hand, steady-state yields of desorbing species with no background interference were high enough to allow techniques such as isotopic fingerprinting to be applied. For example, it was possible to identify B2O2 unequi10) vocally by usingthethecalculated known boron isotopic ratios distribution (~-‘80%BU, “20% B and comparing and measured of B~0O 1OBUO 2(mass 52), B experi2 102 (mass 54). Agreement within 3% was obtained in this (mass 53) and B~ ment.

P.R. Davis, S.A. Chambers/O

2 interaction with LaB6(100) surface

0-

I

I

I

201

I

DESORPTION RATE vs 02 PRESSURE ~—

~

LaB6 (100) T=1600K

z

0.0

0.2

0.4

0.6

02 PRESSURE

0.8

i.0xi06

(torr)

Fig. 2. Relative yields of B2O2 and B2O3 from LaB6 (100) as functions of oxygen pressure at 1600 K. Solid line: B2 02, dashed line: B2O3.

Fig. 2 shows the relative yields of B202 and B203 at 1600 K as functions of the oxygen pressure, while fig. 3 gives the relative yields of B202, B203 and LaO as functions of sample temperature for fixed oxygen pressure, with corresponding curves for elemental La and B from the clean surface included for comparison. These yields have been determined by correcting for the mass spectrometer transmission and electron multiplier response variation with mass, using rare gases for calibration. We assume that the ionization probabilities of B202 and B203 are the same as B and that for LaO is the same as La. This assumption is not strictly correct, since the ionization probability has been found to increase somewhat with increasing molecular weight. Nevertheless, it is clear that the major species desorbing in the range 1000 < T < 1600 K is B2 02. Thus, B202 desorption may be a dominant factor in limiting useful cathode life in this temperature range under conditions of background oxygen pressure. Based upon fig. 3, taking into account the number of boron atoms per desorbing molecule and the sample stoichiometry B/La 6, we conclude that, in the presence of oxygen at 1000 ~ T ~ 1500 K, one would expect boron depletion of the LaB6 (100) surface. Corresponding formation of a lanthanum oxide layer would be expected, and such behavior has been observed or inferred by several investigators [4—7].

I

I

10

I

I

LoB

.

6(l0O)6tort P02~’IxlO

—

I—

~

/

z

g ‘I

~

I

D8

crj

/

B 202,-’ •

i

II II/~ I ,, lit

/

/ / -J Lu

I\

I

.

I I

—

>W4

I’-’

LaOjII~\

/S

I—

I

Il I

-

-J Lu

~

II

2

I

I

I.

L/La

/8203

O

1000

1200

.-

~

~ 1400

I

1600

TEMPERATURE (K) Fig. 3. Relative desorption yields of B202, B~O3and LaO from LaB6 (100) as functions of tenipcraturc for an oxygen pressure of 1 X 10 Torr. The B and La yields from the clean surface are included for comparison. Relative yields calculated as discussed in the text. I

I

I

I

LoB6 (100) ~I0.

B202 EVAPORATION

z /

/

/

“ ‘

‘Il

tji (\ II~/’I I ~ ‘Is III’ II

A’ -

I

/ /

I I I

Lu >-

k

.

I

Lii

I— -J Lii

~

2,.,,-

0

-

1000

~ 200

1400

1600

TEMPERATURE (K) Fig. 4. Behavior of B2O2 desorption as a function of temperature for different oxygen pressures. A: 5 X 10_6 Tort; B: 2 x 10_6 Tort; C: 1 X 10~6Torr; 0: 5 X 10~ Torr; F: 1 X 10~ Torr.

P.R. Davis, S.A. C’hambers/0

2 interaction with LaB6(100~surface

203

Since a LaB(1 10) crystal [3] exhibits BO and LaO desorption behavior similar to what we have observed here for the (100) plane, one might expect its B2O2 desorption to be similar as well. Thus, it may be generally true that extended heating in oxygen with 1000 ~ T ~ 1500 K will result in boron depletion of LaB6 cathode surfaces. The structure indicated in fig. 3 in the temperature range 1400< T< 1600 K is echoed more strongly in fig. 4, where the temperature dependence of B202 removal is plotted for various oxygen pressures. The sharply peaked structure is present for all pressures above 1 X l0~ Torr, and the peak positions shift slightly to higher temperature with increasing pressure. The cause of the highly reproducible structure is not known, but the extreme sharpness of the structure suggests some sort of resonance in the reaction producing B202. In the same temperature range, the Auger data (fig. 5) show a marked increase in oxygen signal, a corresponding decrease in boron signal and a slight increase in lanthanum signal. There is apparently a significant change in the surface layer in this temperature range. These Auger results are similar to those of other investigators [3,4] who studied the surface concentrations of La, B and 0 on LaB6 (110) and (100) by Auger during 02 exposure at various temperatures. These authors also noted that extended heating in 02 produced dramatic changes in gross surface structure of LaB6 crystals, by facetting and surface rearrangement. 50

i

I

I

I

I

I

I

I

AUGER SIGNAL vs TEMPERATURE ~

La B6 C 100) P02 2 x 10-6 torr

40

-

OXYGEN

a-

30

-

-

BORON ~

20-

Lii

a10 ~

o 600

-

I

I

800

I

I

1000

I

I

1200

I

I

I

1400

1600

TEMPERATURE (K) Fig. 5. Variation with temperature of oxygen, boron and lanthanum Auger signals in an oxygen pressure of 2 X 10_6 Torr.

204

P.R. Davis, S.A. Chambers/0

2 interaction with LaB6(l00) surface

Taken together, the Auger data (fig. 5) and desorption data (fig. 3) suggest that, at temperatures 1650 K and P02 ~ I X 10_6 Torr, oxygen interaction with the LaB6(100) surface decreases to a negligible level. For example, the equilibrium concentration of oxygen on the surface approaches zero as the temperature approaches 1650 K as do the desorption rates of the various oxides. Under these conditions, the dominating desorption product is La, as would be observed for the case of zero oxygen pressure. In contrast, as the pressure increases above I X 10_6 Torr we observe a monotonic increase in boron oxide desorption rates at T~ 1650 K, in good agreement with the increase in oxygen surface coverage measured by Auger spectroscopy [4]. It is expected that the yield of boron oxides is relatively insensitive to bulk sample stoichiometry, since it has been shown that B activity varies only slightly with sample composition [8]. Operation of LaB6 (100) cathodes in oxygen at pressures greater than I X 10—6 Torr results in rapid surface degradation and mass loss by boron oxide or simnultaneous boron oxide/lanthanum oxide removal. On the other hand, cathode operation in oxygen pressures ~ I X 10—6 Torr holds more promise. From the data of fig. 3 (P02 = 1 X 10—6 Tort) we conclude that at temperatures 1000 ~ T ~ 1500 K mass loss occurs non-stoichiometrically, mostly as B202. At T 1525 K, stoichiometric mass loss occurs, with total evaporated B/La 6. Above 1525 K, the ratio of evaporated B to La decreases below 6 and La depletion of the sample begins. In our particular case, however, the exceptionally low value of La vaporization energy (4.3 eV) is due to excess La in the sample, so that the sample approaches the nominal LaB6 stoichiometry as La depletion occurs. In studies of polycrystalline LaB6, Storms and Mueller [8] have demonstrated the effect of stoichiometry upon vaporization rates of B and La. Because of large changes in La activity with composition in the region around B/La = 6.0, we would expect that, for a cathode with less excess La than our sample, the La vaporization energy would be higher, shifting the La vaporization curves toward higher temperature. Thus, the precise temperature yielding stoichiometric evaporation in a fixed oxygen atmosphere will depend on bulk cathode stoichiometry. Acknowledgement The authors wish to thank Dr. L.W. Swanson for many helpful discussions. S.A. Chambers was supported by an M.J. Murdock Charitable Trust Grant of Research Corporation. The sample used in this study was supplied by Dr. J. Verhoeven, Ames Laboratory, DOE. References [1] MA. Noack,M.Sc. Thesis, Iowa State University, Ames, Iowa (1979).

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2 interaction with LaB6(100) surface

(21

205

PA. Redhead, Vacuum 12 (1962) 1203. [3] E.B. Bas, P. Hafner and S. Klauser, in: Proc. 7th Intern. Vacuum Congr. and 3rd Intern. Conf. Solid Surfaces (Vienna, 1972) p. 881. [41 S.J. Klauser and E.B. Bas, App!. Surface Sc!. 3 (1979) 356. [5] C. Oshima and S. Kawai, App!. Phys. Letters 23 (1973) 215. [61 B. Goldstein and D.J. Szostak, Surface Sci. 73 (1978) 461. [7] M. Futamoto, S. Hosoki and U. Kawabe, in: Abstractsof26thFieldEmissionSymp.(Berlin, 1979) p. 79. [8] E. Storms and B. Mueller, J. Phys. Chem. 82 (1978) 51.