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Carbon monoxide and carbon dioxide interaction with tantalum
Surface Science 0 North-Holland
72 (1978) 390-404 Publishing Company
CARBON MONOXIDE AND CARBON DIOXIDE INTERACTION
WlTH
TANTALUM
V.D. BELOV, Yu.K. USTINOV and A.P. KOMAR USSR Acaderr~y of Sciences.Lenirzgrad, USSR
Ioffe Ph_vsical Technical Imtitute,
Received
21 November
1975; manuscript
received
in final form
18 July 1977
The adsorption of carbon monoxide and carbon dioxide on tantalum and the dissolution of these gases in the adsorbent at T ;3 300 K have been studied. The flash-filament method (FFM) in a monopole mass-spectrometer and a field em&ion microscopy was used in the same apparatus. Carbon monoxide and carbon dioxide dissociate on the tantalum surface. carbon monoxide being desorbcd in both cases during the flash. The desorption curves of CO reveal three different binding states: two of them (a. and p’l) for the adsorbed pxticles whereas the high temperature desorption state relates to the adsorbate dissolved in the metal. For the P’~ state of CO the activation energy, the pre-exponential factor and the kinetic order in the kinetic equation of dcsorption have been estimated. They turned out to be E = 110 kcal/mol, C = 3 X IO’* WC-‘, and v = 1. The activation cnerpy of diffusion for CO in tantalum and the energy of outgassing for the metal were found to be 9.4 and 49 kcal/mole, respectively.
1. Introduction
Flash filament studies of the carbon monoxide-tantalum interaction [I ,2] revealed the dissolution and diffusion of the adsorbate in the metal. The dissolution seems to begin after dissociation of CO molecules on the tantalum surface, but the discussion of partial (C and 0) solubility in tantalum can not be based on the tlash filament results. In the flash filament experiments (FFM) CO molecules are always detected. The dissociation of CO on tantalum was observed by Klein and Leder in a field emission microscope (FEM) [3]. However, the authors did not discuss the possibility of dissolution and their results are difficult to compare with those mentioned above [ 1,2] because the experimental conditions of the FEM and FEM measurements were quite different. The sticking coefficient. the saturation density and the solution rate of CO on (100) Ta were obtained using Auger electron spectroscopy by Ko and Schmidt [4]. The authors concluded that the carbon monoxide was adsorbed dissociately without solution of either species at 300 K, but at 500 K carbon dissolved leaving oxygen on the surface. The kinetics of dissolution implies that the states of the subsurface region are populated. In this case one has to make an assumption as to the 390
V.D. Belov et al. / CO and CO2 interaction with Ta
391
sensitivity of AES for various depths. The results obtained here will be qualitative at best. Any advance in the mechanism of adsorbate dissolution in a substrate is badly needed. Urgent scientific and practical requirements made us undertake the investigation of CO-Ta and CO,-Ta adlayers by combined FFM and FEM in the same apparatus. We have made an attempt to draw the results of both methods and to associate the kinetics of dissolution and outgassing with adlayer properties.
2. Experimental The experimental vacuum apparatus is shown in fig. 1. The total pressure of residual gases was “1 X 10m9 Torr. A monopole mass-spectrometer with partial sensitivity of 10-l’ Torr was used for partial pressure measurements. The gases CO and COa were admitted to the experimental chamber from the bakeable gas inlet system. The total pumping rate of CO from the chamber (V= 51) was about 33 l/set (for 10m8
Fig. 1. Experimental apparatus: (1) monopole mass-spectrometer, (2) “black chamber” with adsorbents and ion source, (3) FEM, (4) Bayard-Alpert gauge, (5) Ti-getter pump, (6) liquid nitrogen trap, (7) gas inlet system.
392
V.D. Belov et al. / CO ad
The adsorption occurs at a constant titative analysis of the gas desorption balance equations were used [.5]
CO2 irlteraction
with Ta
pressure of the gas under study. For a quandata (b&t) and r(t)) kinetic and mass-
Here N(t) is the number of CO molecules adsorbed: I_‘, V, and To are the pressure, volume, and temperature of the gas in the apparatus; A is the adsorbent surface area; E and C are the activation energy and the pre-exponential factor of desorption; T(t) is the temperature of the adsorbent during the flash; k is Boltzmann’s constant; and u is the kinetic order. At high rates of heating in the FF experiment, one can find the total pumping rate S by the characteristic time of the desorption curve tail. The rate of desorption on the tail can be neglected. The activation energy, the pre-exponential factor and the order of desorption can be estimated by substituting the experimental values -dN/dt.
N(t) = N,,, -
w st%,~d!_ d* AkT,
0
s t
&dt-2
> 0
and T(t) in the kinetic equation; N
s
max
=AkTo os
=
Apdt
is the number of CO molecules adsorbed during the exposure time ta. The visual observation and measurements in FEM were performed at the tip temperature T1 = 300 K. The photo-shots were made at the same total emission of about lop7 A.
3. FEM results After various exposures of Ta ribbon in CO at po(CO) = 2 X lo-’ Torr and T, = 300 K, the desorption of molecular CO in the flash revealed one weakly bound (a) and two strongly bound @; and 0;) states of adsorbed CO (fig. 2). The exponential decline of the experimental desorption curves lagged behind the decline due to pumping. The pumping rate has been estimated from the characteristic time of the tails of CO desorption curves from W and Ir ribbons in the same apparatus. The decay time of the desorption curves in fig. 2 seems to be caused by the evolution of CO from tantalum at T, = const (T2 = maximum flash temperature). The observation on Ta at elevated adsorption temperatures T, makes it clear that this adsorp-
V.D. Belov et al. / CO and CO2 interaction with Ta
Fig. 2. Thermal desorption K and po(C0) = 2 X 20-* desorption of chemisorbed talum.
393
spectra of CO from tantalum for various exposures (tJ at TI = 300 Torr. (1) ta = 60 set; (2) 1, = 120 set; (3) ta = 600 sec. Curve 1’ carbon monoxide; curve 1” desorption of CO dissolved into tan-
tion is accompanied by a dissolution of the adsorbate in the substrate. This leads to a large mean life time of adparticles during the flash desorption (TV= N(dN/dt)-‘). The graphs of Td versus the number of the sorbed molecules N,,(CO) are shown in fig. 3a. Each graph was obtained at T2 = const., TI being a parameter (300, 900, 1250 K). The graphs for T1 = 900, 1250 K consist of two linear parts. It is the dissolution of adsorbate in the bulk of Ta during adsorption that influences the desorption at N,,, > 10” mol/cm*. The dissolution of adsorbate just in the subsurface region takes place at N,,, < 10” mol/cm*, and the rate of the adsorbate surfacing determines the rate of the whole process of desorption during the flash. Varying T, (T, and N being fixed) we can derive the dependence 7d = 7. exp(Eo/kT2) from which E. and r. can be estimated (fig. 3b). They were found to be 49 kcal/mol and 1.6 X 1O-’ set, respectively. How the rate of adsorption can correlate with the rate of dissolution in bulk is clearly seen from fig. 4. Here the total quantities of CO adsorbed during the time t, are plotted. The fact that these quantities for Ta and W at T1 = 300 K are equal and the corresponding desorption curves have the same form ((Y,fll, f12 states of desorption), makes one think of the same primary form of gas adsorption on these samples and implies that the role of dissolution in Ta at T1 = 300 K is negligible. However, a part of the adlayer on Ta dissolves in the subsurface during the flash. This causes the time lag of the experimental desorption curves in fig. 2. This fact suggests a high rate of dissolution of the adsorbate in the subsurface region. At Tl> 900 K the dissolution of CO proceeds during adsorption, and diffusion in the bulk limits the complete transport of CO from the gas to the bulk of tanta-
394
V.D. Belov et al. / CO and CO2 interaction with Ta
a
Fig. 3. (a) Characteristic time (Td) of decline of the desorption curves as a dunction of CO amount adsorbed on Ta at various temperatures T1. (b) Desorption characteristic time 7d versus temperature for CO and Ta.
lum. In fact the characteristic time of diffusion (2 14 min as is seen from fig. 4) is much larger than the time of adsorption (about 3 min at pO(CO) = 2 X lo-’ Torr) and that of dissolution in the subsurface. So, for large times t, the diffusion proceeds in a stationary manner, with a constant adsorbate concentration on the surface less than that of the monolayer. At T, = 1250 K the diffusion flux in the bulk of tantalum equals the gaseous flux of adsorption, the surface coverage being half a monolayer density. This follows from the value of the sticking probability (the initial slope of adsorption curve in fig. 4) because the sticking probability of CO and Ta is constant (and maximum) at 0 < 19G 0.5 [ 11. The absorbed quantity does not increase linearly with I,, and the saturation of
V.D. B&v
et al. /
395
CO and CO2 interaction with Ta
rb
li
Iq
t,(minl
Fig. 4. Adsorption curves of CO on Ta at pg(C0) = 1 X 10e8 Torr. (1) 7’1= 300 K, (2) 850 K, (3) 1260 K, (4) 1630 K. Dotted line: CO on tungsten at 7’1= 300 K, po(C0) = 1 X lo-’ Torr.
the curves requires the longer time at lower T1. 1500 K when the effective value of the diffusion desorption process. To estimate DT~ the amount T1 < 1500 K was taken into account using N,,, being suitable in the first approximation (fig. 5). can be obtained by determining the dependence
Fig. 5. Amount parameter.
of adsorbate
Fig. 6. The dependence
dissolved
of diffusion
The saturation occurs also at T1 > coefficient DT is small due to the of absorption in the range 300 < = B(DT, t,)“2 (B = const), this The activation energy of diffusion of ln(B2DT,) on l/T1 (fig. 6). It
(N”) versus time, adsorption
coefficient
Drl
on the temperature
temperature
T1.
T1 being the
396
V.D. Belov et al. / CO and Cc)2 interaction with Ta
comes out to be 9.5 kcal/mol, which is quite different from the activation energy of the outgassing process (Eu). The desorption of CO from the Ta surface starts at T* 1500 K. Fig. 2 shows that all the desorption curves originate at the same ter~lperature ir~dependently of the quantity of the adsorbate. The heating rate is the same for all the curves in ftg. 2. The initial steepness of the rising parts of the curves increases with the total number of the adparticles, the time (temperature) at the inflection point in the rising part of the curves r~rnaining constant. However, the position of the maximunl on the curves changes (it shifts to higher temperatures) when the sorbed quantity exceeds that of the monolayer (Tr > 900 K). So the course of the initial portion of the desorption curves in fig. 2 leads us to suggest that desorption of CO from Ta begins as a process with first order kinetics [5]. At high heating rates during the flash the mean life time of adparticles in this process (7,) equals the rise time of the desorption curves (T, = t’ - t in fig. 2). This rise time is much smaller than the decay time of diffusion desorption rd. It means that the processes at the rising and falling parts of the desorption curves are quite different. Desorption of chemisorbed layer and outgassing of the bulk are likely to proceed in succession. This naturally results from the different rates of these two successive processes. We tried to isolate the desorption of the clle~llisorbed layer from the experimental desorption curves in fig. 2 by plotting the pumping exponent just after the inflection point in the rise of the curve. The greater the desorption rate in the rise of the curve the less is the error of such an operation. The isolation is done for curve 1 in fig. 2, where the area under the curve I’ is proportional to the number of chemisorbed molecules (Nk,,). Th e area under curve 1” is proportional to the number of CO molecules desorbed out of the adsorbent volume (A&,). Having plotted In [(-l/N) dnr/dt] = F(l/T), we obtained a straight line corresponding to the first order kinetics, from which the activation energy E, = 110 kc~~rnol and pre-exponential term C = 3 X 10 l2 set-’ were calculated. It can be predicted that dissolution of CO in the tantalum during the flash would end before the inception of the desorption (at T < 1500 K) because the stationary concentration of adsorbate (@G 0.5), which provides the nlaxilllurn dissolution rate, has set in. However, one can easily see that the rate of the adlayer depletion (surface cleaning) can become a sum of the desorption rate and the rate of dissolution when the adsorbate quantity is great and the heating rate during the flash is very high. In this case the measurement of the desorption rate (and estimation of its rate constant) will be difficult. But here we can use additional information: the low temperature dependence of the dissolution rate can be measured. The activation energy of dissolution turned out to be very small as compared with that of desorption. So the rate of surface cleaning against temperature, with desorption and dissolution taking place simultaneously, will be governed only by the desorption rate constant. The dissolution will affect the pre-exponent of the surface cleaning rate constant. Thus the activation energy of desorption can be evaluated even for the process of desorption combined with dissolution.
V.D. Belov et al. / CO and CO2 interaction with Ta
397
Consider now the linear dependence of rd (Nr,&. This dependence (or, it might be better to say, the independence of the CO desorption rate from its total adsorbed quantity) may correspond to the desorption of the adsorbate from the ordered structure in the bulk of Ta. The remarkable properties of the subsurface region and the constant concentration of the solute in the bulk of Ta may originate from the stepwise dependence of the adsorbate diffusion coefficient on the distance from the surface. In this case the stationary distribution of the diffusing substance in the bulk will consist of two regions: (1) a subsurface with the concentration smoothly changing from that on the surface to that in the bulk and (2) the bulk of the substrate with maximum solubility concentration.
4. FEM results The FE lo-’ Torr whole area adsorption.
pattern of the Ta tip saturated with CO at Tr = 300 K and pu(C0) = 2 X is shown in fig. 7. During adsorption the changes are observed over the of the emitter. So the whole surface of the tip is subjected to the CO If then the temperature of the tip increases, the emission undergoes a
Fig. 7. Field emission patterns of Ta surface saturated with CO at T, = 300 K and various temperatures of the tip: (a) 300 K, (b) 1000 K, (c) 1250 K, (d) 1500 K, (e) 1800 K, (f) 2100 K.
398
V.D. Below, ct al. / CO artd CO2 interactiotl
with Ta
number of changes. The patterns observed during heating do not look like those obtained during adsorption at Tt = 300 K. This fact and the structure of the pattern in fig. 7a imply that the adlayer of CO on Ta at Tt = 300 K is amorphous. The same has been shown after FE observation of CO on W [6] while the reversibility of adsorption-desorption patterns in the case of H2 on W and Ta at Tt > 300 K ]6] is evidence in favour of migratory equilibrium in the adlayer-. Heating
Fig. 8. Adsorption
of CO on Ta at po(CO)
= 2 X lO_’
Torr
and various
exposures
t,.
T1 =
1000 K, (a) ra = 60 set, (b) ta = 180 set, (c) fa = 600 set; T1 = 1100 K: (d) t, = 60 SW, (e) t, = 180 set, (f) t, = 600 SW; T, = 1250 K: (g) t, = 60 sec. (h) t, = 180 sec. (i) 1, = 600 set
V.D. Belov et al. / CO and CO2 interaction with Ta
399
the tip as high as 1000 K results in bright spots spreading over the surface of the emitter, mostly around the (110) and (111) planes. Also, dark spots of irregular form are localized in the light area surrounding (100). As the tip temperature is raised to 1250 K, the dark and bright spots disappear (fig. 7) and large islands with great specific emission emerge in the picture. As a rule they are localized in the regions with orientation (301) and are separated from the bright areas by the zones with lower emission. In addition the picture shows well developed regions of (2 11) planes which cannot be seen either in the emission pattern of the clean Ta surface, or during adsorption of CO on the tip at T < 900 K. Irreversible changes in the field emission pattern observed during the heating of an adsorbent are caused by dissolution and diffusion of adsorbate in the bulk of Ta, desorption of CO to the gas phase being observed only at T > 1500 K. It is possible to observe the dependence of the FE image of the cluster’s phase on the coverage, the migration rate of the adsorbate and the time of the diffusion process. Indeed, the difference in the pictures (figs. 7 and 8) is probably caused by the fact that at T, = 300 K the dissolution of adsorbate in the substrate begins without a preliminary attainment of equilibrium in the adlayer by surface migration. However, at T, = 1000 K the equilibrium in the adsorbate is set in before the extensive diffusion
Fig. 9. Field emission patterns of Ta surface saturated with oxygen at T1 = 300 K (a) after heating the tip to T = 900 K (b), 1100 K Cc), 1650 K (d), 1250 K (e), 220 K (f).
The structure of the CO monolayer on Ta at high temperatures (T> 1000 K) coincides in many details with that observed for the 0, Ta system (fig. 9) (the (12 I) planes manifestation). From this fact the conclusion can be drawn that the adsorbed CO molecules dissociate on the Ta surface. The comparison of emission patterns of Ta covered with C [3] and those with adsorbed CO makes us think ot concentrating carbon atoms in the vicinity of the (100) plane of the Ta surface during the initial stage of CO adsorption at 1000 < T1 < 1500 K (fig. 8). We are in agreement with Klein [3] that the bright clusters are composed of carbon atoms and the dark ones of oxygen. It is important that dark spots never appear in the vicinity of the (I 11) plane and that bright ones never appear around the (100). The greater the amount of the adsorbed CO on the surface (the less amount dissolved in the bulk). the more clusters can be observed, and the smaller they are. The less the amount of the adsorbate on the surface (the more it dissolves). the less the number of clusters and the bigger they are. We are dealing with the same situation when considering the condensation of a supersaturated solution. The higher the rate of supersaturation, the smaller the radius of the critical nucleus of crystallization and their number because the energy of nucleus formation becomes less. Our data lead is to the conclusion that the clusters are three-dimcnsional nuclei of crystallization of a supersaturated solution in the subsurface region. They appear to be stoichiomctrical compounds of TaO and TaC types. The critical size of a cluster depends on the ratio of the diffusion flux to that of adsorption. At elevated substrate temperatures the flux of the adsorbate into the bulk of Ta increases. So supersaturation in the subsurface diminishes and the critical size ol nucleus of crystallization (cluster) increases. When the fluxes are equal to each other (curve 3 in fig. 4), supersaturation of the subsurface region vanishes and the clusters disappear. Being the surplus of the adsorbate in the solution, the cluster is in equilibrium with the solvable part of the adsorbate on the surface (0 > 0.5). As to the semiconductor cluster surrounded by a metal, the emission current through it can be very large if the electrochemical potential of the electron in the semiconductor is higher than the Fermi level of the metal (C clusters, VT,& < L+). If. on the other hand. the electron electrochemical potential of the semiconductor iS lower thl the Fermi level of the substrate (0 clusters, pTaO > PTa), the emission from the cluster will be less than from the surrounding region. This can be concluded from consideration of the energy diamgrams for metal--semiconductor contacts [8]. The contrast in the field emission will be more pronounced when the TaC region is in contact with TaO and vice versa.
5. C02-Ta
The experiments were carried out at various temperatures of Ta adsorbents and CO, pressure equal to ? X 10-s Torr. Flash desorption experiments indicate that CO2 dissociates on the Ta surface: desorption of CO (T,*,, z 2000 K) is followed by
V.D. Belov et al. / CO and CO2 interaction with Ta
401
that of TaO (T+, Z 2400 K). The kinetics characteristics of CO and TaO desorption from the CO2 adlayer are in agreement with those obtained separately from the study of CO and O2 adsorption (except for the values of the concentrations). The character of CO2 adsorption at elevated temperatures displays dissociation of the adsorbate and dissolution of the C and 0 adatoms in the substrate. At 300 K the concentration of adsorbed CO* molecules does not exceed that of Ta atoms on the surface. It is apparent that in this CO, molecules are adsorbed on Ta surface and diffusion into the sample may be neglected. At 300 K < T< Tdes adsorption of CO* is accompanied by diffusion of C and 0 atoms into the bulk of tantalum. The number of CO and TaO molecules desorbed by the flash increases. At T = 1500 K the diffusion rate reaches a maximum. The rise of the adsorbent temperature (T = 1650, 1800 K) results in a decrease in the amount of the substance dissolved inTa, which is due to the growth of the desorption flux. As a result, the effective flux of adsorption as well as that of diffusion of the adsorbate into the sample decreases. In general, the picture of CO, adsorption on Ta resembles in many details that of CO adsorption on Ta. However, the dissociation of CO* on the Ta surface leads to the formation of a mixed (C + 0) coverage with a
Fig. 10. Field emission patterns of Ta surface saturated with CO2 at T = 300 K (a) after heating the tip to T = 900 K (b), 1100 K (c), 1250 K (d), 1650 K (e), 2000 K (0.
402
V.D. Below et al. /CO arui CO2 interaction
with Ta
I:ig. 11. Adsorption of’ CO2 on the heated Ta surface (pressure of CO,? is 2 X 10e8 Torr). T = 1100 K, (a) t, = 180 sec. (b) ta = 600 set; T = 1050 K, (c) t, = 180 sec. (d) t, = 600 set; T = 1250 K, (e) t, = 180 set, (f, t, = 600 sec. relatively large contribution from the 0 atoms. This fact must apparently affect the way the adsorbate diffuses into tantalum as well. The FE patterns of a Ta tip covered with CO, differ in some details from those obtained in the course of CO adsorption. Fig. 10a shows the CO saturated surface of a Ta tip at T = 300 K, p(CO*) = 2 X IO-’ Torr and for an exposure to the gas of ja = 15 min. Heating the tip with the coverage in vacuum gives rise to changes in the pattern, which are caused by the migration of the adsorbate over the surface and dissolution in the substrate (fig. lob). When the tip temperature rises to 1100 K, dark and bright spots that were observed in the CO-Ta system at the same temperature of the surface emerge in the picture. The concentration of bright spots as compared with that of dark ones in the cases of CO* on Ta is, however, much less than in the case of CO-Ta (fig. 10~). Further increase of temperature (T’ 1250 K and higher) produces the complete disappearance of the clusters. The patterns and emission characteristics are similar in appearance to those of the Ta tip covered with CO ar.d O2 (cf. fig. 10 with figs. 7 and 9). Within the temperature range T = 1800~-2000 K. desorption of a fraction of the adsorbate occurs by CO molecules and the pattern turns out to be typical for a Ta surface with oxygen coverage. The
V.D. Belov et al. / CO and CO2 interaction with Ta
403
complete cleaning of the surface is attained at T = 2400 K. The FE patterns of the Ta tip during adsorption of CO, at T = 1000, 1050 and 1250 K are given in fig. 11. It is easily seen these pictures differ from those obserbed in the CO-Ta system. At T = 1000 K, in the picture of the surface with CO* coverage (figs. 1 la and 11 b), the dark spots emerge along the lines between the planes with orientation of (310). Only at ra = 30-40 min do small rare bright spots appear in the pattern. A slight increase of the tip temperature (T = 1050 K) results in a change of the pattern, first of all in the regions surrounding (111). On the screen there emerge brightly emitting large clusters (“islands”) about 1000 A in size. As was observed in the case of CO on Ta the clusters are formed and remain on the (310) plane and are separated by the area with lower emission from the bright regions surrounding the (100) plane. However, the probability of their formation in the CO,-Ta system is less (the pattern in figs. 1 lc and 1 Id has only one cluster emerged after 10 min adsorption of CO,). At T> 1100 K (figs. 1 le and 1 If) the cluster structure does not appear and the emission pattern in the presence of CO2 on Ta resembles the pattern of oxygen coverage. 6. Conclusion To sum up: CO and COz adsorption on tantalum is accompanied by the dissociation of adsorbates and the dissolution of C and 0 atoms in the substrate at T > 300 K. The comparison of CO and O2 dissolution rates enables us to conclude that the limiting process in CO dissolution is the carbon diffusion in the bulk of Ta. The activation energy for this process turned out to be 9.5 kcal/mol. The rate of outgassing in the case of CO adsorption on Ta is limited by the rate of surfacing of the dissolved C atoms. The activation energy for this process (49 kcal/mol) is in agreement with that for C cluster formation obtained by Klein [3] (53 kcal/mol). Our data point out that at least three consecutive stages must be discerned during the adsorbate dissolution and cleaning the substrate. These are the mass transport from the gas phase to the surface (and in the opposite direction), from the surface to the subsurface region (and backward), from the subsurface to the bulk of the substrate (and backward). The kinetics of mass transport from the gas phase (and backward) at various experimental conditions (p, T) must be studied in order to get the rates of each of the consecutive stages. In this work the CO and CO, adsorption on tantalum, the diffusion of C and 0 atoms in the bulk of the substrate, the surfacing of the solute (C atoms) and the desorption of CO from the surface of tantalum to the gas phase have been studied. The insolubility of CO from the tight binding state of adsorption (0 < 8 < 0.5) is found, the outgassing of tantalum following the desorption from this state. At T> 1000 K, CO and COz adsorption increases the contrast in the field-emission pattern of the Ta surface (the resolution) thus revealing the crystallography of the substrate. This effect is analogous to that of etching, and seems to be connected with the dissolution of the adsorbate in the substrate.
404
V.D. Below et al. / CO and CO2 interaction
with Ta
References [l] E.P. Gasscr and R. Thwaites, Trans. Faraday Sot. 61 (1965) 513. [2] Yu.K. Ustinov. J. Tech. Phys. (USSR), 61 (1971) 411;61 (1971) 643. [ 31 R. Klein and L. Leder, J. Chem. Phys. 38 (1963) 1863, 1866. [4] S.M. Ko and L.D. Schmidt, Surface Sci. 47 (1975) 557. (51 G. Ehrlich, in: Advances in Catalysis, Vol. 14 (Academic Press, New York. 1963-1964) pp. 255-427; V.N. Ageev, N.I. lonov and Yu.K. Ustinov, J. Techn. Phys. (USSR) 24 (1964) 546. [6] Yu.K. Ustinov, Fiz. Tverd. Tela 12 (1971) 558. [7] V.D. Below, Yu.K. Ustinov and A.P. Komar, J. Techn. Phys. 46 (1976) 2403. [ 8] 1.1. Petrovskij, Electronic Theory of Semiconductors (Minsk, 1973).
72 (1978) 390-404 Publishing Company
CARBON MONOXIDE AND CARBON DIOXIDE INTERACTION
WlTH
TANTALUM
V.D. BELOV, Yu.K. USTINOV and A.P. KOMAR USSR Acaderr~y of Sciences.Lenirzgrad, USSR
Ioffe Ph_vsical Technical Imtitute,
Received
21 November
1975; manuscript
received
in final form
18 July 1977
The adsorption of carbon monoxide and carbon dioxide on tantalum and the dissolution of these gases in the adsorbent at T ;3 300 K have been studied. The flash-filament method (FFM) in a monopole mass-spectrometer and a field em&ion microscopy was used in the same apparatus. Carbon monoxide and carbon dioxide dissociate on the tantalum surface. carbon monoxide being desorbcd in both cases during the flash. The desorption curves of CO reveal three different binding states: two of them (a. and p’l) for the adsorbed pxticles whereas the high temperature desorption state relates to the adsorbate dissolved in the metal. For the P’~ state of CO the activation energy, the pre-exponential factor and the kinetic order in the kinetic equation of dcsorption have been estimated. They turned out to be E = 110 kcal/mol, C = 3 X IO’* WC-‘, and v = 1. The activation cnerpy of diffusion for CO in tantalum and the energy of outgassing for the metal were found to be 9.4 and 49 kcal/mole, respectively.
1. Introduction
Flash filament studies of the carbon monoxide-tantalum interaction [I ,2] revealed the dissolution and diffusion of the adsorbate in the metal. The dissolution seems to begin after dissociation of CO molecules on the tantalum surface, but the discussion of partial (C and 0) solubility in tantalum can not be based on the tlash filament results. In the flash filament experiments (FFM) CO molecules are always detected. The dissociation of CO on tantalum was observed by Klein and Leder in a field emission microscope (FEM) [3]. However, the authors did not discuss the possibility of dissolution and their results are difficult to compare with those mentioned above [ 1,2] because the experimental conditions of the FEM and FEM measurements were quite different. The sticking coefficient. the saturation density and the solution rate of CO on (100) Ta were obtained using Auger electron spectroscopy by Ko and Schmidt [4]. The authors concluded that the carbon monoxide was adsorbed dissociately without solution of either species at 300 K, but at 500 K carbon dissolved leaving oxygen on the surface. The kinetics of dissolution implies that the states of the subsurface region are populated. In this case one has to make an assumption as to the 390
V.D. Belov et al. / CO and CO2 interaction with Ta
391
sensitivity of AES for various depths. The results obtained here will be qualitative at best. Any advance in the mechanism of adsorbate dissolution in a substrate is badly needed. Urgent scientific and practical requirements made us undertake the investigation of CO-Ta and CO,-Ta adlayers by combined FFM and FEM in the same apparatus. We have made an attempt to draw the results of both methods and to associate the kinetics of dissolution and outgassing with adlayer properties.
2. Experimental The experimental vacuum apparatus is shown in fig. 1. The total pressure of residual gases was “1 X 10m9 Torr. A monopole mass-spectrometer with partial sensitivity of 10-l’ Torr was used for partial pressure measurements. The gases CO and COa were admitted to the experimental chamber from the bakeable gas inlet system. The total pumping rate of CO from the chamber (V= 51) was about 33 l/set (for 10m8
Fig. 1. Experimental apparatus: (1) monopole mass-spectrometer, (2) “black chamber” with adsorbents and ion source, (3) FEM, (4) Bayard-Alpert gauge, (5) Ti-getter pump, (6) liquid nitrogen trap, (7) gas inlet system.
392
V.D. Belov et al. / CO ad
The adsorption occurs at a constant titative analysis of the gas desorption balance equations were used [.5]
CO2 irlteraction
with Ta
pressure of the gas under study. For a quandata (b&t) and r(t)) kinetic and mass-
Here N(t) is the number of CO molecules adsorbed: I_‘, V, and To are the pressure, volume, and temperature of the gas in the apparatus; A is the adsorbent surface area; E and C are the activation energy and the pre-exponential factor of desorption; T(t) is the temperature of the adsorbent during the flash; k is Boltzmann’s constant; and u is the kinetic order. At high rates of heating in the FF experiment, one can find the total pumping rate S by the characteristic time of the desorption curve tail. The rate of desorption on the tail can be neglected. The activation energy, the pre-exponential factor and the order of desorption can be estimated by substituting the experimental values -dN/dt.
N(t) = N,,, -
w st%,~d!_ d* AkT,
0
s t
&dt-2
> 0
and T(t) in the kinetic equation; N
s
max
=AkTo os
=
Apdt
is the number of CO molecules adsorbed during the exposure time ta. The visual observation and measurements in FEM were performed at the tip temperature T1 = 300 K. The photo-shots were made at the same total emission of about lop7 A.
3. FEM results After various exposures of Ta ribbon in CO at po(CO) = 2 X lo-’ Torr and T, = 300 K, the desorption of molecular CO in the flash revealed one weakly bound (a) and two strongly bound @; and 0;) states of adsorbed CO (fig. 2). The exponential decline of the experimental desorption curves lagged behind the decline due to pumping. The pumping rate has been estimated from the characteristic time of the tails of CO desorption curves from W and Ir ribbons in the same apparatus. The decay time of the desorption curves in fig. 2 seems to be caused by the evolution of CO from tantalum at T, = const (T2 = maximum flash temperature). The observation on Ta at elevated adsorption temperatures T, makes it clear that this adsorp-
V.D. Belov et al. / CO and CO2 interaction with Ta
Fig. 2. Thermal desorption K and po(C0) = 2 X 20-* desorption of chemisorbed talum.
393
spectra of CO from tantalum for various exposures (tJ at TI = 300 Torr. (1) ta = 60 set; (2) 1, = 120 set; (3) ta = 600 sec. Curve 1’ carbon monoxide; curve 1” desorption of CO dissolved into tan-
tion is accompanied by a dissolution of the adsorbate in the substrate. This leads to a large mean life time of adparticles during the flash desorption (TV= N(dN/dt)-‘). The graphs of Td versus the number of the sorbed molecules N,,(CO) are shown in fig. 3a. Each graph was obtained at T2 = const., TI being a parameter (300, 900, 1250 K). The graphs for T1 = 900, 1250 K consist of two linear parts. It is the dissolution of adsorbate in the bulk of Ta during adsorption that influences the desorption at N,,, > 10” mol/cm*. The dissolution of adsorbate just in the subsurface region takes place at N,,, < 10” mol/cm*, and the rate of the adsorbate surfacing determines the rate of the whole process of desorption during the flash. Varying T, (T, and N being fixed) we can derive the dependence 7d = 7. exp(Eo/kT2) from which E. and r. can be estimated (fig. 3b). They were found to be 49 kcal/mol and 1.6 X 1O-’ set, respectively. How the rate of adsorption can correlate with the rate of dissolution in bulk is clearly seen from fig. 4. Here the total quantities of CO adsorbed during the time t, are plotted. The fact that these quantities for Ta and W at T1 = 300 K are equal and the corresponding desorption curves have the same form ((Y,fll, f12 states of desorption), makes one think of the same primary form of gas adsorption on these samples and implies that the role of dissolution in Ta at T1 = 300 K is negligible. However, a part of the adlayer on Ta dissolves in the subsurface during the flash. This causes the time lag of the experimental desorption curves in fig. 2. This fact suggests a high rate of dissolution of the adsorbate in the subsurface region. At Tl> 900 K the dissolution of CO proceeds during adsorption, and diffusion in the bulk limits the complete transport of CO from the gas to the bulk of tanta-
394
V.D. Belov et al. / CO and CO2 interaction with Ta
a
Fig. 3. (a) Characteristic time (Td) of decline of the desorption curves as a dunction of CO amount adsorbed on Ta at various temperatures T1. (b) Desorption characteristic time 7d versus temperature for CO and Ta.
lum. In fact the characteristic time of diffusion (2 14 min as is seen from fig. 4) is much larger than the time of adsorption (about 3 min at pO(CO) = 2 X lo-’ Torr) and that of dissolution in the subsurface. So, for large times t, the diffusion proceeds in a stationary manner, with a constant adsorbate concentration on the surface less than that of the monolayer. At T, = 1250 K the diffusion flux in the bulk of tantalum equals the gaseous flux of adsorption, the surface coverage being half a monolayer density. This follows from the value of the sticking probability (the initial slope of adsorption curve in fig. 4) because the sticking probability of CO and Ta is constant (and maximum) at 0 < 19G 0.5 [ 11. The absorbed quantity does not increase linearly with I,, and the saturation of
V.D. B&v
et al. /
395
CO and CO2 interaction with Ta
rb
li
Iq
t,(minl
Fig. 4. Adsorption curves of CO on Ta at pg(C0) = 1 X 10e8 Torr. (1) 7’1= 300 K, (2) 850 K, (3) 1260 K, (4) 1630 K. Dotted line: CO on tungsten at 7’1= 300 K, po(C0) = 1 X lo-’ Torr.
the curves requires the longer time at lower T1. 1500 K when the effective value of the diffusion desorption process. To estimate DT~ the amount T1 < 1500 K was taken into account using N,,, being suitable in the first approximation (fig. 5). can be obtained by determining the dependence
Fig. 5. Amount parameter.
of adsorbate
Fig. 6. The dependence
dissolved
of diffusion
The saturation occurs also at T1 > coefficient DT is small due to the of absorption in the range 300 < = B(DT, t,)“2 (B = const), this The activation energy of diffusion of ln(B2DT,) on l/T1 (fig. 6). It
(N”) versus time, adsorption
coefficient
Drl
on the temperature
temperature
T1.
T1 being the
396
V.D. Belov et al. / CO and Cc)2 interaction with Ta
comes out to be 9.5 kcal/mol, which is quite different from the activation energy of the outgassing process (Eu). The desorption of CO from the Ta surface starts at T* 1500 K. Fig. 2 shows that all the desorption curves originate at the same ter~lperature ir~dependently of the quantity of the adsorbate. The heating rate is the same for all the curves in ftg. 2. The initial steepness of the rising parts of the curves increases with the total number of the adparticles, the time (temperature) at the inflection point in the rising part of the curves r~rnaining constant. However, the position of the maximunl on the curves changes (it shifts to higher temperatures) when the sorbed quantity exceeds that of the monolayer (Tr > 900 K). So the course of the initial portion of the desorption curves in fig. 2 leads us to suggest that desorption of CO from Ta begins as a process with first order kinetics [5]. At high heating rates during the flash the mean life time of adparticles in this process (7,) equals the rise time of the desorption curves (T, = t’ - t in fig. 2). This rise time is much smaller than the decay time of diffusion desorption rd. It means that the processes at the rising and falling parts of the desorption curves are quite different. Desorption of chemisorbed layer and outgassing of the bulk are likely to proceed in succession. This naturally results from the different rates of these two successive processes. We tried to isolate the desorption of the clle~llisorbed layer from the experimental desorption curves in fig. 2 by plotting the pumping exponent just after the inflection point in the rise of the curve. The greater the desorption rate in the rise of the curve the less is the error of such an operation. The isolation is done for curve 1 in fig. 2, where the area under the curve I’ is proportional to the number of chemisorbed molecules (Nk,,). Th e area under curve 1” is proportional to the number of CO molecules desorbed out of the adsorbent volume (A&,). Having plotted In [(-l/N) dnr/dt] = F(l/T), we obtained a straight line corresponding to the first order kinetics, from which the activation energy E, = 110 kc~~rnol and pre-exponential term C = 3 X 10 l2 set-’ were calculated. It can be predicted that dissolution of CO in the tantalum during the flash would end before the inception of the desorption (at T < 1500 K) because the stationary concentration of adsorbate (@G 0.5), which provides the nlaxilllurn dissolution rate, has set in. However, one can easily see that the rate of the adlayer depletion (surface cleaning) can become a sum of the desorption rate and the rate of dissolution when the adsorbate quantity is great and the heating rate during the flash is very high. In this case the measurement of the desorption rate (and estimation of its rate constant) will be difficult. But here we can use additional information: the low temperature dependence of the dissolution rate can be measured. The activation energy of dissolution turned out to be very small as compared with that of desorption. So the rate of surface cleaning against temperature, with desorption and dissolution taking place simultaneously, will be governed only by the desorption rate constant. The dissolution will affect the pre-exponent of the surface cleaning rate constant. Thus the activation energy of desorption can be evaluated even for the process of desorption combined with dissolution.
V.D. Belov et al. / CO and CO2 interaction with Ta
397
Consider now the linear dependence of rd (Nr,&. This dependence (or, it might be better to say, the independence of the CO desorption rate from its total adsorbed quantity) may correspond to the desorption of the adsorbate from the ordered structure in the bulk of Ta. The remarkable properties of the subsurface region and the constant concentration of the solute in the bulk of Ta may originate from the stepwise dependence of the adsorbate diffusion coefficient on the distance from the surface. In this case the stationary distribution of the diffusing substance in the bulk will consist of two regions: (1) a subsurface with the concentration smoothly changing from that on the surface to that in the bulk and (2) the bulk of the substrate with maximum solubility concentration.
4. FEM results The FE lo-’ Torr whole area adsorption.
pattern of the Ta tip saturated with CO at Tr = 300 K and pu(C0) = 2 X is shown in fig. 7. During adsorption the changes are observed over the of the emitter. So the whole surface of the tip is subjected to the CO If then the temperature of the tip increases, the emission undergoes a
Fig. 7. Field emission patterns of Ta surface saturated with CO at T, = 300 K and various temperatures of the tip: (a) 300 K, (b) 1000 K, (c) 1250 K, (d) 1500 K, (e) 1800 K, (f) 2100 K.
398
V.D. Below, ct al. / CO artd CO2 interactiotl
with Ta
number of changes. The patterns observed during heating do not look like those obtained during adsorption at Tt = 300 K. This fact and the structure of the pattern in fig. 7a imply that the adlayer of CO on Ta at Tt = 300 K is amorphous. The same has been shown after FE observation of CO on W [6] while the reversibility of adsorption-desorption patterns in the case of H2 on W and Ta at Tt > 300 K ]6] is evidence in favour of migratory equilibrium in the adlayer-. Heating
Fig. 8. Adsorption
of CO on Ta at po(CO)
= 2 X lO_’
Torr
and various
exposures
t,.
T1 =
1000 K, (a) ra = 60 set, (b) ta = 180 set, (c) fa = 600 set; T1 = 1100 K: (d) t, = 60 SW, (e) t, = 180 set, (f) t, = 600 SW; T, = 1250 K: (g) t, = 60 sec. (h) t, = 180 sec. (i) 1, = 600 set
V.D. Belov et al. / CO and CO2 interaction with Ta
399
the tip as high as 1000 K results in bright spots spreading over the surface of the emitter, mostly around the (110) and (111) planes. Also, dark spots of irregular form are localized in the light area surrounding (100). As the tip temperature is raised to 1250 K, the dark and bright spots disappear (fig. 7) and large islands with great specific emission emerge in the picture. As a rule they are localized in the regions with orientation (301) and are separated from the bright areas by the zones with lower emission. In addition the picture shows well developed regions of (2 11) planes which cannot be seen either in the emission pattern of the clean Ta surface, or during adsorption of CO on the tip at T < 900 K. Irreversible changes in the field emission pattern observed during the heating of an adsorbent are caused by dissolution and diffusion of adsorbate in the bulk of Ta, desorption of CO to the gas phase being observed only at T > 1500 K. It is possible to observe the dependence of the FE image of the cluster’s phase on the coverage, the migration rate of the adsorbate and the time of the diffusion process. Indeed, the difference in the pictures (figs. 7 and 8) is probably caused by the fact that at T, = 300 K the dissolution of adsorbate in the substrate begins without a preliminary attainment of equilibrium in the adlayer by surface migration. However, at T, = 1000 K the equilibrium in the adsorbate is set in before the extensive diffusion
Fig. 9. Field emission patterns of Ta surface saturated with oxygen at T1 = 300 K (a) after heating the tip to T = 900 K (b), 1100 K Cc), 1650 K (d), 1250 K (e), 220 K (f).
The structure of the CO monolayer on Ta at high temperatures (T> 1000 K) coincides in many details with that observed for the 0, Ta system (fig. 9) (the (12 I) planes manifestation). From this fact the conclusion can be drawn that the adsorbed CO molecules dissociate on the Ta surface. The comparison of emission patterns of Ta covered with C [3] and those with adsorbed CO makes us think ot concentrating carbon atoms in the vicinity of the (100) plane of the Ta surface during the initial stage of CO adsorption at 1000 < T1 < 1500 K (fig. 8). We are in agreement with Klein [3] that the bright clusters are composed of carbon atoms and the dark ones of oxygen. It is important that dark spots never appear in the vicinity of the (I 11) plane and that bright ones never appear around the (100). The greater the amount of the adsorbed CO on the surface (the less amount dissolved in the bulk). the more clusters can be observed, and the smaller they are. The less the amount of the adsorbate on the surface (the more it dissolves). the less the number of clusters and the bigger they are. We are dealing with the same situation when considering the condensation of a supersaturated solution. The higher the rate of supersaturation, the smaller the radius of the critical nucleus of crystallization and their number because the energy of nucleus formation becomes less. Our data lead is to the conclusion that the clusters are three-dimcnsional nuclei of crystallization of a supersaturated solution in the subsurface region. They appear to be stoichiomctrical compounds of TaO and TaC types. The critical size of a cluster depends on the ratio of the diffusion flux to that of adsorption. At elevated substrate temperatures the flux of the adsorbate into the bulk of Ta increases. So supersaturation in the subsurface diminishes and the critical size ol nucleus of crystallization (cluster) increases. When the fluxes are equal to each other (curve 3 in fig. 4), supersaturation of the subsurface region vanishes and the clusters disappear. Being the surplus of the adsorbate in the solution, the cluster is in equilibrium with the solvable part of the adsorbate on the surface (0 > 0.5). As to the semiconductor cluster surrounded by a metal, the emission current through it can be very large if the electrochemical potential of the electron in the semiconductor is higher than the Fermi level of the metal (C clusters, VT,& < L+). If. on the other hand. the electron electrochemical potential of the semiconductor iS lower thl the Fermi level of the substrate (0 clusters, pTaO > PTa), the emission from the cluster will be less than from the surrounding region. This can be concluded from consideration of the energy diamgrams for metal--semiconductor contacts [8]. The contrast in the field emission will be more pronounced when the TaC region is in contact with TaO and vice versa.
5. C02-Ta
The experiments were carried out at various temperatures of Ta adsorbents and CO, pressure equal to ? X 10-s Torr. Flash desorption experiments indicate that CO2 dissociates on the Ta surface: desorption of CO (T,*,, z 2000 K) is followed by
V.D. Belov et al. / CO and CO2 interaction with Ta
401
that of TaO (T+, Z 2400 K). The kinetics characteristics of CO and TaO desorption from the CO2 adlayer are in agreement with those obtained separately from the study of CO and O2 adsorption (except for the values of the concentrations). The character of CO2 adsorption at elevated temperatures displays dissociation of the adsorbate and dissolution of the C and 0 adatoms in the substrate. At 300 K the concentration of adsorbed CO* molecules does not exceed that of Ta atoms on the surface. It is apparent that in this CO, molecules are adsorbed on Ta surface and diffusion into the sample may be neglected. At 300 K < T< Tdes adsorption of CO* is accompanied by diffusion of C and 0 atoms into the bulk of tantalum. The number of CO and TaO molecules desorbed by the flash increases. At T = 1500 K the diffusion rate reaches a maximum. The rise of the adsorbent temperature (T = 1650, 1800 K) results in a decrease in the amount of the substance dissolved inTa, which is due to the growth of the desorption flux. As a result, the effective flux of adsorption as well as that of diffusion of the adsorbate into the sample decreases. In general, the picture of CO, adsorption on Ta resembles in many details that of CO adsorption on Ta. However, the dissociation of CO* on the Ta surface leads to the formation of a mixed (C + 0) coverage with a
Fig. 10. Field emission patterns of Ta surface saturated with CO2 at T = 300 K (a) after heating the tip to T = 900 K (b), 1100 K (c), 1250 K (d), 1650 K (e), 2000 K (0.
402
V.D. Below et al. /CO arui CO2 interaction
with Ta
I:ig. 11. Adsorption of’ CO2 on the heated Ta surface (pressure of CO,? is 2 X 10e8 Torr). T = 1100 K, (a) t, = 180 sec. (b) ta = 600 set; T = 1050 K, (c) t, = 180 sec. (d) t, = 600 set; T = 1250 K, (e) t, = 180 set, (f, t, = 600 sec. relatively large contribution from the 0 atoms. This fact must apparently affect the way the adsorbate diffuses into tantalum as well. The FE patterns of a Ta tip covered with CO, differ in some details from those obtained in the course of CO adsorption. Fig. 10a shows the CO saturated surface of a Ta tip at T = 300 K, p(CO*) = 2 X IO-’ Torr and for an exposure to the gas of ja = 15 min. Heating the tip with the coverage in vacuum gives rise to changes in the pattern, which are caused by the migration of the adsorbate over the surface and dissolution in the substrate (fig. lob). When the tip temperature rises to 1100 K, dark and bright spots that were observed in the CO-Ta system at the same temperature of the surface emerge in the picture. The concentration of bright spots as compared with that of dark ones in the cases of CO* on Ta is, however, much less than in the case of CO-Ta (fig. 10~). Further increase of temperature (T’ 1250 K and higher) produces the complete disappearance of the clusters. The patterns and emission characteristics are similar in appearance to those of the Ta tip covered with CO ar.d O2 (cf. fig. 10 with figs. 7 and 9). Within the temperature range T = 1800~-2000 K. desorption of a fraction of the adsorbate occurs by CO molecules and the pattern turns out to be typical for a Ta surface with oxygen coverage. The
V.D. Belov et al. / CO and CO2 interaction with Ta
403
complete cleaning of the surface is attained at T = 2400 K. The FE patterns of the Ta tip during adsorption of CO, at T = 1000, 1050 and 1250 K are given in fig. 11. It is easily seen these pictures differ from those obserbed in the CO-Ta system. At T = 1000 K, in the picture of the surface with CO* coverage (figs. 1 la and 11 b), the dark spots emerge along the lines between the planes with orientation of (310). Only at ra = 30-40 min do small rare bright spots appear in the pattern. A slight increase of the tip temperature (T = 1050 K) results in a change of the pattern, first of all in the regions surrounding (111). On the screen there emerge brightly emitting large clusters (“islands”) about 1000 A in size. As was observed in the case of CO on Ta the clusters are formed and remain on the (310) plane and are separated by the area with lower emission from the bright regions surrounding the (100) plane. However, the probability of their formation in the CO,-Ta system is less (the pattern in figs. 1 lc and 1 Id has only one cluster emerged after 10 min adsorption of CO,). At T> 1100 K (figs. 1 le and 1 If) the cluster structure does not appear and the emission pattern in the presence of CO2 on Ta resembles the pattern of oxygen coverage. 6. Conclusion To sum up: CO and COz adsorption on tantalum is accompanied by the dissociation of adsorbates and the dissolution of C and 0 atoms in the substrate at T > 300 K. The comparison of CO and O2 dissolution rates enables us to conclude that the limiting process in CO dissolution is the carbon diffusion in the bulk of Ta. The activation energy for this process turned out to be 9.5 kcal/mol. The rate of outgassing in the case of CO adsorption on Ta is limited by the rate of surfacing of the dissolved C atoms. The activation energy for this process (49 kcal/mol) is in agreement with that for C cluster formation obtained by Klein [3] (53 kcal/mol). Our data point out that at least three consecutive stages must be discerned during the adsorbate dissolution and cleaning the substrate. These are the mass transport from the gas phase to the surface (and in the opposite direction), from the surface to the subsurface region (and backward), from the subsurface to the bulk of the substrate (and backward). The kinetics of mass transport from the gas phase (and backward) at various experimental conditions (p, T) must be studied in order to get the rates of each of the consecutive stages. In this work the CO and CO, adsorption on tantalum, the diffusion of C and 0 atoms in the bulk of the substrate, the surfacing of the solute (C atoms) and the desorption of CO from the surface of tantalum to the gas phase have been studied. The insolubility of CO from the tight binding state of adsorption (0 < 8 < 0.5) is found, the outgassing of tantalum following the desorption from this state. At T> 1000 K, CO and COz adsorption increases the contrast in the field-emission pattern of the Ta surface (the resolution) thus revealing the crystallography of the substrate. This effect is analogous to that of etching, and seems to be connected with the dissolution of the adsorbate in the substrate.
404
V.D. Below et al. / CO and CO2 interaction
with Ta
References [l] E.P. Gasscr and R. Thwaites, Trans. Faraday Sot. 61 (1965) 513. [2] Yu.K. Ustinov. J. Tech. Phys. (USSR), 61 (1971) 411;61 (1971) 643. [ 31 R. Klein and L. Leder, J. Chem. Phys. 38 (1963) 1863, 1866. [4] S.M. Ko and L.D. Schmidt, Surface Sci. 47 (1975) 557. (51 G. Ehrlich, in: Advances in Catalysis, Vol. 14 (Academic Press, New York. 1963-1964) pp. 255-427; V.N. Ageev, N.I. lonov and Yu.K. Ustinov, J. Techn. Phys. (USSR) 24 (1964) 546. [6] Yu.K. Ustinov, Fiz. Tverd. Tela 12 (1971) 558. [7] V.D. Below, Yu.K. Ustinov and A.P. Komar, J. Techn. Phys. 46 (1976) 2403. [ 8] 1.1. Petrovskij, Electronic Theory of Semiconductors (Minsk, 1973).