LEED investigation of Sn on Mo (100)

SURFACE

SCIENCE 27 (1971) 197-210 Q North-Holland

AUGER/LEED INVESTIGATION

Publishing Co,

OF Sn ON Mo(100) *

Received 9 February 1971 Growth of tin on the Mo(IO0) surface has been studied using LEED/Auger techniques. Only one structure, a ~(2 x 2), was faund which could be associated with a %-MO compound. In the presence of oxygen, the growth of the c(2 x2) is inhibited unless the MO is heated to 830°C in the presence of a low coverage of oxygen. Tin oxide will form but is decomposed to free tin plus low oxides of Ma. Attempts to form MosSn (P-W structure) were unsuccessful. Carbon inhibits the growth of any Sn-Mo ordered structures.

LEEDJAuger studies of the growth of metal films have been r~~ortedl-~~) for a number of metals on W and Ta, but no studies have been reported of the growth of Sn on MO. In this report we present our results of examining the interaction of Sn with the Mo(100) surface. Bulk studies show that Sn and MO do not react strongly and that Mo,Sn (P-W structure, a LS:5.09 A) can be formed only with great difficulty. 2, Experimental

Auger and LEED data were obtained using a 3-grid LEED optics arranged for the Auger technique. While the resolution of the Auger technique using a 3-grid con~guration is lower than with 4 grids or other techniques, the spectra obtained were clearly observable. Fine structure in the spectra could not be observed, but this was no great problem since our interest here was to determine the presence of Sn on the MO surface under various conditions. 3. Results A number of runs were made to establish reproducibility of the results. The experimental runs divided into two main portions: Sn on clean Mo(100) and oxygen interaction in the MO-& system. * Work supported by OAR Contract F336i5-69-C-fO64, Force Base, Dayton, Ohio 45433, U&A. 197

ARL, Wright-Patte~on

Air

198

A. G. JACKSONAND M. P. HOOKER

(a) Evaporation of the Sn onto the clean Mo(100) at room temperature resulted only in a decrease in the intensity of the MO LEED pattern. The cleanliness of the sample was determined by obtaining a clean pattern, then determining the Auger spectra. Table 1 shows experimental and theoretical MO Auger transitions. A clean pattern and the associated Auger spectra are shown in fig. 1. Successively heating the sample to higher temperatures in several steps of 3 to 5 min each up to 830°C did not significantly change the pattern observed. At 830°C the pattern changed to the c(2 x 2) pattern shown in fig. 2a. Portions of the associated Auger spectrum are given in TABLE 1 Major Transition

MVNINIII

EXP z z+I

125 130 126

MO Auger transitions in eV* MVNINII

MVMINV

153 157 153

166 163 163

* See ref. 16.

MVNINV 163

ENERGY

(Fig. la)

(rV)

MvNmNv 192 190 190

MvNvNv 226 223 223

AUGER/LEED

fig. 2b. Continued

heating

INVESTIGATION

OF .%I ON

Mo(100)

to 1060°C did not change

the pattern.

199

Higher

temperatures degraded the intensity of the pattern until, at about 1210°C the c(2 x 2) disappeared, leaving the clean pattern, which was confirmed by the Auger spectrum. (b) Exposure of the c(2 x 2) structure as obtained under conditions shown in fig. 2 to 54 L (1 L = 1 x 10e6 torr-set) of 0, yielded a somewhat diffuse pattern in which the c(2 x 2) was no longer evident. Heating to 860°C increased the diffuseness, but a high coverage oxide LEED pattern was in evidence. Heating to 965°C yielded a poorly formed (1 x 2) pattern. Higher temperature heating to 1140°C produced strong (1 x 2) domains. The sequence is shown in fig. 3. Auger spectra for each of these cases revealed that Sn was present up to the time of the 1140°C heating, after which Sn was not evident. Oxygen was seen in the spectra in every case after the exposure. (c) Evaporation of Sn onto a high-coverage oxide reduced the intensity of the LEED pattern. Heating of such a surface to 790°C resulted in a combined pattern of high oxide plus a pattern with (100) symmetry. Heating to

(Fig. lb) Fig. 1. (a) Auger spectrum of clean Mo(100). The carbon peak is due to CO adsorbed during the time required for the scan. (b) Clean Mo(100) pattern, 65 V.

200

A.G.JACKSON

AND M.P.HOOKER

11 [

1

(Fig. 2a)

1010°C yielded the p(2 x 2) oxide, but it is poorly formed. Heating to 1120°C yielded a well formed p(2 x 2) oxide. This sequence is shown in fig. 4. Auger spectra for this sequence show traces of Sn up to the 1120°C heating. Comparison of fig. 3b with fig. 4b shows that these patterns are the same structure but with differing intensities. (d) Evaporation onto the p(2 x 2) oxide patterns with the sample at 830 “C yielded the c(2 x 2) structure, which was identical to the c(2 x 2) obtained by evaporating Sn onto clean Mo( 100). Low exposure (1.2 L) of 0, decreased intensity, but heating to 840°C partially degraded the c(2 x 2) and produced an oxide. Heating to 1075°C improved the oxide pattern, but the c(2 x 2) was still present. Heating to 1130°C yielded an oxide pattern with high background. Heating to 1170°C yielded a strong oxide, as shown in fig. 5. This oxide is due to two domains of ,/S(l x 1) 0, rotated +26” 34’ on Mo(100) and is the same pattern observed from direct oxidation by Kan and Feuerstein14) and by Dooley and Haas15). Auger spectra indicated Sn was present up to the last heating. (e) Evaporation onto the p(2 x 2) oxide at room temperature resulted in a

AUGER/LEED

INVESTIGATION

OF fhl ON

201

Mo(100)

MO

dN dE

400

425

450

475

500

525

ENERGY

(Fig. 2b) Fig. 2. (a) Mo(lOO)-~(2x2) pattern obtained after heating sample to 83O”C, at 45 V. Pattern is associated with a Mo-Sn surface compound. (b) Portions of Auger spectrum of MO and Sn for three cases. Dashed lines: Just after evaporation of Sn onto clean MO; solid lines: after heating to 830°C and forming c(2 x 2); pattern fuzzy; dash-dotted lines: clean Mo(100).

decrease in intensity.

Heating

to temperatures

up to 1100 “C did not produce

the c(2 x 2), although Auger spectra showed the presence lists the sequences described in (a) through (e).

of Sn. Table

2

GENERAL RESULTS

In the presence of carbon on the Mo(100) surface, no strong c(2 x 2) pattern could be found. Only a decrease in the intensity of the pattern resulted after evaporation, implying the presence of an amorphous or disordered layer. Heating to approximately 1100°C restored the “clean pattern” (plus carbide). During the time of scanning the Auger spectrum from 0 to 275 V, enough CO absorbed to give a measurable carbon peak. Taking the 270-280 V scan right after heating the sample, however, gave no carbon peak. Since evaporations of Sn were made immediately after cleaning the sample and were

(b)

Fig. 3. After exposing Mo(1OO):Sn ~(2x2) to 54 L:Oe. (b) After heating to 860°C. (c) After heating to 965°C. (d) After heating to 1140°C. All voltages are 75 V in this and following figures.

(b)

AUGER/LEED

INVESTIGATION

OF %I ON

Mo(100)

203

204

A.G.JACKSON

AND

M.P.HCOKER

AUGER/LEED INVESTIGATION OF Sn ON Mo(100)

205

TABLE2 Sequence after evaporating Sn onto Mo(100) (a) Mo(1OO)~loss of intensity+830”C:c(2x2)-+1210°C:Mo(100) (b) c(2 x2)+54 L:02:diffuse+860°C:diffuse+oxide 965”C:(1~2)~114O”C:(1 x2) (MoSn) (c) oxide-tloss of intensity+790”C: high oxide+1120°C:oxide (no Sn) (d) ~(2x2) sample at 83O”C+c(2 x 2)+1.2 L:Oz+loss of intensity-+840°C:c(2x2)+ +oxide+1075”C:oxide+poor c(2x2)+1130”C:oxide+1170°C:strong oxide [1/5(1 x l)Oa*26” 34’ (no Sn)] (e) p(2 x Z)+loss of intensity+1 100°C: oxide only

for short (l-3 min) periods of time at ~2 x lo-’ tort-, CO contamination was minimal. An attempt was made to grow a bulk tin layer on the Mo(100) face. The MO sample was held at room temperature while Sn was evaporated onto the surface for an extended period of time (6 min). The resulting Auger spectrum showed a very large Sn peak and a greatly reduced MO peak. Gradual heating to about 1000°C led to the c(2 x 2) pattern and a reduced Sn peak and increased MO peak, as indicated in fig. 6.

Fig. 6. Auger spectrum of high coverage tin. Solid line: clean MO; dashed line: after 6 min evaporation; dash-dotted line: after heating to approximately 1000°C.

206

A.G.

JACKSON

AND M. P. HOOKER

Holding the MO at a 830°C while depositing the Sn led to similar depression of the MO peaks. After a certain time of deposition had passed (6 min), the MO peaks were not depressed any further, nor was the Sn peak increased in height. Heating to 1000°C to obtain the c(2 x 2) pattern returned both the MO and Sn peaks to the sizes earlier associated with the c(2 x 2) pattern. 4. Discussion Determination of what the c(2 x 2) structure represents is complicated by the fact that MO-CO at about the same temperatures forms a c(2 x 2)15). The specular intensity distribution of this carbonyl compared with that obtained for the c(2 x 2)-Sn differ, however. In addition, the Auger spectrum indicated the presence of Sn only and not CO or carbon of any kind in significant amounts. Since the uniformity of the film was good as determined by moving the beam around on the sample, the fact that two different electron guns were used which may have been hitting different parts of the sample was not a problem. Sn melts at about 232 “C; thus, at 830°C the surface mobility of the Sn must have been high. For these reasons, the ~(2x2) is associated with Mo-Sn. Since the pattern possesses the C, symmetry, the (100) face of Mo,Sn seems a reasonable suggestion. The Sn(OO1) face, however, also possesses this symmetry. Lattice constants for these two cases are Mo,Sn(lOO):5.09 and Sn(OO1): 5.82. Thus, one expects to have a smaller reciprocal lattice if one has Sn(001) on Mo(100) than if the pattern is Mo,Sn(lOO). Comparison of the two patterns referred to Mo(100) can be made in several ways. 1) The extra spots, indexed as (+, 3) relative to Mo(lOO), suggest the possibility of Mo,Sn(lOO). But this assignment cannot hold since the spots expected would not match the true (3,+) spots observed. 2) The extra spots can be indexed as Sn(l0) and Sn(Ol), thus suggesting Sn(OOl)/Mo(lOO) rotated by 45 degrees. Again, the extra spot locations do not fit. 3) Sn(OOl)/Mo,Sn(lOO). Spot locations do not fit properly. Comparison of the clean pattern with the c(2 x 2) and calculation of their ratios showed that the c(2 x 2) is a Mo(lOO)-c(2 x 2)-Sn, and that it cannot be due to Mo,Sn. If one assumes that the c(2 x 2) structure is due to Sn, as outlined in (b) of the previous section, then exposure to oxygen with the sample at elevated temperatures, could produce SnO, amorphous, which when heated to higher temperatures decomposes to MOO, and Sn (see table 3). The free energies involved in such a process favor these schemes as compared to the formation of more complex oxides such as MOO,. This would

AUGER/LEED

INVESTIGAIION

TABLE

(a)

AF (kcal/mole)

OF SIl ON

Mo(100)

3 AF (kcal/mole)

(b)

Mo+Oz+MoOz Sn+Oz+SnOz

-136 124

Mo+Oz+MoOz 2 Sn+02+2 SnO

MO+-SnO2+MoOz+Sn

-

Mo+2 SnO+MoOz+2

12

207

-136 122 Sn

-

14

explain the (100) oxide pattern evident along with a MOO, while Sn still appears in the Auger spectrum. From these data one cannot definitely state, of course, that there was a substitutional alloy of Mo-Sn. One suspects, however, that the low solubility of Sn in MO would make plausible a substitution of Sn atoms for MO atoms in the surface layer only, rather than significant diffusion of Sn into MO. Since no Mo,Sn is formed, whatever the structure is it is clearly a surface compound and does not arise from impurities diffusing to the surface. Because the structure forms on a clean surface one can suggest that the low solubility of Sn in MO is strongly dependent on the interaction of these two elements rather than on the presence of impurities such as CO, 02, etc. The Auger spectrum for Sn has not been published before, so it is useful to mention here how we identified the peaks associated with Sn. The elements In, Sn, and Sb lie next to each other in the periodic table; hence, we expected that the location of peaks could be estimated by extrapolating between these values. Using the tables published by Haas, Grant and Dooley16) the probable major transition would be MvNVNv. This yields a value of 429 V for the major peak. The value is consistent with the estimated values since the major peaks in In and Sb lie at 403 eV and 456 eV respectively. Experimentally, we observed a large peak at about 430 V. A smaller peak was observed at about 315 C. This corresponds to an MvN,N, transition, again based on comparable transitions in this period. The M,N,,,N, transition at 364 V was not observed, presumably due to the limited resolution of the 3-grid system. In order to verify the calculated positions of the Sn Auger peaks, we studied a small (3 in. x + in. x + in.) piece of boat-refined, 99.999 pure Sn. The electron gun put out such a large current at 2000-2500 V that it melted the sample. A larger (1 in. x 3 in. x 4 in.) sample was then used and gun power was reduced. An infrared pyrometer was used to measure the temperature of the area immediately around the spot where the electron beam hit the sample and showed that at high voltages (> 2.5 kV), even this sample

208

A. G. JACKSON

AND M. P. HOOKER

could easily be heated to 200°C. At 1.5 kV, the temperature stabilized at 120°C. The smaller sample evidently could not conduct the heat away fast enough. The positions of the Sn peaks did verify the earlier estimates of 315 eV (M,N,N,), 354 eV, 430 eV (M,N,Nv). Micrographs of the Mo-Sn taken after evaporation but before heating to form the c(2 x 2) showed small islands of Sn oriented in the [ho] and [Ok] directions (see fig. 7a). The

(4 Fig. 7.

(b)

(a) After evaporating Sn, but before heating; 7500x. (b) Clean Mo(100); 30100x.

uniformity of the film is good, but the density of Sn is low. This explains the large MO peaks present in Auger spectra and why these peaks persisted even for very heavy exposures to Sn. The island coverage amounts to about 12%. The actual tin coverage was probably higher than this since very thin films or thin islands would not be observed on the micrograph. Fig. 7b shows the same sample with no tin present. The behavior of Sn on clean MO is interesting because, in this case, the surface behavior is very different from that of the bulk. But as usual in surface studies the quantitative definition of surface compounds does not necessarily follow from the bulk. In the c(2 x 2) structure, for example, the distribution is 1 to 1 or 50% while the solid solubility of Sn in MO is of the order of O.l%la). The effects of carbon and oxygen substantiate the bulk results since no

AUGER/LEED

ordered

structure

of Sn-Mo

INVESTIGATTON

appear

OF Sll ON

Mo(100)

in the presence

of these elements.

209

The

existence of solid solutions of Mo-Sn, however, cannot be postulated on the basis of structure information of from Auger results alone or together. Our results exemplify the problem inherent in LEED/Auger studies of alloys. We know Sn and MO are present; we know there are structures which are formed; we know some of the structures are oxides, some are Mo-Sn compounds. The Auger results are not yet quantitative enough to calculate the concentrations of elements with any significant degree of confidence. On the positive side, Auger is essential in identifying phases of various structures. This aspect is of great importance in understanding the behavior of ordered surface alloy systems. The absence of structures associated with Mo,Sn, even in ultra-thin film form, is interesting. Although bulk Mo,Sn, as mentioned, is very difficult to produce, we expected that surface alloying would be significant. The contrary was obtained. 5. Conclusions From these results we conclude that ordered surface structure of Sn on Mo(100) forms only in a c(2 x 2) structure. In the presence of O,, the growth of the c(2 x 2) structure is inhibited unless the MO is heated to the temperature observed in the clean case plus a low coverage of oxygen. In the c(2 x 2)Sn structure, the formation of molybdenum oxides takes place after heating to various temperatures by a change from Sn to SnO to MOO, plus Sn. 0, will displace Sn leading to formation of MO oxides. While Mo,Sn was expected, no evidence for the P-tungsten structure vould be found. In the presence of carbon, Sn does not grow on Mo(100) in an ordered structure. Acknowlegements We wish to acknowledge helpful G. J. Dooley, and Dr. J. T. Grant.

discussions

with Dr. T. W. Haas,

Dr.

References 1) D. A. Gorodetskii and A. A. Yasko, Soviet Phys.-Solid State 10 (1969) 1812 [Fiz. Tverd. Tela 10 (1968) 23021. 2) A. G. Jackson, M. P. Hooker and T. W. Haas, J. Appl. Phys. 38 (1967) 4998. 3) N. J. Taylor, Surface Sci. 4 (1966) 161. 4) A. R. L. Moss and Blott, Surface Sci. 17 (1969) 240. 5) D. A. Gorodetskii, A. A. Yasko and Fung-Kho, Bull. Acad. Sci. USSR 33 (1969) 436 (Izv. Akad. Nauk SSSR). 6) D. A. Gorodetskii, Yu P. Malnik and A. A. Yasko, Ukr. Fiz. Zh. 12 (1967) 649. 7) A. G. Naumovets and A. G. Fedorus, Soviet Phys.-Solid State 10 (1969) 2029 [Fiz. Tverd. Tela 10 (1968) 25701.

210

A. G. JACKSON AND M. P. HOOKER

8) D. A. Gorodetskii and A. A. Yasko, Soviet Phys.-Solid State 11(1969) 640 [Fiz. Tverd. Tela 11 (1969) 7901. 9) D. A. Gorodetskii and A. A. Yasko, Soviet Phys.-Solid State 11 (1970) 2028 [Fiz. Tverd. Tela 11 (1969) 25131. 10) J. Anderson, P. J. Estrup and W. E. Danforth, Appl. Phys. Letters 7 (1965) 122. 11) P. J. Estrup, J. Anderson and W. E. Danforth, Surface Sci. 4 (1966) 286. 12) J. H. Pollard and W. E. Danforth, in: The Structure and Chemistry of Solid Surfaces, Ed. G. A. Somorjai (Wiley, New York, 1969) p. 71-l. 13) F. A. Shunk, Constitution of Binary Alloys, 2nd Suppl. (McGraw-Hill, New York, 1969) p. 524. 14) H. K. A. Kan and S. Feuerstein, J. Chem. Phys. 50 (1969) 3618. 15) G. J. Dooley III and T. W. Haas, J. Chem. Phys. 52 (1970) 461. 16) T. W. Haas, J. T. Grant and G. J. Dooley, Phys. Rev. Bl (1970) 449.