Adsorption of silicon on molybdenum in a field emission microscope

SURFACE

SCIENCE

44 (1974) 157-169 Q North-Holland

ADSO~TION

OF SILICON

Publishing Co.

ON MOLYBDENUM

A FIELD EMISSION

MICROSCOPE

G. VENKATACHALAM

and M. K. SINHA

Department

of Physics, North Dakota State Universiiy, North Dakotu XWA?, U.S.A.

Received 5 February 1973; revised manuscript

IN

Fargo*

received 1 March 1974

The adsorption, surface diffusion, and thermal desorption of silicon on molybdenum have been investigated by field emission microscopy. The average work function of silicon covered molybdenum field emitter decreases with a simultaneous reduction in the total field emission current. This suggests the resonance tunneling of the field-emitted electrons. With low coverage, boundary free surface diffusion occurs at 565°K on the [Ill ] zones. Above 585°K diffusion occurs with a sharp boundary and an activation energy of 50.9 kcal/mole in the (211)+(100) direction. Adsorption of silicon on molybdenum tips at and above room temperatures is anisotropic. The activation energy of thermal desorption from (111) and (411) planes is 63.3 and 123.9 kcaI/mole respectively. Annealing the silicon covered tip at 1000°K produces a silicon enriched surface phase with new crystal planes.

1. Introduction The adsorption of semiconductor materials such as silicon and germanium on tungsten, in a field emission microscope (FEM) has been studied by

several authorsl-6). There are several reasons for such studies. The presence of an adsorption layer may change the work function of the metal and a field emission microscope is a very sensitive instrument for such a study. Further, it provides us with a tool to determine the activation energies of desorption and surface diffusion, as a function of coverage, crystallographic planes and directions of the substrate. Other reasons for such investigations are the possibiIity of epitaxial growths of these semiconductors on substrates which are easy to fabricate as FEN specimen and the formation of alioys. We, in our laboratory, initiated these investigations with the last two possibilities as objectives. The fact that a FEM is well suited for the epitaxial growth studies7-9) has already been demonstrated and we believe that FEM and/or field ion microscope have a greater potential for alloy formation investigations than demonstrated until now. We have already shown the feasibility of the diffusion of silicon into the interior of tungsten lo); thereby producing probably a tungsten silicidesa). Alloys of molybdenum with silicon have many technical and industrial 157

158

applicationsll,lX)

G. VENKATACHALAM

because

AND

of their attractive

M. K. SINHA

features

such as high melting

point, high strength and high resistance to oxidation and corrosion. Further, MoSi, coatings are used on metallic and nonmetallic substrates for protection against oxidation and corrosion. There is no published work on the FEM study of silicon on molybdenum, but in an unpublished work Cooperls) studied the properties of silicon deposits on molybdenum. Our investigation of the properties of silicon films on molybdenum included the variation of work function of molybdenum with different coverages of silicon, surface diffusion, adsorption at and above room temperature and thermal desorption14). In addition, we have investigated the nucleation and oriented growth of a silicon film on molybdenum. These results will be reported in another paper. 2. Apparatus The field emission microscope used was similar to the one used in this laboratory in earlier studies j,l”). The emitter tip was made from a 99.95% pure 5 mil molybdenum wire. A short length of this wire was spot welded to a 5 mil tungsten loop mounted on nickel leads. The molybdenum wire was then etched electrolytically in concentrated potassium hydroxide solution, applying a few volts dcl5). The ultrahigh vacuum (uhv) system consisted entirely of Pyrex glass, stainless steel Varian ConFlat flanged fittings and Granville-Phillips bakable valves. The fore-pumping system consisted of a Varian VacSorb pump and a 140 liter/set Vacion pump and a titanium sublimation pump. After bakeout the uhv system was pumped with a 8 liter/set ion pump and a 50 liter/set titanium sublimation pump. The total pressure in the uhv system was - 6 x IO-” torr (equivalent N,). Silicon was evaporated on the specimen emitter, using the method of Kilgore and Roberts 16). A 0.1 x 0. I x 2.0 cm silicon bar was held with tantalum clips spot welded to 60 mil molybdenum leads. Good contact was insured by wrapping the silicon with 3 mil molybdenum wire. The bar was of 5 N purity obtained from Electronic Space Products, Inc. The silicon was outgassed for over 20 h at 1300°K with repeated flashes over 1500°K. In most experiments silicon was evaporated at 1475 “K. The pressure in the uhv system during evaporation remained in the 10-i’ torr range. The temperature was measured with a Pyro-micro-optical pyrometer. The specimen tip was spot welded to a standardized loop of tungsten and the tip temperatures above 1100°K were measured with the optical pyrometer. Temperatures below 1100°K were determined by measuring the resistance of the loop for various heating currents and using the known valuesr7) of the resistance ratio RT/R2930K.

ADSORPTION

OF si ON MO

159

IN FEM

3. Experiments and results 3.1. WORK FUNCTION The average work function surface can be found from plots ls, IQ). If q,, and cp are the the F-N plots of a clean and

of the silicon covered molybdenum emitter the slopes of the Fowler-Nordheim (F-N) work functions and S, and S are the slopes of silicon covered surface, respectively, we have

For a clean molybdenum surface we use q0 = 4.2 eV. Before each evaporation, the emitter was cleaned by flashing it at 2000 “K. Silicon was then evaporated at 1475 “K on the emitter tip at room temperature from one side for different times. The tip was then heated at 1000°K to spread silicon uniformly over the entire surface. The emission current and tip voltage readings were then taken. Slopes of the F-N plots were determined by the linear regression equation and average work functions were calculated. Table 1 shows the results. It will be noted that both the work function of the silicon TABLE

I

of work function o, and preexponential term In A with silicon evaporation time t. One monolayer of coverage corresponds to I = 15 set Variation

t (set)

c(eV)

Aln A

0 2 5 IO 20 40

4.20 4.10 4.1 I 3.97 3.90 3.81

0 - 2.16 -2.52 -3.10 -3.82 -4.67

covered surface and the preexponential term in the F-N equation decrease with increasing coverage. Further, the field emission current at a fixed voltage also decreased as the silicon dose was increased. This decrease continues until 10 set evaporation time, when it increased and for still larger doses there was little change in the emission current. 3.2. SURFACE DIFFUSION In these experiments silicon was evaporated from one side of the tip. Figs. 1b-l g show a sequence of surface diffusion. Fig. 1a is a field emission pattern of clean molybdenum. Silicon was evaporated at 1350°K for 5 set, from the left and the tip was then heated to higher temperatures. At 565 “K a boundary free diffusion occurred along [ll I] zones connecting the (21 l}

160

C. VENKATACHALAM

AND

M. K. SINHA

Fig. 1. Surface diffusion of silicon on molybdenum. Silicon evaporated from the left, silicon source temperature 1350”K, evaporation time 5 sec. (a) Clean Mo tip. (b)-(d) Progressive migration of silicon at 610°K. (e)--(f) Further migration at 640°K. (g) Fully covered tip. Note the dark silicon deposits near the {211) planes on the left hand side of the patterns (b) and (c).

ADSORPTION

OF

Si

ON MO

IN FEM

161

Fig. 2. Surface diffusion of silicon on molybdenum. Silicon source temperature 1475”K, evaporation time 3 min. (a) Just after evaporation on tip of fig. la. (b) Tip heated to 540°K. (c)-(d) Progressive migration of silicon at 6OO’K.

162

G. VENKATACHALAM

AND

M. K. SINHA

Fig. 3. Surface diffusion of silicon on molybdenum. Silicon source temperature 1475”K, evaporation time 6 min. (a) Just after evaporation on tip of fig. la. (b)-(c) Same tip heated to 600’K. Note the boundary between the granular silicon and smooth layer.

and (110) planes and the (1 I 1) region nearest the evaporator was covered with silicon. The (11 I) region on the far side was also covered on further heating at the same temperature. Between 585-700°K diffusion with a sharp radial boundary occurred, figs. I b-If. The boundary moved almost radially toward the {loo> planes. It moved faster in the (21 l)-t(lOO) direction, figs. id, le. This particular feature is different from that of silicon surface diffusion on tungstenj). Formation of a well-defined boundary and subsequent migration depends significantly on the dose of silicon. When silicon was evaporated at 1250°K for 10 set, i.e. for very low dose, the boundary diffused quickly on heating, whereas when silicon was evaporated at 1475 “K for 60 set, i.e. for high dose, the boundary formed and moved with a sharp edge for a short distance, but then the rest of the migration was without a well defined boundary. For higher doses a different type of boundary diffusion occurs. Figs. 2a--2d show the diffusion when silicon was evaporated for

ADSORPTION

OF

163

si ON MO IN FEM

3 min at 1475°K. The left hand side region of the pattern

where silicon was

first incident appears dark compared to the rest of the pattern. As the tip is heated from 540°K to 600”K, a boundary is formed and moves to the right hand side, figs. 2b-2d. For still higher doses, 6 min evaporation at 1475”K, silicon formed a large number of bright clusters on a part of tip surface, fig. 3a. When the tip was heated between 580-620”K, the patterns in figs. 3b and 3c resulted. The activation energy for surface diffusion of silicon in (211) --f (100) direction was measured by determining the temperature dependence of spreading rates. In these experiments silicon was evaporated at 1475 “K for 15 set and patterns similar to figs. 1b-le were obtained. Fig. 4 shows a plot of time vs 1/T.The activation energy, calculated from the slope of the plot was 50.9 + 3 kcal/mole.

100 15.5

16.0

16.5

10% (TEMPERATURE Fig. 4.

I70 INOK)

Plot of log t versus l/T for surface diffusion, activation energy = 50.9 k 3 kcal/mole.

3.3. ADSORPTION It was observed by Swenson and Sinha5) that a tungsten tip covered with silicon and annealed at higher temperatures gave a stable silicon enriched surface phase, which was characterized by the appearance of new planes. Molybdenum was found to behave similarly in some respects.

164

G. VENKATACHALAM

AND

M. K. SINHA

When silicon was evaporated at 1350°K for 2 set on the tip at room temperature and the tip annealed at 6lO”K, three dark planes appeared around each of the { 1 I I> planes, fig. Ic. These were identified as (433) planes. These planes vanish on continued heating at the same temperature. If silicon was evaporated for a minimum time of 6 min at 1475 “K and the tip annealed at lOOO”K, a pattern as shown in fig. 5 was obtained. This pattern is characterized by the appearance of four symmetric (411) planes and two { 111) planes.

Fig. 5.

Field emission pattern of silicon enriched surface phase.

If the molybdenum tip was kept at above room temperatures when silicon is evaporated, patterns similar to fig. 5 can be obtained immediately. The pattern of fig. 5 is obtained if the tip was kept at some temperature between 500-850°K during silicon evaporation and if the evaporation temperature was 1475°K and the evaporation time was from 30 set to 6 min. The { I1 I> planes of this pattern disappear about 1750°K and the (41 l} planes vanish at 1920°K. The average work function of this surface was determined from a F-N plot and was 4.07 eV. 3.4. THERMAL DESORPTION The activation energy of thermal desorption can be determined by measuring the desorption rates as a function of temperature. In some cases the emission current can be taken as a measure of the adsorbate coverage. However, if thermal desorption is not uniform over the whole tip this method becomes questionable. In the case of silicon on molybdenum desorption does occur in a non-uniform way. For example, if the surface as shown in fig. 5 is annealed at a higher temperature, first the dark { 111) planes disappear, i.e. the adsorbed silicon desorbs and then silicon from the (411) planes desorbs. When the tip was annealed, visual observations suggested that

ADSORPTION

OF

si

ON

as the (41 l} and { 11 l} planes gradually

MO IN

165

FEM

vanished,

silicon desorbed

instead

of spreading over the surface. We used this observation to estimate the activation energies of thermal desorption of silicon from these planes. In these experiments silicon was evaporated for one minute at 1475°K on the tip which was kept at 850 “K. After evaporation the picture was checked at room temperature to ensure similar appearance in all the experiments. The field was then reduced to zero and the tip was heated at a chosen temperature until the chosen plane disappeared. Arrhenius type plots of time versus l/T were made for each plane and from their slopes, the activation energies, E des, were determined

2000

by the method

Edes for (111) plane

of least squares.

I-

IO00

400 z f ," 200 ii Y F

100

40

20

d

IO 5.2

1

5.4

0

I

5.6

5.6 107~

6!0

(TEMPERATURE

6:2

6!4

~I~K)

Fig. 6. Plot of log t versus I/?” for thermal desorption of silicon from molybdenum. (1) Thermal desorption from (41 l), Edes = 123.9 i 3 kcal/mole. (2) Thermal desorption from (11 l), Edes = 69.3f 1 kcal/mole.

was 69.3 4 1 kcal/mole and for (411) plane it was 123.9 + 3 kcal/mole. Fig. 6 shows the plots of time versus l/T for the two planes. It should be pointed out that these activation energies may not represent the true desorption energies. Surface diffusion of silicon to other crystal planes, which must be occurring simultaneously, will affect the results.

166

G. VENKATACHALAM

AND

M. K. SINHA

4. Discussion 4.1. WORK FUNCTION As the dose of silicon was increased, the work function decreased with a simultaneous reduction in the total emission current. The current reached a minimum and then there was an increase although it was always less than the emission current for a clean tip. Such a simultaneous reduction in the slope of the F-N plot and total emission current was also observed when nitrogen was adsorbed on tungsten”“). The rate of arrival of silicon atoms at the tip can be calculated approximately from the geometry of the tip and the evaporator and the vapor pressure of silicon at the temperature of evaporation. A calculation taking into account that about one-third of the area of the tip is covered during evaporation and this then spreads over the whole surface, a monolayer of coverage gives about I5 set of evaporation at the evaporation temperature of 1475°K. A monolayer of silicon on tungsten was taken as containing 5 x 1Or4 atoms/cm2. The change in work function of silicon covered tips of tungsten was studied by NeumannI), Collinss), and Swenson21). Neumann and Collins both reported that the work function increased as a result of silicon adsorption, whereas Swenson observed a small decrease. However, the work function of a germanium”) covered tungsten tip remained constant and the emission current decreased with increasing coverage. Duke and Alferieffsa) suggested that the effect of an adsorbate is to modify the shape of the potential barrier seen by the tunneling electrons and in the case of a neutral adsorbate their model provides a simultaneous reduction of the work function and emission current. In our experiments the emission current first reached a minimum approximately at monolayer coverage and then there was a small increase at higher doses of silicon. This increase may be the result of local field enhancement. This is supported by the granular structure of the silicon deposit on the tip. The preexponential part of the F-N term decreases on silicon adsorption. This effect was observed in several other cases such as, silicon on W3), germanium on W6) and also when gases are adsorbed on tungsten. The change in the preexponential term cannot always be associated with changes in the work function, nor can it be wholly ascribed to a decrease in effective emitting area. The reduction, in part, can be ascribed to a reduction in the transmission probability as a result of the additional barrier of the silicon layer. 4.2. SURFACE DIFFUSION Surface

diffusion

of silicon

thin films on molybdenum

was found

to be

ADSORPTIONOF

similar to that on tungstens”). the [ 11 I] zones connecting

si

ON

MO IN FEM

At 565 “K, boundary

free diffusion

167

occurs along

the (110) plane to the {2 11} planes. Like tungsten,

MO atoms are most closely packed in these directions and therefore provide low impedance paths for diffusing atoms. Once this diffusion is complete, silicon diffuses with a sharp boundary towards the (100) planes, figs. lb-lf. Again, the explanation is similar to that of silicon on W. The high value obtained for the activation energy (50.913 kcal/mole) indicates that the diffusion involves chemisorbed atoms. The activation energy is higher than that of silicon on W (35 kcal/mole) and thus the silicon-molybdenum atom bond is stronger. The observed value of the activation energy is consistent with the fact that it should be less than or equal to the activation energy of surface self diffusion of molybdenum. This is 60 kcal/mole. When larger doses of silicon are evaporated, a diffusion with sharp vertical boundary is observed, figs. 2a-2d. The silicon film still appears smooth indicating the uniformity of the impinging silicon atom beam and the absence of the surface mobility of silicon atoms. This boundary is curved around the (21 l} planes. This is again the result of the low impedance paths along the line joining (1 IO} to (21 l} planes. Silicon migrates faster in these directions. With still higher dose of silicon, the impinging atoms form clusters, fig. 3a. Similar clustering was observed by Collinss) for silicon on tungsten and by Kim et a1.23) for germanium on tungsten. The clusters or the crystallites appear bright due to local field enhancement. The granular appearance indicates that surface diffusion occurred during deposition although the tip was at room temperature. The granules are observed only on the side of the tip facing the evaporator, thus the surface migration of silicon atoms is confined to quite short distances and the migration takes place over a smooth layer of silicon. The bond between the crystallites and the substrate is weaker than that of the thin layers and when the tip is heated the crystallites melt and “rolling carpet” like diffusion occurs, figs. 3b and 3c. The diffusion occurs with two boundaries, one bright and the other dark, moving in opposite directions. The crystallites of silicon, on heating, form a smooth layer and the boundary between the granular layer and the smooth layer moves to the left and the thick smoother layer migrates towards the right. 4.3. ADSORPTION AND

THERMALDESORPTION

Adsorption of silicon on molybdenum is anisotropic. In fig. If we see that the regions between the (100) and (21 I} planes show a granular appearance and the (11 I) regions appear smooth. This behavior is different from that of silicon on tungstens8), where for low coverage the surface appeared smooth. At low doses, when the tip is annealed, silicon collects around the (433)

168

G.

VENKATACHALAM

AND M. K. SINHA

planes, fig. lc. A similar observation was reported by Muellerz4) when carbon was adsorbed on tungsten. A surface phase, fig. 5, stable up to 1750°K always appeared when the evaporation times were 30-60 set and the tip was kept at 850°K. This pattern was characterized by the appearance of (41 l} and { 1111 planes and was similar to that observed for silicon on tungste+). When larger amounts of silicon were evaporated with the tip both at room temperature and above room temperatures, these planes appeared only after annealing the tip to higher temperatures. This may be associated with the presence of larger amounts of silicon than required for the formation of the planes. Noimann 2) also observed the creation of new planes when silicon is adsorbed on tungsten and he suggests the following two reasons: (a) Presence of an adsorbate on the surface changes the surface energy of the substrate and as a result, a new equilibrium phase with other planes can be created. (b) Since silicon diffuses in the bulk of tungsten, tungsten silicides with a different crystal structure from the structure of tungsten may form. Noimann ruled out the second possibility because he did not observe diffusion of silicon into tungsten. Swenson and Sinhalo) did observe such a diffusion and according to them a tungsten silicide is probably formed. In the present case the possibility of the formation of a molybdenum silicide cannot be ruled out. Existence of MoSi, Mo,Si, and MoSi, has been established by silicides can be X-ray and microscopic investigations 12925) and molybdenum prepared by direct combination of molybdenum and silicon12). Melting pointses) of Mo,Si=2050”C, Mo,Si,~2100”C, and MoSi,= ~2030°C are higher than the temperatures at which the new planes vanish, but the melting points are for bulk material and for surface this is expected to be less. The solubility of silicon in molybdenum is 0.8 wt% at 1098”Kz5). So although

silicon can diffuse into molybdenum,

we have not observed

it in

our investigation. 5. Conclusions (1) The work function of a silicon covered molybdenum field emitter decreases with a simultaneous reduction in the total field emission current. This suggests the resonance tunnelingaa) of the field emitted electrons. (2) With low coverages, boundary free surface diffusion occurs along the [l 1l] zone at 565 “K and diffusion with a sharp boundary occurs between 585-700°K. The activation energy for surface diffusion in the (211) + (100) direction is 50.9 + 3 kcal/mole. (3) Adsorption of silicon on molybdenum field emitters at and above room temperatures is anisotropic. When silicon is evaporated onto an emitter and the emitter then annealed at higher temperatures, new crystal planes develop which are stable up to 1750-1920°K. This suggests possible formation of a

ADSORPTION

OF

Si ON MO IN

169

FEM

molybdenum silicide. Activation energy of thermal desorption and (411) planes is 63.3 + 1 and 123.9 kcal/mole respectively.

from (111)

Acknowledgements The authors thank Charanjit this investigation.

S. Bhatia for help in some experiments

during

References 1) 2) 3) 4) 5)

H. Neumann, Ann. Physik (Leipzig) 18 (I966) 145. Kh. Noimann, Soviet Phys-Solid State 7 (1966) 1624. R. A. Collins, Surface Sci. 26 (1971) 624. R. A. Collins, Surface Sci. 40 (1973) 470. (a) 0. F. Swenson and M. K. Sinha, J. Vacuum Sci. Technol. 9 (1972) 942. (b) M. K. Sinha, 0. F. Swenson and G. Venkatachalam, Surface Sci. 33 (1972) 414. 6) I. L. Sokol’skaya and N. V. Mileshkina, Soviet Phys. Solid State 6 (1964) 1401. 7) A. J. Melmed, J. Appl. Phys. 36 (1965) 3585. 8) H. M. Montagu-Pollock, T. N. Rhodin and M. J. Southon, Surface Sci. 12 (1968) 1. 9) G. D. W. Smith and J. S. Anderson, Surface Sci. 24 (1971) 459. 10) M. K. Sinha and 0. F. Swenson, Appl. Phys. Letters 19 (1971) 493. 11) P. Schwarzkopf and R. Kieffer, Refractory Hard Metals (MacMillan, New York, 1953). 12) Climax Molybdenum Company Bulletin Cdb-6A, May 1963. 13) E. C. Cooper, M. S. Thesis, Pennsylvania State Univ. 1956. 14) G. Venkatachalam and M. K. Sinha, Bull. Am. Phys. Sot. [2] 18 (1973) 56. 15) E. W. Miiller and T. T. Tsong, Field Ion Microscopy, Principles and Applications (Elsevier, Amsterdam, 1969) p. 119. 16) B. F. Kilgore and R. W. Roberts, Rev. Sci. Instr. 34 (1963) 11. 17) W. E. Forsythe and A. G. Worthing, Astrophys. J. 61(1925) 146. 18) R. H. Good and E. W. Miiller, in: Hundbuch der Physik, Vol. 21 (Springer, Heidelberg, 1956) p. 176. 19) R. Gomer, Field Emission and Field Ionization (Harvard Univ. Press, Cambridge, Mass., 1961). 20) T. A. Delchar and G. Ehrlich, J. Chem. Phys. 42 (1965) 2686. 21) 0. F. Swenson, M. S. Thesis, North Dakota State Univ. 1971. 22) C. B. Duke and M. E. Alferieff, J. Chem. Phys. 46 (1966) 923. 23) H. Kim, H. Araki and E. Sugata, Japan. J. Appl. Phys. 9 (1970) 1445. 24) Ref. 18, p. 214. 25) M. Hansen, Constitution of Binary Alloys, 2nd ed. (McGraw-Hill, New York, 1958).