Thin Solid Fi lms- Elsevier Sequoia S.A., Lausanne - Printed in Switzerland 261

THE ADSORPTION AND NUCLEATION OF SILVER ON TUNGSTEN (110)*

C. M. LO AND J. B. HUDSON

Materials Division, Rensselaer Polytechnic Institute, Troy, N. Y. (U.S.A.) (Received May 16, 1972)

A study has been made of the adsorption and nucleation behavior of silver vapor impinging on a clean tungsten (110) crystal face. The mass spectrometric molecular beam technique has been used to measure the thermal accommodation coefficient, the kinetics and thermodynamics of adsorption and the kinetics of the extended growth of crystalline silver. Two adsorbed phases were observed, having heats of desorption of 66 kcal/mol and 41 kcal/mol respectively. Extended growth occurred from the second adlayer phase without observable supersaturation. The deposit so formed was crystalline and epitaxial with the substrate.

INTRODUCTION

In recent years the adsorption and nucleation behavior of metal vapors on metal and semiconductor surfaces has been studied extensively. Several investigators have used the technique of mass spectrometric monitoring of the vapor flux desorbing from a surface which is being bombarded by a flux of thermal, neutral metal atoms. The first study of this sort was that of Hudson and Sandejasl for cadmium on polycrystalline tungsten. More recently, studies have been made by Nguyen e t al . 2 for indium on silicon (111), by Wagner and Voorhoeve a for cadmium on polycrystalline tungsten, and by Voorhoeve e t al . 4 for cadmium on various faces of germanium.

The adsorption and nucleation results reported in all these studies, while more or less consistent with one another, differ markedly from results obtained in previous studies of metal vapor nucleation on insulator or contaminated metal surfaces. The principal differences observed are the occurrence of multilayer adsorption prior to growth of the bulk phase, and the onset of condensation at small or apparently zero supersaturation. One significant difference between the vapor species used in the molecular beam studies and those used in many earlier studies is that in the molecular beam studies the vapor species has been, in all cases, a material of relatively low heat of vaporization and/or low melting point,

* Paper presented at the International Conference on Thin Films, " Application of Thin Films ", Venice, Italy, May 15-19, 1972; Paper 5.7.

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262 c . M . LO, J. B. HUDSON

whereas earlier studies were more inclined to the use of metals of relatively high binding energy and simple crystal structure, such as copper, silver or gold.

In an attempt to determine whether the results obtained in the molecular beam studies to date are limited to materials of low heat of vaporization or low melting point, we have carried out similar measurements for the case of silver vapor impinging on a clean tungsten (110) single-crystal surface. This system offers the combined advantages of a vapor species which is a relatively simple metal, with a high melting point (compared with cadmium or indium) and a well-characterized, metallic substrate. It was hoped that this combination would produce results more easily comparable with those obtained by other techniques than the previous mass spectrometric studies have been.

APPARATUS

The experimental system used in this study was similar in concept to that used by Hudson and Sandejas 1, with a few exceptions. The use of silver as the vapor species necessitated construction of a vapor source assembly capable of operation at temperatures up to 1500 °C. This was accomplished by containing the silver in a molybdenum crucible, welded shut, with an effusion orifice 2 mm in diameter. The cell was heated by electron bombardment.

The substrate used was a (110) oriented tungsten single-crystal ribbon, 25 mm long, 3 mm wide and 0.25 mm thick. The sample was heated resistively by d.c. A temperature programming and control system was developed which permitted the sample temperature to be held at a constant value between ambient and 1500 °C, to within _ 1 °C, or increased linearly from ambient to 1500 °C at sweep rates ranging from 20 °C/sec to 100 °C/sec. This arrangement permitted us to make adsorption measurements by the flash filament technique.

The system was also equipped for LEED studies, which permitted deter- mination of the orientation of the silver deposits formed, relative to that of the tungsten substrate.

EXPERIMENTAL MEASLrI~3~NTS

The measurements made in this study were for the most part similar to those performed previously by Hudson and Sandejas 1 and described in further detail by Hudson 5. They included measurement of the thermal accommodation coefficient for silver vapor on the clean tungsten surface, measurement of the mean stay time for adsorption by the adsorption transient technique, and measurement of the critical supersaturation for nucleation by measuring the change in desorption flux as the substrate was cooled slowly through the point of vapor-solid equili- brium.

In addition, we have studied the adsorption process by the flash filament technique. This technique has been discussed in detail by Redhead 6 and has been used in studies of metal vapor adsorption by Wagner and Voorhoeve a and Bauer and Poppa 7.

We have also used LEED measurements to verify the orientation and cleanli-

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ADSORPTION AND NUCLEATION OF Ag ON (110)W 263

ness of the substrate prior to our studies, and to determine the crystallinity and relative orientation of the silver deposits formed.

RESULTS AND DISCUSSION

The thermal accommodation coefficient for silver vapor impinging on the clean tungsten surface was observed to be unity. This is in agreement with all other studies of metal vapors impinging on clean metal or semiconductor sur- faces 1-3 and will not be discussed further.

Extensive studies have been made of the adsorption processes in the course of this work, using both the adsorption transient and the flash filament desorption techniques. The limitation placed on the length of this article precludes a complete discussion here. We will simply present the results which bear on the nucleation and growth process.

Adsorption transient curves were obtained for a wide range of impingement rates and substrate temperatures. These curves were essentially exponential in shape at high temperatures and showed increasing departure from exponentiality as the temperature was reduced, eventually being resolved into two exponential segments separated by a sharp break. This behavior is essentially the same as that observed for cadmium on tungsten 1' s and indicates that the appearance of multiple adsorbed phases is not limited to vapor species with a small heat of vaporization. Values of the mean stay time for adsorption, z a, calculated for both segments of the desorption transients, are shown in Fig. 1. Solid lines on this figure were determined from a least-squares fit of the data to the equation

za = Zo exp (AH/RT)

Calculated values of AH and z o are shown in the figure. The behavior observed in the flash filament desorption studies was consis-

tent with that described above for the adsorption transient measurements. For short times of exposure of the substrate to the silver flux, the desorption curves showed a single peak at a desorption temperature of 891 °C. At longer exposure times, this peak saturated in area and a second peak appeared at 790 °C. The saturation coverage associated with the high temperature peak was 6 x l014 atom/cm 2, or about 0.44 monolayers. The values of AH associated with the two peaks were calculated by means of the equation suggested by Redhead 6, using the values of Zo determined from the adsorption transient studies. Values so calculated were in excellent agreement with those determined from the transient measurements. Analysis of the shape of the flash desorption curves indicated that both phases desorb by a first order process.

Attempts to observe a critical supersaturation for nucleation, made by slowly cooling the substrate through the vapor-solid equilibrium temperature at a constant impinging flux, yielded a negative result. That is, in no case was a desorption signal observed that was greater than that associated with the equilib- rium evaporation rate of bulk crystalline silver at the existing substrate tempera- ture. In fact, the desorption rate curves appeared to turn down at a temperature higher than that associated with the vapor-solid equilibrium. A typical example

Thin Solid Films, 12 (1972) 261-266

264 c.M. LO, J. ]}. m.rDSON

100

8

~T igh t l y bound phase

o Loosely bound phase

AHd~=65.89 +- l l l kcal /mole _ _ ~o "2.93 x 10 "~J sec

A

O

o o O

~ o - 2 ~ 3 x 1 G g ~ c

0.01 ~ 0.75 0 .80 0.85 0.90 0.95 1.00 1.05

( I/kl'lu b *K) x 103

Fig. 1. Mean stay time for adsorption of silver on tungsten (110).

of this behavior is shown in Fig. 2. It is in contrast to the finite critical super- saturations observed in previous studies of metal vapors condensing on insulating or contaminated metal surfaces 8-11, but is similar to results reported more recently for metals condensing on clean metal or semiconductor surfaces 2-4. It was also observed in the present study that as the equilibrium point was approached, the rate at which the desorption signal approached its steady state value became very slow. This behavior has also been observed by other workers 12, and indicates that either extremely heavy multilayer adsorption is taking place just prior to bulk condensation, or that material is being lost from the area sampled by the mass spectrometer by surface diffusion. Present techniques are unable to resolve this question.

Adsorption transient measurements, made at temperatures just above the temperature at which the desorption rate began to decrease, indicated that the tightly bound adsorbed phase was always filled to saturation when condensation began, and that the loosely bound phase was filled to the extent of about one monolayer. Again, similar behavior has been observed in other systems.

Measurements of the material accommodation coefficient, made during

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ADSORPTION AND NUCLEATION OF Ag ON (110)W 265

4-

,f

72o 7~o Ao 7;0 ~x~ ~ 8~

T~ b (°C)

Fig. 2. gesorption rate vs. temperature at the onset of extended growth.

extended growth by comparing the evaporating flux from the crystal deposit exposed to a finite impingement rate with that from a freely evaporating crystal deposit, indicated complete accommodation. This is in contrast to the behavior observed for cadmium growing on tungsten 3, but in accord with observations made for indium growing on silicon t2.

LEED measurements of the bulk silver deposits indicated that they were essentially single crystals, epitaxial to the substrate, with Ag(lll)IIW(ll0) and Ag[l[0]ll W[Oll].

CONCLUSION

The sequence of steps occurring in the formation of an epitaxial silver deposit on a clean tungsten (110) surface is as follows. Impinging silver atoms are initially accommodated into an adsorbed phase having a heat of desorption of 66 kcal/mol. This phase saturates at a coverage of 6 x 101, atom/cm 2. Subsequently, a second adsorbed phase forms which has a heat of desorption of 41 kcal/mol. When this phase reaches a coverage of approximately one monolayer, a process leading to the extended growth of an epitaxiaUy oriented bulk crystalline deposit sets in. This process takes place without requiring an observable critical super- saturation. The behavior is completely analogous to that observed in earlier studies 1-4 using vapor species which had much lower heats of vaporization

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266 c . M . LO, J. B. HUDSON

and/or melting points. Thus this sequence of events appears to be the general case for metal vapors impinging on clean metal or semiconductor surfaces. Unfortunately, at this time we lack a comprehensive theory to explain the observed behavior.

ACKNOWLEDGEMENTS

This work was supported by the National Science Foundation of the U.S.A., under grants number GK-1731 and GK-13336. Results presented are taken from a thesis presented by C.M. Lo to the Materials Division, Rensselaer Polytechnic Institute, in partial fulfilment of the requirements for the Ph.D. degree in Materials Science,

REFERENCES

1 J.B. Hudson and J. S. Sandejas, J. Vac. ScL TechnoL, 4 (1967) 230. 2 T.T.A. Nguyen, R. C. Cinti and B. K. Chakraverty, Proc. Intern. Conf. Crystal Growth, Marseille,

July 1971. 3 R.S. Wagner and R. J. H. Voorhoeve, J. Appl. Phys., 42 (1971) 3948. 4 R.J .H. Voorhoeve, R. S. Wagner and J. N. Carides, 1971 Intern. Conf. on Solid Surfaces, Boston,

October 1971. 5 J.B. Hudson, J. Vac. Sci. Technol., 7 (1970) 53. 6 P.A. Redhead, Vacuum, 12 (1962) 203. 7 E. Bauer and H. Poppa, Personal communication. 8 J.B. Hudson, J. Chem. Phys., 36 (1962) 887. 9 B.E. Kressner and J. B. Hudson, J. Chem. Phys., 46 (1967) 1740.

10 A.A. Kinawi and J. B. Hudson, J. Vac. Sci. TechnoL, 6 (1969) 68. 11 J.B. Hudson and J. S. Sandejas, Surface Sci., 15 (1969) 257. 12 T .T .A. Nguyen, Personal communication.

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