Adsorption, nucleation, and growth of cadmium on tungsten covered with oxygen or hydrogen
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T H E present s t a t e of affairs in the field of vaporphase heterogeneous nucleation is characterized by the availability of much theoretical work, but t h e r e is still a paucity of experimental results, l'e E s p e cially r a r e are experiments performed under " c l e a n " conditions, and with proper characterization of the most important variables, i . e . , supersaturation, temperature, and surface coverage. Without such experiments it is difficult to test the existlng theories and to properly a s s e s s the effects of adsorbed impurities. Our e a r l i e r studies were concerned w i t h the deposition of cadmium from the vapor onto c l e a n , p o l y crystalline tungsten ribbons.3 In that system, multil a y e r adsorption (3 to 4 monolayers) precedes growth of bulk cadmium. The heat of adsorption d e c r e a s e s , through five adsorption states, from 51 kcal per gatom to a value c l o s e to the heat of sublimation of bulk cadmium, 26.6 kcal per g - a t o m . Growth of bulk c a d m i u m o c c u r s by the evolution of epitaxial islands and r i d g e s from the adsorbed layer. No supersaturation is required for this process. It is well-known that adsorption of contaminants, such as found in the atmosphere or in residual high vacuum, changes the characteristics of tungsten surf a c e s drastically. Not unexpectedly therefore, the adsorption of metals on tungsten is affected. For copper and beryllium the adsorption lifetime on oxygenated tungsten is appreciably h i g h e r than on c l e a n tungsten. For iron and nickel, on the other hand, the lifetime decreases. 4,s Since adsorption of the condensing m e t a l p l a y s such a decisive role in the growth of c a d m i u m on tungsten, it is expected that the nucleation and growth behavior will be strongly influenced by adsorption of contaminants. Indeed, preadsorption of nitrogen has been found to d e c r e a s e the t h e r m a l accommodation coefficient, the lifetime and the adsorption energy, and to i n c r e a s e the c r i t i c a l supersaturation needed for growth.6'7 In the following, a study is presented of the effects of adsorbed hydrogen and oxygen on the adsorption, nucleation, growth, and epitaxy of c a d m i u m on tungR. J. H. VOORHOEVE and R. S. WAGNER are with Bell Telephone Laboratories, Inc., Murray Hill, N.J. Manuscript submitted March 31, 1971. METALLURGICAL TRANSACTIONS
sten. The kinetics of the process were studied w i t h a mass-spectrometric molecular-beam technique in ultra-high vacuum. The topography and morphology of the c a d m i u m growth were observed by scanning electron microscopy.
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EXPERIMENTAL
The apparatus, materials, and procedures used were the same as in our previous work.s Only a general outline will be given here. In the center of an ultra-high v a c u u m chamber a 25~m thick polycrystalline tungsten ribbon is mounted.
This t a r g e t is resistively heated and its temperature is m e a s u r e d and controlled by a tungsten/tungstenrhenium thermocouple welded to the b a c k . A r e s i s t ively heated,
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