ISITTAL STAGES OF OXIDATIOX OF G s n ~ i i l s ~ v ~ i
August, 1O( IO
1017
THE ISTTT_iT, STAGES OF OXID,ITION OF GERR/3[ASIU?rl 13-r. J. It. I,IGF,SZA Bpll Telephone Laboratories, Znc., Murray Hill,iVew Jersey Ileceived J a n u u r y 9, 1960
The initial stages of oxidation on a germanium surface freed of combined and adsorbed oxygen by reduction with carbon monoxide a t 600" was studied with a vacuum microbalance sensitive to 1/3aothof a GeOz layer. These experiments were done a t an oxygen pressure of 7 5 mm. and a t temperatures from 25 to 400". The oxidation laws were found to change with temperature :idtho extent of oxidation. The initial oxygen uptake is consistent with the viewpoint that the surfaces prepared by rediiction in carbon monoxide behave as if they are atomically clean. The first oxygen layer, as one oxygen per surface germmiurn atom, forms in less than a minute on the clean surface. At temperatures below 250' a second layer of one oxygen p'x surface germanium atom forms according to the logarithmic rate law in agreement with the results of Green and Kafalas. Subsequent oxiJat.ion beyond the second layer is not logarithmic but obeys a Mott and Cabrera thin filmtype equation. At temperatures about 250" the second layer forms in less than one minute. The Pubsequent oxidation rates beyond the second layer where only GeO2 is formed, are very slow and even a t 400' only 17.5 A. of GeO, formed in three hoiirs. Actintion energies for the logarithmic law and for the Rlott and Cabrera equations mere calculated.
Introduction Most oxidation studies have been concerned with the growth of oxide films on pre-existing thinner films and most theories of oxidat'ion treat such syst'ems.I The first stages of oxidation mould be concerned with the following problems: given an initial condit'ion a metal surface free of oxide and adsorbed gases, how would the first monolayer of oxide be formed? Would oxygen react wit,h such a surface to produce a monolayer of the oxide directly or would this first layer form by some less direct path:' The first experiment'al answer to these questions was provided by Lanyon and Trapnel12 in the course of a study of the oxygen uptake by ev:tporat.ed metal films. It was found that for a number of met,als t'he oxygen first. very rapidly formed a monolayer of one oxygen atom per surface metal atom followed by a very much slower formation of :t second monolayer of one to two oxygen atoms per original metal surface atom. The rate of formation for the second monolayer was found to be proportional to the square root of the oxygen pressure and the activation energy for the process varied 1ine:trlg with t'he uptake of this second monolayer.3 C:ibrera4 has suggested t,hat this latter F ~ O W uptake is due to the formation of a chemisorbed oxygen monolayer which when completed is able to transform by some nucleation mechanism t o the first monolayer of the stable oxide upon which the normal oxidation could proceed by the high field Mott and Cabrern5 mechanism. The oxygen uptake by clean germanium surfaces wliicli were formed by crushing single crystals ii! zmctio, lias been studied by Green, Kafnlas arid (1) K. Haii'ie, "Oxgdation von h1etallt.n und Metnlle,oierungen." Springer-Verlag, Berlin, l95G; T. B. Crinilry, "Chemistry of the Solid State, ( E d . IT. E . G:uner), Acadeniir Preis. Ken. I'ork, K. Y., 1955: 0 . K . Kohaactieu-ski m d B. E. Hopkins, "Oxidation of l l e t a l s and Alloys," ;iciidmiic Press, S e w Torl;, N. Y . . 1953. ( 2 ) hl. A . [-I. Litnyon a n d B. A l . JV. T r a p n r l l , PTOC.Rot,. SOC. (I,nndon), A227, XS7 (19.54). ( 3 ) T h e equation is 0 = B e x p [ - ( A & 6AEi)/RT], wtiereOis t h e r o v e i a g e a n d 1,he AE't are activation energies. I t is i i s i i a l l y referieii to as t h e Elov c h , or less coriiinoiily as t h e Roginsky-Zelilovicli eqiiation. I n its i r t r g r a t e d form the equation is 6 = 8 In ( t / t n 4- 1) where t is time. Hereafter t l i i j t y p e of Iiljtnke mill be called t h e logwithmic
+
1 LV... (4) N. Cahrera. "Semicondiirtor Surface Physics." University of Pennsylvania ''ress. Philadelphia, Penna., 19.57. p , 3:?7. ( 5 ) S. b'. Blott, Trrtns. F 7 r a d n y Soc., 43, 420 (1917); X. Cabrern, Phil. Mag.. 40 175 (1949); S . Cnbrrra and N. 1:. h l o t t , R e p . I'rog. i n Phus., l a , 163 (191!t).
Robinson6 and for ion bombarded and baked surfaces by W ~ l s k y . ~ Green found that an initial uptake occurred in less than six seconds to form a monolayer of one oxygen atom for every surface germaiiium atom. This was followed by a much slower rate forming a second layer proceeding according to the logarithmic lam-. Since the secoiid layer forms so s l o d y it was decided to increase the rate by increasing the temperature. By raising the temperature while holding the pressure constant the slowest process will be accelerated and any new phenomeiion will be observed in shorter periods of time. The experiments to be described were cnrried out with a vacuum microbalanccl and germanium ponder. This techniqiie was chosen because of its direct nature and sensitivity. Further, oxidations with rates which depend on pressure can be follomd most conveiiientlg by this technique. The precision of the experiments was 11300th of a monolayer as the amorphous form of germanium dioxide (6.47 X 15-11 g.,/cm.'). The germanium surface was freed of oxide by reducing the powder in carbon monoxide at temperatures irom 580 to 615". After outgassing the powder z a vacuo, oxygeii mas admitted to the powder a t a presswe of 75 mm. and a t temperatureq from 25 to 400". The reaction course n-as followed hy measuring the increase in weight of the powder. The powder area was measured by adsorbing nitrogen at 77 "Iparatedeterminations \\-ere made and were interspersrd n.ith one t o two rductions and oxidations Then 0.5Tnx is 421" or greater so that the reducwhich estahlishetl t h r inctc~pendi~ncc! of the arc's, on these tion rate is established by some soit of bulk dittreat meii t a. A typical exp,c.rinirnt was done in the following manner: fusion reaction a t least when more thaii a monoluper of oxide is involred. L4further clue to the the powvtier ww ontgnssed in a t,itaniuni gettercJd vaciium of from 3 X 1 0 - 7 t o lo-+ mm. at, n temperature of 530". reduction mechaiiim is obtained from a plot of the Carbon monoxide was :ttlmittecf to the poivder by nav of :+. total weight loss observed 011 reduction against copper foil filled trap kept a t 77' I
