Contains reports on one research project.Joint Services Electronics Program (Contract DAAB07-71-C-0300

Dabiri, A. E.

Stickney, Robert E.

Contains research objectives, summary of research and reports on one research project.Joint Services Electronics Programs (U. S. Army, U.S. Navy, and U.S. Air Force) under Contract DAAB07-71-C-030

Balooch, Medhi

DSpace@MIT

IX. PHYSICAL ELECTRONICS AND SURFACE PHYSICSAcademic and Research StaffProf. R. E. StickneyDr. M. J. CardilloDr. H. R. PatilGraduate StudentsP. C. Abbot R. II. IIlabert T. E. KenneyM. Balooch R. L. LevinRESEARCH OBJECTIVES AND SUMMARY OF RESEARCHThe general purpose of our research program is to study problems relating to theatomic, molecular, and electronic processes occurring at gas-solid interfaces.Examples of these processes are: thermionic emission, surface ionization, adsorp-tion, absorption, oxidation, catalysis by metals, electrode processes in electrical dis-charges, and the scattering of molecular beams from solid surfaces. Such processesare encountered in thermionic energy converters, electron tubes, ion propulsion engines,lamps, high-speed flight, and high-temperature nuclear reactors. At present, we areconcentrating on the following problems.1. Absorption, Diffusion, and Permeation of Hydrogen Isotopes in MetalsThe absorption, diffusion, and permeation of hydrogen isotopes in solids frequentlylead to technological problems because hydrogen absorption degrades the mechanical,electrical, and magnetic properties of many materials, and hydrogen permeation resultsin unwanted losses or gains of gas, especially in nuclear systems where tritium, theradioactive isotope of hydrogen, must be contained. As a consequence, there have beennumerous studies of the rates of absorption, desorption, and permeation for varioushydrogen-metal systems. The results indicate that, although the rate-controlling pro-cess generally is the diffusion of H atoms within the bulk of the material, there aremany instances in which the rate appears to be influenced by surface processes, suchas: (a) adsorption and dissociation (and the reverse processes, recombination anddesorption); and (b) penetration of Ii atoms from the surface into the interstices of thesolid (and the reverse process). The objective of this project is to establish a moredetailed description of the influence of surface processes on the rates of absorption,desorption, and permeation of hydrogen isotopes in different metals.We have formulated a simple theoretical model that leads to an approximate expres-sion for the permeation rate in terms of the kinetics of the surface processes and thediffusion process. The model appears to provide a reasonable explanation of existinganomalous data, i. e. , data that deviate from the predictions of the classical permeationequation in which diffusion is assumed to be the rate-controlling process. We now planto extend the analysis to the transient cases of absorption and desorption. In addition,we are constructing an experimental apparatus for determining the absorption, desorp-tion and permeation rates of hydrogen in various metals (e. g. , Nb and V) under condi-tions where surface processes are expected to be influential (for example, at lowpressures and temperatures, and with contaminated surfaces).This work is supported by the Joint Services Electronics Programs (U. S. Army,U.S. Navy, and U.S. Air Force) under Contract DAAB07-71-C-0300.QPR No. 108(IX. PHYSICAL ELECTRONICS AND SURFACE PHYSICS)2. Desorption of Hydrogen from MetalsThis study is motivated by the fact that the desorption of gases from solid surfacesis an essential step in a variety of processes, such as catalysis, metal purification, andpermeation. In the many cases where the gas exists predominantly as atoms on the sur-face but desorbs predominantly as molecules, a vital step in the desorption process isthe recombination of adsorbed atoms to form molecules. In an attempt to obtain a moredetailed understanding of the kinetics and energetics of recombination and desorption, wehave measured the spatial distributions of hydrogen molecules desorbed from Ni, Fe,Pt, Nb, Cu, and stainless steel. The results for all of these metals show that, if thesurface is unintentionally contaminated by the common bulk impurities (carbon and sul-fur), the spatial distributions are more narrow than the commonly assumed diffuse pat-tern, cos 0, where 0 is the angle of inspection measured from the surface normal. Thatis, the desorbed H 2 is concentrated in the vicinity of the surface normal to a greaterdegree than in the case of diffuse emission. Except for Cu, however, the spatial dis-tributions broaden and approach the diffuse pattern as the degree of surface contamina-tion is reduced by ion bombardment and monitored by Auger electron spectrometry. Cuappears to be an exceptional case because its spatial distribution is far from diffuse evenwhen the surface contamination is reduced below the limits of detection. We haverecently completed a more detailed investigation of Cu by using single crystals with sur-faces corresponding to different crystal planes, and the results show that the spatialdistribution depends on the crystallographic structure of the surfaces. To aid us indeveloping a theoretical interpretation of these data, we now plan to use molecular-beamtechniques to determine the adsorption probability for H2 on Cu crystals for varyingenergy and angle of incidence of the impinging molecules.3. Dynamics of the Collisions of Atoms and Molecules with Solid SurfacesOver the past eight years we have performed both analytical and experimental studiesof the nature of collisions between gas atoms (or molecules) and solid surfaces. We haveconcentrated primarily on studies of nonreactive collisions of noble gases and of N2 andCO with tungsten crystals, and the results illustrate the dependence of the collisiondynamics (e. g. , the energy and momentum transfer) on molecular mass, angle of inci-dence, interaction potential, temperature, crystallographic structure, and surface impu-rities. During the past year we have decided to change the direction of this programtoward reactive collisions, such as catalytic reactions and gas-solid reactions, and planshave been made to add a mass spectrometer to our molecular-beam apparatus. In thecoming year we propose to begin an investigation of several simple catalytic reactionsinvolving hydrogen (e. g. , isotope exchange, atomic recombination, molecular dissocia-tion) on metal surfaces whose structures and chemical composition will be characterizedby low-energy electron diffraction and Auger electron spectroscopy. These resultsshould be a valuable supplement to those obtained in our studies of hydrogen permeationand desorption.4. Other StudiesWe are now considering several other possible studies, including a fundamental studyof hydrogen embrittlement of iron and steels, measurements of adsorption isotherms bymeans of Auger electron spectroscopy, mass spectrometric measurements of high-temperature gas-solid reactions (for example, oxidation), and the implantation of ionsin solids.R. E. StickneyQPR No. 108(IX. PHYSICAL ELECTRONICS AND SURFACE PHYSICS)A. EXPERIMENTAL MEASUREMENTS OF THE SPATIAL DISTRIBUTIONOF H2 DESORBED FROM COPPER SINGLE CRYSTALSJoint Services Electronics Programs (Contract DAAB07-71-C-0300)M. Balooch, R. E. Stickney1. IntroductionIn a previous experimental investigation of the spatial distribution of H 2 moleculesdesorbed from single crystal surfaces [Ni (110) and Ni (111)] and polycrystalline sur-faces (Ni, Fe, Pt, Cu, Nb, and 304 stainless steel) Bradley et al. 1,2 observed that thedistributions are strongly dependent on surface composition (i. e., surface impurities).The measured spatial distributions could be described approximately by the form cosd 0,where 0 is the angle of inspection measured from the surface normal. These resultsindicated a basic difference between the surfaces of clean Cu and those of clean Ni, Fe,Pt, and Nb. Clean Cu desorbs H 2 nondiffusely (d 4), whereas the other metals havenondiffuse distribution for surfaces composed of more than one element (i. e., impuresurfaces), but the distributions become diffuse (dz 1) as the surfaces are cleaned.Because of this difference we have decided to investigate copper in greater detail byusing single crystals [Cu (110), Cu (100), and Cu (111)] to determine whether the spa-tial distribution depends on the crystallographic structure of the surface.2. Apparatus and ProceduresThe experimental apparatus is identical to that described previously.1 The experi-mental technique involves exposing one side of the Cu membrane to H 2 at approximatelyatmospheric pressure, while the other side is exposed to vacuum. Hydrogen atomsdiffuse through the membrane and recombine on the surface to form molecules thatdesorb into the evacuated chamber. The spatial distribution of the desorbed moleculesis measured by a conventional ionization gauge mounted on a rotatable shaft whose cen-ter line is tangential to the membrane surface at its center. The membrane is mountedin a manner that enables us to rotate it to various working stations, including an Augerelectron spectrometer for determining the surface composition, a rotatable ionizationgauge for measuring the spatial distributions, and an ion bombardment gun for surfacecleaning.a. Sample PreparationTo avoid the possible effect of crystal defects on the spatial distributions, it isdesirable to have smooth surfaces that have a minimum of defects. Conventionalmethods of cutting and polishing Cu single-crystal samples introduce dislocations to adepth of several millimeters below the cut surface.3 In principle, the crystal can beQPR No. 108(IX. PHYSICAL ELECTRONICS AND SURFACE PHYSICS)etched to remove the volume containing the introduced dislocations, but it is very dif-ficult to maintain a smooth surface of the desired orientation during such an etch.Young and Wilson 3 have shown that dislocations are not introduced into copper crys-tals that have been acid-sawed and acid-polished. Therefore we have adopted theirtechniques as refined by Ahfarn et al. 4The acid saw used for cutting the samples was built in Gordon McKay Laboratoriesat Harvard University. The cutting solution was 9 parts concentrated nitric acid to1 part water, and it was fed onto a reciprocating thread. We cut disks, 1. 5 mm thick,from single-crystal rods, 1. 27 cm in diameter (MARZ grade, Materials Research Cor-poration), that had three different orientations of the crystal axis [(100), (110), and(111)]. Each disk was electron beam welded at the end of a polycrystalline copper(Materials Research Corporation, MARZ grade) cylinder, 1. 27 cm OD and 0.317 cm ID,which was brazed to a stainless-steel tube. This tube was mounted on a rotatable gasmanifold and heated by radiation from a concentric tungsten coil, as reported else-where.IA method of chemical and electrochemical polishing was used 4 which allowsflat surfaces to be produced without introducing dislocations. Chemical clothpolishing was done in 2 stages. In the first stage, a solution was formed by mixing90 ml of concentrated hydrochloric acid with 10 ml of polyethylene glycol 400. Thismixture was saturated with cupric chloride and gaseous hydrogen chloride. One milli-liter of a solution of 100 mg of sodium 2-mercaptobenzimidazole 5-sulfonate in 10 mlof concentrated hydrochloric acid was then added to the resultant solution immediatelybefore use, and the solution was vigorously shaken.The specimens were polished on a piece of percale cotton sheet tightly stretchedover a rectangular glass plate. As suggested by Ahfarn et al. ,4 after cleaning the spec-imen with concentrated nitric acid the solution was poured on the cloth and spread uni-formly over the lap. Then the specimen was moved fairly rapidly backward and forwardin gentle contact with the lap, with its axis first rotated 200 clockwise and then 20' coun-terclockwise with respect to the direction of motion. After each 10 transverse motionsthe crystal was washed with a 10% solution of phosphoric acid containing 1 part in 2000by volume of a saturated aqueous solution of chromic acid and having a temperature of~50 0 C, then it was washed with distilled water and dried in a jet of dry nitrogen.The specimens were polished with this solution until there was no visible residualsurface roughness. At this point we started the second stage, and used the followingsolution: 200 ml of glacial acetic acid, 200 ml of concentrated hydrochloric acid,200 ml of orthophosphoric acid, 60 ml of polyethylene glycol 400, and a solution of100 mg of sodium 2-mercaptobenzimidazole 5-sulfonate in 10 ml of concentrated hydro-chloric acid. The mixture was saturated with gaseous hydrogen chloride. Before use,we added 10 ml of a saturated solution of cupric chloride in hydrochloric acid, saturatedQPR No. 108(IX. PHYSICAL ELECTRONICS AND SURFACE PHYSICS)with hydrogen chloride, to 50 ml of the solution. Polishing was done on a new lap and,as in the first stage, after 10 polishing motions the crystals were washed with warm10% phosphoric acid containing 1 part in 2000 by volume of chromic acid solution.This procedure resulted in very smooth surfaces for all three crystal orientations.Final polishing was accomplished electrochemically. We used MIodel 451 crystalfacing instrument (South Bay Technology, Inc.) with the following polishing solution:250 ml of 60% orthophosphoric acid, 10 gr of copper carbonate, and 1 ml of chromicacid, saturated solution in water.b. Surface CleaningThe predominant impurities on the copper single-crystal surfaces were S and C.We found that the most successful procedure for removing these impurities was to bom-bard the membrane surface with argon ions 5 while maintaining the crystals at the ele-vated temperature required for measurement of the spatial distribution (T~ 1100 0 K).-5The argon pressure was -5 X 10-5 Torr, and the ion-bombardment gun was adjusted toprovide -15 i[A/cm of 250 eV argon ions to the membrane.After several hours of ion bombardment, the crystal was heated (T 11000K) in-610 Torr of oxygen while a tungsten ribbon at -2400 0 K was placed directly in front ofthe membrane. The hot tungsten ribbon partially dissociated the 0)2, thereby providingthe atomic oxygen that was required to remove the carbon from the Cu surfaces.This procedure was repeated several times until C and S were reduced below thelimit of detection. Figure IX-1 shows the Auger spectrum of a copper (110) sampleafter cleaning.3. ResultsThe spatial distributions of 112 desorbed from the cleaned copper single crystals areshown in Fig. IX-2. Since the distributions were almost perfectly symmetric, we haveomitted the data for negative values of 0. Although the distributions for all three crys-tal surfaces broadened as the degree of surface contamination was reduced, none becamea diffuse distribution in the limit of a completely clean surface. The limiting distribu-tion appeared to be approximately characterized by d z 2. 7 for Cu (110), d z 4. 8 for Cu(100), and d z 6 for Cu (111). In the case of Cu (110), the distributions were measuredfor 3 azimuthal orientations (b = 00, 450, and 900, as defined in Fig. IX-3). We mea-sured the distributions for Cu (100) at . = 00 and 45' and for Cu (111) at 6 = 00 and 90 °The data are shown in Fig. IX-4, and we note that the differences among thespatial distributions for varying 4 are sufficiently small that they are within thelimits of our experimental uncertainty. We conclude that the influence of the azi-muthal angle on the spatial distribution is quite weak, at best, and perhapsnonexistent.QPR No. 108Fig. IX-l1.Auger spectrum for a nearly clean Cu(110) surface at 1100°K.Fig. IX-2.Spatial distributions of H2 derived fromnearly clean Cu (110), Cu (100), andCu (111) surfaces.1[,bo]-[,oo]() Cu (II0)(b) Cu (100)Fig. IX-3.Atomic models and azimuthal orientationmeasurements for Cu (110), Cu (100),and Cu (111).(c) Cu (III)QPR No. 108ENERGY (eV)8 (deg)I -- - rr0.8 -0.70.60.50.40.30.2 -20 408(deg)20 406 (deg)20 408 (deg)60 8060 8060 80Differences in spatial distributions of 4 on Cu (110),Cu (100), and Cu (111) surfaces.OCu (III)C : 1)0: .=90'0 0Fig. IX-4.QPR No. 10860 80(IX. PHYSICAL ELECTRONICS AND SURFACE PHYSICS)4. Concluding RemarksThe present results demonstrate that the spatial distribution of hydrogen desorbedfrom copper depends significantly on the crystallographic structure of the exposed sur-faces but does not vary significantly with the azimuthal orientation of the crystal sur-faces. It is likely the differences among the crystal surfaces result from the fact thatthe interaction potential of the H-Cu system depends on the crystal structure of the sur-face. In the model suggested by Van Willigen, the spatial and speed distributions dependcritically upon the height of an energy barrier that is assumed to separate the molecularand atomic adsorption states. In Goodman's model,7 the distributions depend stronglyon the difference between the heat of chemisorption and the heat of dissociation. We haveattempted to improve these models by adding the possible influence of the periodic vari-ation of the interaction energy over the surface, but since we have not yet been able toestablish a convincing model, we shall postpone our considerations of theoretical modelsuntil we obtain additional experimental data. We are now preparing to use molecular-beam techniques to study the probability of adsorption of H 2 on the same Cu crystalplanes. By varying the energy and angle of incidence of the impinging molecules andobserving the resulting change in the adsorption probability, we hope to obtain newinformation on the nature of the H-Cu interaction potential.We are extremely grateful to John Peech of Harvard University for his help incutting and polishing the crystals.References1. T. L. Bradley, A. E. Dabiri, and R. E. Stickney, Surface Sci. 29, 590 (1972).2. R. E. Stickney, R. L. Levin, and T. L. Bradley, Quarterly Progress Report No. 106,Research Laboratory of Electronics, M. I. T., July 15, 1972, pp. 29-40.3. F. W. Young, Jr., and T. R. Wilson, Rev. Sci. Instr. 32, 559 (1961).4. J. S. Ahfarn, Jr., J. P. Monaghan, Jr., and J. W. Mitchell, Rev. Sci. Instr. 41,1853 (1970).5. L. H. Jenkins and I1. F. Chung, Surface Sci. 24, 125 (1971).6. W. Van Willigen, Phys. Letters 28A, 80 (1968).7. F. O. Goodman, Surface Sci. 30, 525 (1972).QPR No. 108

Physical Electronics and Surface Physics

II. PHYSICAL ELECTRONICS AND SURFACE PHYSICSAcademic and Research StaffProf. R. E. StickneyDr. M. J. CardilloDr. H. R. PatilGraduate StudentsP. C. Abbott R. H. Habert T. E. KenneyM. Balooch R. L. LevinA. MEASUREMENTS OF ADSORPTION OF OXYGEN ON TUNGSTENAT LOW PRESSURE AND HIGH TEMPERATUREJoint Services Electronics Program (Contract DAAB07-71-C-0300)A. E. Dabiri, R. E. Stickney1. IntroductionThe interaction of oxygen with tungsten has been studied by many investigators1using a variety of techniques. It is encountered in numerous technological devices, forexample, incandescent and arc lamps, ionization gauges, electron tubes, thermionicenergy converters, ion propulsion engines, and high-temperature vacuum furnaces, andit is considered as a classical example of high-temperature oxidation. The chemicalreaction of oxygen with tungsten at high temperatures and low O2 pressures results inthe formation of volatile tungsten oxides (such as WO, WO 2 , WO 3, W 0 6 ) in addition toO and 02. This reaction, which was studied previously in our laboratory, is describedreasonably well by the quasi-equilibrium analysis proposed by Batty and Stickney, 2, 3but that analysis relies upon a semiquantitative estimate of the equilibration probabil-ity, O2' defined as the probability that an impinging 02 molecule will adsorb and beequilibrated at the tungsten surface rather than being scattered without equilibration.Batty and Stickney suggest that t 0 2 depends primarily on the fraction of the tungstensurface that is not covered by oxygen, but existing experimental data are not sufficientto verify this suggestion. Although the results of flash desorption studies 4 ' 5 providevaluable information on the coverage (i. e., number of oxygen atoms adsorbed per unitsurface area) of oxygen on tungsten as a function of temperature, the pressure rangeis not sufficient to enable us to determine the dependence of coverage on pressure.In an attempt to obtain a more complete understanding of the dependence of 0() oncoverage, we have used Auger electron spectroscopy (AES) to perform approximatemeasurements of the coverage of oxygen on a polycrystalline tungsten sample as a fune--9 -6tion of sample temperature (1200-2500°K) and 02 pressure (5 X 10 to 5 X 10 Torr).QPR No. 109(II. PHYSICAL ELECTtON-ICS AND SURIFACE PIHYSICS)We believe that this is the first demonstration that chemical analysis of solid surfacesby AES is possible even at temperatures as high as 2500'K.2. Experimental Apparatus and ProceduresThe apparatus is a conventional ultrahigh vacuum system equipped with an Augerelectron spectrometer (Physical Electronics Industries, Inc. cylindrical analyzer,\Model 10-243). In the present study, an electron beam current of 400 pA at 3000 eV issupplied to the sample fromnt a gun mounted for grazing incidence. The electronic cir-cuitry (30-kHlz analyzer frequency) is essentially identical to that described by Palmberget al.The tungsten sample is a U-shaped polycrystalline ribbon mounted on a manipulatorin such a manner that only the midpoint of the U is at the focal point of the AES unit.The sample is heated resistively by direct current, and the temperature at the midpointis measured with an optical pyrometer. The sample surface is cleaned by a combina-tion of heating in O, and flashing to 2500 K; this procedure successfully reduces theconcentrations of surface impurities (predominantly carbon) to a level that is below thelimit of detection of AES.Both a nude ionization gauge and a partial pressure analyzer are used to measurethe 0 2 pressure in an attempt to minimize the various possible errors associated with0, pressure measurement. 7 The desired steadv-state 0,) prere , po is attainedby adjusting both the inlet valve that controls the flow of research grade 0( into thechamber and the exhaust valve that controls the effective speed of the ion pump.There are 7 steps in the experimental procedure: (i) Clean the surface as describedabove; (ii) Set po at the desired value; (iii) Ilash the sample to 25000 K; (iv) ileducethe sample temperature, T, to the desired value, and record the Auger spectrum aftersteady-state conditions are attained (for t(nemperatures above 12000K, oxygen coverageattained a steady-state value in less than 4 min: at lower temperatures, however, the(,-W reaction rate is so slow that wex did not attempt to obtain completely steady-stateconditions); (v) Iteduce (or increase) T to anothe r value and again record the Augerspectrum after attaining steady-stat(, coinditions; (vi) liepeat the fifth step for varioustemperatures; (vii) iepeat the entire procedure for various values of pO in the range-9 -65 X 10 -5 X 10 Torr.It is assumed here that \, the coveLrage of oxygen on the tungsten surface, isdirectly proportional to S(, the peak-to-peak amplitude of the first derivative of the509-eV oxygen Auger peak. This assumption has proved accurate for a variety of sys-tems, and we expect that it is a valid approximation for the O-\W system if the coverageis sufficiently low (e. g. , less than a monolayer) and if the adsorbed 0 atoms do notpenetrate into the lattice as a result of surface reconstruction (oxidation). TheQPR No. 109(II. PHYSICAL ELECTRONICS AND SURFACE PHYSICS)experimental results of Musket and Ferrante 9 show that the shape and the width of theoxygen Auger peak at 509 eV do not vary significantly with coverage on the (110) face oftungsten.3. Experimental ResultsFigure II-1 shows the first-derivative Auger spectra that we obtained for 4 values-6of the sample temperature, T, at one level of 02 pressure, po = 1 X 10 Torr. The2spectra have been displaced by an arbitrary amount in the vertical direction to preventthe curves from overlapping. A most interesting feature of these results is that thePo =I xlO TorrFig. II-1.2400K Examples of Auger spectra recorded at various2050 temperatures of the tungsten sample and at a1400 constant 02 pressure, po 2 = 1 X 10-6 Torr.300 Electron beam: 400 1LA and 3000 eV. Modula-ztion: 30 kHz and 15 V peak-to-peak. For clarity,the curves have been displaced vertically toprevent overlapping. Arrows indicate the ex-pected positions of tungsten and oxygen peaksbased on Chang' s tabulations. 1 0W W W 00 200 400 600 800E, ELECTRON ENERGY (eV)signal-to-noise ratio does not appearto be degraded when the sample is heated to hightemperature. Notice that the peak-to-peak amplitude of the oxygen peak follows theexpected trend of decreasing with increasing T.From Auger spectra recorded for various choices of T and po 2 , we evaluated therelative change of SO, the peak-to-peak amplitude of the 509-eV oxygen peak. Theresults presented in Fig. II-2 are restricted to the range T > 12000K because it was dif-ficult to obtain reproducible data at lower temperatures where a surface oxide layerforms at a very slow rate. 4 ' 5 The data obtained at T > 1200°K were quite reproducibleand, in the few checks that we performed, did not exhibit hysteresis (i. e., the value ofS O measured for a particular choice of T and po 2 was essentially the same, regardlessof whether the preceding values of T and pO 2 were above or below the chosen values).In some cases we flashed the sample to 2500*K between each measurement at differentT and p 0 2, but the resulting values of S O were essentially identical to those obtainedQPR No. 109 31-(II. PHYSICAL ELECTRONICS AND SURFACE PHYSICS)when the sample was not flashed between measurements.In Fig. 11-2, at 12001K there is an anomalously large gap between the data for1 X 10- 8 Torr and 1 X 10.7 Torr. Although we do not have a definite explanation for thisgap, we can suggest several possibilities. At the lowest pressures, the rate at which0 Z molecules impinge upon the surface may be so low that (a) the steady-state coverage20p02(Torr)o 5xiO15 • I × 1 -815 - 1 x T 8__ ix10o-7 Fig. 11-2.S5x10-6 Dependence of SO, the peak-to-peakz 10 - amplitude of the 509-eV oxygen Augerpeak, on temperature of the tungsteno sample and on )2 pressure. Electronco 5 beam and modulation conditions as inFig. II-1.O1200 1400 1600 1800 2000 2200 2400T(OK)may be lowered significantly because the electron beam used for Auger spectroscopyproduces electron-stimulated desorption (ESD) of oxygen from tungsten;11 and/or (b) theallowed "equilibration period" (-5 min) may not be sufficient to ensure that steady-state conditions are fully attained. Existing data show that the ESD rate increasesmarkedly with increasing coverage, thereby providing a possible explanation of the-8observation (see Fig. II-2) that the gap betxween the data for 1 X 10 Torr and 1 X-710 Torr increases with increasing coverage. Since ESD data are not available for thehigh electron energies and high temperatures employed in this study, we can do no morethan provide a rough estimate of the magnitude of the ESD rate Rt relative to the oxygenimpingement rate Z expressed in terms of O atoms rather than 0, molecules.Assuming an electron beam current of I 400 IiA, a beam cross-sectional area of-2 2 15 2A 1 X 10 cm , a coverage of N 1 X 10 atoms/ cm , and an ESD cross section of-20 2 1Q 1. 5 X 10 cm (see \Iusket ), we obtain9-R e / Z  6 X 10-9 /PO2, (1)where po 2 is the 0 2 pressure in Torr. As shown bVy ladey and Yates,ll R e=( NI/e A,where E is the electronic charge. The very approximate relation (1) predicts that-8l e/Z 0. 6 at p = 1 X 10 Torr, and we would therefore expect that ESD would pro-duce a significant reduction in the coverage (especially because the sticking probabilityQPR No. 109(II. PHYSICAL ELECTRONICS AND SURFACE PHYSICS)is significantly less than unity at high coverages, and hence the adsorption rate is con-siderably smaller than the impingement rate Z). Because of these uncertainties in thedata obtained at the lowest pressures and temperatures, our discussion favors the datafor higher pressures and temperatures.To obtain a rough evaluation of the constant of proportionality between S O and N, weshall use the results of Ptushinskii and Chuikov4 to estimate the coverage at one partic-ular choice of T and po . At 1600'K and 1 X 10 7 Torr their data 1 3 indicate that2IN 1. 1 X 1015 oxygen atoms/cmrn (with all O atoms on the surface counted, regardlessof their molecular state of adsorption). Since S O  9 for the same state in Fig. 1I-2,the resulting estimated relation isN - 1.2 X 1014 S . (2)We emphasize that this relation is very approximate because of various uncertainties;furthermore, a slightly different proportionality constant is obtained if we select a dif-ferent temperature for fitting our data to those of Ptushinskii and Chuikov.20 I P02 (Torr)• X1-815 A x10- 6_JZ lig. II-3.10 Extension of Fig. 11-2 to lower tem-z peratures. Data below 1200"K do notcorrespond to steady-state conditionsand may be influenced by electron-05 - stimulated desorption.U)0300 700 1100 1500 1900 2300 2700T(oK)Although the data taken below 1200'K are inaccurate because steady-state conditions-6were not fully obtained, Fig. II-3 is included, since the results for pO = 1 X 10 Torr2exhibit an interesting maximum at -1100 ° K. This maximum is consistent with resultsbased on flash desorption measurements, 4 , 5 and it is suspected to be a consequence ofthe fact that the surface oxidation rate is highest in the vicinity of 1100K. The absence-8of a maximum in the curve for p 2 = 1 X 10 Torr is most likely to be associated withthe problems just discussed.QPR No. 109(II. PHYSICAL ELECTRIONICS AND SUIFACE PHYSICS)4. DiscussionA principal objective of' the present study was to determine whether 0, the equil-2ibration probability defined by Batty and Stickney, depends primarily on coverage N,as was previously suggested. Since they did not have data on N, they adopted an expres-sion given in terms of T (0K) and p,() (Torr):14-0. 3S 8O(p) exp(- 1 8400/J). (3)(Note: Since values of 2 greater than unity are absurd, we shall assume 2 1when Eq. 3 yields 50 > 1.) \We have used this (equation to construct FiL. II-4 in thefollowing manner. Each data point in F1ig. II-2 represents the value of S) correspondingto particular choices of T and po2. By using Eq. 3 to calculate O9 for the same Tand P() we obtain the values of LCO corresponding to various values of S . The resultsare shown in Fig. II-4. If O d(epe(nds primarily on the coverage N and N is directlyproportional to SO , then the points plotted in Fig. II-4 should form a single curve cor-responding to the functional relationship (O2(N). Therefore we have attempted to drawa smooth curve through the points, but ignored the unreliable points for 5 X 10 - 9 and1 X 10 - 8 Torr in the range S > 5. The scatter is sufficiently large that these resultsprovide only marginal support of the suggestion that %() depends primarily on cov-erage.0 p0o (Torr)j 0o o 5x10 9M * 1x 10-< -0.8 -75o F ig. 11-4.z 0.6 A c 5x1O5le I lationship between the equilibration2 probability and the oxygen coverafe(assumed to be directly proportional to0.2 -o o *0 00 2 4 6 8 10 12 14So , OXYGEN SIGNALSince the results are not conclusive, we shall not include a detailed description ofour attempts to find a model that would yield a -N relationship in agreement with theQ(PI{ No. 109(II. PHYSICAL ELECTRONICS AND SURFACE PHYSICS)results shown in Fig. 11-4. The simplest forms of the popular models suggested byRobertsl 5 and by IKisliukl 6 do not agree closely over the entire range with the presentresults.In view of experimental results obtained previously in our laboratory 17, 18e wouldexpect that the present data (Fig. II-2) could be correlated in terms of the parameterT/T , where T is the sample temperature and T is the saturation temperature cor-responding to the oxygen pressure pO in our experiments (that is, Tit is the temper-ature at which the equilibrium vapor pressure of liquid oxygen is equal to po)Engelmaier and Sticknev 1 7 adopted the following relationship between T and pO basedon available vapor-pressure data for 09:(4)486. 45log 0 P = 9. 25 486.10 TUsing this equation, we have transformed the data in Fig.Notice that T/T can be used very successfully to correlatei p - 9  -8unreliable points for 5 X 10 and X 10 Tor in the range15z0z 08)cII1-2 to obtain Fig. 11-5.the data if we ignore theSO 5.) 30 40 50 60 70 80 90T/TwRFig. 11-5. Correlation of theparameter T / i.oxygen cove(rage data by means of theWe have also attempted to describe the data in Fig. II-2 by means of kinetic models.Since the kinetics of the O-\V interaction appear to be extremely complex, we consideronly the low-coverage region where the predominant desorption product is atomic oxy-1gen.-5gen. Under steady-state conditions, the rates of adsorption and desorption may beQPR No. 109pO2(Torr)o 5x0l-9A 0 I xlOSa x I10O- 6a - 5x10- 60 0o04(II. PHYSICAL ELECTRONICS AND SUIRFACE PHYSICS)equated to give-1/22O%) p(21TmO kT )22 2= VN exp(-E/ T),where m O 2 is the molecular mass of 0 2 , k is Boltzmann's constant, T' is the temper-ature of the 02 gas (-300'K), v is the frequency factor in the assumed first-order formof the desorption rate, E is the desorption energy for O on W, and T is the temper-ature of the tungsten sample. Under the assumption that v is a constant and 0 is afunction of coverage alone, partial differentiation of the logarithm of Eq. 5 leads to[d(ln po 2 )/ 3(1 / T)]N = -E/k. (6)Therefore, by replotting the data in Fig. 11-2 in the form in po 2 vs 1/T for variouschoices of N (choices of SO), we may evaluate E(N) from the slopes of the lines of con-stant coverage. This has been done in Fig. 11-6, where for convenience we haveexpressed the coverage in terms of 0, the fractional coverage, defined by utilizingEq. 2 together with the assumption that a monolayer contains 1. 2 X 1015 atoms/cm-2(as suggested by King et al. 5). That is,T (K)2500 2000 1500 12503 4 5 6 7 8 9104 /T (OK-')Fig. II-6.Pressure-temperature plots of data inFig. II-2 for various values of cover-age. For convenience, the coverageis described in terms of the fractionalcoverage 0, based on the assumptionthat 0 = 1 corresponds to N z 1. 2 X1015 atoms/cn-210 atoms/cm .0 0.2 0.4 0.6 0.8 1.09, FRACTIONAL COVERAGEFig. 11-7.Variation of the desorption energywith coverage, as estimated fromthe slopes of the constant-coveragelines in Fig. 11-6. Bars indicatethe estimated uncertainty in deter-mining the slopes.QPRI No. 109(II. PHYSICAL ELECTRONICS AND SURFACE PHYSICS)15S= N/1. 2 X 10 =0. 1 SO ' (7)From the slopes of the lines in Fig. 11-6, we have evaluated E vs 0 in Fig. 11-7.Notice that extrapolation of E to the limit of zero coverage yields -140 kcal/mole(-6. 1 eV), which is consistent with the values obtained by other investigators using4, 5, 17-20different techniques.5, 10 l 1 1 12p2 (Torr) -Fig. II-8.0 5xlO- 9  10SC ompari son of a simple, semiempiricalo 5x 106 -8 model (solid curves) with experimental data< Eo Z o age on T and pO0 co from Fig. 11-2 on the dependence of cover-age on T and Bars on the curve forU0 20 po 2  1 X 10- 7 Torr are included to illus-2nc2 trate the effects of uncertainties associatedo500 700 1900 200 2300 2500 with the desorption energy (Fig. 11-7).T, TEMPERATURE ('K)With these data on E vs coverage and with Eq. 3 (Batty and Stickney s approximateexpression for ()), Eq. 5 provides a relationship between N, T, and po in terms ofonly one adjustable parameter, v. This relationship is plotted in Fig. II-8 for13 -1v = 4 X 10 s , which was selected because it leads to the best overall agreement ofthe curves with the experimental data. The magnitude of v appears to be reasonablefor a frequency factor in a first-order desorption model. In view of the many assump-tions and uncertainties associated with both the model and the experimental data, theagreement exhibited in Fig. II-8 is about as good as we could hope for. The curveshave not been extended above S > 6 because the model is invalid at higher coverageswhere the desorption rates for oxides (WO,, W"XO 3) are no longer negligible.References1. For example, see the references listed by B. McCarroll, J. Chem. Phys. 46, 863(1967) and by H. G. Musket, J. Less-Common Metals 22, 175 (1970).2. J. C. Batty and R. E. Stickney, J. Chem. Phys. 51, 4475 (1969).3. J. C. Batty and R1. E. Stickney, Oxidation of Metals 3, 331 (1971).4. Y. G. Ptushinskii and B. A. Chuikov, Surface Sci. 6, 42 (1967); 7, 90 (1967).5. D. A. King, T. E. Madey, and J. T. Yates, Jr., J. Chem. Phys. 55, 3236 (1971).6. P. W. Palmberg, C. K. Bohn, and J. C. Tracy, Appl. Phys. Letters 15, 254(1969).QPR No. 109(II. PHYSICAL ELECTRONICS AND SURFACE PHYSICS)7. P. A. Redhead, Vacuum 13, 253 (1963); J. H. Singleton, J. Chem. Phys. 45, 2819(1966); A. E. Dabiri and R. E. Stickney, J. Vac. Sci. Technol. 9, 1032 (1972).8. It. E. Weber and A. IL. Johnson, J. Appl. Phys. 40, 314 (1969); 2I. Perdereau,Surface Sci. 24, 239 (1971).9. R. G. Musket and J. Ferrante, J. Vac. Sci. Technol. 7, 14 (1970).10. C. C. Chang, Surface Sci. 25, 53 (1971).11. T. E. Nladey and J. T. Yates, Jr., J. Vac. Sci. Technol. 8, 355 (1972); T. E.ladey, Surface Sci. 33, 355 (1972).12. H. G. Musket, Surface Sci. 21, 440 (1970).13. Y. G. Ptushinskii and B. A. Chuikov, Surface Sci. 6, 42 (1967), see Figure 7.14. This expression is from J. C. Batty and fI. E. Stickney, Oxidation of \letals 3,331 (1971), see Equation (7), with the aid of Equation (2).15. J. tK. Roberts, see the description in A. It. \Miller, The Adsorption of Gases onSolids (Cambridge University Press, London, 1949).16. P. Kisliuk, J. Phys. Chem. Solids 5, 78 (1958).17. W. Engelmaier and It. E. Stickney, Surface Sci. 11, 370 (1968).18. IW. Greaves and R. E. Stickney, Surface Sci. 11, 395 (1968).19. H. XW. Wassmuth, HI. Werner, and A. 1K. NIazumdar, J. Vac. Technol. 9, 835(1972).20. W. Engelmaier and It. E. Stickney, op. cit. , see Table I for a survey of dataobtained before 1968.QPR No. 109

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