PHYSICAL REVIEW B 15 MAY 1998-IIVOLUME 57, NUMBER 20

Photoemission and x-ray-absorption study of boron carbide and its surface thermal stability

I. Jimenez,* D. G. J. Sutherland, T. van Buuren, J. A. Carlisle, and L. J. TerminelloLawrence Livermore National Laboratory, Livermore, California 94551

F. J. HimpselDepartment of Physics, University of Wisconsin-Madison, Madison, Wisconsin 53706

~Received 12 December 1997!

We present a photoemission and x-ray-absorption study on boron carbide which identifies the spectroscopicfeatures of this material, discussing them in connection with current structural models. A study on the surfacethermal stability is also performed, revealing that despite the high melting point of boron carbide(;2600 K), significant surface modifications occur in the material at about 1800 K. At this temperature thesurface becomes covered with a graphite layer formed from bulk segregation of carbon, leaving boron-richboron carbide buried beneath the graphitic surface.@S0163-1829~98!02520-X#

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I. INTRODUCTION

Boron carbide is a solid whose structure is based on boicosahedra and linear C-B-C chain building blocks.1,2 Al-though the stoichiometric compound is nominally B4C, theboron to carbon ratio can vary over a broad range by pasubstitution of B by C atoms both in the C-B-C chains andthe icosahedra. This rather complex bonding structure gthe material a manifold of thermomechanical properties~lowdensity, refractory, hard! that make it interesting for manapplications as a replacement for metals, as high-temperacoatings, or as a constituent of composite ceramics.3–5 Froman electronic structure point of view, boron carbide is a seconductor with a gap dependent on the boron to carbonichiometry that can be modified continuously from;0.8 eV~carbon rich! to ;2 eV ~boron rich!,6 and appears as a goocandidate for high-temperature microelectronic device7

From a fundamental standpoint, neither the atomic and etronic structures nor the electronic transport mechanismsfully understood.8,9 Also, there is a lack of information concerning the surface composition, structure, and propertiethis material. In particular, very little has been publishedsurface cleaning procedures, reconstructions in single ctals, or surface stoichiometry modifications.

The use of boron carbide with other materials presemany interesting surface and interface aspects. The fortion of composite ceramics based on boron carbide invointerface reactions between metals, oxides, and boridesstability of the material being a major question.10 Anotherarea of interest arises from the technological problem of cditioning the graphite walls of fusion nuclear reactorsboron deposition. The neutron-absorbing boron coating loers significantly the chemical erosion by reducing the intaction of hydrogen and oxygen ions with the carbon wMost of the published results concern the thickness ofboron layer attained, the location in depth of the boronoms, and the resistance of this layer to the plasma-surinteraction.11–13However, the stoichiometry and structurethe boron carbide coating have largely been ignored.interest of boron carbide as a semiconductor for hitemperature applications poses the typical interface probl

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of microelectronic structures, such as the electronic strucand chemical details of metal/semiconductor and insulasemiconductor junctions.

In this work we study the photoemission and photoasorption spectral features of boron carbide in relationshipcompositional and structural changes at the surface duannealing. The aim of the study is to understand the atoand electronic structure of this compound, information thwould be useful in the characterization of boron carbide tfilms, coatings, and composites. A discussion of the specfeatures in terms of a simple picture of hybridization of tboron and carbon atoms is presented.

II. EXPERIMENT

We have studied by photoemission and x-ray absorpthe changes that take place near the surface of a polycryline B4C sample obtained from a commercial sputtering tget, when heated in vacuum to temperatures up to 2100The sample was cleaved in air and immediately insertedan ultrahigh-vacuum chamber; therefore, some surface ctamination is expected. To remove the contaminants we hchosen to anneal the samples in vacuum, avoiding ion stering cleaning procedures. Ion sputtering is not a well-suiprocedure to clean the surfaces of carbon-rich solids, sincresults in the formation of an amorphous graphitic layer.14,15

In particular, to obtain clean diamond surfaces they are pished in air with an abrasive powder followed by annealiin vacuum,16 and graphite surfaces are cleaved with anhesive tape or annealed in vacuum to very high temperatbut never ion sputtered.17 The B4C samples were heated bdirect flow of current, the temperature being controlled wan optical pyrometer previously calibrated. The temperatwas homogeneous along the sample to within 50 K.

A set of reference samples was measured under the sconditions for direct comparison. Photoemission and phoabsorption spectra were obtained from highly orientedrolitic graphite~HOPG! and a diamond single-crystal~111!oriented, providing a reference for the carbon spectra. Cl

13 167 © 1998 The American Physical Society

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surfaces of graphite and diamond were obtained as descabove. Photoabsorption references for B(1s) were obtainedfrom amorphous boron, hexagonal BN (h-BN), and cubicBN (c-BN) powders epoxied to tantalum sheets.

Surface-sensitive structural information was obtainedphotoemission, by monitoring B(1s), C(1s), and valenceband photoelectrons with kinetic energies between 100150 eV, which correspond to mean electron escape deunder 10 Å. Core level photoabsorption measured in the telectron yield mode probes the surface character of a mrial, but is not restricted to the very top atomic layers sinmost of the measured signal is due to low-kinetic-enesecondary electrons that have been excited deeper insolid. Typical values of the electron escape depth in this crange from 50 to 100 Å.18 The photoabsorption measurments were performed at the bending magnet beam lineof the Stanford Synchrotron Radiation Laboratory~SLAC!,and the photoemission data were collected at the undubeam line 8.0 of the Advanced Light Source~LBNL !, bothequipped with spherical grating monochromators.

III. STRUCTURAL MODELS AND BONDINGENVIRONMENTS IN BORON CARBIDE

Since photoemission and photoabsorption are both teniques sensitive to the bonding of the atoms, structural inmation can be obtained if the spectral features can be direrelated to the different bonding environments of boron acarbon atoms in the material. Let us first consider the diffent bonding configurations and review existing models thave been proposed.

Figure 1 sketches the structure of boron carbide. Tforming units are boron icosahedra, located at the vertice

FIG. 1. Boron carbide structure.

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the rhombohedral crystallographic unit cell, and three-atlinear chains located in the main diagonal that, in principshould have the composition CCC to account for a B4C sto-ichiometry. However, the structure is not so simple becaof mixing of boron and carbon atoms both in the chains aicosahedra. This mixing explains the BxC system being ho-mogeneous within the range 4,x,11. X-ray diffractioncannot distinguish between boron and carbon atoms dutheir similar scattering factors, and the first structural studproposed a simple B12-CCC structure for B4C.19 Other tech-niques like infrared and Raman spectroscopy,20,21 nuclearmagnetic resonance,22 and electron spin resonance23 haveprovided support for the following structural model. Near tcarbon-rich limit, i.e., 20 at. % carbon and B4C stoichiom-etry, the solid is composed of B11C icosahedra and CBCchains. As the composition becomes more boron rich, Cchains replace the CBC ones. With further carbon reducthe B11C icosahedra are replaced by B12 units. Some authorspropose a first replacement of the CBC chains by Cchains until the B13C2 stoichiometry is attained with a welldefined B11C-CBB structure, followed by the change of aoms in the icosahedra.24 Other authors propose the coexisance along the whole homogeneity range of B11C, B12, CBC,and CBB units, and the absence of the other types of linchains, i.e., CCC, BBB, and BCB.25

Independent of the details of the structure, there are sogeneral results concerning the bonding environment. Inicosahedra there are six atoms, labeled B~1!, along the equa-tor of the icosahedron, each forming five intraicosahedbonds and one intericosahedral bond to a terminal atomthe linear chain. There are also six atoms of type B~2! form-ing three-atom poles on the top and bottom of the icosahewhich form five intraicosahedral bonds and one intericosadral bond B~2!-B~2!. Note that one of B~1! or B~2! will be acarbon atom in the case of B11C icosahedra. Some worindicates that the carbon atom is located preferentially ipolar B~2! position.26 The hybridization of the boron andcarbon atoms in the icosahedra is unclear, and thereforcontribution to the photoabsorption spectra is similarly ucertain. This will be discussed in the following section.

The atoms in the extreme of the linear chain, eithcarbon—labeled C~4!—or boron—labeled B~4!—have atetrahedral-like coordination to three B~1! atoms in three dif-ferent icosahedra~with a bond length of 1.61 Å! and to thecentral atom in the chain~with a bond length of 1.43 Å!.27

The hybridization of these C~4! or B~4! atoms should besp3-like, and in principle one would expect a near-edgeray-absorption fine-structure~NEXAFS! signal resemblingthat of diamond for C~4! or cubic boron nitride for B~4!. Thecentral atom in the chain, labeled B~3!, is bonded linearly tothe extreme atoms. The hybridization of the central carbatom is unknown, although some authors proposesp2-like hybridization, based on the bond length.22

In summary, near the B4C stoichiometry the structure isupposed to be B11C-CBC. That means that there is a carbatom forming the B11C cluster with an unknown hybridization and two carbon atoms with ansp3-like hybridization atthe extrema of the linear chain. There are 11 boron atoforming the icosahedron and one boron atom in the centethe linear chain, presumably with ansp2-like hybridization.

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57 13 169PHOTOEMISSION AND X-RAY-ABSORPTION STUDY OF . . .

FIG. 2. B(1s) and C(1s) photoabsorption spectra of boron carbide and reference boron,h-BN, c-BN, graphite, and diamond sampleCurves~a! correspond to the untreated B4C sample. Curves~b!–~e! were measured after heating in vacuum to 1000, 1400, 1700, andK. Curves~f! were obtained from reference samples as indicated in the figure.

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IV. RESULTS AND DISCUSSION

A. Photoabsorption: Reference compounds

A simple method of examining the types of bondinpresent in a solid is based on a direct comparison of x-rabsorption spectra with reference compounds. This wowell to distinguishsp2 from sp3 configurations in B, C, andN using as references graphite, diamond,h-BN, andc-BN.However, the spectra of boron-rich solids based on boicosahedra are more difficult to be interpreted in suchsimple manner, and there is a lack of photoemissionphotoabsorption experimental results on these compouPrevious reports of B4C photoabsorption spectra disagreethe overall spectral line shape without attempting any inpretation of the results.28,29Reference compounds containinboron polyhedra can be the molecular boranescarboranes.30,31 In particular, the closo-carboranes are mecules with icosahedral geometry whose unoccupied etronic structure has been studied in detail recently.32 Previ-ous electronic structure calculations for borane icosahewith substituted carbon atoms were limited to occupstates.33

The type of bonding in the icosahedra must be similarboron carbide and the closo-carboranes, neithersp2 nor sp3.However, one must be aware of the following differencbetween the carboranes and the solid B4C. ~1! In the closo-carboranes the icosahedron has the B10C2 composition, com-pared to the B11C icosahedra in boron carbide.~2! Boroncarbide is a refractory material with strong bonds amoicosahedra, and hence is not a molecular solid~like, e.g.,solid C60!. It is unclear how much of the molecular orbitstructure is retained in the solid.~3! In B4C there are not onlyicosahedra, but linear CBC chains as well. Separatingcontribution from linear chains and icosahedra to the speis an additional problem.

Figure 2 shows a series of boron and carbon photoabs

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tion spectra from boron carbide and reference carbon, boand boron nitride samples. All curves are shown to the saheight to illustrate clearly the changes in the spectral shaDiamond and graphite spectra provide a reference forsp3

andsp2 bonding in carbon, andc-BN andh-BN a referencefor sp3 andsp2 bonding in boron compounds, respectiveCurves ~a! represent the untreated B4C sample and curves~b!–~e! correspond to the sample annealed with increastemperatures. The intensities corresponding to the boronbide spectra along the heating process are displayed in3, showing a transition above 1800 K that is discussedlow.

B. B„1s… photoabsorption

The boron spectra~a!–~d! in Fig. 2 are very similar, indi-cating that no significant changes in the bonding of booccur for temperatures under 1700 K. The onset of the

FIG. 3. Development of the B(1s) and C(1s) photoabsorptionintensities with sample heating to increasing temperatures.

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sorption occurs at 189 eV, the intensity raising to formintense peak at 190.9 eV, labeledA, which could be assignedto p* states from comparison with a similar feature appeing in hexagonal BN at 192.0 eV.34,35 Three structures appear at about 196, 200, and 204 eV that could be assignes* -like states from comparison with the BN spectra. Unlithe BN references, the spectra from B4C do not show separate p* and s* features with well-defined edges. This cabe explained by the bonds not being purep ands type.

In addition to the main peak at 190.9 eV in Fig. 2, seveminor features are present in the 191–194 eV regionchange with the annealing temperature. In curve~a! threepeaks at 191.7, 192.3, and 193.7 eV labeledB, C, andD arepresent, featureD being the most intense. In curve~b! peaksB–D are about the same height, and in curve~d! only thepeakB at 191.7 is observed. The origin of these featureunclear, except for featureD, which seems related toB2O3.

28,29 Photoabsorption and photoemission from tO(1s) core level are shown in Fig. 4 and reveal that a smamount of oxygen is present in the sample which is no londetected after annealing to 1500 K.

Significant changes in the boron absorption are obserfor curve ~e!, after heating the sample to 1900 K. The onof absorption is now at 187 eV, peakA, is still present, but isno longer very intense; the prominent feature is now peakB,

FIG. 4. Oxygen photoemission@panel~a!# and photoabsorption@panel~b!# spectra indicating the presence of oxygen contaminain the untreated sample and the desorption with sample heatinvacuum. Panel~c! represents the photoemission intensity vs heattemperature, evidencing the absence of oxygen at the surfaceheating to 1500 K.

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and thes* states have also changed. This evidence revthe surface instability of boron carbide at 1900 K and incates that a new phase or a new stoichiometry is attainThese changes can be associated with pure boron as caseen by comparison to curve (f1) which represents a reference spectrum obtained from a boron powder. The similabetween curves~e! and (f1) at the low-energy side of theNEXAFS spectrum is clear, thus suggesting the formationa boron-rich boron carbide.

According to the above discussion, the typical B(1s)spectrum from an oxygen-free B4C surface is shown in curve~d! of Fig. 2. This spectrum is similar to the one presentedRef. 28. The different B4C spectra shown in Ref. 29 can bexplained as a boron-rich boron carbide in their sputterposited film and to the presence of oxide contaminationtheir B4C powder. Comparison of our boron powder spetrum in Fig. 2 with the boron curve in Ref. 28 indicates tpresence of oxide in our reference sample by the pea193.8 eV. Therefore, the spectrum of clean boron in Ref.is characterized by the lack of any intense and sharp featAt this point it is interesting to compare our results with tcloso-carborane spectra from Ref. 32.

The carborane spectra show an intense peak at;192 eVwhich is actually composed of several features in the 19193 eV range. They correspond to the unoccupied molecstates 10a9, 17a8, 26a, and 27a within the lowest excitedmolecular orbital ~LEMO! picture described in Ref. 32Their character is mostly ofp type, with the higher-energyorbitals (LEMO11,2,...) having mores character than thelow-energy one~LEMO!. The carboranes also show broafeatures at 197 and 202 eV ofs character. Despite the lack osharp edges in the carborane spectra, the states withs andpcharacter are separated by a deep valley at 194 eV.features in the carborane spectra can be associated morectly to the boron and boron carbide spectra when thelecular spectra are shifted;1 eV to lower energy. The samshift between features in closo-carboranes and B4C is ob-served in the C(1s) spectra and is presented and discussethe next section.

The spectrum of solid boron resembles the carborspectra widened in such a way that the deep valley is lThat smooth shape seems characteristic of the boron ichedra. The spectrum of B4C, however, contains an intensand sharp peak at 190.9 eV, with a full width at half mamum ~FWHM! of 1.1 eV. This peak is sharper than the peat 192 eV of the carborane molecules, with a FWHM of 2eV for the ortho and 1.7 eV for the meta and para copounds. In general, the spectral features in the gas phasmuch sharper than in the solid state, therefore suggestidistinct origin of the narrower peak in B4C. The B4C peakmust be related to intericosahedral bonds in the solid. Sithat intense feature is not present in solid boron, it shocorrespond to the bonding between boron icosahedra andextreme carbon atom in the CBC chain or to bonding withthe CBC chain. Another observation of the onset of phoabsorption, which is located at 187 eV for the boron-risamples and 189 eV in B4C, should be noted. This shift of 2eV must correspond to the presence of a carbon atom inicosahedra in the carbon-rich material.

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57 13 171PHOTOEMISSION AND X-RAY-ABSORPTION STUDY OF . . .

C. C„1s… photoabsorption

The carbon photoabsorption spectra measured onsame sample reveal complementary structural informatCarbon spectra in Fig. 2 correspond to the same stage oannealing process as the boron spectra with the same lCurve~e!, the one representing the sample heated at 190looks identical to the graphite reference displayed in cu(f1), but this structure is not seen in curves~a!–~d!. There-fore, the changes occurring at 1900 K are due to segregaof carbon atoms from B4C to the surface, with the formatioof a graphite overlayer. The sample was heated under Uconditions, thereby precluding carbon contamination frthe chamber. The boron carbide in the subsurface regionlow the graphite layer is boron rich, as evidenced byB(1s) absorption spectra~e!, and this is consistent with thcarbon segregation. The identification of the graphite asouter layer is deduced from the strong attenuation ofboron absorption signal after annealing the sample to tperatures above 1800 K as shown in Fig. 3.

The carbon photoabsorption spectrum of B4C before car-bon segregation is dominated by a double feature at 28and 287.7 eV in spectrum~a! that disappears with the sampheating to 1000 K shown in curve~b!. This double feature inthe C(1s) NEXAFS has its counterpart in the O(1s) signal,shown in Fig. 4, with peaks at 529.5 and 532.5 eV. Tcarbon and oxygen peaks correspond to CvO p* reso-nances, revealing the presence of carbon oxide contamincontaining carbonyl groups.36,37 The carbon oxide featuredisappear uniformly in both the C(1s) and O(1s) spectraafter heating the sample to;700 K. We have observed thsame carbon oxide contamination features in many otheexsitu prepared samples including boron nitride, titaniumtride, graphite, silicon, etc., but they always disappear agentle annealing. A similar double peak in the C(1s) NEX-AFS has been reported in several previous studies of casystems and identified as carbon nanotubes features,38 graph-ite interlayer states,39 or the s* exciton of amorphouscarbon.40 Our assignment of these features to some CvOmoiety is commensurate with the O(1s) NEXAFS in all thesamples studied and consistent with the disappearance ofeatures in the C(1s) and O(1s) spectra after sample outgasing.

Representative NEXAFS spectra of C(1s) in B4C arecurves~b!–~d! of Fig. 2, which remain similar for annealintemperatures between 1000 and 1700 K except for a schange in the 285 eV region. In this region there is a doupeak at 284.9 and 285.6 eV, the latter becoming more intewith further annealing. This double structure seems relatep bonding, since 285.4 is the energy of thep* resonance ingraphite. Apart from that double peak, the spectrum showsharp peak at 287.1 eV, and two smooth edges at 288291 eV.

The most significant features in the boron carbide areference carbon spectra have been labeledW–Z for directcomparison. The graphitep* resonance, labeled asW, ap-pears at an intermediate energy between the doubledescribed above. PeakX at 287.1 eV in boron carbide has ncounterpart in the reference spectra. The onset of thes*states insp3-bonded carbon~diamond reference! is labeledasY and coincides with the boron carbide edge at 288.3

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FeatureZ is the onset of thes* states insp2-bonded carbon~graphite! and coincides with the edge of boron carbide291 eV. A possible interpretation of the coincidence of feturesY in B4C and diamond andZ in B4C and graphite couldbe the presence of carbon atoms in boron carbide retaisp2 and sp3 hybridization. However, one expects a mocomplex bonding for carbon in the icosahedra than convtional p ands bonds.

A better explanation is gained by comparison with tC(1s) photoabsorption spectra in closo-carboranes.32 Thecarborane signal presents an intense and sharp peak aeV, a double feature at 291–292 eV with the increase ofabsorption starting at 290 eV with a smooth edge, a brfeature at 294 eV with an apparent edge at 292.5, anbroader feature at;298 eV. The sharper peak has beensigned to 10a9 and 17a8 states, the double feature at 291292 eV to 3p and 4p states, and the two last broader peahave been assigned tos* states. Direct comparison with thB4C spectra is possible if the molecular spectra are shifteeV to lower energy, in which case peakX lines up with the10a9/17a8 molecular peak, edgeY coincides with the apparent edge of 3p/4p states, and edgeZ with the apparent edgeof moleculars* states. According to this, featuresX, Y, andZ correspond to carbon atoms within the icosahedra. Thfore, the features at 284–286 eV would be related tocarbon atoms in the linear CBC chain. In principle, owould expect ansp3-like signal resembling diamond fothese atoms, i.e., an edge at about 288 eV. However, chtransference from carbon to boron atoms and other efflike core hole screening may account for the energy diffence of about 3.5 eV. If this is the case, photoemissionsults should exhibit a similar energy shift for carbon atowith an sp3 hybridization in B4C.

The observed 1 eV shift between the C(1s) and B(1s)spectra of gas phase closo-carboranes and solid B4C seemsrelated to a charge effect, since the icosahedra in boronbide have a composition B11C with an additional electropositive boron compared to B10C2 in closo-carboranes. A gaversus solid state energy shift may also exist, althoughexpects this effect to be rather small as observed, e.g., inC(1s) photoabsorptionp* resonance from gas benzene vesus solid benzene or graphite.36 A comparison of core levephotoemission spectra from boron carbide and clocarboranes should shed light on this issue.

D. Photoemission

Complementary information about the bonding in B4C be-fore and after annealing to 1800 K is derived from phoemission spectra. Figure 5 shows a series of C(1s) andB(1s) core level and valence band photoemission specorresponding to another vacuum annealing experiment.photoemission and photoabsorption measurements wereformed in separate experiments using the same sesamples and the temperature being controlled with the spyrometer.

The C(1s) photoemission spectra exhibit large changduring the annealing process. Since only at 1500 K dostop detecting the oxygen signal, curves~a! and ~b! can re-veal some type of carbon surface contamination. Curve~c! isa representative spectrum for B4C, presenting two peaks a

13 172 57I. JIMENEZ et al.

FIG. 5. B(1s), C(1s), and valence band photoemission spectra from B4C. Curve~a! represents the untreated sample, curves~b!–~e!correspond to different annealing temperatures, and curves~f! represent a graphite reference.

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281.8 and 283.7 eV. For temperatures over 1800 K a graph-ite peak at 284.5 eV dominates the spectrum~d!, althoughthe overlayer is still thin enough to detect the featureslower binding energy associated with subsurface B4C. Incurve ~e!, corresponding to annealing to 1900 K, only tgraphite peak is present.

The position of the two carbon peaks in spectrum~c! issomehow striking, since both of them have a lower bindenergy than the reference graphite. In principle, one woexpect a peak corresponding tosp3 hybridization at a higherbinding energy than the graphite reference, since the C(s)level in diamond has a higher binding energy than in graite. A chemical shift of about 0.8 eV has been obserbetweensp2 andsp3 phases in samples containing a mixtuof both carbon configurations,41,42including the 231 surfacereconstruction of diamond~111!, which presents pbonding.14 Of course, the binding energy of the core levdepends on many factors including the position of the Felevel within the band gap, charge transfer, and core hrelaxation effects. In the case of boron carbide, the bondsno longer nonpolar as in graphite and diamond, and chtransfer may affect strongly the binding energy of the clevel independently of thesp2 or sp3 hybridization of theatoms. Taking into account the structural model for B4C, onecan explain the intensity of the peak at 281.8 eV bedouble than that of peak at 283.7 by assigning the lobinding-energy feature to the two carbon atoms in the Cchain and the high-binding-energy one to the carbon atomthe icosahedron. Following this argument, electrons insp3-like carbon atoms of the CBC chain have a bindingergy 3.5 eV lower thansp3 carbon atoms in carbon. Thisconsistent with the assignment of the C(1s) photoabsorptionfeatures described above.

The B(1s) core level spectra show the desorption of dferent oxides with increasing heating temperature. Foruntreated sample in curve~a!, three different oxide peakcorresponding to oxidation numbers 31, 21, and 11 are

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present. Curve fitting the spectrum~a! with four componentcurves yields chemical shift values of 4.9, 3.4, and 1.7respectively, for the oxides. The FWHM of the B(1s) corelevel is 1.7 eV, much broader than the features in the phabsorption spectrum which are about 0.5 eV FWHM forp* features. With annealing the oxides are desorbed,higher oxidation numbers first, until neither boron oxides nO(1s) is detected over 1500 K. For annealing at 1900 Kboron signal is detected due to the thick graphite overlay

The valence band spectra are also consistent with thescribed scenario. Curve~a! shows three broad peaks at abo2, 7, and 12 eV. Curve~b! shows similar features, withhigher intensity of the feature near the valence band mmum as can be expected from boron carbide coveredoxides that are being desorbed. Curve~c! shows new featuresand a sharper onset at the valence band maximum. Cotently with the previous discussion, we consider this sptrum as typical of the B4C compound. This valence banspectrum looks similar to previous reports of boron carbgrown from deposition of diethylcarborane on Si~111! andsubsequent annealing.43 The valence band spectra~d! and~e!are identical to the valence band of the graphite referenceexpected from the formation of the graphite overlayer. Nthe presence in the spectra of the C(1s) core level excitedfrom third-order light, a feature that moves in the spectrwith a slight variation of the energy of the incident photon

V. CONCLUSIONS

In summary, we have performed a photoemissionphotoabsorption study of boron carbide to identify the sptral features of this compound of potential interest in maareas of basic and applied surface research, for which herto unclear and unconsistent spectra appeared in the liture. We have attempted to relate the spectral featuresthe bonding environments in B4C, based on a comparisowith reference compounds. Additional theoretical work is

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57 13 173PHOTOEMISSION AND X-RAY-ABSORPTION STUDY OF . . .

quired for a more complete interpretation of the NEXAresults, which contain much information on the atomic aelectronic structure of materials.

This qualitative analysis indicates that many of the ftures in the boron carbide NEXAFS spectra can be direrelated to those of the icosahedral closo-carborane mecules, noting that the molecular spectra are shifted by a1 eV to higher energies compared to the spectra fromsolid. The additional structure in the boron carbide speseems related to the linear CBC chains. The onset of abstion from the B(1s) level shifts 2 eV depending on the icoshedra containing or not a carbon atom. Our results sugthat the carbon atoms in the CBC chain, presumably wsp3 hybridization, show a binding energy for the 1s elec-trons;3.5 eV lower than diamond, the prototype solid wsp3 bonding.

We have performed also a study of the spectral chanobserved during annealing, in order to understand surcleaning procedures and the thermal stability of this copound. We conclude that annealing between 1500 and 1

Snd

a-tlyol-outthetraorp--estith

h

gesacem-700

K is required to attain a clean surface, and that heating o1700 K results in carbon segregation to the surface withformation of a graphite overlayer, leaving a boron-rich borcarbide beneath the graphitic surface.

ACKNOWLEDGMENTS

This work has been supported by the Division of Mateals Sciences, Office of Basic Energy Science, and performunder the auspices of the U.S. Department of EnergyLawrence Livermore National Laboratory under ContraNo. W-7405-ENG-48 and the NSF under Contract NDMR-9632527. The work was performed at the StanfoSynchrotron Radiation Laboratory, which is supported by tDepartment of Energy, Office of Basic Energy Sciences, aat the Advanced Light Source, LBNL, under Contract NDE-AC03-76SF00098. I.J. acknowledges financial suppfrom the Spanish CICYT project PB94-53 and NATproject CRG-971539.

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*Present address: Instituto de Ciencia de Materiales de Mad~CSIC!, Cantoblanco, E-28049, Spain. Electronic addresijimenez@icmm.csic.es

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