PHYSICAL REVIEW 8 VOLUME 48, NUMBER 24 15 DECEMBER 1993-II

Adsorption of a monolayer of iron on P-SiC(100) surfaces

Wenchang Lu, Kaiming Zhang, and Xide XieChina Center ofAdvanced Science and Technology (World Laboratory), P.O. Box 8730, Beijing, People's Republic of China

and Physics Department, Fudan University, Shanghai 200433, People's Republic of China*(Received 29 March 1993)

The linear-muffin-tin-orbital method is used to study the adsorption of a monolayer of Fe atoms on theP-SiC(100) surface. Adsorption energies of Fe on both Si- and C-terminated (100) surfaces are calculat-ed. It is found that Fe can adsorb on both the Si- and C-terminated (100) surfaces. In the case of a cleanSi-terminated surface, the 2p3/p core level of the surface Si atoms shifts downward, since the surface Siatom loses more of its electrons than does the bulk Si atom. When a monolayer of Fe atoms adsorbs onthis surface, the 2@3/2 core level shifts upward; this is due to the fact that charge transfers from Fe to thesurface Si atom. In addition, the layer projected density of states is also studied.

I. INTRODUCTION

Silicon carbide (SiC) is an excellent candidate materialfor useful electronic devices operated at high tempera-tures and strong irradiation because of its wide band gap,high electron mobility, and high breakdown field' compa-rable to that of semiconductor silicon and germanium.The difhculty of obtaining su anciently large, uniformsingle-crystal samples with reproducible characteristicshas discouraged interest in electronics applications in thepast. However, this material has attracted much atten-tion since epitaxial growth on a silicon substrate bychemical vapor deposition has been developed andsingle-crystal I3-SiC with the zinc-blende structure can beobtained with high-quality, large area, and reproduciblecharacteristics.

This work was motivated by current interest in appli-cations of SiC electronic devices and high-strength metalcomposite fibers, as well as in the reactivity of metal-compound semiconductor interfaces. Several experimen-tal studies of 13-SiC surfaces and interfaces betweenmetal and P-SiC (Refs. 9—13) have been reported. Ber-mudez has studied thin aluminum film on SiC substrateswith electron spectroscopy and found that small quasime-tallic islands of aluminum are formed at room tempera-ture. After annealing at high temperature, these islandsreact with SiC followed by the formation of A14C3.Theoretical calculations' have confirmed that Al atomsinteract only with the C-terminated surface. However,for the interface of Pd/SiC, ' the opposite behavior hasbeen found, namely, Pd atoms interact only with the Si-terminated surface. For the Ti/SiC system, TiC islandsmay be grown at high temperature. "

For the Fe/SiC interface, experiments have shown thatiron silicide (FezSi) can be formed in the case of the lowcoverage of Fe atoms, whereas formation of both Fe2Siand iron carbide (FeC3) can occur in the case of high cov-erage of Fe atoms, ' ' i.e., Fe atoms can interact with ei-ther Si-terminated surface or C-terminated surface.However, not many theoretical studies have been ad-dressed to metal-/3-SiC interfaces. Anderson's group has

investigated Ti, Cu, and Pt/SiC (Ref. 15) interfaces usingthe atom superposition and electron delocalizationmolecular-orbital method. Using the charge self-consistent extended Huckel theory, we have studied theadsorption properties of gold on the P-SiC(111) surface'and aluminum on the )t3-SiC(100) surface' and found thatAl atoms interact with the C-terminated surface, whereasAu atoms interact with the Si-terminated surface. In thepresent paper, the linear-muffin-tin-orbital (LMTO)method' ' with supercell approach is used to study elec-tronic properties of a monolayer of iron on the P-SiC(100)surface.

II. OUTLINE OF THE METHOD

When the LMTO method with the atomic-sphere ap-proximation (ASA) is used to deal with the relativelyopen zinc-blende structure, the empty spheres are usuallyintroduced at the tetrahedral interstitial sites for provid-ing an adequate description of the charge density and po-tential in the interstitial region. ' Bachelet and Christen-sen have explicitly shown for GaAs that the ASA withinterstitial empty spheres provides results which are ingood agreement with first-principles pseudopotential cal-culations involving no spherical shape approximation forthe potential. We have studied the ground-state proper-ties of P-SiC and have shown that the results agree wellwith other theoretical calculations and experiments. Inthe case of the solid-vacuum supercell, the vacuum regionis also filled with empty spheres according to the samestrncture as the solid. ' The adsorption properties ofNa, Ca atoms on the Si(111)surface have been studied byBisi, Arcangeli, and Ossicini with the solid-vacuum su-percell and the results obtained are in good agreementwith experiment. Therefore, it is believed that theLMTO method with the solid-vacuum supercell can beused to deal with the present problem.

For the adsorption of Fe on a Si-terminated surface, aslab including seven silicon layers and six carbon layers isused to simulate the substrate. There are two Si-terminated surfaces in the slab, therefore the supercell for

0163-1829/93/48(24)/18159(5)/$06. 00 18 159 1993 The American Physical Society

18 160 WENCHANG LU, KAIMING ZHANG, AND XIDE XIE 48

III. RESULTS AND DISCUSSIQN

G.enerally, atoms of the clean p-SiC(100) surface relaxand form dimers, and the surface exists in (2X 1) recon-struction. Our calculations for Al adsorption on the(100) surface have indicated that the Al atom prefers toadsorb on ideal surface, i.e., reconstruction disappearsduring adsorption. Therefore, only the adsorption onideal surface is considered in the present work.

The adsorption energy per Fe atom is calculated byhalf of the di6'erence between the total energy of the ad-sorption system and the sum of the total energy of theclean surface and two single adatoms. The calculated ad-sorption energies for iron on the Si- and C-terminatedsurfaces are listed in Table I. From Table I, it can beseen that adsorption energies on the fourfold site are

Fesi~ csicsir cSl

csir csicSl

Fe

Vacuum

Adsorbed layerr ) Surface layerr ) Sub-surface layerr }Middle layer

r

a monolayer of Fe adsorption should include a layer of Featoms on each surface (see Fig. 1). This supercell hasmirror symmetry with respect to the central layer of thesolid films. For the adsorption of Fe on the C-terminatedsurface, the supercell used here is similar to that for theadsorption of Fe on the Si-terminated surface except thatthe Si atoms are substituted by C atoms and the C atomsare substituted by Si atoms.

Two possible adsorption geometries for a monolayer ofFe on the p-SiC(100) surface, namely, the bridge site andfourfold site, are considered (see Fig. 2). The distance be-tween the Fe monolayer and the surface is chosen in sucha way that the bond length between the Fe atom and itsnearest-neighbor surface atom equals the sum of their co-valent radii.

In the practice calculation, a grid of 16 k points in theBrillouin zone is used in the self-consistent calculationand then a grid of 64 k points is used to improve the self-consistent results. The exchange and correlation poten-tial is approximated by the Barth-Hedin parametriza-tion.

~ ~ ~ ~ ~ ~ ~

~ ~ 4 ~ ~ ~ 4

~ ~ 4 ~ 4 ~ 44 4

a 4 a ~ i(a}Bridge site

4 4 4 4~ a ~ i ~ a 4

4 4 4 4~ k ~ k ~ i ~

~ 4 4 4~ k ~ k 4 k ~

~ 4 4(b}Four-fold site

Surface atom ~ Second layer atom ~ Adatom

FIG. 2. Illustration of the adsorption geometries.

TABLE I. Adsorption energies (in units of eV} of Fe on f3-

SiC(100) surfaces

Si-terminatedsurface

C-terminatedsurface

smaller than that on the bridge site; therefore, bridge siteadsorption is stable in both the cases of Si- and C-terminated surfaces. In the following, we will only dis-cuss the bridge site adsorption. Iron can absorb on boththe Si- and C-terminated surfaces, since the adsorptionenergies on these two surfaces are quite close. This resultis different from the behaviors of Al and Au on the p-SiCsurface. Our previous EHT calculations have shown thatAl atoms mainly interact with the C-terminated surface'and Au atoms mainly interact with the Si-terminated sur-face. ' These results agree qualitatively with the experi-mental observations. For the Fe/p-SiC(100) interface, ex-periments have shown that both Fe silicide and Fe car-bide can be formed. ' ' For the Si-terminated surface, ithas been observed by experiment that an activation ener-gy of 5.2 eV/atom is required for breaking a Si-C bond.In the present work only the adsorption of one monolayerof Fe is considered, and the calculated adsorption energyis quite close to the activation energy of silicon depletion.It might indicate the possibility of iron silicide formationon the Si-terminated surface. As for the C-terminatedsurface, no experimental activation energy is available,since the binding energy for Fe adsorption on this surfaceis even lower than that on the Si-terminated surface andone may conclude that similar to the formation of Fe sili-cide, Fe carbide can also be formed.

The charge transfer is not well defined when theLMTO method is used to treat the open structure prob-lem, since empty spheres have to be introduced in the in-terstitial region. However, the charge inside the atomicsphere still can give some useful information. Qualitativeresults can be obtained from the analysis of the chargedistribution. The value of charge in all empty spheres ofthe vacuum region is very small. The value is about 0.3electrons in the empty sphere near the surface and thevalue is smaller than 0.05 electrons in the other emptyspheres of the vacuum region. Therefore, the solid-vacuum supercell is reasonable to describe the surfaceand adsorption problems.

FIG. 1. Illustration of the present used supershell for Fe ad-sorption on Si-terminated (100) surface.

Bridge siteFourfold site

—5.10—1.22

—6.47—2.34

48 ADSORPTION OF A MONOLAYER OF IRON ON P-SiC(100). . . 18 161

TABLE II. Charges inside the atomic and empty spheres forthe clean Si-terminated (100) surface. The values in brackets arefor the iron-adsorbed surface.

Atoms Surface layer Second layer Middle layer Bulk

SiE,C

12.09{12.36)0.38(0.46)6.75(6.74)0.53(0.51)

12.28( 12.28 )

0.40(0.40)6.80(6.80)0.46(0.46)

12.29( 12.29)0.40(0.40)6.81(6.81)0.48(0.48 )

12.280.426.830.47

Table II gives the charges inside the atomic spheresand empty spheres for the clean and iron-adsorbed Si-terminated (100) surface. The empty Ei sphere is at thesame layer as the Si atom and E2 at the same layer as theC atom. In order to compare with the bulk, the chargedistribution in the bulk P-SiC is also shown in Table II.From Table II, it can be seen that charges in Si, C andE

& E2 of the middle layer are very close to that in bulkp-SiC. The charge in the surface Si atom in the case ofclean surface is less than that in the bulk P-SiC, but afteriron adsorption, the charge in the surface Si atom shownin the bracket of Table II increases and becomes morethan that in the bulk p-SiC, since the adsorbed Fe atomtransfers electrons to the surface Si atom about 0.27 elec-trons.

Table III gives the charge distribution for the cleanand iron-adsorbed C-terminated surface. The charge dis-tribution in the middle layer is also very close to that inthe bulk p-SiC. The charge in the surface C atom in thecase of the clean surface is less than that in the bulk byabout 0.78 electrons. Although the adsorbed Fe adatomgives electrons to the surface C atom, the charge in thesurface C atom is still less than that in bulk p-SiC. Thisis different from the Si-terminated surface.

The surface core-level shift is calculated as thedifference between core-level eigenvalues of the surfaceatoms and that of the bulk atoms. In the present LMTOcalculation, the total core-level shift consists of two parts.One is the intra-atomic contribution and the other is theinteratomic Madelung contribution. The Si 2p core-level shifts in the cases of clean and iron-adsorbed Si-terminated (100) surface are listed in Table IV. It can beseen that the core-level shift in the atoms of the middlelayer is very small and could be neglected. This meansthat the middle layer is quite bulklike and the presentlyused supercell is reasonable to simulate the surface prob-lem. In the case of the clean surface, the core-level Si2p shifts downward, since the surface Si atom has lostelectrons. After the adsorption of Fe on this surface, the

TABLE IV. The core-level shifts (in units of eV) of Si 2p'in the cases of clean and adsorbed Si-terminated {100)surfaces.

Surface layer Second layer Middle layer

Clean surfaceAdsorbed surface

—2.802.13

—0.010.10

0.010.02

Surfa

40

G5

O20Cl

Subsurface

Midd

0-20 -16 -12 -8

E(eV)

100

80-F

60-05

40—n

Surface

Subsurface

core level shifts upward since the surface Si atom gainselectrons from the adatom Fe.

The layer projected density of states (LPDOS's) for theclean and adsorbed Si- and C-terminated surfaces areshown in Figs. 3 and 4, respectively. The zero-point en-ergy (vertical lines in the figures) is aligned at the Fermilevel. From the figures it can be seen that the LPDOS ofthe middle layer is bulklike, particularly there is no statein the band gap. This also indicates that the model of asupercell is reasonable to simulate the system. In the caseof the clean Si-terminated (100) surface, the surface stateS almost completely fills the band gap. By analyzing thewave function, it can be found that these surface statesare mainly contributed by the dangling bonds of the sur-face Si atoms as is expected. Our previous calculationsby the semiempirical extended Huckel band theoryhave shown that the surface state splits into two partsduring surface reconstruction. One part is at the top ofthe valence band and the other part is at the bottom ofthe conduction band. The band gap reappears. FromFig. 3(a), it is found that there are two other surface

TABLE III. Charges inside the atomic and empty spheres forthe clean C-terminated (100) surface. The values in brackets arefor the iron-adsorbed surface.

20Middle

SiEIC

12.35( 12.27)0.38(0.41 )

6.05(6.53)0.28(0.56)

12.28( 12.30)0.40(0.40)6.76{6.80)0.50(0.47)

12.29( 12.30)0.40(0.40)6.78( 6.82)0.46(0.48 )

12.280.426.830.47

Atoms Surface layer Second layer Middle layer Bulk 0-20 -8

E(eV)

FIG. 3. Layer projected density of states of (a) clean and (b)adsorbed Si-terminated (100) surfaces. The vertical lines indi-cate the Fermi level.

18 162 WENCHANG LU, KAIMING ZHANG, AND XIDE XIE 48

Surface

(~)

Subsurface~40L6$

CO

Midd

0-20 -12 -8

E(eV}0 4

100

(b)

601

G$

O 40Cl

20

0-20 -16 -12 -8

E(eV}

FIG. 4. Layer projected density of states of (a) clean and (b)Fe adsorbed C-terminated (100) surfaces. The vertical lines in-dicate the Fermi level.

states: 2 and 8. Peak A in Fig. 3(a) is a surface resonantstate and peak B is a state existing in the interband gapbetween the s- and the p-like bands. Comparing theLPDOS's of the clean and adsorbed surfaces, the follow-ing results can be obtained. The LPDOS in the middlelayer changes slightly, whereas the changes in theLPDOS in the surface layer due to the Fe adsorption arequite noticeable. First, the density of state S in the bandgap decreases and only a small peak at about 0.5 eVabove the Fermi level remains in the fundamental bandgap. Second, the resonance state A decreases drastically,and a high peak appears at about 3.5 eV below the Fermienergy which is the interaction between Fe and surface Siatoms. Finally, peak B disappears after adsorption, andthe densities around 13 eV below the Fermi energy in-crease. The above results are due to the fact that ad-sorbed Fe atoms interact with surface atoms and partiallysaturate the dangling bonds of the surface atoms. In theLPDOS of adsorbed Fe atoms, there are three peaks atabout —1.0, —0.2, and 0.5 eV. It can be found that thepeaks at —1.0 and —0.2 eV are bonding states and thepeak at 0.5 eV is an antibonding state. Moreover, the in-teraction between Fe atoms and the surface atoms is dueto the bonding of Fe 3d states with 3p states of the sur-

face Si atoms. The bonding of the Fe 4s state with Si 3sand 3p states and that of Si 3s with the Fe 4s and 3dstates is weak. This is a general behavior of transitionmetal on the silicon surface.

In the case of the clean C-terminated surface [Fig.4(a)], the surface state also exists in the band gap. Thepeak A just below the valence-band edge is contributedby the dangling bonds and back bonds of the surface Catoms. There is no state in the interband gap, which isdifferent from the clean Si-terminated surface. After Feadsorption, the LPDOS below —4.0 eV almost remainsunchanged. The heights of the surface state S and peakA significantly decrease, whereas the densities around—3.0 eV increase. This is also due to the fact that thedangling bonds of the surface atoms are saturated by theadsorbed Fe atoms. In the LPDOS of Fe atoms, there arealso three peaks around the Fermi energy. The peaks at—1.8 and —0.4 eV are bonding states and the peak at 0.2eV is a nonbonding state. The 3d states of the Fe atomsmainly bond with the 2p states of surface C atoms, andthe other bonding is weak.

In both cases of Si- and C-terminated surfaces, the ad-sorbed Fe atoms have a high density of state at the Fermilevel. This implies that both the Si- and C-terminated(100) surfaces with a monolayer of Fe atoms might have ametallic character.

Experiments have shown that both iron silicide andiron carbide can be formed on the (100) surfaces due tothe interaction between Fe atoms and surface atoms. Thepresent calculations indicate that the Fe atom can adsorbon both Si- and C-terminated surfaces, i.e., the Fe atomcan interact with both Si- and C-terminated surfaces.

IV. SUMMARY

The LMTO method with the ASA is used to study theadsorption of a monolayer of Fe atoms on the p-SiC(100)surfaces. The adsorption energies on both the Si- and C-terminated (100) surfaces only have a slight difference,therefore, the Fe atom can adsorb on these two surfaces.This result is different from that of Al and Au adsorp-tions on p-SiC. The charge distribution and the Si 2pcore-level shift are discussed. It is found that the chargetransfers from the adsorbed Fe atom to the surface atom.As a consequence, the core-level shift of Si 2p changesfrom downward to upward. The LPDOS is also dis-cussed. The densities of surface states in the case of theclean surface decrease significantly after Fe adsorption,since the dangling bonds of the surface atoms are partial-ly saturated by the adsorbed Fe atoms. The 3d states ofthe Fe atoms mainly bond with the p states of the surfaceatoms. The adsorbed Fe monolayers have high density ofstate at the Fermi level, therefore the p-SiC(100} surfacescovered with a monolayer of Fe might have some metal-lic character.

ADSORPTION OF A MONOI. AYER OF IRON ON P-SiC(1001. . . 18 163

*Mailing address.Landolt-Bornstein Tables, edited by O. Madelung, M. Schulz,

and H. Weiss, Landolt-Bornstein New Series, Group X, Vol.17, Pt. a (Springer, Berlin, 1982).

~S. Nishino, J. A. Powell, and H. A. Will, Appl. Phys. Lett. 42,460 (1983).

A. Suzuki, K. Furukawa, Y. Higashigaki, S. Harada, S. Nakaji-ma, and T. Inoguchi, J. Cryst. Growth 70, 287 (1984).

4J. Ryu, H. J. Kim, and R. F. Davis, Appl. Phys. Lett. 47, 850(1985).

5P. Liaw and R. F. Davis, J. Electrochem. Soc. 132, 642 (1985).R. Kaplan, J. Appl. Phys. 56, 1636 (1984).

7M. Dayan, J. Vac. Sci. Technol. A 3, 361 (1985);4, 38 (1986).S. Hara, W. F. J. Slijkerman, J. F. van der Veen, I. Ohdomari,

S. Misawa, E. Sakuma, and S. Yoshida, Surf. Sci. 231, L196(1990).

V. M. Bermudez, Appl. Phys. Lett. 42, 70 (1983).V. M. Bermudez, Appl. Surf. Sci ~ 17, 12 (1983).

' S. Hasegawa, S. Nakamura, N. Kawamoto, H. Bishibe, and Y.Mizokawa, Surf. Sci. 206, L851 (1988).

' R. Kaplan, P. H. Klein, and A. Addamiano, J. Appl. Phys. 58,

321 (1985).I Y. Mizokawa, S. Nakanishi, and S. Miyase, Jpn. J. Appl.

Phys. 28, 2570 (1989);28, 2576 (1989).W. Lu, K. Zhang, and X. Xie, Phys. Rev. B 45, 11048 (1992).A. B. Anderson and C. Ravimohan, Phys. Rev. B 38, 974(1988); S. P. Mehandru and A. B. Anderson, Surf. Sci. 245,333 (1991).

W. Lu, L. Ye, and K. Zhang, Chin. Phys. Lett. 9, 101 (1991).O. K. Andersen, Phys. Rev. B 12, 3060 (1975).H. L. Skriver, The LMTO Method (Springer, Berlin, 1984).D. Glotzel, B. Segall, and O. K. Andersen, Solid State Com-mun. 36, 403 (1980).G. B. Bachelet and N. E. Christensen, Phys. Rev. B 31, 879(1985).

O. Bisi, C. Arcangeli, and S. Ossicini, Phys. Rev. B 43, 9823(1991).U. von Barth and L. Hedin, J. Phys. C 5, 1629 (1972).J. J. Bellina, Jr. and M. V. Zeller, Appl. Surf. Sci. 25, 380(1986).W. Lu, W. Yang, and K. Zhang, J. Phys. Condens. Matter 3,9079 (1991).