Oxidation of thin ErSi1.7 overlayers on Si(111)

Applied Surlil~ Science 5 6 5 8 (113112)5111-5116 Norfll-Holland

aoo~ed

surface science

Oxidation oi' thin ErSi~.7 overlayers on S i ( l l l ) N. G u e t f i , T . A , N g u y e n T a n *, J.Y. V e u i l l e n a n d D . B . L o l l t n a n C N R S - L E P E S ~ *. BP 166, 38042 (;remJhle £~'dex. Frlmce

Received 6 May 1991; accepled Ik~rpublication 5 June 1991

The oxldalion tit ErSij 7 thin films, epi~xially grown oft Sill I I X7 × 7) sur fact,t, hy solid phase epita~, has been investisated by X-ray and UV photoelectron 5pcctrlls~Opie~.Oxidation ha~ heen carried out at rca~mtemperature under hlw pressure ( _<2 x 10 5 robert and 1 lain of oxyg~tn, and nt 71111o C tlllder 2 X 10 5 mbar of oxygen. In all cases, both Si and Er react wilh oxygen. A1 roum temperalure the reaction depend~ orl Ihe pressure. Under low pre,~sure tile silicide ~urta¢~ 15ralher inert: a ehemisurotion phase nf oxygen on Si ~lnd Er is detecled only after expt:sures > la ~ hmgmuir. High oxygen pressure produces a thin layer of mixed Sit3,, Si suhnxides and Er~O~. At 7(10° (', SiP z and Er,Oj are simultaneously formed, Ihns implying the decumposition of the silicide, The t)xide Ixyer has a SiO.~ lerminalion at the surface. Only about half of ale decomposed Si atoms reacl with oxygen. Tile preferential oxidation of Er is altribulcd to the high wdne of the heat of formation of Er~O~. An oxidalion mechanism is proptlsed.

I. Introduction T h e r m a l oxidation of transition-metal silicides has received great attention I1,2[, b e c a u s e of their use in silicon microclcctronic technology. O n the o t h e r hand, rare-earth ( R E ) silieides, a new class of silicides, are currently studied in several laboratories. T h e s e :;!!icides have interesting electrical and structural properties: low Schottky barrier height on n-type silicon [3], low resistivity [3,4], perfect epitaxy on St(111) [4,5]. Possible applications as infrared detectors or ohmic contacts on silicon are predicted [6]. Moreover, as R E are much more reactive with oxygen than normal metals, investigations of t h e oxidation of R E siltt i d e s may bring new information on the general p h e n o m e n o n o f the oxidation of silicides. W e have used photoelectron spectroscopy (XPS and U P S ) and L E E D to study the reaction of i2rSil. 7 with O , at room t e m p e r a t u r e and at 7 0 0 ° C . This work follows the two o t h e r studies performed on WSi,,. [7] and TaSi 2 ['g] overlayers on silicon. * To whom correspondence shtmld he addressed. ** ,' ssoclaled to University Joseph Fourier. Grem~ble.

2. Experiments E x p e r i m e n t s were carried out in an ultrahigh vacnum (base pressure ~ I x Ill -Ill m b a r ) twoc h a m b e r V S W E S C A , e q u i p p e d with XPS, UPS, L E E D and facilities for thin-film preparation and ox3,gen admission. E r b i u m silicide ErSi~ 7 layers, of ~ 50 A thick, were epitaxially grown on Si(l 11 X7 × 7) surfaces by annealing double layers of E r and Si in the proportion o f ~ 1 E r : 2 Si at 7 5 0 - 8 0 0 o C [9]. They were c h a r a c t e r i z e d by surface tcclr -'l.ues, in the s a m e m a n n e r as described before [5j. Oxidation was performed at room temp e r a t u r e under _< 2 x 10 -5 m b a r and a t 1 atm o f oxygen, and at 7 0 0 ° C , under 2 × 10 -5 m b a r o f oxygen. For low-temperature experiments oxygen was a d m i t t e d to She preparation c h a m b e r through a leak valve. For high oxygen pressure the sample was transfcrred to an oxidation chamber, fabric a t e d for superconductor oxide studies and connected to the preparation chamber. A f t e r each oxidation cycle the s a m p l e was transli-~rred to t h e analysis c h a m b e r and measured, T h e electron s p e c t r o m e t e r was calibrated with reference to the Au4f7/7. line ( E b b 8 4 cV). T h r e e X-ray lines were used, the standard M g K a for detection o f

0169-433,'/92/$05.0(I !~ 1992 - Elsevier Science Pgbilsher~ B.V. All right° re:~'rved

502

N. ~tl~r]'i t'l aL / gkhlatloa of lhta l:'r,$~,z ot e&Oer~¢,~ Stl t l l )

the S i 2 p and O Is signals, the mnnochromatized A I K a for measuring the E r 4 d core-level which needs a high resolution in energy in ord¢~ to separate the spin-orbit splitting, and the Z r M~ ( 151.4 eV) which gives a Si 2p signal very sensitive to the surface.

Hell 40.OeV

3, Results and discusslon ]n order to obtain reference XPS spectra for O l s and E r 4 d in erbium oxide E r g o 3 we first performed oxidatlon of a "'thick" ( ~ ltHt A) Er film deposited on a foil of Ta, which does not react with RE. This film was saturated with oxygen at room temperature and post-annealed at 750 ° C to produce the final oxide phase of Er.~O~. We will examine successively in what follows the oxidatiog of epitaxial ErSil7 thin films at room temperature under _~ 2 × It) -5 mbar and I arm of oxygen, a m it 7II0°C under Po: = 2 x 10 mbar. 3. I. Oxid~uion at rrxmz temperature (RT)

At R T and under low oxygen ? r e s s t , e (_< 2 x 10 5 mhar) erbium silicide reacts very weakly with oxygen. After exposures of 10 and 102 lanemuir (I iangmuir ( L ) = I T o r t . s ) no change is o b ~ r v e d on both the electronic and atomic structure of the silicide: the U P S and X P S spectra exhibit the same structures as the clean surface ones and the characteristic v'~ × ~I:~ R30 L E E D pattern is not modified. Changes begin to appear only at l0 s L exposure, where L E E D indicates a faint (1 × 1) diagram with very weak extra "/3 spots, and the U P S H e l l valence band (fig. 1) reveals a small hump at ~ - 6 . 5 eV, inside the E r 4 f energy region, indicated by an arrow (curve b)` The U P S spectra given by the He 11 radiation ( h v = 40.g eV) is extremely sensitive to the surface. U p o n ~ 5 x 109 L oxygen exposure ( P = 2 × 10 -5 tabor, t = 6 rain) L E E D spots disappear and important changes are observed by XPS and UPS. T h e valence band of ErSi 17 is greatly attenuated and a broad and large structure emerges, con,

f _10 -5 EF INITIAL ENERGY (eV)

Fig. 1, LIps spectra (lie II. 40.8 eV): a: dean ErSi: 7 surface; b: oiler lot~ L expt~sur¢ of O2: e: after 5 x Ill 3 L exposure of 0 9 d: after 15 mmnexp,)sure al I aim of O:; e: after 2 rain t,~idalion at 70a~c. Po: ~ 2 x la 5 tabor. The valcnce build remains the same Ior longer oxidalion. tered around ~ 6,5 eV, the energy position of the O 2 p states (fig. I, curve c). At the same time X P S detects both S i - O and E r - O bonds. The S i 2 p core-level spectra, detected at glancing angle (60 ° from the surface normal) in order to enhance the surface contribution, are reported in fig. 2a. After O z exposure this core-level (curve b) presents a shoulder at the left-hand side. This gives, after subtraction of the clean silicide contribution (represented by curve a) a broad structure centered at about 2 e V from the clean Si peak. This energy range corresponds to the Si in silicon suboxides [10]. Similar extra structures were a l ~ observed for WSi, [7] and TaSiz [8] and were attributed to chemisorption of oxygen on the Si atoms. A same interpretation is given for the present case of erbium silicide. As for the E r 4 d core-levels (fig. 2b), in comparison with clean ErSiL7 (curve a)` a 5 x 10 ~ L oxygen exposure enlarges the width of the spectra (cur'~e b) by an extra-emission indicated by the hatched area. As this extra-emission occupies the same energy region as Er:O3 (dotted curve), we att r ~ u t e it to the Er atoms bonded to oxygen.

N. Guerfi et aL / O.ridalion o f dim ErSi i r ol'erla~wra un Sill I I )

Er4d

S~2p

/

....../ i

i

i

I

~

i

i

99 177 167 BINDING ENERGY (eV) Fig. 2. to) XPS IMgK(¢) Si2p cure-leve! ::pc'.l:'::~e'~e.~led al gl,~acingangle (fin ° from the normal Io the surface) in order to enhanL~ the ~urface emi~ion: a: clean ErSi t 7; b: after 5 × 103 L of O,; c: after 15 mln exposure at I aim of O.1. (b) XPS Er4d core-levels, taken wfth wamochromatized AIKa X-ray. in ~lrder Io have a muximumenergy resolu[1on. Dolled line: reference E%Oj spectrum: a: clean ISrSilr: U: after 5 x 103 L oxygen exposure: c: after 15 min .:xposure at I arm ofO z. ~3

These two types of oxygen bonds can be also deduced from the O Is spectra (fig. 3). The curve given by oxygen chemisorption (curve b) is broader than the one measured c;, the reference oxidized

O~

BINDING ENERGY (eV)

Fig. 3. XPS (MgKt~] OIs speclra: a: reference Er20 ~ layer: b: alter 5 × 103 L a~gen exposure; c: after 15 rain exposure at I arm of O z.

5113

Er film (curve a), It gives, after subtracting the contribution of the E r - O bonds (dashed line 1), a second component (dashed line 2) centered at about 533 eV, which covers the binding energy region of s!licon oxides [11]. Fo; this curve decomposition we suppose that the E r - O bonds are represented by a symmetric peak centered at the O l s energy position in E r 2 0 ~. The peak areas of these 2 components give approximately 75% of oxygen banded to Er atoms and 25% bonded to Si atoms. After 15 min exposure under 1 arm of oxygen phote,clcctron spectroscopy detects an oxide phase at the surface. The UPS spectrum (fig. 1, curve d) is characteristic of an insulating oxide surface, the silicide valence states completely disappear and the O 2 p emission dominates the valence band. Important changes are also observed by XPS, in comparison with the chemisorption phase seen above. The oxide part of the S i 2 p core-level is more predominant and is formed of 2 components indicated by 2 and 3 on the figure (fig. 2a, curve c). The first one has an energy shift of ~ 3.4 eV, e.g., the value measured on SiO 2 thin films [101. The last one occupies the energy position of silicon suhoxide [10]. The silicon oxide is thus composed of SiO 2 and siiieon suboxide. A~ for the E r 4 d level (fig. 2b, curve c), it has the shape of Er_,O s oxide, but with a slightly smaller binding energy shift. As the structures of R E core-levels are very complex [12], we have not analysed in detail this spectrum but taken in account only the presence of E r 2 0 3 oxide on the surface. The formation of silicon and erbium oxides is also revealed by the O Is core-level (fig. 3b, curve c) which is decomposed, as above in the case of low oxygen pressure, into Er oxide component (line 1) and silicon oxide component (line 2). Here also we represent first the E r - O bonds by a symmetric peak centered at the O Is energy position in ErzO 3. Then, line 2 is deduced by subtracting line l from the experimental peak. T h e intensities of these components correspond to a number at O atoms bonded to Er slightly greater than the one bonded to Si. In the chemisorption phase we have seen that these two types of oxygen were in the proportion of ~ 1 to 3.

,~ ~hterfi a aL / Omfamm of thin ErXi t ~ ot erla~t,r~ on Sit l l l )

It is imporlaut to underline here that erbium silicide is much less reactive with oxygen than silicon, erbium or normal metals• At r(mm temperature an exposure of several L of oxygen is sufficient to form ehemisorption phase and to modify the electronic properties of silicon [10], erbium [13] and normal metals [14], while ~ 5 × ]03 L are needed to produce detectable effects on ErSiL7 surface. The only other stud~, of oxygen a d ~ r p f i o n on erbium silicide was done by Valeri el al. [15] by A E S and EELS. These authors have found that at exposures up to 103 L bolh components of the silicide react with ox~'gcn to give snccassivel~. Er_,Oj and SI P,, In our case, the formation of SIP., is observed only at I aim of oxygen. A possible cause of the diserepaney could be the nature of the silieide surface, epitaxial ErSil. 7 here and sputter cleaned with a ErSi-like stoiehiometry in Valcri's experiments.

•

Er4d

(b)

3 . 2 Oxidation at 7 0 0 ° C

qC3 99 172 167 BINDING ENEP.GY (eVl Fig. 4. (a) XPS (MgKa) Si2p coreqevels registered at the normal to the surfa~ t[ull line curves) and at 6g ° from this direclion (dashed line culwes), cu~e a: after 2 rain oxidalion. 70ft~C. P(I~ 2 x Ift " rebut; cuwe b: after 25 rain under the ~me condilions; crave c: Si2p given by the ZtM~ fine (hi, 1514 eV) shim'lug siP., on Ihe lop of the oxide laver (see texl). (ft) XPS Er4d core-levels tmonochromatlzed AIK~) afler oxidaliOnat 7UO°U.P ( ~ - 2 × I O 5 mi'~ar,curve ~: I min o~idation; cu~,c b: 25 rain. The dashed lines show the 2 ¢~lmponents:(I )silicide, (2) oxide.

The XPS Si2p, E r 4 d and O Is core level spectra indicate that both silicon dioxide S i P 2 and erbium sesquioxide Er.,O 3 are formed on the silieide surface• In fig. 4a are grouped the S i 2 p spectra taken after 2 rain (curves a} and 25 rain (curves b) expositions under P(r, = 2 × 10 -s mbzr at 7 0 0 ° C . Two detection angles were used. perpondicular to the surface (full lines) and at 60 ° from this direction (dashed lines). A spectrum taken with t h e Z r M ~ X-rxy !ine ( h u = 151.4 cV) is also reported for comparison (curve c). This latter, due to its kinetic t.nergy in the 50 eV range, is "~ery seasitive to the surface. These spectra p r e ~ n t a peak at the left-hand side of the silicide structure, centered at about + 3,4 eV, i.e., the energy position of Si 2p in SIP_,. This supplementary structure increases with the exposition time t indicating a continuous growth of S i P 2. At the same time, E r is also oxidized, as testified by the E r 4 d core-levels given in fig. 4b, for 1 rain (curve a) and 25 rain (curve b) oxidations. We see that the shaoe of the peak changes after each oxidation : the double structure due to s p i n - o r b i t coupling disappears and is replaced by an asymmetric broad feature, which gives after subtrac-

tion of the contribution of the silicide (dashed line 1), a second component (dashed line 2). "/'his component 2 is similar to the structure measured in ErzO 3 oxide and has the same energy position. It is attributed to E r r O 3 oxide formed on the silieide layer. The presence of the two types of oxides, S i P r and Er20~ is also indicated by the O l s XPS spectra given in fig. 5 for 2 oxidation times, 2 and 25 rain, at detection normal to the surface (re. speetively curves a a~d b) and at 60 ° from the surface normal (respectively curves a' and b'). The broad width of these spectra is typical of a multiple chemical state of oxygen. By using the method described above, we have decomposed these spectra in two components, drawn here in dashed lives. T h e second one, labeled 2 in the figure, occupies the energy position of S i P z ( E h 533 cV) and the first one, labeled 1 has the characteristics, of E r r O 3 ( E h ~ 5 3 1 . 6 eV). The simultaneous formaticr of S i P 2 and ErzO ~ is again confirmed. Moreover, at glancing angle detection the S i P z peak is higher than the E r r O 3 one. This indicates an enrichment of S i P 2 on the

Iv~ Gaerfi et aL / Oxidatton a f dt#z ErSi I 7 urcrhn'er.~ t~lr Si( l l I)

5.~

553

551

BINDING ENERGY {eV)

Fig. 5. XPS (Ms Ka ) O IS core-levels after oxidalion al 700 ° C, Po: = 2 X 10 s mbar. Curves a and a': 2 rain: curves b and h': 25 rain. Curves a and b: along the surface normal; curves a' and b' at glancing angle detection (60 ~ t. surface. This observation is in concordance with the S i 2 p core-level excited by the ZrM~: 151.4 e V line (fig. 4a, curve c) where the SiO 2 is the dominant structure. From these XPS results the following thermal oxidation mechanism can be proposed for ErSi 17, The oxidation begins with the upper layer of the silicide, which is generally terminated by Si atoms, when the sillcide is prepared by solid phase epitaxy on S i ( l l l ) [16,17] giving first a thin S i O , layer. It continues afterwards with decomposition of the silicide and simultaneous oxidation of Si and Er to form a mixed layer of S i O z and E r 2 0 s. However, all of the decomposed Si atoms do not oxidize: the O Is components attributed respectively, to SiO z and ErzO 3 are in the proportion slightly greater than unity while a complete oxidation of the Si atoms would give a ratio equal to 2.2. This deficit in SiO~ could be attributed to the high reactivity of E r with oxygen, combined with a limited quantity of oxygen atoms available at the silieide-oxide interface. For oxidation of silicon and of silieides [1,2], this oxygen supply proceeds by oxygen diffusion

505

through the oxide layer, the diffusion being the rate-limiting process. The unreactcd Si atoms would then diffuse back to the Si substratc. A similar deficit of SiO 2 has been detected by RBS in the thermal oxidation of GdSi~ by Sun et al, [18]. The proposed rea:;on was a barrier effect of Gd oxide on the diffusion of Si atoms. For this R E silicide, two types of oxides, G d z O s and SiO2, were also detected, O u r results clearly indicate that the oxidation at high temperatures of the rare-earth silicide ErSiL7 is very different from the one of transition-metal and nca:-uoble-mctal silicides. In both of the latter cases, the oxide overlayers are formed of pure SiO z or SiO 2 with ocgtiglble amounts of metal oxides [1,2], less than a few parts per thousand. The preferential oxidatiot~ of Er, in comparison with Si, can be explained in the frame work of thermodynamics, by means of the heat of formation AHf of metal oxides and SlOe [19]. For Er203, the standard a H f is equal to - 9 3 . 9 kcal/g-atom [20]. whereas for SiO 2 [2] it is only 68.5 kcal/g-atom. The decomposition of the ErSi 17 silicide und e r oxidant ambient could hc an inconvenience for applications in microeleetrooies. But as the mixed SiO2 -~ Er203 overlayer has a good insulator characteristic, the U P S valence band (fig. 1, curve e) being ~,ery similar to the SiO z one [8], and as R E oxides are excellent corrosion protectors [21], this inconvenience seems to be not important.

4. Conclusion The main results of this study are the following: - At room t e m p e r a t u r e and low oxygen pressure (Po, -< 2 × 10 -5 mbar) ErSi~ 7 silicidc is less reactive than silicon, erbium or normal metals. - Both Er and Si react with oxygen. At ro'ola temperature, low-pressure exposure gives rise to a chemisorption phase, while 1 atm of oxygen produces a mixed layer of E r 2 0 3, SiO2 and Si suboxides, At 7 0 0 ° C and under 2 × 10 -5 mbar oxygen pressure the silicide is decomposed with SiO 2 and E r 2 0 3 growing simultaneously on the

5Or,

IK Glter]i el .L / OMdatum off thbl ErSi t 70ledayer~ or1 Si( I I D

surface. T h e u p m o s t layer o f this m i x e d o x i d e is f o r m e d of s i n , . O n l y a h o u t h a l f o f t h e Si a t o m s l i b e r a t e d by t h e d e c o m p o s i t i o n o f t h e silicid¢ is oxidized, t h e r e m a i n i n g a m o u n t w o u l d diffuse b a c k to t h e Si substrafc.

Acknowledgement T h e a u t h o r s wish to t h a n k D r . F. A r n a u d d ' A v i t a y a for h e l p f n l discussions,

R¢[erences [1] F.M. dlieurl¢, A. Cros, R.D. Ftampton and E.A. Irene, Phil. Mug, B 55 (1987) 291. [2J M.A Niggler, in: VLSI Eleclroldcs, Ed. N.G. Einspruch (Academic P~'ess, New York. !9831. 131 Y. Duboz, P.A. Badoz, F. Arnaud d'Avitaya and E. Roscucher, Phy~, Rev. B 40 (1989) l(1007. [4] F. Arnaud d'Avilaya, P.A, []adoz. Y. (.'ampidclli, J.A. Chroboczek, J.Y. Duboz, A . Pcria and J. Pierre, Thin Solid Films I S4 ( ] b~J())283. [Sl J.Y. Vcuillen, T.A. Y¢gttycnTan, D.B. Lollman, N. Guerfi and R. Cinli, Surf. Scl. 251/Z'32 (19911 432. [6] J.A. Knapp, S.]. Picraus. C.S. VCu and S.S. Lau, Appl Phys. Left 44 (19841 747.

[7] T.A. Ngu!,t:n Tan, M. Aziz3n and L Derrien, J. Vac, Sei. Technol. A 5 (1987) 1412. [81 T.A. Nguyen Tan, N. Ouerfi, J.Y. Veuillen and J. Derrlcn, Appl. Surf. Sci. 4] (1989) 266. [9] T.A. Nguyen Tan and J.Y. Veuilh:n, ECO-4. The Hague, The Netherlands, March 1991. lll)] G. Hollins,:r, J,F, Murat, FJ. Himpsel, G. Hushes and J.L. Jordan, Surf. Sci. 168 (1986) 609. [1 ;] J.E. Rowe. G. Margaritundo. H. Ibaeh and H. Fruilzheim, Solid State Commun. 20 (19761 277. [12] M. Campagna and F.U, I [illehrechL in: Handbook oil the Physics and Chemistry of Rate Earths. Vol. 10. Eds. K.A. Gsehn.~ider and L Eyring (North-Holhmd, Amsterdam 1987h 1131 F.P Nctzcr, R.A. Will¢ and M. Gronze, Surf. ScL 102 (19811 75. [14] SCc fur example G. Wilder, W. Kieszling and D Boringmann. Vacuum 41 (19901 93. [15] S. Val¢rl, U. I)el Pen(ruing, G. Otlaviani, P. Sassaroli and K.N. Tu~ Solid State Commun. 0986) 569. [16] C. Pirrl, J.C. Peruchetti, D. Bolmont and O. fit:winner, Phys. Rcv. B 33 ( 1986141S8. [17J R, Baptisl, S. Ferrer, G. Grenet and H.C. Pl:on. Phys. Rev, Len. 64(19901311. [18] H.V. Sun, G. Mezey. G. Pet6, F. Paszti, E. Kotal. A. Manuaba, M, Fried and J. Gynlai, Nud. Instr. Moth. B 15 (1986) 247. [19] R. Beyers, J. Appl. Phys. 56 (19841 147. 12O] B.M. Angelov. J. Phys. C 15 (1982) L 239. [21] F.M. S¢on, J. Legs-Common Mel. 148 (19891 73.