I. INTRODUCTION
The high structural quality that can be obtained with epitax- ial thin films of silicides on silicon has aroused considerable interest. Of particular interest are the silicides NiSi, and CoSi2, which have the CaF2 structure with lattice param¬ eters only 0.4% and 1.3% smaller than that of Si, respective- ly. Two orientations with respect to the Si( 111 ) substrate, denoted as type A and type B, are known to be possible for a film with the CaF, structure. Type-A films share the orienta- tion of the silicon substrate, while a type-B film differs from the substrate by a 180° rotation about the surface normal. With proper recipes of Ni deposition and annealing, one can grow a NiSiz film of either type A or type B, ' whereas CoSiz generally grows as type B. 2 It is possible, however, to grow films with mixtures of type-A and type-B orientations. 3.4 It has been observed that different preparations of epitax- ial CoSi 2 on Si( 111 ) produce different terminal surfaces. 3 . 3 For deposition of ~ 1 monolayer (ML; 1 ML = 7.8 x 1014 cm-2) Co on CoSi2(111), a low-temperature (420 °C) an- neal produces a more cobalt-rich "CoSi2-Co" surface while annealing above 550 °C yields the silicon-rich "CoSiz-Si" surface. Hellman and Tung 3 propose that CoSi2-Co is bulk CoSi 2 truncated between Si-Co-Si trilayers, and that CoSi 2-Si is CoSi 2-Co with an additional Si bilayer. Tung and Batstone 6 have proposed that the formation of pinholes of- ten observed for high-temperature annealing is associated with Si atoms being supplied from the substrate to form the CoSi,-Si surface. An understanding of the electronic structure of cobalt- modified Si( 1 1 1 ) surfaces is important to the scientific and technological interest in epitaxial silicides. As noted in a re- cent theoretical study 7 of the electronic structure of bulk CoSi2, good conduction through a heterojunction requires matching of electronic states between substrate and over- layer; the authors find no such match for CoSi 2 (111) on n- Si(111). 7 It is important to test this calculated band struc¬ ture experimentally. Furthermore, the states observed by angle resolved ultraviolet photoemission spectroscopy (ARUPS), especially those identified as surface states, are used as an indicator of surface atomic structure. We have studied valence-band electronic structure of the Si(111)-epitaxial CoSi, system in three types of samples: (i) thick CoSi, films to examine its bulklike dispersion; (ii) de¬ posits of 0.5, 1.0, and 2.0 ML of Co annealed to form CoSi 2 , to emphasize the effect of surfaces; and (iii) unannealed de- posits up to 0.5 ML to study the initial reaction of Co on Si(111).
II. EXPERIMENTAL PROCEDURE
Lightly-doped n-type Si( 111 ) substrates were cleaned by cycles of Ne+ bombardment (1 keV) and annealing at 900 °C measured by an infrared pyrometer, followed by slow cooling. This yielded samples with sharp, strong 7 × 7 low- energy electron diffraction (LEED) patterns and an Auger electron spectroscopy (AES) peak height ratio C (272 eV):Si (92 eV) of typically 0.002. No measurable oxygen AES signal was detected [O (510):Si (92 eV) <0.0003] using a single-pass cylindrical mirror analyzer (CMA). ARUPS spectra for substrates cleaned in this way showed the three surface states characteristic of a clean Si(lll)- 7 × 7 surface. 0734-2101/89/032449-05$01.00 2449 J. Vac. Sci. Technol. A 7 (3), May/Jun 1989 © 1989 American Vacuum Society 2449 Cobalt was deposited on Si( 1 1 1 )-7 X 7 substrates from a Co pellet heated by electron bombardment and surrounded by a shuttered collimating shroud cooled with liquid nitro- gen. Typical Co deposition occurred at a rate of 0.4 ML/ min; the rate was checked using a quartz-crystal thickness monitor. The sample Co coverage was determined by AES with the total accumulated coverage calibrated using Ruth- erford backscattering spectroscopy (RBS). A 5-min depo- sition caused a pressure rise of < 3 × 10 - 10 Torr. A small increase in carbon contamination (to an AES ratio C:Si of 0.006) was observed after 2 ML of Co deposition, with no oxygen contamination. CoSi, films were grown in situ by depositing Co onto a substrate heated at 450° ± 30 °C by combined radiative and electron-bombardment heating from behind. Co coverages of 0.5, 1, 2, and 10 ML Co were used to produce 1.6, 3.1, 6.3, and 31 Å of CoSi 2 . Below - 525 °C the Si samples were transparent at the infrared (IR) wavelength used by the py- rometer (1.1 pm) so the temperature was estimated by ex- trapolating sample temperture as a function of heating pow- er. The ratio of the low-energy Auger peak heights, Co(MMV)/Si(LVV), was measured for each CoSi, film us- ing a single-pass CMA at a modulation amplitude of 2 eV p.-p. The value for 31 Ã agrees with that for CoSi2-Co on bulk CoSi , , ³ while the ratios for 1.6, 3.1, and 6.3 Å are con ­ sistent with layer-by-layer growth of CoSi 2 -Co. 8 Additional studies on CoSi 2 were done using a thick ( ~ 100-Å CoSi,) layer grown epitaxially on Si(111) in a separate molecular-beam epitaxy (MBE) system by depo- sition of both Co and Si. This layer was capped with amor- phous Si, which was sputtered off after transfer into our sam¬ ple preparation chamber. Sputtering was stopped when the CoSi, layer was reached, to avoid depletion of Si at the sur- face. This was followed by annealing at 600 °C to produce a CoSi -Si termination. Prepared samples were transferred in vacuo into a sepa- rate chamber, with a typical pressure of 1 X 10~ 10 Torr, for ARUPS measurements using linearly polarized synchrotron radiation at the National Synchrotron Light Source. A toroi¬ dal-grating monochromator 9 was used in the photon-energy range 12-80 eV, at a resolution of Ahv<0.05 eV. The hemi- spherical electron energy analyzer 9 was typically operated to give an overall energy resolution of AE<O.1 eV full width at half-maximum (FWHM) height and angle resolution of A0 ~ 1° FWHM. Electron energies are initial-state energies with respect to E f = 0.
III. RESULTS AND DISCUSSION
A. Annealed CoSi² films
Figure 1 shows photoemission spectra taken at normal emission from the MBE-grown CoSi2(111) sample, which show dispersion of bulk states with respect to photon energy variation. They are dominated by a band of A, symmetry, which disperses upward from Ej = — 2.1 eV at hv = 25.5 eV and crosses E f at hv~ 14 and 50 eV, and a A 3 band which disperses between — 1.55 eV at hv = 25.5 eV and — 1.9 eV at hv = 60 eV. There also appears to be another weak feature dispersing toward the Fermi energy for hv = 30-40 eV; its J. Vac. Sci. Technol. A, Vol. 7, No. 3, May/Jun 1989 origin is undetermined. This dispersion with photon energy has been interpreted 10 as dispersion with respect to k 1 , which we estimate by assuming a free-electron final-state band, so that
k 1 = [(2m/ñ²)(hv + E i — Φ + V 0 ] 1 /2.
The parameter Φ = 4.75 eV is the work function for CoSi2- Si 5 and V o  = 14.8 eV is the assumed value of the inner poten¬ tial measured with respect to the vacuum level. 10 The initial- state energy at symmetry points is then — 1.9 eV for F 12, — 2.1 eV for L 1 , and — 1.6 eV for L 3 . These agree very closely with the calculated values presented in Fig. 1 of Ref. 7. As photon energy increases above hv = 50 eV, the photo- electron escape depth decreases to one or two lattice con¬ stants, and the corresponding uncertainty in k, becomes a large fraction of the Brillouin-zone size. The spectra are thus dominated by a k 1 -selective surface density of states (DOS) emphasizing Co (3d) states because of their higher cross section. The maximum for hv = 60 eV lies at  — 1.9 eV, which coincides with the peak in calculated DOS for bulk CoSi, in Fig. 5(a) of Ref. 7. Figure 2 shows off-normal spectra for the same sample at hv = 60 eV in the (1 IO) mirror plane, with polarization that selects even-parity states. At this photon energy, θ ,  =  0° cor- responds to Γ in the extended-zone scheme for a free-elec¬ tron final state, and angle sweeps 0° to  — 32° and 0° to + 20° correspond approximately to F-L-F and to F-K in the bulk Brillouin zone. For the most part the peaks observed can be explained in terms of the calculated band structure in Ref. 7. The CoSi 2 layers grown in situ, with CoSi 2-Co termina¬ tions, are significantly different from CoSi 2 -Si, as we see in Fig. 3(a) for a 31- Ă CoSi 2 layer. Particularly at higher, more surface-sensitive photon energies, the shapes of the spectra are different, with the positions of the maxima shifted to- ward E F . At hv = 60 eV the maximum is at  — 1.5 eV, shift- ed by 0.4 eV toward E F compared with CoSi 2 -Si (Fig. 1) and with calculated bulk DOS. 7 At lower photon energies, which mostly reflect dispersion of bulk states, the spectra are much more similar between CoSi2-Co and CoSi2-Si. The features have the same shape, and while there is still some apparent shift due to increased DOS intensity around — 1.5 eV, it is much smaller. This indicates that the shift at higher photon energies denotes a change in surface electronic struc¬ ture, probably because Co atoms have fewer Si neighbors at the CoSi 2-Co surface than in bulk CoSi 2. The peak in surface DOS lies at the same energy as the surface resonance/Tamm state at  — 1.4 eV reported by Pirri et al.'' Thinner CoSi 2 films also display a shift of the surface DOS peak toward E F , but for both a 6.3-Å layer [Fig. 3(b)] and for a 3.1-Ă layer (not shown) the shift is smaller than for the 31-Ă film. One explanation for this may be that part of the DOS peak at - 1.5 eV is associated with defect sites in the films grown by Co deposition and annealing. If limited mo- bility of Si at the annealing temperature causes films to have regions that are, say, slightly Si deficient, then thicker films will have a greater incidence of these defects. The CoSi2-Si film of Fig. 1, on the other hand, was grown by deposition of both Co and Si and would not have defects associated with J. Vac. Sci. Technol. A, Vol. 7, No. 3, May/Jun 1989 incomplete transport of Si. It should also be noted that a small part of the apparent peak shift, < 0.1 eV, is an artifact of different photon incidence angles used for the spectra of Fig. 1 (45° from normal) and those of Fig. 3 (70°): in the latter case, an increased proportion of E, in the linear photon polarization enhances emission from the A , states, which lie closer to E, than the A, states along most of the F-L  line. The CoSi--Co surfaces also display a new peak A with E, =  — 3.0 eV. This peak is more apparent for the 6.3- å film than for the 31- Ã film, particularly at the more surface¬ sensitive photon energies. While this feature is close to the surface state on the CoSi,-Co surface at — 2.7 eV reported by Pirri et al.,'' it is not the same: the sample used for Fig. 3(b) also displays the — 2.7-eV surface state at proper pho¬ ton energy and incidence angle, and feature A shows the opposite incidence-angle dependence. Feature A is not ac¬ counted for by the bulk band structure of CoSi, nor is it seen in comparable spectra for clean Si( 1 1 1 ). It thus seems to be a state specific to the surface or interface. Off-normal polar- angle scans reveal dispersion qualitatively similar to Pirri's surface state.'' We note also that thinner CoSi, films (3.1 Ä) show peak A only weakly and at a different energy; this is not surprising since these films consist for the most part of a single CoSi, trilayer, which should be electronically differ- ent from a surface layer on bulk CoSi, or even on a two¬ trilayer film. An intriguing possibility is that peak A is actu- ally a state like Pirri's surface state but localized at the Si/ CoSi, interface rather than at the surface. This hypothesis is suggested by our data, since peak A appears in our more surface-sensitive spectra only when the CoSi, layer is thin; but further experimental tests are necessary.
B. Unannealed Co deposits
Figure 4 compares photoemission spectra at normal emis¬ sion for unannealed deposits of 0.1, 0.2, 0.3, and 0.5 ML Co on Si( 1 1 1 ) with corresponding results for annealed deposits of 0.5, 1.0, and 2.0 ML (i.e., CoSi, films). We note first that at low coverage there is increased intensity in the peak near — 1.8 eV, where the S, surface state of Si(111)-7×7 ap- pears. Upon Co deposition, the S, surface state peak is prob- ably quenched and is replaced by emission from Co(3d) states; a slight change in peak shape suggests this. Since this energy corresponds to the DOS peak in bulk CoSi 2 , growth of this peak suggests that the initial 0.1 ML of deposited Co is strongly bonded to Si, in an environment similar to that in CoSi 2 . It would be interesting to perform surface EXAFS measurements as a function of Co coverage to see if there are the expected differences from the early stages of Si/NiSi, interface formation. ' ³ Further deposition causes more growth near — 1.4 eV and above, as seen in the difference curves of Fig. 4(b). This is indicative of Co less strongly reacted with Si. More signifi- cant is the growth of a definite peak B at ~ — 2.6 eV for 0.5- ML deposition; this is also visible as a weak shoulder for 0.3 ML. Peak B appears very similar to the surface state found by Pirri et al. 11 It is easily quenched by contamination, and for 0.5-ML coverage is not visible after ~ 1 h at 1 x 10 - 10 Torr. At normal emission it appears stronger in spectra with glancing photon incidence (70° off normal) than for near- J. Vac. Sci. Technol. A, Vol. 7, No. 3, May/Jun 1989 normal (25°) incidence; this is true also for Pirri's surface state, which has A, symmetry. It apparently does not dis- perse with variations in photon energy; although below 18 eV the changes in the bulk-Si contribution to the spectra make this difficult to test.
IV. CONCLUSIONS
There is a clear electronic difference between the Si- and Co-rich surfaces of CoSi2. In CoSi2-Si even the outermost Co atoms are in an essentially bulklike electronic environ- ment; the states we observe agree well with the calculated bulk band structure. 7 In CoSi2-Co the outermost Co atoms are in a different environment that pushes the (3d) states upward toward the Fermi level. Some of the Co(3d) shift may also be due to incomplete Co bonding in defected re- gions of the films. In addition to altered Co(3d) behavior at the surface, our results also suggest the possibility of an in- terface state A that consists of a Co(3d)-Si(3p) hybrid near the Si/CoSi, boundary as formed at 450 °C. More detailed examination of the behavior of this state should be a fruitful step toward understanding the structure of this interface. Valence-band studies of unannealed deposits of submono¬ layer Co coverage help to confirm and refine the conclusions of previous core-level studies 13 that initially deposited Co ( < 0.5 ML) occupies CoSi 2 -like sites. It appears that rough- ly 0.1-0.2 ML of deposited Co on Si(111)-7X 7 occupies an almost bulklike environment, while further deposited Co reacts less strongly. Some of this less-reacted Co seems to be in an environment similar to that of the outermost Co in CoSiz-Co, with a photoemission peak at  — 1.4 eV. This con¬ clusion is also supported by the presence of a state B similar to the surface state at — 2.7 eV reported by Pirri et al., for the CoSi2-Co surface. '' Further studies of this interface for- mation will focus on the properties of Co/Si hybrid states in this energy range.
ACKNOWLEDGMENTS
Research carried out at the National Synchrotron Light Source, Brookhaven National Laboratory, which is support- ed by the U.S. Department of Energy, Division of Materials Sciences and Division of Chemical Sciences (DOE Contract No. DE-AC02-76CH00016). Chambliss and Rhodin ac- knowledge support by the Materials Science Center at Cor- nell University (NSF-DMR16616-A01). We are grateful to William O'Brien, Paiboon Tangyunyong, and Ron Ozer for help with the experiment and to Raymond Tung and Steve Yalisove for supplying a thick CoSi 2 sample and for per- forming RBS measurements. 10- ML deposited Co. For 1-ML deposited Co, the ratio indicates a surface too cobalt-rich for either a CoSi,-Si termination or large islands thicker than 1 CoSi, trilayer. If the escape depth of 53-eV electrons in CoSi, is 8- 10 A, the quantitative agreement is excellent, but peak heights in the deriv- ative-mode Auger data are misleading because the peak shapes for Si in Si and in CoSi, are markedly different. The AES results were calibrated by RBS measurements. 9P. Thiry, P. A. Bennett, S. D. Kevan, W. A. Royer, E. E. Chaban, et al., Nucl. Instrum. Methods 222, 85 (1984). 10G. Gewinner, C. Pirri, J. C. Peruchetti, D. Bolmont, J. Derrien and P. Thiry, Phys. Rev. B 38, 1879 (1988). "C. Pirri, G. Gewinner, J. C. Peruchetti, D. Bolmont, and J. Derrien, Phys. Rev. B. 38, 1512 (1988). 12F. Comin, J. E. Rowe, and P. H. Citrin, Phys. Rev. Lett. 51, 2402 (1983). 13C. Pirri, J. C. Peruchetti, G. Gewinner, and J. Derrien, Surf. Sci. 152/153, 1106 (1985); F. Boscherini, J. J. Joyce, M. W. Ruckman, and J. H. Weaver, Phys. Rev. B 35, 4216 (1987).