The interrelation between electronic and crystalline structure of ultrathin metal films: Cu on W(110)

358

Surface

THE INTERRELATION BETWEEN ELECTRONIC ULTRATHIN METAL FILMS: Cu ON W(ll0)

G. LILIENKAMP, Ph.vsrkalisches

Received

Institut,

C. KOZIOt Technische

25 July 1989; accepted

AND CRYSTALLINE

Science 226 (1990) 358-370 North-Holland

STRUCTURE

OF

and E. BAUER

Universitiit

ClausthaI, D-3392 Cluusthal-Zeilerfield,

for publication

18 September

Fed. Rep. of Germurq

1989

The evolution of the electronic structure of ultrathin Cu layers on W(110) with increasing thickness is studied by angle resolved photoelectron spectroscopy and related to their crystalline structure and growth mode as determined by reflection high energy electron diffraction, photoelectron diffraction and Auger electron spectroscopy. The band structure is mapped for several thicknesses and symmetry analyzed. The influence of symmetry, packing density and interaction with the substrate is discussed.

1. Introduction Quasi-two-dimensional (2D) systems have attracted increasing attention in recent years because of their unusual physical properties such as the magnetic anisotropy in ferromagnetic films. A large amount of information has already been accumulated on the electronic [l] and on the crystalline structure [2]. Of particular interest is the interrelation between electronic and crystalline structure, its evolution in the transition from zero to three dimensions and the influence of the substrate on this interrelation. The goal of the present study is to deepen the understanding of this interrelation by a detailed study of the electronic structure of a specific system, Cu on W(llO), and by correlating it with its crystalline structure. The system Cu on W(110) was choosen for several reasons. First of all, Cu has a narrow 3d-band [3] in a binding energy range which is well separated from the major band structure features of W. Therefore, it should be particularly easy to separate the emission from very thin layers from that of the substrate. Secondly, the growth of Cu on W(110) has already been studied in detail [4,5]. At room temperature Cu grows in the quasi0039-6028/90/$03.50 (North-Holland)

0 Elsevier Science

Publishers

B.V.

Frank-van der Merwe mode, that is approximately monolayer-by-monolayer, which is essential for the goal of the present study. It passes through several structures within the first three monolayers, which differ not only in packing density but also in symmetry [4-61, similar to the closely related system Cu on Mo(ll0) [7]. This is important for the determination of the connection between crystal structure and electronic structure. Finally, there are already several studies of ultrathin Cu layers on other substrates with which the results for W(110) may be compared [g-13]. In this way some information on the influence of the interaction with the substrate may be obtained. Unlike to the earlier LEED work [4,5] the structural characterization of the Cu films is done here by reflection high energy electron diffraction (RHEED) because this technique not only allows to determine the structure but also simultaneously to monitor deposition rate and growth mode beyond the monolayer range, while Auger electron spectroscopy (AES) is particularly useful for the last purpose in the monolayer range. X-ray photoelectron diffraction is used to determine the crystalline quality in the surface region of thicker films. The electronic structure is obtained from a

G. Lilienkamp et al. / Electronic and crystalline structure of ultrathin metal films

detailed analysis of angle-resolved photoelectron (ARUPS) spectra obtained with unpolarized and polarized He1 light.

2. Experimental The experiments were performed in a VGESCALAB photoelectron spectrometer with an extra RHEED- and preparation chamber. A liquid nitrogen-trapped diffusion pump and titanium sublimators produced a base pressure of 4 x 10-i’ mbar. For photoelectron spectroscopy we used the He1 line from a Leybold cold cathode capillary discharge lamp with a homebuilt triple-reflection polarizer [14], which gave a linear polarization of about 90%. The spectra were recorded with an energy resolution of better than 150 and 100 meV for measurements with and without polarizer and a step width of typically 40 meV. An aperture at the entrance of the analyzer input lens reduced the acceptance to a cone with lo half angle. Angular distributions were sampled by rotating the crystal. The angle between incident light and detected electrons was always 36 O. The polar and azimuthal orientations were set by RHEED and via the symmetry of the photoelectron spectra with an absolute accuracy of better than lo. The relative error of the polar angle was k 0.2”. The position of sharp and isolated peaks could be reproduced to within f30 meV. An additional error of 30 meV had to be taken into account for the uncertainty of the Fermi level. Thus, the absolute binding energies are accurate to within k60 meV. The system was further equipped with an electron gun working at 1.8 keV and 3pA for Auger electron spectroscopy for which the hemispherical analyzer of the ESCALAB was used. Differentiated spectra were obtained by modulation of the target potential with 3 I&, for contamination checks and 1 I&, for monitoring the film growth. For the latter purpose, the differentiated M,,M,, M,, Auger signal was recorded during deposition. A VG X-ray monochromator with Al anode was used for photoelectron diffraction. RHEED was used to determine the structure, the growth mode and the deposition rate. For structural analysis RHEED patterns were taken at

359

various polar angles in the two W(110) azimuths, [llO] and [OOl], in which in previous LEED work [4-61 additional reflections were found. The RHEED data agreed completely with the LEED results and are, therefore, not reported here. Growth mode and deposition rate were monitored by RHEED specular beam intensity oscillations. In this case the glancing angle of incidence was always very small and a Faraday cup with a 500 pm aperture 250 mm from the crystal was used to measure the intensity. The beam energy was usually 15 keV, the beam current 3 PA. A more detailed account of this method is given elsewhere

1151. The tungsten substrate was oriented to within 0.05” of the (110) plane and cleaned by the usual procedures [4] resulting in an impurity to substrate ratio of the heights of the differentiated main Auger peaks of better than 1: 300. The deposition and measurement temperature was 300 K. The Cu deposition rate was monitored in the RHEEDand preparation chamber by a quadrupole mass spectrometer, the film growth by RHEED intensity oscillations as mentioned before. A second smaller evaporator was placed close to the entrance lens of the analyzer which made Auger and photoelectron spectroscopy possible during deposition. With careful outgassing a working pressure of less than 6 X lo-” mbar could be reached. The contamination of the films was in the same range as mentioned above for the clean surface even after a few hours of measurements.

3. Results We start with the flux calibration of our evaporators. Fig. 1 shows the intensity of the specularly reflected RHEED-beam during growth. The oscillations indicate layer by layer growth [15]. The strong damping is caused by the formation of a growth front whose shape is not changing significantly after deposition of a few layers. The degree of damping depends upon deposition temperature and deposition rate but also upon residual gas pressure [15]. Therefore great care should be taken to ensure clean conditions during the deposition. After deposition one can leave the

G. Lilienkamp

360

et al. / Electronic and crystalline structure of ultrathin metal films

mation of more and more three-dimensional islands. The series of superstructures at coverages below 3 ML,, were the same as in refs. [4,6]: up to 1 ML,,, only a (1 x 1) pattern was seen, that is the film grows pseudomorphically. Above 1 ML,, satellite reflections in the [ITO] direction corresponding to an approximate (15 x 1) superstructure form which increase in intensity up to 2.13 and 2.47 ML,,, (= 2 ML,,, . Between 2.13 ML,, ML,,) the (15 x 1) structure is gradually replaced by an approximate (1 X 8) structure which is caused by a double layer with nearly undistorted Cu(l11) periodicity. The quality of thicker layers was checked by photoelectron diffraction [16-181. Fig. 2a shows the polar angle dependence for the Cu[lOi] azimuth. The emission plane was (121). The positions of the features are symmetric about the [ill] direction but not their intensity, which is due to the geometry of the apparatus and differences in the excitation probabilities, because monochromator and analyzer are fixed and only the crystal is rotated. The direction of the close-packed rows of atoms coincides satisfactorily with the measured

r'

DEPOSITION TIME/min Fig. 1. Intensity oscillations of the specular RHEED beam during the growth of Cu on W(110) at room temperature with the [l’IO] azimuth in the plane of incidence. The primary energy was 15 keV.

sample for eight hours in good vacuum (4 x lo-” mbar) without affecting the electronic properties. The period after the first three oscillations in fig. 1 is attributed to the deposition of a monolayer with fee (111) density (ML,,). From this value the time for one pseudomorphic layer with W(110) density (ML,,,) is calculated and used for calibration of small doses. The width of the diffraction peaks increased with thickness, showing the for-

Emiswm plane.

Cu/W~llO)

11211

14ML epitaxial

-80

-60

-40

-20

0

20

40

60

Angle/dgr.

cioil wll lOTI

[11'1

[loll Hill [lZll .

. . +I++-

100’

-80

.

I -60

-40 -20 0 Angle/dgr.

20

40

60

Fig. 2. Polar scans for the Cu 2p 3,2 photoelectrons of 14 ML Cu on W(110) in (a) the [lOi] and (b) the [l?l] angles corresponding to close packed atom rows are indicated. The sketches on the right show schematically and the emission planes of the sample.

azimuth of copper. The the scattering geometry

G. Lilienkamp

et al. / Electronic and crystalline structure of ultrathin metal films

maxima. Additional structure, for instance near normal emission ( + 10 o ) can be due to Kikuchi diffraction [19]. It is so well resolved here because of our good angular resolution of f lo. The situation in the [121] azimuth of the Cu layer is quite different. The peaks are not so well pronounced but are symmetr& about 0” polar angle (fig. 2b), although the (101) emission plane of fee crystals is not symmetric about the normal of the (111) surface. If the directions of the close-packed rows are reflected about the normal, an acceptable coincidence is achieved. The rationale for the reflection is the twinning of the growing crystal, because none of the two possible adsorption sites in the second fee layer is preferred. This twinning is a general phenomenon in the epitaxy of fee metals with (111) orientation on many substrates. The consequences for the electronic structure will be discussed later. The first ARUPS measurements were taken in normal emission. The sample was kept at room temperature, as was the case for all other ARUPS recordings. In fig. 3 a few spectra were plotted as a function of the binding energy E,. The parameter is the coverage in units of the complete pseudomorphic density. The first spectrum was taken from clean tyngsten. Already at a coverage of about 10% of a complete pseudomorphic monolayer (ML,,) the Cu d-band is seen whose shape does not change much in the further submonolayer range. It consists of three different parts, which is better visible in polarized light. The full width is 900 meV, measured by extrapolation of the slope of the inflection points, and the center is located at E, = 2.9 eV. The small width compared to bulk copper is attributed to the smaller overlap of the wavefunctions in the more loosely packed pseudomorphic layer (14.12 x 1014 atoms/cm’) compared to the Cu(ll1) plane (17.67 x 1014 atoms/cm2). After completion of the pseudomorphic layer, a new feature grows at the high energy side of the band, and the band width increases to 1.1 eV. Above 1.5 ML,, it further increases to 1.8 eV, because of a flat shoulder at the higher binding energy side. In the third layer it reaches nearly its bulk value of 1.9 eV. Up to 1.8 ML,, the intensity in the region of the pseudomorphic monolayer band is not much affected. At 1.8

361

cu/w (110)

180

60

1 0.26

0

&

4

Binding

2

0

Energy/eV

Fig. 3. ARUP spectra for Cu/W(llO) as a function of coverage in normal emission. The parameter is the thickness in ML,. The curves are shifted equidistantly in the y-direction for clarity.

ML,, the dominating maximum at lower binding energy stops growing and decreases with increasing coverage. Close to the double layer, the highest intensity is found at the energy of the most tightly bound pseudomorphic layer band. Between 2.4 and 3 ML,, the spectra are nearly identical. Above 5 ML,, (= 4 ML,) they were within the limits of error identical with those of bulk Cu(ll1) single crystal surfaces [3]. The growth was monitored more quantitatively by recording UPS intensities at characteristic energies as a function of deposition time. In fig. 4 the results for the peaks in fig. 3 marked A (2.92 ev), B (2.69 eV) and C (2.41 eV) are plotted without background subtraction. In addition the CuM,,M,,M,, Auger signal curves are shown, in

G. Lilienkamp et al. / Electronic und crystalline structure of ultrathin metal films

362

0

1000

Deposition

2000

3000

Time/s

Fig. 4. Spectroscopic data as a function of deposition time. (a) Differentiated Auger intensity at normal emission. (b) Differentiated Auger signal 42O off normal in [liO] azimuth. (c) ARUPS intensity for 2.69 eV binding energy in normal emission (B in fig. 3). (d) Same as (c) but 2.92 eV (A in fig. 3). (e) Same as (c) but 2.41 eV (C in fig. 3). The monolayer is determined by the first break in the Auger curves.

one case (a) taken in normal emission, in the other (b) at 42” emission angle. In both cases the acceptance angle was + 12”. The first break in the Auger curves indicates the completion of the bee layer, which coincides with the RHEED data within 3%. The plateau or minimum beyond the double layer in normal emission is not seen in measurements with cylindrical mirror analyzers (CMA), which are more widely used for this kind of analysis than the concentric hemispherical analyzer (CHA) used here. The acceptance angle of the CHA is much smaller than that of the CMA, so that the CHA is much more sensitive to changes in angular distribution of the Auger electrons caused by changes in structure (Auger electron diffraction). The 42” curve (b) may be compared with similar curves measured with a CMA but it has to be kept in mind that the CMA integrates over all azimuths while a single azimuth and its environment is selected by the CHA. In fact, different results are obtained at 42” with the CHA in azimuths different from the one shown here ([liO]) [20]. At normal emission (e = 0) a third linear rise starts close to 2.5 ML,,, which

corresponds to 2 ML,,, while at 0 = 42” this rise starts already at 2.13 ML,,, as in the CMA case. The UPS curves show also a very sharp break at the complete bee monolayer. Up to nearly 2 ML,,, the high binding energy curves show only slight changes, while the 2.41 eV curve reflects the formation of the new band due to the second layer. The decrease above 1.8 ML,,, is not accompanied by a change in the RHEED pattern. It is probably caused by the further rearrangement of the film which lasts up to about 2.5 ML,,, or double fee density. Up to 2.13 ML,,, the first monolayer rearranges into the same (15 x 1) structure as the second monolayer, above 2.13 ML,,, the rearrangement into the fee (111) structure takes place, as clearly shown by RHEED. The plateau or minimum after about 2 ML,,, in the 0” AES curve is probably caused by the shift of the Cu atoms further away from the W lattice sites during the transition to the fee structure. This reduces the intensity in the normal direction by the number of Auger electrons backscattered from W atoms lying originally perpendicular below the Cu atoms. The second break in the 42” curve at 2.13 ML,,, indicates the end of the reorganisation into two distorted Cu layers with a nearly 15-fold superstructure in the [liO] azimuth. In the submonolayer region one can find another break in the 2.69 eV and 2.92 eV ARUPS curves at about 40% of a bee monolayer. Here no change of the adlayer symmetry is observable. The origin is the transition from small clusters and 2D islands to large 2D islands which is seen also in the workfunction change [21]. In order to obtain information about the dispersion of the electronic structure ARUPS spectra were measured as a function of emission angle. Fig. 5 shows results for the pseudomorphic monolayer in the [liO] azimuth. The polar angle increases from negative angles (analyzer and source are on the same side of the surface normal) in steps of 3” up to = 12 O. The Cu d-band between 2 and 3.6 eV binding energy is well separated from the W d-bands. Cu features outside this region can be observed at 5 eV around normal emission and between 2 eV and E, near 8 = 55”. A better separation of the d-peaks can be achieved by using polarized light (not shown here). Both,

G. Lilienkamp

et al. / Electronic and crystalline structure of ultrathin metal films

363

60

1 'IO011-

-4-

[ii01

.

-5-

.

-0.6

0.0

.

:

,

r

R

+

0.6

1.6

2.4

k,,/ A-' 60

Fig. 6. Band mapping of 1 ML, Cu/W(llO) in azimuth with data from fig. 5 (squares), s-polarized and p-polarized (triangles) measurements. The insets experimental arrangement with polarizer and the zone of the bee (110) surface.

Binding

Energy/eV

Fig. 5. Angular dependent ARUPS results for 1 ML,, Cu/W(llO) in the [l]O] azimuth. The polar angle is varied in steps of 3O. The curves are shifted for clarity.

unpolarized and polarized data points (peak positions) are plotted in fig. 6 into an E(k,,) dispersion curve, where k,, is calculated with the usual formula: 1k,, 1 = h-‘(2mE,i,)1’2sin

8,

the [ITO] (circles), show the Brillouin

assuming k,, conservation during excitation and emission. The even states (p-polarization) with respect to the (001) emission plane are represented by triangles, the odd (s-polarization) by circles, and squares are used for unpolarized measurements. Around r three d-bands exist. The central one is odd, the outer ones clearly even. For larger k,, vectors the bands merge and separate again. The even and odd bands are symmetric with respect to N. Another odd d-band was detectable as a shoulder at N just below 2 eV. In addition there is a less sharp and intense peak at T with an

Table 1 Binding energies, polarization and transformation properties of Cu states at F and N for the pseudomorphic layer of Cu/W(llO)

[ii01

Binding energy (ev) Polarization Basis function

5.1 P, 1

3.07 PX xz

2.87 s Yr

2.61 PX xz

3.56 S

2.58 P

2.19 S

’ P

EF

G. Lilienkamp et al. / EIectronic and crystalline structure of ultrathin metal film

364

T

Ol a

T

1.73 MLKc Cu I WUlOl s-pol

r1701 1

i

/IN.

r

-5 -0.8

0.0

0.8

1.6

2.4

k,,/i-’ Fig. 8. Same as fig. 6 but for 1.73 ML,,,

Binding

Binding

Energy/eV

As an example for the different measured coverages during the growth of the second layer fig. 7 shows the results for 1.73 ML,,, Cu/W(llO). In s-polarization we find one strong and flat band, which exists in the whole Brillouin zone, a further sharp band for larger polar angles, and a broad shoulder at the higher binding energy side of the first band, on which a third structure is superimposed. In the case of p-polarized light there is also one strong band at larger polar angles, but between about 30” and 15” only weak broad structures exist, which might be an indication for predominating p,-polarization of the dominating band, because in this region the z-component of the electrical field vector is vanishing in our experimental setup. Near 8 = 0 o there are a few even bands which are not well separated and quickly varying in intensity. Directly at 6 = 0 o we can see one strong maximum and two or three shoulders. There is only one further band without obvious d-character at 8 = 45 o (N) close to E,.

Energy/eV

Fig. 7. Same as fig. 5 but for 1.73 ML,, taken with (a) s-polarization and (b) p-polarization.

energy of about 5 eV and another one with high dispersion near s between E_r and 2 eV. The energies of all bands at T and N are listed in table 1 together with their polarization. The bands are labeled by letters with increasing energy. At r the p,- (t 11 surface normal) and p,-polarization (x is in the emission plane) can be distinguished by rotation of the azimuth. A change to odd symmetry indicates p,-polarization while constant p-polarization can only occur for the p,-direction.

Table 2 Binding energies, polarization and transformation properties of Cu states at r and N for 1.73 ML,,

Cu/W(llO)

[liO]

r,

I;,

G

T,

T;,

Nb

N,

N, “G

Binding energy (eV)

2.9 0.55

2.9

2.62

2.38

2.38

2.61

2.55

2.03

Polarization Basis function

PX xz

s

PI x‘?

PX xi

S

P

S

sP

YZ

YZ

G. Lilienkamp

et al. / Electronic and crystalline structure of ultrathin metal films

365

0 2.4ML jCt~lW(llOl/ 20

-0.8

0.0

0.8 k,,/i-’

1.6

2.4

Fig. 10. Same as fig. 8 but for 2.4 ML,. The inset shows the Brillouin zone of the fee (111) surface.

2

4

Binding

Energy/eV

Binding

nounced. Near normal emission there are at least three subbands in s-polarization, which are too close to be resolved completely. At 20” one band splits off from this group and can be followed up to the zone boundary. At larger angles the highest lying band (around G) has the same dispersion as in the two thinner films shown here but is shifted slightly towards E,. The other three odd bands below are like all the other s-polarized bands very flat compared to the monolayer and the even bands. These are not well separated throughout the whole Brillouin zone, but it was possible to reproduce all shoulders which are evaluated in the dispersion curve (fig. 10). In p-polarization there is one more band close to the Brillouin zone boundary and the band between E, = 0.5 eV and

0

Energy/eV

Fig. 9. Same as fig. 7 but for 2.4 ML,.

The low lying states in the monolayer at t9 = 0 o and E B = 5 eV are no longer detectable now. Table 2 lists the energies and polarization, fig. 8 shows the k,,-dependence. States with the same letters in the monolayer are directly comparable. that is after rearrangement of At 2.41 ML,, the first two layers to a nearly hexagonal arrangement, we find data (fig. 9) similar to those for exactly two fee layers but somewhat better pro-

Table 3 Binding energies, polarization

Binding energy (ev) Polarization Basis functions

and transformation

Cu/W(llO)

[liO]

r;,

fb

c

r;,

Fe

c

3.62

2.8

2.8

2.62

2.3

2.33

PX x?

S

S

PX

S

Yr

XY

X,?

Yr

aa Binding energy/eV Polarization

properties of Cu states at T and G for 2.41 ML,

3.39 s

KS

MC

K

K

Mf

M*

2.82

2.82

2.46

2.44

2.00

0.61

P

S

P

S

S

P

366

G. Lilienkamp

et al. / Electronic and crystalline structure of ultrathin metal films

2 eV around M is much better observable and more data points can be extracted than for 1.73 ML,,,. At r the positions and shifts of the even bands are clearer. The energy range of the d-bands is enlarged by a now well visible band below 3.5 eV. All energies at T and E are tabulated in table 3.

4. Discussion For a better understanding of the properties of the bands we performed a symmetry analysis of our data, using dipole selection rules, which can be deduced from group theory [22] or the simple method described by Goldmann [23] under the assumption, that the final state for normal emission belongs to the completely symmetric representation [24]. Comparable methods have become standard for the investigation of adsorbed molecules [25-271. In the pseudomorphic growth range, the r point is attributed to the full symmetry of the point group, which is here C,,. C,, has four different representations, that is four possibilities of different transformation properties of the electronic states. For normal emission they can be classified by the electrical field vectors necessary for excitation. Under this condition one is the completely symmetric representation, whose states can be excited only by E vectors with a non-vanishing z-component, the second is forbidden in dipole transitions, the third is odd with respect to the mirror reflection about the plane of emission and a y-component (s-polarization) of the field is necessary (y is parallel to [OOl] here) to see emission, and the fourth is even and belongs to x-polarized light. The character table of the point groups give basis functions of the states, which describe the transformation properties if their representation is known [22]. The three d-bands of the monolayer at r can therefore be interpreted as xz- (even) or yz-like (odd). The lowest band at 5 eV is shaped like a parabola with strong dispersion, and is excitated by p, light. These characterizations suggest a sstate. This is supported by the comparison with the lowest bulk Cu valence band at E, = 8 eV,

which has the same properties in experiment and is s-like. In measurements of the (111) surface of bulk Cu also a s-like structure is found at 5 eV which is interpreted as a surface state [28]. This structure and the state in our experiment have probably the same character. Therefore we examined our data for other structures, which might be compared with other surface states of the (111) surface of bulk Cu. The Shockley state at T close to E, [29], which was the subject of many studies of Cu(ll1) surfaces [3], is not visible. The higher intensity on the clean W surface at that energy is decreasing with coverage. At 3 ML,,, the count rate is rising again, due to a state comparable to this surface state. The reason of this behaviour is probably the nonvanishing density of bulk states of tungsten in this region, which does not allow a surface state to exist. Another possible explanation is a shift of the surface state above the Fermi energy. A third surface state of Cu(ll1) was found by Heimann et al. [30] at M with a binding energy of about 2 eV. N is the comparable point for C,, symmetry in our case. The atom rows in this direction have already nearly fee density. In fact such a band can be found in the pseudomorphic layer with slightly less energy (2.19 eV for 1 ML,,, and 2.00 eV for 2.41 ML,,), with the same shape and the proper odd symmetry. A further comparison with bulk data [31] shows that the band at the zone boundary (Ed in the monolayer, M, for 2.41 ML,,) has also a counterpart in studies of bulk crystals, which is sp-like. A more detailed discussion requires calculations. In the dispersion curve for 1.73 ML,,, we can see the interesting result that the d-bands are not symmetric about N, but the extrema, which are the mirror reflection centers, are shifted somewhat towards larger k,, values. This phenomenon is visible from less then 1.5 ML,,, up to more than 2.00 ML,,. In this thickness range we observe the 15-fold superstructure in the [liO] direction in the diffraction pattern which reduces the half length of the unit cell in reciprocal space to 0.0942 A- ‘. This is just the shift of the symmetry center which an odd superstructure should cause, in satisfactory agreement with our data within the limits of error. The lowest band at K shows perhaps the double

G. Lilienkomp

et al. / Electronic and crystalline structure of ultrathin metal films

shift. No other hints for a reduced Brillouin zone such as band splitting or a higher periodicity of the bands could be observed. At 2.41 ML,, we have C,, symmetry because of the two hexagonal layers as in bulk material. The substrate may superimpose disorder or a small stretching. Group theory shows that we should have degeneracy between d,,- and d,,-like states at r. This can be seen twice in the dispersion curve for 2.41 ML,,. The symmetry analysis shows d,,-character for both even and d,,-character for both odd bands, so that the demands of group theory are fulfilled. The difference between the bands of the same symmetry is probably the bonding character of the lower and the antibonding character of the upper band. Beside these four bands there is another odd feature in between and an even band below all the others. The odd band disappears near r. This can be an indication of a d,,-type which is not observable in normal emission. The even structure belongs to the completely symmetric representation and should have d,z-character. If the bands in the two twin domains had different dispersion, at r degenerate bands with the same mirror symmetry should exist. This is not the case, so twinning has little influence for this coverage. For thick layers a comparison with [31] for the IYLUX- and [32] for the ILKL-plane shows, that only both directions together can explain our results quite well, in agreement with the photoelectron diffraction data. We turn now to the comparison with other ARUPS data for ultrathin Cu films. Such a comparison is meaningful only if it is ascertained that the layers really grow layer by layer, a condition which certainly is not fulfilled for the system Cu/Ag(lOO). Cu has a much larger specific free surface energy (yc, = 1.93 J m-2) than Ag (yAg = 1.30 J m2) so that Cu cannot wet Ag and, thus, cannot grow monolayer-by-monolayer. Even if this would be the case, it is extremely unlikely that it grows pseudomorphically as reported [9,11] because the mismatch is so large that Ag forms a pseudo-hexagonal layer in the reciprocal system, Ag/Cu(lOO) [33,34]. The larger bond strength in a hypothetical Cu monolayer on Ag(lOO) than in a Ag monolayer on Cu(100) precludes, therefore,

367

pseudomorphy of Cu on Ag(lOO). Thus, it must be concluded that the ARUPS data of Cu on Ag(lOO) [9,11] have been obtained on layers with unknown structure and that a comparison with the present data is not possible, as interesting it would be because of the different substrate symmetry. The second prerequisite for a meaningful comparison is a precise knowledge of the layer thickness. This is a problem in some of the other studies which makes a good comparison difficult. In the first study [S] of the system Cu/Ru(OOOl) an unusual growth mode was found while later AES and LEED studies of this system showed the usual pseudo-Frank-van der Met-we growth at room temperature [35,36] and Stranski-Krastanov mode at high temperatures [35]. Nevertheless, a comparison will be attempted because the ARUPS measurements were made only on high temperature deposits for which the coverages up to 2 ML should be reliable. Reliable growth mode and coverage data are also available for Cu on Ni(lOO) [13] while the coverage scale in the ARUPS study of Cu/Nb(llO) [lo] differs by a factor of about 2 from another AES and LEED study [37]. We start with the most closely related system, Cu/Nb(llO) [lo]. If we use the coverage scale of ref. [37] and take into account the poorer energy resolution in ref. [lo] then the normal-emission spectrum of the incommensurate Cu double layer on Nb(ll0) is quite similar to our three-peak spectra from this layer on W(llO), in which the lowest binding energy state dominates (see fig. 3). The major difference is a shift of 0.4-0.45 eV to larger binding energies in the case of Cu on Nb(ll0) which is compatible with the weaker interaction of Cu with Nb as seen in the desorption temperature. The Cu/Nb(llO) peak near 2 eV is not clearly visible in the case of a tungsten substrate. However, traces of it may exist in the tungsten d-band region. For thick layers, the ARUPS-spectra of [lo] and our data are not significantly different. The symmetry analysis in [lo] is at least in a few points different from our results. Their d,z-state is part of the triple group in the monolayer, but we see this band only near double layer coverage, and also only at lower energies. The assignment of d,2_yz to the 2 eV peak in [lo] seems to be questionable, because bands with this

368

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transformation property should not be visible in normal emission [23].- The other two d-band peaks _ of [lo] should have I’, and I, symmetry as in our data, although the assignment x.. or yx in [lo] is not correct, possibly due to a printing error. In the system Cu on Ru(0001) the Cu d-band is superimposed on the Ru d-band. This fact and the lower resolution in ref. [8] may be responsible for the absence of significant changes of the 3d-peak position and shape in the normal-emission spectra up to 2 monolayers. Also the dispersion of the major structures in the spectra is the same in a 0.82 ML and in a 2 ML thick film, except for a feature in the T direction. This weak dependence upon film thickness is compatible with the fact that there is only a minor re-organization of the layer with increasing thickness. In a more recent study of Cu on Ru(0001) [12] a pronounced peak with binding energy 1.5 eV was found in addition to the two dominating peaks at E, = 2.3 eV and -3.4 eV at an emission angle of 52” in the TK direction. Supported by calculations they assigned this peak to an interface state. Nothing comparable was found in the present study at any thickness, emission azimuth and angle. This may be due to the different substrate - the clean Ru spectrum has a deep minimum at this energy which suggests a gap in the energy range of the interface peak but it can also be a consequence of the different structure of the layer. The packing density of Cu on Ni(lOO) is quite different from that of the bee and fee layers on W(llO), 16.10 X lOI4 cmA2 versus 14.12 X lOi4 cme2 and 17.67 X lOI cme2, respectively, and so is the symmetry, fourfold versus two-fold and six-fold, respectively. A comparison with the data for Cu on Ni(lOO) [13] is, therefore, of interest. The dominant peak in normal emission from the fee Cu(100) monolayer on Ni occurs at a smaller binding energy (2.52 eV) than from the bee Cu(ll0) layer on W(110) (2.9 eV, see fig. 3). The d-band features of the monolayer of ref. [13] extend from 1.65 eV to 4.35 eV (figs. 4 and 6 in ref. [13]) whereas in our monolayer they are limited to the range from 2.2 to 3.6 eV. The fact that the d-band is by a factor of two narrower in our case can be qualitatively attributed to the lower packing den-

metulfilms

sity and weaker hybridization with the substrate. With increasing number of layers no dramatic changes occur in Cu films on Ni(lOO) except for a shift to higher binding energies with increasing number of layers. This is in contrast to the strong changes seen here with increasing thickness (see fig. 3) but not surprising: in Cu on W(110) packing density and symmetry change considerably while in Cu on Ni(lOO) they remain constant. Only the number of Cu neighbour increases and the influence of the Ni atoms decreases so that the hybridization with the substrate, which pulls the Cu bands towards the Ni bands, contributes less to the overall spectrum. The slight decrease of the binding energy of the Cu d-electrons with thickness on W(110) seen here is a consequence of the decrease of the heat of adsorption from the first to the second layer which is evident in thermal desorption [4,38,39] and can be explained qualitatively with the usual Born-Haber cycle arguments

[401. The preceding discussion shows that only a very limited amount of information can be extracted from the comparison, although several substrate symmetries (bee (110) hcp (0001) fee (100)) and different substrates with the same symmetry (W and Nb) could be used. It is obvious that much more detailed information on other systems is needed. As far as the theory is concerned, the situation is not much better. The only calculation which appears useful for comparison is that of Jepsen et al. [41] for an unsupported Cu(ll1) monolayer. The atomic densities in the Cu[ll2] and the W[liO] directions are similar so that one might expect some similarity between free and adsorbed monolayer in this direction. This is true to some extent: the sp-band at E, = 5 eV and its shape is reproduced by the theory, also the sp-band above the d-band region is seen both in experiment and theory. Little agreement can be found for the d-bands: the distance to the lower sp-band is different, the band width is larger in the calculations and ‘the dispersions of the subbands are rather dissimilar. Keeping in mind that theory and experiment are considering two quite different systems, the observed partial agreement is rather surprising than disappointing. For a bet-

G. Lilienkamp et al. / Electronic and crystalline structure of ultrathin metal films

ter comparison, realistic calculations for pseudomorphic Cu layers and Cu(ll1) films 2-3 atomic layers thick are needed.

5. Summary The interrelation between crystalline and electronic structure is determined by several factors: (i) the symmetry of the layer, (ii) the interaction with atoms in the same layer, (iii) the interaction with atoms in the next layer, (iv) the interaction with the substrate. A clear separation of the influence of these factors on the interrelation is, of course, not possible because they are all coupled. Thus the change from bee to fee symmetry (factor (i)) is connected with a change in packing density (factor (ii)) and with the onset of second layer formation (factor (iii)); factor (iv) has some influence too because second layer atoms have much less interaction with the substrate than first layer atoms. Furthermore, the symmetry change is complicated by the transition via the incommensurate approximate (15 x 1) structure. A better separation has to await detailed studies on substrates with the same symmetry such as Mo(ll0) or Nb(ll0) and with other symmetry such as hcp (OOOl), fee (ill), (100) or bee (100). Although some data are already available, they are much too limited to draw general conclusions. For Cu on W(110) the ARUPS data, combined with the RHEED and AES data show clearly the influence of layer symmetry in the symmetry analysis of the spectra obtained with polarized light, the influence of the lateral and normal Cu interactions in the band narrowing and fine structure and the influence of the substrate in the shift of the band centroid towards the location of the substrate bands. The transition from the pseudomorphic to the incommensurate bee structure is evident in a shift of the band extrema to other k,, vectors. Some changes in the electronic structure with layer structure and thickness are so extreme that they provide a more sensitive possibility for monitoring the thickness than AES, once they have been calibrated with AES or another sensitive thickness or coverage measurement method.

369

Acknowledgement This work Foundation.

was supported

by the Volkswagen

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