Photoemission of thin and ultrathin films of calcium

Surface Science 0 North-Holland

86 (1979) 75 -82 Publishing Company

PHOTOEMISSION

OF THIN AND ULTRATHIN

FILMS OF CALCIUM

L. ARMAND, J.L. BOUILLOT and L. GAUDART Ddpartetnerlt FI3331

de Physique

Marseille

Manuscript

ExpCrimerrtale,

UtliversitP de Proveme,

PI. V. llugo.

CPdex 3, Frattce

received

in final form

17 November

1978

We have investigated the differences between thick films and thin films. The thin calcium film was obtained by means of thermal evaporation and condensation under ultrahigh vacuum. The vacuum was in the range of 10-l ’ Torr and was obtained with an ion-pump, a titanium getter-pump and a helium cooled cryopump. The film structure depends on the quantity of metal deposited on the substrate. Three main structures were observed: granular, lacunar and continuous. Photoemission measurements were made with photons of 2 to 5.6 eV. Thus we were able to determine the density of electronic states just below the Fermi level and to study the dependence of the work function on structure.

1. Introduction The study of thin metallic films is rewarding as on the one hand the properties of bulk metal are involved and on the other hand new phenomena can be demonstrated due to surface, roughness, or film structure. The thin films can be studied according to several points of view [ 1 ,?I. For instance, their electrical resistivity [3], optical properties [4-71 or photoemission [8-l I] can be investigated. These last years, calcium has been much studied from a theoretical point of view and its electronic structure is now beginning to be well known [ 12-141. Using photoemission measurements we studied the energy distribution curves obtained from thin calcium films. The measurements were made around the photoelectric threshold (from 2 to 5.6 eV) and we were thus able to study the relationship between the work function and the structure of the film.

2. Experimental

conditions

2.1. Apparatus and experimerltal procedure

The thin film of calcium, which constituted the photocathode was deposited on an optically smooth silica substrate by thermal evaporation and condensation 75

in an ultrahigh vacuum. The ultrahigh vacuum was in the range of lo-” Torr. It was obtained using an ion-pump, a titanium getter-pump and a helium cooled cryopump. The direction of the radiations was normal to the film. The incident photon energy was measured by means of a reference photoelectric cell, taken into account the transmittance of the window. Calibration of this cell was realized with a thermopile. A second photoelectric cell was placed against the exit window in order to determine film transmittance. The mass thicknessd, i.e. the quantity of metal deposited per unit of area, was determined from transmittance of the films using the curves established by Blanc et al. [ 151. Mass thicknesses ranged from 3 to 51 nm and the deposition rate was approximately 0.3 rim/s.. 2.2. Injluerlce

of the residual gases

~k~ine-ea~ll metals are particularly liable to deterioration and the metal must therefore be carefully degassed before evaporation. Bondarenko and Makhov 1161 have shown, with barium, that where degassing was insufficient, the film no longer had the same properties. A second important point is the presence of the residual gases in the vacuum chamber during evaporation of the film, as has been shown by Barna et al. [ 171. The pressure rose again to 5 X lo-” Torr during evaporation of the calcium film and went down again to IO-” Torr a few seconds after evaporation. The helium cooled cryopump is particularly well adapted for rapidly pumping the gases given off by the calcium during heating.

3. Structures of the thin calcium films In the early stages of the deposition of a metal on an insulating substrate, the resulting film structure consists of unconnected metallic islands distributed across the substrate [lS]. The fdm is said to be granular. If the quantity of metal evaporated increases. the number and size of the aggregates increase and the metallic islands join together. Paths form between the aggregates and voids are considerable. The film is then said to be lacunar. If the quantity of deposited metal increases further, the film becomes continuous. The alkaline-earth metals are extremely reactive and electron micrographs of the thin films cannot be effected directly. Two methods can be employed. Firstly, carbon replicas were realized. These replicas were shadowed with a platinum evaporation and the electron micrographs were then undertaken. An electron micrograph obtained by this method from a thin calcium film of 4.8 nm is displayed in fig. 1. Unconnected metallic aggregates are observed. The second method is that described by Hamilton and Loge1 [19]. If zinc is evaporated from a resistance-heated boat, it does not condense on a perfectly clean quartz substrate at room temperature. But if there are active sites on the substrate. for example aggregates of another metal, the zinc vapor condenses on these active sites and the electron micrograph of the zinc thus deposited can then be under-

L. Armand et al. / Photoemission

of films of Ca

Fig. 1. Electron micrograph of a thin calcium film of 4.8 nm, obtained technique (P. Renucci and R. Rivoira, private communication).

17

from

a carbon

replica

taken. Fig. 2 shows the result obtained by this process with a calcium film of 13 nm mass thickness. Whatever the method used, the electron micrographs show that the change from the granular film to the lacunar one occurs between 5 and 6 nm with the calcium. The thin calcium films obtained under these conditions are polycrystalline as has been shown by Reale [20].

4. Photoemission

measurements

The quantum yield, i.e. the number of electrons emitted per absorbed photon, can be calculated from the experimental measurements. The absorbed flux was cal-

78

of 3 thin calcium film of 13 nm, obtained E‘ig. 2. Ele otnJll micrograph tech niqur fP. Renucci and R. Kivoira, private communication).

culated

from

the incident

flux by eliminating

the flux

from

transmitted

:inc cow :rage

and the flux

reflected. At normal incidence, diffusion is negligible. For all the films, the variation of intensity as a function of the potential difference between the anode and the cathode, is similar. The intensity increases and reaches saturation. The measurements of the quantum yield have been undertaken in this saturation range. The quantum yield pg then depends on two variables: the film structure, located by its mass thickness. and the incident photon energy. The dependence of pQ on mass thickness d is displayed in fig. 3. The yield is no longer dependent on d when high values greater than 15 nm are involved. The thick films exhibit

a behavior

analogous

to that of the bulk. With smaller mass thicknesses

two

I.. .4rrnard

et al. / Photoer~2issiotl

of film

0f‘Ca

79

0.021

0.01

Fig. 3. Variations of the quantum yield versus film thickness for t\vo monochromatic tions.

radia-

I:&. 4. Work function @vversusm;1csthicknessd

maxima

were obtained

J, = 3 ? 0.5 nm

at

311ci

do

=

5.5

+ 0.5

11111

These values are independent of incident photon energy. The work function of each thin calcium film has been determined using Fowler’s theory [Zl]. The dependence of the work function on mass thickness cl is displayed in fig. 4. There are two minima of the work function, respectively around mass thicknessesdl and do corresponding to the two maxima of the yield. The corresponding work function values are : 0, = 2.55 CV

and

(“SC, = 2.2 eV

For mass thicknesses stant [22] :

greater than about

10 nm, the work function

&,, is con-

C$,,,= 7.98 t 0.05 ev The energy distribution curves in the range of photon energies chosen are much harder to obtain than the work functions. We have observed experimentally that the small aggregates were more rapidly altered than the lacunar or continuous films. We have therefore limited OUTstudy to lacunar and continuous films. For all thin films with mass thicknesses greater than 7 nm the density of electrons N(e) is the same. The electronic structure of the bulk therefore appears very rapidly in the thin films. With gold, for example. Borzjak et al. [23] observed band structure from a

I‘ig. 5.

d is

tt1;m

1 nnl.

thickness of 1 nm. Fig. 5 shows the variations of N(E) as a function of the energy E of the initial electronic density. for several incident photon energies tzw. Two main electronic structures are observed. Firstly a large peak at approximately 0.4 eV below Fermi level and, subsequently, a shoulder around 1 eV below the Fermi level. This shoulder disappears with the highest photon energies (tlw 2 4.5 eV).

5. Discussion The results to be interpreted are essentially: the minima of the work function and the shape of the energy distribtition curves. There are numerous and very different methods of determining the work function of a metal. In particular, let us quote thermionic emission [ 161, photoemission [14], field emission [Xl, and use of a contact potential difference [L’6]. The variations of the work function possess the same shape. When the quantity of metal deposited increases, the work function decreases and then increases and reaches a constant value. The decrease only occurs if the work function of the adsorbate is smaller than that of the substrate, and the latter can bc a metal [77] or an insulator. With the calcium and the strontium [ 1 l] there are two minima. The first, of abscissa d,, corresponds to a granular structure. It is probably connected with the appearance of conduction. The second minimum is observed for the critical thickness do. This is an intermediate thickness between the granular films and the lacunar ones. It is well known that the work function splits up into several terms and one of them depends on the electronic double layer on the surface. The emitted

L. Armand et al. / Photoemission

offilrr~sof

Ca

81

photoelectrons cross this double layer and the corresponding energy intervenes in the calculation of the work function. The variation of the mass thickness, and therefore of the macroscopic structure, brings on a modification in the size of the surface in relation to the size of the bulk. This modification is particularly great in the neighborhood of the critical thickness de. It brings on a change in the distribution of charges in the neighborhood of the surface and, consequently the variation of the work function [28]. The peak in the energy distribution curves has already been observed by Kress and Lapeyre [29] with calcium bulk films. The interpretation of this peak was done by Lopez-Rios and Sommers [30]. A very detailed interpretation of the density of states in the vicinity of the Fermi level has been given by these authors. An explanation from a direct transition model was given. Indeed, the energy bands show several filled - or nearly filled - s bands near the Fermi level, in the I-X, F-W and F-K directions in the Brillouin zone. The corresponding optical transition is probably a s-d transition, but it is possible that several optical transitions give a contribution.

6. Conclusion We have studied the thin calcium films with a view to distinguish the effects of bulk from those of the surface. The energy distribution curves show that thin calcium-films have an electronic structure identical to that of bulk metal as from a mass thickness of 7 nm. The surface contribution is strong near the threshold [31] and the results which concern the variation of the work function reveals effects specific to aggregates and to the change granular-lacunar films.

Acknowledgements The authors wish to thank P. Renucci and R. Rivoira (Centre d’Etudes des Couches Minces, Marseille, France) for discussion and communication of electron micrographs.

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