Work function of polycrystalline Ag, Au and Al

JOURNAL OF ELECTRON SPECTROSCOPY and Related Phenomena

ELSEVIER

Journal of Electron Spectroscopyand Related Phenomena 88-91 (1998) 643-648

Work function of polycrystalline Ag, Au and A1 M. Uda a'b'*, A. Nakamura a, T. Yamamoto a'b, Y.

Fujirnoto a

aDepartment of Materials Science and Engineering, Waseda University, 3-4-10hkubo, Shinjuku-ku, Tokyo 169, Japan bLaboratory for Materials Science and Technology, Waseda University, 2-8-26 Nishiwaseda, Shinjuku-ku, Tokyo 169, Japan

Abstract

Work functions of polycrystalline Ag, Au and A1 films were measured by the photoelectric method, which were deposited on amorphous quartz. These films were preferentially oriented, and an oriented plane, i.e. (111), and a degree of orientation were determined precisely by X-ray diffraction. The work functions of well-defined polycrystalline Ag increased from 4.35 to 4.64 eV after annealing at 500°C, but those of Au and A1 remain almost unchanged after annealing at 350°C, i.e. 5.40 and 4.30 eV, respectively. The low work function of Ag before annealing was explained by the appearance of the local density of state near the Fermi edge of Ag, which is due to formation of the stacking fault in f.c.c. Ag during deposition. © 1998 Elsevier Science B.V. Keywords: Work function; Polycrystal; Ag; Stacking fault; Preferred orientation; Annealing

1. Introduction

The valence electrons are confined to a solid by a potential barrier at the surface of the solid. The work function is a measure of the strength of this potential barrier. All phenomena having to do with the escape of an electron from a solid or with the transfer of an electron from a solid to another are deeply connected with the work function, which are concerned with the photoelectron, Auger electron, thermionic and field emissions, and the contact potential. Then a reliable measurement of the work function is of fundamental importance to understanding surface-related phenomena. However, much scatter has been observed even for a given material [1-5] because of differences in surface preparations and in measuring methods. One of the most reliable measurements was performed on Ag single crystals after a repeated cycle of ion bombardment and annealing [6]. The work * Corresponding author.

function of 4.3 and 4.25 eV after vacuum evaporation was improved to 4.64 and 4.52 eV for Ag (100) and (110), respectively, after the abovementioned treatment. An Auger electron spectrum has been characterized as the spectrum from a clean Ag surface only after repeated cycles of argon-ion bombardment and annealing [7]. These results tell us that the clean surface is essential to obtain a reliable work function. However, we assumed that the repeated cycles of ion bombardment and annealing are effective, not only for surface purification, but also for reduction in stacking fault densities where vacuum-evaporated thin films of a f.c.c, structure sometimes have atomic sequences of a h.c.p, structure, i.e. the stacking fault. To confirm the assumption, the discrete variational X a (DV-Xa) molecular orbital calculation method [8] has been used, by which the local density of state (LDOS) for purely f.c.c. Ag, Au and A1 and that for Ag, Au and A1 with the stacking fault can be estimated. In the present experiment well-defined polycrystalline Ag, Au and A1 with a highly oriented (111) plane

0368-2048/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved PH S0368-2048(97)00236-3

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M. Uda et al./Journal of Electron Spectroscopy and Related Phenomena 88-91 (1998) 643-648

were prepared by the vacuum-evaporation method followed by annealing. The degree of orientation was determined by X-ray diffraction [9]. The photoelectron yield method has been employed to determine the work function, because LDOS can only be deduced from the yield by differentiating it by an irradiated photon energy.

h=0.000

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=0. 20

2. Experiments and results Thin films of Ag, Au, and A1 were deposited on amorphous quartz plates maintained at room temperature in a vacuum (--10 -9 Ton'), of thickness 1.7, 0.12 and 0.15/xm, respectively. Here the thickness of the films was estimated by the gravimetry. The purity of the starting materials was 99.999% for Ag, 99.95% for Au and 99.99% for A1. After deposition followed by photoelectron measurements the films were maintained at elevated temperatures for a few hours where necessary. All the measurements were made at room temperature by irradiating the films with monochromatized photons through a grating monochromater (Kratos Shoettel Instruments GM 100). Energyrange and energy-interval of photons used were 4.00-6.00 eV and 0.05 eV, which were measured by a photodiode (Hamamatsu Photonics S 1227-101 BQ) installed in a sample position in place of a sample. The number of emitted electrons from the films were counted by a channeltron, where excitation was achieved using a deuterium lamp (Hamamatsu Photonics L1226). The quantum yield of the photoelectrons Y was deduced by dividing the number of observed photoelectrons by the number of incident photons. The photoelectric work function was determined by extrapolating a straight line part of y1/2 to zero photoelectron numbers. After the photoelectron experiments, X-ray diffraction measurements were performed for all the films to determine preferentially oriented polycrystalline planes and to estimate degrees of orientation. Diffracted intensities from polycrystals with the preferred orientation [9] are expressed as: I p r e f = I ° r a n d X exp( - h 2

X

~b2)

(1)

w h e r e /rand is an intensity from randomly oriented polycrystals, h is a Gauss constant to show the degree

0

,

J ,'"r---~ , , 40 60 80

Fig. 1. Degrees of orientation with parameters of the Gaussian constant h as a function of an inclination angle between normals of observedand orientated planes. of orientation and q~ is an angle between normals for preferentially oriented and observed planes. If h is large, the degree of orientation is high as shown in Fig. 1, where h = 0 means random orientation. Observed intensities from the Ag film are shown in Fig. 2(a) schematically as an example. From comparison of Fig. 2(a) with Fig. 2(b) which shows theoretical intensities from randomly oriented Ag polycrystals, an oriented plane of the Ag film was easily determined to be (111). The degree of orientation can be deduced by minimizing the R factor defined by: R = ~ Ilobs-Iprefl lobs

(2)

where lob s m e a n s an observed intensity, h and R were determined to be 0.104 and 0.037 for the Ag film, respectively. The orientated planes were (111) for all the films prepared here, and the degrees of orientation were 54 and 86% for Ag if integrating exp( - h 2 x ~b2) from 0 to 5 ° and to 10° in Fig. 1, 61 and 91% for Au, and 57 and 88% for A1, respectively with inclination angles of 5 ° or less and 10° or less, respectively, referring to the normal of the oriented plane. Now the evaporated polycrystalline films were well defined in view of the oriented plane and the degree of orientation. The work functions of the films heat-treated at the temperatures indicated are shown in Table 1, which are characterized by pronounced change in Ag, but by a tiny or fundamentally no change in Au and A1. Typical YI/Z-E curves are shown in Fig. 3(a), (b) and (c), giving work functions of 4.35 eV at r.t. and 4.64 eV when annealed at 500°C for 1 h for Ag, 5.38 eV at r.t. and 5.42 eV when annealed at

M. Uda et al./Journal of Electron Spectroscopy and Related Phenomena 88-91 (1998) 643-648

respectively. T h e L D O S seems to be at least two states for A g but one for A u and A1. T o explain the states, the m o l e c u l a r orbital ( M O ) calculations w e r e performed.

Table 1 Work functions of vacuum-evaporated Ag, Au and A1 films as a function of annealed temperature, whose oriented planes are (111) for all

r.t. 50°C 100°C 150°C 200°C 250°C 300°C 350°C 400°C 500°C

Ag (eV)

Au (eV)

A1 (eV)

4.35 4.35 4.35 4.35 4.35 4.35 4.35 4.43 4.46 4.64

5.38 ------5.42 5.41 --

4.31 ------4.29 ---

3. M O c a l c u l a t i o n s a n d d i s c u s s i o n

T o understand the n o t i c e a b l e c h a n g e in the v a l e n c e structure o f Ag, the discrete variational X a potential ( D V - X u ) m e t h o d [8] was e m p l o y e d , by w h i c h m o l e cular orbitals located near the F e r m i e d g e or o f the v a l e n c e band w e r e calculated. In this calculation a cluster m a d e o f 13 A g atoms was u s e d for f.c.c. and h.c.p, clusters o f Ag, as s h o w n in Fig. 5. O n l y contributions f r o m m o l e c u l a r orbitals l o c a l i z e d on a central A g atom are, h o w e v e r , c o n s i d e r e d here, w h i c h can o n l y be used as a representation o f bulk A g atoms in this calculation. T h e L D O S was m a d e by c o n v o l u t i n g squares o f coefficients o f m o l e c u l a r orbital w a v e functions with the G a u s s i a n f u n c t i o n o f 0.5 e V width. T h e DV-Xc~ calculations w e r e

The degrees of orientation are written in the text. 350°C for 1 h for A u and 4.31 e V at r.t. and 4.29 e V w h e n annealed at 350°C for 1 h for A1, respectively. T o e m b o s s such features on figures the q u a n t u m yields w e r e differentiated with the incident p h o t o n energies, i.e. the local density o f state ( L D O S ) as s h o w n in Fig. 4(a), (b) and (c) for Ag, A u and A1,

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80

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60

80

100

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20 Fig. 2. Diffracted X-ray intensities from observed pattern (a); randomly oriented polycrystals (theoretical) (b); and calculated pattern using Eq. (1) in the text (c).

646

M. Uda et al./Journal of Electron Spectroscopy and Related Phenomena 88-91 (1998) 643-648

(a) Ag

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5 6 Incident photon energy(eV) Fig. 3. Square roots of the observed photoelectron emission or quantum yields as a function of incident photon energies. performed under the following conditions: basis sets Is--5p, potential well width 4.0 a.u. (0.73 × a0), well depth - 2.0 Hr, discrete sampling points 4000, and symmetry C3v. Ag has a f.c.c, structure in an equilibrium state at room temperature but has sometimes a h.c.p, sequence in the f.c.c, structure i.e., the stacking fault structure, when Ag is crystallized or condensed rapidly. The vacuum-deposition is indeed the case. Then the LDOS for f.c.c, and h.c.p. Ag were calculated, and are compared in Fig. 6(a) and (b), where the Gaussian width for the h.c.p, sequence was selected as 0.3 eV for reproducing experimental data. The LDOS near the Fermi edge alone are shown in Fig. 7(a) and (b) for comparison with experimental data. Here, mixing ratios of f.c.c, to h.c.p, sequences were assumed to be 1.00/0.54 for the spectrum taken at r.t. and 1.00/0.42 for the spectrum taken from Ag annealed at 350°C for 1 h, and 1.00/0.00 for that taken from Ag annealed at 500°C for 1 h for reproducing experimental data. Based on the molecular orbital calculations, the appearance of the local density of state centered at - 4 . 6 eV can be explained by the stacking fault

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4.5

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Binding energy referred to V.L.(eV) Fig. 4. The local density of state (LDOS) deduced from the observed photoelectron emission or quantum yields as a function of binding energies (referred to V.L.). formed in the f.c.c. Ag structure, which contributes to lowering of the work function of Ag. When mixing ratios of the stacking fault to the f.c.c, structure decrease in the polycrystalline Ag film with preferentially oriented (111) plane, as realized by high temperature annealing, the work function increases and finally reaches 4.72 eV, which is the value for a (111) Ag single crystal free from contamination and stacking fault. Different amounts of the presence of the

f.c.c, cluster

h.e.p, cluster

Fig. 5. Cluster models used for the molecular orbital calculations - - DV-Xc~.

M. Uda et aL/Journal of Electron Spectroscopy and Related Phenomena 88-91 (1998) 643-648

Ag (a) f.c.c.

• exp.(r.t.) o exo.(350°C)

~ L--calc.(f.c.c.) t

0

5

10

Relative binding energy(eV) Fig. 6. The local density of state (LDOS) of the valence band of f.c.c, and h.c.p. Ag. The lowest energy peak at the f.c.c. Ag is deeper than that of the h.c.p. Ag by - 1 eV.

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Fig. 8. The local density of state (LDOS) of Au after annealing at r.t. and 350°C for 1 h. Contributions from f.c.c, and h.c.p. Ag (theoretical) are denoted as continuous and dotted lines, respectively.

5

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stacking fault in the f.c.c, structure are the reason why the work function of Ag is sensitive to surface treatment. To confirm the assumption that lowering in the work function of Ag, when vacuum deposited but not annealed, is caused by the stacking fault embedded in the f.c.c, structure, LDOS of Au and A1 were also calculated under similar conditions to those for Ag, i.e., basis sets ls--6p(Au) and ls--3d(A1), potential well width 0.73 a0, well depth -2.0 Hr, discrete sampling points 4000, and symmetry C3v. The energy difference in the highest occupied molecular orbitals (HOMO) of Au with f.c.c, and h.c.p, structures was 0.0 eV, and the LDOS of Au near the Fermi edge are shown in Fig. 8 using a full line for f.c.c, and a dotted line for h.c.p. The observed LDOS in Fig. 8 are well reproduced by the calculated ones, suggesting that the f.c.c, structure of Au free from the stacking fault gives almost the same LDOS as that for mixed structures of f.c.c, and h.c.p, near the Fermi edge. The energy difference of 0.0 eV in the HOMO for f.c.c, and h.c.p. A1 was also obtained by the DV-Xa MO calculations. These facts support the assumption of a decrease in the work function of Ag, which is due to mixing of h.c.p, with f.c.c.

Oo°

4 - ---4.5-

5

5.5

Binding energy referred to V.L.(eV) Fig. 7. The local density of state (LDOS) of Ag after annealing at r.t., 350°C and 500°C for 1 h. Contributions from h.c.p, and f.c.c. Ag (theoretical) are written by chain and broken lines, respectively, where mixing ratios of f.c.c, to h.c.p, are 1.00:0.54 for r.t., 1.00:0.42 for 350°C and 1.00:0.00 for 500°C, respectively.

References [1] R.H. Fowler, Phys. Rev. 38 (1931) 45. [2] S. Fliigge, Encyclopedia of Physics, Vol. 21: ElectronEmission Gas Discharges, Springer, Berlin, 1956, p. 346. [3] D.E. Eastman, Phys. Rev. B 2 (1970) 1.

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M. Uda et al./Journal of Electron Spectroscopy and Related Phenomena 88-91 (1998) 643-648

[4] M. Cardona and L. Ley, Photoemission in Solids - - General Principles, Springer, Berlin, 1978, p. 19. [5] H.B. Michaelson, J. Appl. Phys. 48 (1977) 4729. [6] A.W. Dweydari and C.H.B. Mee, Phys. Stat. Sol. (a) 27 (1975) 223.

[7] A.M. Bradshaw, A. Engelhardt and D. Menzel, Ber. Bunsenges. Phys. Chem. 76 (1972) 500. [8] H. Adachi, M. Tsukad and C. Satoko, J. Phys. Soc. Jpn. 45 (1978) 875. [9] M. Uda, Z. Anorg. Alg. Chem. 350 (1967) 105.