Surface states on the (001) and (100) surfaces of equi-atomic CuAu I

Solid State Communications, Vol. 98, No. 9, pp. 799-802, 1996 Copyright @ 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0038-109W6 $12.00 + .OO

PIISOO38-1098(96)00150-O

SURFACE STATES ON THE (001) AND (100) SURFACES OF EQUI-ATOMIC

Alloy Research Center, Department

CuAu I

Xumou Xu and R.G. Jordan of Physics, Florida Atlantic University, Boca Raton, Florida 3343 l-0991, U.S.A.

(Received 9 January 1996; accepted 5 March 1996 by S. Louie)

We have observed for the first time two Tamm-type surface states on the (00 1) and (100) surfaces of equi-atomic CuAu I, at binding energies of about 1.5 eV and 6.2 eV. By measuring their dispersions in kl l-space we have located symmetry points in the surface Brillouin zones, which has enabled us to deduce the lattice spacings in the surface layer. We find that these values are very similar to those of the bulk. Copyright @ 1996 Elsevier Science Ltd INTRINSIC Tamm-type surface states, split-off from the top of the d-band continuum, have been observed around the a-point on the (100) surfaces of the noble metals Cu [l-5] and Au [6]. Related surface states have also been observed on some Cu-Au alloys [7-l 11. In the case of Cu, for example, the surface state has a binding energy of about 1.8 eV, and if x, y are the nearest neighbor directions in the (100) surface, it consists of d,z_,,z orbitals and is spatially confined to the outermost layer [12-141. Thus, such states may be sensitive probes of the surface potential and hence the local environment at the surface. In this paper we report the observation of two Tamm-type surface states on the (001) and (100) surfaces of equi-atomic CuAu I. By measuring their dispersions we have located the symmetry points in the surface Brillouin zones (SBZ’s), which has allowed us to determine the magnitudes of the surface translation vectors. We find that the lattice spacings on the (001) and (100) surfaces conform closely to those for an ideal truncation of the bulk, and so we conclude that neither surface suffers a lattice reconstruction. Equi-atomic CuAu I has the Llo layered tetragonal structure with a c/a ratio of 0.927 [15]. Thus, if an ideal truncation of the bulk occurs at the (001) and (100) surfaces, the two-dimensional reciprocal lattice of the former is square, whereas the two-dimensional reciprocal lattice of the latter is rectangular, see Fig. l(a) and (c); the corresponding SBZ’s are shown in Fig. l(b) and (d). For convenience, and to assist comparisons, we re-label the origins of the SBZ’s for the (100) surface, rio and ?br,as &f’ and a’, respectively. It should be noted that M is equivalent to %f’ but not to il?‘, since 799

0

0

lOlO

0 K

[1ool-

(a)

w

[o’o1 (4 Fig. 1. (a) The real space and (b) reciprocal space lattice and surface Brillouin zone for the (001) surface of equi-atomic CuAu I; the open circles in (a) correspond to either all Cu or all Au atoms. (c) The real space and (d) reciprocal space lattice and surface Brillouin zones for the (100) surface of equi-atomic CuAu I; in (c), if the open circles represent Cu atoms then the shaded circles represent Au atoms. The heavy lines in (b) and (d) are reciprocal lattice vectors and the lighter lines show the Brillouin zone boundaries.

(001) AND (100) SURFACES OF EQUI-ATOMIC

800

CuAu Z

(b) CuAu( 100)

Emission

-2

0

2

4

6

8

10

Binding Energy (eV)

-2

0

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2

4

6

8

10

Binding Energy (eV)

Fig. 2. (a) Angle-resolved spectra from CuAu(OO1) using unpolarized He1 radiation at an incident angle of 60” with (100) as the mirror plane. (b) Angle-resolved spectra from CuAu(100) using unpolarized He1 radiation at an incident angle of 60” with (001) as the mirror plane. The shaded peaks at about 1.5 eV and 6.0 eV are Tamm-type surface states, referred to as Si and SZ, respectively, in the text. The symmetry lines along r&, fi@’ and r@” are defined as z, & and A-, respectively. From angleresolved Auger electron spectroscopic (AES) measurements [15] we have determined that the (001) surface of clean and ordered CuAu Z is essentially 100% Au and the (100) surface consists of - 70% Au atoms with an additional - 0.15ML of Au atoms on top. Such surfaces are stable and reproducible and are produced by Ar+ bombardment followed by a 30 min anneal at 320 “C, i.e., well below the CuAu Z - CuAu ZZ (order-order) transition at about 390 “C. Corresponding LEED patterns indicated that the surfaces are indeed ordered. In Fig. 2(a) and (b) we show photoemission spectra from the (001) and (100) surfaces of CuAu Z using He1 radiation. We claim that the peaks Si and S2, which are shaded in the spectra, arise from Tamm-type surface states. We conclude that they are surface states for several reasons [15]; for instance, (i) they do not disperse in energy with different photon energies, (ii) their intensities are strongly diminished following contamination with air, whereas bulk features are much less

affected, (iii) their energy dispersions along 2, & and i do not depend on photon energy, (iv) their binding energies are sensitive to the relative concentration of Cu and Au at the surface, showing a sensitivity to modifications of the surface potential, and (v) measurements taken at different azimuthal angles imply elliptical or circular energy contours around the ti and J?’ points, which is a characteristic property of surface states. (Unfortunately, our geometrical arrangement was such that we could not vary the angle of incidence to any great extent and so we were unable to investigate their polarization dependence.) The state Si, at a binding energy of about 1.5 eV, corresponds to the Tamm-states occuring at the a-point of the surface Brillouin zone of Cu( 100) [l-5] and Au(lO0) [6] and the (similar) surface state observed on the (100) face of CusAu [lo, 111. A similar state to S2, at a binding energy of about 6.2 eV, appears to have been observed on Au/Cu( 100) [7-91. In Figs 3 and 4 we show the dispersions of s_l and S2 along the fm (.%), rtif (a) and rZ&‘c (A) directions for different photon energies, determined using

Vol. 98, No. 9

(001) AND (100) SURFACES OF EQUI-ATOMIC

n

z r--1.2

1

I

E

7iz

1.4

1.62

E E 2.0

’ kll(W

lz

E

f-

1.8

0.4 1

0.8 I

CuAu I

1.2 I

801

E

1.62

2.0 I

2.4 I

2.8 I MA-‘)

2.4 I

2.8 I

(4 CuAu( 100) in (001) plane l-6.3

Energy (eV) CuAu(001) in (010) plane Energy

0.4 a

(eV)

7s

IT-------_ 1.3

1.5

I

I

0.8 t

1.2 I

1.62 2.0 I I

iI

1.72

1.9

1

I

-1.3

2.1

j-6.3

I

kll(A-l)

Energy (eV) 77’ 0.5 I

7-i 0.9 I

1.3 I

-,I

M

1.72 2.1 I .r-5.9

x 2.5 I

2.9 I ww

-1.9 t

Energy

(eV)

Fig. 3. Energy dispersion relations of St for (a) CuAu(OO1) in the [OlO](r&!) direction in the SBZ, and CuAu( 100) in the [OlO] (fir?‘) direction in the SBZ, and (b) CuAu(lO0) in the [OOl]@i@“) direction in the SBZ. 0 NeI, l He1 and A He11 radiations.

+ -6.3 Energy (eV)

Fig. 4. Energy dispersion relations of & for (a) CuAu(OO1) in the [OlO](r&?) direction in the SBZ, and CuAu(lO0) in the [OlO] (r@) direction in the SBZ, and (b) CuAu( 100) in the [OOl](r&P) direction in the SBZ. l He1 and A He11 radiations.

the well-known expression -0.59 + 0.03 (for CuAu(lO0)) in Fig. 3(b). These values are somewhat smaller than the values reported for where E is the kinetic energy (in eV) and 0 is the Cu [16] and Cu3Au [11], i.e., about -2.38 and -0.94, emission angle. The behavior of the surface states S1 respectively; we hope these observations will promote shown in Fig. 3 is similar to that observed for the theoretical investigations of the electronic structures Tamm-type states at the a-point in Cu(lO0) [l-4] and at ordered Cu-Au surfaces. Another interesting point to note is that there is a difference of over 0.2 eV in Au(lO0) [6] and Cu3Au(lOO) [lo, 111. From parabolic binding energy between the states on the CuAu(OO1) fits of the form and CuAu( 100) surfaces, which we believe is due to the FL2 different local environments at the two surface [15]. It EEWI) = Eo + =(kll - koj2, has been shown, for example, that short exposures of very low-energy Ar+ bombardment increases the relawhere EB is the binding energy and ko is the position of the symmetry point, the calculated effective elec- tive concentration of Cu in the surface regions -withtron masses (m* /m,) are -0.73?0.03 (for CuAu(OO1)) out attenuating significantly the surface state emission and -0.64 f 0.06 (for CuAu( 100)) in Fig. 3(a) and - which leads to an increase in the binding energy of kl~(A-‘) = 0.512B4 sine,

802

(001) AND (100) SURFACES OF EQUI-ATOMIC

the surface states towards the value of 1.8 eV observed for pure Cu [l-4,15]. Table I. Surface lattice constants for the (100) and (001) surfaces of CuAu I. t determined from LEED patterns. Lattice constant a c eiff

Calculated A 3.88 Yk0.08 3.66 k 0.07 0‘94 + 0.04

X-ray diffraction A 3.968 3.680 0.927 0.93 + 0.02t

CuAu Z

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Acknowledgements-This work was funded by NSF (reference numbers DMR-9120120 and DMR9500654) and we are grateful for that support. We also acknowledge the help of Dr. S.L. Qiu with the photoemission experiments and the assistance of Lilian Masliah. REFJSRENCES

1. F. Heimann, J. Hermanson, H. Miosga and H. Neddermeyer, Phys. Rev. B20, 3059 (1979); Phys. Rev. Left. 42, 1782 (1979). As expected, there is a good correspondence be2. D. Westphal and A. Goldmann, Sur$ Sci. 95, L249 tween the positions of the &Zand M’ symmetry points (1980). 3. SD. Kevan and D.A. Shirley, Phys. Rev. B22, 542 in Fig. 3(a), at a value of 1.62 A-I, but along A., see (1980). Fig. 3(b), the position of &” is at 1.72 A-i. Similarly, 4. R. Courths and S. Hiifner, Phys. Repts. 112, 53 in Fig. 4(a) and (b) the ti and 8’ symmetry points (1984). are 1.62 A-’ from r, but in Fig. 4(c) the A?’ sym5. FL. Wincott, N.B. Brooks, D.&L. Law and G. metry point is 1.72 A-’ from j?. Since the Tamm-type Thornton, Phys. Rev. B33,4373 (1986); F.L. Winsurface state $1 is localized at the surface, we can decott, D.S.-L. Law, N.B. Brooks, B. Pierce and G. termine the surface lattice constants from the values Thornton, Surf Sci. 178,300 (1986). of /l?til, II’lii’l and lf@[. In Table 1 we show the 6. I? Heimann, J. Hermanson, H. Miosga and H. calculated values and compare them with the lattice Neddermeyer, Phys. Rev. Lett. 43, 1757 (1987). 7. G.W. Graham, Surf Sci. 184, 137 (1987). constants determined from the same samples using x8. J.C. Hansen, J.A. Benson, W.D. Clendening, M.T. ray diffraction. The major experimental discrepancy in McEllistrem and J.G. Tobin, Phys. Rev. B36,6186 determining the positions of the symmetry points (1987). the maximum value of 6/k,/ I/ jk~lI = 0.02 -is the un9. J.C. Hansen and J.G. Tobin, J. Vuc. Sci TechnoI. certainty in the work function, which we have taken as A7, 2475 (1989); J.C. Hansen, M.K. Wagner and an average of the values for Cu( 100) and Au( 100) [ 171, J.G. Tobin, Solid St. Commun. 72, 319 (1989). and the alignment of the sample (and hence the angle 10. S. Lobus. M. Lau. R. Courths and S. Halilov. Surf: , II of emission). Sci. 287/288,568 ‘( 1993). We see that the values calculated from the dispersion 11. R. Paniago, R. Matzdorf, A. Goldmann and R. Courths, .Z Phys.: Condens. hurter 7,2095 (1995). relations are in quite good agreement with those de2. J.R. Smith, J.G. Gay and F.J. Arlinghaus, Phys. termined from (bulk) x-ray diffraction measurements. Rev. B21, 2201 (1980). Furthermore, the calculated c/a ratio in the (100) sur13. A. Euceda, D.M. Bylander, L. Kleinman and K. face layer is entirely consistent with that determined Medink, Phys. Rev, B27, 659 (1983). from LEED patterns [ 151. We conclude therefore that 14. T. Fujiwara, .Z Phys. F: Meta1 Phys. 16,869 (1986). the lattice spacings in the (100) and (001~ surface lay- 15. Xumou Xu, Ph.D Disser~ution, Florida Atlantic ers of equi-atomic CuAu Z are very similar to those in University 1995. the bulk. Thus, these surfaces appear not to be recon16. S.D. Kevan, N.G. Stoffel and N.V Smith, Phys. structed, i e., the two-dimensional lattices conform, Rev, B31, 3348 (1985). 17. H.B. Michelson, J: Appf. Phys. 48,4729 (1977). within the experimental error, to an ideal truncation of the bulk.