Surface Science 111 (1981) L701-L704 North-Holland Publishing Company

L701

SURFACE SCIENCE LETTERS

A LEED INQUIRY INTO THE QUESTION OF RECONSTRUCTION OF {001} Nb *

A.J. MELMED and S.T. CEYER Surface Science Division, Nalional Bureau o f Standards, Washington, DC 20234, USA

and

R.T. TUNG ** and W.R. GRAHAM Laboratory for Research on the Structure o f Matter, University o f Pennsylvania, Philadelphia, Pennsylvania 19104, USA

Received 9 July;'accepted for publication 11 August 1981

We have examined the structure of the {001} surface of Nb by LEED and have found that a (1 X 1) diffraction pattern prevails over the entire range of temperature investigated, that is 15-1000 K. This is the first observation of an unreconstructed {1301} surface of a bee metal over such a temperature range. This result is consistent with a reconstruction mechanism requiring the coupling of occupied surface states and is expected to further theoretical devel- opment of the mechanism of surface reconstruction.

In the past two years there have been several experimental and theoretical investigations into the thermal stability of the {001} surface of body centered cubic metals. The {001} faces o f the bcc metals that have been studied (.W [ 1], Me [2], Cr [3], V [4]) have exhibited more complex structures than that expected for a simple termination o f the bulk metal, i.e. they are reconstructed. The underlying driving force for surface reconstruction is a lowering of the total energy of the system, but the reason for the instability and the mechanism by which the total energy is lowered are poorly understood. The {001} surface atomic structure of Nb, a bcc metal, has been unexplored previously as a function of temperature. We have investigated the atomic structure of this crystal face over a temperature range o f ~ 1 5 - 1 0 0 0 K and have found that it is not reconstructed. This experimental result should have an important bearing on further theoretical development relating to the mechanism of reconstruction since Nb is very distinct from the bee metals previously studied in its surface electronic structure and phonon frequencies.

Current theoretical models concerning the mechanism of reconstruction [5 -9 ]

* This work was partially supported by the National Science Foundation under Grant DMR- 804)9056.

** Present address: Bell Laboratories, Murray Hill, New Jersey 07974, USA.

0039-6028/81/0000-0000/$02.50 © 1981 North-Holland

L702 A.J. Melmed et al. / LEED and reconstruction of {001 } Nb

and some recent experimental evidence [10] suggest that surface states lying near the Fermi energy are required for the reconstruction to occur. The correct sym- metry of these surface states on the 2D Fermi surface induces an electronic instabil- ity in the surface which drives the reconstruction via a surface phonon mode "softening" (phonon frequency becomes zero). This creates a gap in the Fermi sur- face at the new BriUouin zone boundary, lowering the energy of the system. There are two proposals for the origin of the temperature dependence of the reconstruc- struction. One proposal [5,6] suggests that the sharpening of the electron Fermi distribution as the temperature is lowered modifies the effective screening between the atoms, which must be compensated for by movement of the surface atoms into the reconstructed phase. A second proposal [9] suggests that the temperature dependence arises from the anharmonic motion of the surface atoms. In this mode[, the surface is unstable for displacements of all wavevectors but the phonon mode which causes gapping of the Fermi surface is responsible for the observed recon- struction, since this configuration is marginally energetically favorable. In either model, the magnitude of the phonon frequency term compared to the magnitude of the temperature dependent term determines the temperature at which the recon- struction occurs.

Unlike the {001} surfaces of W, Mo and Cr, the {001} Nb surface has no occu- pied surface states of the correct symmetry and energy, as indicated by a surface band structure calculation [11]. The Fermi surface does exhibit the required nesting of a set of surface states, but these states are above the Fermi energy and hence are mostly unoccupied. Thus, the correlation between the absence of an occupied surface state and the absence of reconstruction can be tested experi- mentally. In addition, niobium has a lower Debye temperature than the bcc metals previously studied. This may be important in a model [9] where the surface is un- stable for arbitrary displacements of the atoms and where the temperature depen- dence of the reconstruction arises from the phonon frequencies. The low Debye temperature of Nb requires, within this model [9], that the investigation of the stability of the surface be carried out at as low temperatures as possible, giving the highest frequency mode a chance to go "soft".

Several LEED investigations of the {001} Nb surface [12-15] have been pub- lished, but no results for the clean surface as a function of temperature have appeared. A recent investigation of the clean {001} Nb surface by field ion micro- scopy (FIM) [16] showed that the crystal plane is unreconstructed over the tem- perature range 15-450 K. The planes studied were small, on the order of 35 A or less in diameter, as in the FIM study of (001} W [17]. In view of comments [1 ] on the inhibition of reconstruction by nearby plane edges of {001 } W, the question of whether or not an extended (001} Nb surface is reconstructed as studied by LEED arises. The present work was undertaken in an effort to answer the question for clean {001} Nb over the temperature range "~15-1000 K.

Specimens were spark cut from a 99.99% pure Nb single crystal rod and were oriented and mechanically polished such that the final orientation of each specimen

A.J. Melmed et al. / LEED and reconstruction o f {001 ) Nb L703

was within 0.25 ° of exact {001} orientation. In order to test reproducibility, the experiments were conducted in three different LEED systems, using three different Nb specimens. One system is described here. This was a mostly metal commercially manufactured LEED instrument with a system base pressure (LEED apparatus running) of 4 X 10 -9 Pa (3 X 10 -t i Tort). The specimen was heated by electron bom- bardment and cooled by conduction to a reservoir filled with either liquid nitrogen, cooling the specimen to 78 K, or cold helium gas from a liquid helium supply, cooling the specimen to 15 K. The specimen was originally contaminated with S, C, N and O, as determined by Auger electron spectroscopy. Flashing the specimen to 2200°C was sufficient to remove the S, C and N. Oxygen contamination resulted in a complex LEED pattern and was the most difficult contaminant to remove. Heating to 2200°C for a total of 20 min was required to remove most of the oxide. Several three-second flashes removed the final traces of contamination as judged by the absence of additional diffraction features beyond sharp integral order spots and a uniform dark background in the LEED pattern. Considerable diffuse scattering occurred above ~500 K. After the cleaning process, the specimen was again flashed to 2200°C (during which the system pressure remained below 1.3 X 10 -s Pa) and cooled to either 78 K or 15 K. Observations were made and photographs were taken during the cooling process, which was complete in approximately 40 to 60 s. Additional photographs were then taken.

A (1 X 1) LEED pattern prevailed over the entire temperature range of the experimental observations. A typical diffraction pattern is reproduced in fig. 1. We conclude that the clean {001} Nb surface atomic structure has (1 X 1) symmetry. This agrees with the conclusion drawn from the FIM experiments [ 16].

Fig. 1. Low-energy elecUon diffraction pattern for a clean {001} Nb surface at 78 K. Beam energy = 158 eV.

L704 A.J. Melmed et al. / LEED and reconstruction of (O01) Nb

The absence of reconstruction on the (001) face of Nb throughout the tempera- ture region studied is consistent with the suggestion that coupling of surface states crossing the Fermi energy and subsequent opening of an energy gap at the new sur- face Brillouin zone boundary is required for reconstruction to occur. Based on this idea, Tosatti [5] considers the effect of alloying additions of Nb to Me and predicts that before reaching pure Nb, the reconstruction phenomenon should completely disappear. Ingiesfield [9] suggests that even in the absence of surface state coupling some sort of reconstruction via surface phonon mode softening should occur if the (001) surface is unstable to arbitrary lateral displacements. The question of predicted stability of the {001) Nb surface to displacements along all wavevectors awaits a theoretical calculation similar to the one for {001) Me [9] but specifically for ~001) Nb. Our result suggests that the (001) Nb surface is very stable to lateral displacements.

References

[1] See, for example, M.K. Debe and D.A. King, Surface Sci. 81 (1979) 193. [2] T.E. Felter, R.A. Barker and P.J. Estrup, Phys. Rev. Letters 38 (1977) 1138. [3] G. Gewinner, J.C. Peruchetti, A. Jaegle and R. Riedinger, Phys. Rev. Letters 43 (1979)

935. [4] P.W. Davies and R.M. Lambert, Surface Sci. 95 (1980) 571. [5] E. Tosatti, Solid State Commun. 25 (1978) 637. [6] E. Tosatti and P.W. Anderson, Japan. J. Appl. Phys. Suppl. 2, Pt. 2 (1974) 331. [7] K. Terakura, I. Terakura and Y. Teraoka, Surface Sci. 86 (1979) 535. [8] J.E. Inglesfield, J. Phys. C l l (1978) L69. [9] J.E. Inglesfield, J. Phys. C12 (1979) 149.

[10] J.C. Campuzano, D.A. King, C. Somerton and J.E. Inglesfield, Phys. Rev. Letters 45 (1980) 1649.

[11] S.G. Louie, K.M. He, J.R. Chelikowsky and M.L. Cohen, Phys. Rev. Letters 37 (1976) 1289; Phys. Rev. B15 (1977) 5627.

[12] D. Tabor and J. Wilson, Surface Sci. 20 (1970) 203. [13] J.M. Dickey, H.H. Farrell and M. Strongin, Surface Sci. 23 (1970) 448. [14] H.H. Farrell, M. Strongin and J.M. Dickey, Phys. Rev. B6 (1972) 4703. [15] H.H. FarreU and M. Strongin, Surface Sci. 38 (1973) 18. [16] R.T. Tung, PhD Thesis, University of Pennsylvania (1980). [17] A.J. Melmed, R.T. Tung, W.R. Graham and G.D.W. Smith, Phys. Rev. Letters 43 (1979)

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