A comparative study of amorphous and crystalline superconducting molybdenum films by ultraviolet photoemission spectroscopy

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Solid State Communications, Vol. 28, pp. 631-633. © Pergamon Press Ltd. 1978. Printed in Great Britain.

0038-1098/78/1122-0631 $02.00/0

A COMPARATIVE STUDY OF AMORPHOUS AND CRYSTALLINE SUPERCONDUCTING MOLYBDENUM FILMS BY ULTRAVIOLET PHOTOEMISSION SPECTROSCOPY B. SchrOder,* W, Grobman, W. L. Johnson, t C. C. Tsuei and P. Chaudhari IBM Thomas J. Watson Research Center Yorktown Heights, New York 10598 (Received 14 August 1978 by M. Cardona) Ultraviolet photoemission measurements at two photon energies are carried out, to understand the strong enhancement of the superconducting transition temperature T c in the nitrogen stablized amorphous molybdenum films. The rise of T c can be attributed to a 4 0 % increase of the electronic density of states at the Fermi level N(EF) and a 15% softening of the phonon spectrum.

In order to further understand the electronic properties of these Mo films, a comparative study of the bee and the aforementioned amorphous phases has been carried out using the technique of ultraviolet photoemission spectroscopy. In this letter, we show that a significant change occurs in the electronic band structure as one proceeds from films having the bcc structure to those having the amorphous structure. The difference in T c for the two phases can largely be attributed to this change in the band structure. A quantitative analysis of T c based on the McMillan equation 10 is shown to account for the results. The samples used in this study were prepared by electronbeam evaporation of 99.999% pure Mo rods in two UHV systems having a basic pressure of p < 10 "10 Torr. Results of an in situ structural study and T c measurements obtained in one of these systems have been reported. 8 The photoemission spectra were obtained in a second UHV system equipped with a cylindrical mirror electrostatic electron analyzer with a constant pass energy of 15eV (resolution 0.15eV) for hv=21.2eV spectra, and a pass energy of 30eV (resolution 0.3eV) for hv=40.8eV spectra. The amorphous samples were prepared using a substrate temperature T s = 7 7 ° K and a partial nitrogen pressure P(N2) ~ 10 -6 Torr (base pressure = 10 -10 Torr); such conditions were previously shown to be optimal for producing an amorphous phase. The crystalline films were prepared with T s ~ 300°K and P(N2) < 5x 10 -9 Torr (during evaporation). The superconducting transition as previously measured by electrical resistance for Mo films containing various amounts of N 2 and T s = 77°K are shown in Fig. 1 for reference. The photoemission samples were found to have T c < I . 5 ° K (crystalline) and T c ~ 8°K (amorphous) in agreement with the previous work.

The electronic and structural properties of vapor-quenched transition metal films have been investigated by numerous authors. I-5 Systematic investigation of superconductivity and structural properties of "highly disordered" films consisting of metals and alloys of the second and third transition series vapor-quenched onto cryogenic substrates were carried out by Collver and Hammond. 6 The superconducting transition temperature T c of the as-quenched films was found to follow a smooth functional dependence on the average group number (AGN) in each series, exhibiting a broad maximum near midseries. Structural investigations carried out on those films for which the disordered phase was stable up to or above room temperature indicated that these films could be characterized as amorphous. Other films were assumed to be amorphous. Several theoretical attempts have been made to explain these findings. 6-7 In each case, assumptions were made concerning the nature of the electronic band structure in the amorphous state for which no experimental data are available. The role of impurities in stabilizing the amorphous structure was neglected and little attention was paid to the question as to whether all transition metals quenched onto cryogenic substrates are in fact amorphous in the absence of such impurities. Recently, it has been shown that molybdenum (Mo) films prepared in ultra-high vacuum (~ 10 "10 Torr) retain the equilibrum bcc structure for substrate temperatures down to 4.2°K. 8 The amorphous phase was shown to be stablized by the incorporation of chemically active gaseous contamination into the films. In situ investigations of the superconductivity and structure of such amorphous films stabilized by the controlled introduction of nitrogen during evaporation was reported. A stable (up to temperatures of ~ 200°C) amorphous phase containing about 20 at. % of incorporated nitrogen and having a sharply defined superconducting transition at 8.5°K (near to that observed by Ref. 6 for "highly disordered" Mo films) was found to be well described by the dense random packing of hard spheres model for amorphous structure. Linker and Meyer 9 have shown that nitrogen ion implantation into the bcc Mo films kept at low temperatures leads to an amorphous like structure transformation and an enhancement of T c up to 9.2 K. No structural transformation and no increase of T c took place in Mo films by implanting noble gas ions.

The experimental photoemission spectra for the amorphous and crystalline films are shown in Fig. 2b and Fig. 2e for h~ = 2 h 2 e V and hv = 40.SeV respectively. The theoretical one electron density of states I0 for bcc Mo appropriately broadened to account for experimental energy resolution is shown in Fig. 2a for comparison. The three peaked structure theoretically predicted below E F in the binding energy range E F = 0 eV < E B < 5 eV is evident in the photoemission intensity of the crystalline film but is absent for the amorphous film.

* Permanent address: Fachbereich Physik der Universitat Kaiserslautern, 675 Kaiserslautern, Federal Republic of Germany t Present address: California Institute of Technology, Pasadena, California 91125

631

632

STUDY OF SUPERCONDUCTING

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Vol. 28, No. 8

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Fig.l. Normalized resistance as a function of temperature for various Mo films containing different quantities of nitrogen. The inaccuracy of the nitrogen fraction x amounts to +0.05 determined by electron microprobe analysis. Sharp features are observed at E a = 1.75, 2.80, and 4.10eV for the crystalline film, particularly in the hv = 40.8eV spectra. In Fig. 2b and 2c, the emission intensities were normalized so that for 0 < E B < 5eV, the integrated areas (minus a recording electron background) are the same for the crystalline and amorphous samples. This procedure permits one to roughly deduce the difference in the emission intensities at E F which then leads to an estimate of the difference in N(EF), the one electron density of states at E F. This estimate can be used in comparing the superconducting properties. This estimate is made using the hv = 40.8 eV spectra, for which directtransition effects should be weak. The chemical nature of the nitrogen incorporated into the amorphous structure is of considerable interest. Contamination of the surf.ace of the crystalline samples during data acquisition occurs by accumulation of ambient N 2 (p < 5 × 10 -9 Torr) from prior amorphous films preparation. The N 2 absorbed on crystalline film surface gives rise to additional structure in the photoemission spectra not observed in a freshly deposited film. For the hv = 21.2 eV spectra, the N 2 p-orbital gives rise to a peak at El] = 6.1 eV. This peak and additional structure can be seen in the hv = 40.8 eV spectra where the data acquisition time is longer and contamination by surface N 2 is greater. The surface contamination also occurs for amorphous films to a somewhat greater extent since the ambient N 2 pressure P(N2) ~ 10 -6 Torr, cannot be "chopped" instantly following deposition. The effect of surface nitrogen on the films is combined with the effect of nitrogen incorporated in the amorphous phase. The amorphous films exhibit an N 2 orbital which is shifted to El] = 5.3 eV. The decrease of E B (compared with that of surface N 2 on a crystalline film) may indicate electron charge transfer from Mo to incorporated nitrogen, but this transfer is probably much less than one electron per atom and cannot explain the considerable change in N(EF). The increase in N(E F) for amorphous samples can probably be attributed to principle broadening of the d-band density of states in the amorphous phase, and to p-d hybridization. Strong p-d hybridization between the p-electrons of the metalloid (nitrogen) and the d-electrons of transition metals has

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been observed in a series of interestial compounds, and is usually connected with an increase of N(E F) for transistion metals having low N(EF) (Mo,W) and a decrease in N(EF) for transition metals with high N(EF)(Mb,V,Ta). Estimating the increase in N(E F) to be ~ 4 0 % from the photoemission spectra of amorphous Mo as compared to bcc Mo (see Fig. 2 ), one can use McMillan's equation to gain some insight into the enhancement of T c in the amorphous film. Following McMillan, we have Tc= 0/1.45 exp [ - 1 . 0 4 ( l + ~ ) / ( h - / t * ( l + 0 . 6 2 A ) ) ]

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with = N(EF)/M<~o2 > where M is the atomic mass, <~02> is a mean square phonon frequency, <12> is an average square eleetron-phonon matrix element, and 0 is the Debye temperature. With the Coulomb pseudopotential ~* = 0.13, (this value of p* seems to be reasonable for the transition metals I1}. We obtain the observed T c = 8.5°K for the amorphous samples if in addition to the 4 0 % change in N(EF), we also assume that <0:2> amorphous = 0.85bc c. A 15% softening of the phonon frequencies is typically observed for amorphous metals as compared to corre-

Vol. 28, No. 8

STUDY OF SUPERCONDUCTING MOLYBDENUM FILMS

sponding crystalline metals. 12,13 If it were assumed that phonon softening alone accounts for the difference in T c, then an unreasonable softening of nearly 50% would be required to explain the T c of the amorphous film. The change in < I 2 > in going from the bee to the amorphous state should probably also be considered in this discussion. The increase in N(E F) and subsequent enhanced electron screening effects, and the slight increase in the nearest neighbor atom distance in the amorphous phase should both result in a somewhat decreased

<12>. In conclusion, it may be said that the enhancement of T c in amorphous Mo films results from an interplay of several effects. One dominant effect appears to be the increase of N(EF) as one proceeds from the bcc to the amorphous phase. The present photoemission study indicates this increase in N(E F) to be about 40%. Combining this with an estimated 15% softening of phonons allows one to account for the T c of the amorphous Mo.

REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

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Crow J. E., Stronglin M., Thompson R. S., and Kammerer O. F., Physics Letters 30A, 161 (1969). Bond W. L., Cooper A. S., Andres K., Hall G. W., Geballe T. H. and Matthews B. T., Phys. Rev. Lett. 15, 260 (1965). Chopra K. L., Randlett M. R. and Duff R. H., Phil. Mag. 1..66,261 (1967). Bosnell J. R. and Voisey H. C., Thin Solid Films 6, 107 (1970). Schmidt P. H., J. Vac. Sci. Technol. 1_._00,611 (1973). Collver M. M. and Hammond R. H., Phys. Rev. Lett. 3....0, 92 (1973). Kerker G. and Bennemann K. Y., Z. Physik 26_..44, 15 (1973). Schroder B., Johnson W. L., Tsuei C. C., Chaudhari P. and Gracyzk J. F., AIP Conf. Proc. (USA) 31, 353 (1976). Linker G. and Meyer O., Sol. State Comm. 2_.0, 695 (1976). Moruzzi V. L., Janak J. F. and Williams A. R., "Calculated Electronic Properties of Metals", (Pergamon Press, 1978) p. 129. McMillan W. L., Phys. Rev. 16.__.7_7,331 (1968). Golding B., Bagley B. G. and Hsu F. S. L., Phys. Rev. Lett. 2_.99,68 (1972). Buckel W. and Ohlerich C., Proc. 13th Intern. Conf. Low Temp. Phys. 1972, Plenum Press, New York 4,437 (1972).