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Oscillations of the superconducting transition temperature in aluminum films
Volume 53A, number 4
PHYSICS LETTERS
30 June 1975
OSCILLATIONS OF THE SUPERCONDUCTING TRANSITION TEMPERATURE IN ALUMINUM FILMS ~ H. SIXL Physikalisches lnstitut, Teil 3, der Universitiit Stuttgart, D-7000 Stuttgart-80, Germany Received 16 May 1975 Oscillations of the superconducting transition temperature of thin Al-filmshave been observed during deposition of siliconmonoxide SiO. Friedel oscillations as well as the quantum size effect may account for the observations. The properties of thin superconducting films covered with different organic or inorganic dielectrics have been the subject of active investigations [1,2]. Whenever insulating or semiconducting material is deposited onto the clean surface of a superconductor the transition temperature T c is either increased or depressed depending on both, the superconductor and the deposit. The interpretation of the observed effects was restricted to two main causes, namely to positive or negative charging of the films [1 ] and to the alteration of the surface phonons [2]. Recent theoretical considerations [3], concerning the metal-semiconductor interface, showed that large concentrations of electron states in the semiconductor gap may favour superconductivity. The importance of these states was further stressed by Halbritter [4] who pointed out that resonance scattering or coherent tunnelexchange of metal electrons into gap states of the deposit may be the dominant mechanism of the superconductor-dielecticointeraction. In this letter, we report about Tc-oscillations in aluminum fdms during deposition of silicon-monoxide. Care was taken to prevent contamination of the single crystal quartz substrate and of the aluminum film before covering with the dielectric. Thus, in contrast to previously reported results, all experiments are reproducible. The aluminum and SiO films are produced in a cryostat (with pressure less than 10 -7 torr) by vacuum evaporation and condensation of the material at 4.2K. Substrate and Films were protected by turnable, cooled masks (4.2K). The thickness of the films was monitored by the change in the optical transmission and reflectivity. The transition temperature was measured resistively. All films showed the temperature dependence
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TEMPERATURE T IKI
Fig. 1. Transition curves R(T) of an A1 film (thickness: 70A, length: 10 mm, width: 1 ram) uncoated (1), with a SiO-coating of 3A (2), 13A (3), 26A (4) and 70A(5)'. predicted by the fluctuation theories including strong pairbreaking [5] with no dimensionality-change [6] upon coverage with SiO. Figure 1 represents the measured temperature dependent resistance R(T) of an aluminum film as a function of the thickness t(SiO) of the silicon-monoxide coating. Tc changes oscillatory with deposition of SiO, whereas RN, the normal-state resistance, decreases monotonously. The normalized temperature dependence [5] R N [ R = 1 + (ro/r)( 4 - n ) / 2 remains unchanged (~" = T I T c - 1, r o : transition width, n = 2 : dimensionality of the superconductor [6]). In fig. 2 the transition temperature T c is plotted versus the thickness of the SiO-coating for different aluminum films. All curves are unnormalized concerning the initial Tc(t(A1)) of the uncoated AI-Film. The overall
~r Supported by the Deutsche Forschungsgemeinschaft.
333
Volume 53A, number 4
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I'HYSICS LETTERS
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THICKNESS t [~,I OF SiO-COATING
l:i~. 2. l:unctional dependence of the transition temperature Tc of different A1 fihns on the thickness of the SiO-coating t(SiO). The effect decreases with increasing AI fihn thicknL'ss t (AlL characteristic of the coating-effect renrains unchanged. Tire absolute lnaximunr change in T c at 3 A SiO-thickhess increases proportional to 1/t (A1), whereas the nlinimum value at 13A in all cases is identical with the initial transition temperature. The increase o f T c at high SiO thickness is again proportional to 1/t(Al). Finally it should be emphasized clearly that maxima, minima and the final value of T c are all reached at about the same characteristic SiO-fihn thickness, independent on the thickness of the aluminum films. The following discussion has to account for the remarkable oscillative behaviour of T c and its final increase to a constant value. It is unlikely that the observed effects result from three independent origins, which cancel each other at about 13 A SiO-thickness. Therefore, the observed total SiO-thickness dependence is reduced to only one or two independent effects. Because of the initial and final increase of T c and the magnitude of the observed To-changes, charging as well as surface phonon alteration is ruled out. Both effects lead to a monotonous decrease of Tc [2]. Furthermore resonance scattering or coherent exchange of the conduction electrons with localized states, which enhances the resistance and weakens the superconducting interaction is in contradiction to the experiments [4]. Doping of the aluminum at the surface occurs within a few monolayers, because there is no sharp boundary 334
30 June 1975
in the atomic scale. The Al/SiO interface consists of a mixture of SiO and AI particles. Superconductivity of aluminum doped with different organic and inorganic materials depends on the impurity concentration [8]. T c increases monotonously to a maximum at an optimal concentration of about 30 percent. A further increase of the impurity concentration leads to the destruction of superconductivity. This effect is rather independent on the coating materials and may perhaps be responsible to the initial increase of T c. The following two mechanisms may account for the observed oscillating Tc-variation and the decrease of the normal-state resistance R N upon coating with SiO. 1) Quantum size effect (Blatt - T h o m p s o n effect [7]) in the SiO interface. Quantization of the transverse motion of the electrons in the SiO monolayers at the interface results in oscillations of T c at integer multiples of 2 t (SiO)/3`. Assuming a free electron gas, the De Broglie wavelength 3, of the electrons is a function of the electron density. Within the SiO interfacial layers the density of quasi free electrons decreases rapidly with increasing distance to the Al-interface, therefore 3` increases. This results in nonperiodic oscillations of T c as observed experimentally (see fig. 2). 2) Friedel oscillations [9]. Boundary conditions, imposed on the free electron gas of a metal, result in spacial electron density oscillations perpendicular to the metal surface. Depending on the coating material the metal wave function will be allowed to penetrate more or less into the coating. Therefore, during the deposition of SiO, the damped electron density oscillations at the A1 surface will be shifted asymptotically as a whole beyond the A1 surface into the SiO coating, controlled by the penetration depth. The oscillation of the electron density at the interface leads to an oscillation of the electron-phonon coupling, and therefore may be the reason of the To-oscillation. In addition, these two mechanisms account for the different results, using different coating materials, as observed experimentally. They are sensitive to the number of states available as well as to the ability of the dielectric to form an "alloy" with the metal. Such behaviour is strongly favoured by a small semiconductor energy gap. Insulator coating of aluminum shows a weak and only monotonous To-dependence. Preliminary experiments with other semiconducting materials (Se, Ge) indicate that we are dealing with a general effect due to a semiconductor-metal interface.
Volume 53A, number 4
PHYSICS LETTERS
The author wishes to thank H.C. Wolf, J.Halbritter and W. Sanwald for helpful discussions.
References [1] W. Riihl, Z.f. Physik 186 (1965) 190; 196 (1966)464; P. Hllschand D.G' Naugle, Z.f. Physik 201 (1967) 1; B.M. Hoffman, F.R. Gamble and H.M. McConnell, J. Am. Chem. Soc. 89 (1967) 27; D.G. Naugle, Phys. Lett. 25A (1967) 688. [2] D.G. Naugle, J.W. Baker and R.E. Allen, Phys. Rev. B7 (1973) 3028; W. Felsch and R.E. Glover, Solid State Communications 10 (1972) 1033.
30 June 1975
[3] V.L. Shneerson, Soviet Physics JETP 35 (1972) 1209; D. Allender, J. Bray and J. Bardeen, Phys. Rev. B7 (1973) 1020. [4] J. Halbritter, Phys. Lett. 49A (1974) 379 and private communications. [5] L.G. Aslamazov and A.I. Larkin, Soviet Physics Solid State 10 (1968) 875; E. Abrahams and J.W.F. Woo, Phys. Lett. 27A (1968) 117; R.S. Thompson, Phys. Rev. BI (1970) 327. [6] H. Sixl and W. Sanwald, Solid State Communications 16 (1975) 603. [7] J.M. Blatt and C.J. Thompson, Phys. Rev. Lett. 10 (1963) 332. [81 F.R. Gamble and H.M. MeConnell, Phys. Lett. 26A (1968) 162; B. Abeles and J.J. Hanak, Phys. Lett. 34A (1971) 165. [9] N.D. Lang and W. Kohn, Phys. Rev. B1 (1970) 4555.
335
PHYSICS LETTERS
30 June 1975
OSCILLATIONS OF THE SUPERCONDUCTING TRANSITION TEMPERATURE IN ALUMINUM FILMS ~ H. SIXL Physikalisches lnstitut, Teil 3, der Universitiit Stuttgart, D-7000 Stuttgart-80, Germany Received 16 May 1975 Oscillations of the superconducting transition temperature of thin Al-filmshave been observed during deposition of siliconmonoxide SiO. Friedel oscillations as well as the quantum size effect may account for the observations. The properties of thin superconducting films covered with different organic or inorganic dielectrics have been the subject of active investigations [1,2]. Whenever insulating or semiconducting material is deposited onto the clean surface of a superconductor the transition temperature T c is either increased or depressed depending on both, the superconductor and the deposit. The interpretation of the observed effects was restricted to two main causes, namely to positive or negative charging of the films [1 ] and to the alteration of the surface phonons [2]. Recent theoretical considerations [3], concerning the metal-semiconductor interface, showed that large concentrations of electron states in the semiconductor gap may favour superconductivity. The importance of these states was further stressed by Halbritter [4] who pointed out that resonance scattering or coherent tunnelexchange of metal electrons into gap states of the deposit may be the dominant mechanism of the superconductor-dielecticointeraction. In this letter, we report about Tc-oscillations in aluminum fdms during deposition of silicon-monoxide. Care was taken to prevent contamination of the single crystal quartz substrate and of the aluminum film before covering with the dielectric. Thus, in contrast to previously reported results, all experiments are reproducible. The aluminum and SiO films are produced in a cryostat (with pressure less than 10 -7 torr) by vacuum evaporation and condensation of the material at 4.2K. Substrate and Films were protected by turnable, cooled masks (4.2K). The thickness of the films was monitored by the change in the optical transmission and reflectivity. The transition temperature was measured resistively. All films showed the temperature dependence
n~
3
Z
2
5
t(Al)=?0~
"'
/
u'}
n~
7.~
2.9
3.0
i 3.1
'
'
TEMPERATURE T IKI
Fig. 1. Transition curves R(T) of an A1 film (thickness: 70A, length: 10 mm, width: 1 ram) uncoated (1), with a SiO-coating of 3A (2), 13A (3), 26A (4) and 70A(5)'. predicted by the fluctuation theories including strong pairbreaking [5] with no dimensionality-change [6] upon coverage with SiO. Figure 1 represents the measured temperature dependent resistance R(T) of an aluminum film as a function of the thickness t(SiO) of the silicon-monoxide coating. Tc changes oscillatory with deposition of SiO, whereas RN, the normal-state resistance, decreases monotonously. The normalized temperature dependence [5] R N [ R = 1 + (ro/r)( 4 - n ) / 2 remains unchanged (~" = T I T c - 1, r o : transition width, n = 2 : dimensionality of the superconductor [6]). In fig. 2 the transition temperature T c is plotted versus the thickness of the SiO-coating for different aluminum films. All curves are unnormalized concerning the initial Tc(t(A1)) of the uncoated AI-Film. The overall
~r Supported by the Deutsche Forschungsgemeinschaft.
333
Volume 53A, number 4
_~
3.1f
I.IJ 30 ~'P n,-
I'HYSICS LETTERS
,
~
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t(AI ) 70~,
~
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THICKNESS t [~,I OF SiO-COATING
l:i~. 2. l:unctional dependence of the transition temperature Tc of different A1 fihns on the thickness of the SiO-coating t(SiO). The effect decreases with increasing AI fihn thicknL'ss t (AlL characteristic of the coating-effect renrains unchanged. Tire absolute lnaximunr change in T c at 3 A SiO-thickhess increases proportional to 1/t (A1), whereas the nlinimum value at 13A in all cases is identical with the initial transition temperature. The increase o f T c at high SiO thickness is again proportional to 1/t(Al). Finally it should be emphasized clearly that maxima, minima and the final value of T c are all reached at about the same characteristic SiO-fihn thickness, independent on the thickness of the aluminum films. The following discussion has to account for the remarkable oscillative behaviour of T c and its final increase to a constant value. It is unlikely that the observed effects result from three independent origins, which cancel each other at about 13 A SiO-thickness. Therefore, the observed total SiO-thickness dependence is reduced to only one or two independent effects. Because of the initial and final increase of T c and the magnitude of the observed To-changes, charging as well as surface phonon alteration is ruled out. Both effects lead to a monotonous decrease of Tc [2]. Furthermore resonance scattering or coherent exchange of the conduction electrons with localized states, which enhances the resistance and weakens the superconducting interaction is in contradiction to the experiments [4]. Doping of the aluminum at the surface occurs within a few monolayers, because there is no sharp boundary 334
30 June 1975
in the atomic scale. The Al/SiO interface consists of a mixture of SiO and AI particles. Superconductivity of aluminum doped with different organic and inorganic materials depends on the impurity concentration [8]. T c increases monotonously to a maximum at an optimal concentration of about 30 percent. A further increase of the impurity concentration leads to the destruction of superconductivity. This effect is rather independent on the coating materials and may perhaps be responsible to the initial increase of T c. The following two mechanisms may account for the observed oscillating Tc-variation and the decrease of the normal-state resistance R N upon coating with SiO. 1) Quantum size effect (Blatt - T h o m p s o n effect [7]) in the SiO interface. Quantization of the transverse motion of the electrons in the SiO monolayers at the interface results in oscillations of T c at integer multiples of 2 t (SiO)/3`. Assuming a free electron gas, the De Broglie wavelength 3, of the electrons is a function of the electron density. Within the SiO interfacial layers the density of quasi free electrons decreases rapidly with increasing distance to the Al-interface, therefore 3` increases. This results in nonperiodic oscillations of T c as observed experimentally (see fig. 2). 2) Friedel oscillations [9]. Boundary conditions, imposed on the free electron gas of a metal, result in spacial electron density oscillations perpendicular to the metal surface. Depending on the coating material the metal wave function will be allowed to penetrate more or less into the coating. Therefore, during the deposition of SiO, the damped electron density oscillations at the A1 surface will be shifted asymptotically as a whole beyond the A1 surface into the SiO coating, controlled by the penetration depth. The oscillation of the electron density at the interface leads to an oscillation of the electron-phonon coupling, and therefore may be the reason of the To-oscillation. In addition, these two mechanisms account for the different results, using different coating materials, as observed experimentally. They are sensitive to the number of states available as well as to the ability of the dielectric to form an "alloy" with the metal. Such behaviour is strongly favoured by a small semiconductor energy gap. Insulator coating of aluminum shows a weak and only monotonous To-dependence. Preliminary experiments with other semiconducting materials (Se, Ge) indicate that we are dealing with a general effect due to a semiconductor-metal interface.
Volume 53A, number 4
PHYSICS LETTERS
The author wishes to thank H.C. Wolf, J.Halbritter and W. Sanwald for helpful discussions.
References [1] W. Riihl, Z.f. Physik 186 (1965) 190; 196 (1966)464; P. Hllschand D.G' Naugle, Z.f. Physik 201 (1967) 1; B.M. Hoffman, F.R. Gamble and H.M. McConnell, J. Am. Chem. Soc. 89 (1967) 27; D.G. Naugle, Phys. Lett. 25A (1967) 688. [2] D.G. Naugle, J.W. Baker and R.E. Allen, Phys. Rev. B7 (1973) 3028; W. Felsch and R.E. Glover, Solid State Communications 10 (1972) 1033.
30 June 1975
[3] V.L. Shneerson, Soviet Physics JETP 35 (1972) 1209; D. Allender, J. Bray and J. Bardeen, Phys. Rev. B7 (1973) 1020. [4] J. Halbritter, Phys. Lett. 49A (1974) 379 and private communications. [5] L.G. Aslamazov and A.I. Larkin, Soviet Physics Solid State 10 (1968) 875; E. Abrahams and J.W.F. Woo, Phys. Lett. 27A (1968) 117; R.S. Thompson, Phys. Rev. BI (1970) 327. [6] H. Sixl and W. Sanwald, Solid State Communications 16 (1975) 603. [7] J.M. Blatt and C.J. Thompson, Phys. Rev. Lett. 10 (1963) 332. [81 F.R. Gamble and H.M. MeConnell, Phys. Lett. 26A (1968) 162; B. Abeles and J.J. Hanak, Phys. Lett. 34A (1971) 165. [9] N.D. Lang and W. Kohn, Phys. Rev. B1 (1970) 4555.
335