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Surface and interface study on MBE-grown Nd1.85Ce0.15CuO4 thin films by photoemission spectroscopy and tunneling spectroscopy
i
PHYSICA © Physica C 282-287 (1997) 965-966
ELSEVIER
Surface and interface study on MBE-grown Nd1.8SCe0.1SCu04 thin films by photoemission spectroscopy and tunneling spectroscopy M. Naito, H. Yamamoto, H. Sato NIT Basic Research Laboratories, 3-1 Morinosato-Wakarniya, Atsugi-shi, Kanagawa 243-01, Japan Ndl.85CeO.l5Cu04 (NCCO) surfaces and PblNCCO interfaces have been extensively investigated by X-ray / ultraviolet spectroscopies and tunnel spectroscopy using films grown by MBE. The photoelectron spectra obtained in-situ on the surfaces ofMBE-grown NCCO films are free from any dirt peak, and make feasible the first systematic investigation of the surface and interface of cuprates. The results indicate that the surface and interface electronic structures of NCCO are strongly influenced by oxygen non-stoichiometry, and that the precise control of oxygen content at the surface (interface) is essential to obtain the bulk-representative surface (interface). The importance of photoelectron spectroscopies on the surface of cuprate superconductors is two-fold. First, they establish the standard photoelectron spectra of the "intrinsic" surface representing the bulk electronic structure, which is important in understanding the superconducting mechanism of cuprates. Second, they provide accurate information on the surface and interface reaction, which is necessary for fabricating good HTSC based tunnel junctions. The progress in the surface study of cuprates has been hampered by the difficulty in preparing clean and intrinsic surfaces [1]. This is mainly because of the intrinsic surface unstableness of cuprates: oxygens are loosely bound to the lattice and thereby easily evaporate from the surface. In this article, we report the major results of an extensive study of the Nd1.85CeO.15Cu04 (NCCO) surface and the metal! NCCO interface using films grown by molecular beam epitaxy (MBE) [2]. NCCO films were grown on SrTi03(OOI) or Nb(O.5%)-doped SrTi03(001) substrates by reactive coevaporation from metal sources in a customdesigned ultra-high vacuum chamber [3]. All of the films were single-phased and c-axis oriented, and typically show Tcend 22 - 23 K, P(3OOK) 150 - 200 JlOcm, and p(30K) = 30 - 40 JlOcm, if they are properly reduced. After deposition, the films were cooled down to ambient temperature (- 200 0c) under various reducing conditions-specifically, the P02 in reduction was varied from ~1O.8 Torr to 3 X 10.5 Torr. For the purpose of comparison, we also made a few oxygenated films, which were cooled down with the same amount of O2 +0 3 gas supply as during growth. Figures 1 (a) - (d) compare the in-situ Cu 2p XP spectra of the NCCO films cooled down in O2 with (a) Po ~ 10-8 Torr, (b) P~= 1-5 X 10-6 Torr, (c) P02 = 5 Torr, and in (d) O +0 , The intensity ratio 3X 2 3 of the satellite peak to main one (/llm) , which represents the valence of Cu, increases with increasing
=
P0 2 in reduction, and is enlarged further in the oxygenated film. From the comparison with the spectrum of hole-doped LSCO, the valence of Cu at the surface is intermediate between + 1 and +2, and comes closer to +2 with increasing P02• For the film reduced in P~ ~ 10-8 Torr, 111m is very small, indicating that Cu at the surface is close to mono-valent. Our preliminary tunnel measurements shows that this Cu+1 surface is metallic but not superconducting, indicating that the reduction is excessive. On the other hand, the valence of Cu of the oxygenated film is higher than +2. MgKa main
=
10-
0921-4534/97/$17.00 © Elsevier Science B.Y. AU rights reserved. PH S0921-4S34(97)00602-3
960
950
940
Binding Energy (eV)
Figure. 1 Cu2p XPspedraofNCCOsurfaces andPb/NCCOiIlIrrflIres.
966
M. Naito et al. / Physica C 282-287 (1997) 965-966
The FWHM of the main peak also increases with increasing P~. According to the report by Parmigiani et al. [4] on the relation between the width of the Cu 2P3n. main line and the oxygen coordination around Cu in cuprates, our results can be interpreted as follows. The Cu 2p spectrum of the oxygenated NCCO film is close to our preliminary Cu 2p spectrum of an LSCO film in that both show a large 111m ratio and a wide 2P3n. main line. This indicates that a fair amount of excess oxygen atoms probably occupy interstitial sites (0(3», which correspond to the apical sites of the related T-structure. The semiconducting behavior of the resistivity of the oxygenated film suggests that oxygen atoms also occupy apical sites well inside the film, at least partially. A similar tendency in the Cu 2p spectrum was observed to a less pronounced degree in the film reduced in O2 with p~ = 3 X 10-5 Torr, where oxygen atoms seem to occupy the apical sites mainly in the limited region near the surface, with an extremely small percentage inside the film, as suggested from the resistivity data. These results indicate that the intrinsic surface of superconducting NCCO can be obtained only by the reduction in the proper range of P02 =1 - 5 X 10-6 Torr. Figures l(e) and (f) show the change in the Cu 2p XP spectrum of the optimally reduced NCCO film by exposure to (e) 0.2 atm pure O2, and to (f) air. Oxygen exposure distinctly enlarges the 111m ratio and makes the Cu 2p main peak broaden and shift to a higher binding energy, indicating the adsorption of oxygen at the surface: a fair amount of excess oxygen atoms occupy the interstitial apical sites (0(3». Further exposure to air for long time (up to 2 weeks) brings about no essential change in the Cu 2p XP spectrum and also a very slight change in the 0 Is XP spectrum, although there is an indication of H20/C02 adsorption in the valence band UP spectrum. In fact, it was found that the NCCO surface is fairly stable against air exposure [2]. Figures l(g) - (i) show the Cu 2p XP spectra of the PblNCCO interface. These spectra were taken by depositing 1.0 nm thick Pb metal at an ambient temperature on (g) in-situ, (h) oxygen-exposed, and (i) air-exposed surfaces of optimally reduced NCCO films, respectively. For all, Pb deposition apparently diminishes the I/lm ratio. This implies that NCCO is reduced by Pb at PblNCCO interface. The redox reaction between Pb and NCCO is also confirmed by Pb 4fXPS. The Cu valence of Fig. 1(g) is close to +1, indicating oxygen deficiency at the regular sites [0(1) and/or 0(2)] in the interface region. On the other hand, the Cu valence of Fig. l(h) and (i) is intermediate between + 1 and +2, suggesting that oxygen and H20/ CO2 adsorption protects, to some extent, NCCO from suffering from oxygen deficiencies at the regular sites by Pb deposition. Complementary results were obtained from the
PbINCCO/STO(OOl)
1;
1 ···· ..
!
.
············f··········~\T~~;~~~·~J,~(~~)········
'-' i : i: (a') in-situ «1.4K) ~ 0.1 ..................' .... ·~.:;.- .. ·W .. ·/~ ..·....;................· 0.01
·······f··.......... .\·,.:.h:\~:i·:;.:~:·(9.~9 . .. ! ! !«1.4K) :
:
:
-10
0
10
0.OO1.L,---~---=----~~7=-----:;-'
-20
Bias Voltage (mV)
20
Figure. 2 Twmel spectra ofPb / Nca> tunnel junctions.
PblNCCO tunnel spectra. Figure 2 shows the dUdV characteristics of Pb/NCCO tunnel junctions fabricated on the in-situ surface [(a), (a')] and the airexposed (ex-situ) surface [(b), (b')] using optimally reduced NCCO films. The solid lines [(a) ,(b)] represent the data taken at T =9 K, where Pb is normal and the broken lines [(a'), (b')] the data taken at T < 1.4 K, where Pb is superconducting. In either junction, the dominant transport process of electrons seems to be elastic tunneling since the superconducting gap and the phonon structure of Pb are observed clearly at T < 1.4 K. However, the superconducting gap structure of NCCO is hardly observed at all in the in-situ tunnel junction, indicating that the superconductivity of NCCO is completely lost due to oxygen depletion in the interface region. On the other hand, the superconducting gap structure of NCCO with a reasonable gap value (- 4 meV) can be clearly seen in the ex-situ junction, although the high zero-bias conductance (-30 % of the normal conductance at our best) and the smeared shape of dUdV implies the presence of a normal metal region and the distribution of the magnitude of the superconducting gap, probably due to non-uniform oxygen distribution at interface. Summarizing the results, precise control of oxygen stoichiometry at the surface and interface is essential to obtain an intrinsic (i. e., superconducting) NCCO surface and interface, which is indispensable in obtaining reliable data in surface sensitive experiments and also in fabricating tunnel junctions and superlattices with desirable characteristics. REFERENCES 1. R. P. Vasquez, 1. Electron Spectrosc. Relat. Phenom., 66 (1994) 209.
Yamamoto, M. Naito and H. Sato, in preparation for publication. 3. M. Naito and H. Sato, Appl. Phys. Lett., 67 (1995) 2557. 4. F. Parmigiani, L. E. Depero, T. Minerva and J. B.Torrance,1. Electron Spectrosc. Relat. Phenom., 58 (1992) 315.
2. H.
PHYSICA © Physica C 282-287 (1997) 965-966
ELSEVIER
Surface and interface study on MBE-grown Nd1.8SCe0.1SCu04 thin films by photoemission spectroscopy and tunneling spectroscopy M. Naito, H. Yamamoto, H. Sato NIT Basic Research Laboratories, 3-1 Morinosato-Wakarniya, Atsugi-shi, Kanagawa 243-01, Japan Ndl.85CeO.l5Cu04 (NCCO) surfaces and PblNCCO interfaces have been extensively investigated by X-ray / ultraviolet spectroscopies and tunnel spectroscopy using films grown by MBE. The photoelectron spectra obtained in-situ on the surfaces ofMBE-grown NCCO films are free from any dirt peak, and make feasible the first systematic investigation of the surface and interface of cuprates. The results indicate that the surface and interface electronic structures of NCCO are strongly influenced by oxygen non-stoichiometry, and that the precise control of oxygen content at the surface (interface) is essential to obtain the bulk-representative surface (interface). The importance of photoelectron spectroscopies on the surface of cuprate superconductors is two-fold. First, they establish the standard photoelectron spectra of the "intrinsic" surface representing the bulk electronic structure, which is important in understanding the superconducting mechanism of cuprates. Second, they provide accurate information on the surface and interface reaction, which is necessary for fabricating good HTSC based tunnel junctions. The progress in the surface study of cuprates has been hampered by the difficulty in preparing clean and intrinsic surfaces [1]. This is mainly because of the intrinsic surface unstableness of cuprates: oxygens are loosely bound to the lattice and thereby easily evaporate from the surface. In this article, we report the major results of an extensive study of the Nd1.85CeO.15Cu04 (NCCO) surface and the metal! NCCO interface using films grown by molecular beam epitaxy (MBE) [2]. NCCO films were grown on SrTi03(OOI) or Nb(O.5%)-doped SrTi03(001) substrates by reactive coevaporation from metal sources in a customdesigned ultra-high vacuum chamber [3]. All of the films were single-phased and c-axis oriented, and typically show Tcend 22 - 23 K, P(3OOK) 150 - 200 JlOcm, and p(30K) = 30 - 40 JlOcm, if they are properly reduced. After deposition, the films were cooled down to ambient temperature (- 200 0c) under various reducing conditions-specifically, the P02 in reduction was varied from ~1O.8 Torr to 3 X 10.5 Torr. For the purpose of comparison, we also made a few oxygenated films, which were cooled down with the same amount of O2 +0 3 gas supply as during growth. Figures 1 (a) - (d) compare the in-situ Cu 2p XP spectra of the NCCO films cooled down in O2 with (a) Po ~ 10-8 Torr, (b) P~= 1-5 X 10-6 Torr, (c) P02 = 5 Torr, and in (d) O +0 , The intensity ratio 3X 2 3 of the satellite peak to main one (/llm) , which represents the valence of Cu, increases with increasing
=
P0 2 in reduction, and is enlarged further in the oxygenated film. From the comparison with the spectrum of hole-doped LSCO, the valence of Cu at the surface is intermediate between + 1 and +2, and comes closer to +2 with increasing P02• For the film reduced in P~ ~ 10-8 Torr, 111m is very small, indicating that Cu at the surface is close to mono-valent. Our preliminary tunnel measurements shows that this Cu+1 surface is metallic but not superconducting, indicating that the reduction is excessive. On the other hand, the valence of Cu of the oxygenated film is higher than +2. MgKa main
=
10-
0921-4534/97/$17.00 © Elsevier Science B.Y. AU rights reserved. PH S0921-4S34(97)00602-3
960
950
940
Binding Energy (eV)
Figure. 1 Cu2p XPspedraofNCCOsurfaces andPb/NCCOiIlIrrflIres.
966
M. Naito et al. / Physica C 282-287 (1997) 965-966
The FWHM of the main peak also increases with increasing P~. According to the report by Parmigiani et al. [4] on the relation between the width of the Cu 2P3n. main line and the oxygen coordination around Cu in cuprates, our results can be interpreted as follows. The Cu 2p spectrum of the oxygenated NCCO film is close to our preliminary Cu 2p spectrum of an LSCO film in that both show a large 111m ratio and a wide 2P3n. main line. This indicates that a fair amount of excess oxygen atoms probably occupy interstitial sites (0(3», which correspond to the apical sites of the related T-structure. The semiconducting behavior of the resistivity of the oxygenated film suggests that oxygen atoms also occupy apical sites well inside the film, at least partially. A similar tendency in the Cu 2p spectrum was observed to a less pronounced degree in the film reduced in O2 with p~ = 3 X 10-5 Torr, where oxygen atoms seem to occupy the apical sites mainly in the limited region near the surface, with an extremely small percentage inside the film, as suggested from the resistivity data. These results indicate that the intrinsic surface of superconducting NCCO can be obtained only by the reduction in the proper range of P02 =1 - 5 X 10-6 Torr. Figures l(e) and (f) show the change in the Cu 2p XP spectrum of the optimally reduced NCCO film by exposure to (e) 0.2 atm pure O2, and to (f) air. Oxygen exposure distinctly enlarges the 111m ratio and makes the Cu 2p main peak broaden and shift to a higher binding energy, indicating the adsorption of oxygen at the surface: a fair amount of excess oxygen atoms occupy the interstitial apical sites (0(3». Further exposure to air for long time (up to 2 weeks) brings about no essential change in the Cu 2p XP spectrum and also a very slight change in the 0 Is XP spectrum, although there is an indication of H20/C02 adsorption in the valence band UP spectrum. In fact, it was found that the NCCO surface is fairly stable against air exposure [2]. Figures l(g) - (i) show the Cu 2p XP spectra of the PblNCCO interface. These spectra were taken by depositing 1.0 nm thick Pb metal at an ambient temperature on (g) in-situ, (h) oxygen-exposed, and (i) air-exposed surfaces of optimally reduced NCCO films, respectively. For all, Pb deposition apparently diminishes the I/lm ratio. This implies that NCCO is reduced by Pb at PblNCCO interface. The redox reaction between Pb and NCCO is also confirmed by Pb 4fXPS. The Cu valence of Fig. 1(g) is close to +1, indicating oxygen deficiency at the regular sites [0(1) and/or 0(2)] in the interface region. On the other hand, the Cu valence of Fig. l(h) and (i) is intermediate between + 1 and +2, suggesting that oxygen and H20/ CO2 adsorption protects, to some extent, NCCO from suffering from oxygen deficiencies at the regular sites by Pb deposition. Complementary results were obtained from the
PbINCCO/STO(OOl)
1;
1 ···· ..
!
.
············f··········~\T~~;~~~·~J,~(~~)········
'-' i : i: (a') in-situ «1.4K) ~ 0.1 ..................' .... ·~.:;.- .. ·W .. ·/~ ..·....;................· 0.01
·······f··.......... .\·,.:.h:\~:i·:;.:~:·(9.~9 . .. ! ! !«1.4K) :
:
:
-10
0
10
0.OO1.L,---~---=----~~7=-----:;-'
-20
Bias Voltage (mV)
20
Figure. 2 Twmel spectra ofPb / Nca> tunnel junctions.
PblNCCO tunnel spectra. Figure 2 shows the dUdV characteristics of Pb/NCCO tunnel junctions fabricated on the in-situ surface [(a), (a')] and the airexposed (ex-situ) surface [(b), (b')] using optimally reduced NCCO films. The solid lines [(a) ,(b)] represent the data taken at T =9 K, where Pb is normal and the broken lines [(a'), (b')] the data taken at T < 1.4 K, where Pb is superconducting. In either junction, the dominant transport process of electrons seems to be elastic tunneling since the superconducting gap and the phonon structure of Pb are observed clearly at T < 1.4 K. However, the superconducting gap structure of NCCO is hardly observed at all in the in-situ tunnel junction, indicating that the superconductivity of NCCO is completely lost due to oxygen depletion in the interface region. On the other hand, the superconducting gap structure of NCCO with a reasonable gap value (- 4 meV) can be clearly seen in the ex-situ junction, although the high zero-bias conductance (-30 % of the normal conductance at our best) and the smeared shape of dUdV implies the presence of a normal metal region and the distribution of the magnitude of the superconducting gap, probably due to non-uniform oxygen distribution at interface. Summarizing the results, precise control of oxygen stoichiometry at the surface and interface is essential to obtain an intrinsic (i. e., superconducting) NCCO surface and interface, which is indispensable in obtaining reliable data in surface sensitive experiments and also in fabricating tunnel junctions and superlattices with desirable characteristics. REFERENCES 1. R. P. Vasquez, 1. Electron Spectrosc. Relat. Phenom., 66 (1994) 209.
Yamamoto, M. Naito and H. Sato, in preparation for publication. 3. M. Naito and H. Sato, Appl. Phys. Lett., 67 (1995) 2557. 4. F. Parmigiani, L. E. Depero, T. Minerva and J. B.Torrance,1. Electron Spectrosc. Relat. Phenom., 58 (1992) 315.
2. H.