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Spectroscopic study of the surface oxidation of a thin epitaxial Co layer
Applied Surface Science 158 Ž2000. 353–356 www.elsevier.nlrlocaterapsusc
Spectroscopic study of the surface oxidation of a thin epitaxial Co layer R. Mamy ) Laboratoire de Physique de la Matiere ` Condensee ´ de Toulouse, Department de Physique, INSA, SNCMP, CNRS, Complexe Scientifique de Rangueil, 135 AÕenue de Rangueil, 31077 Toulouse Cedex 04, France Received 28 October 1999; accepted 26 January 2000
Abstract We followed the surface oxidation of a 7-nm Co layer epitaxied on sapphire as a function of the oxygen coverage. Ultra-violet photoemission spectroscopy, atomic force microscopy and scanning tunneling microscopy were used to determine the thickness, surface roughness and electronic gap of the Co oxide. q 2000 Elsevier Science B.V. All rights reserved. PACS: 68-55; 79-60 D; 61-16 Ch; 81-65 Mq Keywords: Thin films growth; Surface oxidation; Photoemission spectroscopy; Scanning probe microscopy
1. Introduction Our purpose in this work is to prepare well-characterized thin layers of CoO on Co by introducing oxygen under ultra-high vacuum and checking the CoOrCo interface from the point of view of crystallography Žlow energy electron diffraction: LEED. and electronic states, in particular the formation of the insulator–metal barrier by ultra-violet photoemission spectroscopy ŽUPS.. In reality the knowledge of the interfacial quality is important to understand the magnetic properties of CoO, in particular the exchange coupling between CoO Žwhich is antiferromagnetic under a temperature of 290 K. and Co.
)
Tel.: q33-5-6155-9659; fax: q33-5-6217-1850. E-mail address: [email protected] ŽR. Mamy..
In a previous study w1x we proposed the ultra-violet oxidation in air which gave satisfactory magnetic properties but no check of the interface was done. Other studies of O 2 adsorption on Co by X-rays w2x, ultra-violet w3x and inverse w4x photoemission spectroscopies put in evidence the rapid formation of CoO rather than Co 3 O4 or CoŽOH. 2 at 300 K. From another point of view CoO is believed to be an antiferromagnetic insulator of the Mott type, its 6 eV gap arising from the electronic correlations in the 3d electronic states but more precisely it would be a charge transfer insulator w5x. Anyhow whatever the interpretation of the UPS results, the valence band spectra can be used to detect the formation of the gap between the Co 3d states and the Fermi level during the oxidation process. We propose farther the scanning tunneling spectroscopy ŽSTM. to get an idea of the gap between filled and empty electronic states in CoO.
0169-4332r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 0 0 . 0 0 0 3 6 - 2
354
R. Mamy r Applied Surface Science 158 (2000) 353–356
2. Experiment We choose to deposit 7 nm of Co on sapphire Ž a-Al 2 O 3 . to obtain the Ž111. orientation for Co so as to expect Ž111. layers of alternated magnetization in CoO and indeed Co deposited on sapphire at 700 K gave a good hexagonal diagram for the LEED with sharp spots. O 2 was introduced with an intermediate vacuum stage to the photoemission chamber where the vacuum raised to 10y6 Torr after oxygen introduction and where the gas composition was checked with a quadrupole mass spectrometer. During oxygen exposure the sample was at room temperature. After each O 2 exposure the valence band evolution is controlled by UPS: in this experiment a HeI discharge lamp Ž21.2 eV excitation energy. was used as a radiation source and a photoelectron detection system based on a rotating angle resolved hemispherical analyzer
Fig. 1. Evolution of the density of valence states of Co obtained by photoelectron spectroscopy with the 21.2 eV HeI radiation, as a function of the oxygen coverage in Langmuir Ž1 L s10y6 Torr s.. The binding energies are referred to the Fermi level Ž EF . of the metal.
Ž150 meV resolution. was utilized here. Photoelectrons outgoing along the normal to the surface of the sample were analyzed in energy. On Fig. 1 the photoemission intensities were plotted as a function of electron binding energies referred to the Fermi level. At the end of the oxidation cycles the surface topography was controlled ex situ, in air, by atomic force microscopy ŽAFM. ŽNanoscopeIII Scanning Probe from Digital Instruments. and compared to that obtained before oxidation. The same apparatus can be used with an STM head.
3. Results and discussion The valence bands evolution is given on Fig. 1 with the O 2 exposures given in Langmuir Ž1 L s 10y6 Torr s.. Before any oxidation the spectrum is that of Co where the principal feature at y0.7 eV originates from the Co 3d states. The binding energies are referred to the Fermi level Ž EF . which corresponds to a shoulder in the low energy side of the Co 3d peak, hardly visible on Fig. 1 but more clearly with gold calibration. For 90 L the feature at y5 eV comes from the O 2p states while the Co 3d ones are still present and decrease at 2250 L and still more at 5000 L. The new feature at y2 eV can be attributed to the Co 3d states of CoO, the one at y10 eV is a satellite characteristic of CoO w5x. So at this stage we observe that Ždue to the small oxide thickness with respect to the mean free path of the photoelectrons which is here about 2 nm in our kinetic energy range. the underlying non-oxidized Co is detected by the Co 3d feature at y0.7 eV. The disappearance of this metallic Co feature at 20,000 L discovers entirely the Co 3d states of CoO and a cut-off at y0.5 eV under E F is clearly seen which gives the top of the valence bands of CoO. This disappearance implies a CoO thickness near 2 nm Žthe electron mean free path.. This is in accordance with many previous studies on oxygen adsorption on Co which gave various limit thicknesses around this value w2–4,6x. The images from AFM ŽFig. 2. indicate a rather uniform oxidation with a root mean square ŽRMS. of 0.3 nm instead of 0.6 nm with ultra-violet oxidation in air w1x Žthe RMS was
R. Mamy r Applied Surface Science 158 (2000) 353–356
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density of the electronic states involved in the tunneling, that is, neglecting the effect of the structureless density of s states of the tungsten tip, we can expect to obtain the density of filled and empty surface oxide states for - 0 and ) 0 bias scan, respectively. Obviously, by operating in air the fine structures on the density of the d states are lost, but the important result is that a 1.5 eV gap is obtained. It is also noticeable to find again the 0.5 eV difference between occupied states and the Fermi level Žlocated at zero bias voltage on Fig. 3. of the photoemission results and also the 1 eV value between EF and empty states given by the inverse photoemission results w4x. So this value of the gap is highly probable. This similarity of results shows that the local and perturbing character of STM in air is not capital when only a value of the gap is researched and that no profound chemical reactions are involved which would otherwise change the value of the gap, which is not the case here.
Fig. 2. AFM image on a Ž1 mm=1 mm. zone of the Co surface after oxidation. The profile shown below was taken along the white line so as to cross one of the most prominent hollows.
0.2 nm before oxidation.. The hollows were found as far as we can explore, all less than 2 nm over each line of analysis in the various Ž1 = 1 mm. zones as that of Fig. 2. Finally, we propose to perform ex situ STM spectroscopy in air to measure the gap in the oxide so obtained, as tests we made with layered semi-conductors with inert surfaces ŽMoS 2 , GaSe. permitted to obtain their gap. For a 2 nm oxide formed on a metal, a tunneling current can be obtained between a tungsten tip and the surface of the oxide Žvoltage V: 1 V, current I: 1 pA., however, the I Ž V . spectra are very unstable in air and were averaged 20 times. On Fig. 3 the horizontal flat on the I Ž V . curve Žlower curve. arises from the surface gap of the oxide, the bias scan is the voltage between the sample and the tip. The d IrdV curve Župper curve. would give the
Fig. 3. I Ž V . and d IrdV characteristics obtained with STM in spectroscopic mode where the tip is fixed and the voltage between the sample and the tip is scanned Žbias scan..
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R. Mamy r Applied Surface Science 158 (2000) 353–356
4. Conclusion
References
We have shown here that, with the combined techniques of UPS, AFM and STM, the conditions for the formation of CoO by oxygen adsorption on top of a Co layer epitaxied on sapphire can be well defined. This CoO layer has a thickness of 2 nm, a satisfactory uniformity with a RMS roughness of 0.3 nm and above all the insulating character is evidenced by the formation of a gap of 1.5 eV in the electronic valence states.
w1x B. Raquet, R. Mamy, J.C. Ousset, N. Negre, M. Goiran, C. ` Guerret-Piecourt, J. Magn. Magn. Mater. 184 Ž1. Ž1998. 41. ´ w2x C.R. Brundle, T.J. Chuang, D.W. Rice, Surf. Sci. 60 Ž1976. 286. w3x G.R. Castro, J. Kuppers, Surf. Sci. 123 Ž1982. 456. ¨ w4x L. Duo, ´ M. Finazzi, F. Ciccacci, L. Braicovich, Phys. Rev. B 47 Ž1993. 15848. w5x Z.X. Chen, J.W. Allen, W. Ellis, J.S. Kang, S.J. Oh, I. Landau, W.E. Spicer, Phys. Rev. B 42 Ž1990. 1817. w6x L. Smardz, U. Kobler, W. Zinn, J. Appl. Phys. 71 Ž10. Ž1992. ¨ 5199.
Spectroscopic study of the surface oxidation of a thin epitaxial Co layer R. Mamy ) Laboratoire de Physique de la Matiere ` Condensee ´ de Toulouse, Department de Physique, INSA, SNCMP, CNRS, Complexe Scientifique de Rangueil, 135 AÕenue de Rangueil, 31077 Toulouse Cedex 04, France Received 28 October 1999; accepted 26 January 2000
Abstract We followed the surface oxidation of a 7-nm Co layer epitaxied on sapphire as a function of the oxygen coverage. Ultra-violet photoemission spectroscopy, atomic force microscopy and scanning tunneling microscopy were used to determine the thickness, surface roughness and electronic gap of the Co oxide. q 2000 Elsevier Science B.V. All rights reserved. PACS: 68-55; 79-60 D; 61-16 Ch; 81-65 Mq Keywords: Thin films growth; Surface oxidation; Photoemission spectroscopy; Scanning probe microscopy
1. Introduction Our purpose in this work is to prepare well-characterized thin layers of CoO on Co by introducing oxygen under ultra-high vacuum and checking the CoOrCo interface from the point of view of crystallography Žlow energy electron diffraction: LEED. and electronic states, in particular the formation of the insulator–metal barrier by ultra-violet photoemission spectroscopy ŽUPS.. In reality the knowledge of the interfacial quality is important to understand the magnetic properties of CoO, in particular the exchange coupling between CoO Žwhich is antiferromagnetic under a temperature of 290 K. and Co.
)
Tel.: q33-5-6155-9659; fax: q33-5-6217-1850. E-mail address: [email protected] ŽR. Mamy..
In a previous study w1x we proposed the ultra-violet oxidation in air which gave satisfactory magnetic properties but no check of the interface was done. Other studies of O 2 adsorption on Co by X-rays w2x, ultra-violet w3x and inverse w4x photoemission spectroscopies put in evidence the rapid formation of CoO rather than Co 3 O4 or CoŽOH. 2 at 300 K. From another point of view CoO is believed to be an antiferromagnetic insulator of the Mott type, its 6 eV gap arising from the electronic correlations in the 3d electronic states but more precisely it would be a charge transfer insulator w5x. Anyhow whatever the interpretation of the UPS results, the valence band spectra can be used to detect the formation of the gap between the Co 3d states and the Fermi level during the oxidation process. We propose farther the scanning tunneling spectroscopy ŽSTM. to get an idea of the gap between filled and empty electronic states in CoO.
0169-4332r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 0 0 . 0 0 0 3 6 - 2
354
R. Mamy r Applied Surface Science 158 (2000) 353–356
2. Experiment We choose to deposit 7 nm of Co on sapphire Ž a-Al 2 O 3 . to obtain the Ž111. orientation for Co so as to expect Ž111. layers of alternated magnetization in CoO and indeed Co deposited on sapphire at 700 K gave a good hexagonal diagram for the LEED with sharp spots. O 2 was introduced with an intermediate vacuum stage to the photoemission chamber where the vacuum raised to 10y6 Torr after oxygen introduction and where the gas composition was checked with a quadrupole mass spectrometer. During oxygen exposure the sample was at room temperature. After each O 2 exposure the valence band evolution is controlled by UPS: in this experiment a HeI discharge lamp Ž21.2 eV excitation energy. was used as a radiation source and a photoelectron detection system based on a rotating angle resolved hemispherical analyzer
Fig. 1. Evolution of the density of valence states of Co obtained by photoelectron spectroscopy with the 21.2 eV HeI radiation, as a function of the oxygen coverage in Langmuir Ž1 L s10y6 Torr s.. The binding energies are referred to the Fermi level Ž EF . of the metal.
Ž150 meV resolution. was utilized here. Photoelectrons outgoing along the normal to the surface of the sample were analyzed in energy. On Fig. 1 the photoemission intensities were plotted as a function of electron binding energies referred to the Fermi level. At the end of the oxidation cycles the surface topography was controlled ex situ, in air, by atomic force microscopy ŽAFM. ŽNanoscopeIII Scanning Probe from Digital Instruments. and compared to that obtained before oxidation. The same apparatus can be used with an STM head.
3. Results and discussion The valence bands evolution is given on Fig. 1 with the O 2 exposures given in Langmuir Ž1 L s 10y6 Torr s.. Before any oxidation the spectrum is that of Co where the principal feature at y0.7 eV originates from the Co 3d states. The binding energies are referred to the Fermi level Ž EF . which corresponds to a shoulder in the low energy side of the Co 3d peak, hardly visible on Fig. 1 but more clearly with gold calibration. For 90 L the feature at y5 eV comes from the O 2p states while the Co 3d ones are still present and decrease at 2250 L and still more at 5000 L. The new feature at y2 eV can be attributed to the Co 3d states of CoO, the one at y10 eV is a satellite characteristic of CoO w5x. So at this stage we observe that Ždue to the small oxide thickness with respect to the mean free path of the photoelectrons which is here about 2 nm in our kinetic energy range. the underlying non-oxidized Co is detected by the Co 3d feature at y0.7 eV. The disappearance of this metallic Co feature at 20,000 L discovers entirely the Co 3d states of CoO and a cut-off at y0.5 eV under E F is clearly seen which gives the top of the valence bands of CoO. This disappearance implies a CoO thickness near 2 nm Žthe electron mean free path.. This is in accordance with many previous studies on oxygen adsorption on Co which gave various limit thicknesses around this value w2–4,6x. The images from AFM ŽFig. 2. indicate a rather uniform oxidation with a root mean square ŽRMS. of 0.3 nm instead of 0.6 nm with ultra-violet oxidation in air w1x Žthe RMS was
R. Mamy r Applied Surface Science 158 (2000) 353–356
355
density of the electronic states involved in the tunneling, that is, neglecting the effect of the structureless density of s states of the tungsten tip, we can expect to obtain the density of filled and empty surface oxide states for - 0 and ) 0 bias scan, respectively. Obviously, by operating in air the fine structures on the density of the d states are lost, but the important result is that a 1.5 eV gap is obtained. It is also noticeable to find again the 0.5 eV difference between occupied states and the Fermi level Žlocated at zero bias voltage on Fig. 3. of the photoemission results and also the 1 eV value between EF and empty states given by the inverse photoemission results w4x. So this value of the gap is highly probable. This similarity of results shows that the local and perturbing character of STM in air is not capital when only a value of the gap is researched and that no profound chemical reactions are involved which would otherwise change the value of the gap, which is not the case here.
Fig. 2. AFM image on a Ž1 mm=1 mm. zone of the Co surface after oxidation. The profile shown below was taken along the white line so as to cross one of the most prominent hollows.
0.2 nm before oxidation.. The hollows were found as far as we can explore, all less than 2 nm over each line of analysis in the various Ž1 = 1 mm. zones as that of Fig. 2. Finally, we propose to perform ex situ STM spectroscopy in air to measure the gap in the oxide so obtained, as tests we made with layered semi-conductors with inert surfaces ŽMoS 2 , GaSe. permitted to obtain their gap. For a 2 nm oxide formed on a metal, a tunneling current can be obtained between a tungsten tip and the surface of the oxide Žvoltage V: 1 V, current I: 1 pA., however, the I Ž V . spectra are very unstable in air and were averaged 20 times. On Fig. 3 the horizontal flat on the I Ž V . curve Žlower curve. arises from the surface gap of the oxide, the bias scan is the voltage between the sample and the tip. The d IrdV curve Župper curve. would give the
Fig. 3. I Ž V . and d IrdV characteristics obtained with STM in spectroscopic mode where the tip is fixed and the voltage between the sample and the tip is scanned Žbias scan..
356
R. Mamy r Applied Surface Science 158 (2000) 353–356
4. Conclusion
References
We have shown here that, with the combined techniques of UPS, AFM and STM, the conditions for the formation of CoO by oxygen adsorption on top of a Co layer epitaxied on sapphire can be well defined. This CoO layer has a thickness of 2 nm, a satisfactory uniformity with a RMS roughness of 0.3 nm and above all the insulating character is evidenced by the formation of a gap of 1.5 eV in the electronic valence states.
w1x B. Raquet, R. Mamy, J.C. Ousset, N. Negre, M. Goiran, C. ` Guerret-Piecourt, J. Magn. Magn. Mater. 184 Ž1. Ž1998. 41. ´ w2x C.R. Brundle, T.J. Chuang, D.W. Rice, Surf. Sci. 60 Ž1976. 286. w3x G.R. Castro, J. Kuppers, Surf. Sci. 123 Ž1982. 456. ¨ w4x L. Duo, ´ M. Finazzi, F. Ciccacci, L. Braicovich, Phys. Rev. B 47 Ž1993. 15848. w5x Z.X. Chen, J.W. Allen, W. Ellis, J.S. Kang, S.J. Oh, I. Landau, W.E. Spicer, Phys. Rev. B 42 Ž1990. 1817. w6x L. Smardz, U. Kobler, W. Zinn, J. Appl. Phys. 71 Ž10. Ž1992. ¨ 5199.