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Oxygen diffusion in laser heated YBa2Cu3O7 films
J. P$s. Chem. Solids Vol. 56, No. 12. pp. 18294830. 1995 Copyright @ \99S Elsevier Science Ltd Printed in Great Britain. All rights reserved 0022-3697/95 $9.50 + 0.00
Pergamon
00223697(95)00103-d
OXYGEN DIFFUSION
IN LASER HEATED YBazCu307 FILMS
A, BOCK, R. KURSTEN and U. MERKT fur Angewandte Pbysii und Zentrum fiir Mikrostrukturforschung, UniversitHt Hamburg, Jungiusstrabe 11, D-20355 Hamburg, Germany
Institut
Abstract-Phonons of laser-deposited YBu~CusOr (YBCO7) films on MgO(100) are investigated in a calibrated Raman set-up as a function of the power density of the argon-ion laser. While four Ae modes are visible for low power densities additional modes appear when the power density is increased. These are the defect-induced modes at 230 cm-’ and 585 em-i, normally assigned to infrared active phonons of YBC07_a, and Raman active modes of the oxygen-reduced tetragonal phase YBCO6. At still higher power densities the phonons of the tetragonal phase dominate the spectra. We compare spot temperatures deduced from Stokes and anti-Stokes Raman spectra with those calculated from the solution of the heat equation. By varying the cryostat temperature and the ambient pressure we investigate the oxygen out-diffusion mechanism.
Keywords:Raman spectroscopy, oxygen diffusion, laser heating.
1. INTRODUCTION
Raman spectroscopy has proven to be a proper method for the determination of various YBC06+x film properties, e.g., the chain-oxygen content x, possible impurity phases, and the epitaxial quality [1,2]. However, it has been demonstrated that the experimental conditions, in particular the power density, have to be chosen with care in order to avoid a damage of the sample during the measurement [3]. The heating, induced by the absorbed laser light, can lead to a depletion of the chain-oxygen content, as the chain oxygen O(1) has the highest mobility at a given temperature [4]. With the oxygen loss at the chain site the electrical properties of the sample deteriorate as the critical temperature decreases. Here we present investigations of the damage threshold of the power density when a YBC07 film is heated with the 514 nm line of an argon-ion laser at different ambient temperatures and pressures. In these Raman experiments the laser serves the dual role of heating and investigating the sample simultaneously.
2. EXPERIMENTAL The samples are mounted on the cold finger of a helium cryostat whose temperature 7’s is measured by a silicon diode. The cryostat can be evacuated to residual pressures pe of lob6 mbar. Calibrated and straylight-corrected Raman spectra are taken in quasi-backscattering geometry. Details of the set-up, the calibration, and the straylight correction are given elsewhere [3]. The radii of the laser focus on the sample are determined to be rh = (6 + 1) urn and rV = (11 k 1) Mm in the horizontal and vertical directions, respectively. Given the laser power P on the sample
the maximum power density PL inside the Gaussian spot is PL = P/(TTQJ~), which we will call the “power density” in the following. We investigate a c-axis oriented YBCO7 6lm on MgO(100) prepared by laser deposition [S]. The film has a thickness of 240 nm, a critical temperature Td = 85.2 K, and a critical current density j,(77K) = 4 x lo6 Acxr~-~. Using procedures described elsewhere [3] we determined the thermal boundary resistance at the film-substrate interface tobe~=(0.5+0.1)~10-~Kcm~W-i.
3, RESULTS AND DISCUSSION
In Fig. 1, we present spectra from a Stokes/anti-Stokes series which were taken for increasing power densities. At the lowest power density we observe four Ar phonons of YBC07, i.e., the Ba mode at 117 cm-‘, the Cu(2) mode at 151 cm-t, the Big mode at 339 cm-‘, and the apex oxygen O(4) mode at 502 cm-i, which gives x = 1 [l]. From an analysis of the polarized and depolarized spectra at the lowest power density we determine the c-axis oriented fraction of the film 6 = 87 f 5 % and the in-plane orientation of this fraction Qe = 93 f 5 % [2]. In order to obtain these informations the spectra have been fitted using Fano profiles for the Ba and the Btr mode, Lorentz profiles for the Cu(2) and O(4) mode, and an almost constant background. When the power density is increased to values above 96 kWcmez new modes begin to develop at 230 cm-’ and 585 cm-‘. Up to a power density of 150 kWcm_’ the intensity of these new modes grows continuously. At still higher power densities the intensity of these modes starts to decrease until they almost vanish at the highest power density of 290 kWcme2. In addition to the above findings we observe with increasing power density that the Cu(2) mode shifts to lower frequen-
1829
1830
A BOCK et a/.
ties, the BI, rises in intensity, the Ba and the O(4) mode diminish, and another mode appears at 450 cm-‘. The additional modes were fitted using a Fano profile for the 230 cm-’ mode and Lore& profiles for the 450 cm-’ and the 585 cm-’ mode. To improve the fit we have included a broad Lorentzian at 280 cm-‘. The modes at 230, 280, and 585 err-’ are infrared active modes of YBCO,_6 dominated by vibrations of Cu(l), O(l), and O(4), respectively [6,7]. They become Raman active due to disorder in the occupancy of the chain-oxygen site. Thus, the appearance of these modes indicates the onset of oxygen out diffusion. The mode at 450 cm-’ is a Raman active plane-oxygen mode of YBC06. The remaining Raman active phonons of YBCOe expected in the applied polarization geometry are the Cu(2) mode at 140 cm-’ and the Big mode at 340 cm-’ [8]. Both are clearly visible at the highest power density. Comparison of Stokes and anti-Stokes intensities allows to determine the maximum spot temperature Tma, which is located in the center of the Gaussian laser spot at the film surface [3]. The thus determined temperatures agree well with the temperatures calculated from the solution of the heat equation as can be seen in the inset of Fig. 1. Above the damage threshold at Pd,, = (96 f 5) kWcm-*, i.e., at TrmLI= (500 f 10) K the observed changes of the intensities reflect the decrease of the chain-oxygen content [8]. We have carried out damage measurements at various ambient pressures po and temperatures TO on the same film. The thresholds and concomitant spot temperatures Tma are depicted in Fig. 2. The temperature dependence of the threshold indicates that the oxygen outdiffusion starts at T,, = (490 +- 10) K, independent of the power density. Therefore, we role out a solely photoactivated diffusion. Within the experimental errors, we observe merely a weak dependence of the threshold on the ambient pressure. Also, the amount of out-diffused oxygen at the maximum power density (not shown here) depends only slightly on the pressure. These observations are in contrast to the p-T phase diagram of YBCOG+~ [9]. According to this diagram a pronounced pressure dependence would be expected, and YBC@ would expected to be stable even for our highest power density in an ambient pressure of 1 bar. Hence, we conclude that the reaction of oxygen outdiffusion is of mixed type containing thermal-activated and photo-activated processes. Acknowledgement-This paper was supported by BMFT, contract number 13N5807A
0
200
400
600
Raman shift (cm-‘) Fig. 1. Stokes Raman spectra in polarized (z(y, y)T) and depolarized (2(x, y)P) geometry for different power densities PL. The spectra are o&t as indicated, their intensities are normalized with respect to the power density. The inset shows spot temperatures T,, versus power density. Dots represent experimental Stokes/anti-Stokes temperatures, the solid line has been calculated as described in Ref. [3].
200
, s s 3 ’ I * ’ ’ To= 300 K
I
I
T
p,rrl 0w6 (mbar)
I
p. (mbar)
Fig. 2. Damage thresholds versus ambient pressure and cryostat temperature. The corresponding spot temperatures Tmarin K are indicated.
8. Burns G. et aL, Physicu C 181, 37 (1991). 9. Somekh R. E. et al., Physics and Materials Science of High
TemperatureSuperconductors II, edited by R. Kossowsky et al. (Kluver Academic Publishers, Netherlands, 1992).
REFERENCES 1. Feile R., Physica C 159, 1 (1989). 2. Dieckmann N. et af., Physica C, 245, 2112 (1995). 3. Bock A., Phys. Rev. B, 51, 15506 (1995). 4. Conder K. et aL, Physicu C 210, 282 (1993). 5. Schilling M. et al., Thin Solid Films 235, 202 (1993). 6. Wake D. R. et al., Phys. Rev. Lett. 67, 3728 (1991). 7. Thomsen C. et al., Phys. Rev. B 45, 8154 (1992).
Pergamon
00223697(95)00103-d
OXYGEN DIFFUSION
IN LASER HEATED YBazCu307 FILMS
A, BOCK, R. KURSTEN and U. MERKT fur Angewandte Pbysii und Zentrum fiir Mikrostrukturforschung, UniversitHt Hamburg, Jungiusstrabe 11, D-20355 Hamburg, Germany
Institut
Abstract-Phonons of laser-deposited YBu~CusOr (YBCO7) films on MgO(100) are investigated in a calibrated Raman set-up as a function of the power density of the argon-ion laser. While four Ae modes are visible for low power densities additional modes appear when the power density is increased. These are the defect-induced modes at 230 cm-’ and 585 em-i, normally assigned to infrared active phonons of YBC07_a, and Raman active modes of the oxygen-reduced tetragonal phase YBCO6. At still higher power densities the phonons of the tetragonal phase dominate the spectra. We compare spot temperatures deduced from Stokes and anti-Stokes Raman spectra with those calculated from the solution of the heat equation. By varying the cryostat temperature and the ambient pressure we investigate the oxygen out-diffusion mechanism.
Keywords:Raman spectroscopy, oxygen diffusion, laser heating.
1. INTRODUCTION
Raman spectroscopy has proven to be a proper method for the determination of various YBC06+x film properties, e.g., the chain-oxygen content x, possible impurity phases, and the epitaxial quality [1,2]. However, it has been demonstrated that the experimental conditions, in particular the power density, have to be chosen with care in order to avoid a damage of the sample during the measurement [3]. The heating, induced by the absorbed laser light, can lead to a depletion of the chain-oxygen content, as the chain oxygen O(1) has the highest mobility at a given temperature [4]. With the oxygen loss at the chain site the electrical properties of the sample deteriorate as the critical temperature decreases. Here we present investigations of the damage threshold of the power density when a YBC07 film is heated with the 514 nm line of an argon-ion laser at different ambient temperatures and pressures. In these Raman experiments the laser serves the dual role of heating and investigating the sample simultaneously.
2. EXPERIMENTAL The samples are mounted on the cold finger of a helium cryostat whose temperature 7’s is measured by a silicon diode. The cryostat can be evacuated to residual pressures pe of lob6 mbar. Calibrated and straylight-corrected Raman spectra are taken in quasi-backscattering geometry. Details of the set-up, the calibration, and the straylight correction are given elsewhere [3]. The radii of the laser focus on the sample are determined to be rh = (6 + 1) urn and rV = (11 k 1) Mm in the horizontal and vertical directions, respectively. Given the laser power P on the sample
the maximum power density PL inside the Gaussian spot is PL = P/(TTQJ~), which we will call the “power density” in the following. We investigate a c-axis oriented YBCO7 6lm on MgO(100) prepared by laser deposition [S]. The film has a thickness of 240 nm, a critical temperature Td = 85.2 K, and a critical current density j,(77K) = 4 x lo6 Acxr~-~. Using procedures described elsewhere [3] we determined the thermal boundary resistance at the film-substrate interface tobe~=(0.5+0.1)~10-~Kcm~W-i.
3, RESULTS AND DISCUSSION
In Fig. 1, we present spectra from a Stokes/anti-Stokes series which were taken for increasing power densities. At the lowest power density we observe four Ar phonons of YBC07, i.e., the Ba mode at 117 cm-‘, the Cu(2) mode at 151 cm-t, the Big mode at 339 cm-‘, and the apex oxygen O(4) mode at 502 cm-i, which gives x = 1 [l]. From an analysis of the polarized and depolarized spectra at the lowest power density we determine the c-axis oriented fraction of the film 6 = 87 f 5 % and the in-plane orientation of this fraction Qe = 93 f 5 % [2]. In order to obtain these informations the spectra have been fitted using Fano profiles for the Ba and the Btr mode, Lorentz profiles for the Cu(2) and O(4) mode, and an almost constant background. When the power density is increased to values above 96 kWcmez new modes begin to develop at 230 cm-’ and 585 cm-‘. Up to a power density of 150 kWcm_’ the intensity of these new modes grows continuously. At still higher power densities the intensity of these modes starts to decrease until they almost vanish at the highest power density of 290 kWcme2. In addition to the above findings we observe with increasing power density that the Cu(2) mode shifts to lower frequen-
1829
1830
A BOCK et a/.
ties, the BI, rises in intensity, the Ba and the O(4) mode diminish, and another mode appears at 450 cm-‘. The additional modes were fitted using a Fano profile for the 230 cm-’ mode and Lore& profiles for the 450 cm-’ and the 585 cm-’ mode. To improve the fit we have included a broad Lorentzian at 280 cm-‘. The modes at 230, 280, and 585 err-’ are infrared active modes of YBCO,_6 dominated by vibrations of Cu(l), O(l), and O(4), respectively [6,7]. They become Raman active due to disorder in the occupancy of the chain-oxygen site. Thus, the appearance of these modes indicates the onset of oxygen out diffusion. The mode at 450 cm-’ is a Raman active plane-oxygen mode of YBC06. The remaining Raman active phonons of YBCOe expected in the applied polarization geometry are the Cu(2) mode at 140 cm-’ and the Big mode at 340 cm-’ [8]. Both are clearly visible at the highest power density. Comparison of Stokes and anti-Stokes intensities allows to determine the maximum spot temperature Tma, which is located in the center of the Gaussian laser spot at the film surface [3]. The thus determined temperatures agree well with the temperatures calculated from the solution of the heat equation as can be seen in the inset of Fig. 1. Above the damage threshold at Pd,, = (96 f 5) kWcm-*, i.e., at TrmLI= (500 f 10) K the observed changes of the intensities reflect the decrease of the chain-oxygen content [8]. We have carried out damage measurements at various ambient pressures po and temperatures TO on the same film. The thresholds and concomitant spot temperatures Tma are depicted in Fig. 2. The temperature dependence of the threshold indicates that the oxygen outdiffusion starts at T,, = (490 +- 10) K, independent of the power density. Therefore, we role out a solely photoactivated diffusion. Within the experimental errors, we observe merely a weak dependence of the threshold on the ambient pressure. Also, the amount of out-diffused oxygen at the maximum power density (not shown here) depends only slightly on the pressure. These observations are in contrast to the p-T phase diagram of YBCOG+~ [9]. According to this diagram a pronounced pressure dependence would be expected, and YBC@ would expected to be stable even for our highest power density in an ambient pressure of 1 bar. Hence, we conclude that the reaction of oxygen outdiffusion is of mixed type containing thermal-activated and photo-activated processes. Acknowledgement-This paper was supported by BMFT, contract number 13N5807A
0
200
400
600
Raman shift (cm-‘) Fig. 1. Stokes Raman spectra in polarized (z(y, y)T) and depolarized (2(x, y)P) geometry for different power densities PL. The spectra are o&t as indicated, their intensities are normalized with respect to the power density. The inset shows spot temperatures T,, versus power density. Dots represent experimental Stokes/anti-Stokes temperatures, the solid line has been calculated as described in Ref. [3].
200
, s s 3 ’ I * ’ ’ To= 300 K
I
I
T
p,rrl 0w6 (mbar)
I
p. (mbar)
Fig. 2. Damage thresholds versus ambient pressure and cryostat temperature. The corresponding spot temperatures Tmarin K are indicated.
8. Burns G. et aL, Physicu C 181, 37 (1991). 9. Somekh R. E. et al., Physics and Materials Science of High
TemperatureSuperconductors II, edited by R. Kossowsky et al. (Kluver Academic Publishers, Netherlands, 1992).
REFERENCES 1. Feile R., Physica C 159, 1 (1989). 2. Dieckmann N. et af., Physica C, 245, 2112 (1995). 3. Bock A., Phys. Rev. B, 51, 15506 (1995). 4. Conder K. et aL, Physicu C 210, 282 (1993). 5. Schilling M. et al., Thin Solid Films 235, 202 (1993). 6. Wake D. R. et al., Phys. Rev. Lett. 67, 3728 (1991). 7. Thomsen C. et al., Phys. Rev. B 45, 8154 (1992).