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Thickness dependence of superconductivity for La1.85Sr0.15CuO4 thin film deposited by pulsed electron deposition technique
Journal of Alloys and Compounds 450 (2008) 473–476
Thickness dependence of superconductivity for La1.85Sr0.15CuO4 thin film deposited by pulsed electron deposition technique L.M. Chen a,c , F.G. Zeng a , Y.F. Guo b,c , W.H. Tang b,∗ a
Department of Mathematics and Physics, Zhengzhou Institute of Aeronautical Industry and Management, Zhengzhou, Henan 450015, China b Department of Physics, Center for Optoelectronics Materials and Devices, Zhejiang Sci-Tech University, Xiasha College Park, Hangzhou, Zhejiang 310018, China c National Laboratory for Condensed Matter Physics & Institute of Physics, Chinese Academy of Sciences, Beijing 100080, China Received 30 August 2006; received in revised form 29 October 2006; accepted 1 November 2006 Available online 28 November 2006
Abstract High-quality superconducting La1.85 Sr0.15 CuO4 (LSCO) thin film is deposited on (1 0 0) SrTiO3 substrate by a novel pulsed electron deposition (PED) technique and a thickness-dependent superconductivity is also observed. Parameters including the place of the substrate in the plume, the target-to-substrate distance and the pressure in the deposition chamber, can affect the film thickness in the deposition process. For the place of the substrate in the plume, a Gaussian distribution of the film thickness is found. Analogical relations of the target-to-substrate distance and the deposition pressure in PED as in PLD are also educed in this paper. © 2006 Elsevier B.V. All rights reserved. Keywords: Thin films; High-TC superconductors; Electrical transport
1. Introduction Pulsed electron deposition (PED) technique, an ablation based film deposition technique [1], is a rather novel method for a wide range of materials, including metals, alloys, polymers and oxide films. Its principle is similar to pulsed laser deposition (PLD) technique but has more advantages [2]. One important is that the electrons produced by a channel spark system penetrate into the target material without being confined by the target material’s optical properties. The penetration-depth can be in micrometers of oxide materials, it is enough for most solid-state materials. PED was applied to deposit YBa2 Cu3 O7−x (YBCO) thin films on SrTiO3 (STO) substrate 12 years ago [2], but very limited reports are available and covered high-TC superconducting films are only limited to MBa2 Cu3 Oy (M = Y, Gd, Sm) and Gd1−x Eux Ba2 Cu3 Oy [3–6]. As an important model for investigating the underlying mechanism of the high-TC superconductors, La2−x Srx CuO4 (LSCO) film has the K2 NiF4 type structure with single CuO2 planes, which is one of the simplest crystallographic structures. LSCO ∗
Corresponding author. Tel.: +86 571 86843222; fax: +86 571 86843222. E-mail address: [email protected] (W.H. Tang).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.11.002
film can easily be prepared for the superconductivity occurs in a wide doping range. The best superconductivity can be realized with an optimal doping value of x = 0.15. Up to now, various techniques has been used to grow this film. Achieving the transition temperature (TC ) and the zero-resistance temperature (TC0 ) of the corresponding bulk polycrystalline material has been hindered by the inability of ordered films, additionally, when deposited on the commonly used STO substrate, a strain in the film also results a thickness dependence of TC and TC0 [7]. The reason for this strain is the large mismatch (3.4%) between the film and the substrate. Using PED technique to deposit thin LSCO film is not only an attempt but also a challenge. Whether the film has the same thickness dependence of its TC and TC0 ? What affect the film thickness in the PED process? We pay much attention to these questions in this work. 2. Experimental For a comparison with the results of PLD, we use 5 × 10 mm2 (1 0 0) STO single crystal as substrate. The LSCO target with the optimal strontium doping (x = 0.15) is prepared by solid-state reaction at 1150 ◦ C for 24 h in air. The prepared single phase target is mounted on a target holder which can rotate in the deposition process to form a steady plume. The substrate is glued with silver paste on a heater holder. It is important to control the pulse frequency in order to obtain high-quality thin film, and it is 6 Hz in our experiment. The
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L.M. Chen et al. / Journal of Alloys and Compounds 450 (2008) 473–476
Fig. 1. (a) X-diffraction spectrum and (b) rocking curve measured for (0 0 6) reflection of LSCO film (400 nm, deposited at a 760 ◦ C substrate temperature, 6 Hz pulse frequency, 12 kV high voltage, 6 cm target-to-substrate distance and 12 mTorr pressure) on (1 0 0) STO. pulsed electron beam carrying a 1000 A current with a 100 ns pulse-width is produced by a commercially PEBS-10source. The energy density is as high as ∼10 J/cm2 . When the electron beam hits the target surface, the target material on the surface is heated then leads to its evaporation in a short time. The vapor will get ionized and expands in to a plume perpendicular to the substrate and when the plume contacts the substrate, the material in the plume will stoichiometricly form LSCO film, accordingly, this mechanism is just the same as that of PLD. By varying the deposition times when other parameters are set, thin films with different thickness measured by using a Dektak profilometer can be obtained.
3. Results and discussions The θ − 2θ diffraction spectrum of a selected LSCO film is shown in Fig. 1(a). As expected, the film is c-axis alignment perpendicular to the substrate surface for that only (0 0 2n) peaks are presented. Fig. 1(b) shows a rocking curve measured for the (0 0 6) reflection with a full width at half-maximum (FWHM) of 0.5◦ , indicating good crystallinity. Film obtained with good morphology is demonstrated in Fig. 2(a). The presence of the particulates is common in PED process just as in PLD process, but fewer particulates are found. The electrical transport properties are summarized in Fig. 2(b). The LSCO thin films have temperature-dependent resistances and below TC they will change from metallic to superconducting state. Among the superconducting thin LSCO films, the
film deposited when the substrate temperature is 760 ◦ C has the highest TC and TC0 , of which the value is 32.9 and 29.2 K, respectively. TC and TC0 both increase with the substrate temperature increasing from 740 to 760 ◦ C, then decrease with the continued increasing substrate temperature. The temperature for depositing superconducting LSCO film is 700–810 ◦ C. The inapposite substrate, the unsteady plume and has not adopted the best suitable annealing method may be the main causes responsible for the broader transition width (3.7 K) and FWHM 0.5◦ . Despite this, by using PED technique high-quality LSCO film can be successfully fabricated. The R–T measurement also demonstrates a thickness-dependent TC and TC0, which is illustrated in Fig. 3. It is clearly shown both TC and TC0 decrease with film decreasing thickness, while the transition width TC = TC − TC0 will increases with the decreasing thickness d. For film of 150 nm, TC can be 5.85 K, while as a comparison, for film of 400 nm, TC is only be 4.1 K. This effect is almost the same as what has been observed in PLD process. TC and TC0 reach their maximum value when d is 400 nm and do not change much with d > 400 nm, but they drop significantly when d < 300 nm, and TC0 is only 11.25 K for d = 150 nm. In PED process, besides the deposition times, film thickness is also affected by the place of the substrate in the plume, the target-to-substrate distance (D) and the pressure in the chamber (P). The direction perpendicular to the target surface is set as
Fig. 2. (a) The morphology of LSCO film deposited as that in Fig. 1 and (b) R–T curves of the films deposited at different substrate temperatures with other parameters as in (a).
L.M. Chen et al. / Journal of Alloys and Compounds 450 (2008) 473–476
Fig. 3. The relation between TC (TC0 ) and the film thickness deposited with other parameters as in Fig. 1.
x-axis and thus it fixes the three-dimensional coordinates. The ions density at any point in the plasma can be described by a Gaussian function: ni (x, y, z, t) =√
NT t
2π1.5 X(t)Y (t)Z(t) x2 y2 z2 ×exp − , − − 2X(t)2 2Y (t)2 2Z(t)2
t≤τ
NT is the total number of the ions at any time t = τ. According to this Gaussian distribution of the ions in the plume, the expansion velocity of the ions [8] is v៝ (x, y, z, t) =
y dY (t) ៝ z dZ(t) ៝ x dX(t) ៝ i+ j+ k X(t) dt Y (t) dt Z(t) dt
In the deposition process, the film thickness should be in direct proportion to the number of the ions contacting the substrate. At any point (y, z) the thickness is t d(y, z, t ) 0 ni (y, z, t)v៝ (t)dt. For ni (y, z, t) is a spatial Gaussian variation parameter, d(y, z, t ) is also the same. When PED goes into operation, from the kinetic characters of the ions [9], the picture of the ions in the plume is that the velocities are the lowest with the high accelerations when the expansion of the plume begins and then the velocities reach their maximal
475
Fig. 4. The film thickness measured in different place on the heater holder of LCSO film deposited with other parameters as in Fig. 1.
value with zero acceleration, and thus the plume looks like an ellipsoid being elongated. The heater holder is circular with a diameter of 5 cm. Assume the center of the heater holder being the origin, one diameter being the x-axis and an arbitrary direction along it being set as the positive direction, when this origin is a superposition with the plume center and other parameters are set, the thicknesses of the film in different place along the x-axis are measured, being shown in Fig. 4. In the experiment, the x value is the distance from the center of the substrate to the origin and the x-axis is through the substrate center. We note that when the substrate at the center of the heater holder, after 50,000 pulses, d is about 400 nm, while at a nearer distance of 1.75 cm, d is only about 110 nm. From the shape of Fig. 4, the different d is consistent with that d(y, z) is a spatial Gaussian variation parameter. Accordingly, the substrate should be placed in the plume center to effectively collect the target material. In experiment we find that the profile width of the plume is a function of the target-to-substrate distance D. A compromise is that a higher collection efficiency with a short D and a larger thickness non-uniformity. With the same pulse times of the electron gun of 50,000, the non-uniformity is about 50 nm with D is 4 cm, and is only 20 nm or so when D is 8 cm, obviously, an appropriate D is essential for deposition uniform LSCO thin film. Fig. 5(a) is the relation between d and D, and Fig. 5(b) is TC
Fig. 5. (a) The relation between the film thickness and the target-to-substrate distance D deposited with other parameters as in Fig. 1 and (b) the superconductivity measured on the same samples.
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L.M. Chen et al. / Journal of Alloys and Compounds 450 (2008) 473–476
(TC0 ) and D. The figures clearly show that TC and TC0 begin to increase with the increasing D and then fall dramatically when D exceeds 6 cm. More big grains in the plume will be absorbed in the film at a nearer D, which not only results in a non-uniformity of the LSCO film but also introduces mismatch in the thin film and thus restrain the superconductivity. If the distance exceeds the optimal value too much, the reduced deposition efficiency can also affect the superconductivity by the obtained thinner film. When the distance is 10 cm, after 50,000 pulses, the film thickness is about 120 nm thinner than that with the distance 6 cm. Oxygen molecules could interact with the ions in the plume in the small deposition chamber. By increasing or decreasing the scattering between them, the pressure in the chamber can also affect the thickness of the LSCO thin film. PED source is operable over a limited range of pressures in the absence of differential pumping. The pressure range is 8–20 mTorr, and therefore it more easily results in an unsteady thermal process in the plume. Finely control is essential for PED depositing various films. When the pressures are 8, 10, 12, 15 and 18 mTorr, LSCO films with different thickness are obtained being 477, 432, 400, 353 and 311 nm, respectively. Zhou and co-workers [10] investigated the best deposition parameters for PLD by using the blast model. As has almost the same deposition principle, this model is also used for PED, so the target-to-substrate distance D has a relation with the deposition pressure P. According to PLD, the relation is (E − Eth )D−3 P−1 = K. E is the power density of the electron beam at the target. Eth is the power density for the ablated material escaping from the target and K is an experimental constant. For most material, Eth is far smaller than E and Eth can be neglected. According to our experiments data, the optimal pressure and distance for deposition LSCO is 12 mTorr and 6 cm. In PED, the pulse energy increases strongly and arises approximately linear with the voltage, when operated at 12 kV, with a beam cross-section of 9 × 10−2 cm−2 , the power density E at the target is about 6 J/cm2 , thus the relation in PED is that D−3 P−1 ≈ 1.4 × 10−2 J cm−5 (Pa)−1 .
4. Conclusions By studying the superconducting La0.85 Sr0.15 CuO4 thin film fabricated by the pulsed electron deposition (PED) method, we found almost the same thickness dependence of the superconductivity of the film as in pulsed laser deposition (PED). The parameters which affect the film thickness, such as the place of the substrate in the plume, the target-to-substrate distance and the deposition pressure, were detailed studied. The relation between the target-to-substrate distance and the deposition pressure reveal us the correlation of the parameters in PED process. More efforts should be devoted to getting a comprehensive understanding of this novel film fabricating method. Acknowledgements This work was supported by the key project of Zhejiang Provincial Natural Science Foundation (Z605131) and National Natural Science Foundation of China (60571029, 050672088). W.H. Tang was supported by the Creative Research Group of National Natural Science Foundation of China (Grant no. 60321001). References [1] Mikhail Strikovski, K.S. Harshavardhan, Appl. Phys. Lett. 82 (2003) 853. [2] Q.D. Jiang, F.C. Matacotta, M.C. Konijnenberg, G. M¨uller, C. Schultheiss, Thin solid films 241 (1994) 100. [3] K. Frank, J. Christiansen, Appl. Phys. A 48 (1989) 397. [4] V.I. Dediu, Q.D. Jiang, F.C. Matacott, P. Scardi, M. Lazzarino, G. Nieva, L. Civale, Supercond. Sci. Technol. 3 (1995) 160. [5] H.M. Christen, D.F. Lee, F.A. List, S.W. Cook, K.J. Leonard, L. Heatherly, P.M. Martin, M. Paranthaman, A. Goyal, C.M. Rouleau, Supercond. Sci. Technol. 18 (2005) 1168. [6] Kyoung Pil Ko, Seung Hyum Moon, Kyu Jeong Song, Chan Park, Sang Im Yoo, IEEE Trans. Appl. Supercond. 15 (2005) 3054. [7] M.Z. Cieplak, M. Berkowski, et al., Appl. Phys. Lett. 65 (1994) 3383. [8] Dawson, Phys. Fluids 12 (1969) 875. [9] D.A. Freiwald, R.A. Axford, J. Appl. Phys. 46 (1975) 1171. [10] Yue-Liang Zhou, Physics (in Chinese) 27 (1998) 167.
Thickness dependence of superconductivity for La1.85Sr0.15CuO4 thin film deposited by pulsed electron deposition technique L.M. Chen a,c , F.G. Zeng a , Y.F. Guo b,c , W.H. Tang b,∗ a
Department of Mathematics and Physics, Zhengzhou Institute of Aeronautical Industry and Management, Zhengzhou, Henan 450015, China b Department of Physics, Center for Optoelectronics Materials and Devices, Zhejiang Sci-Tech University, Xiasha College Park, Hangzhou, Zhejiang 310018, China c National Laboratory for Condensed Matter Physics & Institute of Physics, Chinese Academy of Sciences, Beijing 100080, China Received 30 August 2006; received in revised form 29 October 2006; accepted 1 November 2006 Available online 28 November 2006
Abstract High-quality superconducting La1.85 Sr0.15 CuO4 (LSCO) thin film is deposited on (1 0 0) SrTiO3 substrate by a novel pulsed electron deposition (PED) technique and a thickness-dependent superconductivity is also observed. Parameters including the place of the substrate in the plume, the target-to-substrate distance and the pressure in the deposition chamber, can affect the film thickness in the deposition process. For the place of the substrate in the plume, a Gaussian distribution of the film thickness is found. Analogical relations of the target-to-substrate distance and the deposition pressure in PED as in PLD are also educed in this paper. © 2006 Elsevier B.V. All rights reserved. Keywords: Thin films; High-TC superconductors; Electrical transport
1. Introduction Pulsed electron deposition (PED) technique, an ablation based film deposition technique [1], is a rather novel method for a wide range of materials, including metals, alloys, polymers and oxide films. Its principle is similar to pulsed laser deposition (PLD) technique but has more advantages [2]. One important is that the electrons produced by a channel spark system penetrate into the target material without being confined by the target material’s optical properties. The penetration-depth can be in micrometers of oxide materials, it is enough for most solid-state materials. PED was applied to deposit YBa2 Cu3 O7−x (YBCO) thin films on SrTiO3 (STO) substrate 12 years ago [2], but very limited reports are available and covered high-TC superconducting films are only limited to MBa2 Cu3 Oy (M = Y, Gd, Sm) and Gd1−x Eux Ba2 Cu3 Oy [3–6]. As an important model for investigating the underlying mechanism of the high-TC superconductors, La2−x Srx CuO4 (LSCO) film has the K2 NiF4 type structure with single CuO2 planes, which is one of the simplest crystallographic structures. LSCO ∗
Corresponding author. Tel.: +86 571 86843222; fax: +86 571 86843222. E-mail address: [email protected] (W.H. Tang).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.11.002
film can easily be prepared for the superconductivity occurs in a wide doping range. The best superconductivity can be realized with an optimal doping value of x = 0.15. Up to now, various techniques has been used to grow this film. Achieving the transition temperature (TC ) and the zero-resistance temperature (TC0 ) of the corresponding bulk polycrystalline material has been hindered by the inability of ordered films, additionally, when deposited on the commonly used STO substrate, a strain in the film also results a thickness dependence of TC and TC0 [7]. The reason for this strain is the large mismatch (3.4%) between the film and the substrate. Using PED technique to deposit thin LSCO film is not only an attempt but also a challenge. Whether the film has the same thickness dependence of its TC and TC0 ? What affect the film thickness in the PED process? We pay much attention to these questions in this work. 2. Experimental For a comparison with the results of PLD, we use 5 × 10 mm2 (1 0 0) STO single crystal as substrate. The LSCO target with the optimal strontium doping (x = 0.15) is prepared by solid-state reaction at 1150 ◦ C for 24 h in air. The prepared single phase target is mounted on a target holder which can rotate in the deposition process to form a steady plume. The substrate is glued with silver paste on a heater holder. It is important to control the pulse frequency in order to obtain high-quality thin film, and it is 6 Hz in our experiment. The
474
L.M. Chen et al. / Journal of Alloys and Compounds 450 (2008) 473–476
Fig. 1. (a) X-diffraction spectrum and (b) rocking curve measured for (0 0 6) reflection of LSCO film (400 nm, deposited at a 760 ◦ C substrate temperature, 6 Hz pulse frequency, 12 kV high voltage, 6 cm target-to-substrate distance and 12 mTorr pressure) on (1 0 0) STO. pulsed electron beam carrying a 1000 A current with a 100 ns pulse-width is produced by a commercially PEBS-10source. The energy density is as high as ∼10 J/cm2 . When the electron beam hits the target surface, the target material on the surface is heated then leads to its evaporation in a short time. The vapor will get ionized and expands in to a plume perpendicular to the substrate and when the plume contacts the substrate, the material in the plume will stoichiometricly form LSCO film, accordingly, this mechanism is just the same as that of PLD. By varying the deposition times when other parameters are set, thin films with different thickness measured by using a Dektak profilometer can be obtained.
3. Results and discussions The θ − 2θ diffraction spectrum of a selected LSCO film is shown in Fig. 1(a). As expected, the film is c-axis alignment perpendicular to the substrate surface for that only (0 0 2n) peaks are presented. Fig. 1(b) shows a rocking curve measured for the (0 0 6) reflection with a full width at half-maximum (FWHM) of 0.5◦ , indicating good crystallinity. Film obtained with good morphology is demonstrated in Fig. 2(a). The presence of the particulates is common in PED process just as in PLD process, but fewer particulates are found. The electrical transport properties are summarized in Fig. 2(b). The LSCO thin films have temperature-dependent resistances and below TC they will change from metallic to superconducting state. Among the superconducting thin LSCO films, the
film deposited when the substrate temperature is 760 ◦ C has the highest TC and TC0 , of which the value is 32.9 and 29.2 K, respectively. TC and TC0 both increase with the substrate temperature increasing from 740 to 760 ◦ C, then decrease with the continued increasing substrate temperature. The temperature for depositing superconducting LSCO film is 700–810 ◦ C. The inapposite substrate, the unsteady plume and has not adopted the best suitable annealing method may be the main causes responsible for the broader transition width (3.7 K) and FWHM 0.5◦ . Despite this, by using PED technique high-quality LSCO film can be successfully fabricated. The R–T measurement also demonstrates a thickness-dependent TC and TC0, which is illustrated in Fig. 3. It is clearly shown both TC and TC0 decrease with film decreasing thickness, while the transition width TC = TC − TC0 will increases with the decreasing thickness d. For film of 150 nm, TC can be 5.85 K, while as a comparison, for film of 400 nm, TC is only be 4.1 K. This effect is almost the same as what has been observed in PLD process. TC and TC0 reach their maximum value when d is 400 nm and do not change much with d > 400 nm, but they drop significantly when d < 300 nm, and TC0 is only 11.25 K for d = 150 nm. In PED process, besides the deposition times, film thickness is also affected by the place of the substrate in the plume, the target-to-substrate distance (D) and the pressure in the chamber (P). The direction perpendicular to the target surface is set as
Fig. 2. (a) The morphology of LSCO film deposited as that in Fig. 1 and (b) R–T curves of the films deposited at different substrate temperatures with other parameters as in (a).
L.M. Chen et al. / Journal of Alloys and Compounds 450 (2008) 473–476
Fig. 3. The relation between TC (TC0 ) and the film thickness deposited with other parameters as in Fig. 1.
x-axis and thus it fixes the three-dimensional coordinates. The ions density at any point in the plasma can be described by a Gaussian function: ni (x, y, z, t) =√
NT t
2π1.5 X(t)Y (t)Z(t) x2 y2 z2 ×exp − , − − 2X(t)2 2Y (t)2 2Z(t)2
t≤τ
NT is the total number of the ions at any time t = τ. According to this Gaussian distribution of the ions in the plume, the expansion velocity of the ions [8] is v៝ (x, y, z, t) =
y dY (t) ៝ z dZ(t) ៝ x dX(t) ៝ i+ j+ k X(t) dt Y (t) dt Z(t) dt
In the deposition process, the film thickness should be in direct proportion to the number of the ions contacting the substrate. At any point (y, z) the thickness is t d(y, z, t ) 0 ni (y, z, t)v៝ (t)dt. For ni (y, z, t) is a spatial Gaussian variation parameter, d(y, z, t ) is also the same. When PED goes into operation, from the kinetic characters of the ions [9], the picture of the ions in the plume is that the velocities are the lowest with the high accelerations when the expansion of the plume begins and then the velocities reach their maximal
475
Fig. 4. The film thickness measured in different place on the heater holder of LCSO film deposited with other parameters as in Fig. 1.
value with zero acceleration, and thus the plume looks like an ellipsoid being elongated. The heater holder is circular with a diameter of 5 cm. Assume the center of the heater holder being the origin, one diameter being the x-axis and an arbitrary direction along it being set as the positive direction, when this origin is a superposition with the plume center and other parameters are set, the thicknesses of the film in different place along the x-axis are measured, being shown in Fig. 4. In the experiment, the x value is the distance from the center of the substrate to the origin and the x-axis is through the substrate center. We note that when the substrate at the center of the heater holder, after 50,000 pulses, d is about 400 nm, while at a nearer distance of 1.75 cm, d is only about 110 nm. From the shape of Fig. 4, the different d is consistent with that d(y, z) is a spatial Gaussian variation parameter. Accordingly, the substrate should be placed in the plume center to effectively collect the target material. In experiment we find that the profile width of the plume is a function of the target-to-substrate distance D. A compromise is that a higher collection efficiency with a short D and a larger thickness non-uniformity. With the same pulse times of the electron gun of 50,000, the non-uniformity is about 50 nm with D is 4 cm, and is only 20 nm or so when D is 8 cm, obviously, an appropriate D is essential for deposition uniform LSCO thin film. Fig. 5(a) is the relation between d and D, and Fig. 5(b) is TC
Fig. 5. (a) The relation between the film thickness and the target-to-substrate distance D deposited with other parameters as in Fig. 1 and (b) the superconductivity measured on the same samples.
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L.M. Chen et al. / Journal of Alloys and Compounds 450 (2008) 473–476
(TC0 ) and D. The figures clearly show that TC and TC0 begin to increase with the increasing D and then fall dramatically when D exceeds 6 cm. More big grains in the plume will be absorbed in the film at a nearer D, which not only results in a non-uniformity of the LSCO film but also introduces mismatch in the thin film and thus restrain the superconductivity. If the distance exceeds the optimal value too much, the reduced deposition efficiency can also affect the superconductivity by the obtained thinner film. When the distance is 10 cm, after 50,000 pulses, the film thickness is about 120 nm thinner than that with the distance 6 cm. Oxygen molecules could interact with the ions in the plume in the small deposition chamber. By increasing or decreasing the scattering between them, the pressure in the chamber can also affect the thickness of the LSCO thin film. PED source is operable over a limited range of pressures in the absence of differential pumping. The pressure range is 8–20 mTorr, and therefore it more easily results in an unsteady thermal process in the plume. Finely control is essential for PED depositing various films. When the pressures are 8, 10, 12, 15 and 18 mTorr, LSCO films with different thickness are obtained being 477, 432, 400, 353 and 311 nm, respectively. Zhou and co-workers [10] investigated the best deposition parameters for PLD by using the blast model. As has almost the same deposition principle, this model is also used for PED, so the target-to-substrate distance D has a relation with the deposition pressure P. According to PLD, the relation is (E − Eth )D−3 P−1 = K. E is the power density of the electron beam at the target. Eth is the power density for the ablated material escaping from the target and K is an experimental constant. For most material, Eth is far smaller than E and Eth can be neglected. According to our experiments data, the optimal pressure and distance for deposition LSCO is 12 mTorr and 6 cm. In PED, the pulse energy increases strongly and arises approximately linear with the voltage, when operated at 12 kV, with a beam cross-section of 9 × 10−2 cm−2 , the power density E at the target is about 6 J/cm2 , thus the relation in PED is that D−3 P−1 ≈ 1.4 × 10−2 J cm−5 (Pa)−1 .
4. Conclusions By studying the superconducting La0.85 Sr0.15 CuO4 thin film fabricated by the pulsed electron deposition (PED) method, we found almost the same thickness dependence of the superconductivity of the film as in pulsed laser deposition (PED). The parameters which affect the film thickness, such as the place of the substrate in the plume, the target-to-substrate distance and the deposition pressure, were detailed studied. The relation between the target-to-substrate distance and the deposition pressure reveal us the correlation of the parameters in PED process. More efforts should be devoted to getting a comprehensive understanding of this novel film fabricating method. Acknowledgements This work was supported by the key project of Zhejiang Provincial Natural Science Foundation (Z605131) and National Natural Science Foundation of China (60571029, 050672088). W.H. Tang was supported by the Creative Research Group of National Natural Science Foundation of China (Grant no. 60321001). References [1] Mikhail Strikovski, K.S. Harshavardhan, Appl. Phys. Lett. 82 (2003) 853. [2] Q.D. Jiang, F.C. Matacotta, M.C. Konijnenberg, G. M¨uller, C. Schultheiss, Thin solid films 241 (1994) 100. [3] K. Frank, J. Christiansen, Appl. Phys. A 48 (1989) 397. [4] V.I. Dediu, Q.D. Jiang, F.C. Matacott, P. Scardi, M. Lazzarino, G. Nieva, L. Civale, Supercond. Sci. Technol. 3 (1995) 160. [5] H.M. Christen, D.F. Lee, F.A. List, S.W. Cook, K.J. Leonard, L. Heatherly, P.M. Martin, M. Paranthaman, A. Goyal, C.M. Rouleau, Supercond. Sci. Technol. 18 (2005) 1168. [6] Kyoung Pil Ko, Seung Hyum Moon, Kyu Jeong Song, Chan Park, Sang Im Yoo, IEEE Trans. Appl. Supercond. 15 (2005) 3054. [7] M.Z. Cieplak, M. Berkowski, et al., Appl. Phys. Lett. 65 (1994) 3383. [8] Dawson, Phys. Fluids 12 (1969) 875. [9] D.A. Freiwald, R.A. Axford, J. Appl. Phys. 46 (1975) 1171. [10] Yue-Liang Zhou, Physics (in Chinese) 27 (1998) 167.