- Email: [email protected]
Influence of the deposition parameters on the biaxial alignment of MgO grown by unbalanced magnetron sputtering
ARTICLE IN PRESS
Journal of Crystal Growth 271 (2004) 462–468 www.elsevier.com/locate/jcrysgro
Influence of the deposition parameters on the biaxial alignment of MgO grown by unbalanced magnetron sputtering P. Ghekiere, S. Mahieu, G. De Winter, R. De Gryse, D. Depla Department of Solid State Sciences, University of Gent, Krijgslaan 281/S1, B-9000 Gent, Belgium Received 6 July 2004; accepted 18 August 2004 Communicated by M. Kawasaki
Abstract For many years magnesium oxide (MgO) played a key role in the research of buffer layers for high-temperature superconducting copper oxides. For buffer layers, not only the out-of-plane alignment has to be taken into account, but also the in-plane orientation, which is important. The goal of our experiments is to grow biaxially aligned layers, i.e. layers with both out-of-plane alignment and in-plane alignment. The biaxially aligned MgO layers are deposited by unbalanced magnetron sputtering on a non-textured substrate. By varying different deposition parameters (e.g. inclination angle, pressure, reactive gas flow, distance between target and substrate, etc.) it is possible to improve the biaxial alignment. MgO layers with a preferential [1 1 1] out-of-plane orientation and an in-plane alignment with a FWHM of 19.61 in f and 7.81 in w are observed. r 2004 Elsevier B.V. All rights reserved. PACS: 68.55.Jk Keywords: A1. Biaxial alignment; A1. Deposition parameters; A1. Unbalanced magnetron sputtering; B1. MgO
1. Introduction For many years magnesium oxide (MgO) has been the subject of research as buffer layer for the growth of high-temperature superconducting copper oxides (HTSC). To achieve a high current Corresponding author. Tel.: +32-9-264-43-82; fax: +32-9-
264-49-96. E-mail address: [email protected] (P. Ghekiere).
density, HTSC have to be grown on single crystals. Since the growth of single crystals is expensive and size restricted, other techniques are developed to grow highly oriented layers. Biaxially aligned buffer layers on polycrystalline or amorphous substrates can be used as a substitute for a single crystal approximating the properties necessary for the growth of high-temperature superconductors. The critical current density J c is tightly connected to the angle between the orientation of the grains
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.08.010
ARTICLE IN PRESS P. Ghekiere et al. / Journal of Crystal Growth 271 (2004) 462–468
of the buffer layer [1] and so to the quality of the biaxially aligned buffer layer. Different techniques have been developed to grow highly biaxially aligned MgO layers. The first technique is called Ion Beam Assisted Deposition (IBAD) [2–5]. Another method to grow biaxially aligned MgO layers is Inclined Substrate Deposition (ISD) [6,7]. In this paper, we have studied the growth mechanism of biaxially aligned MgO layers grown by unbalanced magnetron sputtering on an inclined substrate. This deposition technique (unbalanced magnetron sputtering on a tilted substrate) has been applied successfully to deposit biaxially aligned Yttria Stabilised Zirconia (YSZ) and Indium Tin Oxide (ITO) [8]. We have mainly focused on the morphological and microstructure of the layer, in order to reveal the mechanism and to improve the biaxial alignment of the MgO thin film.
2. Experimental procedure MgO thin films were deposited by reactive magnetron sputtering. Therefore, a type II planar unbalanced magnetron with an inner to outer magnetic field ratio of 1:9 has been used. The used deposition system is analogous as described in Ref. [9]. A 200 circular metallic Mg-target was sputtered in an argon atmosphere with local oxygen inlet. During deposition a flow of 60 sccm Ar and 3 sccm O2 was used. Amorphous glass and non-aligned stainless steel are used as substrate. The substrate temperature was floating. Several other deposition parameters have been varied and optimised. An overview of the varied deposition parameters with their range and optimised deposition condition is shown in Table 1. To examine the influence of one deposition parameter, all the other parameters were kept constant. Each time a parameter was optimised, the optimised value was used in the following depositions. The starting point of different deposition parameters during the first series is mentioned below. The target substrate distance was 10 cm and the discharge current was fixed at 0.70 A. The working pressure was 0.45 Pa. The non-aligned stainless
463
Table 1 The used deposition parameters, their range and the optimised deposition conditions Deposition parameter
Range
Optimalisation
Inclination angle Target–substrate distance Discharge current Working pressure Substrate bias Layer thickness
01–701 9–13 cm 0.50–0.80 A 0.40–0.60 Pa 40– þ10 V 0.25–3:0 mm
551 10 cm 0.70 A 0.45 Pa Floating 1:5 mm
steel substrate was grounded at zero potential. The MgO thin films have been grown with a thickness of 1:0 mm: The preferential out-of-plane alignment was determined by X-ray angular scans and X-ray pole figures, f-scans and w-scans were performed to confine the in-plane alignment. A FEG-scanning electron microscope (SEM) was used to reveal the topographical and cross-sectional microstructure, and to investigate the texture evolution of the MgO layers.
3. Biaxial alignment The MgO-layers were grown using the deposition parameters as described in Section 2. XRD angular scans reveal that MgO layers have a [1 1 1] out-of-plane orientation. None of the parameters listed in Table 1 cause a drastic change in out-ofplane orientation. However, the fraction of [1 1 1] out-of-plane oriented grains may depend on the changed deposition parameter. A decrease in [1 1 1] out-of-plane oriented grains causes an increase of randomly oriented grains. None of the deposited MgO thin films exhibit another preferential out-ofplane orientation. For each deposition parameter the change in the fraction of [1 1 1] out-of-plane oriented grains as well as the in-plane alignment will be discussed. 3.1. Influence of the inclination angle The first parameter that has been varied is the inclination angle a of the substrate. This is the angle between the substrate normal and the
ARTICLE IN PRESS 464
P. Ghekiere et al. / Journal of Crystal Growth 271 (2004) 462–468
Fig. 1. Influence of the inclination angle of the substrate on the in-plane alignment of MgO-layers.
Fig. 2. Influence of the distance between the target and the substrate on the in-plane alignment of MgO-layers.
incident vapour flux. On a non-tilted substrate the MgO layers have a strong [1 1 1] out-of-plane orientation. The fraction of [1 1 1] out-of-plane oriented grains does not change when the inclination angle a increases. However, the in-plane alignment improves exponentially with increasing inclination angle as shown in Fig. 1. The FWHM goes asymptotic to 24.41 in f: Since the deposition rate decreases rapidly at a large inclination angle, an inclination angle of 551 is a good compromise between deposition rate and biaxial alignment. Therefore, the depositions of all the other series will be carried out with this inclination angle.
which is too high. The optimised distance for depositions is 10 cm. 3.3. Influence of the discharge current All MgO layers have a strong preferential [1 1 1] out-of-plane orientation when the discharge current is varied between 0.50 and 0.80 A. No change in the fraction of [1 1 1] out-of-plane oriented grains has been observed. The FWHM of the (0 0 2) peak increases when the discharge current is too low or too high (not shown). The in-plane alignment reaches an optimum with a discharge current of 0.70 A. 3.4. Influence of the working pressure
3.2. Influence of the target–substrate distance At a target–substrate distance of 13 cm the layers are fully [1 1 1] out-of-plane oriented. The fraction of [1 1 1] out-of-plane oriented grains decreases if the substrate is closer to the target. When the substrate is too close to the target (i.e. 9 cm), the fraction of randomly oriented grains increases rapidly and a preferential [1 1 1] out-ofplane orientation was no longer observed. The in-plane alignment improves when the substrate is closer to the target which is clearly seen in Fig. 2. However, with a relative distance of 9 cm the in-plane alignment disappears completely. This could be the consequence of a plasma density
The working pressure during deposition has been varied between 0.40 and 0.60 Pa. The pressure is changed by changing the pumping speed, so the Ar=O2 -ratio remains constant. All layers have a strong preferential [1 1 1] out-ofplane orientation. However, an increase of randomly oriented grains is observed when the working pressure is 0.40 Pa. Fig. 3 shows that the in-plane alignment reaches an optimum at 0.45 Pa. It is important to remark that the last three parameters (target–substrate distance, discharge current, working pressure) strongly depend on each other. All those parameters have an influence on the mean free path of the Mg adatoms. Assume
ARTICLE IN PRESS P. Ghekiere et al. / Journal of Crystal Growth 271 (2004) 462–468
Fig. 3. Influence of the working pressure on the in-plane alignment of MgO-layers.
that one of the parameters was optimised differently, the other parameters would reach another optimum.
465
Fig. 4. Influence of the substrate bias on the in-plane alignment of MgO-layers. : FWHM of the (0 0 2) peak in f; : intensity of the (0 0 2) peak.
6 V. The floating potential is situated in the region where the biaxial alignment is optimal. This means that the floating potential is optimal to obtain biaxially aligned layers.
3.5. Influence of the substrate bias 3.6. Influence of the layer thickness During the deposition a bias has been applied to the substrate. This bias is varied between 40 V and þ10 V: A strong [1 1 1] out-of-plane orientation is observed when the substrate bias is slightly negative (20 V–0 V). Applying a more negative or a positive bias gives rise to an increase in randomly out-of-plane oriented grains. The change of the in-plane alignment as a function of the substrate bias is shown in Fig. 4. A positive bias results in a strong increase of the FWHM of the (0 0 2) pole, while a negative bias provides a better in-plane alignment. As such, repelling negatively charged O 2 and electrons ðe Þ þ and attracting the positive charged Ar is favourable for the in-plane alignment. However, a strong decrease of the peak intensity is observed when the negative bias is higher than 20 V. So the number of grains with a good biaxial alignment has dropped drastically. Langmuir measurements were carried out to determine the plasma and floating potential. The plasma potential remains constant at þ5 V; independent of the used deposition parameters as mentioned in Table 1. The floating potential is slightly negative and varied between 14 V and
MgO layers with different thicknesses have been grown. The fraction of [1 1 1] out-of-plane oriented grains increases with increasing thickness. The layer is fully [1 1 1] out-of-plane oriented at a thickness of 1:0 mm: It is possible that the surface of the MgO film earlier is fully [1 1 1] out-of-plane oriented because XRD does not only provide information of the surface but gives an average over a certain thickness. When the layers are thicker than 1:5 mm the fraction of randomly oriented grains increases. The in-plane alignment as a function of the layer thickness is shown in Fig. 5. The FWHM of the (0 0 2) peak decreases with increasing layer thickness. The increase of the intensity is a consequence of layer thickness because of a larger amount of reflection planes and the improvement of the inplane orientation. The in-plane alignment reaches an optimum at a layer thickness of 1:5 mm: When the layers are thicker than 1:5 mm the FWHM increases rapidly with increasing thickness. The inplane alignment has completely disappeared at a layer thickness of 3:0 mm (Fig. 5). It is believed that this is due to a temperature increase during
ARTICLE IN PRESS 466
P. Ghekiere et al. / Journal of Crystal Growth 271 (2004) 462–468
Fig. 5. Influence of the layer thickness on the in-plane alignment of a MgO thin film deposited on stainless-steel. : FWHM of the (0 0 2)-peak in f; : intensity of the (0 0 2)-peak.
depositions, causing an increase of the adatom mobility. At a given moment, a threshold temperature is reached by which grain boundary diffusion is possible. This results in the formation of a highly disrupted structure, which is confirmed by SEM results.
Fig. 6. (0 0 2) pole figure of MgO layer deposited with optimised deposition parameters.
4. Crystallographic structure and microstructure Depositions with optimised parameters (Table 1) have been carried out. Fig. 6 shows the (0 0 2) pole figure of such a thin film. The incident vapour flux is indicated with an arrow. It can be calculated from the pole figure that the [1 1 1] direction is not perfectly out-of-plane oriented but is tilted 121 away from the flux. The [0 0 2] direction is 421 tilted towards the incident vapour flux. No difference in crystallographic tilting is observed during optimalisation of the deposition parameters. The MgO layers deposited at optimised conditions have a strong in-plane alignment with a FWHM of 19.61 in f and 7.81 in w: Fig. 7 shows the surface of MgO observed with SEM grown under optimal conditions . It reveals a roof-tile structure. Because of the [1 1 1] out-ofplane orientation and the threefold symmetry, the columns are limited by {0 0 1}-planes. This plane has the highest packing density and lowers the surface free energy. The (0 0 2)-planes are oriented towards the flux.
Fig. 7. Plan view SEM image of MgO thin film deposited under optimal conditions.
Fig. 8 shows a SEM cross-section of a MgO layer with a thickness of 1:5 mm: A clear columnar structure is observed. The columns grow parallel to the substrate normal, indicating a high mobility of the adatoms on the surface of MgO during
ARTICLE IN PRESS P. Ghekiere et al. / Journal of Crystal Growth 271 (2004) 462–468
467
Fig. 8. Cross-section SEM image of MgO thin film deposited at optimal deposition conditions (Table 1).
deposition. At the early stages of the growth, small grains evolve at the substrate. When the layer grows thicker, columns with the [1 1 1] direction out-of-plane oriented, which is the fastest growth direction for a rocksalt structure, overgrow the others [10]. This is called evolutionary growth [11]. It is also visible that the columns expand laterally with increasing thickness.
5. Conclusion Biaxially aligned MgO layers were deposited on a non-aligned tilted substrate using an unbalanced magnetron. To investigate the mechanism of the growth of biaxially aligned layers, different deposition parameters have been changed. The change of the out-of-plane and in-plane alignment with changing deposition parameter is examined. All the MgO layers exhibit a preferential [1 1 1] out-of-plane orientation. The best aligned layer had an in-plane alignment with a FWHM of 19.61 in f and 7.81 in w:
Until now, no HTSC depositions have been carried out on biaxially aligned MgO layers but will be the subject of further research.
Acknowledgements The authors would like to thank the IWT, the Flemish Institute for the Promotion of scientific and Technological Research in the Industry, for its support.
References [1] D. Dimos, P. Chaudhari, J. Mannhart, F.K. LeGoues, Phys. Rev. Lett. 61 (2) (1988) 219. [2] C.P. Wang, K.B. Do, M.R. Beasley, T.H. Geballe, R.H. Hammond, Appl. Phys. Lett. 71 (20) (1997) 2955. [3] J.R. Groves, P.N. Arendt, S.R. Foltyn, R.F. DePaula, C.P. Wang, R.H. Hammond, IEEE Trans. Appl. Supercond. 9 (2) (1999) 1964. [4] R. Huhne, C. Beyer, B. Holzapfel, C.G. Oertel, L.S.L.W. Skrotzki, Cryst. Res. Technol. 35 (4) (2000) 419.
ARTICLE IN PRESS 468
P. Ghekiere et al. / Journal of Crystal Growth 271 (2004) 462–468
[5] R.T. Brewer, J.W. Hartman, J.R. Groves, P.N. Arendt, P.C. Yashar, H.A. Atwater, Appl. Surf. Sci. 175 (2001) 691. [6] M. Bauer, J. Schwachulla, S. Furtner, P. Berberich, H. Kinder, Inst. Phys. Conf. Ser. 158 (1997) 1077. [7] M.P. Chudzik, R.E. Koritala, L.P. Luo, D. Miller, U. Balachandran, C.R. Kannewurf, IEEE Trans. Appl. Supercond. 11 (1) (2001) 3469.
[8] G. De Winter, S. Mahieu, I. De Roeck, R. De Gryse, J. Denul, IEEE Trans. Appl. Supercond. 13 (2) (2003) 2565. [9] G. De Winter, J. Denul, R. De Gryse, Inst. Phys. Conf. Ser. 167 (1999) 635. [10] V.E. Henrich, P.A. Cox, The Surface Science of Metal Oxides, first ed., Cambridge University Press, Cambridge, 1994. [11] A. Van der Drift, Philips Res. Rep. 22 (1967) 267.
Journal of Crystal Growth 271 (2004) 462–468 www.elsevier.com/locate/jcrysgro
Influence of the deposition parameters on the biaxial alignment of MgO grown by unbalanced magnetron sputtering P. Ghekiere, S. Mahieu, G. De Winter, R. De Gryse, D. Depla Department of Solid State Sciences, University of Gent, Krijgslaan 281/S1, B-9000 Gent, Belgium Received 6 July 2004; accepted 18 August 2004 Communicated by M. Kawasaki
Abstract For many years magnesium oxide (MgO) played a key role in the research of buffer layers for high-temperature superconducting copper oxides. For buffer layers, not only the out-of-plane alignment has to be taken into account, but also the in-plane orientation, which is important. The goal of our experiments is to grow biaxially aligned layers, i.e. layers with both out-of-plane alignment and in-plane alignment. The biaxially aligned MgO layers are deposited by unbalanced magnetron sputtering on a non-textured substrate. By varying different deposition parameters (e.g. inclination angle, pressure, reactive gas flow, distance between target and substrate, etc.) it is possible to improve the biaxial alignment. MgO layers with a preferential [1 1 1] out-of-plane orientation and an in-plane alignment with a FWHM of 19.61 in f and 7.81 in w are observed. r 2004 Elsevier B.V. All rights reserved. PACS: 68.55.Jk Keywords: A1. Biaxial alignment; A1. Deposition parameters; A1. Unbalanced magnetron sputtering; B1. MgO
1. Introduction For many years magnesium oxide (MgO) has been the subject of research as buffer layer for the growth of high-temperature superconducting copper oxides (HTSC). To achieve a high current Corresponding author. Tel.: +32-9-264-43-82; fax: +32-9-
264-49-96. E-mail address: [email protected] (P. Ghekiere).
density, HTSC have to be grown on single crystals. Since the growth of single crystals is expensive and size restricted, other techniques are developed to grow highly oriented layers. Biaxially aligned buffer layers on polycrystalline or amorphous substrates can be used as a substitute for a single crystal approximating the properties necessary for the growth of high-temperature superconductors. The critical current density J c is tightly connected to the angle between the orientation of the grains
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.08.010
ARTICLE IN PRESS P. Ghekiere et al. / Journal of Crystal Growth 271 (2004) 462–468
of the buffer layer [1] and so to the quality of the biaxially aligned buffer layer. Different techniques have been developed to grow highly biaxially aligned MgO layers. The first technique is called Ion Beam Assisted Deposition (IBAD) [2–5]. Another method to grow biaxially aligned MgO layers is Inclined Substrate Deposition (ISD) [6,7]. In this paper, we have studied the growth mechanism of biaxially aligned MgO layers grown by unbalanced magnetron sputtering on an inclined substrate. This deposition technique (unbalanced magnetron sputtering on a tilted substrate) has been applied successfully to deposit biaxially aligned Yttria Stabilised Zirconia (YSZ) and Indium Tin Oxide (ITO) [8]. We have mainly focused on the morphological and microstructure of the layer, in order to reveal the mechanism and to improve the biaxial alignment of the MgO thin film.
2. Experimental procedure MgO thin films were deposited by reactive magnetron sputtering. Therefore, a type II planar unbalanced magnetron with an inner to outer magnetic field ratio of 1:9 has been used. The used deposition system is analogous as described in Ref. [9]. A 200 circular metallic Mg-target was sputtered in an argon atmosphere with local oxygen inlet. During deposition a flow of 60 sccm Ar and 3 sccm O2 was used. Amorphous glass and non-aligned stainless steel are used as substrate. The substrate temperature was floating. Several other deposition parameters have been varied and optimised. An overview of the varied deposition parameters with their range and optimised deposition condition is shown in Table 1. To examine the influence of one deposition parameter, all the other parameters were kept constant. Each time a parameter was optimised, the optimised value was used in the following depositions. The starting point of different deposition parameters during the first series is mentioned below. The target substrate distance was 10 cm and the discharge current was fixed at 0.70 A. The working pressure was 0.45 Pa. The non-aligned stainless
463
Table 1 The used deposition parameters, their range and the optimised deposition conditions Deposition parameter
Range
Optimalisation
Inclination angle Target–substrate distance Discharge current Working pressure Substrate bias Layer thickness
01–701 9–13 cm 0.50–0.80 A 0.40–0.60 Pa 40– þ10 V 0.25–3:0 mm
551 10 cm 0.70 A 0.45 Pa Floating 1:5 mm
steel substrate was grounded at zero potential. The MgO thin films have been grown with a thickness of 1:0 mm: The preferential out-of-plane alignment was determined by X-ray angular scans and X-ray pole figures, f-scans and w-scans were performed to confine the in-plane alignment. A FEG-scanning electron microscope (SEM) was used to reveal the topographical and cross-sectional microstructure, and to investigate the texture evolution of the MgO layers.
3. Biaxial alignment The MgO-layers were grown using the deposition parameters as described in Section 2. XRD angular scans reveal that MgO layers have a [1 1 1] out-of-plane orientation. None of the parameters listed in Table 1 cause a drastic change in out-ofplane orientation. However, the fraction of [1 1 1] out-of-plane oriented grains may depend on the changed deposition parameter. A decrease in [1 1 1] out-of-plane oriented grains causes an increase of randomly oriented grains. None of the deposited MgO thin films exhibit another preferential out-ofplane orientation. For each deposition parameter the change in the fraction of [1 1 1] out-of-plane oriented grains as well as the in-plane alignment will be discussed. 3.1. Influence of the inclination angle The first parameter that has been varied is the inclination angle a of the substrate. This is the angle between the substrate normal and the
ARTICLE IN PRESS 464
P. Ghekiere et al. / Journal of Crystal Growth 271 (2004) 462–468
Fig. 1. Influence of the inclination angle of the substrate on the in-plane alignment of MgO-layers.
Fig. 2. Influence of the distance between the target and the substrate on the in-plane alignment of MgO-layers.
incident vapour flux. On a non-tilted substrate the MgO layers have a strong [1 1 1] out-of-plane orientation. The fraction of [1 1 1] out-of-plane oriented grains does not change when the inclination angle a increases. However, the in-plane alignment improves exponentially with increasing inclination angle as shown in Fig. 1. The FWHM goes asymptotic to 24.41 in f: Since the deposition rate decreases rapidly at a large inclination angle, an inclination angle of 551 is a good compromise between deposition rate and biaxial alignment. Therefore, the depositions of all the other series will be carried out with this inclination angle.
which is too high. The optimised distance for depositions is 10 cm. 3.3. Influence of the discharge current All MgO layers have a strong preferential [1 1 1] out-of-plane orientation when the discharge current is varied between 0.50 and 0.80 A. No change in the fraction of [1 1 1] out-of-plane oriented grains has been observed. The FWHM of the (0 0 2) peak increases when the discharge current is too low or too high (not shown). The in-plane alignment reaches an optimum with a discharge current of 0.70 A. 3.4. Influence of the working pressure
3.2. Influence of the target–substrate distance At a target–substrate distance of 13 cm the layers are fully [1 1 1] out-of-plane oriented. The fraction of [1 1 1] out-of-plane oriented grains decreases if the substrate is closer to the target. When the substrate is too close to the target (i.e. 9 cm), the fraction of randomly oriented grains increases rapidly and a preferential [1 1 1] out-ofplane orientation was no longer observed. The in-plane alignment improves when the substrate is closer to the target which is clearly seen in Fig. 2. However, with a relative distance of 9 cm the in-plane alignment disappears completely. This could be the consequence of a plasma density
The working pressure during deposition has been varied between 0.40 and 0.60 Pa. The pressure is changed by changing the pumping speed, so the Ar=O2 -ratio remains constant. All layers have a strong preferential [1 1 1] out-ofplane orientation. However, an increase of randomly oriented grains is observed when the working pressure is 0.40 Pa. Fig. 3 shows that the in-plane alignment reaches an optimum at 0.45 Pa. It is important to remark that the last three parameters (target–substrate distance, discharge current, working pressure) strongly depend on each other. All those parameters have an influence on the mean free path of the Mg adatoms. Assume
ARTICLE IN PRESS P. Ghekiere et al. / Journal of Crystal Growth 271 (2004) 462–468
Fig. 3. Influence of the working pressure on the in-plane alignment of MgO-layers.
that one of the parameters was optimised differently, the other parameters would reach another optimum.
465
Fig. 4. Influence of the substrate bias on the in-plane alignment of MgO-layers. : FWHM of the (0 0 2) peak in f; : intensity of the (0 0 2) peak.
6 V. The floating potential is situated in the region where the biaxial alignment is optimal. This means that the floating potential is optimal to obtain biaxially aligned layers.
3.5. Influence of the substrate bias 3.6. Influence of the layer thickness During the deposition a bias has been applied to the substrate. This bias is varied between 40 V and þ10 V: A strong [1 1 1] out-of-plane orientation is observed when the substrate bias is slightly negative (20 V–0 V). Applying a more negative or a positive bias gives rise to an increase in randomly out-of-plane oriented grains. The change of the in-plane alignment as a function of the substrate bias is shown in Fig. 4. A positive bias results in a strong increase of the FWHM of the (0 0 2) pole, while a negative bias provides a better in-plane alignment. As such, repelling negatively charged O 2 and electrons ðe Þ þ and attracting the positive charged Ar is favourable for the in-plane alignment. However, a strong decrease of the peak intensity is observed when the negative bias is higher than 20 V. So the number of grains with a good biaxial alignment has dropped drastically. Langmuir measurements were carried out to determine the plasma and floating potential. The plasma potential remains constant at þ5 V; independent of the used deposition parameters as mentioned in Table 1. The floating potential is slightly negative and varied between 14 V and
MgO layers with different thicknesses have been grown. The fraction of [1 1 1] out-of-plane oriented grains increases with increasing thickness. The layer is fully [1 1 1] out-of-plane oriented at a thickness of 1:0 mm: It is possible that the surface of the MgO film earlier is fully [1 1 1] out-of-plane oriented because XRD does not only provide information of the surface but gives an average over a certain thickness. When the layers are thicker than 1:5 mm the fraction of randomly oriented grains increases. The in-plane alignment as a function of the layer thickness is shown in Fig. 5. The FWHM of the (0 0 2) peak decreases with increasing layer thickness. The increase of the intensity is a consequence of layer thickness because of a larger amount of reflection planes and the improvement of the inplane orientation. The in-plane alignment reaches an optimum at a layer thickness of 1:5 mm: When the layers are thicker than 1:5 mm the FWHM increases rapidly with increasing thickness. The inplane alignment has completely disappeared at a layer thickness of 3:0 mm (Fig. 5). It is believed that this is due to a temperature increase during
ARTICLE IN PRESS 466
P. Ghekiere et al. / Journal of Crystal Growth 271 (2004) 462–468
Fig. 5. Influence of the layer thickness on the in-plane alignment of a MgO thin film deposited on stainless-steel. : FWHM of the (0 0 2)-peak in f; : intensity of the (0 0 2)-peak.
depositions, causing an increase of the adatom mobility. At a given moment, a threshold temperature is reached by which grain boundary diffusion is possible. This results in the formation of a highly disrupted structure, which is confirmed by SEM results.
Fig. 6. (0 0 2) pole figure of MgO layer deposited with optimised deposition parameters.
4. Crystallographic structure and microstructure Depositions with optimised parameters (Table 1) have been carried out. Fig. 6 shows the (0 0 2) pole figure of such a thin film. The incident vapour flux is indicated with an arrow. It can be calculated from the pole figure that the [1 1 1] direction is not perfectly out-of-plane oriented but is tilted 121 away from the flux. The [0 0 2] direction is 421 tilted towards the incident vapour flux. No difference in crystallographic tilting is observed during optimalisation of the deposition parameters. The MgO layers deposited at optimised conditions have a strong in-plane alignment with a FWHM of 19.61 in f and 7.81 in w: Fig. 7 shows the surface of MgO observed with SEM grown under optimal conditions . It reveals a roof-tile structure. Because of the [1 1 1] out-ofplane orientation and the threefold symmetry, the columns are limited by {0 0 1}-planes. This plane has the highest packing density and lowers the surface free energy. The (0 0 2)-planes are oriented towards the flux.
Fig. 7. Plan view SEM image of MgO thin film deposited under optimal conditions.
Fig. 8 shows a SEM cross-section of a MgO layer with a thickness of 1:5 mm: A clear columnar structure is observed. The columns grow parallel to the substrate normal, indicating a high mobility of the adatoms on the surface of MgO during
ARTICLE IN PRESS P. Ghekiere et al. / Journal of Crystal Growth 271 (2004) 462–468
467
Fig. 8. Cross-section SEM image of MgO thin film deposited at optimal deposition conditions (Table 1).
deposition. At the early stages of the growth, small grains evolve at the substrate. When the layer grows thicker, columns with the [1 1 1] direction out-of-plane oriented, which is the fastest growth direction for a rocksalt structure, overgrow the others [10]. This is called evolutionary growth [11]. It is also visible that the columns expand laterally with increasing thickness.
5. Conclusion Biaxially aligned MgO layers were deposited on a non-aligned tilted substrate using an unbalanced magnetron. To investigate the mechanism of the growth of biaxially aligned layers, different deposition parameters have been changed. The change of the out-of-plane and in-plane alignment with changing deposition parameter is examined. All the MgO layers exhibit a preferential [1 1 1] out-of-plane orientation. The best aligned layer had an in-plane alignment with a FWHM of 19.61 in f and 7.81 in w:
Until now, no HTSC depositions have been carried out on biaxially aligned MgO layers but will be the subject of further research.
Acknowledgements The authors would like to thank the IWT, the Flemish Institute for the Promotion of scientific and Technological Research in the Industry, for its support.
References [1] D. Dimos, P. Chaudhari, J. Mannhart, F.K. LeGoues, Phys. Rev. Lett. 61 (2) (1988) 219. [2] C.P. Wang, K.B. Do, M.R. Beasley, T.H. Geballe, R.H. Hammond, Appl. Phys. Lett. 71 (20) (1997) 2955. [3] J.R. Groves, P.N. Arendt, S.R. Foltyn, R.F. DePaula, C.P. Wang, R.H. Hammond, IEEE Trans. Appl. Supercond. 9 (2) (1999) 1964. [4] R. Huhne, C. Beyer, B. Holzapfel, C.G. Oertel, L.S.L.W. Skrotzki, Cryst. Res. Technol. 35 (4) (2000) 419.
ARTICLE IN PRESS 468
P. Ghekiere et al. / Journal of Crystal Growth 271 (2004) 462–468
[5] R.T. Brewer, J.W. Hartman, J.R. Groves, P.N. Arendt, P.C. Yashar, H.A. Atwater, Appl. Surf. Sci. 175 (2001) 691. [6] M. Bauer, J. Schwachulla, S. Furtner, P. Berberich, H. Kinder, Inst. Phys. Conf. Ser. 158 (1997) 1077. [7] M.P. Chudzik, R.E. Koritala, L.P. Luo, D. Miller, U. Balachandran, C.R. Kannewurf, IEEE Trans. Appl. Supercond. 11 (1) (2001) 3469.
[8] G. De Winter, S. Mahieu, I. De Roeck, R. De Gryse, J. Denul, IEEE Trans. Appl. Supercond. 13 (2) (2003) 2565. [9] G. De Winter, J. Denul, R. De Gryse, Inst. Phys. Conf. Ser. 167 (1999) 635. [10] V.E. Henrich, P.A. Cox, The Surface Science of Metal Oxides, first ed., Cambridge University Press, Cambridge, 1994. [11] A. Van der Drift, Philips Res. Rep. 22 (1967) 267.