- Email: [email protected]
Delafossite CuAlO2 films prepared by reactive sputtering using Cu and Al targets
Journal of Physics and Chemistry of Solids 64 (2003) 1671–1674 www.elsevier.com/locate/jpcs
Delafossite CuAlO2 films prepared by reactive sputtering using Cu and Al targets N. Tsuboia,*, Y. Takahashib, S. Kobayashia, H. Shimizua, K. Katoa, F. Kanekoa b
a Faculty of Engineering, Niigata University, Ikarashi 2-nocho, Niigata 950-2181, Japan Graduate School of Science and Technology, Niigata University, Ikarashi 2-nocho, Niigata 950-2181, Japan
Abstract Composition and structure of CuAlO2 films, deposited through the dc-reactive sputtering method using Cu and Al elemental targets and Ar-diluted oxygen gas, were controlled by the Cu and Al deposition periods and the postannealing temperature. The delafossite CuAlO2 films were successfully prepared by the postannealing of the films with [Cu]/[Al] ¼ 1 at temperatures higher than 700 8C in the nitrogen atmosphere. In comparison with the optical absorption edge of the CuAlO2 films, those of Cu-rich and Al-rich films sifted to the longer and shorter wavelength regions, respectively. The shifts of the optical absorption edge in the off-stoichiometric films are interpretable to be due to the additional copper oxide or aluminum oxide phase. The resistivity of the high-temperature postannealed films with p-type was in the range of 10 – 102 Vcm. q 2003 Elsevier Ltd. All rights reserved.
1. Introduction
2. Experimental
Transparent conducting oxide films with n-type are widely used as transparent electrodes in various devices such as flat panel displays and solar cells. Recently, transparent ternary oxide films, containing Cuþ as a major cationic species, were reported to have p-type conductivity [1 –7]. The pn junction of the transparent conducting oxide films is much attractive as new functional window devices. Preparation of the p-type transparent oxide films have been mainly carried out by pulse laser deposition and rfsputtering using the sintered bulk polycrystal target [1 –5]. However, it is not easy to sinter the polycrystal target from commercial binary oxide sources by solid-state reaction at temperatures higher than about 1000 8C for several days. Very recently, CuAlO2 films including additional phases were prepared by a chemical-vapor deposition technique [6] and solution methods [7]. This report, for the first time, demonstrates preparation of delafossite CuAlO2 films by the reactive dc-sputtering method using Cu and Al elemental targets and Ar-diluted oxygen gas, i.e. the sintered polycrystal oxide was not used as the target material.
The dc-sputtering system used in this work has two pairs of facing targets for Cu and Al metal sources as shown in Fig. 1. In the facing targets sputtering system, the discharge plasma is confined between the facing targets by magnetic field as high as 1400 Oe and the substrate is placed outside of the plasma so that the films can avoid bombardments of high-energy particles such as electrons and negative ions. Moreover, the large plasma confinement enables dcsputtering to be performed stably at a gas pressure as low as 4 mTorr in O2(20%) and Ar(80%) atmosphere. The diameter of the target disks was 33 mm. Input dc-current and dc-voltage for the Cu facing targets are 10 mA and about 750 V, respectively. Those for the Al facing targets are 60 mA and about 350 V, respectively. The sputtered Cu and Al atoms were alternately deposited on the quartz substrate at 300 8C for about 4 h by controlling a pulse motor connected with the substrate holder. The sequential deposition period of Cu ðtCu Þ was fixed at 1 s, while that of Al ðtAl Þ was changed from 3 to 15 s. The deposited thickness per cycle corresponded to about one monolayer of CuAlO2. The total film thickness was in the range of 0.4– 0.8 mm. The films were postannealed at temperatures of 500–1050 8C for 4 h in the nitrogen flow under atmospheric pressure.
* Corresponding author. Fax: þ 81-25-262-7261. E-mail address: [email protected] (N. Tsuboi).
0022-3697/03/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0022-3697(03)00194-X
1672
N. Tsuboi et al. / Journal of Physics and Chemistry of Solids 64 (2003) 1671–1674
Fig. 1. Schematic representation of dc-reactive sputtering system with two pairs of the facing Cu and Al elemental targets.
The composition of the films was determined by electron probe X-ray microanalysis (EPMA). Surface morphology was observed by scanning electron microscope (SEM). Xray diffraction (XRD) was measured using the Cu Ka radiation line. Optical transmission measurements were performed on HITACHI U-3200 spectrophotometer. The resistivity of the films was estimated at room temperature by van der Pauw method using four Au electrodes evaporated on the film surface. The conduction type was checked by the thermal-probe method.
the [Cu]/[Al] ratio was around unity. On the other hand, the molar fraction of oxygen in as-deposited films was around 0.6– 0.7 without regard to the [Cu]/[Al] ratio. The molar fraction of oxygen in films postannealed at 500 8C seemed to be almost equal to that of the as-deposited films. By postannealing at temperatures higher than 700 8C, the molar fraction of oxygen approached to about 0.5. The composition of the high-temperature postannealed films with tAl ¼ 7 – 9 s corresponded to the stoichiometric composition of CuAlO2 In the SEM images of the film surfaces, the films postannealed at 500 8C as well as the as-deposited film seemed to have smooth surface morphology without regard of the [Cu]/[Al] ratio. After postannealing at temperatures higher than 700 8C, the films had slightly rough surfaces, suggesting appearances of grains by crystallization. This fact can be related with the decrease of oxygen content as described above. Fig. 3 shows XRD patterns of as-deposited and postannealed films with [Cu]/[Al] , 1. No XRD lines were observed in the as-deposited films, regardless of the [Cu]/[Al] ratio. This fact indicates that the as-deposited
3. Results and discussion Composition of as-deposited and postannealed films is shown in Fig. 2. The [Cu]/[Al] ratio in the as-deposited films decreased with increasing tAl : At tAl ¼ 7 – 9 s;
Fig. 2. Composition of as-deposited and postannealed films. The postanneal temperatures are shown in the upper-left side. The [Cu]/[Al] ratio in the as-deposited films decreased with increasing tAl :
Fig. 3. X-ray diffraction patterns of as-deposited and postannealed films with [Cu]/[Al] , 1. The diffraction lines marked with arrows correspond to CuAlO2 delafossite structure. The broad peak around 22 8 is due to the quartz substrate.
N. Tsuboi et al. / Journal of Physics and Chemistry of Solids 64 (2003) 1671–1674
films with excessive oxygen content have amorphous structure. In the films postannealed at temperatures higher than 700 8C, there appear XRD lines of CuAlO2 delafossite structure as shown in Fig. 3. The temperature regions indicating the amorphous and crystalline states in the XRD patterns correspond to the temperature regions showing the smooth and rough surfaces in the SEM images, respectively. Fig. 4 shows optical transmission spectra of as-deposeted and postannealed films with [Cu]/[Al] , 1. By the hightemperature postannealing process, optical transmission increased in the short wavelength region, causing changes of the film color from brown to transparency. In the hightemperature postannealed film with [Cu]/[Al] , 1, the photon energy of the low optical transmission (,1021%), suggesting optical absorption edge, corresponds to the direct energy-gap of CuAlO2 (3.5 eV [4]). The above XRD and transmission results are consistent with the EPMA and SEM results, i.e. these results indicate that the high-temperature postannealing process under the nitrogen atmosphere for the as-deposited amorphous films with excessive oxygen content gave rise to the decrease of the oxygen content and the crystallization of the CuAlO2 phase. The high-temperature postannealed film with [Cu]/[Al] , 1 is considered to be CuAlO2 film. Cu-rich films postannealed at 1050 8C exhibited Cu2O XRD line, marked with W, besides the CuAlO2 XRD lines as
1673
Fig. 5. X-ray diffraction patterns of Cu-rich ([Cu]/[Al] ¼ 1.7) and Al-rich ([Cu]/[Al] ¼ 0.7) films postannealed at 1050 8C. The diffraction lines marked with W and arrows correspond to Cu2O and CuAlO2 phases, respectively. The weak line marked with £ in the Cu-rich film seems to correspond to CuSiO3 phase, which could be caused by reaction between the quartz substrate and copper oxides at high temperature.
shown in Fig. 5. In Al-rich, on the other hand, only CuAlO2 XRD lines were observed. Fig. 6 shows optical transmission spectra of the films postannealed at 1050 8C. The optical absorption edge shifts from the longer wavelength region to the shorter wavelength region with decreasing the [Cu]/[Al] ratio. It is reasonable that the copper oxide phase in the Cu-rich films causes the shift of the optical absorption edge to the longer wavelength region. In contrast, the shift of
Fig. 4. Optical transmission spectra at room temperature in asdeposited and postannealed films with [Cu]/[Al] , 1.
Fig. 6. Optical transmission spectra of the films postannealed at 1050 8C. The [Cu]/[Al] ratios of the films are (a) 1.8, (b) 1.7, (c) 1.1 and (d) 0.7. The inset in the figure uses a linear scale for the optical transmission.
1674
N. Tsuboi et al. / Journal of Physics and Chemistry of Solids 64 (2003) 1671–1674
the optical absorption edge of the Al-rich films to the shorter wavelength region is reasonably caused by an inclusion of amorphous aluminum oxide that exhibits no XRD lines. These facts are consistent with the EPMA and XRD results mentioned before. The resistivity of the as-deposited films was higher than 105 Vcm. On the other hand, the resistivity of hightemperature postannealed films was in the range of 10 – 102 Vcm. The resistivity of the postannealed films was independent of the [Cu]/[Al] ratio. The conduction type of the postannealed films had p-type conductivity. Since the resistivity of the postannealed films was independent of the amount of the copper oxide and aluminum oxide phases, the electrical property of the films is interpretable to be dominated by the CuAlO2 phase.
The optical absorption edge of the postannealed films shifted from the long wavelength region (, 0.5 mm) to the short wavelength region (, 0.3 mm) with decreasing the [Cu]/[Al] ratio. This fact is considered to be due to the additional copper oxide or aluminum oxide phase. The resistivity of high-temperature postannealed films with p-type was in the range of 10 – 102 Vcm, which was independent of the [Cu]/[Al] ratio. The electrical property of the films is interpretable to be dominated by the CuAlO2 phase.
Acknowledgements The authors would like to thank Prof. Y. Hoshi, T. Kawakami, T. Siino and Y. Itoh for their kind supports during this study.
4. Conclusion The composition and the structure of the films, deposited through the dc-reactive sputtering method using two pairs of the facing Cu and Al elemental targets and Ar-diluted oxygen gas, were controlled by the Cu and Al sputtering periods and the postannealing temperature. The postannealing at temperatures higher than 700 8C under the nitrogen atmosphere for the as-deposited amorphous films with excessive oxygen content caused the approach of the oxygen molar fraction to 0.5 and the crystallization of delafossite type CuAlO2 phase. The stoichiometric CuAlO2 films, whose optical absorption edge corresponded to the energy gap of CuAlO2, were successfully prepared by the postannealing of the as-deposited films with [Cu]/[Al] , 1.
References [1] H. Kawazoe, M. Yasukawa, H. Hyodo, M. Kurita, H. Yanagi, H. Hosono, Nature 389 (1997) 939. [2] R.E. Stauber, J.D. Perkins, P.A. Parilla, D.S. Ginley, Electrochem. Solid-State Lett. 2 (1999) 654. [3] H. Kawazoe, H. Yanagi, K. Ueda, H. Hosono, Bull. Mater. Res. Soc. 25 (8) (2000) 28. [4] H. Yanagi, S. Inoue, K. Ueda, H. Kawazoe, H. Hosono, N. Hamada, J. Appl. Phys. 88 (2000) 4159. [5] J. Tate, M.K. Jayaraj, A.D. Draeseke, T. Ulbrich, A.W. Sleight, K.A. Vanaja, R. Nagarajan, J.F. Wager, R.L. Hoffman, Thin Solid Films 411 (2002) 119. [6] H. Gong, Y. Wang, Y. Luo, Appl. Phys. Lett. 76 (2000) 3959. [7] K. Tonooka, K. Shimokawa, O. Nishimura, Thin Solid Films 411 (2002) 129.
Delafossite CuAlO2 films prepared by reactive sputtering using Cu and Al targets N. Tsuboia,*, Y. Takahashib, S. Kobayashia, H. Shimizua, K. Katoa, F. Kanekoa b
a Faculty of Engineering, Niigata University, Ikarashi 2-nocho, Niigata 950-2181, Japan Graduate School of Science and Technology, Niigata University, Ikarashi 2-nocho, Niigata 950-2181, Japan
Abstract Composition and structure of CuAlO2 films, deposited through the dc-reactive sputtering method using Cu and Al elemental targets and Ar-diluted oxygen gas, were controlled by the Cu and Al deposition periods and the postannealing temperature. The delafossite CuAlO2 films were successfully prepared by the postannealing of the films with [Cu]/[Al] ¼ 1 at temperatures higher than 700 8C in the nitrogen atmosphere. In comparison with the optical absorption edge of the CuAlO2 films, those of Cu-rich and Al-rich films sifted to the longer and shorter wavelength regions, respectively. The shifts of the optical absorption edge in the off-stoichiometric films are interpretable to be due to the additional copper oxide or aluminum oxide phase. The resistivity of the high-temperature postannealed films with p-type was in the range of 10 – 102 Vcm. q 2003 Elsevier Ltd. All rights reserved.
1. Introduction
2. Experimental
Transparent conducting oxide films with n-type are widely used as transparent electrodes in various devices such as flat panel displays and solar cells. Recently, transparent ternary oxide films, containing Cuþ as a major cationic species, were reported to have p-type conductivity [1 –7]. The pn junction of the transparent conducting oxide films is much attractive as new functional window devices. Preparation of the p-type transparent oxide films have been mainly carried out by pulse laser deposition and rfsputtering using the sintered bulk polycrystal target [1 –5]. However, it is not easy to sinter the polycrystal target from commercial binary oxide sources by solid-state reaction at temperatures higher than about 1000 8C for several days. Very recently, CuAlO2 films including additional phases were prepared by a chemical-vapor deposition technique [6] and solution methods [7]. This report, for the first time, demonstrates preparation of delafossite CuAlO2 films by the reactive dc-sputtering method using Cu and Al elemental targets and Ar-diluted oxygen gas, i.e. the sintered polycrystal oxide was not used as the target material.
The dc-sputtering system used in this work has two pairs of facing targets for Cu and Al metal sources as shown in Fig. 1. In the facing targets sputtering system, the discharge plasma is confined between the facing targets by magnetic field as high as 1400 Oe and the substrate is placed outside of the plasma so that the films can avoid bombardments of high-energy particles such as electrons and negative ions. Moreover, the large plasma confinement enables dcsputtering to be performed stably at a gas pressure as low as 4 mTorr in O2(20%) and Ar(80%) atmosphere. The diameter of the target disks was 33 mm. Input dc-current and dc-voltage for the Cu facing targets are 10 mA and about 750 V, respectively. Those for the Al facing targets are 60 mA and about 350 V, respectively. The sputtered Cu and Al atoms were alternately deposited on the quartz substrate at 300 8C for about 4 h by controlling a pulse motor connected with the substrate holder. The sequential deposition period of Cu ðtCu Þ was fixed at 1 s, while that of Al ðtAl Þ was changed from 3 to 15 s. The deposited thickness per cycle corresponded to about one monolayer of CuAlO2. The total film thickness was in the range of 0.4– 0.8 mm. The films were postannealed at temperatures of 500–1050 8C for 4 h in the nitrogen flow under atmospheric pressure.
* Corresponding author. Fax: þ 81-25-262-7261. E-mail address: [email protected] (N. Tsuboi).
0022-3697/03/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0022-3697(03)00194-X
1672
N. Tsuboi et al. / Journal of Physics and Chemistry of Solids 64 (2003) 1671–1674
Fig. 1. Schematic representation of dc-reactive sputtering system with two pairs of the facing Cu and Al elemental targets.
The composition of the films was determined by electron probe X-ray microanalysis (EPMA). Surface morphology was observed by scanning electron microscope (SEM). Xray diffraction (XRD) was measured using the Cu Ka radiation line. Optical transmission measurements were performed on HITACHI U-3200 spectrophotometer. The resistivity of the films was estimated at room temperature by van der Pauw method using four Au electrodes evaporated on the film surface. The conduction type was checked by the thermal-probe method.
the [Cu]/[Al] ratio was around unity. On the other hand, the molar fraction of oxygen in as-deposited films was around 0.6– 0.7 without regard to the [Cu]/[Al] ratio. The molar fraction of oxygen in films postannealed at 500 8C seemed to be almost equal to that of the as-deposited films. By postannealing at temperatures higher than 700 8C, the molar fraction of oxygen approached to about 0.5. The composition of the high-temperature postannealed films with tAl ¼ 7 – 9 s corresponded to the stoichiometric composition of CuAlO2 In the SEM images of the film surfaces, the films postannealed at 500 8C as well as the as-deposited film seemed to have smooth surface morphology without regard of the [Cu]/[Al] ratio. After postannealing at temperatures higher than 700 8C, the films had slightly rough surfaces, suggesting appearances of grains by crystallization. This fact can be related with the decrease of oxygen content as described above. Fig. 3 shows XRD patterns of as-deposited and postannealed films with [Cu]/[Al] , 1. No XRD lines were observed in the as-deposited films, regardless of the [Cu]/[Al] ratio. This fact indicates that the as-deposited
3. Results and discussion Composition of as-deposited and postannealed films is shown in Fig. 2. The [Cu]/[Al] ratio in the as-deposited films decreased with increasing tAl : At tAl ¼ 7 – 9 s;
Fig. 2. Composition of as-deposited and postannealed films. The postanneal temperatures are shown in the upper-left side. The [Cu]/[Al] ratio in the as-deposited films decreased with increasing tAl :
Fig. 3. X-ray diffraction patterns of as-deposited and postannealed films with [Cu]/[Al] , 1. The diffraction lines marked with arrows correspond to CuAlO2 delafossite structure. The broad peak around 22 8 is due to the quartz substrate.
N. Tsuboi et al. / Journal of Physics and Chemistry of Solids 64 (2003) 1671–1674
films with excessive oxygen content have amorphous structure. In the films postannealed at temperatures higher than 700 8C, there appear XRD lines of CuAlO2 delafossite structure as shown in Fig. 3. The temperature regions indicating the amorphous and crystalline states in the XRD patterns correspond to the temperature regions showing the smooth and rough surfaces in the SEM images, respectively. Fig. 4 shows optical transmission spectra of as-deposeted and postannealed films with [Cu]/[Al] , 1. By the hightemperature postannealing process, optical transmission increased in the short wavelength region, causing changes of the film color from brown to transparency. In the hightemperature postannealed film with [Cu]/[Al] , 1, the photon energy of the low optical transmission (,1021%), suggesting optical absorption edge, corresponds to the direct energy-gap of CuAlO2 (3.5 eV [4]). The above XRD and transmission results are consistent with the EPMA and SEM results, i.e. these results indicate that the high-temperature postannealing process under the nitrogen atmosphere for the as-deposited amorphous films with excessive oxygen content gave rise to the decrease of the oxygen content and the crystallization of the CuAlO2 phase. The high-temperature postannealed film with [Cu]/[Al] , 1 is considered to be CuAlO2 film. Cu-rich films postannealed at 1050 8C exhibited Cu2O XRD line, marked with W, besides the CuAlO2 XRD lines as
1673
Fig. 5. X-ray diffraction patterns of Cu-rich ([Cu]/[Al] ¼ 1.7) and Al-rich ([Cu]/[Al] ¼ 0.7) films postannealed at 1050 8C. The diffraction lines marked with W and arrows correspond to Cu2O and CuAlO2 phases, respectively. The weak line marked with £ in the Cu-rich film seems to correspond to CuSiO3 phase, which could be caused by reaction between the quartz substrate and copper oxides at high temperature.
shown in Fig. 5. In Al-rich, on the other hand, only CuAlO2 XRD lines were observed. Fig. 6 shows optical transmission spectra of the films postannealed at 1050 8C. The optical absorption edge shifts from the longer wavelength region to the shorter wavelength region with decreasing the [Cu]/[Al] ratio. It is reasonable that the copper oxide phase in the Cu-rich films causes the shift of the optical absorption edge to the longer wavelength region. In contrast, the shift of
Fig. 4. Optical transmission spectra at room temperature in asdeposited and postannealed films with [Cu]/[Al] , 1.
Fig. 6. Optical transmission spectra of the films postannealed at 1050 8C. The [Cu]/[Al] ratios of the films are (a) 1.8, (b) 1.7, (c) 1.1 and (d) 0.7. The inset in the figure uses a linear scale for the optical transmission.
1674
N. Tsuboi et al. / Journal of Physics and Chemistry of Solids 64 (2003) 1671–1674
the optical absorption edge of the Al-rich films to the shorter wavelength region is reasonably caused by an inclusion of amorphous aluminum oxide that exhibits no XRD lines. These facts are consistent with the EPMA and XRD results mentioned before. The resistivity of the as-deposited films was higher than 105 Vcm. On the other hand, the resistivity of hightemperature postannealed films was in the range of 10 – 102 Vcm. The resistivity of the postannealed films was independent of the [Cu]/[Al] ratio. The conduction type of the postannealed films had p-type conductivity. Since the resistivity of the postannealed films was independent of the amount of the copper oxide and aluminum oxide phases, the electrical property of the films is interpretable to be dominated by the CuAlO2 phase.
The optical absorption edge of the postannealed films shifted from the long wavelength region (, 0.5 mm) to the short wavelength region (, 0.3 mm) with decreasing the [Cu]/[Al] ratio. This fact is considered to be due to the additional copper oxide or aluminum oxide phase. The resistivity of high-temperature postannealed films with p-type was in the range of 10 – 102 Vcm, which was independent of the [Cu]/[Al] ratio. The electrical property of the films is interpretable to be dominated by the CuAlO2 phase.
Acknowledgements The authors would like to thank Prof. Y. Hoshi, T. Kawakami, T. Siino and Y. Itoh for their kind supports during this study.
4. Conclusion The composition and the structure of the films, deposited through the dc-reactive sputtering method using two pairs of the facing Cu and Al elemental targets and Ar-diluted oxygen gas, were controlled by the Cu and Al sputtering periods and the postannealing temperature. The postannealing at temperatures higher than 700 8C under the nitrogen atmosphere for the as-deposited amorphous films with excessive oxygen content caused the approach of the oxygen molar fraction to 0.5 and the crystallization of delafossite type CuAlO2 phase. The stoichiometric CuAlO2 films, whose optical absorption edge corresponded to the energy gap of CuAlO2, were successfully prepared by the postannealing of the as-deposited films with [Cu]/[Al] , 1.
References [1] H. Kawazoe, M. Yasukawa, H. Hyodo, M. Kurita, H. Yanagi, H. Hosono, Nature 389 (1997) 939. [2] R.E. Stauber, J.D. Perkins, P.A. Parilla, D.S. Ginley, Electrochem. Solid-State Lett. 2 (1999) 654. [3] H. Kawazoe, H. Yanagi, K. Ueda, H. Hosono, Bull. Mater. Res. Soc. 25 (8) (2000) 28. [4] H. Yanagi, S. Inoue, K. Ueda, H. Kawazoe, H. Hosono, N. Hamada, J. Appl. Phys. 88 (2000) 4159. [5] J. Tate, M.K. Jayaraj, A.D. Draeseke, T. Ulbrich, A.W. Sleight, K.A. Vanaja, R. Nagarajan, J.F. Wager, R.L. Hoffman, Thin Solid Films 411 (2002) 119. [6] H. Gong, Y. Wang, Y. Luo, Appl. Phys. Lett. 76 (2000) 3959. [7] K. Tonooka, K. Shimokawa, O. Nishimura, Thin Solid Films 411 (2002) 129.