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sapphire structure
Journal of Physics and Chemistry of Solids 66 (2005) 2123–2126 www.elsevier.com/locate/jpcs
Characterization of CuAlO2 thin film prepared by rapid thermal annealing of an Al2O3/Cu2O/sapphire structure J.H. Shy, B.H. Tseng * Institute of Materials Science and Engineering, National Sun Yat-Sen University, Kaohsiung 804, Taiwan, ROC
Abstract CuAlO2 thin film was successfully prepared by rapid thermal annealing of an Al2O3/Cu2O/sapphire structure in air above 1000 8C. The film was mostly with single crystalline structure as verified by X-ray diffraction methods. We found that crystal quality and electrical conductivity of the films were affected by the cooling rate after annealing. The highest conductivity obtained in this work was 0.57 S/cm. Optical gap of this film was determined to be 3.75 eV. q 2005 Elsevier Ltd. All rights reserved. Keywords: A. Oxides; A. Thin films; CuAlO2
1. Introduction
2. Experimental procedures
CuAlO2 has been known as a p-type transparent conducting oxide (TCO) without intentional doping. Optical transmission over 70% and room-temperature resistivity about 0.95 S/cm had been reported [1]. In addition to combine p-type and n-type TCOs to form all-oxides light emitting device [2], its promising potential is also found in applications such as diluted magnetic semiconductors [3] and high-efficiency solar cells [4]. Thin films of CuAlO2 had been prepared by physical vapor deposition techniques [1,5,6]. Among them, pulsed laser deposition (PLD) technique is the one that may produce CuAlO 2 films with the best properties. However, the preparation of single-phase CuAlO2 target for PLD is a tedious process [7]. In this work, we successfully develop a simple and reproducible method to prepare CuAlO2 by thin-film reaction. Our experiments showed that thin films of CuAlO2 could grow epitaxially on (001) sapphire substrates. In this article, we describe the methods for preparing CuAlO2 thin films and report their structures and properties characterized by X-ray diffraction methods, scanning electron microscope, four-point probe, and spectrophotometer.
CuAlO2 film was prepared by depositing Cu2O and Al2O3 precursor films on a substrate and followed by annealing in an oxygen-containing atmosphere. The precursor films were deposited subsequently by reactive magnetron sputtering in a Ar/O2 gas mixture. Cu and Al metal targets (5 N purity) were used and reacted to form Cu2O and Al2O3, respectively. The sputtering chamber was evacuated to 1!10K6 torr by a turbomolecular pumping system and then back filled a gas mixture of Ar and O2 (4%) to a vacuum pressure of 5!10K3 torr. The precursor films were deposited without substrate heating. Subsequent CuAlO2 formation process was performed in air ambient by rapid thermal annealing (RTA). The specimens were heated up to 1100 8C with a heating rate of 50 8C/s and kept at the designated temperature for 40 min. Quartz, sapphire and Si wafer had been used as a substrate for the film formation. Previous experiments showed that substrate materials containing Si could react with Cu2O and formed CuSiO3 in addition to CuAlO2. Therefore, sapphire was selected as the substrate throughout this work. CuAlO2 films were characterized by the following techniques: (1) X-ray diffractometer (XRD) for the identification of second phases and crystal structure; (2) X-ray Laue method for the verification of epitaxial growth; (3) X-ray double-crystal rocking curve (DCRC) method for the study of crystal quality; (4) Scanning electron microscope (SEM) for the observation of surface morphology and cross-sectional structure; (5) Thermal probe and four-point probe for conductivity type and resistivity
* Corresponding author. E-mail address: [email protected] (B.H. Tseng).
0022-3697/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2005.09.062
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Al2O3(200 nm) >1000°C
CuAlO2(360 nm)
Annealing in air
Sapphire
Cu2O(500 nm) Sapphire
Cu2O + Al2O3→ 2CuAlO2 Fig. 1. A typical sandwich structure of Al2O3 (200 nm)/Cu2O (500 nm)/sapphire before and after annealing.
measurements, respectively; (6) Spectrophotometer for the determination of optical transmission and bandgap. 3. Results and discussion 3.1. Formation of CuAlO2 through thin film reaction Cu2O may react with Al2O3 to form CuAlO2 i.e. Cu2OC Al2O3/2CuAlO2. Our experiments showed that thin film of Cu2O was not stable and might transform into CuO when the temperature was kept below 900 8C in an oxygen containing atmosphere. Therefore, rapid thermal annealing above 1000 8C should be used to prevent phase transformation of Cu2O and promote the above-mentioned chemical reaction. Moreover, we noted that Cu2O melted when the temperature reached 1000 8C. Surface tension of liquid Cu2O caused the formation of droplets on sapphire substrate. With a thin Al2O3 layer coated on Cu2O, we were able to obtain a continuous CuAlO2 film on sapphire. Since the density of Al2O3 is lower than that of Cu2O, it may float on melted Cu2O and stabilize the layer structure. A typical sandwich structure of Al2O3(200 nm)/Cu2O(500 nm) /sapphire before and after annealing is shown in Fig. 1. Comparing CuAlO2 film thickness determined from SEM observation with that of the calculated result based on original thickness of a Cu2O film and the density of both materials indicated that the as-grown
film was considerably thinner than the calculated thickness. We see that the thickness of CuAlO2 film was 380 nm as measured from the cross-sectional SEM image, while the calculated thickness was 1000 nm. Further experiments using another sapphire substrate to cap the whole surface of a sandwich structure, we found that the film thickness of CuAlO2 was very close to the calculated value. It implies that part of Cu2O may evaporate during the film formation process. Oxygen content in the annealing ambient also affected the phase formation. For instances, thermal annealing in pure oxygen could lead to the formation of CuAl2O4. In addition, vacuum annealing could cause the problem of inhibiting the formation of CuAlO2. Annealing in pure argon had a similar problem. In most cases, the successful results were obtained by air annealing. 3.2. Characterizations of CuAlO2 thin films The precursor films were originally amorphous. They crystallized in the annealed film if the reaction was partially completed. For a completely reacted specimen, XRD results shown in Fig. 2 indicate that the film containing no second phases except CuAlO2. The intensity measured from an X-ray diffractometer is magnified using the log scale. We see that the peaks of (003), (006), (009) and (0012) have very strong intensity as compared with the (101) and (012) peaks. Another X-ray experiment using Laue method, as
1012 DoubleCrystal Rocking Curve
Laue Pattern
1011
(006)
1010 Intensity (cps)
109
FWHM 157.2 arc sec
107 106
20
30
0
350
700 1050 1400
(0012)
(009)
(012)
(sec)
(101)
103
(006)
104
−1400 −1050 −700 −350
Sapphire (003)
105
(003)
Intensity
108
102 101 10
50
60
2 Fig. 2. As-grown CuAlO2 film examined by various X-ray diffraction methods.
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30 ˚ Sapphire [1 1 2 0]H CuAlO2 [2 1 0]R Sapphire [1 1 0 0]H CuAlO2 [0 1 0]R
Al
Sapphire cell (Hexagonal)
O
CuAlO2 cell (Rhombohedral)
Fig. 3. Sketch of CuAlO2 lattice stacked over crystal lattice of sapphire. Table 1 Effects of cooling rate on crystalline quality and electrical properties of the films (cooling from 1100 8C to room temperature) Cooling rate (8C/min)
18
54
FWHM (arc s) Electrical conductivity (S/cm) Conductivity type
157.2 0.57 P-type
179.2 2.2!10K3 P-type
– 0.95 [1] P-type [1]
shown in the inset on the left in Fig. 2, indicated that the film was single crystalline in nature. Symmetrical spots in the Laue pattern were contributed from the film and substrate. DCRC X-ray data also shows that FWHM of the (006) peak is 157.2 arc s (FWHM of the substrate peak is 55.3 arc s). It worth to note that crystal structures of CuAlO2 and sapphire are rhombohedral and hexagonal, respectively. The
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film was grown on (0001) basal plane of sapphire. Identification of XRD peaks indicated that all except the ones from sapphire were caused by (003)-series plane of CuAlO2 film. Since, atomic arrangement on (003) plane of rhombohedral structure has a distorted hexagon shape, lattice mismatch was different in difference directions on the plane, see Fig. 3. For a common direction of the two structures, i.e. ½010R==½1100 H, the lattice mismatch was calculated to be 3.98%. As mentioned before, CuAlO2 formed through the reaction of Cu2O and Al2O3. CuAlO2 may form at the top Al2O3/Cu2O interface and the bottom Cu2O/sapphire interface through interfacial reaction. The reactions between Cu2O and sapphire may promote the epitaxial growth. However, the reaction occurred at the top interface can not lead to epitaxial structure. The observed result may be explained in the following. The melted Cu2O layer dissolves CuAlO2 formed at the top Al2O3/Cu2O interface. When the Cu2O melt supersaturates with CuAlO2, it started to nucleate and grow epitaxially on a CuAlO2 film already formed on sapphire substrate. Our experiments also showed that the use of high cooling rate after annealing could degrade the crystal quality and the film conductivity. As can be seen in Table 1, the FWHM of DCRC increases from 157.2 to 179.2 arc s and the film conductivity decreases from 0.57 to 2.20!10K3 S/cm as the cooling rate increases from 18 to 54 8C/min. Surface morphology observed by SEM reveals the terrace feature on the film surface, see Fig. 4. The inset in the Fig. 4 is a cross-sectional SEM micrograph showing a rough film surface. Optical properties of the films were measured by a spectrophotometer. The data was obtained by subtracting the signals of film on substrate from that of bare substrate. Optical
Fig. 4. Surface morphology of as-grown CuAlO2 film, the inset shows the cross section of the film.
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2.0
film. Growth mechanisms possibly involved thin-film reactions and liquid phase epitaxy. The details need further investigation. Crystal quality and electrical conductivity of the films were affected by the cooling rate after annealing. In this work, our best result showed that the film conductivity was 0.57 S/cm. Optical gap of this film was estimated to be 3.75 eV. Optical transmission as low as 60% was attributed to the rough surface and the oxygen content in the film.
100
1.5
Transmission (%)
(αhv)2 × 1011(ev2 / cm2)
80
1.0
60
40
20
0 300
400
500
600
700
800
900
1000
Wavelength (nm)
0.5
Acknowledgements 0.0 1.5
2.0
2.5
3.0
3.5
4.0
4.5
Eg (eV) Fig. 5. (ahv)2 vs. hv plot derived from the optical transmission curve (see the inset) obtained from a CuAlO2 film.
The authors gratefully acknowledge the support of the National Council of Science under Contract No. NSC 92-2216E-110-015. We also thank Prof. T. Y. Dong for helping optical transmission measurement. References
transmission of as-grown CuAlO2 film was about 60%, see Fig. 5. This value was relatively low as compared with other reports and could be attributed to the rough surface as well as the oxygen content in the film. Optical gap was estimated to be about 3.75 eV through the (ahv)2 vs. hv plot, which was close to the reported value. 4. Conclusions For the first time, CuAlO2 epitaxial film is successfully grown on sapphire substrate by rapid thermal annealing of aAl2O3/Cu2O/sapphire sandwich structure in air above 1000 8C. A Al2O3 cap layer was crucial to obtain a continuous CuAlO2
[1] H. Kawazoe, M. Yasukawa, H. Hyodo, M. Kurita, H. Yanagi, H. Hosono, Nature 389 (1997) 939–942. [2] H. Ohta, M. Orita, M. Hirano, H. Hosono, J. Appl. Phys. 89 (2001) 5720–5725. [3] T. Dietl, H. Ohno, F. Matsukura, J. Cibert, D. Ferrand, Science 287 (2000) 1019. [4] A. Lugue, A. Marti, N. Lo´pez, E. Antolin, E. Ca´novas et al. Appl. Phys. Lett. 87 (2005) 083505. [5] A.N. Banerjee, S. Kundoo, K.K. Chattopadhyay, Thin Solid Films 440 (2003) 5–10. [6] R.E. Stauber, J.D. Perkins, P.A. Parilla, D.S. Ginley, Electrochem. SolidState Lett. 2 (1999) 654–656. [7] T. Ishiguro, A. Kitazawa, N. Mizutani, M. Kato, J. Solid State Chem. 40 (1981) 170–174.
Characterization of CuAlO2 thin film prepared by rapid thermal annealing of an Al2O3/Cu2O/sapphire structure J.H. Shy, B.H. Tseng * Institute of Materials Science and Engineering, National Sun Yat-Sen University, Kaohsiung 804, Taiwan, ROC
Abstract CuAlO2 thin film was successfully prepared by rapid thermal annealing of an Al2O3/Cu2O/sapphire structure in air above 1000 8C. The film was mostly with single crystalline structure as verified by X-ray diffraction methods. We found that crystal quality and electrical conductivity of the films were affected by the cooling rate after annealing. The highest conductivity obtained in this work was 0.57 S/cm. Optical gap of this film was determined to be 3.75 eV. q 2005 Elsevier Ltd. All rights reserved. Keywords: A. Oxides; A. Thin films; CuAlO2
1. Introduction
2. Experimental procedures
CuAlO2 has been known as a p-type transparent conducting oxide (TCO) without intentional doping. Optical transmission over 70% and room-temperature resistivity about 0.95 S/cm had been reported [1]. In addition to combine p-type and n-type TCOs to form all-oxides light emitting device [2], its promising potential is also found in applications such as diluted magnetic semiconductors [3] and high-efficiency solar cells [4]. Thin films of CuAlO2 had been prepared by physical vapor deposition techniques [1,5,6]. Among them, pulsed laser deposition (PLD) technique is the one that may produce CuAlO 2 films with the best properties. However, the preparation of single-phase CuAlO2 target for PLD is a tedious process [7]. In this work, we successfully develop a simple and reproducible method to prepare CuAlO2 by thin-film reaction. Our experiments showed that thin films of CuAlO2 could grow epitaxially on (001) sapphire substrates. In this article, we describe the methods for preparing CuAlO2 thin films and report their structures and properties characterized by X-ray diffraction methods, scanning electron microscope, four-point probe, and spectrophotometer.
CuAlO2 film was prepared by depositing Cu2O and Al2O3 precursor films on a substrate and followed by annealing in an oxygen-containing atmosphere. The precursor films were deposited subsequently by reactive magnetron sputtering in a Ar/O2 gas mixture. Cu and Al metal targets (5 N purity) were used and reacted to form Cu2O and Al2O3, respectively. The sputtering chamber was evacuated to 1!10K6 torr by a turbomolecular pumping system and then back filled a gas mixture of Ar and O2 (4%) to a vacuum pressure of 5!10K3 torr. The precursor films were deposited without substrate heating. Subsequent CuAlO2 formation process was performed in air ambient by rapid thermal annealing (RTA). The specimens were heated up to 1100 8C with a heating rate of 50 8C/s and kept at the designated temperature for 40 min. Quartz, sapphire and Si wafer had been used as a substrate for the film formation. Previous experiments showed that substrate materials containing Si could react with Cu2O and formed CuSiO3 in addition to CuAlO2. Therefore, sapphire was selected as the substrate throughout this work. CuAlO2 films were characterized by the following techniques: (1) X-ray diffractometer (XRD) for the identification of second phases and crystal structure; (2) X-ray Laue method for the verification of epitaxial growth; (3) X-ray double-crystal rocking curve (DCRC) method for the study of crystal quality; (4) Scanning electron microscope (SEM) for the observation of surface morphology and cross-sectional structure; (5) Thermal probe and four-point probe for conductivity type and resistivity
* Corresponding author. E-mail address: [email protected] (B.H. Tseng).
0022-3697/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2005.09.062
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Al2O3(200 nm) >1000°C
CuAlO2(360 nm)
Annealing in air
Sapphire
Cu2O(500 nm) Sapphire
Cu2O + Al2O3→ 2CuAlO2 Fig. 1. A typical sandwich structure of Al2O3 (200 nm)/Cu2O (500 nm)/sapphire before and after annealing.
measurements, respectively; (6) Spectrophotometer for the determination of optical transmission and bandgap. 3. Results and discussion 3.1. Formation of CuAlO2 through thin film reaction Cu2O may react with Al2O3 to form CuAlO2 i.e. Cu2OC Al2O3/2CuAlO2. Our experiments showed that thin film of Cu2O was not stable and might transform into CuO when the temperature was kept below 900 8C in an oxygen containing atmosphere. Therefore, rapid thermal annealing above 1000 8C should be used to prevent phase transformation of Cu2O and promote the above-mentioned chemical reaction. Moreover, we noted that Cu2O melted when the temperature reached 1000 8C. Surface tension of liquid Cu2O caused the formation of droplets on sapphire substrate. With a thin Al2O3 layer coated on Cu2O, we were able to obtain a continuous CuAlO2 film on sapphire. Since the density of Al2O3 is lower than that of Cu2O, it may float on melted Cu2O and stabilize the layer structure. A typical sandwich structure of Al2O3(200 nm)/Cu2O(500 nm) /sapphire before and after annealing is shown in Fig. 1. Comparing CuAlO2 film thickness determined from SEM observation with that of the calculated result based on original thickness of a Cu2O film and the density of both materials indicated that the as-grown
film was considerably thinner than the calculated thickness. We see that the thickness of CuAlO2 film was 380 nm as measured from the cross-sectional SEM image, while the calculated thickness was 1000 nm. Further experiments using another sapphire substrate to cap the whole surface of a sandwich structure, we found that the film thickness of CuAlO2 was very close to the calculated value. It implies that part of Cu2O may evaporate during the film formation process. Oxygen content in the annealing ambient also affected the phase formation. For instances, thermal annealing in pure oxygen could lead to the formation of CuAl2O4. In addition, vacuum annealing could cause the problem of inhibiting the formation of CuAlO2. Annealing in pure argon had a similar problem. In most cases, the successful results were obtained by air annealing. 3.2. Characterizations of CuAlO2 thin films The precursor films were originally amorphous. They crystallized in the annealed film if the reaction was partially completed. For a completely reacted specimen, XRD results shown in Fig. 2 indicate that the film containing no second phases except CuAlO2. The intensity measured from an X-ray diffractometer is magnified using the log scale. We see that the peaks of (003), (006), (009) and (0012) have very strong intensity as compared with the (101) and (012) peaks. Another X-ray experiment using Laue method, as
1012 DoubleCrystal Rocking Curve
Laue Pattern
1011
(006)
1010 Intensity (cps)
109
FWHM 157.2 arc sec
107 106
20
30
0
350
700 1050 1400
(0012)
(009)
(012)
(sec)
(101)
103
(006)
104
−1400 −1050 −700 −350
Sapphire (003)
105
(003)
Intensity
108
102 101 10
50
60
2 Fig. 2. As-grown CuAlO2 film examined by various X-ray diffraction methods.
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30 ˚ Sapphire [1 1 2 0]H CuAlO2 [2 1 0]R Sapphire [1 1 0 0]H CuAlO2 [0 1 0]R
Al
Sapphire cell (Hexagonal)
O
CuAlO2 cell (Rhombohedral)
Fig. 3. Sketch of CuAlO2 lattice stacked over crystal lattice of sapphire. Table 1 Effects of cooling rate on crystalline quality and electrical properties of the films (cooling from 1100 8C to room temperature) Cooling rate (8C/min)
18
54
FWHM (arc s) Electrical conductivity (S/cm) Conductivity type
157.2 0.57 P-type
179.2 2.2!10K3 P-type
– 0.95 [1] P-type [1]
shown in the inset on the left in Fig. 2, indicated that the film was single crystalline in nature. Symmetrical spots in the Laue pattern were contributed from the film and substrate. DCRC X-ray data also shows that FWHM of the (006) peak is 157.2 arc s (FWHM of the substrate peak is 55.3 arc s). It worth to note that crystal structures of CuAlO2 and sapphire are rhombohedral and hexagonal, respectively. The
2125
film was grown on (0001) basal plane of sapphire. Identification of XRD peaks indicated that all except the ones from sapphire were caused by (003)-series plane of CuAlO2 film. Since, atomic arrangement on (003) plane of rhombohedral structure has a distorted hexagon shape, lattice mismatch was different in difference directions on the plane, see Fig. 3. For a common direction of the two structures, i.e. ½010R==½1100 H, the lattice mismatch was calculated to be 3.98%. As mentioned before, CuAlO2 formed through the reaction of Cu2O and Al2O3. CuAlO2 may form at the top Al2O3/Cu2O interface and the bottom Cu2O/sapphire interface through interfacial reaction. The reactions between Cu2O and sapphire may promote the epitaxial growth. However, the reaction occurred at the top interface can not lead to epitaxial structure. The observed result may be explained in the following. The melted Cu2O layer dissolves CuAlO2 formed at the top Al2O3/Cu2O interface. When the Cu2O melt supersaturates with CuAlO2, it started to nucleate and grow epitaxially on a CuAlO2 film already formed on sapphire substrate. Our experiments also showed that the use of high cooling rate after annealing could degrade the crystal quality and the film conductivity. As can be seen in Table 1, the FWHM of DCRC increases from 157.2 to 179.2 arc s and the film conductivity decreases from 0.57 to 2.20!10K3 S/cm as the cooling rate increases from 18 to 54 8C/min. Surface morphology observed by SEM reveals the terrace feature on the film surface, see Fig. 4. The inset in the Fig. 4 is a cross-sectional SEM micrograph showing a rough film surface. Optical properties of the films were measured by a spectrophotometer. The data was obtained by subtracting the signals of film on substrate from that of bare substrate. Optical
Fig. 4. Surface morphology of as-grown CuAlO2 film, the inset shows the cross section of the film.
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J.H. Shy, B.H. Tseng / Journal of Physics and Chemistry of Solids 66 (2005) 2123–2126
2.0
film. Growth mechanisms possibly involved thin-film reactions and liquid phase epitaxy. The details need further investigation. Crystal quality and electrical conductivity of the films were affected by the cooling rate after annealing. In this work, our best result showed that the film conductivity was 0.57 S/cm. Optical gap of this film was estimated to be 3.75 eV. Optical transmission as low as 60% was attributed to the rough surface and the oxygen content in the film.
100
1.5
Transmission (%)
(αhv)2 × 1011(ev2 / cm2)
80
1.0
60
40
20
0 300
400
500
600
700
800
900
1000
Wavelength (nm)
0.5
Acknowledgements 0.0 1.5
2.0
2.5
3.0
3.5
4.0
4.5
Eg (eV) Fig. 5. (ahv)2 vs. hv plot derived from the optical transmission curve (see the inset) obtained from a CuAlO2 film.
The authors gratefully acknowledge the support of the National Council of Science under Contract No. NSC 92-2216E-110-015. We also thank Prof. T. Y. Dong for helping optical transmission measurement. References
transmission of as-grown CuAlO2 film was about 60%, see Fig. 5. This value was relatively low as compared with other reports and could be attributed to the rough surface as well as the oxygen content in the film. Optical gap was estimated to be about 3.75 eV through the (ahv)2 vs. hv plot, which was close to the reported value. 4. Conclusions For the first time, CuAlO2 epitaxial film is successfully grown on sapphire substrate by rapid thermal annealing of aAl2O3/Cu2O/sapphire sandwich structure in air above 1000 8C. A Al2O3 cap layer was crucial to obtain a continuous CuAlO2
[1] H. Kawazoe, M. Yasukawa, H. Hyodo, M. Kurita, H. Yanagi, H. Hosono, Nature 389 (1997) 939–942. [2] H. Ohta, M. Orita, M. Hirano, H. Hosono, J. Appl. Phys. 89 (2001) 5720–5725. [3] T. Dietl, H. Ohno, F. Matsukura, J. Cibert, D. Ferrand, Science 287 (2000) 1019. [4] A. Lugue, A. Marti, N. Lo´pez, E. Antolin, E. Ca´novas et al. Appl. Phys. Lett. 87 (2005) 083505. [5] A.N. Banerjee, S. Kundoo, K.K. Chattopadhyay, Thin Solid Films 440 (2003) 5–10. [6] R.E. Stauber, J.D. Perkins, P.A. Parilla, D.S. Ginley, Electrochem. SolidState Lett. 2 (1999) 654–656. [7] T. Ishiguro, A. Kitazawa, N. Mizutani, M. Kato, J. Solid State Chem. 40 (1981) 170–174.