- Email: [email protected]
Role of oxygen in structural properties of annealed CuAlO2 films
Journal of Crystal Growth 314 (2011) 370–373
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Role of oxygen in structural properties of annealed CuAlO2 films W. Lan a,b,n, J.Q. Pan a, C.Q. Zhu a, G.Q. Wang a, Q. Su a,b, X.Q. Liu a,b, E.Q. Xie a,b, H. Yan c a b c
Key Laboratory for Magnetism and Magnetic Materials of Ministry of Education, Lanzhou University, Lanzhou 730000, PR China School of Physical Science and Technology, Lanzhou University, Lanzhou 730000, PR China The College of Materials Science and Engineering, Beijing University of Technology, Beijing 100022, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 September 2010 Accepted 11 November 2010 Communicated by D.P. Norton Available online 10 December 2010
CuAlO2 films were sputtered on quartz substrates at different oxygen partial pressures (OPP) and carried out the annealing at 900 1C for 5 h in N2 ambient. The structural properties of these films have been studied in detail by X-ray diffraction, Raman spectroscopy, and atomic force microscopy. Annealed CuAlO2 films are grown along the (0 0 1) preferential orientation. The film deposited at 20% OPP demonstrates the excellent crystalline behavior and the smallest electrical resistivity (41.8 O cm). At higher OPP, the crystalline behavior begins to degenerate up to the amorphous state at 60% OPP, and some micro-caves presented in the film surface become larger and deeper with the increase in OPP. We believe that the negative thermal expansion behavior associated with excess oxygen atoms is the primary responsibility for the change in structural properties. & 2010 Elsevier B.V. All rights reserved.
Keywords: B1. CuAlO2 films A1. X-ray diffraction A1. structural properties A1. oxygen concentration
1. Introduction As a p-type wide direct bandgap oxide semiconductor, delafossite CuAlO2 could be applied to fabricate transparent oxide optoelectronic devices, such as transparent p–n diodes and transparent thin-film transistors [1], and it has become a multifunctional semiconductor due to the discovery of other properties, such as gas sensitive property of ozone [2], field emission [3], and photocatalysis [4]. Delafossite CuAlO2 has a hexagonal layered crystal structure. There are three structural fragments in CuAlO2: O–Cu–O fragments along the c axis, AlO6 layers, and hexagonal Cu layers parallel to the ab plane. It is found that the electrical and optical properties of CuAlO2 films heavily depend on their structural characterization and oxygen concentration. Via the post-annealing performed in oxygen ambient, Yanagi et al. [5] observed a significant enhancement in the transmittance of CuAlO2 film in the visible region and improved two orders of magnitude in the hole carrier concentration. Wang and Gong [6] obtained a remarkable increase in the conductivity of CuAl O films after annealing in air. In our previous work [7], we also found that adequate oxygen content could improve the transparency and the electrical conductivity of sputtered Cu Al O films but excess oxygen resulted in an increase in electrical resistivity. However, the physical mechanism of the structural change has not been well understood until now, and why excess oxygen in CuAlO2 leads to deteriorative performances, especially the conductivity. Therefore, it is necessary to investigate
the structural changes and the electrical deterioration of CuAlO2 films correlated with oxygen concentration. In the current work, oxygen concentration in CuAlO2 films is modulated by oxygen partial pressure (OPP) during sputtering process. We study the structure and microstructure changes in annealed CuAlO2 films and analyze the physical mechanism.
2. Experimental procedure CuAlO2 films were grown on quartz substrates using radio frequency (r.f.) magnetron sputtering. The deposition parameters were summarized in Table 1. The sputtering procedure was reported elsewhere [8]. Compared with the previous one, only difference was OPP parameter in the sputtering gas (O2 +Ar), which was adjusted by a mass flow meter and varied from 0% to 60%. For the prepared samples post-annealing was carried out at 900 1C for 5 h in N2 ambient in a tube furnace with a sealing Al2O3 pipe and a Pt–Rh electric thermocouple [8]. The structure of CuAlO2 films was identified by a BRUKER-AXS D8 X-ray diffractometer (Cu Ka, l ¼0.154056 nm) and Raman scattering spectrometer with a confocal microscope (Horiba Jobin-Yvon Labram HR800, the laser wavelength of 325 nm). An atomic force microscope instrument (P47-PRO) was used to observe the surface morphology of CuAlO2 films. Electrical conductivity was determined using the measurement system assembled by Agilent E5273 (Denver, CO) and Lakeshore 340 (Westerville, OH).
3. Results and discussion n
Corresponding author at: Key Laboratory for Magnetism and Magnetic Materials of Ministry of Education, Lanzhou University, Lanzhou 730000, PR China. E-mail address: [email protected] (W. Lan). 0022-0248/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2010.11.172
Fig. 1 shows the X-ray diffraction (XRD) fine patterns of annealed CuAlO2 films deposited at different OPPs of 0%, 10%,
W. Lan et al. / Journal of Crystal Growth 314 (2011) 370–373
Table 1 Summary of sputtering parameters. Parameter
Value
Sputtering power (W) Work pressure (Pa) Substrate temperature (1C) Electrode distance (mm) Sputtering gasses (%) Deposition time (h)
100 1 500 40 O2/(O2 + Ar)¼0, 10, 20, 30, 40, 60 2
371
of diffraction peaks first increases with the OPP increment and then gradually decreases up to the complete disappearance. The film with the optimal (0 0 6) diffraction peak is obtained at 20% OPP, which indicates that adequate oxygen concentration can obviously improve the structure of CuAlO2 films, but excess oxygen will lead to an amorphous behavior. The similar features are also demonstrated in the results of grazing incidence scan mode (Fig. 1(b)). Note that (0 0 3) diffraction peak turns into the strongest one, and other diffraction peaks, including (1 0 1), (0 1 2), and (1 0 4), become pretty obvious. It is understandable that different scattering planes were involved in the diffraction actions when two scan modes were used. To demonstrate the effect of OPP parameter on the preferential growth of CuAlO2 films, the lattice constants a and c of delafossitestructure CuAlO2 can be calculated, according to Bragg’s law: 2d sin y ¼ nl
ð1Þ
In the case of the first order approximation n ¼1, the law can be deduced to 2
sin y ¼
l2 4 h2 þ hkþ k2 4
3
a2
þ
l2 c2
ð2Þ
As all the diffraction peaks are along the vertical direction of (0 0 1) crystal plane collected by 2y/y scan mode for CuAlO2 films, we can calculate only the lattice constant c. In accordance with the (0 0 6) diffraction peak at 2y E31.67, the lattice constant c is calculated by c¼
3l sin y
ð3Þ
Fig. 2 shows the lattice constant c of CuAlO2 films as a function of OPP. It can be seen that the c value reduces by about 0.011 A˚ from ˚ corresponding to the 0% OPP and the 20% 16.9381 to 16.9268 A, OPP, respectively. Above 20% OPP, the c value and the (0 0 6) diffraction peak basically arrive at a stabilization, which indicates that oxygen concentration in CuAlO2 films increases with the increment of the OPP, and surplus oxygen atoms force the lattice contraction along the c axis vertical to the substrate surface. On the other hand, the grain size corresponding to (0 0 6) diffraction peak is calculated by means of Scherrer’s equation: D¼
0:9l b cos y
ð4Þ
In Fig. 2, it is found that the largest grain size, around 107 nm, is obtained in the film sputtered at 20% OPP, which implies that oxygen richness is beneficial to increase the grain size to improve the structural properties of CuAlO2 films. 110
20%, 30%, 40%, and 60%. XRD measurement was carried out using 2y/y scan mode (Fig. 1(a)) and grazing incidence scan mode (Fig. 1(b)), with the scan step of 0.011 and scan time of 0.3 s/step. The scan procedure was run two times continuously. As seen from Fig. 1(a), CuAlO2 films exhibit the preferential growth along [0 0 1] orientation, which is perpendicular to the substrate surface. All the diffraction peaks on the (0 0 1) crystal plane of delafossite CuAlO2 are presented simultaneously (JCP2.2CA No. 35-1401), in which (0 0 6) diffraction peak is the strongest. It suggests that the surface energy of (0 0 1) crystal plane might be the lowest in delafossitestructure CuAlO2 crystal. It is worth pointing out that the intensity
105 100
16.935
95 the lattice constant c grain size
16.930
90
Grain Size (nm)
Fig. 1. XRD fine patterns of annealed CuAlO2 films sputtered at different OPPs (0%, 10%, 20%, 30%, 40%, and 60%): (a) 2y/y scan mode and (b) grazing incidence scan mode.
The lattice constant c (Å)
16.940
85 16.925
0
10
20
30
40
80
O2 partial pressure (%) Fig. 2. Lattice constant c and the calculated grain size for annealed CuAlO2 films.
372
W. Lan et al. / Journal of Crystal Growth 314 (2011) 370–373
Delafossite-structure CuAlO2, with four atoms in the unit cell, belongs to R3¯m space group and gives rise to 12 normal modes. Group-theoretical analysis decomposes a general mode at the Brillouin zone center as G ¼ A1g +Eg + 3A2u + 3Eu, among which, the A1g + Eg modes are Raman active [9]. Fig. 3 illustrates typical Raman scattering spectra of annealed CuAlO2 films deposited at different OPPs (0%, 20%, 40%, and 60%). It is obvious that two Raman vibration modes, A1g and Eg, both are present in the annealed CuAlO2 films except the 60% OPP. Moreover, the change in their signal intensity is consistent with the above-depicted XRD results. Note that the central position of A1g and Eg Raman peaks also shifts to high frequency at about 5 cm 1. These cases reveal that the peak shift in the XRD and Raman results is not a systematic error. We know that
Fig. 3. Typical Raman spectra of annealed CuAlO2 films.
A mode implies the movement in the direction of Cu–O bonds (along the hexagonal c axis), whereas double degenerate E mode describes vibration in the perpendicular direction. Annealed CuAlO2 films have a preferred (0 0 6) growth orientation, indicating that the c axis of the grains becomes uniformly perpendicular to the substrate surface. The shifts in A1g and Eg Raman peaks point out that the bonds are constricted in the parallel and vertical orientations of the substrate surface, which might be due to the interior stress in the films. In addition, we observe two weak Raman peaks located at 610 and 656 cm 1, which might be attributed to B5u and A2g high-order Raman active modes [10]. Based on the XRD and Raman results, we know that suitable oxygen richness could apparently improve the structure of CuAlO2 films. However, the crystallized behavior of CuAlO2 films begins to degenerate when the OPP is above 20%. In order to clarify the mechanism of structural deterioration, atomic force microscope (AFM) was used to observe the morphology of annealed CuAlO2 films. Fig. 4 demonstrates typical AFM images of annealed CuAlO2 films prepared at 10%, 20%, 40%, and 60% OPP. CuAlO2 films grown below 20% OPP are with quite smooth surface. Above 20% OPP, some micro-caves begin to appear in the film surface. With the increase in OPP, these micro-caves become gradually larger and deeper. Up to 60% OPP, the micro-caves are developed with 3 mm diameter and 200 nm depth. As far as thin CuAlO2 films (approximately 310 nm in the thickness) are concerned, the influence of the micro-caves distributed in the surface is very obvious on their structure. They eventually lead to the structure transition from the crystalline form to the amorphous one. It is expected that the structural change could be further confirmed by electrical properties. Fig. 5 shows I–V characterizations and room temperature electrical resistivity of annealed CuAlO2 films grown at different OPPs. Ag electrodes were sputtered and then in-situ alloying was performed in vacuum at 400 1C. It is obvious that the linear dependence is obtained in the inset, which indicates that ohmic
Fig. 4. AFM images of annealed CuAlO2 films: (a) 10%, (b) 20%, (c) 40%, and (d) 60% OPP.
W. Lan et al. / Journal of Crystal Growth 314 (2011) 370–373
373
O–Cu–O units along the c axis. This can be confirmed by the experimental results of XRD and Raman spectroscopy. It is well known that the shrinkage of symmetrical unit would reinforce the negative TEB [13]. Therefore, the more the excess of oxygen atoms, the more obvious the negative TEB in CuAlO2, which leads to the stronger stress in the films. In order to relax the internal stress, some micro-caves begin to appear in the film surface when surplus oxygen atoms arrive at a certain concentration. Moreover, these micro-caves become gradually larger and deeper with the increase of excess oxygen.
4. Conclusions
Fig. 5. I–V characterizations and room temperature electrical resistivity of annealed CuAlO2 films.
contacts are accomplished between Ag electrodes and annealed CuAlO2 films. The minimum resistivity is estimated to be 41.8 O cm for the film deposited at 20% OPP. Compared with the 0% OPP, the resistivity of the 20% OPP decreases 4 orders of magnitude, suggesting that the oxygen concentration could evidently adjust the electrical properties of p-type CuAlO2 films. As expected, structural deterioration results in the soaring enhancement of electrical resistivity. Combining the results of XRD, Raman, and AFM, it is concluded that the optimal oxygen concentration can be achieved at 20% OPP for sputtering CuAlO2 films. Delafossite CuAlO2 has a layered crystal structure, in particular O–Cu–O dumbbell units along the c axis. It has been found that CuAlO2 represents an anisotropic thermal expansion behavior (TEB). Ishiguro et al. [11] found that the thermal expansion coefficient (TEC) at a axis in CuAlO2 was 11.0 10 6 K 1, which was about three times that at the c axis. Recently, Li et al. [12] further proved that CuAlO2 exhibits negative TEB along the c axis (TEC, –3.2 10 6 K 1) in the below 500 K condition, which was attributed to the symmetrical O–Cu–O dumbbell units arrayed in the c axis. Above 500 K, the competition between Al–O bonds with the positive TEB and O–Cu–O units with the negative TEB resulted in an overall positive TEB along the c axis. During the postannealing process, the atoms in CuAlO2 films preferentially rearrange on the ab plane with the lowest surface energy, which is parallel to the substrate surface. The anisotropic TEB leads to a difference of expansion behavior in the lattice dynamics. After post-annealing, TEC at the c axis undergoes a transformation from the positive to the negative, which will bring about the internal stress in CuAlO2 films. For the oxygen-rich films, surplus oxygen atoms should be located at interstitial positions in the crystal lattice, such as the interstices in the Al–O octahedrons and those between the O–Cu–O units. These interstitial oxygen atoms will force CuAlO2 lattice to distort itself, which causes the contraction of
CuAlO2 films were sputtered at different OPPs and performed the post-annealing. The annealed films are grown along the preferred (0 0 1) orientation. The optimal crystalline film is deposited at 20% OPP, which has the smallest electrical resistivity. Above 20% OPP, the crystalline behavior of CuAlO2 films begins to gradually degenerate up to the amorphous state at 60% OPP. The internal stress generated by the anisotropic TEB is exacerbated due to the TEB transformation along the c axis from positive to negative during the annealing process. The negative TEB of CuAlO2 films could be strengthened by surplus oxygen atoms located at the interstitial position. To release the internal stress, some microcaves formed in the film surface become larger and deeper with increase in the OPP. The fact suggests that it is very important for appropriate oxygen concentration to improve structural properties of annealed CuAlO2 films.
Acknowledgments The authors would like to acknowledge the financial support by the National Natural Science Foundation of China (no. 50802037) and the Fundamental Research Funds for the Central Universities (no. lzujbky-2009-56). References [1] H. Ohta, H. Hosono, Mater. Today 6 (2004) 42. [2] X.G. Zheng, K. Taniguchi, A. Takahashi, Y. Liu, C.N. Xu, Appl. Phys. Lett. 85 (2004) 1728. [3] A.N. Banerjee, R. Maity, P.K. Ghosh, K.K. Chattopadhyay, Thin Solid Films 474 (2005) 261. [4] J.R. Smith, T.H.V. Steenkiste, X.G. Wang, Phys. Rev. B 79 (2009) 041403. [5] H. Yanagi, S. Inoue, K. Ueda, H. Kawazoe, H. Hosono, N. Hamada, J. Appl. Phys. 88 (2000) 4159. [6] Y. Wang, H. Gong, Adv. Mater. CVD 6 (2000) 285. [7] W. Lan, M. Zhang, G.B. Dong, P.M. Dong, Y.Y. Wang, H. Yan, Mater. Sci. Eng. B 139 (2007) 155. [8] W. Lan, M. Zhang, G.B. Dong, Y.Y. Wang, H. Yan, J. Mater. Res. 22 (2007) 3338. [9] J. Pellicer-Porres, D. Martı´nez-Garcı´a, A. Segura, P. Rodrı´guez-Herna´ndez, ˜ oz, J.C. Chervin, N. Garro, D. Kim, Phys. Rev. B 74 (2006) 184301. A. Mun [10] G.G. Siu, M.J. Stokes, Y.L. Liu, Phys. Rev. B 59 (1999) 3173. [11] T. Ishiguro, N. Ishizawa, N. Mizutani, M. Kato, J. Solid State Chem. 41 (1982) 132. [12] J. Li, A. Sleight, C. Jones, B. Toby, J. Solid State Chem. 178 (2005) 285. [13] A.W. Sleight, Inorg. Chem. 37 (1998) 2855.
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Role of oxygen in structural properties of annealed CuAlO2 films W. Lan a,b,n, J.Q. Pan a, C.Q. Zhu a, G.Q. Wang a, Q. Su a,b, X.Q. Liu a,b, E.Q. Xie a,b, H. Yan c a b c
Key Laboratory for Magnetism and Magnetic Materials of Ministry of Education, Lanzhou University, Lanzhou 730000, PR China School of Physical Science and Technology, Lanzhou University, Lanzhou 730000, PR China The College of Materials Science and Engineering, Beijing University of Technology, Beijing 100022, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 September 2010 Accepted 11 November 2010 Communicated by D.P. Norton Available online 10 December 2010
CuAlO2 films were sputtered on quartz substrates at different oxygen partial pressures (OPP) and carried out the annealing at 900 1C for 5 h in N2 ambient. The structural properties of these films have been studied in detail by X-ray diffraction, Raman spectroscopy, and atomic force microscopy. Annealed CuAlO2 films are grown along the (0 0 1) preferential orientation. The film deposited at 20% OPP demonstrates the excellent crystalline behavior and the smallest electrical resistivity (41.8 O cm). At higher OPP, the crystalline behavior begins to degenerate up to the amorphous state at 60% OPP, and some micro-caves presented in the film surface become larger and deeper with the increase in OPP. We believe that the negative thermal expansion behavior associated with excess oxygen atoms is the primary responsibility for the change in structural properties. & 2010 Elsevier B.V. All rights reserved.
Keywords: B1. CuAlO2 films A1. X-ray diffraction A1. structural properties A1. oxygen concentration
1. Introduction As a p-type wide direct bandgap oxide semiconductor, delafossite CuAlO2 could be applied to fabricate transparent oxide optoelectronic devices, such as transparent p–n diodes and transparent thin-film transistors [1], and it has become a multifunctional semiconductor due to the discovery of other properties, such as gas sensitive property of ozone [2], field emission [3], and photocatalysis [4]. Delafossite CuAlO2 has a hexagonal layered crystal structure. There are three structural fragments in CuAlO2: O–Cu–O fragments along the c axis, AlO6 layers, and hexagonal Cu layers parallel to the ab plane. It is found that the electrical and optical properties of CuAlO2 films heavily depend on their structural characterization and oxygen concentration. Via the post-annealing performed in oxygen ambient, Yanagi et al. [5] observed a significant enhancement in the transmittance of CuAlO2 film in the visible region and improved two orders of magnitude in the hole carrier concentration. Wang and Gong [6] obtained a remarkable increase in the conductivity of CuAl O films after annealing in air. In our previous work [7], we also found that adequate oxygen content could improve the transparency and the electrical conductivity of sputtered Cu Al O films but excess oxygen resulted in an increase in electrical resistivity. However, the physical mechanism of the structural change has not been well understood until now, and why excess oxygen in CuAlO2 leads to deteriorative performances, especially the conductivity. Therefore, it is necessary to investigate
the structural changes and the electrical deterioration of CuAlO2 films correlated with oxygen concentration. In the current work, oxygen concentration in CuAlO2 films is modulated by oxygen partial pressure (OPP) during sputtering process. We study the structure and microstructure changes in annealed CuAlO2 films and analyze the physical mechanism.
2. Experimental procedure CuAlO2 films were grown on quartz substrates using radio frequency (r.f.) magnetron sputtering. The deposition parameters were summarized in Table 1. The sputtering procedure was reported elsewhere [8]. Compared with the previous one, only difference was OPP parameter in the sputtering gas (O2 +Ar), which was adjusted by a mass flow meter and varied from 0% to 60%. For the prepared samples post-annealing was carried out at 900 1C for 5 h in N2 ambient in a tube furnace with a sealing Al2O3 pipe and a Pt–Rh electric thermocouple [8]. The structure of CuAlO2 films was identified by a BRUKER-AXS D8 X-ray diffractometer (Cu Ka, l ¼0.154056 nm) and Raman scattering spectrometer with a confocal microscope (Horiba Jobin-Yvon Labram HR800, the laser wavelength of 325 nm). An atomic force microscope instrument (P47-PRO) was used to observe the surface morphology of CuAlO2 films. Electrical conductivity was determined using the measurement system assembled by Agilent E5273 (Denver, CO) and Lakeshore 340 (Westerville, OH).
3. Results and discussion n
Corresponding author at: Key Laboratory for Magnetism and Magnetic Materials of Ministry of Education, Lanzhou University, Lanzhou 730000, PR China. E-mail address: [email protected] (W. Lan). 0022-0248/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2010.11.172
Fig. 1 shows the X-ray diffraction (XRD) fine patterns of annealed CuAlO2 films deposited at different OPPs of 0%, 10%,
W. Lan et al. / Journal of Crystal Growth 314 (2011) 370–373
Table 1 Summary of sputtering parameters. Parameter
Value
Sputtering power (W) Work pressure (Pa) Substrate temperature (1C) Electrode distance (mm) Sputtering gasses (%) Deposition time (h)
100 1 500 40 O2/(O2 + Ar)¼0, 10, 20, 30, 40, 60 2
371
of diffraction peaks first increases with the OPP increment and then gradually decreases up to the complete disappearance. The film with the optimal (0 0 6) diffraction peak is obtained at 20% OPP, which indicates that adequate oxygen concentration can obviously improve the structure of CuAlO2 films, but excess oxygen will lead to an amorphous behavior. The similar features are also demonstrated in the results of grazing incidence scan mode (Fig. 1(b)). Note that (0 0 3) diffraction peak turns into the strongest one, and other diffraction peaks, including (1 0 1), (0 1 2), and (1 0 4), become pretty obvious. It is understandable that different scattering planes were involved in the diffraction actions when two scan modes were used. To demonstrate the effect of OPP parameter on the preferential growth of CuAlO2 films, the lattice constants a and c of delafossitestructure CuAlO2 can be calculated, according to Bragg’s law: 2d sin y ¼ nl
ð1Þ
In the case of the first order approximation n ¼1, the law can be deduced to 2
sin y ¼
l2 4 h2 þ hkþ k2 4
3
a2
þ
l2 c2
ð2Þ
As all the diffraction peaks are along the vertical direction of (0 0 1) crystal plane collected by 2y/y scan mode for CuAlO2 films, we can calculate only the lattice constant c. In accordance with the (0 0 6) diffraction peak at 2y E31.67, the lattice constant c is calculated by c¼
3l sin y
ð3Þ
Fig. 2 shows the lattice constant c of CuAlO2 films as a function of OPP. It can be seen that the c value reduces by about 0.011 A˚ from ˚ corresponding to the 0% OPP and the 20% 16.9381 to 16.9268 A, OPP, respectively. Above 20% OPP, the c value and the (0 0 6) diffraction peak basically arrive at a stabilization, which indicates that oxygen concentration in CuAlO2 films increases with the increment of the OPP, and surplus oxygen atoms force the lattice contraction along the c axis vertical to the substrate surface. On the other hand, the grain size corresponding to (0 0 6) diffraction peak is calculated by means of Scherrer’s equation: D¼
0:9l b cos y
ð4Þ
In Fig. 2, it is found that the largest grain size, around 107 nm, is obtained in the film sputtered at 20% OPP, which implies that oxygen richness is beneficial to increase the grain size to improve the structural properties of CuAlO2 films. 110
20%, 30%, 40%, and 60%. XRD measurement was carried out using 2y/y scan mode (Fig. 1(a)) and grazing incidence scan mode (Fig. 1(b)), with the scan step of 0.011 and scan time of 0.3 s/step. The scan procedure was run two times continuously. As seen from Fig. 1(a), CuAlO2 films exhibit the preferential growth along [0 0 1] orientation, which is perpendicular to the substrate surface. All the diffraction peaks on the (0 0 1) crystal plane of delafossite CuAlO2 are presented simultaneously (JCP2.2CA No. 35-1401), in which (0 0 6) diffraction peak is the strongest. It suggests that the surface energy of (0 0 1) crystal plane might be the lowest in delafossitestructure CuAlO2 crystal. It is worth pointing out that the intensity
105 100
16.935
95 the lattice constant c grain size
16.930
90
Grain Size (nm)
Fig. 1. XRD fine patterns of annealed CuAlO2 films sputtered at different OPPs (0%, 10%, 20%, 30%, 40%, and 60%): (a) 2y/y scan mode and (b) grazing incidence scan mode.
The lattice constant c (Å)
16.940
85 16.925
0
10
20
30
40
80
O2 partial pressure (%) Fig. 2. Lattice constant c and the calculated grain size for annealed CuAlO2 films.
372
W. Lan et al. / Journal of Crystal Growth 314 (2011) 370–373
Delafossite-structure CuAlO2, with four atoms in the unit cell, belongs to R3¯m space group and gives rise to 12 normal modes. Group-theoretical analysis decomposes a general mode at the Brillouin zone center as G ¼ A1g +Eg + 3A2u + 3Eu, among which, the A1g + Eg modes are Raman active [9]. Fig. 3 illustrates typical Raman scattering spectra of annealed CuAlO2 films deposited at different OPPs (0%, 20%, 40%, and 60%). It is obvious that two Raman vibration modes, A1g and Eg, both are present in the annealed CuAlO2 films except the 60% OPP. Moreover, the change in their signal intensity is consistent with the above-depicted XRD results. Note that the central position of A1g and Eg Raman peaks also shifts to high frequency at about 5 cm 1. These cases reveal that the peak shift in the XRD and Raman results is not a systematic error. We know that
Fig. 3. Typical Raman spectra of annealed CuAlO2 films.
A mode implies the movement in the direction of Cu–O bonds (along the hexagonal c axis), whereas double degenerate E mode describes vibration in the perpendicular direction. Annealed CuAlO2 films have a preferred (0 0 6) growth orientation, indicating that the c axis of the grains becomes uniformly perpendicular to the substrate surface. The shifts in A1g and Eg Raman peaks point out that the bonds are constricted in the parallel and vertical orientations of the substrate surface, which might be due to the interior stress in the films. In addition, we observe two weak Raman peaks located at 610 and 656 cm 1, which might be attributed to B5u and A2g high-order Raman active modes [10]. Based on the XRD and Raman results, we know that suitable oxygen richness could apparently improve the structure of CuAlO2 films. However, the crystallized behavior of CuAlO2 films begins to degenerate when the OPP is above 20%. In order to clarify the mechanism of structural deterioration, atomic force microscope (AFM) was used to observe the morphology of annealed CuAlO2 films. Fig. 4 demonstrates typical AFM images of annealed CuAlO2 films prepared at 10%, 20%, 40%, and 60% OPP. CuAlO2 films grown below 20% OPP are with quite smooth surface. Above 20% OPP, some micro-caves begin to appear in the film surface. With the increase in OPP, these micro-caves become gradually larger and deeper. Up to 60% OPP, the micro-caves are developed with 3 mm diameter and 200 nm depth. As far as thin CuAlO2 films (approximately 310 nm in the thickness) are concerned, the influence of the micro-caves distributed in the surface is very obvious on their structure. They eventually lead to the structure transition from the crystalline form to the amorphous one. It is expected that the structural change could be further confirmed by electrical properties. Fig. 5 shows I–V characterizations and room temperature electrical resistivity of annealed CuAlO2 films grown at different OPPs. Ag electrodes were sputtered and then in-situ alloying was performed in vacuum at 400 1C. It is obvious that the linear dependence is obtained in the inset, which indicates that ohmic
Fig. 4. AFM images of annealed CuAlO2 films: (a) 10%, (b) 20%, (c) 40%, and (d) 60% OPP.
W. Lan et al. / Journal of Crystal Growth 314 (2011) 370–373
373
O–Cu–O units along the c axis. This can be confirmed by the experimental results of XRD and Raman spectroscopy. It is well known that the shrinkage of symmetrical unit would reinforce the negative TEB [13]. Therefore, the more the excess of oxygen atoms, the more obvious the negative TEB in CuAlO2, which leads to the stronger stress in the films. In order to relax the internal stress, some micro-caves begin to appear in the film surface when surplus oxygen atoms arrive at a certain concentration. Moreover, these micro-caves become gradually larger and deeper with the increase of excess oxygen.
4. Conclusions
Fig. 5. I–V characterizations and room temperature electrical resistivity of annealed CuAlO2 films.
contacts are accomplished between Ag electrodes and annealed CuAlO2 films. The minimum resistivity is estimated to be 41.8 O cm for the film deposited at 20% OPP. Compared with the 0% OPP, the resistivity of the 20% OPP decreases 4 orders of magnitude, suggesting that the oxygen concentration could evidently adjust the electrical properties of p-type CuAlO2 films. As expected, structural deterioration results in the soaring enhancement of electrical resistivity. Combining the results of XRD, Raman, and AFM, it is concluded that the optimal oxygen concentration can be achieved at 20% OPP for sputtering CuAlO2 films. Delafossite CuAlO2 has a layered crystal structure, in particular O–Cu–O dumbbell units along the c axis. It has been found that CuAlO2 represents an anisotropic thermal expansion behavior (TEB). Ishiguro et al. [11] found that the thermal expansion coefficient (TEC) at a axis in CuAlO2 was 11.0 10 6 K 1, which was about three times that at the c axis. Recently, Li et al. [12] further proved that CuAlO2 exhibits negative TEB along the c axis (TEC, –3.2 10 6 K 1) in the below 500 K condition, which was attributed to the symmetrical O–Cu–O dumbbell units arrayed in the c axis. Above 500 K, the competition between Al–O bonds with the positive TEB and O–Cu–O units with the negative TEB resulted in an overall positive TEB along the c axis. During the postannealing process, the atoms in CuAlO2 films preferentially rearrange on the ab plane with the lowest surface energy, which is parallel to the substrate surface. The anisotropic TEB leads to a difference of expansion behavior in the lattice dynamics. After post-annealing, TEC at the c axis undergoes a transformation from the positive to the negative, which will bring about the internal stress in CuAlO2 films. For the oxygen-rich films, surplus oxygen atoms should be located at interstitial positions in the crystal lattice, such as the interstices in the Al–O octahedrons and those between the O–Cu–O units. These interstitial oxygen atoms will force CuAlO2 lattice to distort itself, which causes the contraction of
CuAlO2 films were sputtered at different OPPs and performed the post-annealing. The annealed films are grown along the preferred (0 0 1) orientation. The optimal crystalline film is deposited at 20% OPP, which has the smallest electrical resistivity. Above 20% OPP, the crystalline behavior of CuAlO2 films begins to gradually degenerate up to the amorphous state at 60% OPP. The internal stress generated by the anisotropic TEB is exacerbated due to the TEB transformation along the c axis from positive to negative during the annealing process. The negative TEB of CuAlO2 films could be strengthened by surplus oxygen atoms located at the interstitial position. To release the internal stress, some microcaves formed in the film surface become larger and deeper with increase in the OPP. The fact suggests that it is very important for appropriate oxygen concentration to improve structural properties of annealed CuAlO2 films.
Acknowledgments The authors would like to acknowledge the financial support by the National Natural Science Foundation of China (no. 50802037) and the Fundamental Research Funds for the Central Universities (no. lzujbky-2009-56). References [1] H. Ohta, H. Hosono, Mater. Today 6 (2004) 42. [2] X.G. Zheng, K. Taniguchi, A. Takahashi, Y. Liu, C.N. Xu, Appl. Phys. Lett. 85 (2004) 1728. [3] A.N. Banerjee, R. Maity, P.K. Ghosh, K.K. Chattopadhyay, Thin Solid Films 474 (2005) 261. [4] J.R. Smith, T.H.V. Steenkiste, X.G. Wang, Phys. Rev. B 79 (2009) 041403. [5] H. Yanagi, S. Inoue, K. Ueda, H. Kawazoe, H. Hosono, N. Hamada, J. Appl. Phys. 88 (2000) 4159. [6] Y. Wang, H. Gong, Adv. Mater. CVD 6 (2000) 285. [7] W. Lan, M. Zhang, G.B. Dong, P.M. Dong, Y.Y. Wang, H. Yan, Mater. Sci. Eng. B 139 (2007) 155. [8] W. Lan, M. Zhang, G.B. Dong, Y.Y. Wang, H. Yan, J. Mater. Res. 22 (2007) 3338. [9] J. Pellicer-Porres, D. Martı´nez-Garcı´a, A. Segura, P. Rodrı´guez-Herna´ndez, ˜ oz, J.C. Chervin, N. Garro, D. Kim, Phys. Rev. B 74 (2006) 184301. A. Mun [10] G.G. Siu, M.J. Stokes, Y.L. Liu, Phys. Rev. B 59 (1999) 3173. [11] T. Ishiguro, N. Ishizawa, N. Mizutani, M. Kato, J. Solid State Chem. 41 (1982) 132. [12] J. Li, A. Sleight, C. Jones, B. Toby, J. Solid State Chem. 178 (2005) 285. [13] A.W. Sleight, Inorg. Chem. 37 (1998) 2855.