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Effect of post-annealing treatments on the properties of Zn1−xCdxO films on glass substrates
Materials Science and Engineering B 111 (2004) 9–13
Effect of post-annealing treatments on the properties of Zn1−x Cdx O films on glass substrates D.W. Ma, Z.Z. Ye∗ , J.Y. Huang, L.P. Zhu, B.H. Zhao, J.H. He State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou 310027, PR China Received 7 March 2003; accepted 17 December 2003
Abstract Ternary Zn1−x Cdx O (0 ≤ x ≤ 0.53) alloy crystalline films with highly (0 0 2)-preferred orientations have been prepared on glass substrates by using the dc reactive magnetron sputtering method and exposed to annealing treatments in O2 ambient. The crystal quality of the annealed Zn1−x Cdx O films improves much. As a case of the sample with x = 0.53, the diffraction peaks in intensity attain maximum values at the annealing temperature of 500 ◦ C. But for the samples with x ≥ 0.53, the CdO phase separation and re-evaporation from the films are very serious. So for the Zn1−x Cdx O films with higher Cd content, to prevent the segregation of CdO, the annealing temperature should be less than 400 ◦ C, and the annealing time should be less than 1 h. © 2004 Elsevier B.V. All rights reserved. Keywords: Zinc oxide; Film deposition; Annealing; Crystal symmetry; Optical properties; Segregation
1. Introduction The increased interest on short-wavelength optoelectronic devices, such as blue and ultraviolet (UV) light emitters and detectors has stimulated investigations on II–VI semiconductor oxide ZnO and its ternary alloys [1]. As an alternative to GaN, ZnO is of hexagonal wurtzite-type structure with a direct wide band gap of 3.3 eV and an excitonic binding energy of ∼60 meV, which is much larger than that of GaN (25 meV) or the thermal energy at room temperature (25 meV) [2,3]. Moreover, ZnO is a commercially available material with advantages of low cost, non-toxicity and high chemical stability [4,5]. To achieve applicable ZnO film based optoelectronic devices, considerable experimental investigations have been focused on the preparation of p-type ZnO film and its band gap engineering by various doping and alloying methods [6,7]. We concentrate on the latter in this paper. CdO is of a cubic-type structure with a band gap of 2.3 eV, theoretically by alloying with CdO, the band gap of ZnO can be modulated, and the luminescence of ZnCdO alloys can cover green, blue to ultraviolet light spectra. More important, ∗
Corresponding author. Tel.: +86-571-87953139; fax: +86-571-87952625. E-mail address: [email protected] (Z.Z. Ye). 0921-5107/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2003.12.007
the substitution of proper amount of Zn by Cd is not expected to induce a significant change in crystal structure and lattice constant due to their similar ionic radii, therefore the ternary ZnCdO alloy system can construct efficient heterojunction with ZnO, which is a key technique in optoelectronic device applications [7–10]. Several techniques were attempted to deposit Zn1−x Cdx O alloy films, such as sol–gel [11], spray pyrolysis [12,13], MBE [14] and PLD [15] methods. But the prepared Zn1−x Cdx O films were not satisfying due to the coexistence of multiphase or polycrystalline state, and the reported upper limit of Cd substitution was lower than 25 at.% [11–15]. By the dc reactive magnetron sputtering method, we have successfully deposited completely (0 0 2)-oriented ternary Zn1−x Cdx O (0 ≤ x ≤ 0.53) alloy films on Si [8] and glass substrates. In this paper, we will report the structural, optical and electrical properties of as-deposited and annealed Zn1−x Cdx O (0 ≤ x ≤ 1.0) films on glass substrates.
2. Experimental Zn1−x Cdx O films were deposited on glass substrates using the dc reactive magnetron sputtering method. The sputtering targets with different ratios of Zn to Cd were prepared using Zn and Cd metals with high purities of 99.999%. The glass
D.W. Ma et al. / Materials Science and Engineering B 111 (2004) 9–13
substrates were rinsed in acetone accompanied by supersonic wave. The deposition chamber was evacuated to a base pressure of 10−3 Pa by a combination of mechanical-pump and oil-diffusion-pump. High purity (99.995% pure) argon and oxygen were used as sputtering and reactive gases. The specific flux of argon to oxygen was controlled at a ratio of Ar:O2 ∼ 1:4, and the total pressure maintained at 4.0 Pa. The target to substrate distance was fixed at 60 mm. The sputtering current and sputtering voltage were 200 mA and 200 V, respectively. The deposition time was kept 30 min for all samples. Before the experiment of deposition, the Zn–Cd target was presputtered in argon–oxygen atmosphere for about 10 min in order to remove the surface oxide from the target. The post-annealing treatments of the samples were performed in a quartz tube under O2 ambient. The crystal structure of the samples was investigated by the method of X-ray diffraction (XRD), where a Cu K␣ (λ = 0.154056 nm) source was used. The transmittance measurements were performed by a Cary-100-Bio UV-Vis spectrophotometer. The electrical properties were studied by van der Pauw Hall measurements. The film thicknesses were in the range of 300–400 nm as determined by using ␣-step measurements. The film compositions were determined by X-ray photoemission spectroscopy (XPS) measurements, where a Omicron EAC2000-125 spherical deflector analyzer and a Mg K␣ X-ray (1253.6 eV) were used, as will be discussed in detail in other related papers, the compositions × mentioned below all denote the measured film compositions.
3. Results and discussion Fig. 1 shows the XRD spectra of as-deposited Zn1−x Cdx O (0 ≤ x ≤ 1.0) films on glass substrates at temperatures of 375 ◦ C, up to x = 0.53, the spectra nearly exhibit only Zn1−x Cdx O(0 0 2) peaks at 34.51–33.57◦ , depending on Cd contents, which indicates that single-phase Zn1−x Cdx O (0 ≤
x ≤ 0.53) films with a structure of hexagonal wurtzite ZnO were grown without any significant formation of a separated CdO phase. However, for the higher Cd content of x = 0.77, a CdO(2 0 0) peak is also observed, suggesting phase segregation between ZnO and CdO. For the pure CdO, only a (2 0 0) peak with a full-width at half-maximum (FWHM) of ∼0.3◦ is observed, indicating the recovery of high crystalline quality in single phase region. It is also observed that with the increasing Cd content, the intensity of the corresponding XRD peaks is increased, especially the (2 0 0) peak from the pure CdO film is much higher in contrast to the (0 0 2) peak from the pure ZnO film, indicating a much better crystallinity for CdO than that for ZnO, which is consistent with the previous reports [11,12]. There is further unequivocal evidence for Cd incorporation in ZnO: the c-axis lattice constants of the Zn1−x Cdx O films almost linearly increase up to 0.5394 nm (x = 0.77) from 0.5183 nm (x = 0), as shown in Fig. 4 (solid line). Considering that the ionic radius of Cd (0.97 Å) is larger than that of Zn (0.74 Å), it is undoubted that the substitution of Zn by Cd takes place on the equivalent crystallographic position of Zn in hexagonal wurtzite structure. Fig. 2a displays the XRD patterns of annealed Zn0.47 Cd0.53 O films at different temperatures for 1 h. Fig. 2b shows
Intensity(a.u.)
10
(002)hex
30000 15000 0 30000 15000 0 30000 15000 0 20000 10000 0 20000 10000 0 6000 3000 0 20
700˚C (200)cub
500˚C 400˚C 300˚C as deposited 30
40
50
60
70
2θ (deg)
(200)cub (002)hex
x= 1.0
0.534
x= 0.77
0.532
x= 0.53 x= 0.36 x= 0.20
C-axis constant(nm)
Intensity(a.u.)
(a) 90000 60000 30000 0 6000 3000 0 6000 3000 0 3000 1500 0 3000 1500 0 1200 600 0 20
600˚C
30
40
50 2θ (deg)
60
0.530 0.528 0.526 0.524
x= 0 70
Fig. 1. XRD spectra of as-deposited Zn1−x Cdx O (0 ≤ x ≤ 1.0) films on glass substrates.
(b)
as deposited
300
400
500 T(˚C)
600
700
Fig. 2. (a) XRD patterns of annealed Zn0.47 Cd0.53 O films; (b) c-axis constant of the annealed Zn0.47 Cd0.53 O films as a function of annealing temperature.
400000 200000 0 12000 6000 0 30000 15000 0 18000 9000 0 14000 7000 0 8000 4000 0 20
(200)cub
11
0.540 x= 1.0
(002)hex
x= 0.77 x= 0.53 x= 0.36
C-axis constant (nm)
Intensity(a.u.)
D.W. Ma et al. / Materials Science and Engineering B 111 (2004) 9–13
as deposited annealed
0.535 0.530 0.525 0.520
x= 0.20
0.515 x= 0 30
40
50
60
0.0
0.2
0.4 0.6 Cd content (x)
0.8
70
2θ (deg) Fig. 3. XRD spectra of annealed Zn1−x Cdx O (0 ≤ x ≤ 1.0) films at 500 ◦ C.
Fig. 4. Cd content dependence of the c-axis constant of as deposited and annealed Zn1−x Cdx O (0 ≤ x ≤ 0.77) films.
Transmittance(%)
100 80 60
x=0 x=0.20 x=0.36 x=0.53 x=0.77 x=1.0
40 20 0
400
500
600 700 Wavelength(nm)
800
900
Fig. 5. Transmittance spectra of as-deposited Zn1−x Cdx O (0 ≤ x ≤ 1.0) films.
which is consistent with the XRD spectra shown in Fig. 3. As for the pure ZnO film, it is assumed that in-plane tensile stress lies in the as-deposited ZnO film due to a relatively poor crystallinity for ZnO, by the post-annealing treatment the ZnO film is in a comparatively stress-free state, therefore, the c-axis will stretch out according to Poisson’s ratio. Figs. 5 and 6 show the transmittance spectra of the as-deposited and annealed Zn1−x Cdx O (0 ≤ x ≤ 1.0) films, respectively. By the transmittance data, the absorption coefficients (α) of the films can be determined by using the
100
Transmittance(%)
the c-axis constants of the films as a function of the annealing temperatures. When the sample was annealed at 300 ◦ C, the XRD peak in intensity enhances, and the corresponding c-axis constant decreases sharply down to 0.5286 nm from 0.5335 nm (as deposited) likely due to a CdO phase separation from the film, though the (2 0 0) peak corresponding to the CdO phase is hardly observed at the same time; The release of in-plane compressive stress in the film may correlative to the decrease of the c-axis constant also [8,16]. For the annealing temperature up to 400 ◦ C, the CdO(2 0 0) peak can be observed. At 500 ◦ C, the intensity of the XRD peaks attains maximum values, and the segregation between ZnO and CdO phases becomes clearer. However, when the temperature continues to increase, the diffraction peak decreases, and the c-axis constant tends to a smaller value. It is noteworthy that when the annealing temperature increases up to 700 ◦ C, the CdO(2 0 0) peak almost vanishes, which can be ascribed to the Cd re-evaporation in view of a smaller lattice energy for CdO (3806 kJ mol) than that for ZnO (4142 kJ mol). Fig. 3 shows the XRD spectra of annealed Zn1−x Cdx O (0 ≤ x ≤ 1.0) films at 500 ◦ C under O2 ambient for 1 h. Combined with Fig. 1, obviously by post-annealing treatments, in intensity the diffraction peaks of the samples increase much. For the annealed samples with x ≤ 0.36, no distinct segregation of cubic CdO phase is observed from the XRD spectra. However, for the annealed one with x = 0.53, a low (2 0 0) peak corresponding to the CdO phase appears, when the Cd composition increases up to 77 at.%, the separation between the ZnO and CdO phases is very seriously in the annealed film. Fig. 4 (dashed line) shows the Cd content dependences of the c-axis constants of the annealed Zn1−x Cdx O (0 ≤ x ≤ 0.77) films, from x = 0–0.77, the corresponding c-axis lattice constants are 0.5194, 0.5214, 0.5244, 0.5277, 0.5313 nm, respectively, smaller than those of the as-deposited samples except for the pure ZnO film. Moreover, with the increasing Cd content in the film, the decrease of the c-axis constant becomes evident, indicating the CdO phase separation and re-evaporation more seriously,
80 60
x=0 x=0.20 x=0.36 x=0.53 x=0.77 x=1.0
40 20 0
400
500
600 700 Wavelength(nm)
800
900
Fig. 6. Transmittance spectra of annealed Zn1−x Cdx O (0 ≤ x ≤ 1.0) films.
12
D.W. Ma et al. / Materials Science and Engineering B 111 (2004) 9–13
Table 1 Electron concentrations n (per cubic centimeter) of as-deposited and annealed Zn1−x Cdx O (0 ≤ x ≤ 1.0) films x=0
As-deposited films (n) Annealed films (n)
4.31 × 6.45 × 1014 1015
x = 0.20
x = 0.36
x = 0.53
x = 0.77
x = 1.0
6.08 × 8.20 × 1014
1.15 × 5.66 × 1015
2.68 × 2.34 × 1018
7.87 × 3.03 × 1019
2.00 × 1020 8.39 × 1019
1015
Band-gap energy(eV)
3.4 as deposited annealed
3.2 3.0 2.8 2.6 2.4 2.2
0.0
0.2
0.4 0.6 Cd content(x)
0.8
1.0
Fig. 7. Band gap energy of as-grown and annealed Zn1−x Cdx O (0 ≤ x ≤ 1.0) films as a function of Cd content x.
following relation: α = [2.303 × log(1/T)]/d, where T is the transmittance and d is the film thickness. For evaluating the band gaps of the films, the values of α calculated from the relation above are used to plot (αhν)2 versus hν, where ν is the photon energy, by extrapolating the linear portions of the curves to αhν = 0, the direct band gap values of the films can be evaluated [17]. Fig. 7 shows the band gaps of the as-grown and annealed Zn1−x Cdx O (0 ≤ x ≤ 1.0) films as a function of Cd contents. It is noted that when the films were annealed, the band gaps of the samples with x ≤ 0.36 are slightly larger, but for the samples with x ≥ 0.53, the increases of the band gaps become more evident, it sounds reasonable considering that the CdO separation and re-evaporation more seriously in the annealed Zn1−x Cdx O films with higher Cd contents and the smaller band gap of CdO than that of ZnO. According to the previous reports [18–20], other factors, such as the variations of the crystal quality, the grain sizes may play minor rules in the variations of the film band gaps. For example, by post-annealing treatments, the crystal quality tends to be more perfect, and the ions coalesce more tightly, therefore the band gaps
Conduction Band
1016
1019
1019
evaluated incline to bigger values; On the other hand, the band bending is very sensitive to the smaller grain size, as schematically illustrated in Fig. 8, by post-annealing treatments, the grain sizes of the samples become larger, and the bands tend to be flat, thus resulting in the increases of the seemingly band gap energies [19]. For the pure CdO film, although above factors still work, an important phenomenon known as the Burstein–Moss effect observed in n-type semiconductors [20] is likely more responsible for the shift of the band gap. Table 1 lists the electron concentrations of the as-deposited and the annealed Zn1−x Cdx O (0 ≤ x ≤ 1.0) films. For the annealed CdO film, the markedly decreasing electron concentration will make the Fermi level moves away from the conduction band, therefore, the seeming band gap of the annealed CdO film decreases. To investigate the effect of annealing time on the properties of the samples, post-annealing treatments of the Zn1−x Cdx O films were performed at 500 ◦ C in O2 for different periods of 1–4 h, which are orderly designated first, second, third and forth annealing. Fig. 9 shows the XRD spectra of the annealed Zn0.64 Cd0.36 O films as a function of the annealing time. By the first annealed treatment, the crystal quality of the sample increases remarkably; Via the second annealed treatment, a CdO(2 0 0) diffraction peak is observed, suggesting the phase segregation of CdO from the matrix phase. However, no diffraction peak from CdO phase is observed in the sample after the third annealing treatment, it ought to be due to the Cd re-evaporation; The forth annealing treatment also makes the Zn re-evaporate seriously.
(002)
Intensity(a.u.)
Zn1−x Cdx O films
forth annealed
third annealed (200)
second annealed
first annealed eff
Eg
0
Eg
as deposited 20
Valence Band Fig. 8. The schematic illustration of effect of band bending on the band gap energy [19].
30
40
50
60
70
2 θ (deg)
Fig. 9. XRD spectra of annealed Zn0.64 Cd0.36 O films as a function of annealing time.
D.W. Ma et al. / Materials Science and Engineering B 111 (2004) 9–13
4. Conclusions In conclusion, the dc reactive sputtering method is an effective technique for depositing ternary Zn1−x Cdx O alloy films. Even when the Cd composition increases up to 53 at.%, the Zn1−x Cdx O films are single-phase and exhibit a hexagonal wurtzite ZnO structure. By post-annealing treatments in O2 oxygen, the crystal quality of the Zn1−x Cdx O films enhances, but distinct segregation and re-evaporation of CdO from the matrix phase are observed for the films with higher Cd compositions, the optical band gaps of the annealed Zn1−x Cdx O films increase except for the pure CdO film. Different annealing time experiments show that the post-annealing treatment with longer time makes the separation between ZnO and CdO happen as well. So it is important to improve the crystal quality of the Zn1−x Cdx O films by post-annealing treatments in O2 , but for the Zn1−x Cdx O films with higher Cd composition, to prevent the segregation of CdO, the annealing temperature and annealing time should be controlled strictly. Acknowledgements This work was supported by Chinese Special Funds for Major State Basic Research Project G20000683-06 and National Natural Science Foundation of China under Contract No. 90201038. References [1] T. Makino, Y. Segawa, M. Kawasaki, A. Ohtomo, R. Shiroki, K. Tamura, T. Yasuda, H. Koinuma, Appl. Phys. Lett. 78 (2001) 1237.
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[2] J. Narayan, K. Dovidenko, A.K. Sharma, S. Oktyabrsky, J. Appl. Phys. 84 (1998) 2597. [3] A. Ohtomo, M. Kawasaki, Y. Sakurai, I. Ohkubo, R. Shiroki, Y. Yoshida, T. Yasuda, Y. Segawa, H. Koinuma, Mater. Sci. Eng. B56 (1998) 263. [4] H. Kim, A. Pique, J.S. Horwitz, H. Murata, Z.H. Kafafi, C.M. Gilmore, D.B. Chrisey, Thin Solid Films 377–378 (2000) 798. [5] T.K. Suramanyam, B. Srinivasulu Naidu, S. Uthanna, Opt. Mater. 13 (1999) 239. [6] Y. Yan, S.B. Zhang, S.T. Pantelides, Phys. Rev. Lett. 86 (2001) 5723. [7] A. Ohtomo, M. Kawasaki, T. Koida, K. Masubuchi, H. Koinuma, Y. Sakurai, Y. Yoshida, T. Yasuda, Y. Segawa, Appl. Phys. Lett. 72 (1998) 2466. [8] D.W. Ma, Z.Z. Ye, J.Y. Huang, B.H. Zhao, S.K. Wan, X.H. Sun, Z.G. Wang, Chin. Phys. Lett. 20 (2003) 942. [9] Y. Jin, B. Zhang, S. Yang, Y. Wang, J. Chen, H. Zhang, C. Huang, C. Cao, H. Cao, R.P.H. Chang, Solid State Commun. 119 (2001) 409. [10] D. Zhao, Y. Liu, D. Shen, Y. Lu, J. Zhang, X. Fan, J. Cryst. Growth 234 (2002) 427. [11] Y.-S. Choi, C.-G. Lee, S.M. Cho, Thin Solid Films 289 (1996) 153. [12] O. Vigil, L. Vaollant, F. Cruz, G. Santana, A. Moroles-Acevedo, G. Contreras-Puente, Thin Solid Films 361–362 (2000) 53. [13] H. Tabet-Derraz, N. Benramdance, D. Nacer, A. Bouzidi, M. Medles, Solar Energy Mater. Solar Cells 73 (2002) 249. [14] K. Sakurai, T. Takagi, T. Kubo, D. Kajita, T. Tanabe, H. Takasu, S. Fujita, S. Fujita, J. Cryst. Growth 237–239 (2002) 514. [15] T. Makino, Y. Segawa, M. Kawasaki, A. Ohtomo, R. Shiroki, K. Tamura, T. Yasuda, H. Koinuma, Appl. Phys. Lett. 78 (2001) 1237. [16] J. Yin, Z.G. Liu, H. Liu, X.S. Wang, T. Zhu, J.M. Liu, J. Cryst. Growth 220 (2000) 281. [17] D.M. Carballeda-Galicia, R. Castanedo-Pérez, O. Jiménez-Sandoval, S. Jiménez-Sandoval, G. Torres-delgado, C.I. Zúñiga-Romero, Thin Solid Films 371 (2000) 105. [18] O. Vigil, F. Cruz, A. Morales-Acevedo, G. Contreras-Puente, L. Vaillant, G. Santana, Mater. Chem. Phys. 68 (2001) 249. [19] V. Srikant, D.R. Clarke, J. Appl. Phys. 81 (1997) 6357. [20] N. Ueda, H. Maeda, H. Hosono, H. Kawazoe, J. Appl. Phys. 84 (1998) 6174.
Effect of post-annealing treatments on the properties of Zn1−x Cdx O films on glass substrates D.W. Ma, Z.Z. Ye∗ , J.Y. Huang, L.P. Zhu, B.H. Zhao, J.H. He State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou 310027, PR China Received 7 March 2003; accepted 17 December 2003
Abstract Ternary Zn1−x Cdx O (0 ≤ x ≤ 0.53) alloy crystalline films with highly (0 0 2)-preferred orientations have been prepared on glass substrates by using the dc reactive magnetron sputtering method and exposed to annealing treatments in O2 ambient. The crystal quality of the annealed Zn1−x Cdx O films improves much. As a case of the sample with x = 0.53, the diffraction peaks in intensity attain maximum values at the annealing temperature of 500 ◦ C. But for the samples with x ≥ 0.53, the CdO phase separation and re-evaporation from the films are very serious. So for the Zn1−x Cdx O films with higher Cd content, to prevent the segregation of CdO, the annealing temperature should be less than 400 ◦ C, and the annealing time should be less than 1 h. © 2004 Elsevier B.V. All rights reserved. Keywords: Zinc oxide; Film deposition; Annealing; Crystal symmetry; Optical properties; Segregation
1. Introduction The increased interest on short-wavelength optoelectronic devices, such as blue and ultraviolet (UV) light emitters and detectors has stimulated investigations on II–VI semiconductor oxide ZnO and its ternary alloys [1]. As an alternative to GaN, ZnO is of hexagonal wurtzite-type structure with a direct wide band gap of 3.3 eV and an excitonic binding energy of ∼60 meV, which is much larger than that of GaN (25 meV) or the thermal energy at room temperature (25 meV) [2,3]. Moreover, ZnO is a commercially available material with advantages of low cost, non-toxicity and high chemical stability [4,5]. To achieve applicable ZnO film based optoelectronic devices, considerable experimental investigations have been focused on the preparation of p-type ZnO film and its band gap engineering by various doping and alloying methods [6,7]. We concentrate on the latter in this paper. CdO is of a cubic-type structure with a band gap of 2.3 eV, theoretically by alloying with CdO, the band gap of ZnO can be modulated, and the luminescence of ZnCdO alloys can cover green, blue to ultraviolet light spectra. More important, ∗
Corresponding author. Tel.: +86-571-87953139; fax: +86-571-87952625. E-mail address: [email protected] (Z.Z. Ye). 0921-5107/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2003.12.007
the substitution of proper amount of Zn by Cd is not expected to induce a significant change in crystal structure and lattice constant due to their similar ionic radii, therefore the ternary ZnCdO alloy system can construct efficient heterojunction with ZnO, which is a key technique in optoelectronic device applications [7–10]. Several techniques were attempted to deposit Zn1−x Cdx O alloy films, such as sol–gel [11], spray pyrolysis [12,13], MBE [14] and PLD [15] methods. But the prepared Zn1−x Cdx O films were not satisfying due to the coexistence of multiphase or polycrystalline state, and the reported upper limit of Cd substitution was lower than 25 at.% [11–15]. By the dc reactive magnetron sputtering method, we have successfully deposited completely (0 0 2)-oriented ternary Zn1−x Cdx O (0 ≤ x ≤ 0.53) alloy films on Si [8] and glass substrates. In this paper, we will report the structural, optical and electrical properties of as-deposited and annealed Zn1−x Cdx O (0 ≤ x ≤ 1.0) films on glass substrates.
2. Experimental Zn1−x Cdx O films were deposited on glass substrates using the dc reactive magnetron sputtering method. The sputtering targets with different ratios of Zn to Cd were prepared using Zn and Cd metals with high purities of 99.999%. The glass
D.W. Ma et al. / Materials Science and Engineering B 111 (2004) 9–13
substrates were rinsed in acetone accompanied by supersonic wave. The deposition chamber was evacuated to a base pressure of 10−3 Pa by a combination of mechanical-pump and oil-diffusion-pump. High purity (99.995% pure) argon and oxygen were used as sputtering and reactive gases. The specific flux of argon to oxygen was controlled at a ratio of Ar:O2 ∼ 1:4, and the total pressure maintained at 4.0 Pa. The target to substrate distance was fixed at 60 mm. The sputtering current and sputtering voltage were 200 mA and 200 V, respectively. The deposition time was kept 30 min for all samples. Before the experiment of deposition, the Zn–Cd target was presputtered in argon–oxygen atmosphere for about 10 min in order to remove the surface oxide from the target. The post-annealing treatments of the samples were performed in a quartz tube under O2 ambient. The crystal structure of the samples was investigated by the method of X-ray diffraction (XRD), where a Cu K␣ (λ = 0.154056 nm) source was used. The transmittance measurements were performed by a Cary-100-Bio UV-Vis spectrophotometer. The electrical properties were studied by van der Pauw Hall measurements. The film thicknesses were in the range of 300–400 nm as determined by using ␣-step measurements. The film compositions were determined by X-ray photoemission spectroscopy (XPS) measurements, where a Omicron EAC2000-125 spherical deflector analyzer and a Mg K␣ X-ray (1253.6 eV) were used, as will be discussed in detail in other related papers, the compositions × mentioned below all denote the measured film compositions.
3. Results and discussion Fig. 1 shows the XRD spectra of as-deposited Zn1−x Cdx O (0 ≤ x ≤ 1.0) films on glass substrates at temperatures of 375 ◦ C, up to x = 0.53, the spectra nearly exhibit only Zn1−x Cdx O(0 0 2) peaks at 34.51–33.57◦ , depending on Cd contents, which indicates that single-phase Zn1−x Cdx O (0 ≤
x ≤ 0.53) films with a structure of hexagonal wurtzite ZnO were grown without any significant formation of a separated CdO phase. However, for the higher Cd content of x = 0.77, a CdO(2 0 0) peak is also observed, suggesting phase segregation between ZnO and CdO. For the pure CdO, only a (2 0 0) peak with a full-width at half-maximum (FWHM) of ∼0.3◦ is observed, indicating the recovery of high crystalline quality in single phase region. It is also observed that with the increasing Cd content, the intensity of the corresponding XRD peaks is increased, especially the (2 0 0) peak from the pure CdO film is much higher in contrast to the (0 0 2) peak from the pure ZnO film, indicating a much better crystallinity for CdO than that for ZnO, which is consistent with the previous reports [11,12]. There is further unequivocal evidence for Cd incorporation in ZnO: the c-axis lattice constants of the Zn1−x Cdx O films almost linearly increase up to 0.5394 nm (x = 0.77) from 0.5183 nm (x = 0), as shown in Fig. 4 (solid line). Considering that the ionic radius of Cd (0.97 Å) is larger than that of Zn (0.74 Å), it is undoubted that the substitution of Zn by Cd takes place on the equivalent crystallographic position of Zn in hexagonal wurtzite structure. Fig. 2a displays the XRD patterns of annealed Zn0.47 Cd0.53 O films at different temperatures for 1 h. Fig. 2b shows
Intensity(a.u.)
10
(002)hex
30000 15000 0 30000 15000 0 30000 15000 0 20000 10000 0 20000 10000 0 6000 3000 0 20
700˚C (200)cub
500˚C 400˚C 300˚C as deposited 30
40
50
60
70
2θ (deg)
(200)cub (002)hex
x= 1.0
0.534
x= 0.77
0.532
x= 0.53 x= 0.36 x= 0.20
C-axis constant(nm)
Intensity(a.u.)
(a) 90000 60000 30000 0 6000 3000 0 6000 3000 0 3000 1500 0 3000 1500 0 1200 600 0 20
600˚C
30
40
50 2θ (deg)
60
0.530 0.528 0.526 0.524
x= 0 70
Fig. 1. XRD spectra of as-deposited Zn1−x Cdx O (0 ≤ x ≤ 1.0) films on glass substrates.
(b)
as deposited
300
400
500 T(˚C)
600
700
Fig. 2. (a) XRD patterns of annealed Zn0.47 Cd0.53 O films; (b) c-axis constant of the annealed Zn0.47 Cd0.53 O films as a function of annealing temperature.
400000 200000 0 12000 6000 0 30000 15000 0 18000 9000 0 14000 7000 0 8000 4000 0 20
(200)cub
11
0.540 x= 1.0
(002)hex
x= 0.77 x= 0.53 x= 0.36
C-axis constant (nm)
Intensity(a.u.)
D.W. Ma et al. / Materials Science and Engineering B 111 (2004) 9–13
as deposited annealed
0.535 0.530 0.525 0.520
x= 0.20
0.515 x= 0 30
40
50
60
0.0
0.2
0.4 0.6 Cd content (x)
0.8
70
2θ (deg) Fig. 3. XRD spectra of annealed Zn1−x Cdx O (0 ≤ x ≤ 1.0) films at 500 ◦ C.
Fig. 4. Cd content dependence of the c-axis constant of as deposited and annealed Zn1−x Cdx O (0 ≤ x ≤ 0.77) films.
Transmittance(%)
100 80 60
x=0 x=0.20 x=0.36 x=0.53 x=0.77 x=1.0
40 20 0
400
500
600 700 Wavelength(nm)
800
900
Fig. 5. Transmittance spectra of as-deposited Zn1−x Cdx O (0 ≤ x ≤ 1.0) films.
which is consistent with the XRD spectra shown in Fig. 3. As for the pure ZnO film, it is assumed that in-plane tensile stress lies in the as-deposited ZnO film due to a relatively poor crystallinity for ZnO, by the post-annealing treatment the ZnO film is in a comparatively stress-free state, therefore, the c-axis will stretch out according to Poisson’s ratio. Figs. 5 and 6 show the transmittance spectra of the as-deposited and annealed Zn1−x Cdx O (0 ≤ x ≤ 1.0) films, respectively. By the transmittance data, the absorption coefficients (α) of the films can be determined by using the
100
Transmittance(%)
the c-axis constants of the films as a function of the annealing temperatures. When the sample was annealed at 300 ◦ C, the XRD peak in intensity enhances, and the corresponding c-axis constant decreases sharply down to 0.5286 nm from 0.5335 nm (as deposited) likely due to a CdO phase separation from the film, though the (2 0 0) peak corresponding to the CdO phase is hardly observed at the same time; The release of in-plane compressive stress in the film may correlative to the decrease of the c-axis constant also [8,16]. For the annealing temperature up to 400 ◦ C, the CdO(2 0 0) peak can be observed. At 500 ◦ C, the intensity of the XRD peaks attains maximum values, and the segregation between ZnO and CdO phases becomes clearer. However, when the temperature continues to increase, the diffraction peak decreases, and the c-axis constant tends to a smaller value. It is noteworthy that when the annealing temperature increases up to 700 ◦ C, the CdO(2 0 0) peak almost vanishes, which can be ascribed to the Cd re-evaporation in view of a smaller lattice energy for CdO (3806 kJ mol) than that for ZnO (4142 kJ mol). Fig. 3 shows the XRD spectra of annealed Zn1−x Cdx O (0 ≤ x ≤ 1.0) films at 500 ◦ C under O2 ambient for 1 h. Combined with Fig. 1, obviously by post-annealing treatments, in intensity the diffraction peaks of the samples increase much. For the annealed samples with x ≤ 0.36, no distinct segregation of cubic CdO phase is observed from the XRD spectra. However, for the annealed one with x = 0.53, a low (2 0 0) peak corresponding to the CdO phase appears, when the Cd composition increases up to 77 at.%, the separation between the ZnO and CdO phases is very seriously in the annealed film. Fig. 4 (dashed line) shows the Cd content dependences of the c-axis constants of the annealed Zn1−x Cdx O (0 ≤ x ≤ 0.77) films, from x = 0–0.77, the corresponding c-axis lattice constants are 0.5194, 0.5214, 0.5244, 0.5277, 0.5313 nm, respectively, smaller than those of the as-deposited samples except for the pure ZnO film. Moreover, with the increasing Cd content in the film, the decrease of the c-axis constant becomes evident, indicating the CdO phase separation and re-evaporation more seriously,
80 60
x=0 x=0.20 x=0.36 x=0.53 x=0.77 x=1.0
40 20 0
400
500
600 700 Wavelength(nm)
800
900
Fig. 6. Transmittance spectra of annealed Zn1−x Cdx O (0 ≤ x ≤ 1.0) films.
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D.W. Ma et al. / Materials Science and Engineering B 111 (2004) 9–13
Table 1 Electron concentrations n (per cubic centimeter) of as-deposited and annealed Zn1−x Cdx O (0 ≤ x ≤ 1.0) films x=0
As-deposited films (n) Annealed films (n)
4.31 × 6.45 × 1014 1015
x = 0.20
x = 0.36
x = 0.53
x = 0.77
x = 1.0
6.08 × 8.20 × 1014
1.15 × 5.66 × 1015
2.68 × 2.34 × 1018
7.87 × 3.03 × 1019
2.00 × 1020 8.39 × 1019
1015
Band-gap energy(eV)
3.4 as deposited annealed
3.2 3.0 2.8 2.6 2.4 2.2
0.0
0.2
0.4 0.6 Cd content(x)
0.8
1.0
Fig. 7. Band gap energy of as-grown and annealed Zn1−x Cdx O (0 ≤ x ≤ 1.0) films as a function of Cd content x.
following relation: α = [2.303 × log(1/T)]/d, where T is the transmittance and d is the film thickness. For evaluating the band gaps of the films, the values of α calculated from the relation above are used to plot (αhν)2 versus hν, where ν is the photon energy, by extrapolating the linear portions of the curves to αhν = 0, the direct band gap values of the films can be evaluated [17]. Fig. 7 shows the band gaps of the as-grown and annealed Zn1−x Cdx O (0 ≤ x ≤ 1.0) films as a function of Cd contents. It is noted that when the films were annealed, the band gaps of the samples with x ≤ 0.36 are slightly larger, but for the samples with x ≥ 0.53, the increases of the band gaps become more evident, it sounds reasonable considering that the CdO separation and re-evaporation more seriously in the annealed Zn1−x Cdx O films with higher Cd contents and the smaller band gap of CdO than that of ZnO. According to the previous reports [18–20], other factors, such as the variations of the crystal quality, the grain sizes may play minor rules in the variations of the film band gaps. For example, by post-annealing treatments, the crystal quality tends to be more perfect, and the ions coalesce more tightly, therefore the band gaps
Conduction Band
1016
1019
1019
evaluated incline to bigger values; On the other hand, the band bending is very sensitive to the smaller grain size, as schematically illustrated in Fig. 8, by post-annealing treatments, the grain sizes of the samples become larger, and the bands tend to be flat, thus resulting in the increases of the seemingly band gap energies [19]. For the pure CdO film, although above factors still work, an important phenomenon known as the Burstein–Moss effect observed in n-type semiconductors [20] is likely more responsible for the shift of the band gap. Table 1 lists the electron concentrations of the as-deposited and the annealed Zn1−x Cdx O (0 ≤ x ≤ 1.0) films. For the annealed CdO film, the markedly decreasing electron concentration will make the Fermi level moves away from the conduction band, therefore, the seeming band gap of the annealed CdO film decreases. To investigate the effect of annealing time on the properties of the samples, post-annealing treatments of the Zn1−x Cdx O films were performed at 500 ◦ C in O2 for different periods of 1–4 h, which are orderly designated first, second, third and forth annealing. Fig. 9 shows the XRD spectra of the annealed Zn0.64 Cd0.36 O films as a function of the annealing time. By the first annealed treatment, the crystal quality of the sample increases remarkably; Via the second annealed treatment, a CdO(2 0 0) diffraction peak is observed, suggesting the phase segregation of CdO from the matrix phase. However, no diffraction peak from CdO phase is observed in the sample after the third annealing treatment, it ought to be due to the Cd re-evaporation; The forth annealing treatment also makes the Zn re-evaporate seriously.
(002)
Intensity(a.u.)
Zn1−x Cdx O films
forth annealed
third annealed (200)
second annealed
first annealed eff
Eg
0
Eg
as deposited 20
Valence Band Fig. 8. The schematic illustration of effect of band bending on the band gap energy [19].
30
40
50
60
70
2 θ (deg)
Fig. 9. XRD spectra of annealed Zn0.64 Cd0.36 O films as a function of annealing time.
D.W. Ma et al. / Materials Science and Engineering B 111 (2004) 9–13
4. Conclusions In conclusion, the dc reactive sputtering method is an effective technique for depositing ternary Zn1−x Cdx O alloy films. Even when the Cd composition increases up to 53 at.%, the Zn1−x Cdx O films are single-phase and exhibit a hexagonal wurtzite ZnO structure. By post-annealing treatments in O2 oxygen, the crystal quality of the Zn1−x Cdx O films enhances, but distinct segregation and re-evaporation of CdO from the matrix phase are observed for the films with higher Cd compositions, the optical band gaps of the annealed Zn1−x Cdx O films increase except for the pure CdO film. Different annealing time experiments show that the post-annealing treatment with longer time makes the separation between ZnO and CdO happen as well. So it is important to improve the crystal quality of the Zn1−x Cdx O films by post-annealing treatments in O2 , but for the Zn1−x Cdx O films with higher Cd composition, to prevent the segregation of CdO, the annealing temperature and annealing time should be controlled strictly. Acknowledgements This work was supported by Chinese Special Funds for Major State Basic Research Project G20000683-06 and National Natural Science Foundation of China under Contract No. 90201038. References [1] T. Makino, Y. Segawa, M. Kawasaki, A. Ohtomo, R. Shiroki, K. Tamura, T. Yasuda, H. Koinuma, Appl. Phys. Lett. 78 (2001) 1237.
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