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Preparation of Cu2O films on MgO (1 1 0) substrate by means of halide chemical vapor deposition under atmospheric pressure
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Materials Chemistry and Physics 106 (2007) 292–295
Preparation of Cu2O films on MgO (1 1 0) substrate by means of halide chemical vapor deposition under atmospheric pressure Hiroki Kobayashi a , Takato Nakamura a , Naoyuki Takahashi b,∗ a
Department of Materials Science and Technology, Faculty of Engineering, Shizuoka University, 3-5-1 Johoku, Hamamatsu, Shizuoka 432-8561, Japan b Department of Environmental and Biotechnological Frontier Engineering, Fukui University of Technology, 3-6-1 Gakuen, Fukui 910-8505, Japan Received 21 September 2006; received in revised form 21 May 2007; accepted 2 June 2007
Abstract High-quality Cu2 O thin films were grown epitaxially on MgO (1 1 0) substrate by halide chemical vapor deposition under atmospheric pressure (AP-HCVD). The full width at half maximum of X-ray diffraction ω rocking measurement of the (2 2 0) plane was 0.1429◦ and that the of the (1 −1 0) plane was 0.303◦ .This result showed that the Cu2 O films have a high degree of out-of-plane and in-plane crystallinity. Pole-figure and reciprocal space mapping (RSM) of Cu2 O films showed Cu2 O film is grown without strain. Optical band gap energy of Cu2 O film calculated from absorption spectra showed 2.38 eV. These results indicated that AP-HCVD was promising growth method for high-quality Cu2 O film. © 2007 Elsevier B.V. All rights reserved. Keywords: Oxide; Thin films; Vapor deposition; Crystal structure
1. Introduction Nature yields cuprous (Cu2 O) and cupric oxide (CuO) as cuprite and tenorite, respectively. Cu2 O is a reddish p-type semiconductor of nature both ionic and covalent native with a direct forbidden band gap of 2.17 eV. A crystal of Cu2 O has a cubic structure (space group,O4h = pn3m), having two molecules in the unit cell. This material is well known in the history of semiconductor physics and has been used as crystal rectifiers. Since a few of the hydrogen-like excition series in absorption were observed in Cu2 O, its optical properties have been well investigated as a typical crystal for studying properties of excition [1–3]. Recent reports on a possibility of the Bose condensation of excition [4–6] and on the photoconductivity with anomalies at low temperatures [7,8] have attracted attention. Further investigations are being performed intensively and extensively. Thin films of Cu2 O have been prepared by means of chemical vapor deposition (CVD) [9–11], magnetron sputtering [12] and electro-deposition [13,14]. However, it is known that the materials usually include considerable amounts of non-stoichiometry, such as copper vacancies, and oxygen vacancies, but the correlation between theses defects and electrical conductivity has
∗
Corresponding author. Tel.: +81 776 29 2686. E-mail address: [email protected] (N. Takahashi).
0254-0584/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2007.06.008
not been understood in detail. In addition, CVD and sputtering are costly because of the films being deposited in vacuo using expensive set-ups and chemicals. Therefore, it is particularly important to make clear the low-cost preparation process of high-quality Cu2 O film. For this reason, the purpose of this study is to develop an alternative method using CuI and O2 as starting materials for the growth of Cu2 O films. This brings the following advantages: (a) the films are formed by a simple reaction of CuI with O2 in the gas phase under atmospheric pressure; (b) their post-annealing is not necessary; (c) high-purity CuI used as source of copper is cheap compared with the source materials for the deposition techniques described above. 2. Experimental details A schematic illustration of the reactor used in this study is shown in Fig. 1. The Cu2 O films were deposited onto a MgO (1 1 0). CuI in a source boat was evaporated at a temperature of 883 K, and supplied to the growth zone of the reactor by N2 carrier gas, and O2 was also supplied there by the same carrier gas. Partial pressure of CuI and O2 were adjusted independently to 1.24 × 10−2 and 1.25 × 103 Pa. The purity of the N2 and O2 gases was of 99.9999% and 99.9999% (Tomoe shokai Co., Ltd.), respectively, and that of CuI (Kojundo Chemical Lab. Co., Ltd.) was of 99.999%. A typical experimental condition is summarized in Table 1. The film thickness was approximately 1 m. Crystallographic structure of the deposited Cu2 O was examined by Xray diffractometry (XRD) using a Rigaku RINT 2000 X-ray diffractometer with Cu K␣1 radiation. Furthermore, the high resolution X-ray diffractometry
H. Kobayashi et al. / Materials Chemistry and Physics 106 (2007) 292–295
293
Fig. 1. Schematic diagram of the apparatus used in this work. Table 1 Typical growth conditions Substrate
MgO (1 1 0)
CuI partial pressure O2 partial pressure CuI source temperature Carrier gas Total flow rate Growth temperature of Cu2 O film Duration of deposition
1.24 × 10−2 Pa 1.25 × 103 Pa 883 K N2 810 cm3 min−1 1123 K 3600 s
(HR-XRD) measurements were performed with Cu K␣1 radiation using a Rigaku ATX-G X-ray diffractometer, equipped with a curved graded multilayer mirror and a double-crystal Bartels-type Ge (2 2 0) monochromator. The out-plane and in-plane of Cu2 O films on MgO (1 1 0) as shown in Fig. 2 were analyzed by HR-XRD. In addition, the pole-figure and reciprocal space mapping (RSM) of Cu2 O films were measured using a HR-XRD. Scanning electron micrographs (SEM) were recorded on JEOL Ltd., JSM-5500LV in order to estimate the film thickness and to observe the surface morphology. Optical transmission spectra were examined with Shimadzu UV-3150 spectrophotometer, and optical band gap of Cu2 O films were calculated by absorption coefficient.
Fig. 3. (a) θ − 2θ scan spectra of Cu2 O films deposited on MgO (1 1 0) substrate. (b) ω-Scan rocking curve of Cu2 O (2 2 0) peak deposited on MgO (1 1 0) substrate.
3. Results and discussion Fig. 3 shows the θ − 2θ scan spectra of Cu2 O films deposited on MgO (1 1 0) substrate. XRD spectrum shows that the 2θ peak of the Cu2 O (1 1 0) plane at 29.34◦ and Cu2 O (2 2 0) plane at 60.84◦ are very symmetrical and no peaks corresponding to other planes are detectable, as shown in Fig. 3(a). This indicates that the Cu2 O film was epitaxially grown on MgO (1 1 0) in highly
Fig. 2. The out-plane and in-plane of Cu2 O film on MgO (1 1 0).
Fig. 4. (a) φ-Scans for Cu2 O (2 2 0) peak deposited on MgO (1 1 0) substrate. (b) φ-Scan rocking curve of Cu2 O (2 2 0) peak deposited on MgO (1 1 0) substrate.
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H. Kobayashi et al. / Materials Chemistry and Physics 106 (2007) 292–295 Table 2 Lattice spacing of the Cu2 O film deposited on MgO (1 1 0) substrate
Fig. 5. Growth direction model of Cu2 O film deposited on MgO (1 1 0) substrate.
(1 1 0) oriented orientation under the compressive stress that is commonly observed for sputtered Cu2 O [15]. This FWHM value of Cu2 O (2 2 0) peak in ω-scan of Fig. 3(b) was 0.1429◦ . This value was smaller than previous reported, ωFWHM = 0.19◦ [16]. These results show that the Cu2 O film grown on MgO (1 1 0) has a very high out-of-plane crystallinity. We employed the φ-scan of the HR-XRD to evaluate the inplane crystal quality of the Cu2 O films as shown in Fig. 4(a). The peaks of (2 2 0) plane with a 180◦ separation show that the Cu2 O films have a homogeneous in-plane alignment on MgO
Fig. 6. X-ray ϕ − φ pole scan with a 2θ fixed at the (a) Cu2 O (2 0 0) and (b) Cu2 O (2 2 0) reflections.
Lattice spacing
˚ Calculated value (A)
˚ JCPDS-card 5-0667 (A)
MgO (3 3 1) Cu2 O (3 3 1) MgO (2 2 2) Cu2 O (2 2 2)
0.964 0.987 1.217 1.234
0.966 0.979 1.2158 1.233
(1 1 0). It is noteworthy that the Cu2 O film has the (2 2 0) φscan rocking curve with a small FWHM (0.3032◦ ) as shown in Fig. 4(b). The small FWHM of (2 2 0) φ rocking curve indicates that the Cu2 O film is high in-plane crystalline quality as well out-of-plane crystalline quality. These results of in/out-plane measurement using HR-XRD implied the direction of crystal axis was Cu2 O [1 1 0]//MgO [1 0 0], Cu2 O [0 0 1]//MgO [0 0 1] and Cu2 O [1 1 0]//MgO [1 −1 0], as shown in Fig. 5. There-
Fig. 7. (a) RSM of asymmetric around the (4 2 0) lattice point of the MgO (1 1 0) substrate. (b) RSM of asymmetric around the (2 2 0) and (3 3 1) lattice point of the MgO (1 1 0) substrate.
H. Kobayashi et al. / Materials Chemistry and Physics 106 (2007) 292–295
295
where hν is the photon energy, Eg the optical band gap and B constant. For a direct transition, n = 1/2 or 2/3. A value of n = 1/2 was found to be most suitable for Cu2 O films, since it gives the best linear graph in the band edge region. In Fig. 8, the (αhν)2 of the Cu2 O film is plotted as a function of the band gap energy. The value of Eg can be obtained by extrapolating the linear portion to the photon energy axis. The Eg of deposited Cu2 O film was Eg = 2.38 eV. The obtained Eg is similar to those of Cu2 O films prepared by other methods [19–21]. 4. Conclusions
Fig. 8. Band gap energy dependence of the (αhν)2 of the Cu2 O film deposited on MgO (1 1 0) substrate.
fore, FWHM of (2 2 0) φ rocking curve of Cu2 O film describes FWHM of (1 1 0) plane rocking curve of Cu2 O film. Fig. 6 shows X-ray ϕ − φ pole scan with a 2θ fixed at the Cu2 O (2 0 0) and Cu2 O (2 2 0) reflections. In the X-ray ϕ − φ pole scan with a 2θ fixed at the Cu2 O (2 0 0) reflection, two clear spots from (0 2 0) and (2 0 0) were observed in Fig. 6(a). In addition, two pair spots of (0 1 1)–(0 1 1) and (1 0 1)–(1 0 1) were observed in the X-ray ϕ − φ pole scan with a 2θ fixed at the Cu2 O (2 2 0) reflection, as shown in Fig. 6(b). This means that Cu2 O film is grown without rotation domain. Strain and relaxation of Cu2 O film were characterized by reciprocal space mapping (RSM) of the XRD intensity around asymmetrical diffraction spots of Cu2 O. Fig. 7(a and b) shows the RSM around (4 2 0)–(3 1 0) and (3 3 1)–(2 2 2) diffractions from a Cu2 O films on MgO (1 1 0). In the lattice spacing calculated from direction spot, the Cu2 O film is almost perfectly aligned to that of Cu2 O JCPDS-card 5-0667, as shown in Table 2. This result shows Cu2 O film grown without strain. It is, therefore, evident that a Cu2 O film with high quality was grown by means of AP-HCVD.The absorption coefficient α was calculated by T = A exp(−αd)
(1)
where T is the transmittance of the Cu2 O film, A a constant, and d is the film thickness. The optical band gaps of the Cu2 O films were determined by applying the Tauc model [17], and David and Mott [18] in the high-absorbance region: αhν = B(hν − Eg )n
(2)
Thin films of Cu2 O were deposited on a MgO (1 1 0) substrate under atmospheric pressure by a new vapor-phase deposition method using CuI and O2 as starting materials. HR-XRD measurement showed that the Cu2 O films have a high degree of out-of-plane and in-plane crystallinity. Furthermore, pole-figure and RSM of Cu2 O films showed Cu2 O film is grown without strain. Optical band gap energy of Cu2 O film calculated from absorption spectra showed 2.38 eV. Consequently, it is evident that reaction of CuI and O2 under atmospheric pressure yields high-quality Cu2 O films. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
E.F. Gross, Nuovo Cimento Suppl. 3 (1956) 672. S. Nikitine, Optical Properties of Solids, Plenum, Newyork, 1969. V.I. Agekyan, Phys. Status Solidi (a) 43 (1977) 11. D.W. Snoke, J.P. Wolfe, A. Mysyrowicz, Phys. Rev. Lett. 64 (1990) 2543. D.W. Snoke, J.P. Wolfe, Phys. Rev. B 39 (1989) 4030. J.L. Lin, J.P. Wolfe, Phys. Rev. Lett. 23 (1993) 1222. H. Shimada, T. Masumi, J. Phys. Soc. Jpn. 58 (1989) 1717. H. Amekura, T. Masumi, J. Phys. Soc. Jpn. 64 (1995) 2684. M. Ottosson, J.O. Carlsson, Surf. Cort. Technol. 78 (1996) 263. T. Maruyama, Sol. Energy Mater. Sol. Cell 56 (1998) 85. G.G. Condorelli, G. Malandrino, I.L. Fragala, Chem. Vap. Deposition 5 (1999) 21. H. Holzoschuh, H. Suhr, Appl. Phys. A 51 (1990) 486. J. Mrales, L. Sanchez, S. Bijanl, L. Martinez, M. Gabas, J.R. Barrado, Electrochemi. Solid State Lett. 8 (2005) A159. R. Liu, E.W. Bohannan, J.A. Switzer, F. Oba, F. Ernst, Appl. Phys. Lett. 83 (2003) 1944. M. Ottosson, J. Lu, J.O. Carlsson, J. Cryst. Growth 151 (1995) 305. Z.G. Yin, H.T. Zhang, D.M. Goodner, M.J. Bedzyk, R.P.H. Chang, Y. Sun, J.B. Ketterson, Appl. Phys. Lett. 86 (2005) 061901. J. Tauc, Amorphous and Liquid Semiconductor, Plenum, London, 1974. E.A. David, N.F. Mott, Philos. Mag. 22 (1970) 903. S.C. Ray, Sol. Energy Mater. Sol. Cells 68 (2001) 307. V. Georgieva, M. Ristov, Sol. Energy Mater. Sol. Cell 73 (2002) 67. T. Maruyama, Sol. Energy Mater. Sol. Cell 56 (1998) 58.
Materials Chemistry and Physics 106 (2007) 292–295
Preparation of Cu2O films on MgO (1 1 0) substrate by means of halide chemical vapor deposition under atmospheric pressure Hiroki Kobayashi a , Takato Nakamura a , Naoyuki Takahashi b,∗ a
Department of Materials Science and Technology, Faculty of Engineering, Shizuoka University, 3-5-1 Johoku, Hamamatsu, Shizuoka 432-8561, Japan b Department of Environmental and Biotechnological Frontier Engineering, Fukui University of Technology, 3-6-1 Gakuen, Fukui 910-8505, Japan Received 21 September 2006; received in revised form 21 May 2007; accepted 2 June 2007
Abstract High-quality Cu2 O thin films were grown epitaxially on MgO (1 1 0) substrate by halide chemical vapor deposition under atmospheric pressure (AP-HCVD). The full width at half maximum of X-ray diffraction ω rocking measurement of the (2 2 0) plane was 0.1429◦ and that the of the (1 −1 0) plane was 0.303◦ .This result showed that the Cu2 O films have a high degree of out-of-plane and in-plane crystallinity. Pole-figure and reciprocal space mapping (RSM) of Cu2 O films showed Cu2 O film is grown without strain. Optical band gap energy of Cu2 O film calculated from absorption spectra showed 2.38 eV. These results indicated that AP-HCVD was promising growth method for high-quality Cu2 O film. © 2007 Elsevier B.V. All rights reserved. Keywords: Oxide; Thin films; Vapor deposition; Crystal structure
1. Introduction Nature yields cuprous (Cu2 O) and cupric oxide (CuO) as cuprite and tenorite, respectively. Cu2 O is a reddish p-type semiconductor of nature both ionic and covalent native with a direct forbidden band gap of 2.17 eV. A crystal of Cu2 O has a cubic structure (space group,O4h = pn3m), having two molecules in the unit cell. This material is well known in the history of semiconductor physics and has been used as crystal rectifiers. Since a few of the hydrogen-like excition series in absorption were observed in Cu2 O, its optical properties have been well investigated as a typical crystal for studying properties of excition [1–3]. Recent reports on a possibility of the Bose condensation of excition [4–6] and on the photoconductivity with anomalies at low temperatures [7,8] have attracted attention. Further investigations are being performed intensively and extensively. Thin films of Cu2 O have been prepared by means of chemical vapor deposition (CVD) [9–11], magnetron sputtering [12] and electro-deposition [13,14]. However, it is known that the materials usually include considerable amounts of non-stoichiometry, such as copper vacancies, and oxygen vacancies, but the correlation between theses defects and electrical conductivity has
∗
Corresponding author. Tel.: +81 776 29 2686. E-mail address: [email protected] (N. Takahashi).
0254-0584/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2007.06.008
not been understood in detail. In addition, CVD and sputtering are costly because of the films being deposited in vacuo using expensive set-ups and chemicals. Therefore, it is particularly important to make clear the low-cost preparation process of high-quality Cu2 O film. For this reason, the purpose of this study is to develop an alternative method using CuI and O2 as starting materials for the growth of Cu2 O films. This brings the following advantages: (a) the films are formed by a simple reaction of CuI with O2 in the gas phase under atmospheric pressure; (b) their post-annealing is not necessary; (c) high-purity CuI used as source of copper is cheap compared with the source materials for the deposition techniques described above. 2. Experimental details A schematic illustration of the reactor used in this study is shown in Fig. 1. The Cu2 O films were deposited onto a MgO (1 1 0). CuI in a source boat was evaporated at a temperature of 883 K, and supplied to the growth zone of the reactor by N2 carrier gas, and O2 was also supplied there by the same carrier gas. Partial pressure of CuI and O2 were adjusted independently to 1.24 × 10−2 and 1.25 × 103 Pa. The purity of the N2 and O2 gases was of 99.9999% and 99.9999% (Tomoe shokai Co., Ltd.), respectively, and that of CuI (Kojundo Chemical Lab. Co., Ltd.) was of 99.999%. A typical experimental condition is summarized in Table 1. The film thickness was approximately 1 m. Crystallographic structure of the deposited Cu2 O was examined by Xray diffractometry (XRD) using a Rigaku RINT 2000 X-ray diffractometer with Cu K␣1 radiation. Furthermore, the high resolution X-ray diffractometry
H. Kobayashi et al. / Materials Chemistry and Physics 106 (2007) 292–295
293
Fig. 1. Schematic diagram of the apparatus used in this work. Table 1 Typical growth conditions Substrate
MgO (1 1 0)
CuI partial pressure O2 partial pressure CuI source temperature Carrier gas Total flow rate Growth temperature of Cu2 O film Duration of deposition
1.24 × 10−2 Pa 1.25 × 103 Pa 883 K N2 810 cm3 min−1 1123 K 3600 s
(HR-XRD) measurements were performed with Cu K␣1 radiation using a Rigaku ATX-G X-ray diffractometer, equipped with a curved graded multilayer mirror and a double-crystal Bartels-type Ge (2 2 0) monochromator. The out-plane and in-plane of Cu2 O films on MgO (1 1 0) as shown in Fig. 2 were analyzed by HR-XRD. In addition, the pole-figure and reciprocal space mapping (RSM) of Cu2 O films were measured using a HR-XRD. Scanning electron micrographs (SEM) were recorded on JEOL Ltd., JSM-5500LV in order to estimate the film thickness and to observe the surface morphology. Optical transmission spectra were examined with Shimadzu UV-3150 spectrophotometer, and optical band gap of Cu2 O films were calculated by absorption coefficient.
Fig. 3. (a) θ − 2θ scan spectra of Cu2 O films deposited on MgO (1 1 0) substrate. (b) ω-Scan rocking curve of Cu2 O (2 2 0) peak deposited on MgO (1 1 0) substrate.
3. Results and discussion Fig. 3 shows the θ − 2θ scan spectra of Cu2 O films deposited on MgO (1 1 0) substrate. XRD spectrum shows that the 2θ peak of the Cu2 O (1 1 0) plane at 29.34◦ and Cu2 O (2 2 0) plane at 60.84◦ are very symmetrical and no peaks corresponding to other planes are detectable, as shown in Fig. 3(a). This indicates that the Cu2 O film was epitaxially grown on MgO (1 1 0) in highly
Fig. 2. The out-plane and in-plane of Cu2 O film on MgO (1 1 0).
Fig. 4. (a) φ-Scans for Cu2 O (2 2 0) peak deposited on MgO (1 1 0) substrate. (b) φ-Scan rocking curve of Cu2 O (2 2 0) peak deposited on MgO (1 1 0) substrate.
294
H. Kobayashi et al. / Materials Chemistry and Physics 106 (2007) 292–295 Table 2 Lattice spacing of the Cu2 O film deposited on MgO (1 1 0) substrate
Fig. 5. Growth direction model of Cu2 O film deposited on MgO (1 1 0) substrate.
(1 1 0) oriented orientation under the compressive stress that is commonly observed for sputtered Cu2 O [15]. This FWHM value of Cu2 O (2 2 0) peak in ω-scan of Fig. 3(b) was 0.1429◦ . This value was smaller than previous reported, ωFWHM = 0.19◦ [16]. These results show that the Cu2 O film grown on MgO (1 1 0) has a very high out-of-plane crystallinity. We employed the φ-scan of the HR-XRD to evaluate the inplane crystal quality of the Cu2 O films as shown in Fig. 4(a). The peaks of (2 2 0) plane with a 180◦ separation show that the Cu2 O films have a homogeneous in-plane alignment on MgO
Fig. 6. X-ray ϕ − φ pole scan with a 2θ fixed at the (a) Cu2 O (2 0 0) and (b) Cu2 O (2 2 0) reflections.
Lattice spacing
˚ Calculated value (A)
˚ JCPDS-card 5-0667 (A)
MgO (3 3 1) Cu2 O (3 3 1) MgO (2 2 2) Cu2 O (2 2 2)
0.964 0.987 1.217 1.234
0.966 0.979 1.2158 1.233
(1 1 0). It is noteworthy that the Cu2 O film has the (2 2 0) φscan rocking curve with a small FWHM (0.3032◦ ) as shown in Fig. 4(b). The small FWHM of (2 2 0) φ rocking curve indicates that the Cu2 O film is high in-plane crystalline quality as well out-of-plane crystalline quality. These results of in/out-plane measurement using HR-XRD implied the direction of crystal axis was Cu2 O [1 1 0]//MgO [1 0 0], Cu2 O [0 0 1]//MgO [0 0 1] and Cu2 O [1 1 0]//MgO [1 −1 0], as shown in Fig. 5. There-
Fig. 7. (a) RSM of asymmetric around the (4 2 0) lattice point of the MgO (1 1 0) substrate. (b) RSM of asymmetric around the (2 2 0) and (3 3 1) lattice point of the MgO (1 1 0) substrate.
H. Kobayashi et al. / Materials Chemistry and Physics 106 (2007) 292–295
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where hν is the photon energy, Eg the optical band gap and B constant. For a direct transition, n = 1/2 or 2/3. A value of n = 1/2 was found to be most suitable for Cu2 O films, since it gives the best linear graph in the band edge region. In Fig. 8, the (αhν)2 of the Cu2 O film is plotted as a function of the band gap energy. The value of Eg can be obtained by extrapolating the linear portion to the photon energy axis. The Eg of deposited Cu2 O film was Eg = 2.38 eV. The obtained Eg is similar to those of Cu2 O films prepared by other methods [19–21]. 4. Conclusions
Fig. 8. Band gap energy dependence of the (αhν)2 of the Cu2 O film deposited on MgO (1 1 0) substrate.
fore, FWHM of (2 2 0) φ rocking curve of Cu2 O film describes FWHM of (1 1 0) plane rocking curve of Cu2 O film. Fig. 6 shows X-ray ϕ − φ pole scan with a 2θ fixed at the Cu2 O (2 0 0) and Cu2 O (2 2 0) reflections. In the X-ray ϕ − φ pole scan with a 2θ fixed at the Cu2 O (2 0 0) reflection, two clear spots from (0 2 0) and (2 0 0) were observed in Fig. 6(a). In addition, two pair spots of (0 1 1)–(0 1 1) and (1 0 1)–(1 0 1) were observed in the X-ray ϕ − φ pole scan with a 2θ fixed at the Cu2 O (2 2 0) reflection, as shown in Fig. 6(b). This means that Cu2 O film is grown without rotation domain. Strain and relaxation of Cu2 O film were characterized by reciprocal space mapping (RSM) of the XRD intensity around asymmetrical diffraction spots of Cu2 O. Fig. 7(a and b) shows the RSM around (4 2 0)–(3 1 0) and (3 3 1)–(2 2 2) diffractions from a Cu2 O films on MgO (1 1 0). In the lattice spacing calculated from direction spot, the Cu2 O film is almost perfectly aligned to that of Cu2 O JCPDS-card 5-0667, as shown in Table 2. This result shows Cu2 O film grown without strain. It is, therefore, evident that a Cu2 O film with high quality was grown by means of AP-HCVD.The absorption coefficient α was calculated by T = A exp(−αd)
(1)
where T is the transmittance of the Cu2 O film, A a constant, and d is the film thickness. The optical band gaps of the Cu2 O films were determined by applying the Tauc model [17], and David and Mott [18] in the high-absorbance region: αhν = B(hν − Eg )n
(2)
Thin films of Cu2 O were deposited on a MgO (1 1 0) substrate under atmospheric pressure by a new vapor-phase deposition method using CuI and O2 as starting materials. HR-XRD measurement showed that the Cu2 O films have a high degree of out-of-plane and in-plane crystallinity. Furthermore, pole-figure and RSM of Cu2 O films showed Cu2 O film is grown without strain. Optical band gap energy of Cu2 O film calculated from absorption spectra showed 2.38 eV. Consequently, it is evident that reaction of CuI and O2 under atmospheric pressure yields high-quality Cu2 O films. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
E.F. Gross, Nuovo Cimento Suppl. 3 (1956) 672. S. Nikitine, Optical Properties of Solids, Plenum, Newyork, 1969. V.I. Agekyan, Phys. Status Solidi (a) 43 (1977) 11. D.W. Snoke, J.P. Wolfe, A. Mysyrowicz, Phys. Rev. Lett. 64 (1990) 2543. D.W. Snoke, J.P. Wolfe, Phys. Rev. B 39 (1989) 4030. J.L. Lin, J.P. Wolfe, Phys. Rev. Lett. 23 (1993) 1222. H. Shimada, T. Masumi, J. Phys. Soc. Jpn. 58 (1989) 1717. H. Amekura, T. Masumi, J. Phys. Soc. Jpn. 64 (1995) 2684. M. Ottosson, J.O. Carlsson, Surf. Cort. Technol. 78 (1996) 263. T. Maruyama, Sol. Energy Mater. Sol. Cell 56 (1998) 85. G.G. Condorelli, G. Malandrino, I.L. Fragala, Chem. Vap. Deposition 5 (1999) 21. H. Holzoschuh, H. Suhr, Appl. Phys. A 51 (1990) 486. J. Mrales, L. Sanchez, S. Bijanl, L. Martinez, M. Gabas, J.R. Barrado, Electrochemi. Solid State Lett. 8 (2005) A159. R. Liu, E.W. Bohannan, J.A. Switzer, F. Oba, F. Ernst, Appl. Phys. Lett. 83 (2003) 1944. M. Ottosson, J. Lu, J.O. Carlsson, J. Cryst. Growth 151 (1995) 305. Z.G. Yin, H.T. Zhang, D.M. Goodner, M.J. Bedzyk, R.P.H. Chang, Y. Sun, J.B. Ketterson, Appl. Phys. Lett. 86 (2005) 061901. J. Tauc, Amorphous and Liquid Semiconductor, Plenum, London, 1974. E.A. David, N.F. Mott, Philos. Mag. 22 (1970) 903. S.C. Ray, Sol. Energy Mater. Sol. Cells 68 (2001) 307. V. Georgieva, M. Ristov, Sol. Energy Mater. Sol. Cell 73 (2002) 67. T. Maruyama, Sol. Energy Mater. Sol. Cell 56 (1998) 58.