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Photoluminescence and X-ray diffraction studies on Cu2O
ARTICLE IN PRESS Journal of Luminescence 129 (2009) 1483–1487
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Photoluminescence and X-ray diffraction studies on Cu2O ˜ o-Garcı´a c, M. Galva´n-Arellano a, H. Solache-Carranco a,, G. Jua´rez-Dı´az a, A. Esparza-Garcı´a c, M. Brisen b a a ˜ a-Sierra J. Martı´nez-Jua´rez , G. Romero-Paredes , R. Pen a
´xico Departamento de Ingenierı´a Ele´ctrica, SEES, CINVESTAV-IPN, Me´xico, D.F., Me ´n en Dispositivos Semiconductores, BUAP, Puebla, Pue., Me´xico Centro de Investigacio c ´xico, D.F., Me´xico Centro de Ciencias Aplicadas y Desarrollo de Tecnologı´a-UNAM, Me b
a r t i c l e in fo
abstract
Available online 8 April 2009
Cuprous oxide (Cu2O) crystals were grown by the two-step crystallization method in air atmosphere conditions from polycrystalline thin copper foils. The method comprises two stages; in the first one the copper plates are oxidized at 1020 1C by some hours in line with its initial thickness. In the second stage, the growth of large crystalline areas is promoted by annealing the Cu2O samples at 1100 1C for long periods. Raman scattering an X-ray measurements demonstrates the existence of the single-phase Cu2O. The effects on the crystalline structure and photoluminescence (PL) response were studied as a function of the conditions used in the second stage of the synthesis method. PL spectra were taken from 10 to 180 K to define the main radiative recombination paths. Besides the near band excitonic transitions, two strong emission bands at 720 and 920 nm associated with relaxed excitons at oxygen and copper vacancies were detected. Both excitonic-vacancy bond transitions presented similar intensities that are related to the growth method. X-ray and Raman scattering measurements help to assess the samples crystalline quality. & 2009 Elsevier B.V. All rights reserved.
Keywords: Cu2O Semiconductors Raman scattering Photoluminescence X-ray diffraction
1. Introduction The synthesis of good quality cuprous oxide (Cu2O) is important because of its usefulness for several semiconductor devices [1,2]. The use of Cu2O in optoelectronic devices has recently gained impulse because it posses natural p-type conductivity between the great variety of metallic oxide semiconductors, for instance ZnO [3]. Cu2O crystallizes in cubic structure and has direct bandgap energy of 2.1 eV [3]. Studies on the electrical properties demonstrate that it has an acceptor level at EV+0.4 eV, and two donor levels at 1.1 and 1.3 eV below the conduction band [4]. In spite of their interesting properties, the application of Cu2O in optoelectronics is restricted because there are some difficulties to control their electrical and optical properties. An important process to produce high quality Cu2O consists in promoting the secondary crystallization growth mode by annealing thin copper oxide foils in air or oxygen atmospheres [5,15]. The secondary crystallization growth mode is realized inside the Cu2O region according to the Cu–O equilibrium phase diagram [6]; after the crystallization process Cu2O samples with crystalline areas greater than one inch in square can be produced. The starting copper oxide was formerly prepared from polycrystalline copper foils,
Corresponding author. Tel.: +52 55 5747 3800.
E-mail address: [email protected] (H. Solache-Carranco). 0022-2313/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2009.02.033
and the runs can be done at atmospheric pressure [15]. The usefulness of the growth mode has been demonstrated in the synthesis of various materials [7–10]. The Cu2O p-type conductivity is usually linked to the Cu vacancies (VCu), however the existence of O2 vacancies (VO) must be considered to explain the general behavior of their physicochemical properties. Bloem et al. demonstrated the oxygen vacancies give rise to two oxygenvacancy levels within the Cu2O energy gap [12]. Recently, the synthesis of Cu2O samples with n-type conductivity are reported [13], where the authors claim the conductivity could be assigned to the dominance of O2 vacancies. Some authors have related their usual low electrical conductivity to the existence of unidentified donors [11,14]. Therefore, studies to clarify the role of the growth method on the Cu2O characteristics are necessary. In this work we report the structural and optical characterization of crystalline Cu2O grown by the secondary crystallization method. The Cu2O samples were characterized by X-ray diffraction, Raman scattering and photoluminescence (PL). The PL measurements were done from 10 to 180 K, to study the influence of the growth conditions on the optical properties.
2. Experimental details The characterized samples were grown by the secondary crystallization growth method [5,15]. Square shaped (1 1 cm2)
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thin foils of polycrystalline copper with 99.99% of chemical purity and 380 mm in thickness were used as source material. The entire processing consisted of two stages; in the first one the copper foils were oxidized to Cu2O at 1020 1C by a period of 8 h. The period required to realize the copper oxidation was estimated on the basis of reference [16], where the quantity of cuprous oxide (mass in mg/cm2) converted from polycrystalline flat copper foils as a function of time is given. The results of Ref. [16] were confirmed for our laboratory conditions. The crystallization process was completed in a second annealing stage at 1100 1C by a period over 168 h. Long annealing periods are required to promote the formation of large crystalline areas. The runs were done at one atmosphere of pressure in a horizontal reactor operating in air atmosphere conditions. The reactor chamber is a cylindrical quartz tube heated by resistive elements. Due to the high chemical reactivity of the Cu2O at the processing temperatures, the samples were supported with platinum wires to avoid contact with the chamber walls. After the crystallization stage the Cu2O samples were polished to attain optically finished surfaces. Both sides of the Cu2O samples were chemomechanical polished according to the standard methods [17]. Afterwards, the samples were thoroughly cleaned with organic solvents, followed by a soft chemical etch with a H2O:HNO3 solution (10:1) for 5 s, and a final rinsing with deionized water. The X-ray diffractograms were measured in a BRUKER 08 DISCOVER using the Cu Ka radiation (1.5406 A˚), with 40 kV and current of 40 mA. The Raman dispersion studies were done in a Horiba–Jobin Yvon equipment, model LabRAM HR800 with a resolution of 1 cm1. The PL studies were done between 10 K and room temperature, using a SPEX double monochromator. For low temperature PL measurements a He closed cycle Janis cryostat was used. The PL signal was excited with two distinct lasers; a 50 mW He–Cd laser with the 325 nm line, and an Ar laser of 50 mW with the 488 nm line. The PL signal from the sample was focused into the entrance slit of the monochromator, and detected with a S1 type photomultiplier tube.
3. Results and discussion In Fig. 1, photomicrographs of the surface of two Cu2O samples crystallized at distinct periods are shown. Fig. 1(a) is the surface of an as-grown sample crystallized by 24 h; in this case crystalline areas of 150 mm in average diameter are clearly visible. Fig. 1(b) shows the surface feature of a polished sample crystallized at 1040 1C by 168 h; the average diameter for the crystalline area was
over 1000 mm, therefore the photomicrograph shows only a small area of a single crystal. These results clearly show the increase in size of the crystallites as the annealing time is increased. These results were reproducible at the selected growth conditions; furthermore, the growth rate of the crystalline areas can be controlled with the crystallization temperatures [15]. In Fig. 2, the X-ray diffraction patterns taken at several points of a Cu2O sample crystallized at 1040 1C at two distinct periods are included; Fig. 2(a) shows the X-ray diffraction patterns of a sample crystallized by 24 h, and Fig. 2(b) shows the corresponding patterns for a sample crystallized by 72 h. The diffraction patterns corresponds to single phase Cu2O (JCPDS card file no. 5-0667). No observable traces of Cu or CuO were detected. On the sample annealed by 24 h, four diffraction peaks of the Cu2O phase were observed and identified as the (11 0), (111), (2 0 0), (2 11) and (2 2 0) directions. This result demonstrates the Cu2O sample is constituted by a set of small crystallites with distinct orientations. Conversely, on samples crystallized by 72 h only the (2 11) orientations were detected, with a single and narrow peak. The lattice parameter was a ¼ 4.27 A˚, which is closely related to the Cu2O crystalline standard. The similar results for the X-ray diffraction patterns on five different points of the sample demonstrate the homogeneity of the synthesized material. The prevailing frequency of the (2 11) planes can be explained because those are the Cu2O planes with the highest atomic density, and thus with the minimum free energy [18]. Fig. 3 shows the Raman dispersion spectra of a Cu2O sample crystallized during 96 h. A detailed review on distinct points of the sample gave the same Raman spectra. The Raman spectra show the characteristic phonon frequencies of crystalline Cu2O. The phonon modes at 109 cm1 (G 12) and the 1 G(1) are inactive Raman mode and an only 15 (LO) at 154 cm infrared (ir) allowed mode in perfect Cu2O crystals, respectively. The most intense Raman signal is the second-order overtone 2 G12 1 (1) (218 cm ). Additionally, other second-order overtone 2 G15 (308 cm1), one fourth-order overtone 4 G12 (436 cm1), a 1 + Raman-allowed mode G25 (515 cm ), one red-allowed mode 1 1 G(2) ) and a second order combination 15 (TO, 635 cm , LO, 665 cm (2) 1 ) [19–25], are detected. The inactive [G(1) 15 (LO)+G15 ] (820 cm Raman or allowed-ir modes observed in our Cu2O samples can be explained by small crystalline structure changes induced by the chemomechanical surface polishing. The Raman spectra in Fig. 3 fit the main characteristics of the Compaan and Powell’s spectra after the damage of Cu2O single crystals by ion implantation [26,27]. From this point of view, the likeness between the spectra is a sign of the crystal quality of the crystallized Cu2O. Furthermore, the sharply defined mode at 218 cm1 demonstrates
Fig. 1. Photomicrographs of Cu2O samples crystallized at 1040 1C for periods of (a) 24 h and (b) 168 h.
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Fig. 2. X-ray diffraction patterns of the Cu2O samples crystallized at T ¼ 1040 1C. (a) 24 h and (b) 72 h. The diffraction patterns were taken on different sites of the sample area. The pattern taken at the center is marked with x ¼ 0, the one marked at x ¼ 2 was taken in the middle and marked with x ¼ 4 was taken at the sample edge.
Fig. 3. Raman spectra on a Cu2O sample crystallized by 96 h.
the high structural quality of the synthesized samples, which correlates very well with the X-ray diffraction results. The Fig. 4 shows the 10 K PL spectrum of a Cu2O sample crystallized by 192 h. The PL in Cu2O has been extensively studied for a long time [13,28,29]. Two main groups of PL signals have been identified; the free excitons and bound excitonic region extend from 450 to 650 nm. The region of relaxed excitons at oxygen and copper vacancies extend from 630 to 1200 nm [29]. The direct band gap recombination transitions can be observed only at very low temperatures (2 K) in high-quality material from 600 to 630 nm [28]. The 10 K PL spectra in Fig. 4, clearly shows the 610 nm line, which is identified as the direct exciton recombination (X0-line) without phonons participation [29]. The Fig. 5 shows the PL response for a sample crystallized during 96 h. The measurements were done from 30 to 180 K because in this temperature range the most noticeable changes on the PL signal are detected [28–30]. The observed PL signals corresponds to the next transitions: The band at 720 nm is due to the recombination of excitons bound to double charged oxygen
Fig. 4. 10 K PL spectra of a Cu2O sample crystallized by 192 h.
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Fig. 5. PL spectra taken from 30 to 180 K on a Cu2O sample crystallized by a period of 96 h at 1040 1C.
Fig. 6. Behavior of the intensity of the PL bands related to oxygen and copper vacancies from 30 to 180 K.
vacancies (V2+ O ), the band at 810 nm is produced by the recombination of bound excitons at single charged oxygen vacancy (V+O), and the band at 920 nm is due to the recombination of bound excitons at copper vacancies (VCu) [28–30]. In spite of the huge number of scientific work on this issue, the behavior of the relative intensity of the distinct bands as a function of the measurement temperature, and thus the limiting mechanism of the PL recombination kinetics is even a matter of debate [31]. A central factor of this discussion is the nature of the characterized samples [29]. It must be pointed out that the Cu2O samples used to study the PL behavior are usually synthesized with distinct methods, therefore the dominance of either the peak at 720 nm, mainly dominated by V2+ O , or the one at 920 nm, mainly dominated by VCu, depends on the relative vacancy concentration. Furthermore, the relative peak intensities of each peak depend also on the corresponding radiative recombination efficiency. The behaviors of the PL intensity of distinct bands as a function of the measurement temperature are represented at the Fig. 6. As can be observed, the most noticeable changes are experimented by the radiative transitions at the oxygen vacancies (V2+ O ). As can be seen from Fig. 6, the band at 810 nm, related to single charged oxygen vacancy (V+O) remains almost constant in the examined temperature range, with small changes occurring at 65 K. Otherwise, the band at 720 nm shows markedly changes at around the same temperature. In all our samples, the band at 920 nm, related to copper vacancies (VCu) was the dominant band
of the PL spectra, in this the most marked changes occurred at around 55 K. The 920 nm band show two minima at 45 and 70 K, each one coincides with the two maxima of the 720 nm band; meanwhile the 720 nm band show two minima at 65 and 80 K, each one coincides with the two maxima of the 920 nm band. This correspondence clearly shows the correlation between the PL bands and the relative concentration of the oxygen and copper vacancies in the synthesized material. All these excitonicrelated transitions are clearly dependent on the material preparation method [13,29]. The PL spectra measured on Cu2O samples crystallized over 72 h presented characteristics similar to the ones included above, the PL signal was resulted intense and well defined. All these results correlate very well with the X-ray diffraction and Raman studies.
4. Conclusions In summary, we have successfully synthesized Cu2O with good structural and optical properties. The used processing conditions were useful to obtain Cu2O samples with single crystallites of more than 2 mm in diameter over extended areas. The crystallization process was realized in two stages. In the first one, copper plates are converted to Cu2O. In the second stage Cu2O samples were crystallized during an annealing process a 1040 1C by several
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tens of hours to promote the secondary crystallization. The dominant (2 11) planes in the diffractograms on the crystallized samples are explained because they posses the highest atomic density, and thus the minimum free energy. The X-ray diffraction patterns, Raman-scattering, and the PL spectra of the Cu2O samples demonstrate that the produced material has enough quality to be used in the research of semiconductor devices.
Acknowledgements This work is supported in part by the CONACYT-Me´xico under the contract 49860. The technical support of Ing. Miguel ˜ o is also acknowledged. Avendan References [1] K. Akimoto, S. Ishizuka, M. Yanagita, Y. Nawa, Goutam K. Paul, T. Sakurai, Sol. Energy 80 (2006) 715. [2] D.K. Zang, Y.C. Liu, Y.L. Liu, H. Yang, Phys. B 351 (2004) 178. [3] B. Balamurunga, B.R. Mehta, Thin Solid Films 396 (2001) 90. [4] A.E. Rakshani, Solid State Electron. 29 (1986) 7. [5] R.S. Toth, K. Kilkson, D. Trivich, J. Appl. Phys. 31 (1960) 1117. [6] F. Humpreys, M. Hatherly, Recrystallization and Related Annealing Phenomena, 2nd Edition, Pergamon Press, Oxford, 2004. [7] T. Sadoh, K. Nagatomo, I. Tsunoda, A. Kenjo, T. Enokida, M. Miyao, Thin Solid Films 464–465 (2004) 99. [8] El-Hang Lee, G.A. Rozgonyi, J. Cryst. Growth 70 (1984) 223.
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A.S.A.C. Diniz, C.J. Kiely, Renewable Energy 29 (2004) 2037. C.V. Thompson., Annu. Rev. Mater. Sci. 20 (1990) 245. H. Raebiger, S. Lany, A. Zunger, Phys. Rev. B 76 (2007) 045209. J. Bloem, A.J. Van der Houven van Oordt, F.A. Kro¨ger, Physica 22 (1956) 1254. R. Garuthara, W. Siripala, J. Lumin. 121 (2006) 173. S. Ishizuka, S. Kato, Y. Okamoto, K. Akimoto, J. Cryst. Growth 237 (2002) 616. ˜ a Sierra, G. Juarez Dı´az, 2007. International H. Solache-Carranco, R. Pen Conference on Electrical and Electronics Engineering (ICEEE 2007), pp. 337–340., Mexico City, Mexico. [Online Available: /http://ieeexplore.ieee.org/iel5/4344970/4344971/ 04345036.pdf?tp=&arnumber=434036&isnumber=4344971S]. A. Ro¨nquist, H. Fishmeister, J. Instrum. Methods 89 (1960–61) 65. W.R. Runyann, Semiconductor Measurements and Instrumentation, McGraw Hill, New York, 1975, pp. 187–216. Landolt-Bo¨rnstein, Group III Condensed Matter, Non-Tetrahedrally Bonded Elements and Binary Compounds I, Volume III/17E-17F-41C, ‘‘Cuprous Oxide (Cu2O) crystal structure, lattice parameters,’’ Published by Springer, Jun 2006. C. Carabatos, Phys. Status Solidi 37 (1970) 773. E.F. Gross, F.I. Kreingol’d, V.L. Makarov, ZhETF Pis. Red. 15 (1972) 383. P.F. Williams, S.P.S. Porto, Phys. Rev. B 81 (1973) 782. A. Compaan, H.Z. Cummins, Phys. Rev. B 12 (1972) 4753. K. Reimann, K. Syassen, Phys. Rev. B 39 (1989) 11113. P.Y. Yu, Y.R. Shen, Phys. Rev. B 12 (1975) 1377. P.Y. Yu, Y.R. Shen, Phys. Rev. B 17 (1978) 4017. A. Compaan, Solid State Commun. 16 (1975) 293. D. Powell, A. Compaan, J.R. Macdonald, R.A. Foreman, Phys. Rev. B 12 (1975) 20. S.V. Gastev, A.A. Kaplyanskii, N.S. Sokolov, Solid State Commun. 42 (1982) 389. T. Ito, T. Masumi, J. Phys. Soc. Jpn. 66 (1997) 2185. C.K. The, F.L. Weichman, Can. J. Phys. 61 (1983) 1423. J.I. Jang, Y. Sun, B. Watkins, J.B. Ketterson, Phys. Rev. B 74 (2006) 235204.
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Photoluminescence and X-ray diffraction studies on Cu2O ˜ o-Garcı´a c, M. Galva´n-Arellano a, H. Solache-Carranco a,, G. Jua´rez-Dı´az a, A. Esparza-Garcı´a c, M. Brisen b a a ˜ a-Sierra J. Martı´nez-Jua´rez , G. Romero-Paredes , R. Pen a
´xico Departamento de Ingenierı´a Ele´ctrica, SEES, CINVESTAV-IPN, Me´xico, D.F., Me ´n en Dispositivos Semiconductores, BUAP, Puebla, Pue., Me´xico Centro de Investigacio c ´xico, D.F., Me´xico Centro de Ciencias Aplicadas y Desarrollo de Tecnologı´a-UNAM, Me b
a r t i c l e in fo
abstract
Available online 8 April 2009
Cuprous oxide (Cu2O) crystals were grown by the two-step crystallization method in air atmosphere conditions from polycrystalline thin copper foils. The method comprises two stages; in the first one the copper plates are oxidized at 1020 1C by some hours in line with its initial thickness. In the second stage, the growth of large crystalline areas is promoted by annealing the Cu2O samples at 1100 1C for long periods. Raman scattering an X-ray measurements demonstrates the existence of the single-phase Cu2O. The effects on the crystalline structure and photoluminescence (PL) response were studied as a function of the conditions used in the second stage of the synthesis method. PL spectra were taken from 10 to 180 K to define the main radiative recombination paths. Besides the near band excitonic transitions, two strong emission bands at 720 and 920 nm associated with relaxed excitons at oxygen and copper vacancies were detected. Both excitonic-vacancy bond transitions presented similar intensities that are related to the growth method. X-ray and Raman scattering measurements help to assess the samples crystalline quality. & 2009 Elsevier B.V. All rights reserved.
Keywords: Cu2O Semiconductors Raman scattering Photoluminescence X-ray diffraction
1. Introduction The synthesis of good quality cuprous oxide (Cu2O) is important because of its usefulness for several semiconductor devices [1,2]. The use of Cu2O in optoelectronic devices has recently gained impulse because it posses natural p-type conductivity between the great variety of metallic oxide semiconductors, for instance ZnO [3]. Cu2O crystallizes in cubic structure and has direct bandgap energy of 2.1 eV [3]. Studies on the electrical properties demonstrate that it has an acceptor level at EV+0.4 eV, and two donor levels at 1.1 and 1.3 eV below the conduction band [4]. In spite of their interesting properties, the application of Cu2O in optoelectronics is restricted because there are some difficulties to control their electrical and optical properties. An important process to produce high quality Cu2O consists in promoting the secondary crystallization growth mode by annealing thin copper oxide foils in air or oxygen atmospheres [5,15]. The secondary crystallization growth mode is realized inside the Cu2O region according to the Cu–O equilibrium phase diagram [6]; after the crystallization process Cu2O samples with crystalline areas greater than one inch in square can be produced. The starting copper oxide was formerly prepared from polycrystalline copper foils,
Corresponding author. Tel.: +52 55 5747 3800.
E-mail address: [email protected] (H. Solache-Carranco). 0022-2313/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2009.02.033
and the runs can be done at atmospheric pressure [15]. The usefulness of the growth mode has been demonstrated in the synthesis of various materials [7–10]. The Cu2O p-type conductivity is usually linked to the Cu vacancies (VCu), however the existence of O2 vacancies (VO) must be considered to explain the general behavior of their physicochemical properties. Bloem et al. demonstrated the oxygen vacancies give rise to two oxygenvacancy levels within the Cu2O energy gap [12]. Recently, the synthesis of Cu2O samples with n-type conductivity are reported [13], where the authors claim the conductivity could be assigned to the dominance of O2 vacancies. Some authors have related their usual low electrical conductivity to the existence of unidentified donors [11,14]. Therefore, studies to clarify the role of the growth method on the Cu2O characteristics are necessary. In this work we report the structural and optical characterization of crystalline Cu2O grown by the secondary crystallization method. The Cu2O samples were characterized by X-ray diffraction, Raman scattering and photoluminescence (PL). The PL measurements were done from 10 to 180 K, to study the influence of the growth conditions on the optical properties.
2. Experimental details The characterized samples were grown by the secondary crystallization growth method [5,15]. Square shaped (1 1 cm2)
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thin foils of polycrystalline copper with 99.99% of chemical purity and 380 mm in thickness were used as source material. The entire processing consisted of two stages; in the first one the copper foils were oxidized to Cu2O at 1020 1C by a period of 8 h. The period required to realize the copper oxidation was estimated on the basis of reference [16], where the quantity of cuprous oxide (mass in mg/cm2) converted from polycrystalline flat copper foils as a function of time is given. The results of Ref. [16] were confirmed for our laboratory conditions. The crystallization process was completed in a second annealing stage at 1100 1C by a period over 168 h. Long annealing periods are required to promote the formation of large crystalline areas. The runs were done at one atmosphere of pressure in a horizontal reactor operating in air atmosphere conditions. The reactor chamber is a cylindrical quartz tube heated by resistive elements. Due to the high chemical reactivity of the Cu2O at the processing temperatures, the samples were supported with platinum wires to avoid contact with the chamber walls. After the crystallization stage the Cu2O samples were polished to attain optically finished surfaces. Both sides of the Cu2O samples were chemomechanical polished according to the standard methods [17]. Afterwards, the samples were thoroughly cleaned with organic solvents, followed by a soft chemical etch with a H2O:HNO3 solution (10:1) for 5 s, and a final rinsing with deionized water. The X-ray diffractograms were measured in a BRUKER 08 DISCOVER using the Cu Ka radiation (1.5406 A˚), with 40 kV and current of 40 mA. The Raman dispersion studies were done in a Horiba–Jobin Yvon equipment, model LabRAM HR800 with a resolution of 1 cm1. The PL studies were done between 10 K and room temperature, using a SPEX double monochromator. For low temperature PL measurements a He closed cycle Janis cryostat was used. The PL signal was excited with two distinct lasers; a 50 mW He–Cd laser with the 325 nm line, and an Ar laser of 50 mW with the 488 nm line. The PL signal from the sample was focused into the entrance slit of the monochromator, and detected with a S1 type photomultiplier tube.
3. Results and discussion In Fig. 1, photomicrographs of the surface of two Cu2O samples crystallized at distinct periods are shown. Fig. 1(a) is the surface of an as-grown sample crystallized by 24 h; in this case crystalline areas of 150 mm in average diameter are clearly visible. Fig. 1(b) shows the surface feature of a polished sample crystallized at 1040 1C by 168 h; the average diameter for the crystalline area was
over 1000 mm, therefore the photomicrograph shows only a small area of a single crystal. These results clearly show the increase in size of the crystallites as the annealing time is increased. These results were reproducible at the selected growth conditions; furthermore, the growth rate of the crystalline areas can be controlled with the crystallization temperatures [15]. In Fig. 2, the X-ray diffraction patterns taken at several points of a Cu2O sample crystallized at 1040 1C at two distinct periods are included; Fig. 2(a) shows the X-ray diffraction patterns of a sample crystallized by 24 h, and Fig. 2(b) shows the corresponding patterns for a sample crystallized by 72 h. The diffraction patterns corresponds to single phase Cu2O (JCPDS card file no. 5-0667). No observable traces of Cu or CuO were detected. On the sample annealed by 24 h, four diffraction peaks of the Cu2O phase were observed and identified as the (11 0), (111), (2 0 0), (2 11) and (2 2 0) directions. This result demonstrates the Cu2O sample is constituted by a set of small crystallites with distinct orientations. Conversely, on samples crystallized by 72 h only the (2 11) orientations were detected, with a single and narrow peak. The lattice parameter was a ¼ 4.27 A˚, which is closely related to the Cu2O crystalline standard. The similar results for the X-ray diffraction patterns on five different points of the sample demonstrate the homogeneity of the synthesized material. The prevailing frequency of the (2 11) planes can be explained because those are the Cu2O planes with the highest atomic density, and thus with the minimum free energy [18]. Fig. 3 shows the Raman dispersion spectra of a Cu2O sample crystallized during 96 h. A detailed review on distinct points of the sample gave the same Raman spectra. The Raman spectra show the characteristic phonon frequencies of crystalline Cu2O. The phonon modes at 109 cm1 (G 12) and the 1 G(1) are inactive Raman mode and an only 15 (LO) at 154 cm infrared (ir) allowed mode in perfect Cu2O crystals, respectively. The most intense Raman signal is the second-order overtone 2 G12 1 (1) (218 cm ). Additionally, other second-order overtone 2 G15 (308 cm1), one fourth-order overtone 4 G12 (436 cm1), a 1 + Raman-allowed mode G25 (515 cm ), one red-allowed mode 1 1 G(2) ) and a second order combination 15 (TO, 635 cm , LO, 665 cm (2) 1 ) [19–25], are detected. The inactive [G(1) 15 (LO)+G15 ] (820 cm Raman or allowed-ir modes observed in our Cu2O samples can be explained by small crystalline structure changes induced by the chemomechanical surface polishing. The Raman spectra in Fig. 3 fit the main characteristics of the Compaan and Powell’s spectra after the damage of Cu2O single crystals by ion implantation [26,27]. From this point of view, the likeness between the spectra is a sign of the crystal quality of the crystallized Cu2O. Furthermore, the sharply defined mode at 218 cm1 demonstrates
Fig. 1. Photomicrographs of Cu2O samples crystallized at 1040 1C for periods of (a) 24 h and (b) 168 h.
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Fig. 2. X-ray diffraction patterns of the Cu2O samples crystallized at T ¼ 1040 1C. (a) 24 h and (b) 72 h. The diffraction patterns were taken on different sites of the sample area. The pattern taken at the center is marked with x ¼ 0, the one marked at x ¼ 2 was taken in the middle and marked with x ¼ 4 was taken at the sample edge.
Fig. 3. Raman spectra on a Cu2O sample crystallized by 96 h.
the high structural quality of the synthesized samples, which correlates very well with the X-ray diffraction results. The Fig. 4 shows the 10 K PL spectrum of a Cu2O sample crystallized by 192 h. The PL in Cu2O has been extensively studied for a long time [13,28,29]. Two main groups of PL signals have been identified; the free excitons and bound excitonic region extend from 450 to 650 nm. The region of relaxed excitons at oxygen and copper vacancies extend from 630 to 1200 nm [29]. The direct band gap recombination transitions can be observed only at very low temperatures (2 K) in high-quality material from 600 to 630 nm [28]. The 10 K PL spectra in Fig. 4, clearly shows the 610 nm line, which is identified as the direct exciton recombination (X0-line) without phonons participation [29]. The Fig. 5 shows the PL response for a sample crystallized during 96 h. The measurements were done from 30 to 180 K because in this temperature range the most noticeable changes on the PL signal are detected [28–30]. The observed PL signals corresponds to the next transitions: The band at 720 nm is due to the recombination of excitons bound to double charged oxygen
Fig. 4. 10 K PL spectra of a Cu2O sample crystallized by 192 h.
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Fig. 5. PL spectra taken from 30 to 180 K on a Cu2O sample crystallized by a period of 96 h at 1040 1C.
Fig. 6. Behavior of the intensity of the PL bands related to oxygen and copper vacancies from 30 to 180 K.
vacancies (V2+ O ), the band at 810 nm is produced by the recombination of bound excitons at single charged oxygen vacancy (V+O), and the band at 920 nm is due to the recombination of bound excitons at copper vacancies (VCu) [28–30]. In spite of the huge number of scientific work on this issue, the behavior of the relative intensity of the distinct bands as a function of the measurement temperature, and thus the limiting mechanism of the PL recombination kinetics is even a matter of debate [31]. A central factor of this discussion is the nature of the characterized samples [29]. It must be pointed out that the Cu2O samples used to study the PL behavior are usually synthesized with distinct methods, therefore the dominance of either the peak at 720 nm, mainly dominated by V2+ O , or the one at 920 nm, mainly dominated by VCu, depends on the relative vacancy concentration. Furthermore, the relative peak intensities of each peak depend also on the corresponding radiative recombination efficiency. The behaviors of the PL intensity of distinct bands as a function of the measurement temperature are represented at the Fig. 6. As can be observed, the most noticeable changes are experimented by the radiative transitions at the oxygen vacancies (V2+ O ). As can be seen from Fig. 6, the band at 810 nm, related to single charged oxygen vacancy (V+O) remains almost constant in the examined temperature range, with small changes occurring at 65 K. Otherwise, the band at 720 nm shows markedly changes at around the same temperature. In all our samples, the band at 920 nm, related to copper vacancies (VCu) was the dominant band
of the PL spectra, in this the most marked changes occurred at around 55 K. The 920 nm band show two minima at 45 and 70 K, each one coincides with the two maxima of the 720 nm band; meanwhile the 720 nm band show two minima at 65 and 80 K, each one coincides with the two maxima of the 920 nm band. This correspondence clearly shows the correlation between the PL bands and the relative concentration of the oxygen and copper vacancies in the synthesized material. All these excitonicrelated transitions are clearly dependent on the material preparation method [13,29]. The PL spectra measured on Cu2O samples crystallized over 72 h presented characteristics similar to the ones included above, the PL signal was resulted intense and well defined. All these results correlate very well with the X-ray diffraction and Raman studies.
4. Conclusions In summary, we have successfully synthesized Cu2O with good structural and optical properties. The used processing conditions were useful to obtain Cu2O samples with single crystallites of more than 2 mm in diameter over extended areas. The crystallization process was realized in two stages. In the first one, copper plates are converted to Cu2O. In the second stage Cu2O samples were crystallized during an annealing process a 1040 1C by several
ARTICLE IN PRESS H. Solache-Carranco et al. / Journal of Luminescence 129 (2009) 1483–1487
tens of hours to promote the secondary crystallization. The dominant (2 11) planes in the diffractograms on the crystallized samples are explained because they posses the highest atomic density, and thus the minimum free energy. The X-ray diffraction patterns, Raman-scattering, and the PL spectra of the Cu2O samples demonstrate that the produced material has enough quality to be used in the research of semiconductor devices.
Acknowledgements This work is supported in part by the CONACYT-Me´xico under the contract 49860. The technical support of Ing. Miguel ˜ o is also acknowledged. Avendan References [1] K. Akimoto, S. Ishizuka, M. Yanagita, Y. Nawa, Goutam K. Paul, T. Sakurai, Sol. Energy 80 (2006) 715. [2] D.K. Zang, Y.C. Liu, Y.L. Liu, H. Yang, Phys. B 351 (2004) 178. [3] B. Balamurunga, B.R. Mehta, Thin Solid Films 396 (2001) 90. [4] A.E. Rakshani, Solid State Electron. 29 (1986) 7. [5] R.S. Toth, K. Kilkson, D. Trivich, J. Appl. Phys. 31 (1960) 1117. [6] F. Humpreys, M. Hatherly, Recrystallization and Related Annealing Phenomena, 2nd Edition, Pergamon Press, Oxford, 2004. [7] T. Sadoh, K. Nagatomo, I. Tsunoda, A. Kenjo, T. Enokida, M. Miyao, Thin Solid Films 464–465 (2004) 99. [8] El-Hang Lee, G.A. Rozgonyi, J. Cryst. Growth 70 (1984) 223.
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