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Self-trapped exciton emission in crystalline anatase
Journal of Luminescence 87}89 (2000) 290}292
Self-trapped exciton emission in crystalline anatase I. Sildos *, A. Suisalu , J. Aarik, T. Sekiya, S. Kurita Institute of Physics, Tartu University, Riia Street 142, 51014 Tartu, Estonia Institute of Materials Science, University of Tartu, U$ likooli 18, 50090 Tartu, Estonia Laboratory of Applied Physics, Faculty of Engineering, Yokohama National University, Hodogaya, Yokohama 240, Japan
Abstract A comparative spectroscopic investigation of self-trapped exciton (STE) emission in anatase single crystals and thin "lms was carried out at 5 K. In the single crystals, a strongly polarized STE emission with an electrical vector perpendicular to the c-axis was observed. In#uence of annealing on the spectral characterictics of samples was tested. 2000 Elsevier Science B.V. All rights reserved. Keywords: Anatase; Self-trapped-exciton; Polarized emission
1. Introduction Rutile and anatase are the most common polymorphs of TiO . Both modi"cations have a tetragonal unit cell but they are built up on the networks of di!erently coordinated TiO octahedra. This leads to di!erent material density and di!erent conditions for localization of charge-transfer excitons. Rutile is a denser phase than anatase. In rutile, free exciton states are present while in anatase exciton}phonon coupling leads to self-trapping of excitons [1]. The most basic optical properties of rutile [2] and anatase [3}5] as well as TiO thin "lms [4,6,7] have been studied earlier by several research groups. However, because of their applications in photovoltaic cells [8] and photocatalysts [9] a more detailed optical characterization of anatase is still a topical matter whereas an understanding of the optical processes in thin "lms is of particular interest. In the applications referred to, the grain size in polycrystalline thin "lms has a signi"cant e!ect on the material performance. Several methods have been developed to reduce the crystallite size down to the nanometer level [9,10]. The atomic layer deposition (ALD) method is one of the most promising techniques for producing such
* Corresponding author. Tel.: 00372-7-428882; fax: #003727-383033. E-mail address: [email protected] (I. Sildos)
nanocrystalline materials. The method allows one to get TiO thin "lms in di!erent modi"cations [10]. Besides, the "lm thickness is very uniform while the thickness and the grain size in polycrystalline "lms can be accurately controlled. A study based on the X-ray di!raction and spectroscopic characterization of nanocrystalline TiO thin "lms grown by the ALD, has been published earlier [7]. The purpose of this work is to compare the spectroscopic characteristics of anatase thin "lms and single crystals whereas the polarization of photoluminescence is under our special attention.
2. Experimental The single crystals studied in this work were synthesized by the chemical vapor transport method [5].The thin "lms were grown in a #ow-type ALD reactor described elsewhere [10] at the substrate temperature of 470 K with titanium ethoxide and water as precursors, and at 700 K with titanium tetrachloride and water as precursors. The thickness of the "lms studied ranged from 100 to 240 nm. The structure of the "lms was determined by X-ray di!raction (XRD) and re#ection high-energy electron di!raction (RHEED) methods. According to XRD data anatase was the only phase detected and the apparent size of crystallites was 48 and 45 nm in the "lms grown at 470 and 700 K, respectively. The RHEED studies con"rmed that the "lms grown at 470 K
0022-2313/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 3 1 3 ( 9 9 ) 0 0 3 1 8 - X
I. Sildos et al. / Journal of Luminescence 87}89 (2000) 290}292
were of anatase structure whereas no preferable orientation of crystallites was observed. However, di!erently from the XRD study, RHEED showed the existence of TiO -II phase in the "lm grown at 700 K. Also, RHEED studies revealed some texture development in the latter "lms. Photoluminescence (PL) was measured in a Utrekstype helium bath optical cryostat with a possibility for temperature regulation. The 351 and 333 nm (i.e., 3.53 and 3.71 eV) lines of an Ar> laser were used for continuos-wave excitation while in decay measurements photo-luminescence (PL) was excited with 10 ns pulses of a 337 nm (3.67 eV) wavelength N laser and the decay time was recorded with a 2 GHz digital oscilloscope. PL spectra were measured using an experimental set-up where the direction of the incident laser beam was perpendicular to the emission observation direction. In the case of single-crystalline samples, the c-axis of anatase was always perpendicular to the laser beam whereas the electrical (E) vector of the incident light was vertical, i.e., parallel to the c-axis. The emission was observed in the direction perpendicular to the c-axis of an anatase single crystal. To determine the polarization of PL, an analyzer was used to measure separately the emission spectrum with the E-vector perpendicular to the c-axis (I, ) and that with the E-vector parallel to the c-axis (I ). During the measurements of the thin "lms a sample was positioned so that the "lm surface was vertical and the angle between the direction of PL observation and "lm surface was about 453. All the PL measurements were performed at 5 K. In order to study the possible e!ect of structure defects on PL spectra, some samples were annealed in air at 700 K for 2 h.
3. Results The emission spectra of the single-crystalline anatase as well as those of the thin "lm, consisted of a broad band at about 2.25}2.45 eV and of sharp but rather weak lines at 3.31 and 3.37 eV (Fig. 1). Such spectra were recorded at the excitation energy of 3.53 as well as of 3.71 eV. The spectra were in good agreement with those measured for TiO thin "lms earlier [7]. A speci"c feature was the dependence of the position of the broad band maximum in the thin "lms on the deposition conditions. In the case of "lms grown at 470 K, the maximum was at about 2.30}2.35 eV. This value will coincide with that measured for the single crystals (Fig. 1). By contrast, the maximum was at about 2.40}2.45 eV in the case of "lms grown at 700 K and, according to RHEED data, they contained a TiO -II phase. This fact shows that the PL spectrum is sensitive even to small structural changes. Indeed, as the TiO -II phase was not recorded by XRD, the relative amount of that must have been very small. Nevertheless, a remarkable change in the position of the broad emission band appeared.
291
Fig. 1. Spectra of polarized emission of as-grown anatase thin "lm (a) and (b) single crystal. Spectra were recorded at 5 K.
The polarization ratio p of emission was determined as p"(I !I, )/(I #I, ). In the case of as-grown single crystals the 2.35 eV emission band occurred to be strongly negatively (p&!0.4) polarized while the peaks at 3.31 and 3.37 eV were unpolarized (Fig. 1). The polarization of the emission was independent of the direction of the E vector of the excitation laser beam. Expectedly, the emission was not polarized in the case of the anatase "lms which were grown at 470 K and showed no evidence of a preferable orientation of crystallites (Fig. 1). However, some polarization e!ects were observed in the emission of the "lms grown at 700 K. Therefore, the results of the polarization measurements agreed with the RHEED data which indicated that the crystallites were oriented in the latter "lms. The PL decay curves measured for the 2.35 eV band of thin "lms can be decomposed to three components with q &2.5 ns, q &25 ns and q &300 ns, respectively. For single crystals the decay times occurred to be much shorter ((1 ns, i.e., shorter than the time resolution of the equipment we used). Annealing of the monocrystals led to a strong suppression of the intensity of 2.35 eV band, and a rise of the
292
I. Sildos et al. / Journal of Luminescence 87}89 (2000) 290}292
Fig. 2. Emission spectra of anatase single crystal recorded before annealing (solid line) and after annealing at 700 K during 2 h (dotted line).
sion after annealing (Fig. 2). Quenching of the STE emission in annealed single crystals could be related to the appearance of a new absorption band. The latter, in turn, can be interpreted as an e!ect of oxygen vacancies [5]. However, to unravel this problem, some additional experiments should be carried out similar to those performed for STE in MgF [12]. In the present stage of investigations there are not enough data to build up a model for STE state formation in anatase. Yet, one can suggest that due to the minimum distance between Ti and O atoms in the plane which is perpendicular to the c-axis, a STE con"guration appears more likely in the same plane. Therefore, the E-vector of the emission should be parallel to the same plane independently of the polarization of excitation light.
Acknowledgements
intensity at about 2.9 eV (Fig. 2). Annealing of the thin "lms resulted sometimes in similar changes in the emission spectra but in some cases the intensity of the broad band remained almost unchanged. Neither did the PL decay time constants change in the latter case. The reasons for the di!erent in#uence of annealing are not yet clear and can be an object of further studies.
The authors are thankful to Aleks Aidla for his assistance in the growing of thin "lm, to Hugo MaK ndar for the XRD studies and to Teet Uustare for the RHEED measurements. The work was supported by the Estonian Science Foundation (grant No. 3453).
4. Discussion
References
We can conclude from our spectroscopic studies that in both single-crystal anatase and polycrystalline "lms the photoexcitation to conduction band leads to the self-trapping of the excitonic state, which causes quite a strong STE emission at &2.35 eV. Sharp lines at 3.31 and 3.37 eV are present in both kinds of samples and could be explained as an emission of excitons bound to shallow defects or impurities, analogously to some additional peaks in the emission spectra of SiC [11]. Besides, the 3.31 and 3.37 eV lines are unpolarized which once more indicates that these lines are not of the band-toband transition origin. For single crystals the STE emission is strongly polarized in the direction perpendicular to the c-axis of the crystal (p&!0.4) and independent of the polarization of the excitation light. The rather small decay time (in nanosecond scale) and relatively high intensity of emission indicates a rapid formation of the STE state and high oscillator strength of the STE emission. Annealing leads to the creation of a new absorption band at 3.0 eV [5]. The same band appears in the emis-
[1] H. Tang, F. Levy, H. Berger, P. Schmid, Phys. Rev. B 52 (11) (1995) 7771. [2] A. Amtout, R. Leonelli, Phys. Rev. B 51 (1995) 6848. [3] H. Tang, H. Berger, P. Schmid, F. Levy, Solid State Commun. 92 (3) (1994) 267. [4] H. Tang, H. Berger, P. Schmid, F. Levy, Solid State Commun. 87 (9) (1993) 847. [5] N. Hosaka, T. Sekiya, S. Kurita, J. Lumin. 72}74 (1997) 874. [6] K.M. Krishna, Md. Mosaddeq-ur-Rahman, T. Miki, T. Soga, K. Igarashi, S. Tanemura, M. Umeno, Appl. Surf. Sci. 113/114 (1997) 149. [7] A. Suisalu, J. Aarik, H. MaK ndar, I. Sildos, Thin Solid Films 336 (1998) 295. [8] H. Deng, Z. Lu, H. Mao, H. Xu, J. Vac. Sci. Technol. B 15 (1997) 1460. [9] K. Kawasaki, J.F. Despres, S. Kamei, M. Ishikawa, O. Odawara, J. Mater. Chem. 7 (1997) 2117. [10] J. Aarik, A. Aidla, V. Sammelselg, H. Siimon, T. Uustare, J. Crystal Growth 169 (1996) 496. [11] A. Itoh, T. Kimoto, H. Matsunami, Jpn. J. Appl. Phys. 35 (1996) 4373. [12] K. Tanimura, N. Itoh, J. Appl.Phys. 69 (11) (1991) 7831.
Self-trapped exciton emission in crystalline anatase I. Sildos *, A. Suisalu , J. Aarik, T. Sekiya, S. Kurita Institute of Physics, Tartu University, Riia Street 142, 51014 Tartu, Estonia Institute of Materials Science, University of Tartu, U$ likooli 18, 50090 Tartu, Estonia Laboratory of Applied Physics, Faculty of Engineering, Yokohama National University, Hodogaya, Yokohama 240, Japan
Abstract A comparative spectroscopic investigation of self-trapped exciton (STE) emission in anatase single crystals and thin "lms was carried out at 5 K. In the single crystals, a strongly polarized STE emission with an electrical vector perpendicular to the c-axis was observed. In#uence of annealing on the spectral characterictics of samples was tested. 2000 Elsevier Science B.V. All rights reserved. Keywords: Anatase; Self-trapped-exciton; Polarized emission
1. Introduction Rutile and anatase are the most common polymorphs of TiO . Both modi"cations have a tetragonal unit cell but they are built up on the networks of di!erently coordinated TiO octahedra. This leads to di!erent material density and di!erent conditions for localization of charge-transfer excitons. Rutile is a denser phase than anatase. In rutile, free exciton states are present while in anatase exciton}phonon coupling leads to self-trapping of excitons [1]. The most basic optical properties of rutile [2] and anatase [3}5] as well as TiO thin "lms [4,6,7] have been studied earlier by several research groups. However, because of their applications in photovoltaic cells [8] and photocatalysts [9] a more detailed optical characterization of anatase is still a topical matter whereas an understanding of the optical processes in thin "lms is of particular interest. In the applications referred to, the grain size in polycrystalline thin "lms has a signi"cant e!ect on the material performance. Several methods have been developed to reduce the crystallite size down to the nanometer level [9,10]. The atomic layer deposition (ALD) method is one of the most promising techniques for producing such
* Corresponding author. Tel.: 00372-7-428882; fax: #003727-383033. E-mail address: [email protected] (I. Sildos)
nanocrystalline materials. The method allows one to get TiO thin "lms in di!erent modi"cations [10]. Besides, the "lm thickness is very uniform while the thickness and the grain size in polycrystalline "lms can be accurately controlled. A study based on the X-ray di!raction and spectroscopic characterization of nanocrystalline TiO thin "lms grown by the ALD, has been published earlier [7]. The purpose of this work is to compare the spectroscopic characteristics of anatase thin "lms and single crystals whereas the polarization of photoluminescence is under our special attention.
2. Experimental The single crystals studied in this work were synthesized by the chemical vapor transport method [5].The thin "lms were grown in a #ow-type ALD reactor described elsewhere [10] at the substrate temperature of 470 K with titanium ethoxide and water as precursors, and at 700 K with titanium tetrachloride and water as precursors. The thickness of the "lms studied ranged from 100 to 240 nm. The structure of the "lms was determined by X-ray di!raction (XRD) and re#ection high-energy electron di!raction (RHEED) methods. According to XRD data anatase was the only phase detected and the apparent size of crystallites was 48 and 45 nm in the "lms grown at 470 and 700 K, respectively. The RHEED studies con"rmed that the "lms grown at 470 K
0022-2313/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 3 1 3 ( 9 9 ) 0 0 3 1 8 - X
I. Sildos et al. / Journal of Luminescence 87}89 (2000) 290}292
were of anatase structure whereas no preferable orientation of crystallites was observed. However, di!erently from the XRD study, RHEED showed the existence of TiO -II phase in the "lm grown at 700 K. Also, RHEED studies revealed some texture development in the latter "lms. Photoluminescence (PL) was measured in a Utrekstype helium bath optical cryostat with a possibility for temperature regulation. The 351 and 333 nm (i.e., 3.53 and 3.71 eV) lines of an Ar> laser were used for continuos-wave excitation while in decay measurements photo-luminescence (PL) was excited with 10 ns pulses of a 337 nm (3.67 eV) wavelength N laser and the decay time was recorded with a 2 GHz digital oscilloscope. PL spectra were measured using an experimental set-up where the direction of the incident laser beam was perpendicular to the emission observation direction. In the case of single-crystalline samples, the c-axis of anatase was always perpendicular to the laser beam whereas the electrical (E) vector of the incident light was vertical, i.e., parallel to the c-axis. The emission was observed in the direction perpendicular to the c-axis of an anatase single crystal. To determine the polarization of PL, an analyzer was used to measure separately the emission spectrum with the E-vector perpendicular to the c-axis (I, ) and that with the E-vector parallel to the c-axis (I ). During the measurements of the thin "lms a sample was positioned so that the "lm surface was vertical and the angle between the direction of PL observation and "lm surface was about 453. All the PL measurements were performed at 5 K. In order to study the possible e!ect of structure defects on PL spectra, some samples were annealed in air at 700 K for 2 h.
3. Results The emission spectra of the single-crystalline anatase as well as those of the thin "lm, consisted of a broad band at about 2.25}2.45 eV and of sharp but rather weak lines at 3.31 and 3.37 eV (Fig. 1). Such spectra were recorded at the excitation energy of 3.53 as well as of 3.71 eV. The spectra were in good agreement with those measured for TiO thin "lms earlier [7]. A speci"c feature was the dependence of the position of the broad band maximum in the thin "lms on the deposition conditions. In the case of "lms grown at 470 K, the maximum was at about 2.30}2.35 eV. This value will coincide with that measured for the single crystals (Fig. 1). By contrast, the maximum was at about 2.40}2.45 eV in the case of "lms grown at 700 K and, according to RHEED data, they contained a TiO -II phase. This fact shows that the PL spectrum is sensitive even to small structural changes. Indeed, as the TiO -II phase was not recorded by XRD, the relative amount of that must have been very small. Nevertheless, a remarkable change in the position of the broad emission band appeared.
291
Fig. 1. Spectra of polarized emission of as-grown anatase thin "lm (a) and (b) single crystal. Spectra were recorded at 5 K.
The polarization ratio p of emission was determined as p"(I !I, )/(I #I, ). In the case of as-grown single crystals the 2.35 eV emission band occurred to be strongly negatively (p&!0.4) polarized while the peaks at 3.31 and 3.37 eV were unpolarized (Fig. 1). The polarization of the emission was independent of the direction of the E vector of the excitation laser beam. Expectedly, the emission was not polarized in the case of the anatase "lms which were grown at 470 K and showed no evidence of a preferable orientation of crystallites (Fig. 1). However, some polarization e!ects were observed in the emission of the "lms grown at 700 K. Therefore, the results of the polarization measurements agreed with the RHEED data which indicated that the crystallites were oriented in the latter "lms. The PL decay curves measured for the 2.35 eV band of thin "lms can be decomposed to three components with q &2.5 ns, q &25 ns and q &300 ns, respectively. For single crystals the decay times occurred to be much shorter ((1 ns, i.e., shorter than the time resolution of the equipment we used). Annealing of the monocrystals led to a strong suppression of the intensity of 2.35 eV band, and a rise of the
292
I. Sildos et al. / Journal of Luminescence 87}89 (2000) 290}292
Fig. 2. Emission spectra of anatase single crystal recorded before annealing (solid line) and after annealing at 700 K during 2 h (dotted line).
sion after annealing (Fig. 2). Quenching of the STE emission in annealed single crystals could be related to the appearance of a new absorption band. The latter, in turn, can be interpreted as an e!ect of oxygen vacancies [5]. However, to unravel this problem, some additional experiments should be carried out similar to those performed for STE in MgF [12]. In the present stage of investigations there are not enough data to build up a model for STE state formation in anatase. Yet, one can suggest that due to the minimum distance between Ti and O atoms in the plane which is perpendicular to the c-axis, a STE con"guration appears more likely in the same plane. Therefore, the E-vector of the emission should be parallel to the same plane independently of the polarization of excitation light.
Acknowledgements
intensity at about 2.9 eV (Fig. 2). Annealing of the thin "lms resulted sometimes in similar changes in the emission spectra but in some cases the intensity of the broad band remained almost unchanged. Neither did the PL decay time constants change in the latter case. The reasons for the di!erent in#uence of annealing are not yet clear and can be an object of further studies.
The authors are thankful to Aleks Aidla for his assistance in the growing of thin "lm, to Hugo MaK ndar for the XRD studies and to Teet Uustare for the RHEED measurements. The work was supported by the Estonian Science Foundation (grant No. 3453).
4. Discussion
References
We can conclude from our spectroscopic studies that in both single-crystal anatase and polycrystalline "lms the photoexcitation to conduction band leads to the self-trapping of the excitonic state, which causes quite a strong STE emission at &2.35 eV. Sharp lines at 3.31 and 3.37 eV are present in both kinds of samples and could be explained as an emission of excitons bound to shallow defects or impurities, analogously to some additional peaks in the emission spectra of SiC [11]. Besides, the 3.31 and 3.37 eV lines are unpolarized which once more indicates that these lines are not of the band-toband transition origin. For single crystals the STE emission is strongly polarized in the direction perpendicular to the c-axis of the crystal (p&!0.4) and independent of the polarization of the excitation light. The rather small decay time (in nanosecond scale) and relatively high intensity of emission indicates a rapid formation of the STE state and high oscillator strength of the STE emission. Annealing leads to the creation of a new absorption band at 3.0 eV [5]. The same band appears in the emis-
[1] H. Tang, F. Levy, H. Berger, P. Schmid, Phys. Rev. B 52 (11) (1995) 7771. [2] A. Amtout, R. Leonelli, Phys. Rev. B 51 (1995) 6848. [3] H. Tang, H. Berger, P. Schmid, F. Levy, Solid State Commun. 92 (3) (1994) 267. [4] H. Tang, H. Berger, P. Schmid, F. Levy, Solid State Commun. 87 (9) (1993) 847. [5] N. Hosaka, T. Sekiya, S. Kurita, J. Lumin. 72}74 (1997) 874. [6] K.M. Krishna, Md. Mosaddeq-ur-Rahman, T. Miki, T. Soga, K. Igarashi, S. Tanemura, M. Umeno, Appl. Surf. Sci. 113/114 (1997) 149. [7] A. Suisalu, J. Aarik, H. MaK ndar, I. Sildos, Thin Solid Films 336 (1998) 295. [8] H. Deng, Z. Lu, H. Mao, H. Xu, J. Vac. Sci. Technol. B 15 (1997) 1460. [9] K. Kawasaki, J.F. Despres, S. Kamei, M. Ishikawa, O. Odawara, J. Mater. Chem. 7 (1997) 2117. [10] J. Aarik, A. Aidla, V. Sammelselg, H. Siimon, T. Uustare, J. Crystal Growth 169 (1996) 496. [11] A. Itoh, T. Kimoto, H. Matsunami, Jpn. J. Appl. Phys. 35 (1996) 4373. [12] K. Tanimura, N. Itoh, J. Appl.Phys. 69 (11) (1991) 7831.