- Email: [email protected]
Photoluminescence of β-FeSi2 thin film prepared by ion beam sputter deposition method
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 242 (2006) 673–675 www.elsevier.com/locate/nimb
Photoluminescence of b-FeSi2 thin film prepared by ion beam sputter deposition method K. Shimura a
a,*
, K. Yamaguchi a, H. Yamamoto a, M. Sasase b, S. Shamoto a, K. Hojou
a
Department of Materials Science, Japan Atomic Energy Research Institute, 2-4 Shirakata-Shirane, Tokai-mura, Ibaraki-ken 319-1195, Japan b The Wakasa-wan Energy Research Center, 64-52-1 Nagatani, Tsuruga 914-0192, Japan Available online 9 November 2005
Abstract In this work, photoluminescence (PL) from b-FeSi2 thin film grown by ion beam sputter deposition (IBSD) method is investigated. For the first time, several small PL peaks are observed in the as-grown IBSD films at around 0.77 and 0.83 eV below 100 K. By thermal annealing at 1153 K for more than 24 h, these films showed a strong peak at around 0.81 eV with increased intensity by more than an order of magnitude at 6 K. These annealed samples showed luminescence up to room temperature, while no PL was observed above 100 K for the as-grown films. Ó 2005 Published by Elsevier B.V. PACS: 78.55.Ap; 78.55.Hx; 81.15.Cd; 81.65.Cf Keywords: Ion beam sputter deposition; b-FeSi2; Si; Photoluminescence
1. Introduction It is reported that ‘‘ion beam sputter deposition’’ (IBSD) method, with appropriate surface treatment of Si substrate, has been applied to grow a highly oriented b-FeSi2 film on a single crystal Si substrate [1,2]. It has been reported that b-FeSi2 has a band gap of about 0.8 eV which matches the transmission window of SiO2 fiber [3]. The crucial issues for realizing b-FeSi2 as an optoelectronic device are the growth of single crystal film, the increase of photoluminescence (PL) intensity and the luminescence at higher temperatures. PL has been observed for the b-FeSi2 films prepared by ion beam synthesis (IBS) [4–7], molecular beam epitaxy (MBE) [8], magnetron-sputtering deposition [9] and metal-organic chemical vapor deposition (MOCVD) [10] methods. However, these spectra differ significantly from film to film and usually high temperature annealing is required to obtain strong PL intensity.
*
Corresponding author. Tel.: +81 29 282 6474; fax: +81 29 282 6716. E-mail address: [email protected] (K. Shimura).
0168-583X/$ - see front matter Ó 2005 Published by Elsevier B.V. doi:10.1016/j.nimb.2005.08.175
In this work, for the first time, several minor PL peaks are observed in the as-grown IBSD films below 100 K. Moreover, upon annealing the films at 1153 K for 24 h in a vacuum (<10 4 Pa), a drastic increase of PL intensity was observed. In addition, the obtained results were compared with other b-FeSi2 films prepared by different methods, in order to identify the characteristics of IBSD films. 2. Experimental The b-FeSi2 films were prepared by IBSD method, in which sputtered Fe atoms were deposited onto a Si(1 0 0) substrate heated at 973 K. Under this condition, the Fe atoms form b-FeSi2 film through reaction with Si, where the thickness of the film was controlled to be 100 nm. Most of the substrates were treated with in situ sputter etching by several keV mass-separated Ne+ ion beam prior to deposition, while some substrates were chemically etched based on RCA method (WE: wet etching) [1]. These etchings were then followed by thermal annealing at 1073 K in vacuum (<10 6 Pa). After film deposition, some of the films were
K. Shimura et al. / Nucl. Instr. and Meth. in Phys. Res. B 242 (2006) 673–675
thermally annealed in a vacuum (<10 4 Pa) at 1153 K for more than 24 h. PL measurements were performed in the temperature range of 6–300 K by exciting the samples by a 532 nm (2.33 eV) solid state laser. Luminescence was analyzed by a 25 cm focal length monochrometer (Ritsu, MC-25 N) and detected by a liquid nitrogen cooled InP/ InGaAs photomultiplier (Hamamatsu Photonics, R550972) in the wavelength region of 1100–1700 nm. 3. Results and discussion
0.003
PL Intensity (a.u.)
674
0.002 6K 50 K
0.001
150 K
0 0.7
Typical PL spectra of as-grown IBSD films exhibit two distinct peaks at around 0.77 and 0.83 eV below 100 K, in addition to the bound exciton peak of Si at 1.1 eV [11], as shown in Fig. 1. After thermal annealing of the as-grown film in a vacuum at 1153 K for 24 h, a drastic change in the PL spectra was observed. One distinct peak at around 0.81 eV was observed, where its intensity increased by a factor of 50, in comparison with the PL spectra of as-grown film at 6 K. Although not shown in the figure, the PL was observed at as high as room temperature. It should be mentioned that in general the as-grown films of b-FeSi2 appear to exhibit unique PL spectra depending on the preparation conditions, although they are not reproducible. Nevertheless, they showed almost the same PL spectra after these films were fully annealed at 1153 K. For bulk Si, several PL peaks are known to be associated with dislocations; namely, D1 (0.812 eV), D2 (0.875 eV), D3 (0.934 eV) and D4 (1.000 eV) [12]. It was found that these peaks were too weak to be observed in the as received substrates. On the other hand, the Si substrates sputter-etched by Ne+ ions exhibited strong peaks at around 0.96 and 1.01 eV. These peaks are close to D3 and D4 lines, respectively. Thermal annealing at 1153 K was found to have the following effects on the PL of Si substrates, see Fig. 2: (1) The peak of Si at 1.1 eV grew intensively as a result of annealing. It is probably connected with the recovery of crystallinity of the substrate 0.4
PL Intensity (a.u.)
6K
0.3
annealed
0.2
0.1
as grown
× 50
0 0.7
0.8
100 K
0.9
1.0
1.1
1.2
Photon Energy (eV) Fig. 1. Photoluminescence (PL) spectra of b-FeSi2 films before and after thermal annealing at 1153 K. These spectra were measured at 6 K. Where the PL spectrum for as deposited film is plotted with 50 times magnification.
0.8
0.9 1.0 Photon Energy (eV)
1.1
1.2
Fig. 2. Photoluminescence (PL) spectra of Si substrate irradiated by 3 keV Ne+ ion after thermal annealing at 1153 K.
as confirmed by reflection high-energy electron diffraction (RHEED) analysis [13]. (2) After annealing, a broad peak in the region of 0.7–1.0 eV overshadowed D1–D3 lines, although the D4 line was no longer observed. Besides, the intensity of the peak was not so strong as compared with the 0.81 eV peak of b-FeSi2 film. Hence, it can be considered that Si substrate alone does not result in the emergence of 0.81 eV peak. Moreover, the WE treatment of the substrate also resulted in the enhancement of PL intensity due to thermal annealing. These results indicate that the effect of sputter etching was of minor importance to the enhancement of PL intensity. One of the critical issues regarding the origin of 0.81 eV peak of b-FeSi2 films formed on Si substrate is that it coincides with the D1 line of Si. Other peaks may also originate from Si substrate, so that their assignment should be done carefully. The 0.77 eV peak observed in the as-grown films are also observed in the Si substrate sputter-etched by Ne+ ions. It is likely that they are of the same origin, although the details are not yet clear. The 0.83 eV peak of as-grown films, on the other hand, does not coincide with the 0.81 eV peak of the annealed films, so it is not likely that they are of the same origin. Maeda et al. [7] claim that their PL spectra of IBS films consist of 4 characteristic peaks, that are labeled as A (0.802–0.81 eV), B (0.85–0.87 eV), C (0.75– 0.78 eV) and D (0.89–0.90 eV) bands. Of these four peaks, ‘‘A’’ band is assigned as an intrinsic peak of b-FeSi2, which may correspond to the 0.81 eV peak observed in the annealed IBSD film, whereas ‘‘B’’ and ‘‘C’’ bands to higher and lower energy peaks associated with the 0.81 eV peak, respectively. The temperature dependence of the peak position of PL in the annealed samples is examined. Fig. 3 shows that the peak position of the IBSD film shifts to slightly higher energy with increasing temperature from 6 to 100 K, then to lower energy, from 0.82 to 0.77 eV, above 100 K. It should be noted that the PL characteristics of the IBSD films do not resemble any of the reported PL properties of the films synthesized by ion or plasma processes. While thermal annealing greatly improved PL, a drastic change in film morphology was also revealed by transmission electron microscopy (TEM), where the film
K. Shimura et al. / Nucl. Instr. and Meth. in Phys. Res. B 242 (2006) 673–675
References
0.84
Peak Energy (eV)
675
0.82 This Work Martinelli et al.
0.80 Terai and Maeda
0.78
Chu et al.
0.76 0
50
100
150
200
250
300
Temperature (K) Temperature (K) Temperature (K) Fig. 3. The position of photoluminescence peak of b-FeSi2 films as a function of temperature. The films of Martinelli et al. [5] and Maeda et al. [7] were prepared by IBS method, whereas those of Chu et al. [9] by magnetron-sputtering method.
disintegrated into granules of several ten nanometers in size. It is not clear at present, however, how this change of morphology is related to the observed change in PL. Acknowledgement This work was in part supported by the Grant-in-Aid for Scientific Research (#15560731), Japan Society for the Promotion of Science (JSPS).
[1] M. Haraguchi, H. Yamamoto, K. Yamaguchi, et al., Nucl. Instr. and Meth. B 206 (2003) 313. [2] K. Shimura, T. Katsumata, K. Yamaguchi, et al., Thin Solid Films 461 (2004) 22. [3] H. Lange, Phys. Status Solidi B 201 (1997) 3. [4] N. Kobayashi, H. Katsumata, H.L. Shen, et al., Thin Solid Films 270 (1995) 406. [5] L. Martinelli, E. Grilli, D.B. Migas, et al., Phys. Rev. B 66 (2002) 085320. [6] D.N. Leong, M.A. Harry, K.J. Reeson, et al., Appl. Phys. Lett. 68 (1996) 1649. [7] Y. Maeda, Y. Terai, M. Itakura, et al., Thin Solid Films 461 (2004) 160. [8] T. Suemasu, T. Fujii, K. Takakura, et al., Thin Solid Films 381 (2001) 209. [9] S. Chu, T. Hirohada, H. Kan, Jpn. J. Appl. Phys. 41 (2002) L299. [10] K. Akiyama, T. Kimura, T. Suemasu, et al., Jpn. J. Appl. Phys. 43 (2004) L551. [11] G. Davies, Phys. Rep. 176 (3–4) (1989) 83. [12] N.A. Drozdov, A.A. Patrin, V.D. Tkachev, Sov. Phys. JETP Lett. 23 (11) (1976) 597. [13] S. Igarashi, T. Katsumata, M. Haraguchi, et al., Trans. MRS-J 28 (4) (2003) 1153.
Nuclear Instruments and Methods in Physics Research B 242 (2006) 673–675 www.elsevier.com/locate/nimb
Photoluminescence of b-FeSi2 thin film prepared by ion beam sputter deposition method K. Shimura a
a,*
, K. Yamaguchi a, H. Yamamoto a, M. Sasase b, S. Shamoto a, K. Hojou
a
Department of Materials Science, Japan Atomic Energy Research Institute, 2-4 Shirakata-Shirane, Tokai-mura, Ibaraki-ken 319-1195, Japan b The Wakasa-wan Energy Research Center, 64-52-1 Nagatani, Tsuruga 914-0192, Japan Available online 9 November 2005
Abstract In this work, photoluminescence (PL) from b-FeSi2 thin film grown by ion beam sputter deposition (IBSD) method is investigated. For the first time, several small PL peaks are observed in the as-grown IBSD films at around 0.77 and 0.83 eV below 100 K. By thermal annealing at 1153 K for more than 24 h, these films showed a strong peak at around 0.81 eV with increased intensity by more than an order of magnitude at 6 K. These annealed samples showed luminescence up to room temperature, while no PL was observed above 100 K for the as-grown films. Ó 2005 Published by Elsevier B.V. PACS: 78.55.Ap; 78.55.Hx; 81.15.Cd; 81.65.Cf Keywords: Ion beam sputter deposition; b-FeSi2; Si; Photoluminescence
1. Introduction It is reported that ‘‘ion beam sputter deposition’’ (IBSD) method, with appropriate surface treatment of Si substrate, has been applied to grow a highly oriented b-FeSi2 film on a single crystal Si substrate [1,2]. It has been reported that b-FeSi2 has a band gap of about 0.8 eV which matches the transmission window of SiO2 fiber [3]. The crucial issues for realizing b-FeSi2 as an optoelectronic device are the growth of single crystal film, the increase of photoluminescence (PL) intensity and the luminescence at higher temperatures. PL has been observed for the b-FeSi2 films prepared by ion beam synthesis (IBS) [4–7], molecular beam epitaxy (MBE) [8], magnetron-sputtering deposition [9] and metal-organic chemical vapor deposition (MOCVD) [10] methods. However, these spectra differ significantly from film to film and usually high temperature annealing is required to obtain strong PL intensity.
*
Corresponding author. Tel.: +81 29 282 6474; fax: +81 29 282 6716. E-mail address: [email protected] (K. Shimura).
0168-583X/$ - see front matter Ó 2005 Published by Elsevier B.V. doi:10.1016/j.nimb.2005.08.175
In this work, for the first time, several minor PL peaks are observed in the as-grown IBSD films below 100 K. Moreover, upon annealing the films at 1153 K for 24 h in a vacuum (<10 4 Pa), a drastic increase of PL intensity was observed. In addition, the obtained results were compared with other b-FeSi2 films prepared by different methods, in order to identify the characteristics of IBSD films. 2. Experimental The b-FeSi2 films were prepared by IBSD method, in which sputtered Fe atoms were deposited onto a Si(1 0 0) substrate heated at 973 K. Under this condition, the Fe atoms form b-FeSi2 film through reaction with Si, where the thickness of the film was controlled to be 100 nm. Most of the substrates were treated with in situ sputter etching by several keV mass-separated Ne+ ion beam prior to deposition, while some substrates were chemically etched based on RCA method (WE: wet etching) [1]. These etchings were then followed by thermal annealing at 1073 K in vacuum (<10 6 Pa). After film deposition, some of the films were
K. Shimura et al. / Nucl. Instr. and Meth. in Phys. Res. B 242 (2006) 673–675
thermally annealed in a vacuum (<10 4 Pa) at 1153 K for more than 24 h. PL measurements were performed in the temperature range of 6–300 K by exciting the samples by a 532 nm (2.33 eV) solid state laser. Luminescence was analyzed by a 25 cm focal length monochrometer (Ritsu, MC-25 N) and detected by a liquid nitrogen cooled InP/ InGaAs photomultiplier (Hamamatsu Photonics, R550972) in the wavelength region of 1100–1700 nm. 3. Results and discussion
0.003
PL Intensity (a.u.)
674
0.002 6K 50 K
0.001
150 K
0 0.7
Typical PL spectra of as-grown IBSD films exhibit two distinct peaks at around 0.77 and 0.83 eV below 100 K, in addition to the bound exciton peak of Si at 1.1 eV [11], as shown in Fig. 1. After thermal annealing of the as-grown film in a vacuum at 1153 K for 24 h, a drastic change in the PL spectra was observed. One distinct peak at around 0.81 eV was observed, where its intensity increased by a factor of 50, in comparison with the PL spectra of as-grown film at 6 K. Although not shown in the figure, the PL was observed at as high as room temperature. It should be mentioned that in general the as-grown films of b-FeSi2 appear to exhibit unique PL spectra depending on the preparation conditions, although they are not reproducible. Nevertheless, they showed almost the same PL spectra after these films were fully annealed at 1153 K. For bulk Si, several PL peaks are known to be associated with dislocations; namely, D1 (0.812 eV), D2 (0.875 eV), D3 (0.934 eV) and D4 (1.000 eV) [12]. It was found that these peaks were too weak to be observed in the as received substrates. On the other hand, the Si substrates sputter-etched by Ne+ ions exhibited strong peaks at around 0.96 and 1.01 eV. These peaks are close to D3 and D4 lines, respectively. Thermal annealing at 1153 K was found to have the following effects on the PL of Si substrates, see Fig. 2: (1) The peak of Si at 1.1 eV grew intensively as a result of annealing. It is probably connected with the recovery of crystallinity of the substrate 0.4
PL Intensity (a.u.)
6K
0.3
annealed
0.2
0.1
as grown
× 50
0 0.7
0.8
100 K
0.9
1.0
1.1
1.2
Photon Energy (eV) Fig. 1. Photoluminescence (PL) spectra of b-FeSi2 films before and after thermal annealing at 1153 K. These spectra were measured at 6 K. Where the PL spectrum for as deposited film is plotted with 50 times magnification.
0.8
0.9 1.0 Photon Energy (eV)
1.1
1.2
Fig. 2. Photoluminescence (PL) spectra of Si substrate irradiated by 3 keV Ne+ ion after thermal annealing at 1153 K.
as confirmed by reflection high-energy electron diffraction (RHEED) analysis [13]. (2) After annealing, a broad peak in the region of 0.7–1.0 eV overshadowed D1–D3 lines, although the D4 line was no longer observed. Besides, the intensity of the peak was not so strong as compared with the 0.81 eV peak of b-FeSi2 film. Hence, it can be considered that Si substrate alone does not result in the emergence of 0.81 eV peak. Moreover, the WE treatment of the substrate also resulted in the enhancement of PL intensity due to thermal annealing. These results indicate that the effect of sputter etching was of minor importance to the enhancement of PL intensity. One of the critical issues regarding the origin of 0.81 eV peak of b-FeSi2 films formed on Si substrate is that it coincides with the D1 line of Si. Other peaks may also originate from Si substrate, so that their assignment should be done carefully. The 0.77 eV peak observed in the as-grown films are also observed in the Si substrate sputter-etched by Ne+ ions. It is likely that they are of the same origin, although the details are not yet clear. The 0.83 eV peak of as-grown films, on the other hand, does not coincide with the 0.81 eV peak of the annealed films, so it is not likely that they are of the same origin. Maeda et al. [7] claim that their PL spectra of IBS films consist of 4 characteristic peaks, that are labeled as A (0.802–0.81 eV), B (0.85–0.87 eV), C (0.75– 0.78 eV) and D (0.89–0.90 eV) bands. Of these four peaks, ‘‘A’’ band is assigned as an intrinsic peak of b-FeSi2, which may correspond to the 0.81 eV peak observed in the annealed IBSD film, whereas ‘‘B’’ and ‘‘C’’ bands to higher and lower energy peaks associated with the 0.81 eV peak, respectively. The temperature dependence of the peak position of PL in the annealed samples is examined. Fig. 3 shows that the peak position of the IBSD film shifts to slightly higher energy with increasing temperature from 6 to 100 K, then to lower energy, from 0.82 to 0.77 eV, above 100 K. It should be noted that the PL characteristics of the IBSD films do not resemble any of the reported PL properties of the films synthesized by ion or plasma processes. While thermal annealing greatly improved PL, a drastic change in film morphology was also revealed by transmission electron microscopy (TEM), where the film
K. Shimura et al. / Nucl. Instr. and Meth. in Phys. Res. B 242 (2006) 673–675
References
0.84
Peak Energy (eV)
675
0.82 This Work Martinelli et al.
0.80 Terai and Maeda
0.78
Chu et al.
0.76 0
50
100
150
200
250
300
Temperature (K) Temperature (K) Temperature (K) Fig. 3. The position of photoluminescence peak of b-FeSi2 films as a function of temperature. The films of Martinelli et al. [5] and Maeda et al. [7] were prepared by IBS method, whereas those of Chu et al. [9] by magnetron-sputtering method.
disintegrated into granules of several ten nanometers in size. It is not clear at present, however, how this change of morphology is related to the observed change in PL. Acknowledgement This work was in part supported by the Grant-in-Aid for Scientific Research (#15560731), Japan Society for the Promotion of Science (JSPS).
[1] M. Haraguchi, H. Yamamoto, K. Yamaguchi, et al., Nucl. Instr. and Meth. B 206 (2003) 313. [2] K. Shimura, T. Katsumata, K. Yamaguchi, et al., Thin Solid Films 461 (2004) 22. [3] H. Lange, Phys. Status Solidi B 201 (1997) 3. [4] N. Kobayashi, H. Katsumata, H.L. Shen, et al., Thin Solid Films 270 (1995) 406. [5] L. Martinelli, E. Grilli, D.B. Migas, et al., Phys. Rev. B 66 (2002) 085320. [6] D.N. Leong, M.A. Harry, K.J. Reeson, et al., Appl. Phys. Lett. 68 (1996) 1649. [7] Y. Maeda, Y. Terai, M. Itakura, et al., Thin Solid Films 461 (2004) 160. [8] T. Suemasu, T. Fujii, K. Takakura, et al., Thin Solid Films 381 (2001) 209. [9] S. Chu, T. Hirohada, H. Kan, Jpn. J. Appl. Phys. 41 (2002) L299. [10] K. Akiyama, T. Kimura, T. Suemasu, et al., Jpn. J. Appl. Phys. 43 (2004) L551. [11] G. Davies, Phys. Rep. 176 (3–4) (1989) 83. [12] N.A. Drozdov, A.A. Patrin, V.D. Tkachev, Sov. Phys. JETP Lett. 23 (11) (1976) 597. [13] S. Igarashi, T. Katsumata, M. Haraguchi, et al., Trans. MRS-J 28 (4) (2003) 1153.