- Email: [email protected]
dc dual magnetron sputtering
Applied Surface Science 212–213 (2003) 279–286
Fabrication and characterization of metal and semiconductor SmS thin films by rf/dc dual magnetron sputtering S. Tanemuraa,*, S. Koidea, Y. Senzakia, L. Miaoa, H. Hiraia, Y. Morib, P. Jinc, K. Kanekod, A. Teraie, N. Nabatova-Gabaine a
Department of Environmental Technology & Urban planning, Nagoya Institute of Technology, Gokisho-cho, Showa-ku, Nagoya 466-8555, Japan b Materials Research Laboratory, NGK Insulators Ltd., Mizuho-ku, Nagoya 467-8530, Japan c Institute of Structural and Engineering Materials, National Institute of Advanced Industrial Science and Technology (AIST), Moriyama-ku, Nagoya 463-8560, Japan d Department of Materials Science and Engineering, HVEM Laboratory, Kyushu University, Higasi-ku, Fukuoka 812-8581, Japan e Horiba Jobin-Yvon Co. Ltd., Chiyoda-ku, Tokyo 101-0031, Japan
Abstract SmS thin films have been individually fabricated on either a-SiO/Si or NaCl substrates at a room temperature by dual targets (dc for metal Sm and rf for pressed powdered chalcogenide Sm2S3) magnetron sputtering of concurrent power control. The fabricated films were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and spectroscopic ellipsometry (SE), respectively, to identify phase formation, structure, and optical band gap. The followings are summarized: (1) polycrystalline metal, intermedium, or semiconductor SmS thin films were identified by XRD, TEM and the phase formation was achieved by controlling the ratio of dc to rf power; (2) the obtained lattice constant of ˚ from electron diffraction (ED) and 5.91 A ˚ from lattice image. The former value is contraction by intermedium phase was 5.85 A ˚ ), while the later one is contraction by 1% compared with semiconducting one, 2% compared with bulk semiconductor (5.97 A although this being probably semiconductor which is suggested by the dominated Sm2þ valence state in Sm 3d of XPS; (3) XPS depth profile result confirm that metal Sm and samarium oxide exist near the film–substrate boundary in intermedium case, while stoichiometric SmS is dominant at the surface layer; (4) in semiconductor case, optical band gap is 2.67 eVobtained by Tauc plot from SE results. # 2003 Elsevier Science B.V. All rights reserved. PACS: 68.55; 81.55.C; 61.16.B; 61.10.N; 79.60; 71.20 Keywords: SmS thin films; Dual magnetron sputtering; TEM; XRD; XPS; Optical band gap
1. Introduction
*
Corresponding author. Tel.: þ81-52-735-5024; fax: þ81-52-735-5024. E-mail address: [email protected] (S. Tanemura).
Since the observation of metal to semiconductor transition of SmS polished crystal by annealing at some 100 8C in vacuum [1,2], and SmS thin films on sapphire, quartz, or glass by heating to either 300 8C in
0169-4332/03/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0169-4332(03)00113-2
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S. Tanemura et al. / Applied Surface Science 212–213 (2003) 279–286
vacuum [3] or 400 8C [4,5], SmS became very interesting materials in the field of exclusive memory, light switching, or hologram recording materials [2]. This transition is accompanied with color change from golden yellow to blue-black and the phenomenon is called as ‘‘thermo coloration’’. We have already reported the growth of SmS film on Si substrate kept at 400 8C by dual targets (dc for metal Sm and rf for pressed powdered chalcogenide Sm2S3) magnetron sputtering system [6] in which we fabricated either metallic or semiconducting phase with concurrent adjustment of the applied power to respective target as well as continuous intermediate phase from metal to semiconductor on a substrate at a constant applied power to the respective target. In this report, we have examined the growth conditions of metallic and semiconducing phase of SmS thin films on Si(1 0 0), and NaCl(1 0 0) crystal substrates even at room temperature individually by the same sputtering system [6]. Three typical sputtering conditions by changing the ratio between applied rf power and that of dc power were adopted to fabricate films on above designated substrates. Then, the obtained films were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), Xray photoelectron spectroscopy (XPS), respectively, to identify the grown crystal phase and to evaluate the crystallinity of the films. The optical band gap of the sample with semiconductor phase was evaluated by Tauc plot [7] using extinction coefficient obtained by spectroscopic ellipsometry (SE) and the value being compared with those appeared on the literatures [8,10].
Fig. 1. Schematics of the dual-target sputter system for SmS thin film.
meter) was dc-powered and the pressed powdered chalcogenide Sm2S3 target (99.9% in purity, 1000 nm in particle size, and 50 mm in diameter) being rf-powered, as previously reported [6], is shown in Fig. 1. The used substrates were Si(1 0 0) with naturally oxidized surface, and NaCl(1 0 0) single crystal, respectively. The later one was mainly used for the TEM observation of films by dissolving the substrate in water. The deposition system was evacuated to a background pressure down to 3 104 Pa, and pure gas of Ar (99.999%) with a certain flow rate to fill the working pressure of 1.00 Pa was introduced during deposition. The target was water-cooled and the distance between the center of the target to that of the substrate being held at 10 cm. The substrates were kept at room temperature throughout the experiment. The three sputtering conditions denoted as A, B, and C with the different ratio of applied rf power to dc power
2. Experimental The schematics of the machine in which the metallic Sm target (99.99% in purity, and 50 mm in diaTable 1 Sputtering conditions for different phase growth Conditions
Applied rf power Sm2S3 (Prf) (W)
Applied dc power Sm (Pdc) (W)
Prf/Pdc
Pressure (Pa)
A (metal) B (quasi metal) C (semiconductor)
20 20 35
10 6.3 6.4
2 3.2 5.46
1 1 1
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Fig. 2. XRD diffraction pattern of metallic, intermediate and semiconducting phase of SmS film (M: metal; I: intermedium; S: semiconductor) in comparison with JCPDS standard for S-SmS.
were summarized in Table 1. The sputtering rate was ˚ /min. estimated to be about 93 A Firstly, the deposited films were characterized by X-ray diffraction (Cu Ka radiation) with a thin-film
scan at a fixed y of 28 to confirm the crystal structure. Then, the microcrystal structure and the crystallinity were examined for the films peeled off from NaCl substrate by high-resolution transmission electron
Fig. 3. Electron diffraction pattern of metal phase of SmS film.
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Fig. 4. Electron diffraction pattern and corresponding HRTEM of intermediate phase of SmS film: (a) diffraction pattern and (b) lattice image.
microscope (TEM) (Philips Tech. I electron micro˚ in resolution and fully scopy at 200 KV with 1.94 A digitalized image system). For the sample identified as intermediate phase on Si substrate, XPS was carried out by ESCA-5700ci (Physical Electronics, Inc.) using non-monochromatic Mg Ka line at 1253.6 eV (14 kV, 400 W) to evaluate both the valence state of Sm atom by a spectrum of Sm 3d5/2 and Sm 3d3/2. The depth profile was also obtained by XPS to characterize Sm and S contents throughout the films thickness.
The optical band gap was extrapolated from ellipsometry results using Bðhu Eg Þ2 ¼ ð4pk/l)hu formula. The details on optical properties will be reported elsewhere. 3. Results and discussion 3.1. XRD pattern Metallic and semiconducting phase of SmS are known as NaCl-type cubic crystal of which lattice
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Table 2 Lattice constant calculated from ED and LI in TEM observation for the films fabricated on NaCl substrate under three sputtering conditions in comparison with bulk materials SmS phase
˚) Film a from ED (A
˚) Film a from LI (A
˚) Bulk a (A
(FEB)/B (%)
(FLB)/B (%)
Metal Intermedium Semiconductor
5.69 5.85 6.01
– 5.91 5.97
5.7 – 5.97
0.18 2.99 (M), 2(S) 0.67
– 4.05 (M), 1 (S) 0
˚ [11] and 5.97 A ˚ [8], respectively. The constant is 5.68 A fingerprint of XRD for the films grown on Si fabricated under conditions A, B, and C are exemplified in Fig. 2 where the 2y angle of principal peaks for phase calculated from the above described lattice constant are also given as a reference. There is no significant difference in the XRD patterns for the different substrates with the same sputtering conditions. Diffraction pattern of the sample A is almost the same with those calculated by the lattice constant for metallic phase [11]. This agrees with the color observation (gold color) of the sample. The color of the sample B is changed to dark purple and at the same time XRD peaks of the sample B are only slightly shifted to lower angles from metal peaks. Although the color of the sample C is blue-black which is typical for semiconductor phase, XRD pattern does not exactly agree with semiconductor phase. Consequently, we leave the problem of phase identification of the samples B and C until electron diffraction (ED) and TEM observation. It is worthwhile to note that any crystals of samarium compound such as SmO, Sm2S3, Sm2O3, and Sm3S4, as well as Sm metal are not observed in the XRD results of all samples.
˚ , as shown in Table 2 with 0.18% discrepancy in 5.69 A comparison with literature cited bulk metal one [8]. Because the film thickness is too thick to observe clearly lattice image of SmS, it was omitted in the text. The lattice image and ED ring pattern of sample B are given in Fig. 4. The d-spacings were calculated using the same method with sample A and the lattice plane indices are also assigned in Fig. 4. The calculated
3.2. Diffraction ring pattern and lattice image by TEM The ED ring pattern of sample A is shown in Fig. 3. The d-spacings of the respective rings were calculated from the diameter of the ED rings, using the camera length of 2.99 m and the wavelength of electron beam ˚ and these obtained values were compared of 0.0251 A with the corresponding bulk metal phase. The film fabricated under condition A was identified as polycrystalline SmS metal phase, as assigned in Fig. 3. Furthermore, the lattice constant was calculated as
Fig. 5. Electron diffraction pattern and corresponding HRTEM of semiconductor phase of SmS film: (a) diffraction pattern and (b) lattice image.
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lattice constants, both from ED and lattice image are also given in Table 2. Lattice constant calculated from ˚ with expansion by 2.99% the ring pattern was 5.85 A from the metallic phase and contraction by 2% compared with semiconducting one. While the lattice con˚ with 4.05% stant measured from lattice image is 5.91 A expansion, in comparison with metallic bulk phase and 1% contraction with semiconducting one. Consequently, we identify this film as intermediate phase between metal and semiconductor. Similarly, ED ring pattern and lattice image of sample C are given in Fig. 5. The lattice spacing calculated from ring pattern and from (2 0 0) lattice ˚ , respectively. Although the image was 6.01 and 5.97 A former showed expansion by 0.67% compared with the bulk semiconductor [8], the later agreed exactly with bulk semiconductor [8]. Hence, sample C was proved as SmS semiconducting phase. Throughout the TEM observation, we can conclude that samples A, B and C are metallic, intermediate and
semiconducting phase of polycrystalline SmS, respectively, with granular structure of about 10–20 nm in size. By above phase identification, the ratio between rf power to dc power is suggested to be a crucial sputtering parameter for the growth of either metal, semiconductor, or intermedium structure of SmS films. 3.3. Sm valence state by XPS It has been reported [12–14] in a spectrum of Sm 3d5/2 and Sm 3d3/2 that a mixed valence state of semiconductor phase have been dominated by Sm2þ state, and in contrast, a mixed valence state of metallic phase has been dominated by Sm3þ state. Spectra of Sm 3d5/2 and Sm 3d3/2 taken for the film on Si for intermediate phase are given in Fig. 6 to characterize whether the intermedium phase is existing as likely metal or semiconductor by the peak ratio
Fig. 6. XPS spectra of Sm2þ and Sm3þ valence state from Sm 3d in the intermedium phase of SmS film.
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Fig. 7. Depth profile of XPS for the film with intermedium phase.
between Sm2þ state and Sm3þ state. In Fig. 6, Sm 3d5/2 peaks appeared at 1083.3 eV for Sm3þ and 1074.0 eV for Sm2þ, respectively. The peak ratio of Sm2þ/Sm3þ is about 3. This domination of Sm2þ state implies that the obtained intermediate phase is likely semiconductor and this coincides with the previously deduced results by ED and TEM. Atomic concentration versus sputter time in the depth profile for the same sample is given in Fig. 7. From the surface (corresponding to sputter time 0) to the middle of the film (about 40 nm in depth), the Sm and S concentration is approximately the same and this results in the stoichiometric SmS, while Sm and O concentration is increasing by 20 and 18% near the film and substrate boundary. This suggests that Sm oxides and metal Sm exist near the film and substrate boundary. Then, there are two possibilities for the no metal Sm and oxide appearance in XRD and TEM observations. The first is Sm metal is soluble in water and it may be difficult to identify in the above described observation process of TEM; the second one is they are amorphous.
3.4. Optical band gap of semiconducting phase As the details of the complex refractive indices of both metallic and semiconducting phase of the present SmS samples, which is analyzed by the spectroscopic ellipsometry will be reported elsewhere, we only summarize the optical band gap value of the semiconducting phase of SmS as follows: We tried four transition modes (indirect allowed, indirect forbidden, direct allowed, and direct forbidden), and confirmed that indirect allowed mode is the best fitting. Consequently, the optical band gap between conduction band and valence band was calculated using the formula of Bðhu Eg Þ2 ¼ ð4pk/l)hu by Tauc [7] from indirect allowed transitions mode, where B: constant; hu: photon energy; 4pk/l: absorption coefficient at wavelength l; k: extinction coefficient. We evaluate Eg as 2.67 eV, which is larger than the assumed value of 2.3 eV by Batlogg et al. [8] and that of 1.9 eV by Davis [9], and smaller than 3.6 eV, derived from the optical spectra by Lashkarev and Ivanchenko [10].
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4. Conclusions
References
SmS thin films have been fabricated on a-SiO2/Si and NaCl substrates at a room temperature by dual targets (dc for metal Sm and rf for pressed powdered chalcogenide Sm2S3) magnetron sputtering. Metal, semiconductor and intermedium phase of SmS films were grown under the optimized ratio of dc to rf power. Phase formation of metal, intermedium, or semiconductor was identified by TEM and XRD. The obtained lattice constant of intermedium phase was ˚ from ED and 5.91 A ˚ from lattice image. The 5.85 A former value is contraction by 2% compared with bulk ˚ ), while the later one is consemiconductor (5.97 A traction by 1% compared with semiconducting one. The peak ratio of Sm2þ/Sm3þ obtained from XPS of Sm 3d5/2 and Sm 3d3/2, taken for the film on Si with intermediate phase is about 3 and this confirms that obtained intermedium phase is likely semiconductor phase. In intermedium case, metal Sm and samarium oxide are exsisting near the film–substrate boundary by depth profile of XPS, while stoichiometric SmS is dominant at the surface layer. In semiconductor case, the optical band gap value of semiconductor film is 2.67 eV from Tauc plot and this is slightly larger than theoretically estimated 2.3 eV.
[1] E. Kaldis, P. Wachter, Solid State Commun. 11 (1972) 907. [2] W. Pohl, R. Badertscher, K.A. Muelar, P. Wachter, Appl. Opt. 13 (1974) 95. [3] S.G. Shullman, T.L. Bzhalava, T.B. Zhukova, T.T. Dedegkaev, I.A. Smilnov, Soviet Phys.—Solid State 17 (1975) 1984. [4] T.L. Bzhalava, T.B. Zhukova, I.A. Smilnov, S.G. Shullman, N.A. Yakovleva, Soviet Phys.—Solid State 16 (1974) 2428. [5] T.L. Bzhalava, S.G. Shullman, T.T. Dedegkaev, T.B. Zhukova, I.A. Smilnov, Phys. Lett. A 55 (1975) 161. [6] P. Jin, M. Tazawa, J.F. Huang, S. Tanemura, J. Cryst. Growth 191 (1998) 285. [7] J. Tauc, Optical Properties of Solids, in: F. Abeles (Ed.), North-Holland, Amsterdam, p. 303. [8] B. Batologg, E. Kaldis, A. Schlegel, P. Wachter, Phys. Rev. B 14 (1976) 5503. [9] H.L. Davis, in: Proceedings of the 9th Rare Earth Research Conference, vol. 1, Blacksburg, VA, 1971, p. 3. [10] G.V. Lashkarev, L.A. Ivanchenko, J. Non-Cryst. Solids 8–10 (1970) 670. [11] B. Batologg, A. Schlegel, P. Wachter, Physica B–C 86–88 (1977) 229. [12] M. Cmpaqna, E. Bucher, G.K. Wertheim, L.D. Longinotti, Phys. Rev. Lett. 33 (1974) 165. [13] G. Krill, J.P. Senateur, A. Amamou, J. Phys F: Metal Phys. 10 (1980) 1889. [14] D.W. Pohl, R. Jaggi, K. Gisler, H. Weibel, Solid State Commun. 17 (1975) 705.
Fabrication and characterization of metal and semiconductor SmS thin films by rf/dc dual magnetron sputtering S. Tanemuraa,*, S. Koidea, Y. Senzakia, L. Miaoa, H. Hiraia, Y. Morib, P. Jinc, K. Kanekod, A. Teraie, N. Nabatova-Gabaine a
Department of Environmental Technology & Urban planning, Nagoya Institute of Technology, Gokisho-cho, Showa-ku, Nagoya 466-8555, Japan b Materials Research Laboratory, NGK Insulators Ltd., Mizuho-ku, Nagoya 467-8530, Japan c Institute of Structural and Engineering Materials, National Institute of Advanced Industrial Science and Technology (AIST), Moriyama-ku, Nagoya 463-8560, Japan d Department of Materials Science and Engineering, HVEM Laboratory, Kyushu University, Higasi-ku, Fukuoka 812-8581, Japan e Horiba Jobin-Yvon Co. Ltd., Chiyoda-ku, Tokyo 101-0031, Japan
Abstract SmS thin films have been individually fabricated on either a-SiO/Si or NaCl substrates at a room temperature by dual targets (dc for metal Sm and rf for pressed powdered chalcogenide Sm2S3) magnetron sputtering of concurrent power control. The fabricated films were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and spectroscopic ellipsometry (SE), respectively, to identify phase formation, structure, and optical band gap. The followings are summarized: (1) polycrystalline metal, intermedium, or semiconductor SmS thin films were identified by XRD, TEM and the phase formation was achieved by controlling the ratio of dc to rf power; (2) the obtained lattice constant of ˚ from electron diffraction (ED) and 5.91 A ˚ from lattice image. The former value is contraction by intermedium phase was 5.85 A ˚ ), while the later one is contraction by 1% compared with semiconducting one, 2% compared with bulk semiconductor (5.97 A although this being probably semiconductor which is suggested by the dominated Sm2þ valence state in Sm 3d of XPS; (3) XPS depth profile result confirm that metal Sm and samarium oxide exist near the film–substrate boundary in intermedium case, while stoichiometric SmS is dominant at the surface layer; (4) in semiconductor case, optical band gap is 2.67 eVobtained by Tauc plot from SE results. # 2003 Elsevier Science B.V. All rights reserved. PACS: 68.55; 81.55.C; 61.16.B; 61.10.N; 79.60; 71.20 Keywords: SmS thin films; Dual magnetron sputtering; TEM; XRD; XPS; Optical band gap
1. Introduction
*
Corresponding author. Tel.: þ81-52-735-5024; fax: þ81-52-735-5024. E-mail address: [email protected] (S. Tanemura).
Since the observation of metal to semiconductor transition of SmS polished crystal by annealing at some 100 8C in vacuum [1,2], and SmS thin films on sapphire, quartz, or glass by heating to either 300 8C in
0169-4332/03/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0169-4332(03)00113-2
280
S. Tanemura et al. / Applied Surface Science 212–213 (2003) 279–286
vacuum [3] or 400 8C [4,5], SmS became very interesting materials in the field of exclusive memory, light switching, or hologram recording materials [2]. This transition is accompanied with color change from golden yellow to blue-black and the phenomenon is called as ‘‘thermo coloration’’. We have already reported the growth of SmS film on Si substrate kept at 400 8C by dual targets (dc for metal Sm and rf for pressed powdered chalcogenide Sm2S3) magnetron sputtering system [6] in which we fabricated either metallic or semiconducting phase with concurrent adjustment of the applied power to respective target as well as continuous intermediate phase from metal to semiconductor on a substrate at a constant applied power to the respective target. In this report, we have examined the growth conditions of metallic and semiconducing phase of SmS thin films on Si(1 0 0), and NaCl(1 0 0) crystal substrates even at room temperature individually by the same sputtering system [6]. Three typical sputtering conditions by changing the ratio between applied rf power and that of dc power were adopted to fabricate films on above designated substrates. Then, the obtained films were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), Xray photoelectron spectroscopy (XPS), respectively, to identify the grown crystal phase and to evaluate the crystallinity of the films. The optical band gap of the sample with semiconductor phase was evaluated by Tauc plot [7] using extinction coefficient obtained by spectroscopic ellipsometry (SE) and the value being compared with those appeared on the literatures [8,10].
Fig. 1. Schematics of the dual-target sputter system for SmS thin film.
meter) was dc-powered and the pressed powdered chalcogenide Sm2S3 target (99.9% in purity, 1000 nm in particle size, and 50 mm in diameter) being rf-powered, as previously reported [6], is shown in Fig. 1. The used substrates were Si(1 0 0) with naturally oxidized surface, and NaCl(1 0 0) single crystal, respectively. The later one was mainly used for the TEM observation of films by dissolving the substrate in water. The deposition system was evacuated to a background pressure down to 3 104 Pa, and pure gas of Ar (99.999%) with a certain flow rate to fill the working pressure of 1.00 Pa was introduced during deposition. The target was water-cooled and the distance between the center of the target to that of the substrate being held at 10 cm. The substrates were kept at room temperature throughout the experiment. The three sputtering conditions denoted as A, B, and C with the different ratio of applied rf power to dc power
2. Experimental The schematics of the machine in which the metallic Sm target (99.99% in purity, and 50 mm in diaTable 1 Sputtering conditions for different phase growth Conditions
Applied rf power Sm2S3 (Prf) (W)
Applied dc power Sm (Pdc) (W)
Prf/Pdc
Pressure (Pa)
A (metal) B (quasi metal) C (semiconductor)
20 20 35
10 6.3 6.4
2 3.2 5.46
1 1 1
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Fig. 2. XRD diffraction pattern of metallic, intermediate and semiconducting phase of SmS film (M: metal; I: intermedium; S: semiconductor) in comparison with JCPDS standard for S-SmS.
were summarized in Table 1. The sputtering rate was ˚ /min. estimated to be about 93 A Firstly, the deposited films were characterized by X-ray diffraction (Cu Ka radiation) with a thin-film
scan at a fixed y of 28 to confirm the crystal structure. Then, the microcrystal structure and the crystallinity were examined for the films peeled off from NaCl substrate by high-resolution transmission electron
Fig. 3. Electron diffraction pattern of metal phase of SmS film.
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Fig. 4. Electron diffraction pattern and corresponding HRTEM of intermediate phase of SmS film: (a) diffraction pattern and (b) lattice image.
microscope (TEM) (Philips Tech. I electron micro˚ in resolution and fully scopy at 200 KV with 1.94 A digitalized image system). For the sample identified as intermediate phase on Si substrate, XPS was carried out by ESCA-5700ci (Physical Electronics, Inc.) using non-monochromatic Mg Ka line at 1253.6 eV (14 kV, 400 W) to evaluate both the valence state of Sm atom by a spectrum of Sm 3d5/2 and Sm 3d3/2. The depth profile was also obtained by XPS to characterize Sm and S contents throughout the films thickness.
The optical band gap was extrapolated from ellipsometry results using Bðhu Eg Þ2 ¼ ð4pk/l)hu formula. The details on optical properties will be reported elsewhere. 3. Results and discussion 3.1. XRD pattern Metallic and semiconducting phase of SmS are known as NaCl-type cubic crystal of which lattice
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283
Table 2 Lattice constant calculated from ED and LI in TEM observation for the films fabricated on NaCl substrate under three sputtering conditions in comparison with bulk materials SmS phase
˚) Film a from ED (A
˚) Film a from LI (A
˚) Bulk a (A
(FEB)/B (%)
(FLB)/B (%)
Metal Intermedium Semiconductor
5.69 5.85 6.01
– 5.91 5.97
5.7 – 5.97
0.18 2.99 (M), 2(S) 0.67
– 4.05 (M), 1 (S) 0
˚ [11] and 5.97 A ˚ [8], respectively. The constant is 5.68 A fingerprint of XRD for the films grown on Si fabricated under conditions A, B, and C are exemplified in Fig. 2 where the 2y angle of principal peaks for phase calculated from the above described lattice constant are also given as a reference. There is no significant difference in the XRD patterns for the different substrates with the same sputtering conditions. Diffraction pattern of the sample A is almost the same with those calculated by the lattice constant for metallic phase [11]. This agrees with the color observation (gold color) of the sample. The color of the sample B is changed to dark purple and at the same time XRD peaks of the sample B are only slightly shifted to lower angles from metal peaks. Although the color of the sample C is blue-black which is typical for semiconductor phase, XRD pattern does not exactly agree with semiconductor phase. Consequently, we leave the problem of phase identification of the samples B and C until electron diffraction (ED) and TEM observation. It is worthwhile to note that any crystals of samarium compound such as SmO, Sm2S3, Sm2O3, and Sm3S4, as well as Sm metal are not observed in the XRD results of all samples.
˚ , as shown in Table 2 with 0.18% discrepancy in 5.69 A comparison with literature cited bulk metal one [8]. Because the film thickness is too thick to observe clearly lattice image of SmS, it was omitted in the text. The lattice image and ED ring pattern of sample B are given in Fig. 4. The d-spacings were calculated using the same method with sample A and the lattice plane indices are also assigned in Fig. 4. The calculated
3.2. Diffraction ring pattern and lattice image by TEM The ED ring pattern of sample A is shown in Fig. 3. The d-spacings of the respective rings were calculated from the diameter of the ED rings, using the camera length of 2.99 m and the wavelength of electron beam ˚ and these obtained values were compared of 0.0251 A with the corresponding bulk metal phase. The film fabricated under condition A was identified as polycrystalline SmS metal phase, as assigned in Fig. 3. Furthermore, the lattice constant was calculated as
Fig. 5. Electron diffraction pattern and corresponding HRTEM of semiconductor phase of SmS film: (a) diffraction pattern and (b) lattice image.
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lattice constants, both from ED and lattice image are also given in Table 2. Lattice constant calculated from ˚ with expansion by 2.99% the ring pattern was 5.85 A from the metallic phase and contraction by 2% compared with semiconducting one. While the lattice con˚ with 4.05% stant measured from lattice image is 5.91 A expansion, in comparison with metallic bulk phase and 1% contraction with semiconducting one. Consequently, we identify this film as intermediate phase between metal and semiconductor. Similarly, ED ring pattern and lattice image of sample C are given in Fig. 5. The lattice spacing calculated from ring pattern and from (2 0 0) lattice ˚ , respectively. Although the image was 6.01 and 5.97 A former showed expansion by 0.67% compared with the bulk semiconductor [8], the later agreed exactly with bulk semiconductor [8]. Hence, sample C was proved as SmS semiconducting phase. Throughout the TEM observation, we can conclude that samples A, B and C are metallic, intermediate and
semiconducting phase of polycrystalline SmS, respectively, with granular structure of about 10–20 nm in size. By above phase identification, the ratio between rf power to dc power is suggested to be a crucial sputtering parameter for the growth of either metal, semiconductor, or intermedium structure of SmS films. 3.3. Sm valence state by XPS It has been reported [12–14] in a spectrum of Sm 3d5/2 and Sm 3d3/2 that a mixed valence state of semiconductor phase have been dominated by Sm2þ state, and in contrast, a mixed valence state of metallic phase has been dominated by Sm3þ state. Spectra of Sm 3d5/2 and Sm 3d3/2 taken for the film on Si for intermediate phase are given in Fig. 6 to characterize whether the intermedium phase is existing as likely metal or semiconductor by the peak ratio
Fig. 6. XPS spectra of Sm2þ and Sm3þ valence state from Sm 3d in the intermedium phase of SmS film.
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Fig. 7. Depth profile of XPS for the film with intermedium phase.
between Sm2þ state and Sm3þ state. In Fig. 6, Sm 3d5/2 peaks appeared at 1083.3 eV for Sm3þ and 1074.0 eV for Sm2þ, respectively. The peak ratio of Sm2þ/Sm3þ is about 3. This domination of Sm2þ state implies that the obtained intermediate phase is likely semiconductor and this coincides with the previously deduced results by ED and TEM. Atomic concentration versus sputter time in the depth profile for the same sample is given in Fig. 7. From the surface (corresponding to sputter time 0) to the middle of the film (about 40 nm in depth), the Sm and S concentration is approximately the same and this results in the stoichiometric SmS, while Sm and O concentration is increasing by 20 and 18% near the film and substrate boundary. This suggests that Sm oxides and metal Sm exist near the film and substrate boundary. Then, there are two possibilities for the no metal Sm and oxide appearance in XRD and TEM observations. The first is Sm metal is soluble in water and it may be difficult to identify in the above described observation process of TEM; the second one is they are amorphous.
3.4. Optical band gap of semiconducting phase As the details of the complex refractive indices of both metallic and semiconducting phase of the present SmS samples, which is analyzed by the spectroscopic ellipsometry will be reported elsewhere, we only summarize the optical band gap value of the semiconducting phase of SmS as follows: We tried four transition modes (indirect allowed, indirect forbidden, direct allowed, and direct forbidden), and confirmed that indirect allowed mode is the best fitting. Consequently, the optical band gap between conduction band and valence band was calculated using the formula of Bðhu Eg Þ2 ¼ ð4pk/l)hu by Tauc [7] from indirect allowed transitions mode, where B: constant; hu: photon energy; 4pk/l: absorption coefficient at wavelength l; k: extinction coefficient. We evaluate Eg as 2.67 eV, which is larger than the assumed value of 2.3 eV by Batlogg et al. [8] and that of 1.9 eV by Davis [9], and smaller than 3.6 eV, derived from the optical spectra by Lashkarev and Ivanchenko [10].
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4. Conclusions
References
SmS thin films have been fabricated on a-SiO2/Si and NaCl substrates at a room temperature by dual targets (dc for metal Sm and rf for pressed powdered chalcogenide Sm2S3) magnetron sputtering. Metal, semiconductor and intermedium phase of SmS films were grown under the optimized ratio of dc to rf power. Phase formation of metal, intermedium, or semiconductor was identified by TEM and XRD. The obtained lattice constant of intermedium phase was ˚ from ED and 5.91 A ˚ from lattice image. The 5.85 A former value is contraction by 2% compared with bulk ˚ ), while the later one is consemiconductor (5.97 A traction by 1% compared with semiconducting one. The peak ratio of Sm2þ/Sm3þ obtained from XPS of Sm 3d5/2 and Sm 3d3/2, taken for the film on Si with intermediate phase is about 3 and this confirms that obtained intermedium phase is likely semiconductor phase. In intermedium case, metal Sm and samarium oxide are exsisting near the film–substrate boundary by depth profile of XPS, while stoichiometric SmS is dominant at the surface layer. In semiconductor case, the optical band gap value of semiconductor film is 2.67 eV from Tauc plot and this is slightly larger than theoretically estimated 2.3 eV.
[1] E. Kaldis, P. Wachter, Solid State Commun. 11 (1972) 907. [2] W. Pohl, R. Badertscher, K.A. Muelar, P. Wachter, Appl. Opt. 13 (1974) 95. [3] S.G. Shullman, T.L. Bzhalava, T.B. Zhukova, T.T. Dedegkaev, I.A. Smilnov, Soviet Phys.—Solid State 17 (1975) 1984. [4] T.L. Bzhalava, T.B. Zhukova, I.A. Smilnov, S.G. Shullman, N.A. Yakovleva, Soviet Phys.—Solid State 16 (1974) 2428. [5] T.L. Bzhalava, S.G. Shullman, T.T. Dedegkaev, T.B. Zhukova, I.A. Smilnov, Phys. Lett. A 55 (1975) 161. [6] P. Jin, M. Tazawa, J.F. Huang, S. Tanemura, J. Cryst. Growth 191 (1998) 285. [7] J. Tauc, Optical Properties of Solids, in: F. Abeles (Ed.), North-Holland, Amsterdam, p. 303. [8] B. Batologg, E. Kaldis, A. Schlegel, P. Wachter, Phys. Rev. B 14 (1976) 5503. [9] H.L. Davis, in: Proceedings of the 9th Rare Earth Research Conference, vol. 1, Blacksburg, VA, 1971, p. 3. [10] G.V. Lashkarev, L.A. Ivanchenko, J. Non-Cryst. Solids 8–10 (1970) 670. [11] B. Batologg, A. Schlegel, P. Wachter, Physica B–C 86–88 (1977) 229. [12] M. Cmpaqna, E. Bucher, G.K. Wertheim, L.D. Longinotti, Phys. Rev. Lett. 33 (1974) 165. [13] G. Krill, J.P. Senateur, A. Amamou, J. Phys F: Metal Phys. 10 (1980) 1889. [14] D.W. Pohl, R. Jaggi, K. Gisler, H. Weibel, Solid State Commun. 17 (1975) 705.