Relationship between Surface Cracks and Distribution ... - ScienceDirect

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ScienceDirect Procedia Engineering 139 (2016) 112 – 116

MRS Singapore – ICMAT Symposia Proceedings 8th International Conference on Materials for Advanced Technologies

Relationship between Surface Cracks and Distribution of Crystalline Silicon Grains in RF-6SXWWHUHGȕ-FeSi2 Films

T.Uematsua and K.Nakamurab * a

Department of Electrical and Electronic Engineering, Graduate School of Science and Engineering b Department of Electrical and Electronic Engineering, Faculty of Engineering Science,

Kansai University, 3-3-35 Yamate, Suita, Osaka 564-8680, Japan

Abstract Applications in high-HIILFLHQF\6Lȕ-FeSi2 heterojunction solar cells require the preparation of crack-IUHHȕ-FeSi2 films on silicon. In this study, we obtained crack-IUHHȕ-FeSi2 films on silicon(100) just substrate by RF-Sputtering using FeSi4 target even though we annealed the films at temperatures as high as 900°C after deposition. These films contained numerous silicon crystalline grains because of using FeSi4 target. Furthermore, FeSix films were deposited on silicon(100) substrate 4° off the [110] direction at room temperature through the RF sputtering method using an FeSi3 target. The film annealed at 900°C also contained a large amount of silicon. Different from the films on silicon(100) just substrate, however, the film developed cracks on its surface. © 2016 2015The TheAuthors. Authors. Published Elsevier © Published by by Elsevier Ltd.Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and/or peer-review under responsibility of the scientific committee of Symposium 2015 ICMAT. Selection and/or peer-review under responsibility of the scientific committee of Symposium 2015 ICMAT Keywords: iron disilicide (ȕ-FeSi2); RF sputtering; distribution; silicon grains; cracks

1. Introduction ,URQVLOLFLGHVKDYHPDQ\SKDVHVVXFKDVĮȕȖDQGİSKDVHV$PRQJWKHVHSKDVHVRQO\WKHȕSKDVHFDQEH used for solar cells because it has semiconducting properWLHV ȕ-FeSi2 has attracted attention because it is a good

* Corresponding author. Tel.: +81-6-6368-1121; fax: +81-6-6388-8843. E-mail address: [email protected]

1877-7058 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and/or peer-review under responsibility of the scientific committee of Symposium 2015 ICMAT

doi:10.1016/j.proeng.2015.08.1101

T. Uematsu and K. Nakamura / Procedia Engineering 139 (2016) 112 – 116

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candidate for silicon-based optoelectronic materials. Furthermore, from an ecological point of view, FeSi2 consists of Fe and Si, the resources of which are abundant, and it is harmless to the enviURQPHQWDQGWRKXPDQV>@ȕ-FeSi2 is a promising material for thin solar cells because its absorption coefficients are larger than those of silicon by two orders of magnitude [2]. In addition, it can absorb infrared light because its band gap is ~0.85 eV [3]. Its reported conversion efficiency of 22.4% was calculated by computer simulation using a p-Si/n-FeSi2 heterojunction solar cell; this value is higher than that of a p-Si/n-Si homojunction solar cell [4]. 3UHSDUDWLRQ RI ȕ-FeSi2 through molecular beam epitaxy and metal–organic vapor-phase epitaxy is difficult because Fe and Si have high melting points. Therefore, reactive deposition epitaxy [5], ion beam synthesis [6], and RF sputtering [7–9] are generally used to pUHSDUH ȕ-FeSi2. RF-sputtering, in particular, is among the simplest of these methods. ,QWKLVVWXG\FRQGXFWHGLQRXUODERUDWRU\ZHSUHSDUHGȕ-FeSi2 films that had been sputtered using FeSi4 target on a silicon(100) just substrate . With did not perform faEULFDWLRQ RI ȕ-FeSi2 films on other substrates in our laboratory. A wide variety of FRQGLWLRQVIRUȕ-FeSi2 fabrication need to be investigated to meet its challenges. So we fabricated FeSix films which were deposited on silicon(100) substrate 4° off the [110] direction at room temperature through the RF sputtering method using an FeSi3 target. After deposition, FeSix films were annealed at 400–900°C. After annealing, the crystallinity of the films was analyzed by XRD measurements. Film surfaces were observed under an optical microscope, and film thicknesses were measured by scanning electron microscopy. 2. Experiments FeSix films were deposited on silicon(100) just substrates and silicon(100) substrate 4° off the [110] direction at room-temperature through the RF sputtering method using an FeSi4 target and FeSi3 target (99.9% purity) respectively. The RF power during sputtering was 100 W. The distance between the substrate and target was set at 30 mm. Post-annealing in Ar gas at 400–900°C for 10 min was carried out in a quartz furnace. 7KH FU\VWDOOLQLW\ RI WKH ILOPV ZDV DQDO\]HG E\ ;5' PHDVXUHPHQW XVLQJ &X.Į UDGLDWLRQ 0HDVXUHPHQW ZDV performed at an incident angle of 3°, and the sample holder was rotated during measurements. The incident X-ray angle was kept small because only the diffraction signal from the film and not from the substrate needed to be detected. This technique is known as grazing-incidence XRD method. The film surface was observed under an optical microscope (VF-7510: Keyence). 3. Results and Discussion Figure 1 (a) shows XRD patterns of the films deposited using FeSi4 target on the just substrate. Figure 1 (b) shows XRD patterns of the films deposited using FeSi3 target on the off substrate. As shown in Figure 1, in both conditions, the films annealed at temperatures RYHU&FRPSOHWHO\WUDQVIRUPHGWRȕ-FeSi2, and other phases such DVİ-FeSi did not form on the surface. Figure 2 presents results of peak decomposition oIȕ-FeSi2(202/220) at around 29° for the films annealed at &)RUERWKILOPVD6L SHDNDWZDVVXSHULPSRVHGDJDLQVWWKHȕ-FeSi2(202/220) peak. Figure 3 depicts the deposition-WLPHGHSHQGHQFHRIWKHLQWHQVLW\UDWLREHWZHHQȕ-FeSi2(202/220) and Si(111) peaks. In both conditions, the ratio increased when deposition time was short. This indicates that a layer containing silicon crystals formed near the substrate under both conditions. Figure 4 shows the film surfaces observed by optical microscopy. Figure 4 (a) shows the surface of the film deposited using FeSi4 target on the just substrate and annealed at 900°C for 10 min in Ar gas. As shown in Fig. 4 (a), no cracks formed on the surface of the film annealed at 900°C. Figure 4 (b) displays the film surface that was deposited using FeSi3 target on the off substrate and annealed at 900°C for 10 min in Ar gas. Cracks were observed on the surface of the film annealed at 900°C, as marked by bold lines in Fig. 4 (b). Many cracks were generated during FRROLQJ7KLVPD\EHSULPDULO\GXHWRWKHGLIIHUHQFHLQH[SDQVLRQFRHIILFLHQWVEHWZHHQVLOLFRQDQGȕ-FeSi2, which are 2.6 × 10í6/°C and 6.7 × 10í6/°C respectively [10].

800Υ 700Υ 600Υ 500Υ

Si(311)

ș(313/331) ș(004/040) ș(041) ș(422) ș(133/332) ș(024/042)

Ta=900Υ 800Υ 700Υ 600Υ 500Υ 400Υ

400Υ

20

30

40 50 60 2ȟ (deg.)

70

80

26

Si(111) SiO2

27

28 29 30 2ȟ (deg.)

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Si(111) SiO2

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28 29 30 2ȟ (deg.)

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Fig. 2 (b) Decomposition of the peak for ȕ-FeSi2 (202/220) peak around 29° for the film deposited using FeSi3 target on the off substrate.

50 Ta=900Υ off substrate just substrate

40 30 20 10 0 0

80

ș(202/220)

Ta=900Υ

26

32

Fig. 2 (a) Decomposition of the peak for ȕ-FeSi2 (202/220) peak around 29° for the film deposited using FeSi4 target on the just substrate. Intensity ratio of XRD peaks between Si(111) and ș-FeSi2(202/220) [%]

70

Fig. 1 (b) Annealing-temperature dependence of XRD patterns of the films deposited using FeSi3 target on the off substrate.

ș(202/220)

Ta=900Υ

60 40 50 2ȟ (deg.)

30

20

X-ray intensity (arb.units)

Fig. 1 (a) Annealing-temperature dependence of XRD patterns of the films deposited using FeSi4 target on the just substrate.


ș(222)

Ta=900Υ

ș(202/220)


Si(311)

ș(222)

ș(202/220)

ș(313/331) ș(004/040) ș(041) ș(422) ș(133/332) ș(024/042)



114

10

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30

Deposition time (min) Fig. 3 Deposition-WLPHGHSHQGHQFHRIWKHLQWHQVLW\UDWLREHWZHHQIRUȕ-FeSi2(202/220) and Si(111) peaks


Fig. 4 (a) Surface of the film depositedusing FeSi4 target on the just substrate.

Fig. 4 (b) Surface of the film deposited using FeSi3 on the off substrate. In this figure, cracks are emphasized by bold lines.

No cracks were observed on the VXUIDFHRIWKHȕ-FeSi2 film deposited using FeSi4 target on the just substrate and annealed at 900°C because of UDQGRPO\GLVWULEXWHGVLOLFRQFU\VWDOOLQHJUDLQVQHDUWKHȕ-FeSi2/Si interface. The ȕ-FeSi2 film deposited using FeSi3 target on the 4° off silicon substrate and annealed at 900°C had many cracks on the surface although it contained silicon crystalline grains near the substrate.

Fig. 5 Schematic of the deposition process for the films deposited using FeSi4 target on the just substrate and using FeSi3 target on the off substrate at room temperature.

Figure 5 shows a schematic of the deposition process using FeSi4 on the just substrate and using FeSi3 on the off substrate at room temperature. As previously reported [11], when FeSi3 is used as sputtering target the ratio of Fe and Si atoms in the films deposited at room temperature is almost 1:2 because of the difference in sputtering yield between Fe and Si in Ar. On the other hand, Si atoms of the films deposited using FeSi4 target on the just substrate at room temperature tended to bond to Si substrates at the initial stage of deposition because of the high ratio of Si for FeSi4 target. Films deposited using FeSi3 target on the off substrate at room temperature were the same as those deposited using FeSi4 target on the just substrate at room temperature in that a layer contained more silicon atoms near the substrate. However, this formation was not effective to reduce surface cracks on the films deposited using FeSi3 target on the off substrate at room temperature. This may be due to the selective accumulation of silicon crystalline grains at steps of the off substrate.

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4. Summary FeSix films were deposited using FeSi4 target on silicon(100) just substrate and using FeSi3 target on silicon(100) substrate 4° off the [110] direction at room temperature through the RF sputtering method. Postannealing was carried out at 400–&$OOILOPVFRPSOHWHO\WUDQVIRUPHGWRȕ-FeSi2 after annealing at >500°C in both conditions. For the film deposited using FeSi3 target on the off substrate at room temperature, a layer containing silicon crystals formed near the substrate. 7KLVUHVXOWLVVLPLODUWRWKDWIRUȕ-FeSi2 film deposited using FeSi4 target on silicon(100) just substrate at room temperature. No cracks formed on the film surface deposited using FeSi4 target on the just substrate annealed at 900°C. In contrast, cracks formed on the film surface deposited using FeSi3 target on the off substrate annealed at 900°C. These cracks may be due to the difference in the distribution of crystalline silicon grains. References [1] N. Otogawa, S. Wang, S. Kihara, X. Liu, Y. Fukuzawa, Y. Suzuki, M. Osamura, T. Ootsuka, T. Mise, K. Miyake, Y. Nakayama, H. Tanoue, Y. Makita, Thin Solid Films 461, 223 (2004). [2] T. Yoshitake, Y. Inokuchi and A. Yuri, Appl. Phys. Lett. 88, 182104 (2006). [3] M. C. BOST and J. E. MAHAN, J. Appl. Phys. 64, 2034 (1988). [4] T. Kawaguchi and K. Nakamura, Tech. Digest of 2009 International Meeting for Future of Electron Devices, Kansai 92 (2009). [5] J. E. Mahan, K. M. Geib, Y. Robinson, R. G. Long, Y. Xinghua, G. Bai, M. A. Nicolet and M. Nathan, Appl. Phys. Lett. 56, 2126 (1990). [6] K. Radermacher, S. Mantl, Ch. Dieker and H. Luth, Appl. Phys. Lett. 59, 2145 (1991). [7] T. Ehara, Y. Sasaki, K. Saito, S. Nakagomi and Y. Kokubun, Appl. Surf. Sci. 175, 96 (2001) [8] A. S. W. Wong, G. W. Ho, S. L. Liew, K. C. Chua and D. Z. Chi, Prog. Photovolt: Res. Appl. 19, 464 (2011) [9] M. Komabayashi, K. Hijikata and S. Ido, Jpn. J. Appl. Phys. 29,1118 (1990) [10] K. Nakamura, H. Shimizu, J. Kodera and K. Yokota, Jpn. J. Appl. Phys. 39, 4554 (2000). [11] N. Kawabata and K. Nakamura, Trans. Mat. Res. Soc. Japan, 36(2), 269