Formation of poly-crystalline BaSi2 thin films by pulsed laser deposition for solar cell applications

Journal Pre-proofs Formation of poly-crystalline BaSi2 thin films by pulsed laser deposition for solar cell applications Rui Du, Kaiwen Yang, Xudan Gao, Wangzhou Shi, Weijie Du, Yiwen Zhang, Takashi Suemasu PII: DOI: Reference:

S0167-577X(19)31568-X https://doi.org/10.1016/j.matlet.2019.126936 MLBLUE 126936

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Materials Letters

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23 August 2019 30 September 2019 1 November 2019

Please cite this article as: R. Du, K. Yang, X. Gao, W. Shi, W. Du, Y. Zhang, T. Suemasu, Formation of polycrystalline BaSi2 thin films by pulsed laser deposition for solar cell applications, Materials Letters (2019), doi: https://doi.org/10.1016/j.matlet.2019.126936

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Formation of poly-crystalline BaSi2 thin films by pulsed laser deposition for solar cell applications Rui Dua, Kaiwen Yanga, Xudan Gaoa, Wangzhou Shia, Weijie Dua,*, Yiwen Zhanga,*, Takashi Suemasub aKey

Laboratory of Optoelectronic Material and Device, Shanghai Normal University, Shanghai

200234, China bInstitute

of Applied Physics, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan

Abstract A new growth method for BaSi2 thin films by using pulsed laser deposition (PLD) on transparent SiO2 and CaF2 substrates has been developed. X-ray diffraction and Raman spectroscopy revealed the poly-crystalline property of the deposited films. By introducing a thin Si buffer layer on SiO2 substrate, the crystalline quality of BaSi2 thin films were improved. BaSi2 thin films exhibited a Ba/Si ratio very close to 0.5, indicating the good stoichiometry control of PLD growth. The absorption coefficient of the poly-BaSi2 reached 105 cm-1 and its band gap was deduced to be 1.32 eV, which are similar to those grown by molecular beam epitaxy or sputtering. A maximum photoresponsivity of 12.5 mA/W was achieved in the BaSi2 thin film, which implies the potential of PLD-deposited BaSi2 for thin film solar cell applications. Keywords: BaSi2; Solar energy materials; Thin films; Pulsed laser deposition

* Corresponding authors: [email protected] (W. Du), [email protected] (Y. Zhang) 1

1. Introduction Thin film solar cells like CuInxGa(1-x)Se2 (CIGS), CdTe and perovskite are attracting more and more attention owing to their advantages such as the high power conversion efficiency (PCE) and flexibility [1-3]. However, using toxic and expensive elements, and the long-term stability are critical problems for sustainable developments of these solar cells. Si-based new material barium disilicide (BaSi2) seems to be a good candidate for thin film solar cells owing to its suitable band gap of ~1.3 eV and the large absorption coefficient () of over 105 cm-1 at visible light region, which are key parameters for thin film solar cell materials [4,5]. In last few years, superior properties such as the long minority carrier lifetime and diffusion length, large grain size and high photoresponsivity, have been achieved in high quality BaSi2 thin films grown on Si(111) substrates by molecular beam epitaxy (MBE) method [6,7]. BaSi2 based devices such as the metal/n-BaSi2 Schottky junction and the p-BaSi2/n-BaSi2 homojunction diodes have been fabricated. The evidence of photogenerated carriers separated by the build-in electric field in BaSi2 thin film has been confirmed [8]. Sputtering is another method to form BaSi2 thin films, Yoneyama et al. reported the growth of polycrystalline BaSi2 thin films on Si(111) and glass substrates by radio frequency (RF) magnetron sputtering using a BaSi2 target [9,10]. Besides, thermal evaporation is also a rapid and cost effective way to form BaSi2 thin films. Hara et al. formed single phase poly-crystalline BaSi2 thin films by directly evaporating BaSi2 powder source on Si and glass substrates [11,12]. Pulsed laser deposition (PLD) is also a mature thin film growth technique where a high power pulsed laser beam is focused inside a vacuum chamber to strike a target of the material that is to be deposited. Optoelectronic materials such as ZnO deposited by PLD have been reported [13]. In this letter, we adopted this method to grow BaSi2 thin films on transparent substrates, which has not been reported before. The crystalline quality, optical and photoresponse properties were

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investigated. 2. Experimental SiO2 and CaF2(111) substrates were firstly ultrasonically cleaned in acetone, ethanol and deionized water, and loaded into the chamber of a PLD system (Seinan Industries PLD200) equipped with a 248-nm krypton fluoride excimer laser (Coherent CompexPro). The pulsed laser energy and frequency were kept constant at 300 mJ and 5 Hz. The laser beam was focused into a 1 mm × 2 mm point in the chamber on a stoichiometric BaSi2 target, where the BaSi2 source was ablated and evaporated onto the substrate. The growth temperature was varied from room temperature (RT) to 400-600 °C. BaSi2 thin films were directly deposited on SiO2 substrates in samples A1-A4. For comparison, a 15-nm-thick Si buffer layer was deposited on the SiO2 substrate prior to the deposition of the BaSi2 thin films in samples B1-B3. The growth temperature for the Si buffer layer was 600 °C. In samples C1-C3, CaF2(111) substrates were used. The growth duration was 60 min for each sample. The thicknesses of the films were measured by stylus profiler (Bruker Dektak XT) and the Ba/Si ratios were measured by energy dispersive spectrometer (EDS), as summarized in table I. Table I. Thickness and the Ba/Si ratio of the samples. Sample

Substrate

A1

Thickness (nm)

Ba/Si ratio

304

0.531

306

0.535

307

0.525

A4

295

0.523

B1

316

0.531

320

0.511

B3

314

0.496

C1

301

0.532

296

0.531

290

0.520

A2 A3

B2

C2 C3

SiO2

Si/SiO2

CaF2

3

3. Results and discussions Figure 1 (a), (b) and (c) show the -2 XRD patterns of BaSi2 thin films deposited on SiO2, Si/SiO2 and CaF2 substrates at various growth temperatures. It is obvious that the crystalline quality is poor when BaSi2 thin films were directly grown on the SiO2 substrate even at 600 °C. Considering this problem, we deposited a 15-nm-thick Si buffer layer between the SiO2 substrate and the BaSi2 layer, the XRD results are shown in Fig. 1(b). Orthorhombic BaSi2(301) and (132) peaks were detected at the growth temperature higher than 500 °C, other small peaks such as BaSi2(013) and (203) were also prominent. Compared this result with samples without the Si buffer layer, BaSi2 peaks became much more distinguishable, indicating a dramatic improvement of the crystalline quality by applying the Si buffer layer. Besides, we also grew BaSi2 thin films on CaF2(111) substrate, which has the similar lattice constant to Si(111) substrate, the lattice mismatch between BaSi2 and CaF2 has been illustrated in Fig. S2 in the supplementary part. Toh et al. has reported the growth of BaSi2 thin films on CaF2(111) substrate by MBE [14]. In our research, remarkable BaSi2(301) and (132) peaks also appeared at 600 °C as shown in Fig. 1(c). The preferred growth orientation of BaSi2 thin film on CaF2(111) substrate is very similar to that of grown on the Si/SiO2 substrate. The surface morphologies of the samples are shown in Fig. S1. The PLD-deposited BaSi2 exhibited a smooth and compact surface with good adhesion to the substrates. Figures 1 (a′), (b′), and (c′) show the corresponding Raman spectra of samples. Peaks that are identified as the Ag, Eg and Fg vibration modes of [Si4]4− anion in BaSi2 were observed in all samples [15]. Even for sample A1 deposited at RT, peaks of BaSi2 were still distinguishable, revealing that the orthorhombic phase of BaSi2 still remained during the PLD process at RT. The peak intensity increased with the growth temperature, indicating the improvement of the crystalline quality of the films.

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A3@500ºC A2@400ºC A1@RT 15

20

Fg

Intensity (arb. unit)

(a')

25

200

30

35

40

2 (deg)

45

50

Ag on SiO2

Eg+Fg

A4@600°C A3@500°C A2@400°C

A1@RT

300

400

500

600

-1

Raman shift (cm )

on Si/SiO2

(301)

B3@600ºC

(013)

(203) (301)

(132)

B2@500ºC B1@400ºC

15

20

(b')

25

Fg

30

35

2 (deg)

Eg+Fg

40

45

2

B2@500°C B1@400°C 200

300

50

Ag on Si/SiO

B3@600°C

400

SiTo

500

-1

Intensity (arb.unit)

A4@600ºC

Orthorhombic BaSi2

(b)

Intensity (arb.unit)

on SiO2

Intensity (arb. unit)

Intensity (arb.unit)

(112) (211)

Intensity (arb.unit)

Orthorhombic BaSi2

(a)

Orthorhombic BaSi2

(c) CaF2(111)

on CaF2

(301)

C3@600ºC (132) C2@500ºC

15

20

(c')

25

Fg

30

C1@400ºC 35 40 45

2 (deg)

Eg+Fg

Ag

50

on CaF2

C3@600°C C2@500°C

C1@400°C

600 200

Raman shift (cm )

300

400

500 -1

Raman shift (cm )

600

Fig.1. -2 XRD patterns and Raman spectra of BaSi2 thin films grown on (a)( a′) SiO2, (b)(b′) Si/SiO2

6

10

Sample A4 Sample B3 Sample C3

5

10

1/2

4

10

eV



10

dh

-1

Absorption coefficiency (cm )

and (c)( c′) CaF2 substrates.

3

10 1.0

5

0 1.0

1.5

2.0

1.5 2.0 2.5 Photon energy (eV)

2.5

3.0

3.0

Fig. 2. Absorption spectra of samples A4, B3 and C3 deposited on SiO2, Si/SiO2 and CaF2 substrates at 600 °C, the inset is the (dh)1/2 versus photon energy curves for deducing the band gap.

Figure 2 shows the absorption spectra of the samples. The deposited BaSi2 thin films achieved a large absorption coefficient of over 105 cm-1, implying the potential application in the thin film solar

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cell field. Owing to the improvement of crystalline quality by introducing the thin Si buffer layer, sample B3 exhibited the largest absorption coefficient among these samples. The band gap was deduced to be 1.32 eV as indicated in the (dh)1/2 vs photon energy (h) curves in the inset figure, which is in good agreement with previous reports [4,5].

Photoresponsivity (mA/W)

14

Sample A4 Sample B3 Sample C3

12 10

Chopped light Lock-in

8

1 V bias

amplifier

A

6 4 2 0

400 500 600 700 800 900 1000 1100 1200

Wavelength (nm) (a)

(b)

Fig.3. (a) Photoresponse spectra of samples measured at RT under 1 V bias voltage. (b) An illustration of the photoresponse measurement.

Figure 3(a) shows the photoresponse spectra of these samples. To measure the photoresponsivity, 1-mm-spaced Au stripe electrodes were deposited on the surface of the samples. A bias voltage of 1 V was applied between two stripe electrodes, and the photocurrent was read out by a lock-in amplifier under the irradiation of a chopped monochromatic light, as illustrated Fig. 3(b). Photocurrent was observed and increased sharply for wavelength shorter than 950 nm (h >1.3 eV) and reached a maximum at 800 nm (h = 1.55 eV), this phenomenon is in good agreement with previous reports for BaSi2 [6]. A maximum value of 12.5 mA/W was achieved in sample B3, in which the sample has a best Ba/Si ratio close to 0.5. We speculate that the excessive Ba atoms may cause defects in the BaSi2

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crystal and lead to a deterioration on the photoelectric property. The photoresponsivity of PLD-deposited BaSi2 is one order of magnitude larger than that in the sputtered BaSi2 [9], moreover, it is also comparable to other optoelectronic materials such as CdS and TiO2 [16-18]. On the basis of these results, we conclude that PLD is also an efficient and alternative way to form high quality poly-crystalline BaSi2 thin films.

4. Conclusions We successfully formed poly-crystalline BaSi2 thin films on the SiO2 substrate with a thin Si buffer layer and on the CaF2 substrate by PLD. The Ba/Si ratio was very close to 0.5, indicating the excellent stoichiometry control of PLD growth. The band gap was measured to be about 1.32 eV. A large absorption coefficient of over 105 cm-1 and photoresponsivity of 12.5 mA/W have been achieved. The results imply the great potential of PLD-deposited BaSi2 thin films in thin film solar cell applications. Acknowledgement This work was financially supported by the Natural Science Foundation of Shanghai, China (Grant No. 17ZR420600), the National Natural Science Foundation of China (Grant No. 61704108), the Program for the Professor of Special Appointment (Eastern Scholar) at the Shanghai Institutions of Higher Learning, the Shanghai “1000 Talents Plan” and the Science and Technology Commission of Shanghai Municipality (Grant No. 18070502800). References [1] P. Jackson, R. Wuerz, D. Hariskos, E. Lotter, W. Witte, and M. Powalla, Phys. Status Solidi RRL 10, (2016) 583-586. [2] J. M. Burst, J. N. Duenow, D. S. Albin, E. Colegrove, M. O. Reese, J. A. Aguiar, C.-S. Jiang, M. K. Patel, M. M. Al-Jassim, D. Kuciauskas, S. Swain, T. Ablekim, K. G. Lynn and W. K. Metzger,

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Nature Energy 1, (2016) 16015. [3] G. Niu, X. Guo, and L. Wang, J. Mater. Chem. A 3, (2015) 8970-8980. [4] T. Nakamura, T. Suemasu, K. Takakura, F. Hasegawa, A. Wakahara, and M. Imai, Appl. Phys. Lett. 81, (2002) 1032-1034. [5] K. Toh, T. Saito, and T. Suemasu, Jpn. J. Appl. Phys. 50, (2011) 068001. [6] W. Du, M. Suzuno, M. A. Khan, K. Toh, M. Baba, K. Nakamura, K. Toko, N. Usami, and T. Suemasu, Appl. Phys. Lett. 100, (2012) 152114. [7] T. Suemasu, and N. Usami, J. Phys. D: Appl. Phys. 50, (2017) 023001. [8] W. Du, M. Baba, K. Toko, K. O. Hara, K. Watanabe, T. Sekiguchi, N. Usami, and T. Suemasu, J. Appl. Phys. 115, (2014) 223701. [9] T. Yoneyama, A. Okada, M. Suzuno, T. Shibutami, K. Matsumaru, N. Saito, N. Yoshizawa, K. Toko, and T. Suemasu, Thin Solid Films 534, (2013) 116-119. [10] S. Matsuno, R. Takabe, S. Yokoyama, K. Toko, M. Mesuda, H. Kuramochi, and T. Suemasu, Appl. Phys. Express 11, (2018) 071401. [11] K. O. Hara, J. Yamanaka, K. Arimoto, K. Nakagawa, T. Suemasu, N. Usami, Thin Solid Films 595, (2015) 68-72. [12] Y. Nakagawa, K. O. Hara, T. Suemasu, and N. Usami, Jpn. J. Appl. Phys. 54, (2015) 08KC03. [13] E. Manikandan, M. K. Moodley, S. S. Ray, B. K. Panigrahi, R. Krishnan, N. Padhy, K. G. M.

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[15] T. Sato, H. Hoshida, R. Takabe, K. Toko, Y. Terai, and T. Suemasu, J. Appl. Phys. 124, (2018) 025301. [16] K. C. Wilson, E. Manikandan, M. B. Ahamed, and B. W. Mwakikunga, J. Alloys Compd.

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Conflict of interest The authors declared that they have no conflicts of interest to this work. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.

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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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Highlights: 1.

The first study to apply pulsed laser deposition (PLD) on the growth of semiconducting BaSi2 thin films.

2.

Poly-crystalline BaSi2 thin films were obtained with a large absorption coefficient of over 105 cm-1 and the photoresponsity of 12.5 mA/W.

3.

The PLD-deposited BaSi2 thin films keep a good stoichiometric ratio close to 1:2.

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