Photovoltaic property of sputtered BiFeO3 thin films

Photovoltaic property of sputtered BiFeO3 thin films

Journal of Alloys and Compounds 574 (2013) 402–406 Contents lists available at SciVerse ScienceDirect Journal of Alloys and Compounds journal homepa...

851KB Sizes 0 Downloads 41 Views

Journal of Alloys and Compounds 574 (2013) 402–406

Contents lists available at SciVerse ScienceDirect

Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom

Photovoltaic property of sputtered BiFeO3 thin films H.W. Chang a,⇑, F.T. Yuan b, Y.C. Yu a, P.C. Chen a, C.R. Wang a, C.S. Tu c, S.U. Jen d a

Department of Applied Physics, Tunghai University, Taichung 407, Taiwan Isentek Ltd., Advanced Sensor Laboratory, Taipei 221, Taiwan c Department of Physics, Fu Jen Catholic University, Taipei 242, Taiwan d Institute of Physics, Academia Sinica, Taipei 115, Taiwan b

a r t i c l e

i n f o

Article history: Received 29 April 2013 Received in revised form 21 May 2013 Accepted 23 May 2013 Available online 1 June 2013 Keywords: Photovoltaic effect Multiferroic BiFeO3 films Sputtering method

a b s t r a c t Photovoltaic (PV) property of sputter-deposited BiFeO3 (BFO) polycrystalline films on Pt/Ti/SiO2/Si(1 0 0) substrates has been studied. Isotropic single phase perovskite BFO is obtained in the growth temperature (Tg) range of 350–450 °C. The increase of Tg and film thickness promote grain growth resulting in roughened surface. Significant PV effect under laser illumination with wavelength of 405 nm is obtained. Shortcircuit photocurrent density (Jsc) increases with the increase of laser intensity (I). The Jsc at I = 220 mW/ cm2 (Jmax) are increased with the increase of the growth temperature and BFO thickness. The correlation between Jmax and the size of coherent scattering domain indicates that PV property is highly related to the structural defects. Furthermore, PV effect is reduced at higher Tg = 500 °C or for larger t = 400 nm due to the appearance of the impurity phase and rougher surface. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction Multiferroic (MF) materials, in which ferroelectric (FE) and ferromagnetic (FM) behaviors are coexisted and these two orderings may couple to each other, have drawn much attention due to their interesting physics and immense potential for new multifunctional applications [1–13]. The bismuth ferrite BiFeO3 (BFO) with perovskite structure is one of the most studied MF and a potentially important lead-free FE material because its transition temperatures are well above room temperature, including FE Curie temperature (TC) of 830 °C and antiferromagnetic Neel temperature (TN) of 370 °C [1–13]. Epitaxial BFO thin films display spontaneous electrical polarization an order of magnitude higher than that bulk BFO single crystal [5]. Accordingly, comprehensive studies have been made on the BFO films grown on oxide single crystal substrates, such as SrTiO3(1 0 0), SrTiO3(1 1 0) SrTiO3(1 1 1), MgO(1 0 0), TbScO3(1 1 0), and DyScO3(1 1 0) on various aspects [5–11]. In addition, the sputter-prepared BFO films also considered of interests due to the attractive FE properties [9,12,13]. Recently, significant photovoltaic (PV) effects, originated from the narrower direct energy band gap of 2.2–2.8 eV as compared to other ferroelectric perovskite oxides, in BFO films under visible light illumination have been observed [14–22]. The findings open additional probabilities in the design of the spintronic,

⇑ Corresponding author. Tel.: +886 4 23594643; fax: +886 4 23594643. E-mail address: [email protected] (H.W. Chang). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.05.161

optoelectronic, and magneto-optical devices [14–21]. PV effect of BFO films was dominated by photo-excited electrical carrier across bulk optical gap [15] due to the much larger short-circuit photocurrent under green light than red light illumination, and was also sensitive to the ferroelectric domain wall configuration [16–18], oxygen vacancy concentration [14], and electrodes [20,21], etc. However, most PV studies mainly focused on current–voltage correlation under a constant light intensity, and to date limited study on PV property of polycrystalline BFO thin films prepared by sputtering method, widely used in industry, is available. In this study, we study the sputter-prepared BFO films on commercial Pt(1 1 1)/Ti/SiO2/Si(1 0 0) substrates. Effects of BFO thickness (t) and growth temperature (Tg) on the phase structure, surface morphology, and PV properties under various illumination intensity are systematically investigated. 2. Experiment BFO films with 50–400 nm in thickness (t) were deposited on the Pt/Ti/SiO2/ Si(1 0 0) substrates at substrate temperatures (Tg) in the range of 300–500 °C by radio frequency sputtering from a commercial Bi1.1FeO3 target. The base pressure was better than 5  10 7 Torr, and the working pressure and rf power were maintained at 40 W and 10 m Torr, respectively. The ratio of Ar to O2 is 3:1. Crystallographic structure was identified by conventional X-ray diffractometry (XRD) with Cu Ka radiation. Surface morphology was observed by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Before the photovoltaic measurement, the transparent conductive films of indium tin oxide (ITO) electrode with 150 nm in thickness were deposited onto BFO films. A laser with wavelength of 405 nm was incident normal to the film surface for photo-excitation. No external electric field was applied previously or during the measurement. The details of the photovoltaic measurement were described elsewhere [22].

403

o

450 C o

400 C



BFO(210)

BFO(110)

Pt(111) BFO(002) Pt(200)

o

500 C

♦Si

BFO(111)



Intensity (a. u.)



BFO(001)

(b)

◊ Fe2O3

BFO(210)



♦Si ∇ Bi2O3

BFO(002) Pt(200)





BFO(111)

BFO(110)



Intensity (a. u.)

BFO(001)

(a)

Pt(111)

H.W. Chang et al. / Journal of Alloys and Compounds 574 (2013) 402–406

300 nm 200 nm

o

100 nm 50 nm

o

300 C 24

28

32

36

40

44

48

52

2θ (degree )

400 nm



350 C

20



20

24

28

32

36

40

44

48

52

2θ (degree )

Fig. 1. XRD patterns of (a) 200-nm-thick BFO films deposited on deposited on Pt/Ti/SiO2/Si(1 0 0) substrates at various growth temperature and (b) BFO films grown on Pt/Ti/ SiO2/Si(1 0 0) substrates at Tg = 450 °C with different BFO thickness.

3. Results and discussion Fig. 1a shows XRD patterns of 200 nm BFO films deposited on Pt/Ti/SiO2/Si(1 0 0) substrates at various growth temperature. For

BFO films deposited at Tg = 300 °C, the pattern shows the signals of Pt and Si only, indicating poor crystallization of BFO. When Tg is elevated to 350–500 °C, the diffraction peaks corresponding to perovskite phase emerge, which is consistent with the pseudocubic

Fig. 2. SEM images of various t-thick BFO films deposited on Pt/Ti/SiO2/Si(1 0 0) substrates at different Tg with (t, Tg) = (a) (200 nm, 300 °C), (b) (200 nm, 450 °C), (c) (200 nm, 500 °C), (d) (50 nm, 450 °C), (e) (300 nm, 450 °C), and (f) (400 nm, 450 °C).

404

H.W. Chang et al. / Journal of Alloys and Compounds 574 (2013) 402–406

20

220

(a)

25

160 140

5

Roughness (nm)

10

20

280 260

15

240 220

10

200 180 160

5

140 120

120

0

350

400

450

0

500

Grain Size (nm)

180

Grain Size (nm)

Roughness (nm)

200

15

320 300

(b)

100 50

100

150

200

250

300

350

400

BFO thickness (nm(

Growth Temperature (o C)

Fig. 3. The root-mean-square surface roughness and the average grain size of (a) 200-nm-thick BFO films deposited at various growth temperature and (b) BFO films grown on Pt/Ti/SiO2/Si(1 0 0) substrates at Tg = 450 °C with different BFO thickness.

structure lattice [5]. The films exhibit isotropic orientation. With the increment of Tg from 300 °C to 450 °C, the peak intensity grows and the full width at half maximum (FWHM) narrows, indicative of enhanced crystallinity. The results confirm that the use of a Pt electrode reduces the formation temperature of perovskite BFO phase (350–450 °C) by about 200 °C as compared to oxide underlayers (670–690 °C) [5–7,9,10], agreeing well with the reported result [8,11–13]. With further increment of Tg to 500 °C, large amount of Fe2O3 and Bi2O3 phases forms. The phase change may result from the vaporization of Bi during deposition at higher temperature. To further examine the quality of crystallization, we compare the size of coherent scattering domain (dcsd) in which the unit cells are perfectly aligned without defects and strain. In the present case, the value of dcsd can be determined by Scherrer’s formula because the main contributions to the diffraction line width are from

grain and substructure boundaries. Larger dcsd corresponds to better crystallinity and vice versa. The measured dcsd values increase from 20.4 nm at Tg = 350 °C to 31.1 nm at Tg = 450 °C, confirming the enhancement of crystallinity. Fig. 1b shows XRD patterns of BFO films grown on Pt/Ti/SiO2/ Si(1 0 0) substrates at Td = 450 °C with different BFO thickness. Well-crystallized single phase of perovskite with random orientation is identified in the entire thickness range. When BFO thickness is increased, dcsd is gradually enlarged from 19.1 nm for t = 50– 40.1 nm for t = 400 nm, revealing significant grain growth. Fig. 2 shows SEM images of BFO films grown on Pt/Ti/SiO2/ Si(1 0 0) substrates at various substrate temperatures. All studied BFO films displays dense morphology. When BFO films are deposited at lower Tg of 300 °C, the surface is rather smooth as shown in Fig. 2a. Comparing to the results from Fig. 1a, BFO layer might be either amorphous or nanocrystalline. For 200-nm-thick BFO films

15 1.2

(a)

0.9 0.6 0.3

5

Tg = 500 oC Tg = 450 oC

5

5

Jsc (μA/cm2 )

t = 300 nm

10

Jsc(μA/cm2)

10

t = 400 nm

0 15

0.0 15

(b)

10

0 15

Tg = 400 oC

10

0 15

t = 200 nm

10 5 0 15

t = 100 nm

10

5

5

0 15

0 15

Tg = 350 oC

10

2

220

unit : mW/cm

5

0.2

0 0

0.9

3.9

9.6

18

56

91

125

1000

Time (sec)

1500

unit : mW/cm2

5 0

500

t = 50 nm

10

2000

0.2 0

0.9

3.9 500

9.6

18

56

1000

91 1500

125

220

2000

Time (sec)

Fig. 4. Time-dependent photocurrent under laser illumination of k = 405 nm at various intensity for (a) 200-nm-thick BFO films deposited on Pt/Ti/SiO2/Si(1 0 0) substrates at various Tg and (b) BFO films grown on Pt/Ti/SiO2/Si(1 0 0) substrates at Tg = 450 °C with different BFO thickness.

405

H.W. Chang et al. / Journal of Alloys and Compounds 574 (2013) 402–406

15

(a)

15

Tg = 350 ºC

12

(b)

12

t = 50 nm t = 100 nm t = 200 nm t = 300 nm t = 400 nm

Tg = 400 ºC Tg = 450 ºC

Jsc(μA/cm2)

Jsc (μA/cm2)

9

Tg = 500 ºC 6

3

9

6

3

0

0

0

50

100

150

200

250

0

50

100

150

200

250

Illumination Intensity (mW/cm2)

2

Illumination Intensity (mW/cm )

Fig. 5. The average photocurrent versus intensity for (a) 200-nm-thick BFO films deposited on deposited on Pt/Ti/SiO2/Si(1 0 0) substrates at various Tg and (b) BFO films grown on Pt/Ti/SiO2/Si(1 0 0) substrates at Tg = 450 °C with different BFO thickness.

with increasing Tg as shown in Fig. 2b and c, the average grain size estimated from SEM images increases from 123 nm for Tg = 350 °C to 204 nm for Tg = 500 °C as summarized in Fig. 3a. BFO deposited at 500 °C exhibits bimodal size distributions: the larger grains with the size of 240 nm and smaller grain of 90 nm, which are related to the co-existence of secondary phases of Bi2O3 and Fe2O3 with perovskite phase. Grain growth which is driven by high temperature deposition increases surface roughness. As shown in Fig. 3a, the root-mean-square roughness estimated from AFM increases with Tg from 4.5 nm for Tg = 350 °C to 13.2 nm for Tg = 500 °C. For BFO films grown at Tg = 450 °C as shown in Fig. 2d–f, with the increase of thickness, the estimated grain size increases from 101 nm for t = 50–293 nm for t = 400 nm, and the root-mean-square roughness increases with t from 4.3 nm for t = 50–23.5 nm for t = 400 nm as shown in Fig. 3b. Fig. 4 shows time-dependent short-circuit photocurrent density (Jsc) under laser illumination of k = 405 nm at various intensities for the BFO films. Significant photovoltaic phenomena are obtained in the perovskite BFO films. The PV responses are mainly induced by the optical-excited charge carriers because the energy of photon emitted from laser (3.06 eV) is larger than the energy band gap in BFO (2.2–2.8 eV) [14,18]. PV response strongly depends on the intensity of laser illumination where ‘‘ON’’ and ‘‘OFF’’ indicate

16

J max (μA/cm2)

12

with and without illumination, respectively. The photocurrent flows from ITO to Pt in this present studied ITO/BFO/Pt films, which well agree with the result reported by Hung [22]. In the films with lower values of (t, Tg) = (200 nm, 350–400 °C) and (50 nm, 450 °C), Jsc instantaneously appears a sharp initial peak then keeps constant at fixed illumination. Differently, in the samples with higher values of (t, Tg) = (200 nm, 45–500 °C) and (100–400 nm, 450 °C), Jsc increases with time. Distinct surface condition could be one of the causes for the different PV behaviors. The average short-circuit photocurrent density as a function of illumination intensity is shown in Fig. 5. With the increase of laser intensity to 220 mW/cm2, Jsc increases linearly. As shown in Fig. 5a, with the increase of Tg, the average short-circuit photocurrent density at laser intensity of 220 mW/cm2 (Jmax) are enhanced from 5.1 lA/cm2 for Tg = 350 °C to 11.7 lA/cm2 for Tg = 450 °C, and then drastically reduced to 1 lA/cm2 for higher Tg = 500 °C. The sudden drop is attributed to the appearance of secondary phase and rougher surface. Similarly, as shown in Fig. 5b, when BFO thickness is increased, Jmax is improved from 4.3 lA/cm2 for t = 50 nm to 14.2 lA/ cm2 for t = 300 nm, and then decreased to 8.2 lA/cm2 for thicker t = 400 nm. The decrease might be related to the rougher surface. The presented values of Jsc are comparable to the reported results of polarized epitaxial films [15,18] but at higher intensity of light illumination reaching the order of lA/cm2. The relation between the structure and PV property is revealed. Jmax is found to increase linearly with increasing dcsd as shown in Fig. 6. Jmax increases from 4.3 lA/cm2 to 11.7 lA/cm2 as dcsd grows from 19 nm to 40.1 nm. The results indicate that the structural defects especially planar defects suppress the PV phenomenon of the BFO films.

4. Conclusions 8

4

16

20

24

28

32

36

dscd (nm)) Fig. 6. The photocurrent density at intensity of 220 mW/cm2 versus dcsd plot of BFO films in this present study.

Effect of growth temperature and thickness of BFO films on the photovoltaic properties of sputter-deposited ITO/BFO films on Pt/ Ti/SiO2/Si(1 0 0) substrate are reported. Tg at which isotropic single phase of proveskite BFO forms depends on film thickness. For 200 nm-thick films, it is in the range of 350–450 °C; for 50– 400 nm-thick samples it is 450 °C. Impurity phases Bi2O3 and Fe2O3 extensively form at higher Tg of 500 °C. The grain size and roughness of BFO films increase with increasing Tg and film thickness. Significant PV effect is obtained in the presented BFO films under laser illumination with wavelength of 405 nm. Jsc linearly increases with the illumination intensity. The Jsc at I = 220 mW/cm2

406

H.W. Chang et al. / Journal of Alloys and Compounds 574 (2013) 402–406

(Jmax) are increased with the increase of the growth temperature and BFO thickness. The correlation between Jmax and the size of coherent scattering domain suggests that the structural defects suppress the PV property seriously. In addition, the formation of the secondary phases Fe2O3 and Bi2O3 and roughened surface morphology also lead to drastic decrease of Jsc. Acknowledgements This research was supported by National Science Council, Taiwan under Grant No. NSC-100-2112-M-029-002-MY3. The authors thank Drs. S.N. Hsiao, and H.Y. Lee, National Synchrotron Radiation Research Center (NSRRC), for technical assistance and useful discussion. References [1] W. Prellier, M.P. Singh, P. Murugavel, J. Phys.: Condens. Matter. 17 (2005) R803. [2] H. Bea, M. Gajek, M. Bibes, A. Barthelemy, J. Phys.: Condens. Matter. 20 (2008) 434221. [3] C.W. Nan, M.I. Bichurin, S. Dong, D. Viehland, G. Srinivasan, J. Appl. Phys. 103 (2008) 031101. [4] J.R. Teague, R. Gerson, W.J James, Solid State Commun. 8 (1970) 1073. [5] J. Wang, J.B. Neaton, H. Zheng, V. Nagarajan, S.B. Ogale, B. Liu, D. Viehland, V. Vaithyanathan, D.G. Schlom, U.V. Waghmare, N.A. Spaldin, K.M. Rabe, M. Wuttig, R. Ramesh, Science 299 (2003) 1719.

[6] G. Catalan, J.F. Scott, Adv. Mater. 21 (2009) 2463. [7] Y.H. Chu, L.W. Martin, M.B. Holcomb, R. Ramesh, Mater. Today 10 (2007) 16. [8] S. Ryu, J.Y. Son, Y.H. Shih, H.M. Jang, J.F. Scott, Appl. Phys. Lett. 95 (2009) 242902. [9] J. Wu, J. Wang, J. Appl. Phys. 106 (2009) 104111. [10] C.M. Folkman, S.H. Baek, H.W. Jang, C.B. Eom, C.T. Nelson, X.Q. Pan, Y.L. Li, L.Q. Chen, A. Kumar, V. Gopalan, S.K. Streiffer, Appl. Phys. Lett. 94 (2009) 251911. [11] H.W. Chang, F.T. Yuan, C.W. Shih, W.L. Li, P.H. Chen, C.R. Wang, W.C. Chang, S.U. Jen, J. Appl. Phys. 111 (2012) 07B105. [12] H.W. Chang, F.T. Yuan, C.W. Shih, C.R. Wang, W.C. Chang, S.U. Jen, J. Appl. Phys. 111 (2012) 07D918. [13] H.W. Chang, F.T. Yuan, C.W. Shih, C.S. Ku, P.H. Chen, C.R. Wang, W.C. Chang, S.U. Jen, H.Y. Lee, Nano. Res. Lett. 7 (2012) 435. [14] S.R. Basu, L.W. Martin, Y.H. Chu, M. Gajek, R. Ramesh, R.C. Rai, X. Xu, J.L. Musfeldt, Appl. Phys. Lett. 92 (2008) 091905. [15] T. Choi, S. Lee, Y.J. Choi, V. Kiryukhin, S.W. Cheong, Science 324 (2009) 63. [16] S.Y. Yang, J. Seidel, S.J. Byrnes, P. Shafer, C.H. Yang, M.D. Rossell, P. Yu, Y.H. Chu, J.F. Scott, J.W. Ager, L.W. Martin, R. Ramesh, Nat. Nanotechnol. 5 (2010) 143. [17] R. Guo, L. Chen, D. Wu, J. Wang, Appl. Phys. Lett. 99 (2011) 122902. [18] S.Y. Yang, L.W. Martin, S.J. Bymes, T.E. Conry, S.R. Basu, D. Paran, L. Reichertz, J. Ihlefeld, C. Adamo, A. Melville, Y.H. Chu, C.H. Yang, J.L. Musfeldt, D.G. Schlom, J.W. Ager III, R. Ramesh, Appl. Phys. Lett. 95 (2009) 062909. [19] W. Ji, K. Yao, Y.C. Liang, Adv. Mater. 22 (2010) 1763. [20] B. Chen, M. Li, Y. Liu, Z. Zuo, F. Zhuge, Q.F. Zhan, R.W. Li, Nanotechnology 22 (2011) 195201. [21] Y. Zang, D. Xie, X. Wu, Y. Chen, Y. Lin, M. Li, H. Tian, X. Li, Z. Li, H. Zhu, T. Ren, D. Plant, Appl. Phys. Lett. 99 (2011) 132904. [22] C.M. Hung, M.D. Jiang, J. Anthoninappen, C.S. Tu, J. Appl. Phys. 113 (2013) 17D905.