Effect of Al components on the properties of CuAlO2 thin films deposited by RF magnetron sputtering

Effect of Al components on the properties of CuAlO2 thin films deposited by RF magnetron sputtering

Journal of Alloys and Compounds 581 (2013) 488–493 Contents lists available at ScienceDirect Journal of Alloys and Compounds journal homepage: www.e...

2MB Sizes 1 Downloads 17 Views

Journal of Alloys and Compounds 581 (2013) 488–493

Contents lists available at ScienceDirect

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

Effect of Al components on the properties of CuAlO2 thin films deposited by RF magnetron sputtering Zhaoqi Sun a,b,⇑, Xishun Jiang a,c, Junlei Li a, Gang He a,b, Xueping Song a,b a

School of Physics & Material Science, Anhui University, Hefei 230039, PR China Anhui Key Laboratory of Information Materials and Devices, Hefei 230039, PR China c School of Mechanical and Electronic Engineering, Chuzhou University, Chuzhou 239000, PR China b

a r t i c l e

i n f o

Article history: Received 27 May 2013 Accepted 21 July 2013 Available online 31 July 2013 Keywords: Semiconductor Functional composites Optical properties Wetting properties

a b s t r a c t CuAlO2 (CAO) thin films with Al volume fraction of 1–5% were prepared by RF magnetron sputtering and were annealed at 1173 K for 4 h in the air. The microstructures, optical and hydrophily properties of the as-deposited films were studied. XRD measurement shows that all films are polycrystalline and present as Cu–Al–O phase. 3D profilometer measurement shows the minimum of surface roughness is 11.6 nm for 1%Al–CAO film, and the maximum is 38.6 nm for 5%Al–CAO film. The transmittance in the visible range decreases from 60% to 40% after the annealing process as a result of impurity and rough surface. The minimum value of average refractive index and extinction coefficient is about 1.41 and 0.031 for 1%Al–CAO film. The estimated direct band gaps of CAO films are 3.37–3.53 eV. CAO films changes from hydrophobicity to hydrophily after annealing, and water contact angle of the annealed film increases from 31.1° to 70.5°. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction Known as a p-type transparent conducting oxide (TCO) without intentional doping, delafossite CuAlO2 (CAO) has attracted increasing attention to fabricate transparent oxide optoelectronic devices such as transparent p–n junction diodes and transistors, which are very important for the realization of the so-called ‘Transparent Electronics’ [1]. Furthermore, CAO has become a multifunctional semiconductor due to the discovery of other properties, such as gas sensitive property of ozone, field emission, and photo-catalysis [2–4]. Since p-type TCO CAO has been reported firstly by Kawazoe and co-authors using pulse laser deposition, the preparation and characterization of CAO thin films using various film-deposition techniques such as sputtering deposition, chemical vapor deposition, dip coating, hydrothermal, spray technique, e-beam evaporation and even biosynthesis technique have been reported [5–12]. Among all the film-deposition techniques, the sputtering technique has a high potential for industrial applications, because it offers in principle a simple and flexible control of the film stoichiometry over a large scale at relatively low cost and is suitable for the mass production of integrated circuit devices. The physical properties of the sputtered films mainly

⇑ Corresponding author at: School of Physics & Material Science, Anhui University, Hefei 230039, PR China. Tel./fax: +86 551 63861767. E-mail address: [email protected] (Z. Sun). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.07.143

depend on deposition parameters such as substrate temperature, working pressure, and oxygen partial pressure [13,14]. Apart from what have mentioned above, Cu/Al ratio plays a very important part in CAO thin film. Cai has optimized the Cu/Al ratio for the p-type conductivity and transmittance in copper aluminum oxide thin films deposited by plasma-enhanced metal–organic chemical-vapor deposition, and the best conductive film, with a room-temperature conductivity of 0.289 S/cm and a transparency of 80%, was found to have a Cu/Al ratio of 1.4 ± 0.3 [15]. Recently, Hsieh has reported that the resistivity of Cu–Al–O films ranged from 0.5 to 4.8  103 X cm, which is attributed to the incorporated Al3+ ions substituting the Cu2+ ions, thus decreasing the hole concentration [16]. However, the effect of Cu/Al atomic ratios on structure, optical and wetting properties has seldom been studied. In this report, a series of carefully designed experiments were carried out to investigate the dependence of structural, optical, and wetting properties of the CAO films on the Cu/Al atomic ratio.

2. Experimental CAO films were prepared on glass or single-sided polished silicon substrates by RF magnetron sputtering. The sputter target was a CAO ceramic disk (diameter: 60 mm; thickness: 5 mm) prepared by solid-state reaction between stoichiometric ratios of 99% Cu2O and 99.99% Al(OH)3 powders. Prior to film deposition the substrates were ultrasonically cleaned in acetone, ethanol and deionized water and the target was pre-sputtered in Ar atmosphere for 10 min to clean the surface and to remove any possible contamination.

489

Z. Sun et al. / Journal of Alloys and Compounds 581 (2013) 488–493

Fig. 1. EDS spectra of CAO films with various Al concentrations. (a) 0, (b) 1%Al, (c) 3%Al and (d) 5%Al.

To modulate the Cu/Al atomic ratio, various numbers of Al strips were stuck to the target surface. The Al volume fraction of the film can be calculated by the following equations:

qM ¼ xmM qD SMD =ðxmM qD SMD þ mD qM Þ x ¼ AM =AD

ð1Þ ð2Þ

where qM is Al volume fraction; mM and mD are the mol masses of Al and CAO; qM and qD are the densities of Al and CAO; is the sputtering yield ratio of Al to CAO, and its experimental value is 14.8; x is the ratio of Al exposure area (denoted by AM) to CAO exposure area (denoted by AD). The summary of the deposition conditions is shown in previous paper [17]. All the films were annealed at 1173 K for 4 h in the air. CAO films were characterized by the following techniques: (1) Energy dispersive X-ray spectroscopy (S-4800, Hitachi, Tokyo, Japan) for the detection of surface composition variation; (2) X-ray diffractometer (MXP18AHF, MacScience, Yokohama, Japan) for the identification of second phases and crystal structure; (3) 3D profilometer (PRO500, CETR, Campbell, USA) for the observation of surface morphology and surface roughness; (4) Ultraviolet–visible spectrophotometer (UV-2550, Shimadzu, Kyoto, Japan) for the determination of optical transmission and band gap; (5) Home-made water contact angle apparatus for WCA measurements and wetting properties, respectively.

Table 1 The results of EDS spectra of CAO films with various Al concentrations. Sample no.

Area of Cu (a.u.)

Area of Al (a.u.)

Cu:Al

CAO 1% Al 3% Al 5% Al

11,560 3510 18,148 10,678

11,089 3695 21,384 12,842

1.04:1 0.95:1 0.85:1 0.83:1

3. Results and discussion Fig. 1 and Table 1 show the EDS results of CAO films with various Al concentrations, and the results in Table 1 are calculated by fitting the curves in Fig. 1. The composition analysis of films reveals that the atomic ratio of Cu and Al is from 1.04 to 0.83 with the increasing of Al concentration. The deviation from the stoichiometric ratio of pure CAO film might originate from the different deposition rate of Cu and Al [18]. The EDS analysis shows that Al concentration in CAO films arise as Al strips on the target surface increasing. Fig. 2 shows the XRD spectra of CAO films with various Al concentrations. All the films are polycrystalline and there are phases of Al2O3, CuO and CuAl2O4 besides CAO phase. The result is well in agreement with the observations of Jacob [19]. With increasing amounts of the Al element, the (0 0 6) peaks slightly shift toward smaller angles. To demonstrate the effect of Al function on the microstructure of CAO film, the lattice constant c of delafossite structure CAO can be calculated, according to Scherrer and Bragg equation,



kk b cos h

ð3Þ

Fig. 2. The XRD spectra of CAO films with various Al concentrations.

Table 2 The results of XRD spectra of CAO films with various Al concentrations. Sample no.

Diffraction peak/ 2h (°)

FWHM/ b

Grain size/D (nm)

Lattice constant/ c (Å)

CAO 1% Al 3% Al 5% Al

31.354 31.228 31.123 31.118

0.360 0.332 0.295 0.279

22.66 24.57 27.64 29.23

17.098 17.175 17.238 17.239

490

Z. Sun et al. / Journal of Alloys and Compounds 581 (2013) 488–493

Fig. 3. 3D scan images of CAO films with various Al concentrations (a)–(d): 0–5%Al.

ð4Þ

a d ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 2 2 4 ðh þ hk þ k Þ þ ac2l 3

ð5Þ

In accordance with the (0 0 6) diffraction, the lattice constant c is calculated by



3k sin h

ð6Þ

The calculated grain size D and lattice constant c are shown in Table 2. With increasing amounts of the Al element, both grain size and lattice constant become large, which implies that Al richness is beneficial to the increasing of grain size and the increasing of lattice constant is due to the interstitial of Al3+. The ionic radius of Al3+ is 0.53 Å, smaller than Cu+ (0.96 Å). However, with the adding of Al component Al2O3 phase was observed as a major phase. Fig. 3 shows the 3D scan images of CAO films with various Al concentrations. The minimum of surface roughness is 11.6 nm for 1%Al–CAO film, and the maximum is 38.6 nm for 5%Al–CAO film. Some surface intervals are filled up with second phase atoms, which make the surface smooth. Fig. 4 shows the optical transmittance of CAO films with various Al concentrations. The transmittance goes down from 60% to 40% after the annealing process as a result of impurity and rough surface. As shown in Fig. 4, the

100

Transmittance (%)

2d sin h ¼ nk

80

CuAlO2

60

1% Al 3% Al 5% Al

40

20

0

300

400

500

600

700

800

900

Wavelength (nm) Fig. 4. Transmission spectra of CAO films with various Al concentrations.

average transmittance of 1%Al–CAO film reaches peak value within the wavelength range of 400–760 nm. The variations of average transmittance are due to surface roughness and impurity. The rough surface enhances optical scattering and absorption, which

491

Z. Sun et al. / Journal of Alloys and Compounds 581 (2013) 488–493

80

100

1% Al

CuAlO2 60

80

True value Fitted value

True value Fitted value

60

40

Transmittance (%)

40 20

0

20

300

400

500

600

700

800

900

100

0

400

500

600

700

800

900

600

700

800

900

5% Al

3% Al 80

80 True value Fitted value

60

40

20

20

300

400

500

True value Fitted value

60

40

0

300

100

600

700

800

900

0

300

400

500

Wavelength (nm) Fig. 5. True and fitted transmittance spectra of CAO films with various Al concentrations. Table 3 Fitted parameters for transmittance spectra of CAO films with various Al concentrations. Sample no.

Average transmittance (%)

CAO 1% Al 3% Al 5% Al

RMSE

39.9 48.3 46.2 36.4

0.696 0.960 0.949 0.550

Thickness (nm)

Average optical constant

297.7 302.6 307.5 294.2

n

k

1.49 1.41 1.47 1.50

0.063 0.031 0.044 0.073

1.5

1.6

CuAlO2

CuAlO2 1% Al 3% Al 5% Al

1.0

n

1.5

1% Al 3% Al 5% Al 3.50eV 3.37eV 3.48eV 3.53eV

1.4 0.5 1.3 10 0.0 1.5

-2

k ( 10 )

8

2.0

2.5

3.0

3.5

4.0

4.5

Energy (eV)

6

Fig. 7. Direct band gap estimation of CAO films with various Al concentrations.

4 CuAlO2 1% Al 3% Al 5% Al

2 0 300

400

500

600

700

800

900

Wavelength (nm) Fig. 6. Optical constant of CAO films with various Al concentrations.

reduces the transmittance. On the contrary, the transmittance increases. These are in good agreement with the results of 3D scan images. With increasing amounts of the Al element, more impurities and grain boundaries appear. The transmittance decreases as a result of the scattering of impurity and grain boundary. To obtain the optical constant, optical transmittance was fitted by Film Wizard using Drude–Lorentz model shown in Fig. 5. Experimental and fitted transmittance spectra of films agreed with each

492

Z. Sun et al. / Journal of Alloys and Compounds 581 (2013) 488–493

Fig. 8. Wetting property photos of CAO films with various Al concentrations before and after annealing.

Table 4 Water contact angels of CAO films with various Al concentrations before and after annealing. Sample no.

WCA before annealing (°)

WCA after annealing (°)

CAO 1% Al 3% Al 5% Al

97.5 104.2 103.4 102.8

31.1 61.4 66.0 70.5

with the film directly [20]. For another, the larger grains enhance the surface roughness, as is mentioned in the results of 3D scan. As is known, rough surface can improve hydrophilicity of film [21]. Furthermore, the annealed film exhibits CAO phase along with second phase, thus the interfacial energy between solid and liquid decreases and the WCA reduces rapidly. The presence of impurity is considered to be responsible for the decrease in the wetting angle. 4. Conclusions

other, and root mean square error (RMSE), average film thickness and average optical constant are listed in Table 3. Fig. 6 shows the refractive index (n) and extinction coefficient (k) of CAO films with various Al concentrations. The minimum value of average refractive index and extinction coefficient is about 1.41 and 0.031 for 1%Al–CAO film, respectively. The variations of optical constant are due to electron trap caused by impurity. For transparent medium, as a first approximation, thickness (d), transmittance (T), reflectance (R) and absorption coefficient (a) are related by the following equation:

1 d

a ¼ ln

  1R T

ð7Þ

Besides, the relation between the absorption coefficient (a) and the incident photon energy (hm) can be written as: 1=m

ðahmÞ

¼ Aðhm  Eg Þ

ð8Þ

where A is a constant, Eg is the band gap of the material and exponent m depends on the type of transition. For direct allowed transition, m = 1/2, for indirect allowed transition, m = 2. The estimated direct band gaps of CAO films are shown in Fig. 7, which is determined to be 3.37–3.53 eV. The decrease of the band gap for 1%Al– CAO film originates from quantum confinement, and it is well known that quantum confinement in semiconductor nanocrystals leads to the band gap widening. With the adding of Al component, the increasing direct band gap is due to Al2O3 phase (Eg = 5.55 eV), and this can be further identified by the result of XRD. Fig. 8 shows the wetting property photos of CAO films with various Al concentrations before and after annealing. The water contact angle (WCA) h was calculated by the following equation:

h ¼ arctan

4HL 2

L  4H2

ð9Þ

where L and H are diameter and height of the spherical crown of the droplet and the results are shown in Table 4. The WCA increases from 97.5° to 104.2°, which indicates that the as-deposited CAO films are hydrophobic. The variation of WCA depends on the second phase and the surface roughness. After annealing, the WCA decreases rapidly from 97.5° to 31.1° for CAO film. All the annealed CAO films are hydrophilic. The evolution of WCA originating from annealing may be governed by the following reason. For one thing, organic pollutants on the surface can be removed by annealing, then the water molecules contact

CuAlO2 films with exceed Al volume fraction of 1–5% were prepared by RF magnetron sputtering on silicon and quartz glass using a sintered CuAlO2 ceramic target. Due to the formation of crystal grain and impurity, surface roughness first decreases and then increases with increasing Al component. The average transmittance and refractive index of films are about 40% and 1.5%, respectively. The variation of the determined direct band gap is due to quantum confinement and Al2O3 phase. CuAlO2 film changes from hydrophobicity to hydrophily after annealing, and water contact angle of the annealed film increases from 31.1° to 70.5°, as a result of impurity. Acknowledgements This work is supported by the National Natural Science Foundation of China (Nos. 51072001, 51272001); National Science Research Foundation for Scholars Return from Overseas, Ministry of Education, China; and Science Research Program of Institutions of Higher Education of Anhui Province, China (KJ2010B148). Thanks for Yonglong Zhuang and Zhongqing Lin, Experiment and Technology Center, Anhui University, for electronic microscope test and discussion. References [1] G. Thomas, Nature 389 (1997) 907–908. [2] X.G. Zheng, K. Taniguchi, A. Takahashi, Y. Liu, C.N. Xu, Appl. Phys. Lett. 85 (2004) 1728–1729. [3] A.N. Banerjee, C.K. Ghosh, S. Das, K.K. Chattopadhyay, Appl. Surf. Sci. 225 (2004) 243–246. [4] J.R. Smith, X.G. Wang, Phys. Rev. B 79 (2009) 041403–041406. [5] H. Kawazoe, M. Yasukawa, H. Hyodo, M. Kurita, H. Yanagi, H. Hosono, Nature 389 (1997) 939–942. [6] A.N. Banerjee, C.K. Ghosh, K.K. Chattopadhyay, Sol. Energy Mater. Sol. C 89 (2005) 75–83. [7] H.F. Jiang, X.B. Zhu, H.C. Lei, G. Li, Z.R. Yang, W.H. Song, J.M. Dai, Y.P. Sun, Y.K. Fu, Thin Solid Films 519 (2011) 2559–2563. [8] J. Ding, Y.M. Sui, W.Y. Fu, H.B. Yang, S.K. Liu, Y. Zeng, W.Y. Zhao, P. Sun, J. Guo, H. Chen, M.H. Li, Appl. Surf. Sci. 256 (2010) 6441–6446. [9] S. Gao, Y. Zhao, P. Gou, N. Chen, Y. Xie, Nanotechnology 14 (2003) 538–541. [10] C. Bouzidi, H. Bouzouita, A. Timoumi, B. Rezig, Mater. Sci. Eng. B 118 (2005) 259–263. [11] D.S. Kim, S.J. Park, E.K. Jeong, H.K. Lee, S.Y. Choi, Thin Solid Films 515 (2007) 5103–5108. [12] A. Ahmad, T. Jagadale, V. Dhas, S. Khan, S. Patil, R. Pasricha, V. Ravi, S. Ogale, Adv. Mater. 19 (2007) 3295–3299. [13] G.B. Dong, X.P. Zhao, Y.C. Li, H. Yan, J. Cryst. Growth 311 (2009) 1256–1259. [14] Y.J. Zhang, L.P. Feng, D.Y. Zang, Appl. Surf. Sci. 258 (2012) 5354–5359. [15] J.L. Cai, H. Gong, J. Appl. Phys. 98 (2005) 033707-1–033707-5.

Z. Sun et al. / Journal of Alloys and Compounds 581 (2013) 488–493 [16] P.H. Hsieh, Y.M. Lu, W.S. Hwang, J.J. Yeh, W.L. Jang, Surf. Coat. Technol. 205 (2010) S206–209. [17] J.L. Li, X. Wang, S.W. Shi, X.P. Song, J.G. Lv, J.B. Cui, Z.Q. Sun, J. Am. Ceram. Soc. 95 (2012) 431–435. [18] A.S. Reddy, P.S. Reddy, S. Uthanna, G.M. Rao, J. Mater. Sci.-Mater. El. 17 (2006) 615–620.

493

[19] K.T. Jacob, C.B. Alcock, J. Am. Ceram. Soc. 58 (1975) 192–195. [20] Q. Ye, Z.F. Tang, L. Zhai, Vacuum 81 (2007) 627–631. [21] Y.K. Du, P. Yang, F. Zhao, N.P. Hua, L. Jiang, Thin Solid Films 491 (2005) 133– 136.