Effect of substrate bias voltage on the physical properties of dc reactive magnetron sputtered NiO thin films

Materials Chemistry and Physics 125 (2011) 434–439

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Effect of substrate bias voltage on the physical properties of dc reactive magnetron sputtered NiO thin films A. Mallikarjuna Reddy a , A. Sivasankar Reddy b , P. Sreedhara Reddy a,∗ a b

Department of Physics, Sri Venkateswara University, Tirupati 517 502, India Mechanical Engineering Department, Coimbra University, Coimbra, Portugal

a r t i c l e

i n f o

Article history: Received 24 June 2010 Received in revised form 30 September 2010 Accepted 25 October 2010 Keywords: Sputtering Oxides Optical properties Electrical properties

a b s t r a c t Nickel oxide (NiO) thin films were prepared on glass substrates at various bias voltages using dc reactive magnetron sputtering technique. The influence of substrate bias voltage on structural, optical and electrical properties was systematically investigated using X-ray diffraction (XRD), SEM, EDS, spectrophotometer and Hall effect studies. The NiO films are crystalline with preferential growth along (2 0 0) plane. The NiO films exhibit optical transmittance of 55% and direct band gap of 3.78 eV at the substrate bias voltage of −75 V. The electrical resistivity decreases as substrate bias voltage increases from 0 to −75 V thereafter it was slightly increased. © 2010 Elsevier B.V. All rights reserved.

1. Introduction

2. Experimental

Nickel oxide (NiO) thin films with NaCl-type structure have a wide range of applications due to their excellent chemical stability and durability, as well as optical, electrical and magnetic properties, large spin optical density, and possibility of manufacturing by variety of techniques. They have been used as an anti-ferromagnetic material [1], material for electrochromic display devices [2], and a part of functional sensor layers in chemical sensors [3]. Furthermore it is considered to be a model semiconductor with p-type conductivity with wide band-gap energy ranging from 3.6 to 4.0 eV [4]. It is evident that the improvement of the material properties can be reached by the optimization of the preparation conditions. NiO films can be fabricated by different physical and chemical vapour deposition techniques, such as sputtering [5–7], electron beam evaporation [8,9], sol–gel [10], and spray pyrolysis [11]. Among these techniques, dc reactive magnetron sputtering is one of the most useful techniques having high deposition rates, uniformity over large areas of the substrates and easy control over the composition of the deposited films. The properties of the deposited films mainly depend on the deposition parameters such as substrate temperature, oxygen partial pressure, sputtering power, sputtering pressure and substrate bias voltage. In the present study, NiO thin films were deposited using dc reactive magnetron sputtering technique and studied the effect of bias voltage on the structural, optical and electrical properties.

NiO thin films were grown on corning 7059 glass substrates by using the dc reactive magnetron sputtering from a homemade circular planar magnetron sputtering system. The sputtering system is capable of creating an ultimate vacuum of 5 × 10−4 Pa. The sputter chamber was pumped with diffusion pump and rotary pump combination. The pressure in the sputter chamber was measured using digital Pirani and Penning gauge combination. A circular planar magnetron of 100 mm diameter was used as the magnetron cathode. The magnetron target assembly was mounted on top of the sputter chamber such that the sputtering could be done by sputter down configuration. A continuously variable dc power supply of 1000 V and 1 A was used as a power source for sputtering. A 100 mm diameter and 3 mm thick pure nickel (99.98%) was used as sputter target. Pure argon was used as sputter gas and oxygen as reactive gas. The flow rates of both argon and oxygen gases were controlled individually by Tylan mass flow controllers. Before deposition of each film, the target was sputtered in pure argon atmosphere for 10 min to remove oxide layer if any on the surface of the target. NiO thin films were deposited at various substrate bias voltages (0 to −125 V) by keeping the other deposition conditions such as oxygen partial pressure, sputtering pressure and sputtering power as constant. The sputtering conditions maintained during the growth of NiO films are given in Table 1. The deposited films were characterized by studying crystallographic structure, optical and electrical properties. The crystallographic structure of the films was analyzed by X-ray diffractometer (XRD) using Cu K␣ radiation ( = 0.1546 nm) of model 3003TT manufactured by Seifert. The surface morphology was studied by scanning electron microscope (SEM) of model EVO MA 15 manufactured by Carl Zeiss, for which an EDS is attached of model Inca Penta FETx3 manufactured by Oxford Instruments was used for composition analysis. The optical properties of the films were determined by Perkin Elmer Lambda 950 UV-VIS-NIR double beam spectrophotometer. The electrical resistivity and Hall mobility were studied by employing the van der Pauw method [12]. The sheet resistance (s ) of the films was calculated using the equation:

∗ Corresponding author. Tel.: +91 877 2289472; fax: +91 877 2249611. E-mail address: [email protected] (P. Sreedhara Reddy). 0254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2010.10.035

 s =

 ln2

  f

R1 + R2 2

 (1)

A. Mallikarjuna Reddy et al. / Materials Chemistry and Physics 125 (2011) 434–439 Table 1 Deposition parameters maintained during the deposition of NiO films by dc reactive magnetron sputtering at various substrate bias voltages. 100 mm diameter and 3 mm thick 70 mm Corning 7059 glass 5 × 10−4 Pa 6 × 10−2 Pa 4 Pa 303 K 150 W 0 to −125 V

where f is the van der Pauw correction factor, which depends on the position of electrical contacts on the film surface: 1 − ln2 f = 2



R1 − R2 R1 + R2

2

0.4205

0.4200

Lattice parameter (nm)

Sputtering target pure nickel (99.98%) Target to substrate distance Substrates Ultimate pressure (PU ) Oxygen partial pressure (pO2 ) Sputtering pressure (PW ) Substrate temperature (TS ) Sputtering power Substrate bias voltage

0.4195

0.4190

0.4185

(2)

0.4180

The electrical resistivity () of the films was determined from the relation  = s × t

(3)

-140

where t is the film thickness. The Hall mobility () of the films was calculated from the relation  = (R × 108 )Bs

-120

-100

-80

-60

-40

-20

0

Substrate bias voltage (V)

(4) Fig. 2. Variation of lattice parameters of NiO films with the substrate bias voltage.

where R is the change of resistance with the applied magnetic field (B).

3. Results and discussion 3.1. Structural properties The X-ray diffraction patterns of the NiO films deposited at the different bias voltages are shown in Fig. 1. The crystal structures of the biased and unbiased film were identified to be polycrystalline and retain NaCl structure. The unbiased film exhibits preferred orientation of (2 0 0). The intensity of (2 0 0) peak was increased and becomes sharper with increase of bias voltage up to −75 V. This may be due to the increase of energy to the atoms/molecules which increases the diffusion mobility of the particles, which will help the films to crystallize [13]. Further increasing the bias voltage to −125 V, the peak width became broader and the intensity

(200) -125 V

Intensity (arb.units)

-100 V

-75 V

-50 V

of the peak was sharply decreased. The similar behavior was also observed in dc magnetron sputtered Cu2 O [14] and TiO2 [15] thin films. The 2 value was slightly shifted to higher angle with increase of substrate bias voltage up to −75 V thereafter it shifted to lower angle. The lattice parameter of the films was also influenced by the bias voltage. The variation of the lattice parameter with substrate bias voltage is shown in Fig. 2. The lattice parameter of the films was decreased from 0.4191 to 0.4180 nm with increasing the bias voltage from 0 to −75 V. Thereafter lattice parameter was increased to 0.4203 nm at higher bias voltage of −125 V. The variation in the lattice parameter with the applied bias voltage was due to the stresses developed in the films [16]. The present obtained lattice parameter value of 0.4180 nm at −75 V was close to the standard value [JCPDS No: 78-0643]. It was also observed that the variation in lattice parameter related to the Ni/O ratio. As Ni/O ratio increases the lattice parameter decreases gradually up to substrate bias voltage of −75 V, thereafter lattice parameter increases sharply. This result means that nickel to oxygen (Ni/O) ratio observed from energy dispersive spectroscopy (EDS). i.e As Ni/O ratio increases the lattice parameter decreases and vice-versa due to existence of interstitial oxygen atoms in NiO lattice[17] =

30

40

50

60

70

2θ (degrees) Fig. 1. X-ray diffraction patterns of NiO films deposited at different substrate bias voltages.

−E(a − ao ) 2 ao

(5)

where E is the Young’s modulus of the NiO (200 GPa), a is the lattice parameter of the bulk material, ao is the measured lattice parameter and is the Poisson’s ratio (0.31). The stress developed in the films is obtained by the shift in the interplanar spacing hence change in the lattice parameter. The tensile stress in the films decreased from 1.1587 GPa to 0.3087 GPa with increased substrate bias voltage from 0 V to −75 V, thereafter it increased to 2.0729 GPa at −125 V for (2 0 0) peak, The tensile stress developed in the films is due to the existence of microscopic voids incorporated in the films during condensation [18]. The grain size of the films was calculated from (2 0 0) peak by using Scherer’s equation,

0V 20

435

L=

K ˇ cos 

(6)

where K is the correction factor,  is the wavelength of the incident beam, ˇ is the full width at half maximum corresponding to diffraction angle .

436

A. Mallikarjuna Reddy et al. / Materials Chemistry and Physics 125 (2011) 434–439

Fig. 3. SEM images of NiO films as a function of substrate bias voltage (a) 0 V, (b) −50 V, (c) −75 V, (d) −100 V and (e) −125 V.

The grain size of the films increased from 28 nm to 43 nm with increasing of bias voltage from 0 to −75 V, thereafter it decreased to 19 nm at higher bias voltage of −125 V. The increasing of grain size with increasing of bias voltage was due to the improvement in the crystallinity of the films. The detailed structural information is shown in Table 2. 3.2. Surface morphology and composition The scanning electron microscopy images of NiO films at different substrate bias voltages are shown in Fig. 3. It was observed that, smooth surface in the films was observed up to bias voltage of −50 V and the fine grains appeared when the films formed at a substrate bias voltage −75 V. The size of the grains decreased when the films deposited beyond this bias voltage. The energy sispersive spectroscopy (EDS) was employed to identify the composition

of the as deposited NiO films at different substrate bias voltages. EDS results revealed that the films consist of nickel and oxygen. Fig. 4 shows the EDS spectra of NiO films deposited at various substrate bias voltages. The quantitative analysis of NiO thin films deposited at various substrate bias voltages is shown in Table 3.

Table 2 Structural information of dc reactive magnetron sputtered NiO films at various substrate bias voltages. Substrate bias voltage (V)

Orientation

Lattice parameter (nm)

Grain size (nm)

Stress (GPa)

0 −50 −75 −100 −125

(2 0 0) (2 0 0) (2 0 0) (2 0 0) (2 0 0)

0.4191 0.4190 0.4180 0.4201 0.4203

28 30 43 20 19

1.1587 1.0781 0.3087 1.9201 2.0729

A. Mallikarjuna Reddy et al. / Materials Chemistry and Physics 125 (2011) 434–439

437

Fig. 4. EDS spectra of NiO films deposited at various substrate bias voltages (a) 0 V, (b) −50 V, (c) −75 V, (d) −100 V and (e) −125 V.

3.3. Optical properties Fig. 5 shows the optical transmittance spectra of NiO films as a function of substrate bias voltage. The optical transmittance of the films increased from 44 to 55% (at  = 670 nm) with increasing of bias voltage from 0 to −75 V. Further increasing the bias voltage to −125 V the transmittance of the films decreased to 31%. The absorption edge was shifted towards lower wavelength with the increase of substrate bias voltages up to −75 V. The interstitial oxy-

gen atoms in non-stoichiometric oxygen rich NiO films will cause to scatter or absorb the incident light that results in the reduction in transmittance of NiO films [19] and also increase in transmittance of the films is related to an increase in grain size of the films. The optical absorption coefficient (˛) was calculated from the optical transmittance (T) and reflectance (R) data using the relation 1 ˛= t



ln T (1 − R)2



(7)

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A. Mallikarjuna Reddy et al. / Materials Chemistry and Physics 125 (2011) 434–439

Table 3 The compositional analysis of dc reactive magnetron sputtered NiO films at various substrate bias voltages by energy dispersive spectroscopy (EDS).

12

1.4x10

0V -50 V

−75 −100 125

Atomic%

Ni/O ratio

1.2x10

OK Ni K OK Ni K OK Ni K OK Ni K OK Ni K

37.05 62.95 32.03 67.97 27.74 72.26 32.45 67.55 34.75 65.25

68.35 31.65 63.36 36.64 58.48 41.52 63.81 36.19 66.15 33.85

0.46

1.0x10

-75 V -100 V

12

-125 V

-1

−50

Weight%

0.57 0.70

11

8.0x10

2

0

12

Element

(αhν) (eVcm )2

Substrate bias voltage (V)

0.56 0.51

11

6.0x10

11

4.0x10

11

2.0x10

0.0

where t is the thickness of the films. The dependence of ˛ on the photon energy (h ) fitted to the relation for direct transition ˛h = A(h − Eg )

1/2

(8)

where Eg is the optical band gap of the films. The plots of (˛h )2 versus photon energy (h ) of the NiO films formed at various substrate bias voltages are shown in Fig. 6. The optical band gap of the films was evaluated from the extrapolation of the linear portion of the plots of (˛h )2 versus (h ) to ˛ = 0. The optical band gap of the films increased from 3.67 to 3.78 eV with the increase of substrate bias voltage from 0 to −75 V. Beyond this substrate bias voltage the optical band gap of NiO films was decreased to 3.71 eV. This widening of the optical band gap with substrate bias voltage was attributed to the increase in the carrier concentration and transparent nature of the films [20]. The optical band gap obtained at substrate bias voltage of −75 V in the present investigation is in good agreement with the rf magnetron sputtered NiO films reported by Nandy et al. [21]. However in the literature large optical band gap of 3.88 eV was reported by Kamal et al. [22] in spray pyrolysis deposited NiO films at 420 ◦ C. Hakim et al. [23] reported the lower energy gap of 3.11 eV in solution growth NiO films. The optical transmittance and band gaps variation with substrate bias voltage is listed in Table 4.

2.8

3.2

3.4

3.6

3.8

4.0

4.2

Photon energy (eV) Fig. 6. Plot of (˛h )2 and (h ) for the NiO films deposited at different substrate bias voltages.

Table 4 Optical information of dc reactive magnetron sputtered NiO films at various substrate bias voltages. Substrate bias voltage (V)

Transmittance (%)

Optical band gap (eV)

0 −50 −75 −100 −125

44 46 55 35 31

3.67 3.69 3.78 3.72 3.71

3.4. Electrical properties The electrical properties of the films were highly influenced by the substrate bias voltage. Fig. 7 shows the electrical resistivity of NiO films formed at different substrate bias voltages. The electrical properties of NiO films are associated with their microstructure, composition, and consequently on the deposition environment. However, crystalline NiO film with (2 0 0) orientation is formed with near stoichiometric ratio. The stoichiometric

100

140 120

80

100

Resistivity (Ωcm)

Transmittance (%)

3.0

60

40 0V -50 V 20

80 60

40

-75 V -100 V

20

-125 V 0

400

600

800

1000

1200

Wavelength (nm) Fig. 5. Optical transmittance spectra of NiO films as a function of substrate bias voltage.

-140

-120

-100

-80

-60

-40

-20

0

Substrate bias voltage (V) Fig. 7. Variation of electrical resistivities of NiO films as function of substrate bias voltage.

A. Mallikarjuna Reddy et al. / Materials Chemistry and Physics 125 (2011) 434–439 Table 5 Hall effect data of dc reactive magnetron sputtered NiO films at different substrate bias voltages. Substrate bias voltage (V)

Resistivity () ( cm)

Carrier concentration (n) (cm−3 )

Mobility () (cm2 V−1 s−1 )

0 −50 −75 −100 −125

135.8 70.7 25.4 47.1 68.9

2.6 × 1016 3.0 × 1016 7.5 × 1016 5.3 × 1016 4.1 × 1016

1.8 2.9 3.3 2.5 2.2

439

preferred orientation of (2 0 0) in all bias conditions, and the lattice parameter of the films varied from 0.4191 nm to 0.4180 nm with increase of substrate bias voltage from 0 to −75 V. p-Type conduction of low electrical resistivity of 25.4 cm was obtained at substrate bias voltage of −75 V. The optical transmittance of the films increased from 44 to 55% and optical band gap of the films increased from 3.67 eV to 3.78 eV with increasing of substrate bias voltage from 0 to −75 V. References

NiO is an insulator with high electrical resistivity (>1013 cm) at room temperature [24]. The films showed high electrical resistivity of 135.8 cm at unbiased condition. The electrical resistivity of the films gradually decreased to 25.4 cm with increasing of substrate bias voltage to −75 V. This may be due to increase of crystallinity of NiO films and also increase of Ni/O ratio. Further increasing of the substrate bias voltage to −125 V the electrical resistivity of the films increased to 68.9 cm. The increase of electrical resistivity at higher substrate bias voltages may be due to the entrapment of argon ions [25], it results in excess oxygen in the films. Chen and Yang [26] achieved low electrical resistivity of 14.7 cm in rf sputtered (2 0 0) orientation NiO films at substrate temperature of 400 ◦ C. All the films showed p-type conductivity. The Hall mobility of the films increased from 1.8 to 3.3 cm2 V−1 s−1 with increase of substrate bias voltage from 0 to −75 V thereafter it decreased to 2.2 cm2 V−1 s−1 . The carrier concentration of the films increased from 2.6 × 1016 to 7.5 × 1016 cm−3 with the increase of substrate bias voltage from 0 to −75 V thereafter it decreased to 4.1 × 1016 cm−3 at higher substrate bias voltages. The increasing of Hall mobility and carrier concentration with increase of bias voltage was due to the improvement in grain size and the alignment of grains at the grain boundaries which minimizes the trapping and scattering of the charge carriers at the grain boundaries [27]. The changes in mobility at higher substrate bias voltages may be due to high resistivity of the films. The Hall effect measurements are given in Table 5. 4. Conclusions NiO films have been deposited by dc reactive magnetron sputtering at different substrate bias voltages. The films exhibited the

[1] E. Fujji, A. Tomozawa, H. Torii, R. Takayama, Jpn. J. Appl. Phys. 35 (1996) L328. [2] M. Kitao, K. Izawa, K. Urabe, T. Komatsu, S. Kuwano, S. Yamada, Jpn. J. Appl. Phys. 33 (1994) 6656. [3] H. Kumagai, M. Matsumoto, K. Toyoda, M. Obara, J. Mater. Sci. Lett. 15 (1996) 1081. [4] H. Sato, T. Minami, S. Takata, T. Yamada, Thin Solid Films 236 (1993) 27. [5] W. Estrade, A.M. Andersson, C.G. Granqvist, J. Appl. Phys. 64 (1988) 3678. [6] D.A. Wruck, A.M. Dixon, M. Rubin, S.N. Bogy, J. Vac. Sci. Technol., A 9 (1991) 2170. [7] J. Scarminio, W. Estrade, A. Andersson, A. Gorenstein, F. Decker, J. Electrochem. Sci. 139 (1992) 1236. [8] C.R. Otterman, A. Temmink, K. Bange, Thin Solid Films 193–194 (1990) 409. [9] T. Seike, J. Nagai, Sol. Energy Mater. 22 (1991) 107. [10] L. Wang, Z. Zhang, Y. Cao, J. Ceram. Soc. Jpn. 101 (1993) 227. [11] S.A. Mahmoud, A.A. Akl, H. Kamal, K. Abdel-Hady, Physica B 311 (2002) 366. [12] L.J. van der Pauw, Philips Res. Rep. 13 (1958) 1. [13] B. Stjerna, E. Olsson, C.G. Granqvist, J. Appl. Phys. 76 (1994) 3797. [14] A. Sivasankar Reddy, S. Uthanna, P. Sreedhara Reddy, Appl. Surf. Sci. 253 (2007) 5287. [15] M.C. Barnes, S. Kumar, L. Green, N.M. Hwang, A.R. Gerson, Surf. Coat. Technol. 190 (2005) 321. [16] Ch. Sujatha, G. Mohan Rao, S. Uthanna, Mater. Sci. Eng., B 94 (2002) 106. [17] M. Ohring, The Material Science of Thin Solid Films, Academic Press, New York, 1992. [18] W. Buckel, J. Vac. Sci. Technol., A 6 (1969) 606. [19] J.L. Yang, Y.S. Lai, J.S. Chen, Thin Solid Films 488 (2005) 242. [20] V.I. Fistual, V.M. Vainshtein, Sov. Phys. Solid State 8 (1967) 2769. [21] S. Nandy, U.N. Maiti, C.K. Ghosh, K.K. Chattopadhyay, J. Phys.: Condens. Matter 21 (2009) 115804. [22] H. Kamal, E.K. Elmaghraby, S.A. Ali, A. Hady, J. Cryst. Growth 262 (2004) 424. [23] A. Hakim, J. Hossain, K.A. Khan, Renew. Energy 34 (2009) 2625. [24] B. Subramanian, M.M. Ibrahim, V. Senthilkumar, K.R. Murali, V.S. Vidhya, C.S. Raja, M. Jayachandran, Physica B 403 (2008) 4104. [25] R.K. Waits, J. Vac. Sci. Technol. 15 (1978) 179. [26] H.L. Chen, Y.S. Yang, Thin Solid Films 516 (2008) 5590. [27] P. Mohan Babu, G. Venkata Rao, P. Sreedhara Reddy, S. Uthanna, J. Mater. Sci. Mater. Electron. 15 (2004) 389.