Physics Procedia 49 ( 2013 ) 9 – 14

1875-3892 © 2013 The Authors. Published by Elsevier B.V.Selection and peer-review under responsibility of the Department of Physics, School of Science and Humanities, Kongu Engineering Collegedoi: 10.1016/j.phpro.2013.10.005

ScienceDirect

Spintronic Materials: Nanostructures and Devices (SMND-2011)

Annealing effect on the physical properties of dc reactive magnetron sputtered nickel oxide thin films

A. Mallikarjuna Reddya*, A. Sivasankar Reddyb, P. Sreedhara Reddya*

a Department of Physics, Sri Venkateswara University, Tirupati 517 502, India b Division of Advanced Materials Engineering, Kongju National University, Cheonan City 330 717, South Korea

Abstract

Nickel Oxide (NiO) thin films have been deposited by dc reactive magnetron sputtering technique on unheated substrates and subsequently annealed from 473 to 673 K. The influence of annealing temperature on the structural, morphological, optical and electrical properties was analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), spectrophotometer and Hall effect studies respectively. The as deposited and annealed films were polycrystalline with preferential growth along (200) plane. The NiO films annealed at 573 K exhibited an optical transmittance of 46% and a direct band gap of 3.70 eV. The electrical resistivity of the films decreased as the annealing temperature increased from 473 to 573 K. © 2013 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of SMND-2011 Keywords: Magnetron sputtering; Annealing temperature; Surface morphology; Optical properties; Electrical properties.

1. Introduction

Nickel Oxide (NiO) is usually taken as a model for p-type semiconductors and is an attractive material well known for its chemical stability, as well as for its excellent optical and electrical properties. It is a promising material for various applications such as functional layers for solar cells [1, 2], antiferromagnetic layer [3], p-type transparent conducting thin films [4], material for electrochromic display devices [5]. Thin films of p-type semiconductors are required in many optoelectronic device applications. NiO is an interesting candidate of this class with a wide band gap in the range of 3.6-4.0 eV [6]. It is evident that the improvement of the material properties can be reached by the optimization of the preparation conditions. There are several methods to prepare nickel oxide films, which include sputtering [7, 8], electron beam evaporation [9, 10], plasma-enhanced chemical vapour

*Corresponding author: Tel.: +91877 2289472, Fax: +91877 2249611. E-mail address: psreddy4@gmail.com (P. Sreedhara Reddy).

: amreddy81@gmail.com (A.Mallikarjuna Reddy).

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© 2013 The Authors. Published by Elsevier B.V.Selection and peer-review under responsibility of the Department of Physics, School of Science and Humanities, Kongu Engineering College

Open access under CC BY-NC-ND license.

Open access under CC BY-NC-ND license.

10 A. Mallikarjuna Reddy et al. / Physics Procedia 49 ( 2013 ) 9 – 14

deposition [11], chemical bath deposition [12], and sol-gel [13]. Among these methods, dc reactive magnetron sputtering has been most widely useful technique 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, substrate bias voltage, and post deposition conditions. In this work, thin films of NiO have been deposited using dc reactive magnetron sputtering technique at room temperature and effect of annealing the films at different temperatures has been investigated on structural, morphological, optical and electrical properties.

2. Experimental details

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 x 10-4 Pa. 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 on unheated substrates by keeping the other deposition conditions such as oxygen partial pressure at 6 x 10-4 mbar, sputtering power at 150 W and sputtering pressure at 4 x 10-2 mbar as constant. The as deposited films were annealed in air in the temperature range of 473 to 673 K.

The deposited films were characterized by studying film thickness measurement, crystallographic structure, surface morphology, optical and electrical properties. The film thickness of the as deposited sample was measured by X-ray reflectivity technique. A Scinco made instrument of model SMD 3000 was used for the measurement of film thickness. The crystallographic structure of the films was analyzed by X-ray diffractometer using Cu K radiation Carl Zeiss EVO MA 15 scanning electron microscope (SEM). The surface roughness was studied by atomic force microscopy (AFM). 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 [14].

3. Results and Discussion

3.1. Structural properties

Fig.1. X-ray diffraction patterns of NiO films deposited at room temperature and annealed at various temperatures.

The thickness of NiO thin film deposited in the present investigation was around 350 nm. Fig. 1 shows the XRD patterns of NiO films annealed at different temperatures. The crystal structure of the films annealed at various temperatures was identified to be polycrystalline and retain cubic structure. The films prepared on unheated substrates exhibited (200) orientation. With the annealing temperature, the intensity of (200) peak was decreased and new plane (111) appeared at 573 K. At higher annealing temperature of 673 K the intensity of (200) orientation was

A. Mallikarjuna Reddy et al. / Physics Procedia 49 ( 2013 ) 9 – 14 11

3

(a) (b)

sharply decreased and the intensity of (111) was increased. Tannealing temperature. It has been found that the development of preferred orientation in NiO film is primarily governed by surface energy [15]. The orientation of the films is thought to be dependent on the mobility of adatoms and clusters on the substrate [16]. The crystalline nature of the nickel oxide film with a NaCl-type structure isusually affected by the arrangement of O-2. The lattice parameter of the films was also influenced by the annealingtemperature. The lattice parameter was calculated for (200) reflection and its value at room temperature is 0.4191 nm. It was observed that the lattice parameter of the NiO films was decreased from 0.4178 to 0.4167 nm with increasing the annealing temperature from 473 to 573 K, thereafter it was increased to 0.4179 nm at annealing temperature of 673 K. -ray diffraction data employing the relation [17]

-E(a-ao o (1)

WW o is is 0.4176 nm.

The stress developed in the films was obtained by the shift in the interplanar spacing hence change in the latticeffparameter. The stress developed in the films at room temperature is 1.1587 GPa. The stress in the films varied from0.1544 to -0.6968 GPa with increasing the annealing temperature from 473 to 673 K for (200) peak. The grain size

cosKL (2)

WW

The as deposited film has a grain size of 28 nm, which progressively increased to 33 nm with increasing the annealing temperature to 573 K, thereafter it was decreased to 24 nm at higher annealing temperature of 673 K. By comparing XRD and SEM results, it was apparent that the (200) peak of NiO films is related with the grain growthof the film.

3.2. Surface morphology

The scanning electron microscopy images of NiO films at different annealing temperatures were shown in Fig. 2. It was observed that, smooth surface was observed in the films deposited at room temperature. The fine grains were appeared when the films were annealed at a temperature of 573 K. The size of the grains decreased when the films annealed beyond this temperature.

The atomic force micrographs of NiO films annealed at different temperatures were shown in Fig.3. It wasclearly seen that both the RMS roughness and grain size of the films increased as the annealing temperatureincreases. The decreasing of grain size and the surface roughness was observed at higher annealing temperatures.

12 A. Mallikarjuna Reddy et al. / Physics Procedia 49 ( 2013 ) 9 – 14

(c) (d)

Fig.2. SEM images of NiO films as a function of annealing temperature (a) 303 K (b) 473 K (c) 573 K (d) 673 K.

(a) (b)RMS Roughness =8.9 nm RMS Roughness = 9.3 nm

(c) (d)RMS Roughness =9.7 nm RMS Roughness = 8.2 nm

Fig.3. AFM images of NiO films as a function annealing temperature (a) 303 K (b) 473 K (c) 573 K (d) 673 K.

A. Mallikarjuna Reddy et al. / Physics Procedia 49 ( 2013 ) 9 – 14 13

3.3. Optical properties

The optical transmittance of the as deposited films is 42 % at a wavelength of 650 nm. The transmittance of the films increased from 44 % to 46% with the increase of annealing temperature from 473 to 573 K. On further increasing the annealing temperature to 673 K, the transmittance of the films was decreased to 37 %. The absorption edge was shifted towards the lower wavelength with the increase of annealing temperature. The increase in transmittance of the films is related to an increase in grain size of the films. The optical band gap of the films increased from 3.67 to 3.72 eV with the increase of annealing temperature from 473 to 673 K. This may be related to the gradual annealing out of the unsaturated bonds producing a large number of saturated bonds. The reduction in the number of unsaturated bonds decreased the density of localized states in the band structure, consequently increased the optical band gap [18]. Similar result was also observed in sputtered NiO films by Zhou et al. [19]. The optical band gap obtained in the present study at annealing temperature of 573 K was in good agreement with the reported rf magnetron sputtered NiO films [20].

3.4. Electrical properties

The electrical resistivity of NiO thin films has a strong dependence on the microstructural defects existing in NiO crystallites, such as nickel vacancies and interstitial defects [21]. It was clear that electrical properties of NiO thin films were influenced by the annealing temperature. The films showed high electrical resistivity of 135.8 cm at room temperature (303 K). The electrical resistivity of the films decreased to 17.3 cm with increasing the annealing temperature to 573 K. Thereafter it increased to 31.5 cm at higher annealing temperature of 673 K. This decrease in electrical resistivity was due to increase in carrier concentration. The Hall mobility measurements indicated that the films were p-type conduction. The Hall mobility of the as deposited films is 1.8 cm2V-1s-1, it increased to 3.7 cm2V-1s-1 with increase of annealing temperature to 573 K, thereafter it decreased to 3.3 cm2V-1s-1 at higher annealing temperature of 673 K. The higher carrier concentration of 9.8 x 1016 cm-3 was observed at annealing temperature of 573 K. 4. Conclusions

The Nickel Oxide polycrystalline thin films were successfully deposited by dc reactive magnetron

sputtering at room temperature and annealed at various temperatures. The films showed (200) as preferred orientation at all annealing temperatures. The lattice parameter of the films decreased with increasing the annealing temperature up to 573 K. The optical studies revealed that with increasing the annealing temperature the transmittance of the films was increased besides widening the optical band gap. From the electrical studies, it was observed that the electrical resistivity of the films decreased with increasing the annealing temperature up to 573 K. The films exhibited low electrical resistivity of 17.3 cm, high carrier concentration of 9.8 x 1016 cm-3, with high mobility 3.7 cm2V-1s-1 at the annealing temperature of 573 K.

Acknowledgements

The authors thank the University Grants Commission, New Delhi for (File No. F-40-419/2011 (SR)) providing the financial assistance to carry out the above work.

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