Electronic, Optical and Electrical Properties of Nickel Oxide Thin Films ...

≪Research Paper≫

Applied Science and Convergence Technology Vol.24 No.3, May 2015, pp.72~76 http://dx.doi.org/10.5757/ASCT.2015.24.3.72

Electronic, Optical and Electrical Properties of Nickel Oxide Thin Films Grown by RF Magnetron Sputtering a

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Chanae Park , Juhwan Kim , Kangil Lee , Suhk Kun Oh , Hee Jae Kang *, and Nam Seok Park a

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Department of Physics, Chungbuk National University, Cheongju 361-763, Korea Department of Semiconductor Electroengineering, Chungbuk Health&Science University, Cheongju 363-794, Korea (Received April 29, 2015, Revised May 8, 2015, Accepted May 29, 2015)

Nickel oxide (NiO) thin films were grown on soda-lime glass substrates by RF magnetron sputtering method at room temperature (RT), and they were post-annealed at the temperatures of 100oC, 200oC, 300oC and 400oC for 30 minutes in vacuum. The electronic structure, optical and electrical properties of NiO thin films were investigated using X-ray photoelectron spectroscopy (XPS), reflection electron energy spectroscopy (REELS), UV-spectrometer and Hall Effect measurements, respectively. XPS results showed that the NiO thin films grown o

o

at RT and post annealed at temperatures below 300 C had the NiO phase, but, at 400 C, the nickel metal phase became dominant. The band gaps of NiO thin films post annealed at temperatures below 300oC were about 3.7 eV, but that at 400oC should not be measured clearly because of the dominance of Ni metal phase. The NiO thin films post-annealed at o

temperatures below 300 C showed p-type conductivity with low electrical resistivity and high optical transmittance of 80% in the visible light region, but that post-annealed at 400oC showed n-type semiconductor properties, and the average transmittance in the visible light region was less than 42%. Our results demonstrate that the post-annealing plays a crucial role in enhancing the electrical and optical properties of NiO thin films. Keywords : NiO thin film, RF magnetron sputtering, XPS, REELS, Optical properties, Electrical Properties

I. Introduction

alternatives for electro-chromic display devices, smart windows and functional layer materials

Nickel oxide (NiO) materials with NaCl-type

applicable for chemical sensors, hetero-junction

structure have 4.177 Å of lattice parameter [1,2]. NiO

solar cells, and photo electrolysis due to their

has been found applications as a ferromagnetic

excellent chemical stability as well as optical,

materials [3], a p-type transparent conducting film

electrical and magnetic properties [7-10]. Whilst

[4], a material for electrochromic display devices [5],

most common transparent conducting oxides such as

and a functional sensor layer in chemical sensors [6].

zinc oxide-based materials are n-type semiconductors,

NiO thin film, especially, is one of the possible

the NiO generally is a p-type semiconductor having a

* [E-mail] hjkang@cbu.ac.kr

Electronic, Optical and Electrical Properties of Nickel Oxide Thin Films Grown by RF Magnetron Sputtering

wide band gap ranging from 3.6 to 4.0 eV by creating

The physical thickness of the thin films was about 40

nickel vacancies and by forming interstitial oxygen

nm. The NiO thin films were post-annealed at the

atoms or by adding cation atoms to crystalline NiO,

temperatures of 100 C, 200 C, 300 C and 400 C for 30

which is more useful for optoelectronic device

minutes in vacuum. The electrical properties of NiO

applications because of hole injection [11,12]. NiO

thin films were studied via Hall effect measurement

thin films have been fabricated using various physical

with the Van der Pauw geometry at room temperature

and chemical vapor deposition techniques, including

(RT). XPS spectra were obtained by using a Ulvac-

spray pyrolysis [13], sol-gel [14], electron beam

PHI Quantera II equipment. XPS measurements were

evaporation [15], pulsed laser deposition [16],

performed using a monochromatic Al Kα X-ray

plasma-enhanced chemical vapor deposition [17] and

source and the energy analyzer pass energy of 55 eV.

reactive sputtering [18]. The most widely used are

The binding energies were referenced to the C 1s peak

sputtering techniques.

of hydrocarbon contamination at 284.5 eV [19]. The

o

o

o

o

In this study, NiO thin films were fabricated by the

REELS spectra were measured with primary electron

radio frequency (RF) magnetron sputtering method.

energies of 1.0 keV and with a constant analyzer pass

We focused on the effect of annealing process on

energy of 20 eV in VG ESCALAB 210. The

electronic structure, optical and electrical properties

transmittance spectra of the NiO thin films were

of NiO thin films via X-ray photoelectron spectroscopy

measured by utilizing Genesys 6 model from Thermo

(XPS), reflection electron energy spectroscopy (REELS),

Electron Corporation in the wavelength range of 300

UV-spectrometer, and Hall Effect measurements.

to 1000 nm at RT with an increment of 1.0 nm.

II. Experimental

III. Results and Discussions

The NiO thin films were deposited on soda-lime

Fig. 1 shows the XPS results of Ni 2p and O1s

glass substrates by RF magnetron sputtering method.

states of NiO thin films grown at RT and post-

Figure 1. (Color online) XPS spectra of Ni 2p (a) and O 1s (b) for NiO thin films grown at different annealing temperatures.

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Chanae Park, Juhwan Kim, Kangil Lee, Suhk Kun Oh, Hee Jae Kang, and Nam Seok Park

annealed at various temperatures. The binding energies of the doublet Ni 2p photoelectron core-level spectra for NiO thin films are at 853.7 and 871.6 eV for Ni 2p3/2 and Ni 2p1/2, respectively. These peaks correspond to the binding energies of Ni-O bonds in 2+

the NiO phase (Ni ) [20,21]. The spin-orbit splitting of 17 eV, and the satellite peak due to a shake-up process around 861.0 eV (Ni 2p3/2) and 880.1 eV (Ni 2p1/2) were found in NiO thin films. Furthermore, NiO o

thin films post-annealed at 400 C showed another peak from Ni 2p clearly, located at 851.9 eV. This peak attributes to the presence of metal NiO bonds [20,21], which implies that Ni metal phase is dominant in NiO thin films. The Ni 2p spectrum of NiO thin films were decomposed by GaussianLorentzian line shape with a Shirley background. Another peak at 855.4 eV in the Ni 2p spectra of the 3+

NiO thin films results from the presence of a Ni

Figure 2. (Color online) Reflection electron energy loss spectra with the primary energy of 1.0 keV at different annealing temperatures for NiO thin films.

ion

in the Ni2O3 phase [20,21]. The XPS quantification

of the conduction band. It is well known that the

shows the stoichiometry of NiO at annealing

nickel oxide has the lowest conduction band state

o

temperature of blew 300 C, and the composition of Ni

from the Ni 3d states and the highest valence band

and O was about 80% and 20% at the annealing

state is from the O 2p states. Electronic structures of

o

temperature of 400 C.

NiO thin films grown at RT and post-annealed at

The band gap energy, Eg, of NiO thin film and its

annealing temperature below 300oC were NiO phase,

electronic structure near the band gap were

but it was changed to Ni metal- rich phase at 400oC.

investigated using REELS. As shown in Fig. 2, the

Fig. 3 shows the transmittance as a function of

plasmon loss peaks of the NiO films were located at

wavelength for the NiO thin films. The transmittances

the energy around 22.4 eV. The REELS spectra were

of the NiO thin films grown at RT, 100oC, 200oC, and

used to estimate the energy band gaps of the NiO thin

300oC are 73%, 77%, 76% and 81% in the visible light

films. The onset values of REELS correspond to the

region, respectively. In contrast, the transmittance

band gap, which was described in our previous work

of NiO thin films annealed at 400oC was drastically

[22,23]. The results show that the measured band

decreased to 42%, which could be attributed to the Ni

gaps of the NiO thin films are 3.69, 3.69, 3.67 and

metal phase as shown in Fig. 1. Therefore, the

o

o

o

3.67 eV for RT, 100 C, 200 C, and 300 C, respectively,

annealing process is responsible for the decrease in

within an uncertainty of ±0.1 eV. But the onset value

the transmittance of the NiO thin films.

of loss spectrum obtained from NiO thin film posto

In order to examine the effect of annealing on the

annealed at 400 C is not clear, therefore the band gap

electrical properties of NiO thin films, we measured

value cannot be obtained. Generally, the energy band

the resistivity, carrier concentrations and the Hall

gap of a transition metal oxide exists between the O

mobility using Hall-effect measurements with the Van

2p states of the valence band and the metal d states

der Pauw geometry at RT. As shown in Table 1, the

74

Appl. Sci. Conv. Technol. 24(3), 72-76 (2015)

Electronic, Optical and Electrical Properties of Nickel Oxide Thin Films Grown by RF Magnetron Sputtering

Table 1. The Resistivity of NiO thin films. Films

Resistivity (Ωㆍcm)

NiO as deposited

24.70

o

4.23

o

2.97

o

6.22

NiO 100 C NiO 200 C NiO 300 C o

5.48×10-4

NiO 400 C

Table 2. The Carrier concentration and Mobility of NiO thin films. Films

Figure 3. (Color online) Transmittance spectra obtained by UV-Spectrometer for NiO thin films as a function of wave lengths.

Carrier -3 concentration (cm )

Mobility 2 (cm /Vㆍs)

NiO as deposited

2.41×1015

o

1.78×1016

82.85

o

2.96×1016

70.76

o

1.48×1016

67.69

-3.50×1020

32.54

NiO 100 C NiO 200 C NiO 300 C o

NiO 400 C

104

resistivity of the NiO thin films grown at RT, post annealed at 100oC, 200oC, and 300oC was about 24.7,

IV. Conclusions

4.23, 2.97, and 6.22 Ω·cm, respectively. As shown in Table 2, the NiO thin films grown at room o

o

temperature and post annealed at 100 C, 200 C, and o

300 C, exhibit p-type conductivity with the hole 15

16

16

concentration of 2.41×10 , 1.78×10 , 2.96×10 , and 16

-3

1.48×10 cm , and the mobility of 104, 82.85, 70.76, 2 -1 -1

The electronic, optical and electric properties of NiO thin films grown on soda-lime glass substrates by RF magnetron sputtering method have been studied via XPS, REELS, UV-spectrometer, and Hall-effect measurement, respectively. Electronic

respectively. However, for the

structure obtained with XPS and REELS spectra

NiO thin films post- annealed at temperature of

showed that NiO and Ni2O3 phases co-existed on NiO

and 67.69 cm V s o

400 C, the majority carrier type changes to electrons

thin films deposited at RT and post-annealed at

20

and the carrier concentration increases to 3.50×10

below 300oC, but it changed to Ni metal-rich phase

cm-3 with the resistivity of 5.48×10-4 Ω·cm, and the

after post-annealing at 400oC. The band gaps of the

mobility of 32.54 cm2V-1s-1. It showed that a phase

NiO thin films deposited at RT, 100oC, 200oC, 300oC

transition from p type to n type occurred between

were about 3.7 eV within the accuracy of 0.1 eV, but

300 C and 400 C. The n-type conductivity shown in

the band gap of NiO thin film post annealed at 400oC

the NiO thin films post-annealed at 400oC could be

was not measured because of the dominance of metal

o

o

0

attributed to the Ni metal- rich phase, which was

Ni phase. The optical transmittance spectra showed

observed in electronic structure properties.

that the transmittance of NiO thin films was about 80% in the visible light region, but drastically decreased to 42% after post-annealing at 400oC due to the existence of Ni0. The Hall-effect measurements

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75

Chanae Park, Juhwan Kim, Kangil Lee, Suhk Kun Oh, Hee Jae Kang, and Nam Seok Park

suggest that the NiO thin films prepared at low o

o

temperatures (room temperature, 100 C, 200 C, and o

300 C) are suitable for device applications requiring p-type semiconductors.

[10] H. Hosono, Vacuum 66, 419 (2002). [11] K. G. Gopchandran, B. Joseph, J. T. Abraham, P. Koshy, V. K. Vaidyan, Vacuum 48, 547 (1997). [12] Y. Tawada, H. Okamoto, and Y. Hamakawa, Appl. Phys. Lett. 39, 237 (1981). [13] B. A. Reguig, A. Khelil, L. Cattin, M. Morsli, J.

Acknowledegments

C. Bernede, Appl. Surf. Sci, vol. 253, no. 9, 4330 (2007).

This research was supported by Basic Science

[14] B. A. Reguig, A. Khelil, L. Cattin, M. Morsli, J.

Research Program through the National Research

C. Bernede, Appl. Surf. Sci, vol. 253, no. 9, 4330

Foundation of Korea (NRF) funded by the Ministry of

(2007).

Education,

Science and Technology (2012R1A1A

[15] K. J. Patel, M. S. Desai, C. J. Panchal, Bharati Rehani, J. Nano- Electron. Phys, 3, no. 1, 376

2009590).

(2011). [16] M. Tanaka, M. Mukai, Y. Fujimori, M. Kondoh,

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Appl. Sci. Conv. Technol. 24(3), 72-76 (2015)