Estimation of the annealing effect on optical, structural, and electrical properties of NiO thin films deposited by cost-effective sol-gel technique

1. Introduction

The development in the field of nanocShemistry has led the foundation of new materials, which are synthesized by different methods^1,2. The most notable exception is low-cost solution processed synthesis of metal oxide, which is widely used for the fabrication of nanodevices in photovoltaic industry³. In particular, NiO is a potentially used p-type semi-transparent metal oxide having band gap energy (Eg) values from 3.60 eV to 4.0 eV⁴. Recently, NiO have attracted much attention in the research community due to their numerous potential areas of application such as conductive films⁵, electrochromic material based display devices⁶, organic light emitting diodes as anode materials⁷, chemical sensors⁸ and so on so forth. NiO transparent films have been designed and developed by wide variety of methods which include electrochemical deposition⁹, sol-gel spin coating¹⁰, sputtering¹¹, thermal evaporation deposition¹², chemical solution deposition¹³, etc. Among those, sol-gel method combined with spin-coating deposition is very important technique by virtue of its low-cost, low-temperature and also films can be easily coated on different substrates for various applications^14,15. Most of literature reports indicate that the optical and electrical properties of NiO based films depend on the intrinsic holes induced from nickel vacancies and interstitial oxygen atoms that are achieved upon increasing the annealing temperature ¹⁶.

In this study, an in-expensive strategy for the fabrication of NiO transparent thin films was adopted using wet chemical sol-gel technique with spin coating deposition. The annealing effects in different temperatures under ambient conditions on optical, structural and electrical effects of NiO thin films were characterized.

2. Experimental details

2.1. Chemicals

Absolute ethanol, nickel acetate tetrahydrate [(C₄H₆NiO₄).2H₂O] and mono-ethanolamine (MEA) were used in their as-received forms without further purification or modification.

2.2. Synthesis of NiO precursor ink

NiO ink was synthesized using a simple sol-gel based method which could be found elsewhere¹⁷, however the process parameters were modified in present study. In a typical synthesis procedure, 1.28 g C₄H₆NiO₄).2H₂O was added slowly in the preheated (70 ^oC) mixture of 50 mL absolute ethanol and 5 mM monoethanolamine (MEA) under vigorous stirring. After 12h of stirring, an emerald clear and transparent blue solution was obtained. The NiO precursor solution was left for further aging for 24h and thereafter filtered (through a PTFE syringe filter with pore size of 0.2 μm). An absolute ethanol was chosen and [(C₄H₆NiO₄).2H₂O] as a solvent, while the MEA was added as a stabilizer and the base.

2.3. NiO thin film deposition

To deposit NiO thin films, the glass substrates were sonicated in ethanol, acetone, isopropyl alcohol (IPA), and deionized water for 10 min each. Subsequently, the NiO ink was dropped onto glass substrate and spin-coated at 4000 rpm for 90s. Spinning process was repeated 10 times to make the film thickness of ~250 nm. These substrates were rapidly transferred to a heating plate for drying at 150 °C for 5 min in order to evaporate the solvents and remove residuals. Finally, the as-deposited NiO films were transferred into the tube furnacefor annealing in ambient air from 300 to 600 °C for 60 min.

2.4. Characterization

The structural investigations were carried out by FESEM (Hitachi S4700) and the crystallinity of the NiO films were examined by XRD (Rigaku) equipped with the Cu-Kα radiation (λ=1.54178Å) in the range of 20-90 at 40 kV. The chemistry of the as-deposited NiO films was analyzed by energy-dispersive X-ray spectroscopy (EDS). The optical characterizations were carried out by UV-Vis spectrophotometer and room-temperature photoluminescence (PL) emission spectra was measured with 325 nm line of a He-Cd laser as an excitation source. Carrier concentration (n), carrier mobility (µ), and resistivity (ρ) were determined by Hall measurement method at room temperature.

3. Results and discussion

3.1. Structural properties of NiO thin films

Fig.1 is showed FESEM images for morphologies of NiO films. Fig. 1(a) and 1(b) depict the top and side-view of the thin film, which reveal a uniform and dense NiO film. It can be further seen from the both figures, the presence of monocrystalline grains are uniformly distributed onto entire substrate surface with ~250 nm average film thickness. The distribution of chemical elements was observed by EDS as shown in Fig. 1 (c), which exhibits two high intensity peaks of Ni and O, thereby confirming the high purity of as-prepared NiO films. This type of film is highly suitable for efficient charge conduction and estimation of optical properties for optoelectronic devices. Fig. 1(d) shows the patterns of NiO thin films that were processed at diverse range of annealing temperatures (300 and 600˚C). Obtained XRD patterns exhibit very strong diffraction peaks, indicating good crystallinity since face-centered cubic crystalline NiO structure for both of the samples was observed. Peaks at 2θ = 37.27, 43.3 and 62.90 can be indexed to be (111), (200) and (220), respectively (JCPDS-ICDD no. 78-0429) ¹⁸. In addition, there was no any other impurity (nickel oxide hydrates) detected, further confirming the high purity of NiO films. However, a change in intensity and position of 2θ was noted upon increasing the annealing temperatures from 300 to 600 °C. Moreover, the deposited material exhibited an improved crystallite size due to increase in annealing temperature ¹⁹.

Fig. 1. FESEM figures of NiO film (a) Top view, (b) cross-sectional view, (c) EDX spectra and  (d) XRD measurements of NiO films after annealing at 300 ^oC and 600 ^oC.

3.2. Optical properties of NiO thin films

The UV/Vis absorption and transmittance spectrum of NiO thin films treated at 300 ^oC and 600 ^oC are displayed in Fig. 2. No absorption band is observed in the visible region for both NiO films and only characteristic spectrum was noted in an ultraviolet range of 370 to 390 nm (Fig. 2(a)) with fundamental absorption peak of Ni-O bond. A sharp absorbance indicates the uniformity or continuity of NiO thin films with no extra peaks. The prior band in the range of 300 nm to 355 nm is attributed to the distinctive onset absorption band of NiO. However, upon annealing at higher temperature (300 and 600 ^oC), a slight red-shift in the absorption spectra (3.26 eV to 3.13 eV) was observed, which indicates a marginal increment in density of states for holes upon increasing of temperature²⁰. Fig. 2(b) shows the transmittance spectra of NiO thin films deposited on glass substrate. Both NiO films due to annealing treatment were found to be opaque for wavelengths below 300 nm. The maximum transmittance of 91.25 % is observed around 800 nm for both of these films. Moreover, increment of annealing temperature range from 300 ^oC to 600 ^oC, the transmittance decreased, verifying the transformation from a reflective Ni-metal state reverts to transmissive NiO state as wide band gap semiconductor²¹. The optical band gap energy (Eg) was estimated after applying Tauc’s graphs between absorption (y-axis as αhυ ²) against photon energy (x-axis). The formulated line of linear region on y-axis was intercepted until zero value on the x- axis (photon energy axis) which results in optical band-gap energy of NiO, as shown in Fig. 2(b)-inset.  The band-gap energy of NiO thin films is found between 3.63 eV and 3.44 eV at annealed temperatures 300 ^oC and 600 ^oC, respectively. Thus, the band-gap energy (~0.19 eV) decreases with increase in annealing temperature. The reduction of band-gap energy with changing temperature caused by oxygen ion vacancies engaged by electrons which is acting as donor centers, resulted in broad band absorption due to level of energy  lies close to valance band and results in a red shift of the band energy²².

Fig. 2. (a) UV/Vis absorption and (b) transmittance spectra of NiO thin films after annealing at 300 ^oC and 600 ^oC

In order to interrogate the impact of annealing temperature on the optical emission for NiO thin film; we measured PL spectrum of NiO thin films at annealing temperature between 300 ^oC and 600 ^oC in the range of wavelength from 300 nm (starting) to 800 nm (finishing), as presented in Fig. 3. It is noted that emission spectrum after excited at 325 nm showed three clearly visible peaks at 385, 516 and 715 nm. A peak at wavelength of 385 nm is associated to the near-band-edge (NBE) as UV emission in NiO due to electronic transitions of the Ni²⁺ ions²³. Other broad peaks at wavelength of 516 nm and 715 nm are correlates to green emission spectrum in the visible region. Generally, the oxide of nanomaterials consisting origan The origin of visible emission in these oxide nanomaterials is generally attributed to the presence of defects in the NiO lattice, such as the cation vacancies, interstitial oxygen trapping and nickel vacancies produced by charge transfer between Ni²⁺ and Ni³⁺ ions²⁴. When the annealing temperature of NiO films is increased from 300 to 600 °C, a small change in the emission intensity is observed.

Fig. 3. PL emission spectrum of NiO thin films after annealing at 300 ^oC and 600 ^oC

3.3. Electrical properties of NiO thin films

            Important electrical characteristics of NiO films like resistivity, Hall mobility and charge density or carrier concentration was obtained by Hall-Effect measurement method at an ambient temperature. As shown in Fig. 4, the electrical resistivity decreases drastically from 1020 ohm-cm to 700 ohm-cm upon increasing annealing temperature. This is due to annealing at 600 oC under air (oxygen-rich), which causes production of larger grains that lead to high crystallinity and low oxygen vacancies in sol-gel deposited NiO films, which is key factor of lower resistivity²⁵.  The Hall mobility of NiO thin film after heated 300 oC was 5.33 cm2/V.s, however, a film heated at 600 oC exhibited lower mobility (3.36 cm2/V.s). The carrier concentrations of films heated at 300 oC and 600 oC were 2.59 × 1015 and 1.26 × 1016 cm-3, respectively. These observed changes may lead to the homogenization and stabilization of the NiO films. This is because of the larger grain size and reduction in native defects caused by the change in annealing temperature²⁶.

Fig. 4. Electrical characteristics measured form NiO thin films.

4. Conclusions

            In summary, a low cost and easily process able sol-gel technique has been demonstrated for the deposition of NiO films using spin coating on glass substrates. The deposited films resulted in this research exhibited smooth surface morphology with good adhesion on substrate. This was attributed to an ideal stability of NiO solution. The impact of annealing conditions on crystallinity, optical and electrical characteristics of NiO films were also studied. Interestingly, annealing treatment of the NiO films enhanced the absorption spectrum and XRD and FESEM analyses confirmed that the as-deposited NiO films were face-centered cubic crystalline in nature. While the band gap energies obtained for these films were found to be in the range of 3.63-3.44 eV. Reduction in electrical resistivity and increment in charge carrier were found after increase in annealing temperature (from 300 oC to 600 oC) under ambient atmosphere. Moreover, PL measurements of NiO thin films further confirmed the existence of defects in the structure. The present results motivate its further application in the emerging solar cell, light-emitting diode and other optoelectronic devices since the p-type substitute available presently limits its performance.

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